(Co)polymers of chlorotrifluoroethylene: synthesis, properties, and applications.

Review pubs.acs.org/CR

(Co)polymers of Chlorotrifluoroethylene: Synthesis, Properties, and Applications Frédéric Boschet* and Bruno Ameduri* Ingénierie et Architectures Macromoléculaires, Institut Charles Gerhardt UMR (CNRS) 5253, Ecole Nationale Supérieure de Chimie de Montpellier, 8 Rue de l’Ecole Normale, 34296 Montpellier, Cedex 5, France 3.2.11. Poly(CTFE-co-(per)fluoroalkenes) Copolymers 3.2.12. Poly(CTFE-co-perfluoroalkyl vinyl ethers) Copolymers 3.2.13. Poly(CTFE-co-fluorinated dioxolane) Copolymers 3.2.14. Poly(CTFE-co-acrylonitrile) Copolymers 3.2.15. Kinetics of Radical Copolymerization of CTFE 3.2.16. Poly(CTFE-ter-M1-ter-M2) Terpolymers 3.2.17. Poly(CTFE-ter-VDF-ter-HFP) Terpolymers 3.2.18. Conclusions 3.3. Copolymers Achieved by Controlled Radical Polymerization 3.3.1. Theoretical Concepts on Controlled Radical Polymerizations 3.3.2. Controlled Radical Copolymerization of CTFE 4. Well-Defined Copolymers Based on CTFE 4.1. Fluorinated Alternating Copolymers 4.2. Telechelics Containing CTFE Base-Units 4.2.1. From Telomerization 4.2.2. From Dead-End Polymerization 4.3. Fluorinated Block Copolymers Based on CTFE 4.4. Fluorinated Graft Copolymers 4.4.1. From Traditional Radical Polymerization 4.4.2. From Controlled Radical Polymerization 4.5. Fluorinated Dendrimers 4.6. Chemical Modification of PCTFE or Poly(CTFE-co-M) Copolymers 4.6.1. Chemical Modification of PCTFE 4.6.2. Chemical Modification of Poly(CTFE-altvinyl ether) Copolymers 4.6.3. Chemical Modification of Poly(CTFE-coM) Copolymers 4.7. Cross-Linking 5. Applications of CTFE-Based Fluoropolymers 5.1. Fuel Cells 5.2. Coatings and Films 5.3. Piezoelectric/Ferroelectric/Dielectric Devices 5.4. Optical Applications 5.5. Thermoplastic Elastomers 6. Conclusions Author Information Corresponding Authors

CONTENTS 1. Introduction 2. Synthesis and Homopolymerization of Chlorotrifluoroethylene 2.1. Introduction 2.2. Synthesis of Chlorotrifluoroethylene (CTFE) 2.3. Homopolymerization of Chlorotrifluoroethylene 2.3.1. Structure of PCTFE 2.3.2. Properties of PCTFE 2.3.3. Telomerization of CTFE 2.3.4. Applications of Oligo(chlorotrifluoroethylene)s and CTFE Telomers 3. Copolymers of Chlorotrifluoroethylene 3.1. Introduction 3.2. Copolymers by Conventional Radical Polymerization 3.2.1. Poly(CTFE-co-Ethylene) Copolymers 3.2.2. Poly(CTFE-co-Propylene) and Poly(CTFEco-isobutylene) Copolymers 3.2.3. Poly(CTFE-co-allylic monomers) Copolymers 3.2.4. Poly(CTFE-co-(meth)acrylates or (meth)acrylic Monomers) Copolymers 3.2.5. Poly(CTFE-alt-vinyl ethers) Alternating Copolymers 3.2.6. Poly(CTFE-co-Vinylene Carbonate) Copolymers 3.2.7. Poly(CTFE-co-vinyl esters) Copolymers 3.2.8. Poly(CTFE-co-aromatic co-monomers) Copolymers 3.2.9. Poly(CTFE-co-vinyl chloride) or Poly(CTFE-co-vinylidene chloride) Copolymers 3.2.10. Poly(CTFE-co-vinylidene fluoride) Copolymers © XXXX American Chemical Society

B B B C D D F J J N N O O O P P P S U U

AA AA AB AB AB AC AD AD AD AD AE AE AE AF AF AF AF AG AG AH AJ AJ AJ AK AM AN AN AN AQ AR AR AS AS AS AS

V Received: July 30, 2011

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dx.doi.org/10.1021/cr2002933 | Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews Notes Biographies Acknowledgments Dedication List of Symbols and Abbreviations References

Review

The first review on PCTFE by Chandrasekaran was published in 1985.15 This was followed by Ruesch and Ferstandig’s review in 199316 (updated twice since then17,18), Taylor et al.’s chapter on the modification of PCTFE in 1997,19 a book chapter in 1999 (Hougham et al.5), and more recently another chapter by Millet and Kosmala.20 Considering the activity in this field, it seems timely to write an up-to-date and comprehensive review, which also, for the first time, includes CTFE copolymers. The objectives of this review are to present quasi-exhaustive data on the synthesis, homopolymerization, and copolymerization of CTFE with an emphasis on the resulting (co)polymers including their kinetics. Properties and applications will also be covered. After first summarizing the different synthetic approaches to CTFE, the second part of this review deals with its homopolymerization and telomerization. In the third and principal part, various CTFE copolymers are presented. In this section, the emphasis is on the kinetics of radical copolymerization of CTFE with various co-monomers. The fourth section first focuses on strategies to design well-defined copolymers based on CTFE and subsequently the modification of these CTFEcontaining copolymers. The final section is devoted to the applications of CTFE-based copolymers.

AS AS AT AT AT AU

1. INTRODUCTION Fluorinated polymers1−7 are niche macromolecules that attract much interest because of the versatility of a wide range of applications. Their use ranges from thermoplastics, elastomers, and plastomers to thermoplastic elastomers as well as from semicrystalline to fully amorphous. Furthermore, largely due to the properties of the fluorine element (strong electronegativity, low polarizability, and small van der Waals radius (1.32 Å)) and to the strong C−F bonds (485 kJ mol−1), they exhibit unique and remarkable properties. Hence, fluoropolymers with a high fluorine content exhibit exceptional properties. These include high chemical, thermal, aging, and weather resistance as well as outstanding inertness to hydrocarbons, solvents, acids, and bases. Other highly desirable properties include low dielectric constant, low surface energy (water and oil repellency), low flammability, low refractive index, and low moisture absorption. Furthermore, the high strength of the C−F bond in these polymers has a critical impact on their great resistance to both hydrolytic and oxidation stability. As a result of this long list of positive attributes, these specialty polymers1−7 have found many important applications in the construction and automobile industry. Some examples are UV and graffiti resistant paints and coatings, the preservation of stone monuments, the petrochemical industry, aerospace and aeronautics (as seals, O-rings, and gaskets for hydrazine or liquid hydrogen tanks in space shuttle boosters), chemical engineering (high performance membranes and tubings), optics (claddings and cores of optical fibers), textile treatments, microelectronics, and electrical insulation (cables and wires). These are all areas in which fluoropolymers have been found to be useful. Therefore, despite the high price of their production, mostly related to the purification costs of the gas monomers, to the uncommon polymerization processes and to the small production scales involved, fluoropolymers play an important role in the development of recent technologies. Nonetheless, it must be recognized that fluoropolymers also have their drawbacks. High crystallinity is often encountered in the homopolymers with, as a consequence, a poor solubility in common organic solvents. They are also not easy to cure or cross-link. Hence, the synthesis of specially designed fluorinated copolymers6−11 has been the focus of much research. Such copolymers are composed of at least one co-monomer that leads to the insertion of bulky dangling groups that ultimately induces disorder in the macromolecular structures, reducing the high crystallinity of the homopolymer. The range of such customized fluoropolymers, which do not have the disadvantages of the homopolymers mentioned above, has grown over recent years. Poly(chlorotrifluoroethylene) (PCTFE) is one such fluoropolymer, although its volume production is somewhat less than either PTFE12 or PVDF.13 PCTFE has attracted much interest for its specific properties. It is one of the best gas barrier materials known;14 it has superior inertness and good film-forming properties and is easily processed.15 However, as discussed above, its high crystallinity induces insolubility in most common organic solvents.

2. SYNTHESIS AND HOMOPOLYMERIZATION OF CHLOROTRIFLUOROETHYLENE 2.1. Introduction

The very first fluoropolymer discovered was polychlorotrifluoroethylene (PCTFE; CAS no. 9002-83-9). Its synthesis was pioneered by Schlöffer and Scherer at IG Farbenindustrie AG in 1934. However, the corresponding patent was only issued in 1937.21,22 Coincidentally, this is the exact same year of the discovery of polytetrafluoroethylene (PTFE, Teflon) by Plunkett at the DuPont Company.23 Because of its excellent combination of properties, PCTFE has attracted much interest over the years, although, it must be recognized, to a lesser extent than either poly(vinylidene fluoride) (PVDF) or PTFE. The 3M Company commercialized PCTFE under the Kel-F trademark but it was discontinued in 1995. Currently, PCTFE is only available commercially from Daikin and Honeywell under the trade names Neoflon and Aclar, respectively. However, it is possible that Russian, Indian, or Chinese companies also produce it. Homopolymers derived from CTFE are semicrystalline long chain macromolecules that contain 48.9 wt % of fluorine and 30.5 wt% of chlorine. Their crystallinity ranges from 30 to 70%. PCTFE, endowed with a high level of intrinsic crystallinity (70%), exhibits good mechanical strength and low elongation, whereas amorphous PCTFE (crystallinity ca. 30%) is optically clear, more elastic, and less dense. They are both suitable thermoplastics. PCTFE is a nonflammable polymer24 with excellent chemical resistance, it has both excellent barrier and electrical properties. It is endowed with a unique combination of physical and mechanical properties, such as excellent low creep over a useful temperature range of −250 to +200 °C. This particular property is not found in any other fluorinated polymer. Furthermore, due to extremely low outgassing, PCTFE is also suitable for use in the aerospace and aviation industries.3 It must be noted, however, that in spite of the many interesting high-tech applications for PCTFE such as cryogenic components, oils (low molecular weights PCTFE), valves, seals, gaskets, gas barrier films, and energy-related applications these polymers also exhibit some disadvantages. These will be discussed in more B


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detail later but three major disadvantages to be noted are (i) they have high melting temperatures, (ii) PCTFE is only soluble in 2,5-dichloro-trifluoromethyl benzene at 150 °C,25,26 and (iii) it is difficult to cure. With respect to (i) above, high melting temperatures in turn generate high-energy costs for polymer processing. In addition, this is coupled with a nearby similar decomposition temperature. Fluorinated copolymers based on CTFE have attracted much interest since the pioneering work of Thomas and O’Shaughnessy27 in 1953. Although the copolymers of CTFE, as well as ECTFE,28,29 and the vinyl ethers have attracted a lot of attention (more than 300 patents have been assigned to Asahi Glass Co.)30,31 other copolymers appear to remain of marginal interest. It appears that further work needs to be carried out in order to develop other CTFE-based copolymers. Before describing the synthesis, properties, and applications of PCTFE- and CTFEcontaining copolymers, it is probably useful to summarize the preparation of that monomer.

When working with CTFE, in order to avoid an explosion, oxygen should be avoided since this can generate peroxides as well as other oxygenated products33−36 that may promote its autopolymerization. CTFE is stable in degassed water but hydrolysis is possible in the presence of dissolved oxygen and is enhanced in the presence of alkali.33 CTFE also reacts with halogens, amines, alcohols, chloroform, and halomethanes. The formation of CTFE dimers (4,4-dichlorohexafluoro-1-butene)12 can be catalyzed either by sintered porous PTFE, by a mixture of PTFE and sodium, or by potassium fluoride at 300−450 °C giving yields above 90%. Traces of trimer impurities are also observed. In addition, thermally at temperatures above 400 °C, CTFE can form cyclic dimers such as cis- and trans-1,2dichlorohexafluorocyclobutanes.37 Although CTFE is one of the most widely used monomers, after tetrafluoroethylene and vinylidene fluoride, its availability could very well be endangered because the production of its precursor 1,1,2-trichloro-1,2,2-trifluoroethane (ClCF2CFCl2 also known as CFC113 or Freon 113) was phased out a decade ago in many countries. This is largely due to the fact that this widely used chemical depletes the ozone layer. In fact, CTFE is obtained from this halogenated ethane either by dehalogenation in the gas phase at 500−600 °C,38,39 by reaction with hydrogen40−42 or ethylene,43,44 or by dehalogenation in solution using zinc.45−47These different routes are described in Scheme 1. These syntheses yield various byproducts such as chlorodifluoroethylene, trifluoroethylene, dichlorotrifluoroethane, methyl chloride, dimethyl ether, and CTFE dimers. Thus, the purification of CTFE requires several steps including distillation to get rid of these contaminant byproducts. First, both the methyl chloride and dimethyl ether impurities are removed by passing CTFE gas through sulfuric acid. Water and hydrochloric acid are then eliminated using an alumina column. Finally, the CTFE is condensed and the remaining gases purged. CTFE is also formed as a byproduct during the synthesis of HFC-125 (CF3CHF2) from HCFC-124 (CF3CFHCl) as described in Scheme 2.48 In fact, the side-reaction between HFC-125 and AlF3 yield CTFE in minor quantities. Honeywell (formerly Allied Signal, USA) and Daikin (Japan) are the main commercial suppliers of CTFE, although some producers such as P and M Invest Ltd. have emerged in Russia and more recently in China, the Jiangsu Kangtai Fluorine Chemical Company, Ltd., Changshu 3F Fluorochemical Industry, Shanghai Hanhong Chemical Company, Ltd., and Hangzhou Verychem Science and Technology Company, Ltd. It

2.2. Synthesis of Chlorotrifluoroethylene (CTFE)

Chlorotrifluoroethylene, (CAS no. 9036-80-0) is also known as trifluorovinyl chloride, 1-chloro-1,2,2-trifluoroethene, trifluorochloroethylene, and monochlorotrifluoroethylene (Figure 1). It

Figure 1. 19F−19F COSY NMR spectrum of CTFE (recorded in CDCl3, 400 MHz).

is a colorless, odorless gas with a boiling point of −28 °C.15 It is a toxic gas with a measured LC50 on rats of 4 h at 4000 ppm.32

Scheme 1. Preparation of Chlorotrifluoroethylene from 1,1,2-Trichlorotrifluoroethane (CFC113) by Various Routes38−47

C


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Scheme 2. Preparation of Chlorotrifluoroethylene as a Byproduct during the Synthesis of Pentafluoroethane from 1Chloro-1,2,2,2-tetrafluoroethane48

Table 1. Tentative Assignment of PCTFE Infrared Vibrational Bands51,52 assignmenta CFCl deformation CF2 deformation CF2 wagging CC stretch

is estimated that the overall volume production is about a thousand tons per year.

CCl stretch

2.3. Homopolymerization of Chlorotrifluoroethylene CF2 asymmetrical stretch CF2 symmetrical stretch CF stretch Δυ=3 CF2 wagging

The homopolymerization of CTFE was discovered and pioneered by Schlöffer and Scherer in 1934.22 This PCTFE was of low molecular weight and exhibited poor mechanical properties. Following its discovery, scientists at Cornell University involved in the Manhattan Project developed more useful higher molecular weight PCTFE as well as CTFE oils, greases, and waxes to be used as inert lubricants for mechanical equipment needed in uranium isotope separation. Subsequently, these lubricants were commercialized by Hooker Electrochemical Co. After World War II, the need for inert lubricants increased due to a growing demand in industry for materials capable of sustaining an aggressive environment (Cl2, F2, O2, H2, UF6, peroxides, acids, etc). Their unique properties such as high density, low compressibility, and low vapor pressure led to other applications for PCTFE lubricants.16−18 High molecular weight PCTFE homopolymer was commercialized in the late 1950s by the Kellogg Co. This was later purchased by Minnesota Mining & Manufacturing now better known as 3M. Until 1995 PCTFE was later marketed under the Kel-F trademark, when 3 M sold it to Daikin. They market it under the Neoflon and Daifloil (oligomers) trademarks. In the 1960s, Allied Signal (now Honeywell) started commercializing PCTFE under the Aclon trademark. Other commercially available PCTFE trade names include Voltalef (resin and oil/ grease) from Arkema (formerly Elf Atochem), Fluorolube from Gabriel Performance Products (formerly the product line of Occidental Chemical Corporation), and HaloVac oils, greases, and waxes from Halocarbon Products Corporation. 2.3.1. Structure of PCTFE. Infrared spectral band assignments were first reported in 1956.49,50 Later Kawano and De Araújo revisited51,52 the attributions of the near and mid infrared spectra of PCTFE (see Table 1). Their studies on various samples (monomer, liquid, wax, and solid) reveal the presence of at least two chain conformations, with the predominance of one of them in the solid state.51 Using nuclear magnetic resonance (NMR) spectroscopy, Tiers and Bovey53 demonstrated that PCTFE has isotactic sequences; although the syndiotactic sequences predominate. The preference for syndiotactic conformation was attributed to the lower activation entropy for the latter although the activation enthalpies are identical. Reverse additions, i.e., tail-to-tail sequences, can occur to a small degree (5−10%) when the temperature of the telomerization (see section 2.3.3) exceeds 120 °C or when severe activation conditions are used to promote the telomerization.54 19F transverse relaxation has also been used to characterize PCTFE.55,56 However, the model developed by the authors, which involved flexible polymer chain-ends characterized by bends and twists between segments, was not

Δυ=2 CCl stretch Δυ=2 CF2 symmetrical stretch Δυ=4 CF2 wagging Δυ=2 CF2 asymmetrical stretch Δυ=2 CF stretch 2125 + 1202 Δυ=3 CF2 symmetrical stretch Δυ=3 CF2 assymmetrical stretch 2349 + 1202 Δυ=3 CF stretch Δυ=4 CF2 symmetrical stretch 3486 + 937 3544 + 937 Δυ=4 CF2 asymmetrical stretch Δυ=4 CF stretch

a

band (cm−1) 439 490 520 583 598 649 666 698 902 937 970 1202 1130 1285 1646 1783 1860 2125 2255 2319 2349 2484 3214 3255 3328 3398 3486 3544 3565 3619 3704 4329 4416 4468 4568 4749 5274 6926

υ = vibrational quantum number.

applicable unless the polymer folds were taken into consideration. PCTFE was also studied using 19F solid state NMR spectroscopy, using CRAMPS (combined rotational and multiple pulse spectroscopy),57 high-speed magic angle spinning (MAS),58−60 and 13C-[19F] CPMAS (cross-polarization magicangle spinning).61,62 Although it was possible to observe two peaks assigned to CF2 and CFCl, the resolution was too low to resolve the tacticity. The broadening of the CFCl signal at low temperature was attributed to second-order interactions with the quadrupolar chlorine nuclei. Thanks to a dipolar-filter pulse sequence, Tatsuno et al.60 were able to preferentially observe the amorphous domains (Figure 2, left). The sharp signals noted at 140 °C reflected the polymer chain tacticity. On the other hand, the T1ρ-filter showed the crystalline signals at lower temperature and the rubbery ones at higher T (Figure 2, right). The authors were then able to show the crystalline dependence of the 19F NMR spectrum and concluded that PCTFE crystallites are more mobile than those of other semicrystalline fluorinated polymers D


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Figure 2. Dipolar-filter (left) and T1ρ-filter (right) 19F MAS NMR spectra of PCTFE observed at various temperatures (MAS stands for magic angle spinning). [Reprinted with permission from ref 60. Copyright 2007 Wiley Interscience.]

Table 2. Correlation of Density (d), Viscosity (η), and Vapor Pressure (Pvap) of Chlorotrifluoroethylene Liquid Polymers with Molecular Weights (Mw) at 38, 60, and 99 °C75 T = 38 °C

a

T = 60 °C

T = 99 °C

Mw

d, g/mL

η, cs

Pvap, mm Hg

d, g/mL

η, cs

Pvap, mm Hg

d, g/mL

η, csa

Pvap, mm Hg

550 600 650 700 750 800 850 900 950 1000 1050 1100 1150 1200 1250 1300

1.878 1.899 1.917 1.930 1.942 1.951 1.960 1.969 1.976 1.982 1.988 1.993 1.998 2.004

4.97 9.37 18.9 36.7 76.2 145 268 520 902 1520 2830 4370 7560 12300

20 15 11 8.0 5.7 3.8 2.2 1.0 0.32 0.05

1.8283 1.8490 1.8669 1.8810 1.8917 1.9014 1.9100 1.9184 1.9253 19315 1.9373 1.9424 1.9478 1.9528 1.9574 1.9617

2.60 4.26 7.42 12.9 22.4 36.7 60.3 104 164 251 385 575 892 1330

110 76 52 36 25 16 9.6 5.2 2.3 0.68

1.742 1.762 1.780 1.794 1.804 1.814 1.822 1.831 1.837 1.844 1.849 1.854 1.860 1.864

1.21 1.68 2.45 3.76 5.17 7.27 10.2 15.0 21.2 28.5 36.6 50.3 67.8 91.2

720 560 420 310 220 105 95 54 27 12 4.1 1.1 0.29

a

a

1 cs = 1 centistroke = 1 Centipoise = 1 mPa s.

(such as PVDF) especially at temperatures above 80 °C. This suggests the presence of structural imperfections in the PCTFE crystallites, and as a result of DFT calculations, these were ascribed to conformational changes (i.e., more helical or twisted). The tacticity of PCTFE was also investigated using infrared spectroscopy as well as calculations based on Kramers−Kronig relations.63 This made it possible to investigate the isotactic and syndiotactic configurations of PCTFE. A comparison between the experimental and simulated spectra revealed the preferred syndiotactic configuration of the PCTFE chains. The ESR spectra of PCTFE were recorded following γradiolysis under vacuum, both at room temperature and at 77 K,64 at which temperature the spectral results from four different types of radicals were assigned to both mid-chain and end-chain radicals. It was shown that the mid-chain radicals were formed by defluorination, whereas the end-chain radicals resulted from a C−C bond cleavage.

