Scope of controlled synthesis via chain-growth condensation polymerization: from aromatic polyamides to π-conjugated polymers.

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FEATURE ARTICLE

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Scope of controlled synthesis via chain-growth condensation polymerization: from aromatic polyamides to p-conjugated polymers Tsutomu Yokozawa* and Yoshihiro Ohta Conventional condensation polymerization proceeds in a step-growth polymerization manner, in which the generated polymers possess a broad molecular weight distribution, and control over molecular weight and polymer end groups is difficult. However, the mechanism of condensation polymerization of some monomers has been converted from step-growth to chain-growth by means of activation of the polymer end group, either due to the difference in substituent effects between the monomer and the polymer, or due to successive intramolecular transfer of catalyst to the polymer end. In this article, we review recent developments in chain-growth condensation polymerization (CGCP) in these two areas. The former approach has yielded many architectures containing aromatic polyamides and aromatic polyethers, with unique properties. In the latter case, the mechanism, catalysts, and initiators of Ni- and

Received 14th May 2013, Accepted 24th July 2013

Pd-catalyzed coupling polymerizations leading to poly(alkylthiophene)s and poly(p-phenylene)s have

DOI: 10.1039/c3cc43603a

polymers, and alternating aryl polymers, have also been synthesized by means of catalyst-transfer

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transfer condensation polymerization are not covered in this article.

been extensively investigated. Other well-defined p-conjugated polymers, such as polyfluorenes, n-type condensation polymerization. Many p-conjugated polymer architectures prepared by utilizing catalyst-

Introduction Living polymerization can control not only molecular weight but also polymer end groups, and therefore allows us to synthesize a variety of architectures, including block copolymers, graft copolymers, and star polymers. Addition polymerization and ring-opening polymerization, which proceed in a chain-growth polymerization manner, can involve living polymerization as long as chain transfer and termination do not take place. Polycondensation, however, generally proceeds in a step-growth polymerization manner.1 Consequently, high-molecular-weight polymers with a degree of polymerization (DP) of more than 100 are not obtained unless the extent of reaction exceeds 99%, and the polydispersity reaches 2.0 at high conversion, according to the basic principle of step-growth polymerization established by Carothers and Flory.2–4 This principle is known to be applicable to many polycondensations, but was derived from statistical theory on the assumption that the functional groups of monomers and polymers show the same reactivity. However, it has been reported that some polycondensations do not follow the principle of Carothers and Flory. For example, in cases where a change of substituent effects is induced by Department of Material and Life Chemistry, Kanagawa University, Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan. E-mail: [email protected]; Fax: +81-45-413-9770; Tel: +81-45-481-5661

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bond formation of the monomer, reaction of a monomer with the polymer end functional group can be faster than that of the monomer with another monomer due to enhancement of the reactivity of the polymer end group resulting from the change of substituent effects.5–15 When the rate ratio of the former reaction to the latter becomes sufficiently high, the polycondensation mechanism is changed from step-growth to chaingrowth. Since the end group of the condensation polymer is a stable functional group, not a reactive species such as a radical or an ion, chain transfer and termination should not take place. In these circumstances, chain-growth polycondensation† can show living polymerization behaviour. Indeed, we found that condensation polymerization of phenyl 4-(octylamino)benzoate 1a in the presence of a base and phenyl 4-nitrobenzoate 2 as an initiator yielded well-defined aromatic polyamides with very low polydispersity (Mw/Mn r 1.1) (Fig. 1).16 The Mn values increased in proportion to monomer conversion, indicating that this condensation polymerization proceeded as a chaingrowth polymerization. This can be attributed to the difference in substituent effects between the monomer and the polymer,

† The term polycondensation should be used for only step-growth polymerization via condensation according to an IUPAC recommendation: I. Mita, R. E. T. Stepto, U. W. Suter, Pure Appl. Chem., 1994, 66, 2483. Therefore, chain-growth polymerization via condensation is termed chain-growth condensation polymerization (CGCP).