Several studies report the crystal structure of PCTFE.65,66 They confirm a pseudo hexagonal lattice (lattice parameter a = 0.644 nm and c = 4.15 nm). The polymer chains adopt a helical structure consisting on average of 16.8 CTFE units per turn. Skeletal angles on the CF2 and CFCl atoms were found to differ by 5−7°. This agrees with the assumption that the polymer is atactic with random positioning of the chlorine substituents. This randomness affects the microstructure to such an extent that the polymer diffracts as if it were constituted of continuous helices. The degree of crystallinity is also affected by several parameters including molecular weight, quench rate, and thickness. In commercially available grades this ranges from 40 to 80%. Crystal growth is spherullic and consists of folded-chain crystallites.67 When formed from solution, crystals consist of multisectional disk-shaped lamellae with void-like regions encapsulated between adjacent sectors.68 The densities of fully crystalline and fully amorphous PCTFE are 2.187 and 2.077 g cm−3, respectively.69,70 From this data, it is then possible to determine E


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agents, and in addition, it is a cryogenic polymer. Replacing a fluorine atom in PTFE by a chlorine atom improves the mechanical properties (rigidity, low temperature toughness, resistance to creep, and cold flow), its impermeability to moisture and gases, the optical clarity, and its film-forming properties.15 However, it has also been established that the presence of chlorine increases the surface tension and the coefficient of friction and decreases the thermostability, the electrical properties, and to some extent the chemical resistance.15 As explained above, the mechanical and optical properties result from the crystallinity, which is also linked to the molecular weight, and this endows PCTFE with a wide range of properties. For example, crystallinity decreases with molecular weight, and thus a low molecular weight PCTFE becomes brittle, but, on the other hand, it becomes easier to process (lower melting point). Wear properties were studied using neutron activation analysis, scanning electron microscopy,79 surface tension, and by the assessment of the friction coefficient. Its friction coefficient, like that of FEP, is low, and this results in excellent antistick properties which are comparable to those of polytetrafluoroethylene.80 As mentioned earlier, PCTFE is insoluble in most common organic solvents, and high temperatures (above 120 °C) are often required to achieve solubility.25,26,81,82 Suitable solvents include 1,2-dichloro and 2,5-dinitrotrifluorobenzene (130 °C), 2,5-dichlorotrifluoromethylbenzene (130 °C),83 benzene (200 °C),84 cyclohexane (>235 °C),84 toluene (142 °C),84 carbon tetrachloride (114 °C),84 1,1,1-trichloroethane (120 °C),84 and 1,2,3-trifluoropentachloropropane.81,82,84,85 2.3.2.1. Thermal Properties. PCTFE is a thermally stable and chemically inert polymer.86 Frey et al.87 reported that below 100 °C only chlorine induced a color change in PCTFE. Watson et al.88 showed that PCTFE does not show any significant thermal decomposition up to 250 °C when heated for 3 h in a stream of moist air. However, at higher temperatures, the effect of various chemicals can be observed by discoloration or a release of hydrogen chloride.86 Therefore, it appears that degradation reactions can be observed in the presence of some organic substances that may be used as dispersion media or solvent and metals involved as substrates for films.89 Various values of the glass transition temperature for PCTFE have been reported: 52 °C,90,91 64 °C,92 47−77 °C (depending on crystallinity),93 71−99 °C,20,70,94 and 150 °C.95,96 With the specific intent of clarifying its glass transition temperature (Tg), molecular motions and transitions of PCTFE were studied over the −150 to +200 °C range by thermal/mechanical techniques94,97−100 or dielectric relaxation measurements.76,95,101−103 Both techniques show three transitions at +150, +90, and −37 °C, which were assigned to α, β, and γ relaxations, respectively. In samples of low crystallinity, the γ transition is not observed and the transition becomes a doublet at −20 and −40 °C This doublet was assigned to the amorphous and crystalline phases, respectively. More recently,94 an extremely broad Γ relaxation, centered at ∼ −15 °C, associated with an activation energy of 17 kcal mol−1, was observed. It was proposed that this is the result of a predominantly amorphous phenomenon involving small-scale motions. A β relaxation peak at ∼95 °C with an activation energy of 64 kcal mo1−1 represents the amorphous segmental mobility. Its onset at 75 ± 2 °C is assigned as the Tg. This Tg value for PCTFE is supported by dynamic mechanical, thermomechanical and differential scanning calorimetric data. Khanna and Kumar94 were unable to obtain data to support the existence of an α relaxation, related to the motions within the crystal, at 140−150

the crystallinity of a PCTFE sample by measuring its density at 30 °C (d30) using the following equation: θ=

d30 − 2.077 2.187 − 2.077

The usual crystallinity of PCTFE ranges between 40 and 80%.71 Napolitano and Puccirariello72 studied the inclusion of defects into poly(tetrafluoroethylene). The results of their calculations of both conformational and packing energy suggested that the chlorine side groups are easily tolerated in the crystal lattice. This is in contrast to CF3 (hexafluoropropylene) and CF3O(perfluoromethyl vinyl ether) that are tolerated even with deformation. As a result, an efficient packing mode is possible for poly(TFE-co-CTFE) copolymers. These differences were explained in terms of bulkiness of the substituent rather than repulsive interactions. The results shown in Table 2 clearly indicate that the density of the oligo(CTFE)s increases with molecular weight and decreases with temperature at constant molecular weight. An early positron annihilation study on PCTFE73 showed that the o-Ps lifetime and intensity varied systematically with aging time. The free volume exhibited Doolittle-type free volume relaxation which indicates that the physical aging appears to yield close packing of the polymer chains without concomitant crystallization. In the case of a copolymer of CTFE (96 mol %) and VDF (4 mol %), the structure looks like a twisted ribbon after cooling at temperatures above 79 °C, whereas the nucleation is mostly homogeneous. Below 79 °C, heterogeneous nucleation dominates and a spherullic morphology is obtained.74 The less crystalline materials are optically clear, tough, and ductile as well as being endowed with higher elasticity and lower modulus. On the other hand, the more crystalline ones are less transparent, with high tensile modulus and less elongation. But they do exhibit better impermeability to liquids and vapors20 The result is that these particular characteristics make PCTFE the best vaporbarrier polymer. 2.3.2. Properties of PCTFE. The relationships between molecular weight, viscosity, density, and vapor pressure of a number of fractions of liquid PCTFE polymers are summarized in Table 2.75 The dielectric properties of low molecular weight PCTFE were studied by Hara.76 This author established that these characteristics vary greatly with the molecular weight, and that the crystalline region contributes to the polarization rather than the coexistence of the amorphous and crystalline phases as described by the Maxwell−Wagner model. The polarization arises from two dielectric polarizable modes in the crystalline phase: the main chain dipoles and the end groups dipoles. Other electrical properties summarized in Table 3 have been reported by several authors.20,77,78 PCTFE is endowed with good vapor-barrier properties, superior thermostability, and excellent resistance to oxidizing Table 3. Electrical Properties of PCTFE20,77,78

a

properties

ASTM method

value

dielectric strengtha arc resistance volume resistivityb surface resistivityb

D149 D495 D257 D257

20 V μm−1 360 s 10 Ω cm2 cm−1 (ref 78) 10 Ω (ref 77)

Short time. b50% relative humidity at 25 °C. F


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Scheme 3. Mechanism for the Thermal Degradation of PCTFE According to Zulfiqar et al.104

°C (i.e., Tg < T < Tm), as mentioned in the other studies. The melt temperature (Tm) values for PCTFE are typically 211−216 °C and the polymer is thermally stable up to 250 °C.20 Zulfiqar et al.104−106 studied the thermal decompositions of PCTFE and of its copolymers with VDF and methyl methacrylate (MMA) and found that PCTFE decomposes mostly into the CTFE monomer with traces of C2F2Cl2, C3F5Cl, and C2F3Cl3 (Scheme 3). In the case of copolymers with VDF, HF was also detected, as well as some traces of HCl. The decomposition of PCTFE starts at 300 °C and accentuates at 400 °C. The homolysis of a C−Cl bond seems to be the first step in the degradation of PCTFE. Hence, chain scission and depolymerization can occur (Scheme 3). The proposed mechanism does not involve the end group in the degradation mechanism. For poly(CTFE-co-MMA) copolymers, thermal decomposition led to the formation of mainly the monomers but also CO and CO2.This can be explained by the generation of lactone as observed for the degradation of PMMA homopolymers. Gianetti107,108 tried to rationalize the thermal degradation of PCTFE and poly(CTFE-co-E) (ECTFE) and to compare them to those of other fluoropolymers (Figure 3). He showed that PCTFE has poor thermal stability compared to other partially fluorinated polymers, a consequence of the presence of C−Cl bonds in the backbone. On the other hand, ECTFE has a 40% char residue at 550 °C. This results from a different degradation mechanism compared to the traditional homolytic cleavage postulated for PCTFE. The author also indicates that the degradation is strongly dependent on the atmosphere and the nature of the end groups. He also calculated the C−C bond dissociation energies of various fluorinated linear vinyl polymers and showed that, in contrast to the above-mentioned thermogravimetric analyses, they do not differ much from each other.108 A more recent study109 on the thermolysis of fluoropolymers indicates that the thermolysis of PCTFE yields 3-chloropentafluoropropene CF2CFCF2Cl, CTFE monomer, and some haloacetic acids (chlorodifluoroacetic acid mostly and traces of tri-, di-, and monofluoroacetic acid). Halopropenes can be further degraded (in the troposphere for example) into haloacetic acids. It was also shown that copolymers of CTFE

Figure 3. Thermogravimetric analysis of commercially available fluorinated polymers (pellets; PVF = film), at 10 °C min−1 under nitrogen (PE, ETFE, and PVF stands for poly(ethylene), poly(TFE-coethylene), and poly(vinyl fluoride), respectively). [Reprinted with permission from ref 107. Copyright 2001 Wiley Interscience.]

(with ethylene for example) undergo the same degradation route. 2.3.2.2. Manufacturing and Processing of PCTFE. The polymerization of CTFE can be carried out either in bulk,12,110−114 solution,115−117 suspension,12,107,118−126 or emulsion107,127−130 either in the presence or absence of surfactants.125 PCTFE obtained in bulk is usually less stable than that obtained using other techniques. Although the pressure is usually low, it is recommended that the polymerization of CTFE be carried out in high-pressure reactors, and thus quantitative CTFE conversion can be achieved. After polymerization, the polymer is isolated from the latex or slurry, washed to remove initiator and other residues, and finally dried. Further treatments are possible to remove residual impurities that can lead to degradation. Treatment with carboxylic acids,131 ozone,132 or chlorine133 can all be carried out to convert the end groups resulting in improved thermostability, color, and light transmission of the final PCTFE. Since PCTFE is a melt-processable polymer, it can be extruded, injection molded, compression molded, and dispersion cast.12 Compression molding, the most suitable technique, is usually achieved at temperatures below 275 °C and requires G


H

CHI3 ClCF2CCl2I CF3CCl2I CF3CFClI CF3CFClI ClCF2CFClI ClCF2CFClI Cl2CFCFClI ICF2CFClI CF3I CF3I C2F5I CnF2n+1I iC3F7I or C2F5I CnF2n+1I n = 4,6

C2F5OF IBr ICl I2

SO2Cl2 CCl3−F SO2ClF Cl3CSO2Br CF3S−SCF3 C6F13(CH2)2SH RSH HO(CH2)2S(VDF)xH HSiCl2CH3 POCl3 H−PO(OEt)2 H−PO(OEt)2 Cl−PO(OMe)2 Cl2CH−P(O)Cl2 Cl3C−P(O)Cl2 Cl3C−P(O)Cl2 RR′CH−OH CH3OH CH3OH iPrOH boranes CF3OF

telogen

catalyst

UV TiCl4, ZnCl2, YrCl3 with H2N−C2H4OH CuCl/CuCl2

(C2F5CO)2O2 none none TBPPi none none UV BPO sunlight, UV or γ-ray UV BPO or TBPPi

UV Δ, hν, redox catalysts, radical initiator γ-rays

BPO AlCl3 DTBP no initiator UV TBPPi NR3, NR4+ BPO H2PtCl6 BPO DTBP DTBP BPO BPO BPO FeCl3/benzoin UV UV Peroxides γ-rays DTBP

RT 150 °C 150 °C

RT 40 °C 12 h, 50−55 °C 18 h, 140 °C 17 h, 145 °C 16 h, 70 °C 18 h, 180 °C 5 h, 180 °C RT 140−150 °C RT RT 8 h, 100 °C

−72 °C CH2Cl2, RT

7 h, 134 °C RT 4 h, 135 °C −78 °C

CH3CN, 70 °C 25−45 °C CH3CN, 80 °C 160 °C 95 °C 6 h, 130 °C 140 °C 95 °C CH3CN, 110 °C CH3CN, 110 °C CH2Cl2, 100 °C

various temperatures

CCl4, 95 °C CCl4, 20 °C

conditions

DPn

1−5, 2(yield=25%) 1−10

≤3 1−20 1−3 1 variable F(CF2CFCl)nF (7%) F3CO(CF2CFCl)nF (43%) 1−4 1−6 monoadduct + byproducts ClCF2CFClI (90−100) + Cl2CFCF2I (0−10) 1−6 2−10 13 10−15 4−6 1−10 1−3 1−20 1−11 1 2−10 1 (85%) 1−4 1−6

2−5 1 1−10 1−10 1−3 1 1 4−5 >1 3−4 1 (mainly) 1−5 2−20 35 100

Table 4. Telogens (or Chain Transfer Agents) and Initiating Systems Used in the Radical Telomerization of CTFEa ref

193 167 150,194−197 158 158,198 157 164 157,199 153 153 146 146 200 158 156,201 155,202 202,203 204 205 153,206

171,172 173 174 175 176,177 178 179,180 181 182 183 184 185 183 159 159 186 187 187,188 189 190 191 192

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a

CuCl2 CuCl2

CuCl CuCl2

I

conditions

18 h, 44 °C CH3CN, 130 °C CH3CN, 110 °C CH3OH, 110 °C CH3CN, 110 °C CCl4, 50 °C CH3CN, 110 °C CH3CN, 110 °C CH3CN, 130−150 °C CH3CN, 110−150 °C CH3CN, 150−200 °C CH3CN, 110−150 °C CH3CN, 110−150 °C RT CF2Cl−CFCl2, RT CH3CN, 120 °C RT CH3CN, 110 °C 100 °C RT RT RT RT CH3CN, 110−130 °C RT

24 h, 140 °C monoadducts (33%) telomers (52%) mono and diadducts 1 1−20 1−20 1−10 1 30−50 9−10 1−5 1−10 1−5 1−10 1−3 1 1 1−10 n = 1 (50%) 1 1−10 n = 1 (52%) 6−50 5−20 1 1 1,2 1 1

DPn

DPn stands for average degree of telomerization). DTBP, di-tert-butyl peroxide; BPO, benzoyl peroxide; TBPPi, t-butylperoxypivalate; n, CTFE units; RT, room temperature.

thermal CuBr2/Cu UV FeBr3/Ni BPO boranes AlCl3 UV UV FeCl3/benzoin CuCl2, DBP UV

AlCl3 CuCl or CuCl2 FeCl3/benzoin FeCl3/benzoin FeCl3/Ni AlCl3 BPO BPO FeCl3/benzoin or FeCl3/benzoin or FeCl3/Ni FeCl3/benzoin or FeCl3/benzoin or UV

iPrCl or tBuCl CCl4 CCl4 CCl4 CCl4 CCl4 CCl4 CHCl3 CF3CCl3 RCCl3 CFCl2−CF2Cl RO2C−CCl3 Cl−(CFCl−CF2)n−CCl3 HBr Br2 BrCF2−CFClBr BrCF2−CFClBr CF3−CClBr2 CF2Br2, CF2ClBr CF2Br2 CCl3Br CCl3Br CCl3Br CCl3Br CCl2Br2

catalyst

DTBP

cycloalkanes

telogen

Table 4. continued ref

208 168,209−211 160,161,170,212 168 143 213 159 159 142 214 215 216 217 218,219 220,221 222 223,224 225 226 227 201 218 223,228 144 223

207

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BrCF2CFClBr can be further dehalogenated which leads to the formation of trifluorovinyl monomers (section 2.3.4.2). Among those telogens containing C−I bonds, perfluoroalkyl iodides is the most prominent group. In contrast to TFE and VDF, these have not been extensively studied for the telomerization of CTFE, and it remains an area that is both complex and difficult to perform. This can be explained by the fact that the CFCl−I end groups borne by RF(CF2CFCl)nI in the resulting telomers are more reactive than the CF2−I end groups of RFI CTA.146,150−154 For example, the use of CF3I as the CTA for the telomerization of CTFE led to the formation of two isomers.155,156 Interestingly, Gumbrech and Dettre157 used a mixture of (C2F5CO2)2 and CHI3 (as the iodine donor) to obtain C2F5(CF2CFCl)nI telomers with n ≅ 13. Elemental iodine was used to prepare I(CTFE)nI telomers that were subsequently used as interesting telogens in other telomerization reactions.158 Redox catalysis appears to improve the selectivity and control of the reaction.153,159−161 Although transfer from the catalyst (CuClx and FeCl3) could lead to byproducts, especially for CCl3Br, that leads to disproportionation and gives CCl2Br2 and CCl4.144 In order to enhance the reactivity of telogens bearing C−I bonds, chlorine or bromine atoms were introduced into the iodine moiety.162−167 The kinetics of CTFE telomerization with various chain transfer agents has been extensively investigated by Boutevin and co-workers.160,168−170 However, it should be noted that already in 1955, Haszeldine146 had observed the pseudo-living character (see section 3.3) of the CTFE telomerization. According to the following reaction, that involves the iodine transfer polymerization of CTFE:6

annealing after molding to increase the dimensional stability. In injection molding, PCTFE is often degraded to lower the molecular weight and thus the viscosity of the melt. This degradation is accompanied by the formation of corrosive and toxic byproducts such as HCl or halogenated acetic acid. Extrusion leads to weaker objects than compression molding but is a useful technique for producing PCTFE-insulated wires or PCTFE thin films (25 μm). Films can be obtained from PCTFE dispersions using temperatures ranging from 100 to 250 °C.134 Although most fluoropolymers are inert to thermal and chemical degradation, they are sensitive to high energy radiation.135 The γ-irradiation of PCTFE below its crystalline melt temperature yields almost entirely new chain ends. This indicates that main chain scission was the predominant process of the radiolysis.136,137 Above the crystalline melting temperature, the byproducts include new chains ends and branches.137,138 Under the above irradiation conditions, surprisingly the loss of fluorine over chlorine atom was preferred in spite of the C−F bond energy being higher than that of C−Cl. This observation was explained in terms of the higher electronegativity and smaller size of the fluorine atoms. Unlike PTFE,139 no network structure was obtained after irradiation of PCTFE. 2.3.2.3. Health, Safety, and Environment. In contrast to its monomer, PCTFE has low toxicity and irradiation potential under normal handling conditions. It has been shown that it does not support fungus growth. The Food & Drugs Administration (FDA) for food applications has approved PCTFE. Clinical animal trials with PCTFE as body implants did not cause any reaction after two years. However, it should be pointed out that toxicity may vary depending on the supplier and the manufacturing process. 2.3.3. Telomerization of CTFE. The telomerization of CTFE with various telogens has been previously reviewed several times,6,140,141 however, an updated summary is supplied in Table 4. Compounds containing at least one carbon−halogen bond were often used as the chain transfer agents (CTA) for the telomerization of CTFE although telogens containing cleavable C−H, S−H, P−H, Si−H, and S−S bonds led to most comprehensive studies. For the former series, the redox telomerization of CTFE with RCCl3 telogens (R = Cl, CCl3, CHCl2, CH2Cl, and CH3), has been thoroughly studied either in the presence of copper salts (copper of various oxidation states) or iron salts (mainly Fe3+ involving a reducing agent such as benzoin142 or nickel143). The presence of copper salts usually favors the formation of the monoadduct, which is in contrast to iron salts that lead to polydispersed telomeric distributions. One thing in particular with CCl4 as the telogen is worthy of note. This is the possibility for the CCl3 moiety to reinitiate another telomerization, of either CTFE or other monomers, yielding diblock cotelomers where both blocks are separated by a −CCl2− group (see section 2.3.4). Among telogens that contain C−Br bonds, CCl3Br,144 CF3CClBr2, and BrCF2CFClBr are the most frequently employed. Interestingly, the latter is easily obtained by direct reaction of CTFE with Br2145 and has been employed for the telomerization of various fluoroolefins.146,147 The reason that there is such interest in these telogens compared to the chlorinated ones is the fact that the C−Br bond can be easily chemically modified into other functional groups (RCO2H, R− OH, etc.).148,149 In addition, telomers synthesized from

ClCF2CFCl − I + F2CCFCl → Cl(CF2CFCl)n − I

Although the redox telomerization of CTFE in the presence of CuCl has been reported, only one article claims that the atom transfer radical polymerization of CTFE failed, although it did lead to hyperbranched copolymers with chloromethyl styrene.229 In conclusion, the telomerization of CTFE has been well studied in the presence of numerous telogens and under many reaction conditions. The reaction leads to various telomers with functional end groups that can be used for further reactions or as the functional group of a chain transfer agent in other telomerization or polymerization reactions. 2.3.4. Applications of Oligo(chlorotrifluoroethylene)s and CTFE Telomers. The telomers obtained by the reactions described in the previous section are often used as starting materials for other reactions. For example, they can be used as chain transfer agents in a cotelomerization to obtain block cotelomers or to prepare macromonomers. 2.3.4.1. Oligo(chlorotrifluoroethylene)s as Chain Transfer Agents. The addition of IBr to CTFE was found to lead to four isomers. The major products are BrCF2CFClI and ICF2CFClI, while ICF2CFClBr and BrCF2CFClBr are the minor products.167 Its radical addition to allyl alcohol followed by the selective reduction of the iodine atom and dehalogenation led to the formation of F2CCFC3H6OH which is a potential cure site monomer.167,230 On the contrary, the addition of ICl to CTFE led to a mixture of two isomers: ClCF2CFClI as the major and Cl2CFCF2I the minor.197 The terminal CFCl-I end group is quite reactive (more reactive than CF2−I206), making ClCF2CFClI an efficient telogen for the telomerization of fluoroolefins196,200 and nonhalogenated olefins. ClCF2CFClI was used as a telogen in the J


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recently also used successfully for its telomerization with CTFE.232 CTFE telomers were also obtained by radical telomerization in the presence of carbon tetrachloride as the telogen followed by a chlorination step to produce bis(trichloromethyl) end group telomers. A regioselective attack of the CCl3• radical onto the CF 2 site of CTFE was observed. Such telomers were subsequently used as precursors for producing telechelic compounds by stepwise (or sequential) cotelomerization (Scheme 5).233−237 Such reactions lead to a wide variety of well-defined polymers such as polyesters,238 polyurethanes,239 and polyamides.240 By following a similar pathway using the CTFE-monoadduct two telogens CCl3CF2CCl3 and CCl3CF2CCl2CH2CHClCH2OCOCH3 are formed (Scheme 6). The second was obtained by the redox addition of allyl acetate in the presence of a ruthenium complex, copper or iron-based catalysts to obtain new telomeric compounds.241 Other modifications of CTFE telomers using either phosphonated compounds, vinylidene chloride,242 or allyl acetate236 have also been investigated. Telechelic telomers bearing CCl3 end groups can also be obtained by coupling two telomers in the presence of zinc and acetic anhydride (Scheme 7).155 It is worth noting that the CCl3 end groups of a fluorinated telomer can be chemically modified to give a carboxylic acid140,233 in the presence of mercuric oxides at temperatures below 150 °C. In this case leading to original fluorosurfactants,243−246 dispersing agents,247 polyesters,140,238 polyamides,240 or polyurethanes.140,239 It has also been reported that −CF2−CFCl2 end groups led to −CF2−CO2H.248 With the intention of forming the diacids, the simultaneous hydrolysis of both the CCl3 and CFClX end groups was found to be incomplete. However, advantageously, the modification of CFClX into CCl3 was necessary in order to obtain complete hydrolysis.140 This can be achieved in the presence of AlCl3 and led to the preparation of diacids, diesters (esterification of the

telomerization of trifluoroethylene initiated by benzoyl peroxide at 110 °C.231 Chambers et al.158 prepared I−CF2−CFCl−I by reacting CTFE with elemental iodine in the presence of UV or γ-ray irradiation although ultimately it led back to molecular iodine (I2) and CTFE by β-elimination.158,197 Further exposure with an excess of CTFE led to the formation of I−(CF2−CFCl)n−I polyadducts with n = 2−10. Such telomers can also be brominated to obtain Br−(CF 2 −CFCl) n −Br. I−(CF 2 − CFCl)n−I could be ethylenated or used as original chain transfer agents in the radical telomerization of HFP and TFE.202 Br−CTFE−Br is easily prepared by bubbling CTFE into elemental bromine Br2.166 When the reaction is complete, the brown medium turns clear and colorless. The monoadduct is formed principally and can be distilled under atmospheric pressure at 99 °C.145,147 Br−CTFE−Br can be used as a telogen in the radical telomerization of fluoroolefins such as VDF or CTFE (Scheme 4). Its telomerization with VDF147 can be Scheme 4. Br-CTFE-Br as a Telogen in the Radical Telomerization of VDF147 or CTFE232

initiated either thermally, photochemically, or by using a redox system or a peroxide initiator such as tert-butylperoxypivalate. Thermal or peroxide initiation led to the best results with a degree of polymerization n ≥ 9.147 The same methodology was

Scheme 5. Radical or Redox Telomerizations of CTFE in the Presence of CCl4, Followed by Chlorination and Addition of Allyl Acetate to Obtain Telechelic Difunctional Telomers or Its Oxidation to Yield (Bis)carboxylic acidsa 233−237

a

These can be further transformed into alcohols, amides, and esters. These telechelics can in turn lead to polyurethanes, polyesters, and polyamides. K


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Scheme 6. Modification of CTFE Telomers into Functional CTFE Telomers140,184,241,242,251−253

Scheme 7. Coupling of Brominated Telomers to Obtain Telechelic Telomers Bearing CCl3 End Groups155

group.184,185,251,252 The best results were obtained with the last mentioned CTA in the presence of benzoyl peroxide leading to H−(CTFE)n−PO(OR)2.185 These phosphonates can be hydrolyzed to yield acids and salts as surfactants or for protective coatings via phosphonic acid adhesion groups. The fluorination of Cl−(CTFE)x−CCl3 telomers was also carried out using KF in DMSO at 150−190 °C. This lead to an unexpected perfluorinated CF3−(CF2)p−CFCF−(CF2)m− CF3 compound in 80% yield and minor chlorinated products (head-to-head dechlorination of the CTFE units).254−257 When

diacids), monoalcohols and diols (by reduction with LiAlH4).149,234 Fluorophosphonic CTFE telomers were also synthesized (Scheme 6) using either the Kinnear−Perren249 or Michaelis− Arbuzov250 reaction to modify Cl−(CTFE)n−CCl3 into Cl− (CTFE)n−CCl2−CH2−CHCl−PO(OEt)2 and Cl−(CTFE)n− CCl2−CH2−CHCl−CH2−PO(OEt)2140,251 (Scheme 6). Fluorophosphonic CTFE telomers have also been successfully synthesized directly by telomerization of CTFE in the presence of CCl3POCl2, CHCl2POCl2, or HPO(OR)2 where R is an alkyl L


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Scheme 8. Preparation of Monofunctional or Diblock (Co)Telomers from Cl(CTFE)nCCl3140,235,258−264

the reaction was continued, the double bond shifted toward the extremity of the molecule, leading to the formation of F2C CF−RF,Cl alkenes. CTFE telomers bearing a CCl3 end group were also used as telogens in the preparation of diblock telomers with a wide range of monomers (Scheme 8).140,235,258−264 Most of the resulting products have found applications in medicinal formulations, dyes, insecticides, and fluorosurfactants. These halogenated telomers led to diols and diacids for the preparation of original chlorofluorinated polyesters238 and polyurethanes,239,261 whereas the diacids and diesters are suitable precursors for the formation of polyamides240 (Scheme 9). 2.3.4.2. Oligo(chlorotrifluoroethylene)s as Precursors of Monomers. CTFE telomers with chlorine end groups were used to prepare (meth)acrylate macromonomers, first by radical addition onto allyl alcohol and then by reaction with (meth)acryloyl chloride (Scheme 10). Such macromonomers were photopolymerized to obtain materials suitable for use in optic fibers (see section 5.4).265 The radical addition of iodine monochloride onto CTFE has been extensively investigated.111,147−150 The resulting 1,2dichloro-2-iodotrifluoroethane was used to prepare various trifluorovinyl monomers bearing functional side groups as

Scheme 9. Oligo(CTFE) Telechelic Telomers As Building Blocks for the Preparation of Chlorofluorinated Polyesters, Polyurethanes, and Polyamides238−240,261

described in Scheme 11.266−272 This enabled, for example, the cross-linking of polymers.230,271 Another trifluorovinyl monomer bearing a bromine side-group was prepared from the radical monoaddition of bromine onto M


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Scheme 10. Preparation of (Meth)acrylate Macromonomers Bearing an Oligo(CTFE) Side Chain265

Scheme 11. CTFE Telomers for the Preparation of Functional Trifluorovinyl Monomers167,232,266−273

Daikin Company276 patent, these fluorinated acyl peroxides were used as initiators for the preparation of thermostable high molecular weight PCTFE.

CTFE followed by an ethylene end-capping and further dehalogenation as described in Scheme 11.145 α,β,β-Trifluoro vinyl styrenic monomers have been recently prepared (Scheme 12) by the Pd-catalyzed cross-coupling of CTFE with arylboronic acids in the presence of base and water.274 2.3.4.3. Oligo(chlorotrifluoroethylene)s as Initiators. The DuPont Company used telechelic low molecular weight CTFE telomers for the preparation of oligo(CTFE)-based peroxides (Scheme 13).275 They were obtained by coupling two PCTFE chains terminated by an acyl chloride group. According to a

3. COPOLYMERS OF CHLOROTRIFLUOROETHYLENE 3.1. Introduction

Many co-monomers have been involved in radical copolymerizations with CTFE and their copolymers containing various comonomers can be divided into two categories: (i) the proportion of co-monomers in the copolymer is small; the resulting materials are also thermoplastic with a lower crystallinity than that of PCTFE; (ii) ethylene or electron-donating co-monomers: they tend to alternate due to an acceptor−donor copolymerization. The aim of the following section is to review the radical copolymerization of CTFE starting with hydrogenated comonomers and going onto fluorinated co-monomers. First, we will cover conventional copolymerization (the kinetics of radical

Scheme 12. Preparation of α,β,β-Trifluoro Vinyl Styrenes via the Arylation of CTFE274

N


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Scheme 13. Preparation of Bis(perchlorofluoroacyl) Peroxide with n = 1−10 from CTFE Oligomers275

chemical processing industry,291−294 especially in caustic environments.292,293 Such harsh environments are known to cause stress cracking in PVDF. In contrast to PCTFE which has excellent barrier properties,14 ECTFE has a low permeability to various gases including moisture, chlorine and HCl. However, the largest market for ECTFE, developed by DuPont and AT&T, is as an insulation material in data transmission, automotive, and aerospace cables. This is largely due to its low dielectric constant (2.6 at 1 MHz) and its excellence resistance to fire (it has a tendency to char).24 ECTFE copolymers are used as protection materials in the front or back-sheets of photovoltaic cells295,296 where the main objectives are durability, weatherability, low surface tension (hence self-cleaning and dirt-repelling), high light transparency, and does not yellow. The dielectric properties of Halar, were investigated by Khanna et al.297 as a function of the temperature (−100 to +175 °C). The dielectric loss index shows no change between +25 and +175 °C, but below 25 °C, a significant increase was observed. The three relaxations (α, β, and γ) were found to be similar as measured by both mechanical and dielectric experiments and this suggests a similar origin for the molecular relaxations. However, results show that the relaxation strength from the two experiments is of reverse magnitude. Although in the mechanical experiment, all molecules participate in the relaxation; in the dielectric one, only the polar groups are active (CTFE). Hence, in this latter case, the dissipation is stronger at lower temperatures (γ relaxation) due to lower mobility. More detailed information about the properties and applications of ECTFE copolymers can be found in Stanitis’s excellent review.28 3.2.2. Poly(CTFE-co-Propylene) and Poly(CTFE-co-isobutylene) Copolymers. As in the above section devoted to ECTFE, Ragazzini et al.298 demonstrated that the copolymers of CTFE and propylene were alternating. Similarly, Ishigure et al.299 prepared some poly(CTFE-alt-P) alternating copolymers using γ-ray irradiation. Over a wide range of feed ratios, they studied the microstructures using 19F NMR spectroscopy and showed regular head-to-tail sequences which yield an isotactic polymer. This is especially true when the polymerization temperature was kept low. Tabata et al.300 investigated the kinetics of the radical copolymerization of CTFE with ethylene, propylene, and isobutylene (IB). They observed that the rate of polymerization is in the decreasing order: ethylene > propylene > isobutylene. The reactivity ratios were assessed as being 0 °C (rCTFE = 0.04 and rIB = 0.0). However, the copolymerization was also investigated at −78 and −35 °C, and the results showed different mechanisms due to interactions with the solvent (methanol or amine) at these temperatures. At 0 °C, the poly(CTFE-alt-IB) copolymers are both highly alternating (over a wide range of compositions) and highly crystalline. Carcano et al.301 also studied the bulk copolymerization of CTFE with isobutylene at 0 °C. The reactivity ratios were also assessed and found to be rCTFE = 0.004 ± 0.001 and rIB = 0.004 ±

copolymerization will be discussed in section 3.2.15) and then the controlled radical copolymerization will be highlighted. 3.2. Copolymers by Conventional Radical Polymerization

3.2.1. Poly(CTFE-co-Ethylene) Copolymers. The copolymers of CTFE with ethylene (E), commonly referred to as ECTFE, were first prepared by Hanford at The DuPont Company in 1946.277 Allied Signal (now Honeywell) first commercialized ECTFE in 1974 under the Halar trade name. In 1986, Halar business was sold to Ausimont, USA, Inc., which in turn was bought by Solvay in 2002 that then became Solvay Solexis. Today, it is part of Solvay Specialty Polymers. Ragazzini et al.278 reported the copolymerization of CTFE with ethylene in the presence of a trialkylboron catalyst at various temperatures (from −78 to +60 °C). It was found that the resulting poly(CTFE-alt-E) copolymers exhibit an alternating structure over a wide range of compositions, especially for those obtained at low polymerization temperatures. In 1967, Garbuglio et al.279 confirmed this alternating structure and the physical properties in relation to the composition of the poly(CTFE-alt-ethylene) copolymers and the polymerization temperature. When not alternating, they found that the crystallinity of the poly(CTFE-alt-E) copolymer decreased with the ethylene content. Five years later, Sibilia et al.280 again confirmed the highly alternating structure of these copolymers. The preferred conformation is a kinked structure having ethylene units from one chain lining up opposite the CTFE units of another chain. The high melting point of the alternating ECTFE copolymers can be attributed to the polar association that can occur between fluorine and hydrogen atoms. Industrially, ECTFE is produced by aqueous free radical suspension polymerization. It usually contains a 1:1 molar ratio of CTFE and E and is mostly purely alternating with less than 10% of blocks.281 As the copolymer is insoluble in its monomers (both CTFE and E), a fine powder is obtained which is available in various forms and molecular weights.28 To produce ECTFE-based coatings, special terpolymers were developed. They contain a small amount of termonomer (usually because such termonomers are expensive) such as hexafluoroisobutylene (HFIB), perfluorohexyl ethylene,281 perfluoroisoalkoxy or perfluoroalkyl ethylenes,282 perfluoropropyl vinyl ether (PPVE),283 or organic silanes.284 Other formulations include a mixture of ECTFE with PCTFE homopolymer to obtain a minimum of 90 mol % of CTFE in the thermoprocessable product to ensure improved mechanical properties, good electrical insulation as well as better aging.285 ECTFE has a better ignition resistance than other fluoropolymers.24,28,29 In the case of ECTFE, the degradation mechanism is based on hydrogen halide elimination (dehydrohalogenation) rather than on chain scission. ECTFE has excellent chemical and thermal stability,286 and a fine crystalline structure.287 ECTFE is also available in various types of fibers288−290 and sheets. Due to its high chemical resistance, it is widely used in the O


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copolymers that were subsequently employed as binders for solid alkaline fuel cell membranes.312 Valade et al.313 studied the radical copolymerization of CTFE with diallyldimethylammonium chloride (DADMAC) in solution under various experimental conditions (Scheme 16).

0.007. These differ somewhat from the above-mentioned study due to the copolymerization process. In contrast to ECTFE, the study of poly(CTFE-alt-propylene) copolymers has been a less actively researched area and hence has not led to commercially available poly(CTFE-co-P). 3.2.3. Poly(CTFE-co-allylic monomers) Copolymers. Several manufacturers produce a range of formulations based on CTFE and 2-hydroxyethyl allyl ether. These have at least a third co-monomer such as: acrylates (Central Glass),302,303 isobutylene (Daikin),304 vinyl acetate (Fluorobase from Ausimont),305 or VDF and TFE (Elf Atochem).306−309 Central Glass303 patented a formula for curable electrodeposition paints based on CTFE (25−75 mol %). This is a hydroxyl-containing allyl ether (ethylene or propylene glycol allyl ether, 6hydroxybutyl vinyl ether), a carboxylic acid vinyl ester (such as vinyl acetate, vinyl pivalate, or vinyl caproate) in the presence of a unsaturated long chain carboxylic acid (>C6).310 CTFE copolymerized readily with allyl glycidyl ether (AGE) under radical copolymerization conditions (Scheme 14).311 The

Scheme 16. Radical Copolymerization of Diallyldimethylammonium Chloride (DADMAC) and Chlorotrifluoroethylene (CTFE)313

Poly(CTFE-co-DADMAC) copolymers were obtained in yields up to 85%, and as expected for hydrogenated co-monomers, DADMAC percentages in the copolymers were higher than those in the feed compositions. To obtain insoluble copolymers, high CTFE feed contents (>70 mol %) were required. 3.2.4. Poly(CTFE-co-(meth)acrylates or (meth)acrylic Monomers) Copolymers. Surprisingly, this area is underpresented in the literature and only a few examples can be found. In the first example, Thomas and O’Shaughnessy27 reported that the radical copolymerization of CTFE with methyl methacrylate (MMA) in all proportions. This is shown in Figure 6. From the reactivity ratios it can be seen that MMA is much more reactive, and the resulting product consists mainly of PMMA. Suzuki et al. 314 reported a coupling reaction between a poly(dimethylsiloxane) PDMS end-functionalized with an epoxy group and a poly(CTFE-ter-tBuA-ter-HEAE) terpolymer. This terpolymer, provided by Central Glass Co., is composed of 43 CTFE units, 43 tert-butyl acrylate (tBuA), and 14 units of 2hydroxyethyl allyl ether (HEAE). The reaction yielded a poly(CTFE-ter-tBuA-ter-HEAE)-g-PDMS graft copolymer.302 Chou and Xu315 carried out the emulsion copolymerization of CTFE with vinyl acetate and butyl acrylate or methacrylic acid and observed that the temperature had little effect on the fluorine content of the latex but a strong effect on the particle size. Subsequently, coatings were successfully prepared from these latexes. Ausimont316 claimed the successful radical copolymerization of CTFE with hydroxypropyl acrylate or butyl acrylate, but the acrylate yield was always lower than 1.3 mol %. This strategy shows that the incorporation of a small amount of acrylate monomer improved both mechanical and barrier properties and can be used for producing packaging films for which a suitable barrier to water, nitrogen, and oxygen is required. 3.2.5. Poly(CTFE-alt-vinyl ethers) Alternating Copolymers. Monomers with the general formula CH2CH−O−R which are alkoxy ethenes are commonly called alkyl vinyl ethers (where R stands for an alkyl group). That nomenclature will be used throughout this review. It is important to note that a basic

Scheme 14. Radical Co-oligomerization of Chlorotrifluoroethylene (CTFE) with Allyl Glycidyl Ether (AGE)a 311

a

TBPPi stands for tert-butylperoxypivalate, x is close to 1.

copolymer composition, determined by NMR spectroscopy, elemental analyses, and epoxy equivalent values, matched reasonably well. Results showed that the resulting poly(CTFEco-AGE) co-oligomers exhibit a tendency for alternation that was confirmed by the assessment of the reactivity ratios: rCTFE = 0.20 ± 0.03 and rAGE = 0.15 ± 0.08 at 74 °C. The lower Mn values (ca. 2000−8000 g·mol−1) indicated the formation of co-oligomers rather than copolymers. This was attributed to the chain transfer to AGE (by hydrogen abstraction from AGE) via allylic transfer. The poly(CTFE-co-AGE) cooligomers were soluble in most common organic polar solvents. An optimization of co-oligomer yields was investigated by using ethyl vinyl ether as the third comonomer and using different initiators. The thermal stabilities of the co-oligomers, obtained by thermal gravimetric analysis, showed a 5% weight loss at temperatures over 225 °C under air. Such co-oligomers can be interesting precursors of epoxy resins. 3-Chloro-2-methylpropene was reacted with 1,4diazabicyclo[2.2.2]octane (DABCO) to prepare a new monomer bearing both ammonium and amine functional groups (Scheme 15). Its radical copolymerization with CTFE was initiated by tertbutylperoxypivalate or potassium persulfate in 1,1,1,3,3-pentafluorobutane or deionized water, respectively, to lead to random

Scheme 15. Modification of 3-Chloro-2-methylpropene Modified with DABCO (1,4-Diazabicyclo[2.2.2]octane) and Its Radical Copolymerization with CTFE to Prepare SAFC Binders Therefrom312

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Scheme 17. Molecular Structure of Lumiflon Copolymers from the Radical Copolymerization of CTFE with Various Functional Vinyl Ethersa

a

These bear various functional groups designed for specific properties30,321−323,326

breakthrough in the coating industry.324,325 Lumiflon paints are still the only 20-year guaranteed outdoor paints in the industry.30,326 Takakura has reviewed the synthesis of these copolymers and their applications.30 The copolymerization between CTFE and vinyl ether leads to alternating copolymers as evidenced by the acceptor−donor copolymerization, since CTFE and VE are acceptor and donor monomers, respectively. This topic will be discussed in more detail in section 4.1. One of the early fundamental investigations into the mechanism of this copolymerization was carried out by Boutevin et al.327 They investigated the reactivity and mechanism of the copolymerization of chlorotrifluoroethylene (CTFE) with several vinyl ethers (VE) (hydroxyethyl vinyl ether, glycidyl vinyl ether, and 2-chloroethyl vinyl ether). CTFE is an acceptor monomer (e = +1.56328), whereas the VEs are highly donating (e ≅ −2). Hence, they can copolymerize and lead to alternating copolymers as demonstrated by analysis of the composition. 19F

reaction medium is preferential as most vinyl ethers are sensitive to acidic conditions. The copolymerization of CTFE with vinyl ethers was pioneered in the late 50s317 when Firestone claimed the successful copolymerization of CTFE with several vinyl ethers (butyl and 2-ethylhexyl) and the terpolymerization of CTFE with vinyl ethers and vinyl chloride. The copolymerizations were carried out using an emulsion process with various types of surfactants (sodium alkyl sulfonate and alkylated aryl polyether alcohols), borax, and initiated by potassium persulfate. The original methodology described in the patent is still in use today. For example, it is used to prepare poly(CTFE-co-EVE) copolymers as the core in core−shell materials.318,319 Further work by Tabata and Du Plessis appeared in 1971.320 Although the Firestone patent317 already mentioned coating applications, it was only during the 1980s that Asahi Glass Co. Ltd. developed a series of poly(CTFE-alt-VE) copolymers under the Lumiflon trade name321−323 (Scheme 17). This created a Q


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Scheme 18. Mechanism of the Radical Copolymerization of CTFE with VE Initiated by A• Radical329

considering a complex mixture of diastereoisomers. This allowed an average assignment to be determined. A density functional theory (DFT) computational study of the isotropic magnetic shielding was also performed. The intention was to explore the diasteriomeric relationships between the single building blocks and their mutual influences along the polymer chain. The calculated results totally supported the assignment of the experimental chemical shifts of two diastereoisomeric sets of resonances. This clearly indicates chiral center inversion, and not spin−spin J coupling interactions, as the main cause of the spectral complexity. In 2009, fluorinated copolymers bearing ammonium groups were prepared by the radical copolymerization of fluorinated olefins such as chlorotrifluoroethylene or hexafluoropropylene, with different alkyl vinyl ethers. In the latter, the alkyl group was cyclohexyl, n-butyl, ethyl, or 2-chloroethyl. This was followed by the cationization of the resulting copolymers with trimethylamine (Scheme 19).330 Copolymers were obtained in high yields (>85%). Molecular weights ranged from 8000 to 25 000 g mol−1. It was possible to confirm the alternating structure of these polymers from elemental analysis, as well as from 1H and 19F NMR spectroscopy. A range of other poly(CTFE-alt-VE) copolymers was obtained using the same technique but with 2-chloroethyl vinyl ether and ethyl vinyl ether (Scheme 20). The alternating structure made it possible to incorporate a high degree of chlorine atoms into the side chains. This was monitored by incorporating a controlled amount of ethyl vinyl ether as a termonomer.331,332 Subsequently, the chlorine atoms in the side chains were converted into iodine atoms using nucleophilic substitution. This step was monitored using 1H NMR spectroscopy.332 These copolymers exhibited molecular weight values of about 25 000 g mol−1, and thermal analysis showed degradation started about 220 °C, with glass transition temperatures in the range 34−41 °C.332 A recent strategy describes the synthesis of the original 3chloro-2,2-dimethyl propyl vinyl ether involved in the radical copolymerization with CTFE (Scheme 21).312 It is worth noting that in this case the absence of hydrogen on the β-carbon next to the ammonium group should prevent any Hofmann degradation333 from occurring. Similar strategies made it possible to synthesize original vinyl ethers that bear functional groups such as imidazole (Scheme 22), carbonate (Scheme 23), and phosphonate (Scheme 25) or even containing an end chlorine atom that could be further modified to give an ammonium functional group. This will now be discussed briefly. We334−338 reported the synthesis of a vinyl ether containing an imidazole (Imi) function protected by a benzyl group (BVI). Starting from imidazole the desired product was obtained after a three step-reaction (Scheme 22). It was then copolymerized, in solution, with CTFE by a conventional radical copolymerization reaction and led to a series of alternating poly(BVI-alt-CTFE) copolymers in fairly reasonable yields. Deprotection of the benzyl group produced the poly(CTFE-alt-Imi) fluorinated copolymer. The copolymer was subsequently blended with sulfonated poly(ether ether ketone) (sPEEK) with a view to

NMR spectra made it possible to assess the equilibrium constant for the formation of the charge transfer complex (CTC). A high value (K = 1.4) was found. In addition, the cotelomerization of CTFE and a VE in the presence of a fluorinated thiol group (C6F13C2H4SH178) as a chain transfer agent produced both the mono adducts and the dimer structure. This is, of course, the opposite of the result expected from that CTC because they contradict this kind of polymerization mechanism involving propagation of the transfer complex. Thus, an alternative polymerization mechanism involving free monomers was proposed. The authors believe that the alternating behavior of the copolymer arises from the great difference in polarity between the different kinds of monomers. A further clue to the mechanism of that particular reaction as shown in Scheme 18 was suggested more recently by Carnevale et al.329 These authors offered a structural interpretation of poly(CTFE-alt-VE) alternating copolymers supported by comprehensive 13C, 1H, and 19F NMR spectroscopic data (Figure 4). Actually, each CTFE-VE dyad contains two asymmetric carbon atoms that lead to complex spectra. Observations were rationalized by

Figure 4. 400 MHz (a) 1H (19F decoupled), (b) 19F (1H decoupled), and (c) 13C (1H decoupled) NMR spectra of poly(CTFE-alt-ethyl vinyl ether) copolymer in CDCl3. [Reprinted with permission from ref 329. Copyright 2009 American Chemical Society.] R