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Fig. 1 Chain-growth condensation polymerization through the change of substituent effects.

because the phenyl ester moiety of amide anion 1a 0 , generated from 1a in the presence of a base, is deactivated owing to its strong electron-donating ability through the resonance effect, thus blocking reaction of the monomer with another monomer. Accordingly, 1a 0 reacts with initiator 2 to afford an amide. Since the amide linkage is a weak electron-donating group, the phenyl ester moiety of the amide is more reactive than that of 1a 0 . Consequently, the next monomer selectively reacts with the phenyl ester moiety of the amide. Thus, growth continues in a chain polymerization manner via the selective reaction of 1a 0 with the terminal phenyl ester moiety of the polymer. Another approach to chain-growth condensation polymerization (CGCP) is based on intramolecular transfer of catalyst along the polymer chain. We17,18 and McCullough19 independently found that Kumada–Tamao coupling polymerization of Grignard bromothiophene monomer 3 with Ni(dppp)Cl2 (dppp = 1,3-bis(diphenylphosphino)propane) yielded well-defined

Tsutomu Yokozawa was born in Chiba in 1957. He received his BS (1981), MS (1983), and PhD (1987) in Organic Chemistry from the Tokyo Institute of Technology under the direction of Prof. Nobuo Ishikawa and Prof. Takeshi Nakai. In 1985, he had already started an academic career in the Research Laboratory of Resources Utilization, Tokyo Institute of Technology as a Research Associate, and was Tsutomu Yokozawa promoted to Assistant Professor in 1988. He joined the Department of Applied Chemistry, Kanagawa University, as a Lecturer in 1991, and was promoted to Associate Professor in 1993. During 1997–1998, he worked as a visiting scientist at the University of Illinois at Urbana-Champaign with Prof. J. S. Moore. He was promoted to Full Professor in 1999. He was also a researcher for PRESTO, JST, during 2001–2005 and a guest professor at Wuppertal University in 2010. He received the Award of the Society of Polymer Science, Japan, in 2007. His research interest covers controlled synthesis of polymers, supramolecular chemistry of polymers, as well as synthetic organic chemistry. 8282

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ChemComm poly(hexylthiophene) (P3HT), the molecular weight of which was controlled by the feed ratio of the monomer to the catalyst (Fig. 2). We have proposed the following catalyst-transfer mechanism.20 Ni(dppp)Cl2 reacts with 2 equiv. of 3, and the coupling reaction occurs with concomitant generation of a zerovalent Ni complex. The Ni(0) complex does not diffuse into the reaction mixture, but is inserted into the intramolecular C–Br bond. Another molecule of 3 reacts with this Ni, followed by coupling reaction and intramolecular transfer of the Ni catalyst to the next C–Br bond. Growth continues through transfer of the Ni catalyst to the polymer end group until the monomer is depleted. In this Feature Article, we will focus on recent developments of CGCP involving polymerization based on the substituent effect and catalyst-transfer polymerization. We have previously reviewed early work in these areas, as well as other approaches to CGCP.21,22

CGCP based on the substituent effect New monomers and reactions The polyamide synthesis mentioned above, shown in Fig. 1, has been made more convenient by using the methyl ester monomer, which can be derived from commercially available methyl 4-aminobenzoate in one step, and a commercially available base, lithium 1,1,1,3,3,3-hexamethyldisilazide (LiHMDS).23 However, polymerization of the methyl ester monomer bearing a tri(ethylene glycol) monomethyl ether (TEG) side chain instead of an alkyl group on the nitrogen atom was very slow, affording a polymer with a broad molecular weight distribution. The observed slow polymerization is probably accounted for by

Yoshihiro Ohta was born in Kanagawa, Japan, in 1983. He received his BS (2006), MS (2008), and PhD (2011) in applied chemistry from Kanagawa University under the direction of Prof. Tsutomu Yokozawa. After working as a Research Assistant Professor with Prof. Atsushi Takahara at Kyusyu University (2011), he joined the faculty of Kanagawa University, where he is presently Assistant Professor. Yoshihiro Ohta His research interests include synthesis and characterization of hyperbranched polymers, functional block copolymers, and graft copolymers with welldefined structures.

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Fig. 2

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Chain-growth condensation polymerization by catalyst transfer.

coordination of the TEG side chain to the Li cation derived from LiHMDS, resulting in reduction of the reactivity of the amide anion. Accordingly the more reactive phenyl ester monomer 1b was polymerized at 20 1C, successfully affording polyamide with low polydispersity. An initiator with a weak electron-donating methyl group did not affect the polydispersity (Fig. 3).24 The molecular weight was controlled by the feed ratio of the monomer to the initiator (up to 50). Interestingly, scanning electron microscopy (SEM) images of samples prepared from a THF solution of poly1b revealed the formation of 10–25 mm spherical aggregates, whose diameter was independent of the molecular weight of poly1b (Fig. 4). Other N-alkyl poly( p-benzamide)s did not afford spherical aggregates from a THF solution. Therefore, interactions between the TEG side chain in poly1b and THF and/or differential solubility of the TEG side chain and the aromatic polyamide backbone in THF may underlie the formation of these aggregates. Since resonance effects can work between functional groups not only at the para position of benzene, but also at the 1,5- or

Fig. 3 Chain-growth condensation polymerization of phenyl ester monomer 1b bearing the TEG side chain.