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Scheme 19. Radical Copolymerization of Fluorinated Monomers (Chlorotrifluoroethylene and Hexafluoropropylene) and Vinyl Ethers and the Cationization of 2-Chloroethyl Group into Quaternary Ammonium330

inertness, and demonstrate encouraging stability to both low and high electrochemical potentials.339−341 Using similar strategies, original vinyl ethers bearing dialkyl phosphonate end groups were also successfully synthesized342 and copolymerized343 with CTFE (Scheme 25). Recently, poly(CTFE-alt-EVE) copolymers were prepared in emulsions. Molecular weights up to 190 000 g mol−1 were claimed, and their alternating structure was confirmed by the usual analytical methods.318,319 These copolymers were used to “seed” the emulsion copolymerization of styrene, butyl acrylate, and acrylic acid, leading to core−shell materials. Traditionally, poly(CTFE-alt-VE) copolymers have found industrial applications in paints and coatings.30 But more recently, original vinyl ether monomers have been synthesized or poly(CTFE-alt-2-chloroethyl vinyl ether) copolymers were chemically modified into functional materials suitable for energy applications: alkaline and medium temperature fuel cell membranes, polymeric electrolytes for lithium-ion batteries, as well as photovoltaic materials (see section 4.6.2). 3.2.6. Poly(CTFE-co-Vinylene Carbonate) Copolymers. Krebs and Schneider investigated the radical copolymerization of CTFE with vinylene carbonate (VCA) initiated by AIBN in DMF at 65 °C.344 Statistical poly(CTFE-co-VCA) copolymers with a slight tendency to alternation were obtained and the reactivity ratios were assessed (rCTFE = 0.48, rVCA = 0.42 at 65 °C). The resulting copolymers were soluble in common organic solvents but had rather low molecular weights (Mn ≈ 2000 g mol−1) due to the tendency of the VCA radical to undergo chain transfer. Interestingly, an increasing VCA ratio led to an increase in the reaction rate as evidenced by Figure 5. The maximum reaction rate was reached at 40 mol % of CTFE in the feed (about

Scheme 20. Radical Copolymerizations of Chlorotrifluoroethylene and Vinyl Ethers (2-Chloroethyl Vinyl Ether and Ethyl Vinyl Ether) Initiated by TertButylperoxypivalate (1 mol %) at 75 °C331

using it as medium temperature proton-conducting fuel cell membranes (see section 5.1). A vinyl ether bearing a carbonate side-group has also been prepared (Scheme 23). The transetherification of ethyl vinyl ether and glycerol carbonate using a palladium complex gave an in situ yield of 65%.339,340 Subsequently, this monomer was further copolymerized with various fluoroolefins including: CTFE, hexafluoropropylene and perfluoromethyl vinyl ether (Scheme 24).339−341 This radical copolymerization also consists of an “acceptor-donor” copolymerization and yields alternating structures. Such copolymers are useful as polymer electrolytes in lithium-ion batteries and have potential for Li+ ion conduction thanks to the coordination of the Li+ with the cyclic carbonate. Those copolymers exhibit good thermal properties, chemical

Scheme 21. Preparation of 3-Chloro-(2,2-Dimethylpropyl) Vinyl Ether and Its Radical Copolymerization with CTFE to Achieve Original Binders for Solid Alkaline Fuel Cells312

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Scheme 22. Preparation of a Vinyl Ether Bearing an Imidazole Group and Its Radical Copolymerization with 2-Chloroethyl Vinyl Ether (CTFE)a

a

TBPPi is tert-butylperoxypivalate.334,338

Scheme 23. Synthesis of 2-Oxo-1,3-dioxolan-4-yl-methyl Vinyl Ether by the Transetherification of Ethyl Vinyl Ether with Glycerol Carbonate339,341

Scheme 24. Radical Copolymerization of 2-Oxo-1,3-dioxolan4-yl-methyl Vinyl Ether with Chlorotrifluoroethylene (CTFE), Hexafluoropropylene (HFP), or Perfluoromethylvinyl Ether (PMVE) Initiated by tertButylperoxypivalate (TBPPI)339,340

Figure 5. Conversion after 45 min polymerization time vs the molar fraction of CTFE in the feed for the copolymerization of CTFE with vinylene carbonate (initiated by AIBN in DMF at 65 °C). [Reprinted with permission from ref 344. Copyright 1975 American Chemical Society.]

In this case, the electron-donating vinylene carbonate also promoted copolymerization in an alternating way with the CTFE. This cyclic monomer was used to increase the glass transition temperature of the copolymers in order to prepare transparent amorphous materials. Hence, a wide range of functional terpolymers containing various carbonate or hydroxyl side groups was produced. The interest in such a group lies in the

4 times that of CTFE in homopolymerization). This confirmed a previous study reported in 1971320 by Tabata and Du Plessis on the copolymerization of CTFE with VEs. Copolymers of CTFE and VCA with a vinyl ether containing hydroxyl side groups, as a cure-site monomer were prepared (Scheme 26) for use in optical fiber waveguide applications.345

Scheme 25. Preparation of Vinyl Ether Containing Phosphonate Groups and Their Radical Copolymerization with CTFE342,343

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More recently, Baradie and Soichet348 carried out the radical copolymerization of CTFE with VAc in supercritical CO2, in various compositions, using a surfactant-free reaction and with high yields (91%). The kinetics of the radical copolymerization enabled them to assess the reactivity ratios rCTFE = 0.014 ± 0.050 and rVAc = 0.44 ± 0.03 at 45 °C. Molecular weights and polydispersities varied from 200 000 to 300 000 g mol−1, and from 2.1 to 2.4, respectively. The glass transition temperature of the poly(CTFE-co-VAc) copolymers ranged from 42 to 53 °C. Similarly, Liu et al.349,350 performed the copolymerization of CTFE with VAc in supercritical CO2. They studied the influence of the pressure and the monomer ratio in that particular reaction. They found that the fluorine content in the copolymers (and thus the feed ratio) was the critical factor influencing the surface properties of their films (lowest surface energy at 29.2 wt % of CTFE). 3.2.8. Poly(CTFE-co-aromatic co-monomers) Copolymers. Thomas et al.27 investigated the kinetics of the radical copolymerization of CTFE with styrene. The reactivity ratios were assessed and found to be rCTFE = 0.001 and rStyrene = 7.0 at 60 °C. As expected, styrene (Q = 1) is much more reactive than CTFE (Q = 0.025), and thus, the composition of poly(CTFE-costyrene) copolymers always contained a higher amount of styrene than that of the feed (Figure 6). As a result, various strategies were investigated to discover suitable aromatic comonomers that favor a better copolymerization of the CTFE. A recent strategy attempted to overcome that difference in reactivity by inserting a bulky group into the α-position of the styrene. Hence, the radical copolymerization of CTFE with αfluoromethylstyrene (AFMS) was successfully carried out (Scheme 27),351 although the latter did not homopolymerize

Scheme 26. Preparation of Terpolymers Based on CTFE, Vinylene Carbonate, and Hydroxypoly(Methylene) Vinyl Ether for Optical Fiber Applications345

chemical modification to produce acrylate which in turn can lead to photo-cross-linkable terpolymers.345 3.2.7. Poly(CTFE-co-vinyl esters) Copolymers. Although one example of radical copolymerization of CTFE with vinyl proprionate346 has been reported, most published studies deal with the radical copolymerization of CTFE with vinyl acetate. We will now present three examples. Thomas and O’Shaughnessy27 investigated the kinetics of CTFE polymerization, either in bulk or in solution, as well as its copolymerization with vinyl acetate (Figure 6). Reactivity ratios

Scheme 27. Radical Copolymerization of Chlorotrifluoroethylene (CTFE) with α-Fluoromethylstyrene (AFMS)351

under radical conditions.352 The assessment of the reactivity ratios was also reported rFMB = 3.7 ± 1.8 and rCTFE = 0.4 ± 0.2 at 74 °C). This demonstrated that, as in the reaction above, CTFE is less reactive than the corresponding hydrogenated aromatic monomer. More recently, an even bulkier and electron-withdrawing αgroup was synthesized for possible co- and terpolymerization involving CTFE. However, even if the copolymerization of αfluoromethylstyrene with fluoroolefins such as CTFE proved possible, that of α-trifluoromethylstyrene (ATFMS) with the same fluoroolefins failed systematically (Scheme 28).353 So far, it

Figure 6. Kinetics of radical copolymerization of CTFE with other comonomers (1, 2, or 3). [Reprinted with permission from ref 27. Copyright 1953 Wiley Interscience.]

were assessed and found to be rCTFE = 0.01 and rVAc = 0.6 at 60 °C. This lead to the determination of the Alfrey−Price347 parameters for CTFE (Q = 0.025, e = 2.0). Central Glass has claimed the successful terpolymerization of CTFE (25−75 mol %) with a vinyl or isopropenyl ester of fatty acid (1−12 carbons, 10−70 mol %) and allyl glycidyl ether (AGE, 3−40 mol %) to prepare epoxy-curable transparent coatings. Murray et al.346 reported the radical copolymerization of CTFE with vinyl acetate (VAc) or vinyl propionate (VPr) in an emulsion under redox conditions. The reactivity ratios were assessed as rCTFE = 0.04 ± 0.02 and rVAc = 0.68 ± 0.11 for the CTFE/VAc system and rCTFE = 0.08 ± 0.01 and rVPr = 0.63 ± 0.04 for CTFE/VPr at 25 °C. The authors also used sequence distribution to assess the concentration of monomer at the polymerization site.

Scheme 28. Unsuccessful Radical Copolymerization of αTrifluoromethylstyrene (ATFMS) with CTFE353

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Scheme 29. Radical Terpolymerization of CTFE, VDF, and α-Trifluoromethylstyrenea

a VDF is vinylidene fluoride; chlorotrifluoroethylene, CTFE; tert-butylperoxypivalate, TBPPi; and 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane, BTBPDMM.353.

has only been possible to copolymerize α-fluoromethylstyrene in a terpolymerization with CTFE and VDF by keeping the αfluoromethylstyrene content below 10%. Initiation by a mixture of TBPPi and 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane (Scheme 29) is also needed. The radical copolymerization of chlorotrifluoroethylene (CTFE) with different monomers bearing isocyanato groups such as 3-isopropenyl-α,α′-dimethylbenzyl isocyanate (mTMI), and initiated by 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane has also been reported (Scheme 30).354 Although the mTMI

distributions (from 1.2 to 2.1) were also measured. Though cooligomers were obtained, their thermal properties showed satisfactory thermal stability (higher than 250 °C under air). Wang et al.229 reported the controlled radical copolymerization of CTFE with chloromethylstyrene (CMS) using atom transfer radical polymerization (ATRP) between an electron-rich inimer (i.e., CMS monomer and initiator) and an electrondeficient monomer (CTFE). Surprisingly, that reaction did not allow the radical homopolymerization of CMS with respect to the unsaturated vinyl group (because of its high Q value), but rather yielded hyperbranched poly(CTFE-co-CMS) copolymers (Scheme 31). The authors claimed that ATRP of CMS occurs via the cleavage of the C−Cl bond from CMS but excluded a similar cleavage in CTFE. Attempts to homopolymerize CTFE by ATRP systematically failed. This strongly supports the idea that the poly(CTFE-co-CMS) copolymers had an alternating structure and this was supported by elemental analyses. This hyperbranched polymer is soluble in common organic solvents. In addition, it is amorphous with a glass transition temperature of 88 °C (lying between those of PCTFE and PS, 57 and 100 °C, respectively). This value does not fulfill the Fox equation. 3.2.9. Poly(CTFE-co-vinyl chloride) or Poly(CTFE-covinylidene chloride) Copolymers. The first report of CTFE being used with vinyl chloride is found in a patent by the Firestone Tyre & Rubber Co.317 already in 1957. It describes the polymerization of vinyl chloride in the presence of poly(CTFEco-EHVE) copolymer seeds (EHVE = 2-ethylhexyl vinyl ether). To improve the thermostability of chlorinated polyolefins, Kliman et al.356 carried out the copolymerization of CTFE with chlorinated olefins such as vinyl chloride (VC) and vinylidene chloride (VDC) (Scheme 32). By using the whole composition range this made it possible to determine the reactivity ratios (rCTFE = 0.01 and rVC = 2.53 and rCTFE = 0.02 and rVDC = 17.14 at 60 °C, respectively; see section 3.2.14 and Figure 11). From these reactivity ratios, the authors were able to make the first calculations of the Q and e parameters for CTFE (Q = 0.013 and 0.021; e = 1.9 and 1.6 for VC and VDC, respectively). 3.2.10. Poly(CTFE-co-vinylidene fluoride) Copolymers. Copolymers based on CTFE and vinylidene fluoride (VDF) can be either thermoplastics or elastomers. They can be synthesized either by conventional or controlled radical copolymerization13 and there is a wide range of applications for such copolymers. The first copolymers from 1955 based on VDF and chlorotrifluoroethylene (CTFE) were produced for military applications.357−361 In contrast to poly(CTFE-alt-VE) copolymers, poly(CTFE-co-VDF) copolymers are statistical with the following chemical structure: [(CF2CFCl)x(CH2CF2)y]n. In the mid-1950s, these macromolecules were also the first commercially available fluoroelastomers. They were marketed by the Kellogg Company under the Kel F trademark and after crosslinking, exhibited more favorable properties, greater than any of those already existing. Actually, these CTFE-based fluoroelastomers are unique fire retardants and are resistant to chemicals,

Scheme 30. Alternating Radical Copolymerization of CTFE with 3-Isopropenyl-α,α′-dimethylbenzyl Isocyanate354

monomer does not homopolymerize under radical initiation, the resulting poly(CTFE-co-mTMI) copolymers were characterized and the reactivity ratios of both the co-monomers, rCTFE and rmTMI, were found to be 0.076 and 0.034 at 130 °C, respectively. In all cases, the rCTFE × rmTMI product was close to zero. This confirmed that poly(CTFE-co-mTMI) copolymers exhibit a high tendency toward alternation (Figure 7). This was also confirmed by Igarashi’s theory355 which revealed a high content of CTFEm-MTI heterodyads. The Q and e values for the mTMI monomer were also assessed (Qm‑TMI = 0.007 and emTMI = 0.99). In addition to the kinetics of the copolymerization, the molecular weights (ranging from 1500 to 8200 g mol−1) and the molecular weight

Figure 7. Composition curve for the radical copolymerization of CTFE with 3-isopropenyl-α,α′-dimethylbenzyl isocyanate (mTMI).354 V


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Scheme 31. Synthesis of Dendritic Copolymers Based on CTFE with p-Chloromethylstyrene (CMS) in the Presence of Copper(I)−Bipyridine (bpy) Complex via ATRP of CMS with CTFE229

by Mandelkern et al.363 in 1957. According to the composition, values range between −40 °C (Tg of PVDF) to +45 °C (or +50 °C) (Tg of PCTFE). In contrast, poly(CTFE-co-VDF) copolymers that contain only a small amount of VDF are semicrystalline with a hexagonal structure.364 Those containing 25−70 mol % VDF are amorphous. Although it has also been recently noted365 that poly(CTFE-co-VDF) copolymers with more than 16.6 mol % of CTFE units also display an amorphous state (Figure 9).

Scheme 32. Radical Copolymerization of CTFE with Vinyl Chloride (VC) or Vinylidene Chloride (VDC)356

solvents, heat, and oxidation. These materials can be cross-linked using bis-nucleophiles such as telechelic diamines, bis-phenols, or peroxides.362 Currently, depending on the intended applications and hence the properties required, various poly(CTFE-co-VDF) copolymers are available. The amount of CTFE is crucial in determining the suitability (Figure 8). For example, its content can influence the glass transition temperature (Tg) of the poly(CTFE-co-VDF) copolymers. This was first investigated

Figure 8. Relationship between the CTFE content and the thermoplastic or elastomeric behavior of poly(CTFE-co-VDF) copolymers. [Reprinted with permission from ref 25. Copyright 1967 Wiley Interscience.]

Figure 9. Differential scanning calorimetry thermograms of poly(VDFco-CTFE) copolymers containing various amounts of CTFE. [Reprinted with permission from ref 366. Copyright 2007 Electrochemical Society.] W


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Scheme 33. Preparation of Poly(CTFE-ter-TrFE-ter-VDF) Terpolymers by Radical Copolymerization of VDF and CTFE, Followed by Partial Reduction of the Chlorine Atoms of the CTFE Units378,380,382,387−389,391

To enhance the dielectric properties of poly(CTFE-co-VDF) copolymers, an increasingly popular strategy was to selectively reduce the chlorine atom of CTFE units. This approach was initially pioneered by Cais and Kometani387,388 in 1984, then later in collaboration with Lovinger’s group.389 More recently, it has been revisited by Lu et al.378,380,382 It leads to poly(CTFE-terTrFE-ter-VDF) terpolymers (where TrFE stands for trifluoroethylene, Scheme 33). Zhang et al.390 used Lu’s recipe to fully hydrogenate poly(CTFE-co-VDF) to make poly(TrFE-co-VDF) copolymers and study their crystal phases. Using the same technique, Zhen et al.391 prepared a poly(VDF-ter-CTFE-terTrFE) terpolymer (79/14.9/7 mol %) that presents more microstructural defects, hence giving rise to a higher dielectric constant (∼50 at 1 kHz) than those obtained by direct terpolymerization. Lu et al.378,380,382 comprehensively characterized the microstructure of these poly(CTFE-ter-VDF-ter-TrFE) terpolymers using 19F NMR spectroscopy. They reported the head-to-head VDF-TrFE dyads in these “reduced” poly(CTFE-co-VDF) copolymers in contrast to head-to-tail VDF-TrFE dyads reduced from the direct radical copolymerization of VDF with TrFE.381 This is an issue of some concern since TrFE is dangerous to both transport and storage.392 The authors also noted that such ferroelectric fluoropolymers exhibit high dielectric constants. This very interesting study led to a wide range of new copolymeric structures that enabled these authors to establish the “polymer structure-thermal and dielectric properties” relationship that rationalizes the factors governing responses in these organic electroactive materials. Indeed, many poly(CTFEter-TrFE-ter-VDF) terpolymers were synthesized, and led to a wide range of materials having various Curie temperatures, ranging from 22 to 106 °C. These are much lower than that reported for PVDF which is 195−197 °C.393,394 In addition, these terpolymers display room temperature dielectric constants varying from 11 to 50 at 1 kHz.385 Indeed, the highest dielectric constant (50) and low dielectric loss (tgδ < 0.05)378,380 were achieved for the poly(CTFE-ter-TrFE-ter-VDF) terpolymer containing 78.8, 7.2, and 14.0 mol %, respectively. Such values are higher than the corresponding values for those terpolymers prepared by direct radical terpolymerization of VDF with CTFE and TrFE.395,396 Zhen et al.391 confirmed a dielectrical constant of 37.5397 for the resulting terpolymer. Chung’s group pioneered an original methodology365,398,399 for a similar application. These authors also reported the controlled radical copolymerization of CTFE with VDF from a borane/oxygen system at room temperature. It is worth noting that the borinate generated in situ acted as a counter-radical that actually controls such a radical copolymerization. This will be discussed in more detail in section 3.3.1. The molecular weights of these poly(CTFE-co-VDF) copolymers were found to be greater than 20 000 g mol−1 with narrow copolymer composition distributions. In addition, all experimental trials indicated a CTFE molar percentage higher than that of the feed. The selective reduction of chlorine atoms in CTFE units which yielded poly(CTFE-ter-TrFE-ter-VDF) terpolymers385 has also been reported by the same group.381,399 The consecutive VDF

It has also been shown that CTFE percentages lower than 30% lead to thermoplastic copolymers. The crystalline structure was reported to be monoclinic367 and such copolymers are called flexible PVDFs.368 The thermal properties of poly(CTFE-co-VDF) copolymers have been investigated by various research teams.104,369 They showed that the thermal degradation occurs in a single step following a power law and diffusion model. The mean activation energy for the degradation of the samples tested is about 197 kJ mol−1.369 The degradation products consist of the homopolymers (see section 2.3.2.1 for PCTFE) as well as HCl. This results from an elimination reaction at a CTFE-VDF dyad (and an unsaturated −CFCH− double bond in the backbone).104 Similar to many other copolymers based on CTFE, poly(CTFE-co-VDF) copolymers can also be synthesized either in emulsion370 or in suspension.126 In 1979, the microstructure of poly(CTFE-co-VDF) copolymers was nicely established for the first time by Murasheva et al.371,372 Their careful 19F NMR spectroscopic characterization was surprisingly not acknowledged or referenced when revisited recently by Chung’s team.373,374 Li et al.375 carried out the radical copolymerization of CTFE with VDF in 1,1,2-trichloro-1,2,2trifluoroethane (F113) and they demonstrated that the incorporation of CTFE in the copolymer was twice the theoretical amount. In addition, they showed that by raising the proportion of CTFE, the main microstructures are VDFCTFE-VDF and −(CTFE)n− triads or units. Gee et al.376 performed molecular dynamics simulations of poly(CTFE-co-VDF) copolymers over a wide range of compositions and temperatures. The X-ray peak associated with interchain spacing was shown to shift toward a larger scattering angle as the CTFE content decreased. This observation indicates that interchain spacing increases with increasing CTFE content (due to the bulky chlorine atoms). However, this increase is not uniform throughout the chain because the methylene spacing remains almost constant, whereas that of CF2 rapidly increases between 25 and 50 mol % CTFE. In 1982, it was discovered that poly(CTFE-co-VDF) copolymers also exhibit piezoelectric properties. This is similar to PVDF.377,378 It is noteworthy that currently these copolymers are undergoing a renewed interest.373,374,378−382 PVDF, VDF telomers,383−385 and copolymers385 are the most interesting ferroelectric polymers. This arises from the strong polarization assigned to C−F bonds and the spontaneous orientation of dipoles in the crystalline phases.377 Indeed, the poly(CTFE-coVDF) copolymer shows a dielectric permittivity of 13.379 In addition, poly(CTFE-co-VDF) copolymers containing 9 and 12 mol % CTFE386 exhibit a high electromechanical response. When the percentage is 12 mol %, it exhibited an electrostrictive strain response higher than 15%. Compared with other fluoropolymer-based electroattractive polymers, the poly(CTFE-co-VDF) copolymer displays a higher strain response. However, a higher driving electric field is required. It should be noted that the poly(CTFE-co-VDF) copolymer is cheaper and commercially available. X


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Scheme 34. Sketch of the Dipole Moments in Poly(CTFE-co-VDF) Copolymers [Reprinted with permission from ref 381. Copyright 2007 American Chemical Society.]

units provided strong polarization while the TrFE units distributed randomly, which were cocrystallizable with the VDF units, directed the VDF sequence to an all-trans (polar) conformation. In addition, the low amount of bulky Cl atoms in the CTFE units induced some kinks, which reduced the crystalline size without significantly reducing the overall crystallinity (Scheme 34). As a result, compared to poly(CTFE-co-VDF) copolymers, these poly(CTFE-ter-TrFE-ter-VDF) terpolymers display potential ferroelectric properties385,399 such as high dielectric constant and diffuse phase transition at room temperature (Figure 10). They also exhibit dielectric relaxation (large frequency dependence) for potential high-pulsed capacitors with low energy loss and high energy density. The authors reported similar bulky CTFE effects in these terpolymers with a decrease of both the melting and Curie temperatures.400 Claude et al.401,402 reported the terpolymerization of VDF with both CTFE and TrFE. They also investigated the relationship between the composition of these terpolymers and the dielectric permittivity. They noted that for the same VDF content in the terpolymer, the higher the TrFE content, the higher the Tm and the lower the dielectric permittivity. Indeed, for a large energy storage capacity, both high dielectric permittivity and electrical breakdown strength were required. They also assess the permittivity of poly(CTFE-ter-TrFE-terVDF) terpolymers up to 50 (measured at 1 kHz and at ambient T).366 By adding small amounts of zinc sulfide (ZnS) nanoparticles, it is possible to tune the refractive index of relaxor ferroelectric poly(CTFE-ter-TrFE-ter-VDF) terpolymer nanocomposites between about 1.4 and 1.5 while maintaining a large electro-optic effect and high transparency.403 Tunable long-period fiber gratings have been prepared with the nanocomposite as the second cladding. Over 50 nm of resonant wavelength shift was noted under a change of electric field of 30 V μm−1; this corresponds to a 3-fold increase in tuning range. Functional density calculations suggest that this change in the refractive index arises from the dipolar reorientation of the nanopolar regions in the poly(CTFE-ter-TrFE-ter-VDF) terpolymer.404 Therefore, the changes in the observed polarization have been attributed to the nanopolar regions in the relaxor ferroelectric terpolymers, as well as the large electrostrictive strain, and the EO properties. Li et al.405 prepared dielectric nanocomposites composed of ferroelectric polymers and surface functionalized TiO2 nanoparticles. They found that nanoscale fillers favor the formation of smaller crystalline domains but result in a higher degree of crystallinity in the polymer. Another highlight of such applications with respect to these controlled structure and molecular weight poly(CTFE-co-VDF) copolymers was also reported by researchers at Penn State University.381 They investigated two types of fuel cell membranes. As described above, the copolymers were

Figure 10. Dielectric constant of (a) poly(CTFE-co-VDF) (92/8 mol %) copolymer (melting temperature, Tm = 145.0 °C; Curie temperature, Tc = nonassessed) and (b) poly(CTFE-ter-TrFE-ter-VDF) (65.6/26.7/ 7.7 mol %) terpolymer (Tm = 123.6 °C and Tc = 23.8 °C). [Reprinted with permission from ref 399. Copyright 2009 Elsevier.]

synthesized by controlled radical copolymerization involving borinate as the counter radical (see section 3.3.1). When H3OZr2(PO4)3 particles were used in a 20% ratio and the Nafion wt. content was 60%, a poly(CTFE-co-VDF) copolymer/Nafion/inorganic fillers composite yielded a satisfactory conductivity of 25 mS cm−1 at 120 °C for 70% relative humidity (RH). This is an interesting value since it is known that perfluorosulfonic acid polymers (e.g., Nafion or Aquivion-types) only operate well at high relative humidities and when the temperature is lower than 85 °C (i.e., below the dew point).406 A second family not containing any Nafion, but rather membranes-containing sulfonic acid functions which were added Y


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Scheme 35. Radical Terpolymerization between VDF, CTFE, and a Trifluorovinyl Monomer Bearing a Silane Groupa366

a

X represents H or C2H4Si(OEt)3.

Scheme 36. Use of α,ω-Diiodo Poly(CTFE-co-VDF) Copolymers As Chain Transfer Agent in the Radical Copolymerization of Ethylene and Tetrafluoroethylene or CTFE to Achieve Triblock Thermoplastic Elastomers411

Another strategy to control the radical copolymerization of CTFE with VDF involves an iodine transfer copolymerization that was pioneered by the Daikin Company.411 This is carried out in the presence of IC4F8I or another telechelic diiodide as the chain transfer agent (Scheme 36). To the best of our knowledge, that discovery was, in fact, the first investigation of controlled radical (co)polymerization. The resulting I-poly(CTFE-coVDF)-I contained 45 and 55 mol % of CTFE and VDF, respectively. It behaved as an elastomeric block, the Tg value of which was −7 °C. This soft telechelic diiodide was successfully involved in further radical copolymerizations of ethylene (E) and CTFE (or E and tetrafluoroethylene). Hence, original Hard− Soft−Hard triblock thermoplastic elastomers were successfully produced. The melting points of the hard blocks containing either poly(E-alt-CTFE) or poly(E-alt-TFE) copolymers were 247 or 252 °C, respectively (Scheme 36). The intended application was artificial lenses.411 Besides those original block copolymers based on CTFE, very few graft copolymers have been reported as TPEs. The Central Glass Company is currently producing a terpolymer based on CTFE, VDF, and a peroxide-functionalized allylic monomer that subsequently undergoes a “grafting from” polymerization of PVDF to form a tough, flexible poly(CTFE-co-VDF)-g-PVDF graft thermoplastic material. This is marketed under the Cefral trademark412−415 (see section 4.4.1). In conclusion, the above examples show that poly(CTFE-coVDF) copolymers have found relevant high-tech applications as capacitors, electroactive polymers, or, thanks to their piezoelectric properties, actuators. Additional applications include as nanocomposites, fuel cell membranes, surfactants, optical devices

via sulfonated silicon dioxide or 2-(4-sulfonic phenyl) ethyl trimethoxysilane (Scheme 35) showed satisfactory conductivities. The ionic exchange capacity was found to be 2.25 mmol g−1. In addition, termination by recombination of the radical led to telechelic α,ω-trialkoxysilane poly(CTFE-co-VDF) (see also section 4.2). These end-functionalized fluoropolymers behave as efficient surfactants407 that displayed a high interfacial activity in exfoliated F-polymer/clay nanocomposites. The tri(alkoxy)silane is capable of anchoring the polymeric chain to the clay interface while the hydrolytic polymer exfoliates the layered clay structure.407 A similar approach, in order to obtain original telechelic poly(CTFE-co-VDF) copolymers bearing phosphonic acid end groups (functionality ca. 95%) was carried out by Li et al.408 This team initiated the radical copolymerization of CTFE and VDF using dibenzoyl peroxide that had diethyl phosphate end groups. These end groups were used to induce the direct coupling with the zirconium oxide fillers during the preparation of the nanocomposites. The choice of phosphonic end groups versus side groups (via a co-monomer) was dictated by the desire to maintain the crystallinity and hence the ferroelectric properties. Other original poly(CTFE-co-VDF) copolymer-based nanocomposites409,410 were achieved by mixing that copolymer with carbon nanotubes (CNTs) or C60 fullerene using a solutioncasting method. The CNT or C60 volume fraction ranged from 0.1% to 1.0%. The influence of the CNT and C60 on the crystallization behavior of poly(CTFE-co-VDF) copolymers was highlighted by XRD and DSC. The authors clearly demonstrated that the CNT or C60 increased both the β-phase and the crystallinity. Z


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Scheme 37. Termonomer-Induced Copolymerization of Trifluorovinyl Oxyparabromobenzene (TFVOPBB) with CTFE and VDF421

Scheme 38. Radical Terpolymerization of PFSVE with VDF and CTFEa 422

a PFSVE, VDF, and CTFE stand for perfluoro(4-methyl-3,6-dioxaoct-7-ene) sulfonyl fluoride, vinylidene fluoride, and chlorotrifluoroethylene, respectively.

performances419 than PVDF (higher Tg, lower surface energy, higher toughness). 3.2.12. Poly(CTFE-co-perfluoroalkyl vinyl ethers) Copolymers. Although one patent from Ausimont420 claims the successful copolymerization of CTFE with perfluoroalkyl vinyl ethers (PAVE), to the best of our knowledge, none have actually been produced on an industrial scale. This is in contrast to poly(VDF-co-PAVE) copolymers. Yet, a few academic research groups, pioneered by Souzy et al.,421 have reported original investigations on the radical copolymerization of CTFE with trifluorovinyl oxyparabromobenzene (TFVOPBB). However, this monomer showed poor reactivity. Thus, a more efficient reaction dealt with the terpolymerization of CTFE with VDF and TFVOPBB leading to poly(CTFE-ter-VDF-ter-TFVOPBB) terpolymers (Scheme 37). The sulfonation of these terpolymers led to aryl sulfonic acid tethered to poly(CTFE-co-VDF) copolymers from which novel fuel cell membranes were processed by casting. Unfortunately, the low ion exchange capacity (0.7 meq g−1) and conductivity (ca. 1 mS cm−1) meant that such membranes were not efficient competitors with respect to Nafion or Aquivion. Copolymers of CTFE and perfluoro(4-methyl-3,6-dioxaoct-7ene) sulfonyl fluoride (PFSVE) were prepared by radical copolymerization initiated by Trigonox 101 at 134 °C in 1,1,1,3,3-pentafluorobutane.422 It was noted that CTFE was more reactive than PFSVE and that the resulting poly(CTFE-coPFSVE) copolymers were thus insoluble in the usual organic solvents such as acetone, THF, acetonitrile, DMF, or DMSO. This was attributed to the high amount of CTFE and the high crystallinity of the resulting copolymer. However, these copolymers were soluble in trifluorotoluene. Since copolymerization yield is low, it shows that the CTFE does not copolymerize efficiently with perfluorinated vinyl ether (in contrast with the hydrogenated vinyl ethers, see section 3.2.4) which may explain the absence of a commercially available poly(CTFE-co-PAVE) copolymer. More recently, Flach et al.423 reported the preparation of interesting membranes, the morphology of which they investigated. They were obtained

(artificial lenses), and thermoplastic elastomers. Indeed, the piezo-, ferro-, or pyroelectric properties when a chlorine atom has selectively been reduced are clearly demonstrated. This leads to very interesting poly(CTFE-ter-TrFE-ter-VDF) terpolymers. Such a strategy, initially pioneered by Lovinger in the early 1980s, has been recently revisited and subsequently has led to many more applications for ferroelectric polymers. 3.2.11. Poly(CTFE-co-(per)fluoroalkenes) Copolymers. Only a few studies report the synthesis of poly(CTFE-coperfluoroalkene) copolymers. Dow Corning Co.54 claimed the iodine transfer polymerization (ITP) of CTFE in the presence of perfluoroalkyl iodides and diiodides as well as the stepwise cotelomerization of CTFE and HFP in the presence of perfluoroalkyl iodides and chlorobromodifluoromethane to prepare hydrogen-free, thermostable, chemically inert, highly fluorinated fluids and waxes. In 1996, Napolitano and Pucciariello72 reported packing energy calculations for poly(TFE-co-CTFE) copolymers with a low CTFE content. They suggested that the chlorine group is easily tolerated in the crystal phase. However, no experimental data was provided to support these findings. To obtain CTFE copolymers with higher transparency, The Daikin Company developed the successful copolymerization of CTFE with a RF−CFCF2 co-monomer (including hexafluoropropylene, HFP). RF is a perfluorinated group with up to eight carbons, and it can be either linear or branched.416 Copolymerizations were achieved either in suspension or in the bulk and were initiated by di(acyl) peroxides ((RFCO)2O2 with RF = CnF2n+1, CnF2n+1O, and Cl(CTFE)nCF2) between −30 and +40 °C. Another patent417 claims the radical terpolymerization of CTFE with tetrafluoroethylene (TFE) and a third monomer, the molar percentage of which was kept below 10 mol %. This third monomer could be either another olefin such as ethylene, VDF, or perfluoroalkyl vinyl ether in order to reduce the stress cracking of the PCTFE. More recently, Honeywell patented the radical copolymerization of CTFE with CH2CF−CF3.418 This copolymer can be blended with poly(meth)acrylate esters to prepare fluorocarbon/(meth)acrylate coatings with better AA


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reactivity of acrylonitrile (AN) is higher than that of CTFE. In fact, this behavior is directly linked to the Q value attributed to the resonance of these monomers and indeed QAN ≫ QCTFE. Hence, such a copolymerization would have led to a very high incorporation of acrylonitrile into the copolymers. 3.2.15. Kinetics of Radical Copolymerization of CTFE. Many kinetics of the radical copolymerization of CTFE with various co-monomers (M) have been investigated and the results are summarized in Table 5 and in Figure 11. Thomas and O’Shaughnessy27 also pioneered kinetic studies of CTFE copolymerizations by examining its copolymerization with vinyl acetate, styrene, methyl methacrylate, vinyl chloride, butadiene, and 2-vinylpyridine. It has been claimed that PCTFE is insoluble in its monomer and in most other solvents. This makes it difficult to explain its polymerization in solution. Indeed, it is possible to establish a reactivity series of fluorinated monomers with CTFE using the traditional method for the determination of the relative reactivity of a macroradical toward several monomers. It is common to compare 1/r1 or k12/ k11 values as the ratio of the rate constants for cross-propagation (k12) to that for homopropagation (k11). Thus, the higher the 1/r value, the more the radical is able to react with the second monomer. From the data presented in Table 5 it is possible to suggest the following decreasing series of relative reactivities of monomers to the ⇝CTFE• macroradical: TFE430−432 < Fdioxolane425 < VDF428,429 < α-fluoromethylstyrene351 < NVP426 < allyl glycidyl ether311 < vinyl propionate346 < m-TMI354 < vinyl trialkoxysilane427 < isobutylene300,301 < VDC356 < vinyl ether30,320,327 < vinyl chloride356 < MMA27 < ethylene231,232 ≈ styrene27 < propylene.299 However, it should be made clear that many other cases of kinetics involving CTFE still deserve to be investigated. From Figure 11, it should be noted that none of the comonomers employed in the copolymerization shows a lower reactivity than CTFE, though, as mentioned above, the molar percentage of perfluoro(4-methyl-3,6-dioxaoct-7-ene) sulfonyl fluoride in the copolymer seems to be higher than that in the feed.422 Alfrey and Price347 established a model to describe the reactivity of two co-monomers according to two parameters, Q and e. These take into account the resonance stabilization and the polar effects of both monomers, respectively. Such a model has the advantage of identifying each co-monomer (acceptor, A, and donor, D) by associating both these parameters to represent its activity in the copolymerization:

from the bulk radical copolymerization of CTFE (up to 80 mol % in the copolymer) with PFSVE initiated by a perfluorinated peroxide (perfluorobenzoylperoxide). The resulting copolymers were modified, by hydrolysis, into dangling sulfonic acid groups for possible fuel cell applications (see section 5.1). They could also be transformed into sulfonamides groups (−SO2−NHR where R = benzyl or hexyl). In contrast, the radical terpolymerization of CTFE, VDF, and PFSVE is much more efficient and can be carried out with yields above 60 wt % (Scheme 38).422 This is a typical case of a “termonomer induced copolymerization” (TIC) in which VDF, that is quite reactive with both CTFE and PFSVE, makes the copolymerization of CTFE with PFSVE possible. A low glass transition temperature of −20 °C was observed. This value was higher than that of PVDF at −40 °C but lower than that of PCTFE at +45−50 °C. These PFSVE-containing co- and terpolymers were hydrolyzed to exchange the SO2F moiety by SO3H. The first step was by reaction with lithium carbonate in methanol and the second with hydrochloric acid. A Japanese patent417 claims the radical terpolymerization of CTFE with TFE and a PAVE. The latter is known to reduce the stress cracking of PCTFE even with a molar content below 10 mol %. 3.2.13. Poly(CTFE-co-fluorinated dioxolane) Copolymers. An interesting review reports the synthesis of various copolymers based mainly on fluorinated dioxolanes and tetrafluoroethylene. However, it does not mention any work involving CTFE.424 Fluorinated dioxolanes were prepared and then homopolymerized or copolymerized with fluoroolefins in an attempt to develop amorphous fluorinated polymers for optical applications (Scheme 39).425 The kinetics of the radical Scheme 39. Radical Copolymerization of Fluorinated Olefins (CTFE, PMVE, PPVE, and VDF) with Fluorinated Dioxolanesa 425

a

PMVE = perfluoromethylvinyl ether, PPVE = perfluoropropylvinyl ether, and VDF = vinylidene fluoride.

copolymerization of the fluorinated dioxolane with CTFE led to establishing the reactivity ratios of both co-monomers: rF‑Dioxolane = 3.63 and rCTFE = 0.74 at 74 °C.425 This is strong evidence of the greater reactivity of fluorinated dioxolane compared to CTFE. The resulting copolymers were soluble in hexafluorobenzene and perfluoro-2-butyltetrahydrofuran. Their Tg values ranged from between +84 and +145 °C depending on the copolymer composition. This is somewhat lower than that of homopoly(F-dioxolane) (Tg = 180−190 °C). The copolymeric films were flexible and clear with a low refractive index (1.3350−1.3770 at 532 nm), which makes these materials an interesting choice for optical applications such as in optical fibers. These homopolymers and copolymers exhibit high Tg values and also showed a high thermal stability. 3.2.14. Poly(CTFE-co-acrylonitrile) Copolymers. Thomas and O’Shaughnessy27 reported that the copolymerization of CTFE with acrylonitrile failed. This is not unexpected since the

rA =

Q kAA = A exp[−eA(eA − eD)] kAD QD

rD =

Q kDD = D exp[−eD(eD − eA )] kDA QA

To attempt the synthesis of an alternating copolymer, kAA and kDD are small (e.g., in radical conditions, a vinyl ether does not propagate) about kAD or kDA. In order to attempt to synthesize an alternating polymer, kAA and kDD should be small because, for example, under radical conditions, a vinyl ether does not propagate. Hence, rA and rD are low and the rA × rD product tends toward zero. According to both the above equations, it can be seen that rA × rD is proportional to exp[−(eA − eD)2]. Hence, it can be deduced that |eA − eD| must be as high as possible. This shows that, once again, the polarity between two monomers must be as opposite different as possible. Tables of Q and e values are AB


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Table 5. Reactivity Ratios for the Radical Copolymerization of CTFE with a co-monomer (M) co-monomer M ethylene

propylene isobutylene allyl glycidyl ether ethyl vinyl ether 2-chloroethyl vinyl ether vinyl acetate

vinyl propionate methyl methacrylate N-vinyl-2-pyrrolidone styrene α-fluoromethylstyrene vinyl trialkoxysilane 3-isopropenyl-α,α′-dimethylbenzyl isocyanate vinylene carbonate vinyl chloride vinylidene chloride vinylidene fluoride tetrafluoroethylene

F-dioxolane

rCTFE

rM

0.001 ± 0.001 0.004 ± 0.001 0.009 ± 0.001 0.029 ± 0.001 0.00 0.00 0.04 0.004 ± 0.001 0.20 ± 0.03 0.008−0.012 0.008 0.011 0.04 ± 0.02 0.01 0.014 0.08 ± 0.01 0.005 0.30 0.001 0.40 0.05 0.076 0.48 0.01 0.02 0.75 0.52 1.00 1.10 1.04 0.737

0.070 ± 0.001 0.116 ± 0.001 0.173 ± 0.001 0.250 ± 0.002 0.06 0.24 0.00 0.038 ± 0.007 0.15 ± 0.08 0.00 0.002 0.005 0.68 ± 0.11 0.60 0.44 0.63 ± 0.04 75.00 0.38 7.00 3.70 0.20 0.034 0.42 2.53 17.14 0.73 0.17 1.00 0.80 0.75 3.635

readily available in the literature.6,11,328,433 When considering CTFE copolymers, it is interesting to recall that, depending on the bibliographic source, both the Q and e values for CTFE are QCTFE = 0.020−0.031 and eCTFE = 1.48−1.84, respectively. Consequently, it could be expected that co-monomers showing low Q values, and hence poorly stabilized by resonance and whose double bonds bear electron-donating group(s) (i.e., negative values of e) will lead to alternating copolymers. For further details, the reader can access a recently published interesting review on that topic by Braun and Hu.433 See section 3.2.5 for poly(CTFE-alt-VE) copolymers and section 4.1 for further details. 3.2.16. Poly(CTFE-ter-M1-ter-M2) Terpolymers. In addition to radical copolymerization involving CTFE, several terpolymerizations have also been reported. Some representative examples are listed below. The first one, mentioned above, deals with the terpolymerization of CTFE with VDF and either trifluorooxyaryl bromide,421 sulfonyl chloride,434 or PFSVE.422 The choice of VDF is a key point in favor of this reaction. Using the same strategy, a recent investigation in our group353 showed that α-trifluoromethyl styrene could be successfully copolymerized with a mixture of CTFE and VDF whereas the copolymerization of the aromatic styrene monomer with either CTFE or VDF alone failed. For coating applications, special terpolymers based on CTFE and ethylene were developed. They contained a small amount of a termonomer such as hexafluoroisobutylene (HFIB), perfluorohexyl ethylene,281 perfluoroisoalkoxy perfluoroalkyl ethyl-

rCTFE × rM

T (°C) −78 −40 0 +60 −35 −78 0 0 +74

+50 +80 +45 scCO2

+74 +74 +65 +60 +60

+74

1/rCTFE

0.00007 0.000464 0.001557 0.00725 0

1000 250 111 34.5 ∝

0 0.000152 0.03 0 0.000016 0.000055 0.0272 0.0060 0.0062 0.05 0.3750 0.1140 0.007 1.48 0.01 0.0026 0.2016 0.0025 0.3428 0.55 0.09 1.00 0.88 0.78 2.68

25.00 250 5.00 83.3−125 125 90.9 25.00 100 71.4 12.50 200.00 3.30 1000.00 2.50 20.00 13.20 2.083 100 50 1.33 1.92 1.00 0.91 0.96 1.36

ref 278, 279 278 278 278, 279 299 300 301 311 320 327 30, 327 346 27 346 346 27 426 27 351 427 354 344 356 356 428 429 430 431 432 425

enes,282 perfluoropropyl vinyl ether (PPVE),283 or organic silanes.284 For the same intended applications, several companies developed copolymers based on CTFE and 2-hydroxyethyl allyl ether with at least a third co-monomer such as an acrylate (Central Glass),302 isobutylene (Daikin),304 vinyl acetate (Fluorobase from Ausimont),305 or VDF and TFE (Elf Atochem).306−308 Other epoxy curable coatings were developed from a terpolymer of CTFE (25−75 mol %) vinyl or an isopropenyl ester of a fatty acid (10−70 mol %) and allyl glycidyl ether (AGE, 3−40 mol %) by Central Glass.310 It was found that the copolymerization of CTFE with vinyl ether and a maleimide did not lead to an alternating terpolymer.435 Monitoring the poly(CTFE-ter-VE-ter-maleimide) terpolymer structure showed that CTFE incorporation was low in the initial step of the terpolymerization and increased with conversion. This clearly demonstrates that the maleimide is more reactive than CTFE. This can be explained by the high Q value for maleimide (2.81) in comparison to those of CTFE or VE (0.026 and 0.038, respectively). This indicates that the maleimide is stabilized by resonance and thus attracts electrons leading to higher reactivity. In contrast, Cersosimo et al.436 carried out the terpolymerization of CTFE with vinyl ether and N-vinylpyrrolidone in the following proportion, 50/25/25, and obtained a terpolymer with a random distribution of poly(CTFEalt-VE) alternating sequences. Terpolymers of CTFE and vinylene carbonate (VCA) with a vinyl ether bearing a hydroxyl side group as the cure site monomer were prepared (Scheme 26) as precursors for optical AC


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Figure 11. Composition curves for the radical copolymerization of CTFE with various co-monomers (E = ethylene, IB = isobutylene, MMA = methyl methacrylate, NVP = N-vinylpyrrolidone, P = propylene, TFE = tetrafluoroethylene, VAc = vinyl acetate, VC = vinyl chloride, VDC = vinylidene chloride, VDF = vinylidene fluoride, and VE = vinyl ether).

fiber waveguides.345 The incorporation of fluorine into the structure enhanced the thermostability and reduced both optical losses and water absorption. The hydroxyl group was acrylated to prepare photo-cross-linkable materials. Poly(CTFE-ter-TrFE-ter-VDF) copolymers have also been prepared and for applications where their ferroelectrical and dielectrical properties are desirable. These are reviewed in section 3.2.10.365,366,401−403 3.2.17. Poly(CTFE-ter-VDF-ter-HFP) Terpolymers. SaintLoup et al.437 reported the radical terpolymerization of CTFE, VDF, and hexafluoropropylene (HFP) initiated by hydrogen peroxide using the dead-end polymerization technique. The radical copolymerization of VDF and HFP, initiated by H2O2 in an emulsion process, led to poor monomer conversion. This was less than 5% with respect to the fluoroolefins. Interestingly, the terpolymerizations carried out in acetonitrile showed that the presence of CTFE enabled both alkene conversions and the terpolymerization yields to be higher than those of the corresponding copolymerization of VDF with HFP. As in previous cases, the IR spectra show the presence of the carboxyl end groups arising from the radical addition of HO• radicals onto the CF2 side of these fluoroalkenes. This yielded HO−CF2 end groups that were immediately transformed into an acid fluoride end group, subsequently leading to a carboxylic acid. In a suspension reaction, Lannuzel et al.438 inserted CTFE as the termonomer in the radical terpolymerization of VDF with HFP. The resulting poly(CTFE-ter-VDF-ter-HFP) thermoplastic contained 95.0 mol % VDF, 1.6 mol % CTFE, and 3.4 mol % HFP. It had a melt flow index of 2.26 (21.6 kg) and a melting temperature (Tm) of 141 °C. 3.2.18. Conclusions. A wide range of functionalized monomers, both specifically synthesized or commercially available, were successfully copolymerized with CTFE. Accord-

ing to their reactivity, various thermoplastics or elastomers were obtained with the functional group designed to target the necessary properties for the required application. The kinetics of these radical copolymerizations made it possible to propose a classification of the monomers that to date had not been achieved. Hence, an increasing order of monomer reactivity, with respect to the macroradical bearing the CTFE end groups, has been successfully established. This demonstrates the difficulty out of establishing co-monomers less reactive than CTFE in order to allow the production of a CTFE-rich copolymer. In contrast, vinyl ethers led to alternating copolymers which enabled the production of materials containing 50 mol % CTFE. 3.3. Copolymers Achieved by Controlled Radical Polymerization

3.3.1. Theoretical Concepts on Controlled Radical Polymerizations. In 1955, Haszeldine146 reported the first study of a possible controlled radical polymerization involving CTFE (see section 2.3.1 for the radical telomerization of CTFE). In section 3.2.10 we mentioned some interesting syntheses of thermoplastic elastomers based on the sequential iodine transfer copolymerization439 of various co-monomers. A simplified mechanism involving the dormant and “living” chains is shown in Scheme 40. However, little other work has been carried out on the controlled radical copolymerization (CRP) of CTFE. One Scheme 40. Iodine Transfer Radical Polymerization (ITP) Mechanism439

AD


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monomers with both corresponding reactivity ratios, rA and rD, four equations can be written as seen in Scheme 42.

example is the CRP mechanism based on trialkyl boranes developed by Chung’s group.309,310 The mechanism is described in Scheme 41 and some applications are detailed in Scheme 35.

Scheme 42. Propagation and Cross-Propagation Equations for the Radical Copolymerization between the Acceptor (A) and the Donor (D) Monomers441−448

Scheme 41. Mechanism of Radical Copolymerization of CTFE with a Co-monomer M Controlled by a Boron Species373,374

This situation is really only achieved when both co-monomers do not (or only poorly) homopolymerize. Accordingly, even for known alternating copolymers, short homopolymeric sequences are produced; with the result that the overall properties of the copolymers may be disturbed. It has been established that in order to promote the alternation,441−444,446,448 both comonomers must bear groups of opposing polarity (Scheme 43). The presence of both withdrawing and donating groups enables the macroradical to react more or less easily with the comonomer. This is shown in Scheme 43.

3.3.2. Controlled Radical Copolymerization of CTFE. The first successful iodine transfer copolymerization of VDF and CTFE was achieved by the Daikin Company411 using IC4F8I as the chain transfer agent. This led to the production of I− poly(VDF-co-CTFE)−I (Scheme 36) where the mol % of VDF and CTFE was found to be 55 and 45, respectively, in the form of an elastomeric block (Tg = −7 °C). This soft telechelic diiodide was able to reinitiate the radical copolymerization of ethylene (E) and CTFE (or E and tetrafluoroethylene) and the original hard− soft−hard triblock thermoplastic elastomers. The melting points of the hard blocks which contained either poly(E-alt-CTFE) or poly(E-alt-TFE) copolymers were 247 or 252 °C, respectively (Scheme 36). They were produced with a view to making artificial lenses.411 Wang et al.229 published the synthesis of hyperbranched poly(CTFE-co-CMS) copolymers by the atom transfer radical copolymerization (ATRP) of CTFE with chloromethylstyrene (CMS) (Scheme 31). This was in spite of the fact that all their attempts to homopolymerize CTFE by ATRP systematically failed (see further description in section 3.2.8). Liu et al.440 reported the living/controlled radical copolymerization of CTFE with butyl vinyl ether, which was initiated under 60Co γ-ray irradiation in the presence of S-benzyl O-ethyl dithiocarbonate. Although this work is original, the presence of a xanthate end group was not pointed out. Surprisingly, a very good agreement between the Mn assessed from NMR spectroscopy and that from GPC (calibrated by polystyrene standards) was noted. In addition, the authors did not supply any evidence needed to clarify the nature of the monomeric unit adjacent to the xanthate end group. That would have provided specific information on the ease of getting a chain extension, through the polymerization of VAc to lead to poly(CTFE-alt-BVE)-bpoly(VAc) diblock copolymers.

Scheme 43. Expected Copolymerization Involving Acceptor and Donor Monomers

The mechanism of alternating radical copolymerization is a subject of great debate which has led to various controversies.436 However, one invariable property of the different pairs of comonomers required to yield the alternating systems concerns the complementarity between the electron-donor/electron-acceptor behaviors of the co-monomer pairs. Indeed, it is essential that a highly electron-donating monomer (D) must copolymerize in an alternating fashion with a highly electron-accepting monomer (A), see Scheme 44. Scheme 44. Mechanism of Acceptor−Donor (A-D) Copolymerization

4. WELL-DEFINED COPOLYMERS BASED ON CTFE A range of custom-designed PCTFE-based copolymers have been reported with various types of architectures such as alternating, telechelic (or α,ω-difunctional), block, graft, and dendrimers. The following section describes a nonexhaustive range of the possible strategies used to synthesize such structures. 4.1. Fluorinated Alternating Copolymers

Altogether there have been three independent theories that have been suggested to justify this statement. Bartlett and Nosaki449 proposed the first mechanism in which the monomer couple forms a donor−acceptor complex [DA] that has a higher reactivity than either of the free monomers. This mechanism was confirmed by several studies in particular that of Butler et al.450

Alternating copolymers based on CTFE have been reviewed in section 3.2.5. A copolymerization reaction lies on depends on the competition between the four propagation equations from two monomers.441−448 In the case of the acceptor−donor (A-D) copolymerization between the accepting (A) and donating (D) AE


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Later, Walling et al.451 suggested a second theory. This involved electrostatic interactions and differences in polarity between the radical end-chain and the inserting monomer. Consequently, the activation energy of the alternating propagation is lowered compared to that of homopropagation. In a third mechanism, proposed by Tsuchida and Tomono452 and later confirmed by Shirota et al.,453 both the free monomers and the charge transfer complex (CTC) participate in the propagation. These three theories are summarized in Scheme 44. The latter mechanism received strong support from many authors, in particular that of Butler.450 The participation of the DA complex is emphasized when the charge transfer complex is maximum and when the involved monomeric ratio is 1:1. This scenario efficiently leads to a maximum copolymerization rate. The [AD] charge transfer complex is indeed an equilibrium as represented in Scheme 45:

telomers. In fact, the telomerization of CTFE with CCl4 led to Cl3C−(CTFE)x−CF2CCl3 in two steps. This telomer yielded RO 2 C−(CTFE) x −CF 2 CO 2 R (R = H or alkyl) or GCH2CHClCH2−(CTFE)x−CH2CHClCH2G (G = functional group). More details are presented in sections 2.3.3 and 2.3.4, in particular in Schemes 5 and 9. 4.2.2. From Dead-End Polymerization. Saint-Loup et al.437 successfully obtained telechelic poly(CTFE-ter-VDF-terHFP) teroligomers bearing terminal carboxylic group using the dead-end terpolymerization of these co-monomers initiated by H2O2. The titration of the carbonyl end groups showed a general carboxylic functionality of 1.85 ± 0.15. SEC analysis showed that an initial [H2O2]0/([CTFE]0 + [VDF]0 + [HFP]0) molar ratio of 35% yielded a molecular weight for the resulting terpolymer of 800 g mol−1. The glass transition temperatures of these products were around −44 °C. The carboxylic end groups were then reduced using LiAlH4 to give alcohol groups thus producing telechelic fluorinated diols. After reduction, the absence of any frequencies at 1730 cm−1 in the FTIR spectra revealed that the reduction was complete. Starting from a functionalized dibenzoyl peroxide, Li et al.455 prepared some telechelic poly(CTFE-co-VDF) copolymers, bearing well-defined carboxyl, amino or hydroxyl end groups (Scheme 46), as precursors for cross-linked membranes.

Scheme 45. Acceptor− Donor Complex where CTFE Plays the Role of Acceptor Monomer

These charge transfer complexes have been established by various authors, either by NMR spectroscopy327,454 or by UV data.320 The tendency to produce alternating copolymers can be estimated using the Alfrey−Price347 Q and e parameters as described in section 3.2.15. In the case of CTFE copolymers, it is expected that monomers with negative e and low Q values, which are typical values for electron-donating monomers, should give alternating structures when copolymerized with CTFE. Among the various donating monomers, vinyl ethers are the prime electron-donating candidates. These have already led to many results reported in sections 3.2.5 and 4.5. Olefins such as ethylene, propylene and isobutylene are also good candidates and these are described in sections 3.2.1 and 3.2.2.

4.3. Fluorinated Block Copolymers Based on CTFE

The most common path for the preparation of a block copolymer is the sequential polymerization of the monomer using controlled radical copolymerization techniques. Iodine transfer polymerization439 is often employed to prepare block copolymers based on fluorinated olefins. The sequential iodine transfer copolymerization of fluoromonomers which was pioneered at the Daikin Company in order to produce fluorinated thermoplastic elastomers (TPFEs) has been reviewed in a book.6 These TPFEs are composed of a soft central block which is responsible for introducing the elastomeric properties while both the hard end-blocks favor the physical cross-linking456 (Scheme 36). Poly(CTFE-alt-BVE)-b-poly(VAc) diblock copolymers were prepared by sequential radical copolymerization using 60Co γ-ray irradiation in the presence of S-benzyl O-ethyl dithiocarbonate as the chain transfer agent (CTA).440 The first block was obtained by copolymerization of CTFE and butyl vinyl ether and the resulting poly(CTFE-alt-BVE) copolymers were used as macroCTA for the polymerization of VAc under the same conditions.

4.2. Telechelics Containing CTFE Base-Units

Two main strategies have efficiently led to telechelics that contain CTFE units: radical telomerization, which requires a key chain transfer agent and dead end polymerization. 4.2.1. From Telomerization. In section 2.3.4 can be found reports covering the comprehensive results of functional telomers involving α,ω-difunctional (or telechelic) CTFE

Scheme 46. Preparation of Telechelic Poly(CTFE-co-VDF) Copolymers Bearing Amino or Hydroxyl End Groupsa 455

a

Ri, R′i, and Di represent the initial functional group, the unprotected functional group, and the reagent used for the deprotection, respectively. AF


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Scheme 47. Strategy of Synthesis of Fluorinated Thermoplastic Elastomers via a Two-Step Procedurea

a

CTFE = chlorotrifluoroethylene and VDF = vinylidene fluoride.412−415

Scheme 48. Preparation of Graft Copolymers Based on CTFE232

4.4. Fluorinated Graft Copolymers

NMR spectra of the resulting graft copolymers. Interesting moisture barrier properties were achieved.232 Radiografting films, achieved by grafting monomers from either a macroradical or an irradiated polymer (from ozone, electron beam, gamma, or 60Co rays) were reviewed a few years ago.6 Many films based on a whole series of polymers such as PVDF, poly(VDF-co-HFP), poly(TFE-co-HFP) (FEP), poly(TFE-co-perfluoroalkyl vinyl ether) (PFA), and PTFE have all been successfully activated by such sources. To the best of our knowledge, only a couple of articles report the activation of PCTFE: Zhao457 prepared some PCTFE-g-poly(styrene) graft copolymers using irradiation by fast electrons in the presence of styrene to obtain the corresponding graft copolymer. Okubo et al.458 used a cold plasma to graft poly(acrylic acid) onto the surface of PFA, PTFE, and PCTFE thin films. Grafted films show enhanced adhesion with peeling strengths up to 30 times greater than the untreated films. Various authors459,460 used plasmainduced radical polymerization of VDF and CTFE to graft fluorinated chains onto poly(ethylene). One of the compounds they obtained was poly(ethylene)-g-PCTFE which has better hydrophobic properties and better biocompatibility. This technique was also applied to synthetic fibers.461,462 In a related approach, various authors463,464 demonstrated that the peroxy radicals generated on PCTFE by irradiation are persistent and can survive for more than 6 years. Evidence of the slow decay resulted from the successful graft polymerization of styrene initiated by the remaining peroxy radicals onto preirradiated films after various storage periods.

4.4.1. From Traditional Radical Polymerization. There is an early report, already from 1957, which deals with the synthesis of core−shell materials by emulsion copolymerization of vinyl chloride in the presence of poly(CTFE-co-EHVE) copolymer seeds.317 These copolymers have been used as fabric coatings. One interesting approach deals with the preparation of the Cefral graft copolymer marketed by Central Glass.412−415 It consists of the radical terpolymerization of CTFE, VDF and an allylic monomer bearing a peroxide group (e.g., tert-butylperoxy allyl carbonate, TBPAC). At low temperatures, this reaction yielded a novel macromolecular percarbonate initiator (Scheme 47). This was subsequently used to initiate the graft polymerization of VDF at higher temperatures. As a result, a tough, flexible graft thermoplastic elastomeric material was formed. It was based on an elastomeric backbone structure and thermoplastic PVDF grafts.415 This original synthetic strategy is still applied today on an industrial scale by the aforementioned company. Daikin456 also commercialized some poly(CTFE-co-VDF)-gpoly(VDF) graft copolymers as fluorinated thermoplastic elastomers (Tg = −21 °C). Our team has also prepared various graft copolymers based on CTFE (Scheme 48)232 by copolymerizing CTFE with XYC CZ(CTFE)nBr macromonomers. These were obtained by chemical modification such as the reduction of the telomers of α,ω-dibromo-CTFE (see Schemes 4 and 11). Grafting was confirmed by the absence of the characteristic signals assigned to the three fluorine atoms of the (trifluoro) vinyl groups in the AG


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Scheme 49. Poly(CTFE-co-VDF)-g-poly(M) Graft Copolymers by Atom Transfer Radical Polymerization of the Co-monomer (M) from the Poly(CTFE-co-VDF) Macroinitiator467,473−490

Kolb et al.465,466 grafted polystyryl lithium anions onto PCTFE thin films under various conditions using different types of polystyryl anions. The reaction is not very efficient in THF since the polystyryl anion reacts predominantly by reduction to produce compounds with double or triple carbon−carbon bonds. However, this reduction was not observed in benzene, and the polystyryl anion was grafted by an addition reaction. Another advantage was that the grafted copolymers tended to dissolve in THF but not in benzene. 4.4.2. From Controlled Radical Polymerization. Since the mid 2000s, CTFE-containing copolymers have been used to initiate the atom transfer radical polymerization (ATRP) of various vinyl monomers such as styrene and (meth)acrylates. Reaction led to graft copolymers bearing a fluorinated backbone. However, oligo(CTFE) telomers showed a high chemical inertness in both basic or acid media.260 It has been argued that the grafting occurred through cleavage of the C−Cl bond, although no clear evidence was shown at the time. Most of these graft copolymers have been developed for fuel cell applications. More details regarding this application can be found in section 5.1. Zhang and Russell467 grafted polystyrene or poly(tert-butyl acrylate) (poly(tBA) onto poly(CTFE-co-VDF) copolymers using ATRP in NMP in the presence of CuCl and PMDETA or bipyridine at 120 °C (Scheme 49). The hydrolysis of the acrylate function in poly(CTFE-co-VDF)-g-poly(tBA) into acrylic acid (AA) moieties led to poly(CTFE-co-VDF)-g-poly(AA) amphiphilic graft copolymers that proved useful as pH sensitive membranes. By using this same technique, Guan et al.468−472

prepared several poly(CTFE-co-VDF)-g-poly(styrene)s, followed by the dechlorination of the end groups to prepare coatings with confined ferroelectric properties (see section 5.3). Tan et al.473 synthesized various poly(CTFE-co-VDF)-g-poly(M)s by the ATRP of methyl methacrylate, methyl acrylate, butyl methacrylate, and styrene in order to study the side-reactions of transfer to the solvent and ligand in various nitrogen-containing solvents (DMF, DMSO, and NMP). Low temperatures and short reaction times were found to be the most desirable in order to minimize the chain transfer reaction rate for ATRP reactions conducted in solution. This study was confirmed by Tsang et al.,474,475 who reported the preparation of poly(CTFE-co-VDF)g-poly(SSA) by the ATRP of styrene from a poly(CTFE-coVDF) macroinitiator followed by sulfonation of the styrenic units (Scheme 49; SSA stands for styrene sulfonic acid). In this case, the graft copolymer exhibited higher ionic content than those of diblock copolymers without excessive swelling. This led to the successful production of proton-exchange membranes with highly concentrated ionic domain and thus greater protonic conductivities, which could be used in fuel cells. Park et al.476 also described the synthesis of poly(CTFE-coVDF)-g-poly(SSA) copolymers formed by the surface-initiated ATRP of styrene. This was followed by a sulfonation, and they then prepared a hybrid proton-conducting membrane with TiO2 nanoparticles for use in high temperature fuel cells. TGA thermograms showed that these membranes were stable up to 280 °C. Similarly, Koh et al.477 prepared poly(CTFE-co-VDF)-gpoly(SSA) copolymers and used them as the topcoat in ultrafiltration membranes. Both the rejections and the flux of AH


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Scheme 50. Synthesis of Graft Copolymers by the ATRP of Styrene from the Poly(CTFE-alt-EVE) Alternating Fluoropolymer Macroinitiator Which Were Subsequently Chloromethylated and Quaternized494

branes by surface initiated ATRP of poly(ethylene glycol) monomethyl ether methacrylate (PEGMeMA)490 (Scheme 49). The permeability and fouling properties of these graft copolymer films were evaluated. Koh et al.480,485,489 used a similar procedure to graft poly(ethylene oxide) methacrylate (PEOMA) via ATRP using poly(CTFE-co-VDF) as the macroinitiator. They successfully obtained poly(CTFE-co-VDF)-g-poly(PEOMA) graft copolymers. Those graft copolymers were then used as templates to prepare nanocomposite films containing silver nanoparticles which in turn were obtained by the in situ reduction of AgCF3SO3. They prepared porous TiO2 thin films or used them as additives in ultrafiltration membranes with a view to improving the antifouling properties. The same authors also successfully grafted 4-vinylpyridine (4VP) onto poly(CTFE-co-VDF) and obtained poly(CTFE-coVDF)-g-poly(4VP) (Scheme 49) which was used to stabilize AgBr nanoparticles (20−40 nm). The formation of such nanocomposites used to prepare Ag-loaded TiO2 nanostructures was also carried out involving poly(CTFE-co-VDF)-g-poly(SSA).483 This was obtained as previously described. The preparation of poly(CTFE-co-VDF)-g-poly(SPMA) (SPMA is sulfopropyl methacrylate) by ATRP using the same direct initiation of secondary chlorine atoms (Scheme 50) was reported by Seo et al.481 The graft copolymer was then blended with a heteropolyacid (HPA < 45 wt %) to form protonconducting membranes. It was found that, the water uptake decreased and the conductivity increased with increasing HPA content. Roh et al.493 developed the technique of grafting glycidyl methacrylate (GMA) onto poly(CTFE-co-VDF) by ATRP which led to poly(CTFE-co-VDF)-g-poly(GMA). This could be further sulfonated by sodium bisulfite and cross-linked with sulfosuccinic acid via esterification. These membranes, with conductivities up to 0.11 S cm−1 at 80 °C, are used as protonconducting membranes in fuel cells. The grafting “from” polymerization of styrene initiated by the radicals arising from the C−Cl bond cleavage of poly(CTFE-altVE) and poly[(CTFE-alt-VE)-co-(HFP-alt-VE)] copolymers followed by the chemical modification of the polystyrene grafts was reported by Valade et al.494 HFP and VE are hexafluoropropylene and vinyl ether, respectively. First, the fluorinated alternating copolymers were produced by the radical copolymerization of CTFE (with HFP) and VE. Second, the atom transfer radical polymerization of styrene, in the presence of the catalytic system CuCl/HMTETA, promoted the grafting polymerization of styrene. The reaction was carried out in the presence of an alternating macroinitiator such as poly(CTFE-altVE) using the chlorine atom of CTFE as the initiation site. The kinetics of the styrene polymerization indicated that such a grafting was more or less controlled (Figure 12). This is the first time that, the grafting of polystyrene onto alternating fluorinated

composite membranes increased with increasing PSSA, in other words with the membrane hydrophilicity. Kim et al.478 also synthesized poly(CTFE-co-VDF)-g-poly(SSA) graft copolymers using ATRP, however, in this case, directly from 4-styrene sulfonic acid with CuCl/1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) in NMP at 120 °C (Scheme 49). The chlorine atoms at the extremity of the graft were converted into azido groups to promote the efficient UV-cross-linking of the resulting membranes. A similar approach was also used to prepare PEMFC membranes consisting of poly(CTFE-co-VDF)-g-poly(HEA-coSSA) graft copolymers. This time the hydroxyl moiety of hydroxyl ethyl acrylate (HEA) was used in an esterification reaction with a diacid (sulfosuccinic acid).491 This photo-crosslinking strategy was recently confirmed on original PEMFC.492 Patel et al.484 used the same technique described above to prepare poly(CTFE-co-VDF)-g-poly(SSA) graft copolymers. The intention was to blend them with zeolites to prepare proton-conducting membranes. Poly(CTFE-co-VDF)-g-poly(SSA) was also prepared by Chung et al.479 First, they copolymerized CTFE and VDF, which was initiated and controlled by functional borane/oxygen. Then, polystyrene was grafted by ATRP as described previously. Finally, they carried out a sulfonation to yield the graft copolymer. The composition and sulfonation were optimized to balance the conductivity, water-swelling and mechanical properties. Tan et al.488 also grafted poly(styrene sulfonate) from a poly(CTFE-co-VDF) macroinitiator by first carrying out a partial hydrogenation of CTFE to obtain a poly(CTFE-ter-TrFE-coVDF) terpolymer. Then, using an ATRP of styrene they obtained the poly(CTFE-ter-TrFE-co-VDF)-g-PS graft copolymer. After sulfonation of the PS grafts, poly(CTFE-ter-TrFE-co-VDF)-gpoly(SSA)s were produced. These authors claim that the grafting was tunable and that the graft length could be controlled. These two parameters govern the phase segregation in PEMFC membranes, and hence, conductivities higher than Nafion at 50% relative humidity (RH) were achieved. Roh et al.487 reported the synthesis of a poly(CTFE-co-VDF)g-poly(SSA-co-TMSPMA) comb-like amphiphilic graft copolymer from the ATRP of 4-styrene sulfonic acid and 3(trimethoxysilyl)propyl methacrylate (TMSPMA) onto poly(CTFE-co-VDF) copolymers (Scheme 49). This graft copolymer was combined with tetraethoxysilane (TEOS) under acidic conditions to produce organic−inorganic nanocomposite membranes through an in situ sol−gel reaction between TEOS and PTMSPMA. Although after the introduction of silica the proton conductivity and water uptake of the membranes decreased slightly, the thermal and mechanical properties were significantly enhanced. In addition, poly(CTFE-co-VDF)-g-poly(PEGMeMA) graft copolymers were prepared from poly(CTFE-co-VDF) memAI


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loss of control of the polymerization in the presence of poly(CTFE-co-VDF) as macroinitiators was observed compared to the homopolymerization of acrylonitrile, quite well-defined graft copolymers were still obtained. This loss was attributed to the low solubility of the PAN grafts in DMSO. The light yellow color of the graft copolymers is also an indication that no degradation (by N-containing compounds) of the PVDF unit occurred. Degradation would result in brown to black colored products. The NMR analyses indicated that the grafting densities remained low which in turn indicates that the activation energy needed to dissociate the C−Cl bond in CTFE is quite high. Thermal analyses showed that grafting poly(AN) side chains reduces the stability and the crystallinity of poly(CTFE-co-VDF) copolymers. 4.5. Fluorinated Dendrimers

To the best of our knowledge, the only CTFE hyperbranched fluorinated materials were prepared by Wang et al.229 Their strategy involved a combination of a charge transfer complex inimer and a self-condensing vinyl polymerization between CTFE and chloromethylstyrene (CMS) in the presence of copper bromide and bipyridine at 100 °C (Scheme 31). 4.6. Chemical Modification of PCTFE or Poly(CTFE-co-M) Copolymers

Many investigations have been carried out on the chemical modification of poly(CTFE-co-M) copolymers, either from the ATRP via the C−Cl bond cleavage of CTFE unit or from any halogen atom reaction or another functional group contributed by the M monomer. It is commonly assumed that the chlorine atom of CTFE has a certain reactivity in order to allow modification. This makes PCTFE, or poly(CTFE-co-M) copolymers quite unique compared with other fluoropolymers. Owing to this reactivity, several modifications either by nucleophilic substitution or by elimination are possible. This reactivity has been successfully used for the grafting of PCTFE (co)polymers as discussed in section 4.4. The goal in all of the modifications is to introduce functional groups suitable for the targeted application. 4.6.1. Chemical Modification of PCTFE. Taylor et al.19 extensively reported the modification of PCTFE particularly in the presence of a cobalt(II) trapping agent generated in situ from the chloro(pyridine)cobaloxime(III)/magnesium redox couple (Schemes 51 and 52). Using a substitution of the PCTFE chlorine atom, it was possible to introduce various functional groups such as carboxylic acid,497−499 aldehyde,497 alcohol,497,500 aromatic,501−503 pyridine,504 alkyl,501,502,505 allylic,19,506 and mercapto.507 Cais and Kometani387 also used a displacement mechanism for the dechlorination of PCTFE in order to obtain poly(CTFE-co-trifluoroethylene) or poly(trifluoroethylene) using tributyl tin hydride. As mentioned in section 4.4.1, Kolb et al.465,466 grafted polystyryl lithium anions onto PCTFE thin films under various conditions and using different types of polystyryl anions. Unlike when benzene is the solvent, the reaction was very inefficient in THF. Here a reduction reaction competed with the addition, and thus the graft polymer tended to dissolve in the THF. However, an elimination reaction that yields a double bond on the polymer backbone is also possible. Both the competing substitution and elimination mechanisms are summarized in Scheme 52.508 The substitution is a one-electron process whereas the elimination is a two-electron process. Since they both compete, finding suitable conditions for obtaining a clean substitution with a high degree of functionalization is not easy.

Figure 12. Kinetic plots of poly(styrene) grafting from poly[(CTFE-altEVE)-co-(HFP-alt-EVE)] copolymer T = 110 °C, [styrene]:[Cl]: [CuBr]:[HMTETA] = 104:1:0.4:0.4): (A) ln([M]0/[M]) versus time. (B) Evolution of molecular weights (◆) and PDI (▲) versus styrene conversion. The dotted line represents the theoretical line of the molar masses/styrene conversion dependence (CTFE, HFP, EVE, and HMTETA stand for chlorotrifluoroethylene, hexafluoropropylene, ethylvinyl ether, and 1,1,4,7,10,10-hexamethyltriethylenetetramine, respectively).494

copolymers was successfully achieved. These graft copolymers exhibited two Tg’s. These were assigned to the amorphous domains of the polymeric fluorobackbone (ranging from −20 to +56 °C) and the polystyrene grafts (ca. 95 °C). The thermostability of these copolymers was found to increase upon grafting. While the polystyrene content varying from 81 to 27%, the 5% weight loss degradation temperatures were found to range from 193 to 305 °C. In addition, chloromethylation of the polystyrene grafts followed by the cationization of the chloromethyl dangling groups led to the formation of original ammonium-containing graft copolymers. These reached an ionic exchange capacity higher than 3 meq g−1. In spite of the success of ATRP in preparing graft copolymers from poly(CTFE-co-VDF) copolymers, undesirable side-reactions do occur. Examples are chain transfer reactions to solvents or ligands and dehydrochlorinations catalyzed by nitrogencontaining compounds. These were observed during the ATRP process at relatively high temperatures.473 Since single electron transfer living radical polymerizations (SET-LRP)495 are often carried out at low temperatures, this should avoid the inconvenience of the ATRP side-reactions. Using SET-LRP, Hu et al.496 were able to prepare poly(CTFE-co-VDF)-gpoly(AN) graft copolymers in DMSO at 40 °C, while successfully avoiding the typical side-reactions. Although a slight AJ


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Scheme 51. Modification of PCTFE by Various Methods Leading to Functional PCTFE and PCTFE-g-PS Graft Copolymers387,465,466,497−507

(CTFE-alt-VE) copolymers are found. They yield a wide range of materials for energy applications as depicted in Scheme 53. Fluorinated copolymers bearing ammonium groups were prepared by the modification of CTFE, HFP, and 2-chloroethyl vinyl ether (CEVE) co- or terpolymers.330 The chemical modification of these copolymers consisted of a two-step reaction: (i) replacement of the chlorine by an iodine atom332 followed by (ii) the substitution of the halogen atom by trimethylamine which leads to a quaternary ammonium. The compositions and structures of the resulting copolymers were characterized by the usual analytical techniques. Thermal analyses, in dynamic mode, under air, exhibited decomposition temperatures (Td,10%) higher than 200 °C. The electrochemical properties of some of these polymers were also studied. These exhibited ion exchange capacities (IECs) ranging from 0.50 to 0.75 meq g−1 (theoretical IEC up to 3.63 meq g−1) and water uptake values of 13−25%. Using the same strategy, the chlorine atoms in the dangling CH2CH2Cl groups of poly[(CTFE-alt-CEVE)-co-(CTFE-altEVE)] terpolymers were converted into iodine atoms using nucleophilic substitution.331,332 Hence, a series of five poly[(CTFE-alt-IEVE)-co-(CTFE-alt-EVE)] terpolymers with different degrees of iodine atoms in the side chains were obtained. These copolymers exhibited molecular weights of about 25 000 g mol−1. Thermal analysis of the copolymers showed that under air degradation started from about 220 °C. The Tg values were in the range 34−41 °C and showed a linear dependence with respect to the content of iodine atoms (Figure 13).332

Scheme 52. Mechanisms of Substitution and Elimination Occurring during the Modification of PCTFE Homopolymers508

The reduction of PCTFE often occurs in the presence of magnesium salts or lithium salts such as alkyl lithium or phenyl lithium.508 Using a similar technique, Saneinejad and Shoichet509 successfully grafted lithium poly(ethylene glycol) alkoxide onto PCTFE films. This was followed by sputter-coating with titanium and gold, where the gold regions were modified with peptides with the view to preparing nerve regeneration substrate materials. The XPS data indicated that the grafting occurred via a metal−halogen exchange followed by elimination rather than simple substitution reactions with the PCTFE chlorine. Additional analyses indicated the presence of conjugated carbon−carbon double bonds, which supports an elimination mechanism. 4.6.2. Chemical Modification of Poly(CTFE-alt-vinyl ether) Copolymers. Various modification pathways of polyAK


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Scheme 53. Chemical Modification of Poly(CTFE-alt-CEVE) Alternating Copolymers by Iodination and Further Modifications to Introduce Functional Side Groups for Various Applications Including Fuel Cells and Photovoltaic Materialsa

a

PEMFC = proton exchange membrane fuel cell, SAFC= solid alkaline fuel cell, PV = photovoltaic, and QAFC = quasi-anhydrous fuel cell..313,331,332,334−338,510−514

irrespective of the phosphonic acid content, the principal degradation started from 250 °C and revealed a high thermooxidative stability of these copolymers. Proton-conducting membranes were also cast and their electrochemical properties were evaluated as indicated in section 5.1 (Figure 18). A similar poly(CTFE-alt-IEVE) copolymer was used as the starting material in the grafting of 1-benzyl-2-(hydroxymethyl)imidazole337 or 2-mercaptobenzimidazole. The degree of grafting was controlled by the reactant ratios (Scheme 53).337,511 Deprotection by hydrogenation-generated pendant imidazole functions containing both >NH and >N−R groups that are required for proton hopping (as shown in Scheme 22). The polymer was subsequently blended with sulfonated poly(ether ether ketone) (sPEEK) to form membranes. The NH/SO3H ratio varied between 1 and 100 and reached a maximum conductivity of 14 mS cm−1 at 120 °C and 50% relative humidity. More recently, it was reported that mercaptotriazole reacted with poly(CTFE-alt-IEVE) copolymers to yield original triazolecontaining CTFE copolymers (see Scheme 53).514 These were blended with sulfonated PEEK and yielded membranes. Their thermal, physicochemical (swelling and water uptake), and electrochemical (conductivities) properties were assessed and compared to those containing imidazole and benzimidazole functionalities. Those based on triazole displayed the best

Figure 13. Evolution of the glass transition temperature (Tg) versus the iodine molar content in copolymers based on chlorotrifluoroethylene and 2-iodoethyl vinyl ether. [Reprinted with permission from ref 332. Copyright 2010 Elsevier.]

Phosphonate groups were inserted into the copolymers via the Arbuzov reaction.331,510 It was possible to carry out the hydrolysis of the phosphonate moieties to form phosphonic acid groups quantitatively under mild conditions, in the presence of bromotrimethylsilane (Scheme 53). The thermal and electrochemical properties of the resulting membranes (processed by casting) were investigated. It was shown that, AL


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Figure 14. Expansion of the various regions in the 19F NMR spectrum of the trifluorovinyl group obtained by dehalogenation of Br-CTFE-spacer-G (where G is a functional group). [Reprinted with permission from ref 145. Copyright 2005 Wiley Interscience.]

Scheme 54. Modification of Poly(CTFE-co-VDF) Copolymers into a Sulfonic Acid-Containing Fuel Cell Membrane [Reprinted with permission from ref 366. Copyright 2007 Electrochemical Society.]

conductivities (Figure 17), up to 7 mS cm−1 at 140 °C at 30% relative humidity. Recently, an ω-alkynyl-phthalocyanine compound bearing CF3 groups was successfully grafted onto a series of poly(CTFEalt-CEVE) copolymers for photovoltaic applications.513 As above, the poly(CTFE-alt-CEVE) copolymers were modified to form poly(CTFE-alt-IEVE) by exchanging the CEVE chlorine atom by iodine. Subsequently these iodine side-atoms were chemically changed into azido functions, and finally the phthalocyanine was incorporated by using a copper-catalyzed Huisgen dipolar 1,3-addition. This gave fluorinated copolymers with different degrees of grafting (10−72%) of the phthalocyanine side groups (Scheme 53).513 Poly(CTFE-alt-VE)-g-PS and poly[(CTFE-alt-VE)-co-(HFPalt-VE]-g-PS copolymers have been modified in order to introduce ammonium side groups (Scheme 50).494 Polystyrene side chains were modified, first by chloromethylation and then by the cationization of the chloromethyl dangling groups. Poly(CTFE-alt-3-chloro-2,2-dimethylpropyl vinyl ether) copolymers have also been modified (Scheme 21) according to the above-mentioned technique: first, iodination of the chlorine sideatoms of the vinyl ether and, then, quaternization using trimethylamine. After the counteranion exchange, hydroxyl

anion conducting membranes were produced suitable for use in solid alkaline fuel cells.312 4.6.3. Chemical Modification of Poly(CTFE-co-M) Copolymers. The principal chemical modification of poly(CTFE-co-M) copolymers consists of dechlorination of the CTFE units. As mentioned in section 3.2.10, chlorotrifluoroethylene units can be reduced to give trifluoroethylene. Intended use is essentially for piezoelectric applications. For example, Lu et al.380 prepared some poly(TrFE-co-VDF) copolymers via the reductive dechlorination of poly(CTFE-co-VDF) copolymers. The reaction consisted in reacting tri(n-butyl)tin hydride onto a poly(CTFE-co-VDF) copolymer in the presence of AIBN at 60 °C for 24 h before quenching with methanol. Wang et al.365 hydrogenated poly(CTFE-co-VDF) using AIBN and tri(n-butyl) tin hydride at 60 °C to produce a poly(CTFE-ter-TrFE-ter-VDF) terpolymer with hydrogenation yields ranging from 36 to 100% with respect to the CTFE (see Scheme 33). However, the composition of the resulting terpolymer is not fully controllable, and the efficiency of Bu3SnH is not really precise due to its high moisture sensitivity. Most importantly, the presence of residues of the highly toxic reagents (such as Bu3SnH and its byproduct tributyltin chloride) in the final product is most undesirable for industrial applications. Thus, Tan et al.515 developed a more AM


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environmentally friendly and controllable process for the hydrogenation of poly(CTFE-co-VDF) copolymers. This process consists in an ATRP in the presence of a chain transfer reagent such as a proton donor solvent (NMP, DMF, or DMAc,). More recently, the same authors516 reported the “controlled” dehydrochlorination of poly(CTFE-co-VDF) by tertiary monoamines (of head-to-tail VDF-CTFE dyads exclusively), which did not exhibit the typical side-reactions catalyzed by amines (such as Michael addition and chain scission), as encountered with dehydrofluorination reactions, with the proper choice of monoamine (trimethylamine in particular) and solvent. The dehydrochlorination led to a copolymer containing tunable unsaturations that could be readily cured in the presence of dibenzoyl peroxide (BPO). Co-telomers of CTFE and VDF were dehalogenated to prepare a PVDF-based macromonomer bearing a trifluorovinyl end group (Scheme 48). The reaction was usually carried out in the presence of zinc and gave fair to satisfactory yields. For the preparation of the CF2CF-Spacer-G (G is a functional group), the dehalogenation is confirmed by the presence of three doublets of doublets (of triplets) in the 19F NMR spectrum centered at −98.6, −118.5, and −173.5 ppm. These correspond to the three fluorine atoms of the trifluorovinyl group (Figure 14).145 Chung et al.366 prepared some poly(CTFE-co-VDF) compounds terminated by borinate groups. Under certain conditions, the borinate end groups could be modified into tri(ethoxy)silanes, leading to a tris[tri(alkoxy)silane]-poly(VDF-co-CTFE) copolymer capable of reacting with 2-(4-chlorosulfonylphenyl) ethyltri(methoxy)silane (CSPETMS) or 4-ethyl benzenesulfonyl chloride-grafted silica via hydrolysis and condensation. After hydrolysis of −SO2Cl into a sulfonic acid groups, composite fuel cells membranes were able to be produced as illustrated by Scheme 54.

Table 6. Nonexhaustive List of Applications of PCTFE Plasticsa industry chemical cryogenic and aerospace medical food electrical

packaging

uses baskets, diaphragms; lining and coatings for tanks, pumps, valves, pipes, packaging, valve seats, optical glasses and centrifuge tubes pump liners, seals, gaskets, valve seats, fittings, gaskets for liquid oxygen and hydrogen, windows for infrared guns and missiles, radome covers, gage facings and high vacuum seals blood filters, suture packages, syringe tubes, sterilizable packages, body implants and oxygen barrier materials in blisters coating and components for food-handling equipment, e.g., valve seats for beverage-dispensing equipment hookups, cable and computer wire insulation, switch plates and gears, connectors, coil forms, terminals, resistor sleeves, potentiometer slider assemblies, tubing, parts with metal inserts, battery cases and flexible printed circuits corrosive materials, pharmaceuticals, valuable documents, seaborne instruments, liqueurs and essences

a

Reprinted with permission from ref 25. Copyright 1967 Wiley Interscience.

in aeronautic and space exploration as hydraulic fluids, oils, or lubricants.16,17,51718 Yadav et al.518 studied the thermoreversible gelation of poly(CTFE-co-VDF) copolymers in the presence of alkyl phthalates (C1−8) and showed that the gelation rate increased with alkyl chain length. Although the thermostability is not affected, the solvent retention power of the gels increased with alkyl chain length. Such materials can find suitable applications in lithium ion batteries. Besides many applications in the fuel cell area, poly(CTFE-coVDF)-g-poly(SSA) graft copolymers are also used in ultrafiltration membranes.477 PCTFE-g-PEG grafted films also find applications as substrates for direct primary nerve cell adhesion and neurite outgrowth (nerve regeneration).509

4.7. Cross-Linking

5.1. Fuel Cells

Quite a few (co)polymers based on CTFE are cross-linkable. In contrast to bromine-containing fluorinated copolymers that can be successfully cross-linked via peroxide and triallyl isocyanurate,362 those bearing chlorine atoms have not been reported. This may be a result of the strong C−Cl bond that cannot be easily cleaved in the presence of a radical. However, CHR-Cl groups can be modified into CHR-N3 in the presence of sodium azide. This was achieved by Kim et al.478 They modified the chlorine atoms at the end of the graft of poly(CTFE-co-VDF)-gpoly(SSA) graft copolymers into azido groups which then enabled UV-cross-linking of these copolymers to form membranes. The cross-linked membranes exhibited reduced swelling (83 vs 300%) and better mechanical properties (tensile strength = 26.2 vs 21.1 MPa) while maintaining the proton conductivity.

Although various fuel cell membranes have been developed which are aimed at covering both low and high temperature applications, there is still a lack, especially in the automobile industry, of suitable membranes. What is required is that the membrane sustains its performances at both low relative humidities (RH = 25−30%) and high temperatures (ca. 100− 130 °C; Figure 15). Currently, there is no suitable protonic membrane material, including those based on sulfonic acid, available that has sufficient conductivity at low RH for the desired operating temperature of 120 °C.406 To overcome this limitation, various research teams have recently proposed new membranes. These bear other functional groups that can ensure proton transport. Two mechanisms can be considered: proton conduction (Grotthus mechanism)519 and proton hopping (diffusion mechanism).520 The former is the traditional mechanism that involves sulfonic groups or phosphonic groups. The latter is an anhydrous or quasi-anhydrous mechanism involving mostly imidazole or benzimidazole derivatives. Tsang et al.474 demonstrated that, in the case of poly(CTFEco-VDF)-g-poly(SSA) copolymers, the graft copolymer exhibited higher ionic content than diblock copolymers without excessive swelling. This led to membranes with highly concentrated ionic domains and thus better conductivities. They also prepared blends of these graft copolymers with PVDF to study the effects of the blending on the fuel cell properties.521 One blend (70:30) showed reduced water uptake leading to a higher acid concentration without a decline in proton mobility.Conse-

5. APPLICATIONS OF CTFE-BASED FLUOROPOLYMERS PCTFE plastics are often used in many high-tech areas such as aeronautics, space, electronics, and chemical and medical industries.20 Relevant examples (see Table 6) include their use as chemically resistant electrical insulation, seals, gaskets, valve seats and liners, medical packaging (such as blisters) and instrumentation parts, seals for the petrochemical industry, optical fibers, sample containers, electrodes, column packaging in analytical chemistry, and cryogenic devices. Liquid oligo(CTFE)s have often been used for military purposes, such as AN


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cross-linked membranes, produced via the introduction of azido groups, exhibited less swelling (83 vs 300%) and better mechanical properties (tensile strength = 26.2 vs 21.1 MPa). Again, this was achieved while keeping a satisfactory proton conductivity (20−60 mS cm−1). Park et al. 476 processed a hybrid proton-conducting membrane based on poly(CTFE-co-VDF)-g-poly(SSA) copolymers and TiO2 nanoparticles for high temperature fuel cells. TGA experiments showed that these membranes were stable up to 280 °C. Although the IEC of the membrane was not significantly affected, both water uptake and conductivity increased with increasing TiO2 and PSSA content. The same team also performed a similar study in the presence of titanium isopropioxide (TTIP),522 which is a precursor for TiO2. They showed that the mechanical properties were improved, both the water uptake and the IEC decreased, but proton conductivity was maintained. In a similar way, Patel et al.484 blended their poly(CTFE-co-VDF)-g-poly(SSA) graft copolymers with zeolites to prepare proton-conducting membranes. They showed that a strong interaction between the zeolite and the sulfonic group took place. This reduced the conductivity to 0.011 S cm−1 However, there was another consequence: the water uptake decreased with increasing zeolite content. Roh et al.523 used poly(CTFE-co-VDF)-g-poly(SSA) graft copolymers to prepare composite membranes with embedded phosphotungstinic acid also for use as proton-exchange membranes. They observed a conductivity loss upon incorporation of the phosphotungstinic acid while the thermal and mechanical properties were enhanced. Tan et al.488 prepared membranes based on poly(CTFE-coTrFE-co-VDF)-g-poly(SSA) and obtained higher conductivities at 50% RH than that of Nafion 112. This was achieved by varying the number and length of the poly(styrene sulfonate) grafts in order to tune the phase segregation.

Figure 15. Status on membrane conductivity. [Reprinted with permission from ref 406. Copyright 2004 Wiley Interscience.]

quently, this enhanced the proton conductivity by up to 2-fold. The higher the molecular weight of the PVDF, the better the mechanical strength and the phase segregation; thus, the proton transport was enhanced. Poly(CTFE-co-VDF)-g-poly(SSA) copolymers prepared by Kim et al.478 were used to prepare fuel cell membranes. These showed an increase in ionic exchange capacity (IEC) with increasing PSSA content and thermostability up to 300 °C. The

Figure 16. Conductivity of poly(CTFE-co-VDF) blended with Nafion/H3OZr2(PO4)3 at 120 °C and different RH. [Reprinted with permission from ref 366. Copyright 2007 The Electrochemical Society.] AO


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Figure 17. Alternating current proton conductivities of three different blend membranes composed of poly(CTFE-alt-VE azole) copolymer/s-PEEK: M1: imidazole (35)/s-PEEK (65), M2: benzimidazole (35)/s-PEEK (65), and M3: triazole (40)/s-PEEK (60), and s-PEEK membranes versus reciprocal temperature and RH < 25% (n stands for the amount of nitrogenous heterocycle about sulfonic acid ratio). [Reprinted with permission from ref 514. Copyright 2013 American Chemical Society.]

Roh et al.487 combined poly(CTFE-co-VDF)-g-poly(SSA-coTMSPMA) (where TMSPMA stands for 3-(trimethoxysilyl)propyl methacrylate) with tetraethoxysilane (TEOS) under acidic conditions, to produce organic−inorganic nanocomposite membranes. This involved an in situ sol−gel reaction between TEOS and PTMSPMA. Upon introduction of silica, the proton conductivity and water uptake of the membranes decreased slightly but the thermal and mechanical properties of the membranes were enhanced significantly. Seo et al.481 blended poly(CTFE-co-VDF)-g-poly(SPMA) (SPMA stands for sulfopropyl methacrylate) with an heteropolyacid (HPA < 45 wt %) to form proton-conducting membranes. In these, the water uptake decreased whereas the conductivity increased with increasing HPA content. A poly(CTFE-co-VDF)-g-poly(GMA) (GMA stands for glycidyl methacrylate) copolymer cross-linked by sulfosuccinic acid was prepared and used as a proton-conducting membrane. Conductivities reached up to 0.11 S cm−1 at 80 °C.493 The membrane exhibited good mechanical properties (Young’s modulus >400 MPa) and good thermal stability (up to 300 °C). A series of poly(CTFE-co-VDF) bearing sulfonic acid side groups was prepared by condensation and hydrolysis between a poly(CTFE-co-VDF) bearing tri(ethoxy)silane end groups and 2-(4-chlorosulfonylphenyl) ethyltri(methoxy)silane (CSPETMS) or 4-ethyl benzenesulfonyl chloride-grafted silica (see Scheme 54).366 Composite membranes were produced from these copolymers and showed conductivities up to 7 mS cm−1 at 70% relative humidity (RH). Conductivity results are presented in Figure 16. Frutsaert et al.334−338,511 produced even more elaborate protonic membranes by blending poly(CTFE-alt-VE) copoly-

mers bearing imidazole side groups (Scheme 53) with sulfonated poly(ether ether ketone) (sPEEK; IEC = 1.6 meq g−1) while varying the imidazole/SO3H ratio between 1 and 100. These authors note that the resulting conductivities show a clear dependence on both the fluorinated copolymer amount in the blend and the relative humidity. A more recent study has compared the influence of the nature of the nitrogenous heterocycle. They demonstrated that triazole led to the best conductivities, as shown in Figure 17. Tayouo et al.331,332,510 grafted phosphonic groups onto poly[(CTFE-alt-IEVE)-co-(CTFE-alt-EVE)] terpolymers as explained in section 4.6.2 (Scheme 53). Proton-conducting membranes were then cast and their electrochemical properties were assessed. The IECs, determined by potentiometric titrations, ranged from 2.9 to 6.8 meq g−1. At 25 °C and 100% relative humidity (RH), the level of conductivity was found in the 0.02−20 mS cm−1 range and highly dependent on the IEC (Figure 18). Finally, fair to good conductivity values (about 0.25 mS cm−1) at higher temperatures (120 °C) and lower RHs (25%) were observed. This demonstrates a limited dependence toward both temperature and RH. Interesting membranes with a CTFE content of up to 80 mol %, based on modified poly(CTFE-co-PFSVE) copolymers, with IECs ranging from 0.85 to 1.81 meq g−1, were prepared by bulk radical copolymerization followed by modification of SO2F into SO3H side groups.423 The copolymers exhibited an improved thermostability compared to that of the poly(PFSVE) and one similar to that of Nafion. Membranes for solid alkaline fuel cells (SAFC)512,524,525 differ from protonic membranes because the migrating species is not a proton but a hydroxide anion. Thus, the functional group in the AP


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nanoparticles were prepared using these core−shells and their film properties were investigated.319 The presence of the acrylate shell was found to enhance the compatibility of the fluorinated core with the SiO2 particles. The result led to better mechanical properties for the resulting transparent films. Honeywell, because of its Aclar PCTFE films, is a leader in the production of protective films and moisture barrier films for the packaging of consumer goods and pharmaceuticals in tablets or blisters14,134 Poly(CTFE-co-E), also known as ECTFE or Halar (formerly the property of Ausimont, now Solvay), due to its flame and chemical resistance, has been used in coatings for fume exhaust ducts since the early 90s.24,29 In fact, it was shown that ECTFE has a higher ignition resistance than other fluoropolymers (Table 7).24,29 For a polymer to ignite under heat, the polymer must

Figure 18. Proton conductivity of phosphonic acid grafted fluorinated membrane M47 as a function of Relative Humidity at 90 (●) and 120 °C (■). [Reprinted with permission from ref 510. Copyright 2010 American Chemical Society.]

Table 7. Ignition Resistance of Fluoropolymers29

polymeric chains that is used to promote the diffusion/migration of the active species is not an acid (in the Brønsted context) but rather a cationic group bearing an hydroxide counterion. To prepare such membranes, fluorinated copolymers bearing ammonium groups were prepared by the modification of coor terpolymers of CTFE and 2-chloroethyl vinyl ether (CEVE) (see section 4.6.2 and Scheme 53).330 Their thermal analyses, under air in dynamic mode, exhibited decomposition temperatures (Td,10%) higher than 200 °C. Hence, they fulfill the requirements for SAFC membranes. The electrochemical properties of some of these polymers were also studied and exhibited ionic exchange capacities (IECs) ranging from 0.50 to 0.75 meq g−1 (theoretical IEC up to 3.63 meq g−1) and water uptake values of 13−25%.

compound namea

LOIb (min)

critical heat flux (kW m‑2)

ECTFE ETFE PVDF

52 30−32 42−44

74 17 43

a

ECTFE, ETFE, and PVDF stand for poly(ethylene-co-chlorotrifluoroethylene) copolymer, poly(ethylene-co-tetrafluoroethylene) copolymer, and poly(vinylidene fluoride), respectively. bLOI is the limiting oxygen index.

degrade at a sufficient rate to produce the required flammable gas. In the case of ECTFE, the degradation mechanism is based on hydrogen halide elimination rather than on chain scission. This elimination results in char production and that prevents the penetration of heat and air, and thus retards the ignition. However, an oxidation ablation is observed. ECTFE coatings also exhibit excellent chemical resistance.286 Halar ECTFE has been used to support and protect photovoltaic modules.295,296 The main interest lies in the interesting properties of fluoropolymers compared to glass: lightness, flexibility, selfcleaning/antifouling due to low surface energy, high transmittance, excellent chemical resistance, excellent thermostability, and both excellent weathering and aging to UV. No aging or coloration was observed after exposure to UV over 1000 h at 85 °C and with 85% relative humidity.295,296 To produce ECTFE coatings, special terpolymers have been developed that contain a small amount of termonomer such as hexafluoroisobutylene (HFIB), perfluorohexyl ethylene,281 perfluoroisoalkoxy perfluoroalkyl ethylenes,282 perfluoropropyl vinyl ether (PPVE),283 and organic silanes (for adhesion to glass).284 Highly transparent films based on a copolymer of CTFE with a perfluoroalkene (such as HFP) have been claimed by Daikin.416 PCTFE itself is too crystalline and tends to loose transparency, while its copolymers with VDF result in a loss of thermostability,104 chemical resistance, and mechanical properties (though transparency is enhanced). On the other hand, those with TFE do not exhibit satisfactory mechanical properties. Poly(CTFE-co-TeFP) compounds, where TeFP is CH2 CF−CF3 (2,3,3,3-tetrafluoro-1-propene also called 1234yf), were prepared418 and blended with poly(meth)acrylate esters with a view to preparing fluorocarbon/acrylate coatings endowed with higher Tg than PVDF and thus better performance capabilities, namely having lower surface energies and greater toughness.419 Ausimont316 claimed that the incorporation of a small amount (1.5 yr at room temperature). Guan et al.385,468,469,471,472 reported some poly(CTFE-coVDF)-g-PS graft copolymers based on the synthesis technique used by Zhang and Russell,467 followed by the dechlorination of the end groups. Coatings based on these materials showed interesting confined ferroelectric properties for dielectric capacitors. This requires a difficult combination of high energy density, low losses and a rapid discharge speed in order to be a reliable electrical power system. Both high energy density (10 J cm−3 at 600 MV m−1) and low dielectric/ferroelectric losses (tan δ = 0.006 at 1 kHz) were obtained for a graft copolymer containing 34 wt % PS. Segregation between the PS phase and the crystalline PVDF phase yielded a low polarizable interface, which confined the ferroelectric PVDF crystal. For further insights, the reader can consult Zhu and Wang’s recently published comprehensive review article on this growing topic.385

acrylate) can enhance the mechanical and barrier properties of PCTFE, thus improving the overall properties of the films. Photo-cross-linkable cotelomers of CTFE and acrylic acid were chlorinated to transform some of the acrylic acid groups into the corresponding acyl chlorides. These were then esterified using unsaturated alcohols (allylic or acrylic; see Scheme 8).264 The resulting cotelomers showed great promise, not only as result of their photo-cross-linkable character but also because of the presence of the residual carboxylic groups. These lead to adhesion and solubility in water. Other curable copolymers compositions are based on CTFE (25−75 mol %), an hydroxylcontaining allyl ether (ethylene or propylene glycol allyl ether or 6-hydroxybutyl vinyl ether), a carboxylic acid vinyl ester (vinyl acetate, vinyl pivalate, or vinyl caproate) obtained in the presence of a unsaturated long chain carboxylic acid (>C6).303 Emulsion copolymers of CTFE with vinyl acetate and butyl acrylate or methacrylic acid have been used to prepare fluorinated coatings.315 Poly(CTFE)-g-poly(acrylic acid)458 or poly(CTFE)-g-poly(methyl acrylate)526 were used to enhance the adhesion properties of both thin films and grafted films. They exhibited peeling strengths up to 30-times larger than those of untreated PCTFE films. The maximum adhesive strength was achieved when the surface was covered with a pure graft polymer layer. The thickness of that layer increased with increasing radiation up to the point where radiochemical degradation of the layer occurred. CTFE has often been used to promote the radiation-assisted cross-linking of poly(ethylene) films.527−530

5.4. Optical Applications

The great advantage of using CTFE in optical applications is its absence of protons. The C−H harmonic vibrations are actually located in the near-infrared, which increases optical loss. For example, fluorophosphate-glass-fiber-reinforced PCTFE transparent composites exhibit a 80% transmittance from the visible to the mid-IR. Therefore, this makes them capable of transmitting light to longer wavelengths, and hence they have improved properties compared to simpler glass-fiber PMMA composites.531 Terpolymers based on CTFE, vinylene carbonate (VCA) and HFP were developed as materials for optical fibers.532 Such materials are transparent oligomers with a glass transition temperature above 60 °C. They are transparent from the visible light region to near-infrared light and can be used as optical fiber waveguides.533 This terpolymer is a good example of the concept that was pioneered by Weise,534 a termonomerinduced copolymerization (for which electron-donating VCA is easily copolymerized with electron-accepting CTFE or HFP monomers). This was achieved despite the difficult radical copolymerization of CTFE with HFP. Cyclic VCA made it possible to increase the Tg of these terpolymers. On the other hand, terpolymers of CTFE, VCA, and a vinyl ether bearing hydroxyl side groups (which acts as a cure site monomer) were prepared (Scheme 26) for use as optical fiber waveguides.345 The incorporation of fluorine into the structure enhanced the thermostability and reduced both optical losses and water absorption. The hydroxyl group can be acrylated to prepare photo-cross-linkable materials.535 Such terpolymers345 led to higher molecular weights than those achieved by Krebs and Schneider,344 who reached 2000 g mol−1. Zinc sulfide (ZnS) nanoparticles-poly(CTFE-ter-TrFE-terVDF) terpolymer nanocomposites were prepared for use as tunable long-period fiber gratings functioning as the second cladding in photonic devices.403 A resonant wavelength shift of over 50 nm has been noted under a 30 V.μm−1electric field change; this corresponds to a 3-fold increase in the tuning range compared to the neat terpolymer.

5.3. Piezoelectric/Ferroelectric/Dielectric Devices

Several methods have been used to improve the ferroelectric/ piezoelectric properties of PVDF or poly(VDF-co-TrFE) (co)polymers. One approach was to use the copolymerization of VDF with CTFE followed by eventual reduction of the chlorine atoms. The introduction of CTFE increases the dipole mobility under high fields, broadens the dipole switching transition and renders it fully reversible.385 The intended field of application is mainly in electric energy storage applications. To enhance the dielectric properties of poly(CTFE-co-VDF) copolymers, various teams have selectively reduced the chlorine atom of the CTFE units. This approach was first pioneered by Cais and Kometani387,388 in 1984, then further developed in collaboration with Lovinger’s group,389 and more recently has been revisited by Lu et al.378,380 It leads to poly(CTFE-ter-TrFEter-VDF) terpolymers (Scheme 33). They also pointed out that ferroelectric fluoropolymers exhibit high dielectric constants. Tuning the structure of the copolymers enabled these authors to establish a “polymer structure-thermal and dielectric properties” correlation that provides insight into the parameters governing the responses of these organic electroactive materials. Various poly(CTFE-ter-TrFE-ter-VDF) terpolymers have been synthesized and led to a wide range of materials that display various Curie temperatures (ranging from 22 to 106 °C) and dielectric constants at room temperature varying from 11 to 50 at 1 kHz. The Curie temperature of PVDF itself is 195−197 °C.393,394 Indeed, the highest RT dielectric constant of 50 and a low dielectric loss (tg δ< 0.05)378,380 were found for the poly(CTFEter-TrFE-ter-VDF) terpolymer containing 78.8, 7.2, and 14.0 mol %, respectively. Such values are higher than those of the terpolymers prepared by direct radical terpolymerization of VDF with CTFE and TrFE395,396 (for which the dielectric constant is 37.5397). AR


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An acrylated oligo(CTFE) with the general formula CH2 CH−CO2−CH2−(CF2−CFCl)x−Cl (where x = 2−3) were photopolymerized to obtain amorphous materials endowed with good mechanical properties (Tg > 100 °C).The intended use was as a core for optic fibers.265 The refractive indices at 20 °C were determined to be 1.416 and 1.418 for x = 2 and 3, respectively. The attenuation was found to be lower than that of poly(butyl acrylate). Also, the higher halogen content, the lower the attenuation which is, of course, a desirable effect. Copolymers of CTFE with fluorinated dioxolane were also prepared (Scheme 39). Their Tg values ranged between 84 and 145 °C depending on the copolymer composition which is somewhat lower than that of the homopoly(F-dioxolane) (Tg = 180−190 °C).425 The copolymer films were flexible and clear with a low refractive index (1.3350−1.3770 at 532 nm). This makes these materials an interesting choice for optical applications such as waveguides in optic fibers.

copolymerization of CTFE with vinyl ethers, which led to alternating copolymers, many of the resulting products have been developed for the coatings and paints market. There is also an emerging field for poly(CTFE-alt-VE) copolymers in specific applications where the price of the functional vinyl ether is not so critical. Thus, alternating poly(CTFE-alt-VE) copolymers are interesting since they are composed of 50 mol % of CTFE. This induces thermostability, film-forming capabilities and chemical inertness. The 50 mol % content of VE, brings the functionality required for the intended application. To date, this approach has yielded interesting energy-related materials, and the outlook is excellent. However, very little has been achieved in the controlled radical polymerization of CTFE. The pioneering work of Haszeldine in the 1950s is the first example of iodine transfer polymerization. More recently, the work of Chung, that involved alkyl borane, led either to piezoelectric materials or to fuel cell membranes. Other co-monomers with lower reactivities than CTFE are thus highly sought with the view to obtaining functional PCTFEs with enhanced properties. Monomers that can lead to crosslinking are also of interest. Hence, it is anticipated that a collaboration between organic and macromolecular chemists should go some way to solving such challenges. In addition, a growing interest over the past decade in the preparation of new CTFE-based materials for energy-applications such as in piezoelectric and electrostrictive devices, fuel cell membranes, and PV has emerged. It can be expected that such areas, and probably many other novel challenges, will also be explored and should attract the interest of industrial and academic researchers.

5.5. Thermoplastic Elastomers

Two strategies for producing thermoplastic elastomers (TPEs) have already found relevant developments. The first was pioneered by Daikin in the early 1980s, and is still used to produce such TPEs as hard−soft−hard triblock copolymers. The technique involves the sequential iodine transfer copolymerization of fluoroalkenes (Scheme 36) which leads to Dai-El TPEs. Similar strategies are currently used by DuPont and Solvay (Viton and Technoflon trade names). The soft blocks are either poly(VDF-co-HFP) or poly(VDF-co-CTFE) in appropriate molar ratios while the hard block can be PVDF, PCTFE, or PTFE.6,412 The second approach was developed by the Central Glass Company to provide with CTFE-containing graft copolymers386,387 via a two-step process as illustrated by Scheme 47.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

6. CONCLUSIONS Although PCTFE contains many chlorine atoms that can induce drawbacks with respect to regiostability and toxicity, it is still a very good material. In addition to its moisture barrier properties14 at low molecular weights, it is a good lubricant. PCTFE materials are endowed with excellent chemical and thermal stabilities in spite of an unzipping degradation, initiated from the end groups which occurs at temperatures above 350 °C. In spite of this, Daikin276 has claimed a quite thermostable PCTFE in which the unzipping has been minimized. PCTFE has two main limitations. First, the homopolymer is insoluble in most common organic solvents, and second it is difficult to cross-link due to the absence of a functional group. The telomerization of CTFE with a wide range of different chain transfer agents (or telogens) led to many documented investigations for the synthesis of low molecular weight polymers. These telomers can be monofunctional, telechelic, or polyfunctional intermediates. All have been successfully modified into various architecturally designed polymers (such as diblock copolymers) or new polycondensates (polyurethanes, polyesters, and polyamides). Hence, copolymers based on CTFE prove really interesting. This is because the co-monomer brings complementary properties through its functions. Examples are cross-linking via hydroxyl or epoxide and increased solubility via a cyclohexyl. However, conventional CTFE copolymers also contain a low amount of CTFE that is always less reactive and thus less incorporated, and hence this leads to poor mechanically and thermally stable materials. In the case of the radical

Notes

The authors declare no competing financial interest. Biographies

Frédéric Boschet earned his doctorate degree at the Université du Sud, Toulon-Var (France) working on the synthesis and characterization of associative fluorinated water-soluble polymers under the supervision of Prof. A. Margaillan. In 2003, as postdoctoral associate in the Loker Hydrocarbon Research Institute (California, USA), he collaborated with T. E. Hogen-Esch and G. K. S. Prakash on the development of direct methanol fuel cells and hydrogels. In 2007, he was appointed Associate Professor at the Universidad del Mar (Oaxaca, Mexico) in the Department of Environmental Engineering. In 2008, as part of the team leaded by B. Améduri at the Institut Charles Gerhardt in AS


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Montpellier (France), he worked on the synthesis of fluoropolymers in collaboration with both the industry (Honeywell and Tosoh) and other academic institutions. In 2011, he joined the Chloro-Vinyls Services as part of the PVC research team at Solvay in Brussels (Belgium). His main interests focus on the synthesis and characterization of polymers by anionic, controlled or free radical copolymerization either in solution or in disperse media.

ATRP BPO BVE CEVE CHVE CRP CSM CTA CTFE DABCO DADMAC DMF DMFC DPn DS DSC DT DTBP E EB ECTFE EVE EHVE FEP FTIR GMA GPC HEA HEAE HFIB HFP HMTETA IB IEVE IRFI ITP M MADIX

atom transfer radical polymerization dibenzoyl peroxide butyl vinyl ether 2-chloroethyl vinyl ether cyclohexyl vinyl ether controlled radical polymerization cured site monomer chain transfer agent chlorotrifluoroethylene 1,4-diazabicyclo[2.2.2]octane diallyldimethylammonium chloride dimethyl formamide direct methanol fuel cell number average degree of polymerization degree of sulfonation differential scanning calorimetry degenerative transfer di-tert-butyl peroxide ethylene electron beam poly(CTFE-co-E) ethyl vinyl ether 2-ethylhexyl vinyl ether poly(TFE-co-HFP) Fourier transformed infrared spectroscopy glycydil methacrylate gel permeation chromatography 2-hydroxyethyl acrylate 2-hydroxyethyl allyl ether hexafluoroisobutylene hexafluoropropylene 1,1,4,7,10,10-hexamethyltriethylenetetramine isobutylene 2-iodoethyl vinyl ether α,ω-diiodoperfluoroalkane iodine transfer polymerization monomer (or co-monomer) macromolecular design through interchange of xanthates MAF α-trifluoromethacrylic acid MAS magic angle spinning MEA membrane electrode assembly m-TMI 3-isopropenyl-α,α′-dimethylbenzyl isocyanate Mn number-average molecular weight Mw weight-average molecular weight NMP nitroxide-mediated polymerization NMP N-methylpyrrolidone NMR nuclear magnetic resonance P propylene PAAVE perfluoroalkoxyalkyl vinyl ether PAVE perfluoroalkyl vinyl ether PDI polydispersity index PDMS poly(dimethylsiloxane) PEGMeMA poly(ethylene glycol) monomethyl ether methacrylate PEMFC proton exchange membrane fuel cell PEO poly(ethylene oxide) PEOMA poly(ethylene oxide) methacrylate PFA poly(TFE-co-alkoxyethylene) PFOA perfluorooctanoic acid PFOS perfluorooctane sulfonic acid PFP pentafluoropropylene

Directeur de Recherches at CNRS, Bruno Ameduri leads the “Fluoropolymers and Energy” team at the Laboratory “Engineering and Macromolecular Architectures” of Institute Charles Gerhardt in Montpellier, France. His main interests focus on the synthesis and the characterization of fluorinated monomers (including cure site monomers and telechelics), telomers, and (co)polymers for various applications such as surfactants, elastomers, fuel cell membranes, and solvents, binders, and polymer electrolytes for lithium ion batteries. Coauthor of one book, 30 reviews or chapters of books, and more than 240 peer review publications and coinventor of more than 65 patents, he is also a member of the American and French Chemical Societies and is a member of the Editorial Boards of the Journal of Fluorine Chemistry, European Polymer Journal, Polymer Bulletin, and Polymer Journal (Japan). Out of research, Bruno enjoys cycling, soccer, tennis, and jogging and is an active member of the “Rire” Association and, dressed as a clown, visits sick children in hospitals of Montpellier.

ACKNOWLEDGMENTS The authors would like to acknowledge Prof. Bernard Boutevin, Prof. Georgi Kostov, Dr. Alagapan Thenappan, and Eric Rainal for their valuable input. Past and actual postdoctoral researchers and Ph.D. students (mentioned as coauthors in the list of references) are thanked for their contributions. Authors are also grateful to colleagues in the industry for their comments, and their corresponding companies for helping in the building of valuable collaborations are also aknowledged as well as for sponsoring various studies and supplying free samples. Dr. Gerald Lopez’s help in recording 19F−19F COSY NMR spectrum of CTFE (Figure 1) is also appreciated. DEDICATION Dedicated to Professors Bernard Boutevin and Georgi Kostov (especially for his 70th birthday). LIST OF SYMBOLS AND ABBREVIATIONS AGE allyl glycidyl ether AIBN α,α′ azobisisobutyronitrile AFMS α-fluoromethylstyrene AN acrylonitrile ATFMS α-trifluoromethylstyrene AT


Chemical Reviews PFSVE PMMA PMVE PPVE PSSA PTFE PVDF R RAFT RF RFI RH S SEC SEM semi-IPN SPMA SSA TBPAC tBuA TeFP TEM TFE TFP TFS Tg TMA TMSPMA TPE TrFE UV VAc VC VCA VDC VDF VE VPr Δ

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perfluoro(4-methyl-3,6-dioxaoct-7-ene) sulfonyl fluoride poly(methyl methacrylate) perfluoromethyl vinyl ether perfluoropropyl vinyl ether poly(styrene sulfonic acid) poly(tetrafluoroethylene) poly(vinylidene fluoride) alkyl group reversible addition−fragmentation chain transfer polymerization perfluorinated group perfluoroalkyl iodide relative humidity styrene size exclusion chromatography scanning electron microscopy semi-interpenetrated polymer network sulfopropyl methacrylate styrene sulfonic acide tert-butylperoxy allyl carbonate tert-butyl acrylate 2,3,3,3-tetrafluoropropene transmission electron microscopy tetrafluoroethylene 3,3,3-trifluoropropene α-trifluorostyrene glass transition temperature trimethyl amine 3-(trimethoxysilyl)propyl methacrylate thermoplastic elastomer trifluoroethylene ultraviolet vinyl acetate vinyl chloride vinylene carbonate vinylidene chloride (or 1,1-dichloroethylene) vinylidene fluoride (or 1,1-difluoroethylene) vinyl ether vinyl propionate heating

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