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Fig. 4 SEM images of poly1b self-assembled from a THF solution on a glass plate: (A) and (B) 10 wt% solution of poly1b (Mn NMR = 6400, Mw/Mn = 1.15); (C) 5 wt% of the same polymer; (D) 5 wt% of poly1b (Mn NMR = 13 400, Mw/Mn = 1.10).

2,6-positions of naphthalene, we extended the range of monomers that can provide well-defined aromatic polyamides from benzene monomers to naphthalene monomers (Fig. 5).25 Polymerization of the methyl ester monomer 4a bearing a 3,7-dimethyloctyl side chain in the presence of an initiator and LiHMDS gave welldefined poly(naphthalenecarboxamide), together with a very small amount of a cyclic trimer, formed by self-condensation of 4a in the early stage of the polymerization. Attempts to obtain highermolecular-weight polymers resulted in the formation of polymers that were insoluble in the reaction solvent. On the other hand, polymerization of phenyl ester monomer 4b with the TEG side chain yielded poly(naphthalenecarboxamide) with high solubility, but the molecular weight was well controlled only in the case of [4b]0/[initiator]0 = 10. Polymerization at higher feed ratios was accompanied by self-condensation to afford polyamides via Chem. Commun., 2013, 49, 8281--8310

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Fig. 5

Chain-growth condensation polymerization of naphthalene-based monomers.

chain-growth and step-growth polymerization, so that the Mn value of the polymer did not reach the theoretical value. The undesirable self-condensation is accounted for by insufficient deactivation of the electrophilic ester moiety by the electrondonating resonance effect of the amide anion due to the greater distance between the 2 and 6 positions of the naphthalene ring in comparison with the corresponding para-substituted benzene monomer, which can undergo CGCP without self-condensation until the feed ratio reaches 100.16 In CGCP leading to aromatic polyamides, a strongly, electrondonating amide anion can also deactivate an ester moiety at the meta-position through the inductive effect (+I effect). Thus, polymerization of meta-alkylaminobenzoic acid esters, which are derived from commercially available 3-aminobenzoic acid esters in one or two steps, results in well-defined poly(m-benzamide)s.26–28 As an extension of this work, we investigated polymerization of 3-(4-octyloxybenzylamino)benzoic acid ester 5 bearing an alkoxy group at the ortho-position of the amino group, since we anticipated the formation of polyamides having a specific conformation

Fig. 6

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owing to intramolecular CONH OR hydrogen bonding (Fig. 6).29 Polymerization of 5 with LiHMDS in the presence of an initiator and LiCl at 0 1C afforded a polymer with a narrow molecular weight distribution, but it was accompanied by a small amount of a cyclic trimer that stemmed from self-condensation of 5. The accompanying cyclization of monomer 5 is a specific feature of the polymerization of 5, because CGCP of metasubstituted monomers without the alkoxy group did not afford cyclic triamides.26–28 The difference may be accounted for by the lower acidity of the amino group of 5 due to intramolecular hydrogen bonding between the amine proton and the alkoxy oxygen; slow proton abstraction of the amino group would induce the reaction of deprotonated 5 with non-deprotonated 5, leading to self-condensation. Indeed, the amino proton of 5 appears at a lower magnetic field (4.64–4.57 ppm), compared to that of meta-substituted monomers without the alkoxy group (4.09–4.02 ppm). The 4-octyloxybenzyl (OOB) group on the amide nitrogen of poly5 was removed by treatment of trifluoroacetic acid (TFA) in CH2Cl2 at ambient temperature. The obtained N-H

Chain-growth condensation polymerization of 5 bearing an alkoxy group at the ortho-position of the amino group.

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polybenzamides were only soluble in TFA and CH2Cl2 and had lower solubility than poly(N-H-m-benzamide), which is soluble in dimethylsulfoxide (DMSO) and N,N-dimethylacetamide (DMAc).27 This result is considered to be due to intramolecular hydrogen bond formation between amide N-H and the oxygen atom at the ortho position of the benzene ring in the polymer, which leads to the formation of a planar sheet structure that is likely to favour strong intermolecular interactions among polymer chains. When LiHMDS was added to a solution of 5 in a reverse mode of polymerization,30 the cyclic trimer was selectively obtained in 70% yield. The OOB group was not removed with TFA, but when the cyclic triamide was refluxed in neat TFA with triisopropylsilane (TIPS) as a scavenger,31 the N-H cyclic triamide was formed. Monomer 6 with a methoxyethoxymethoxy-protected hydroxyl group in place of the alkoxy group of 5 similarly underwent CGCP. The protecting groups on the oxygen and amide nitrogen of poly6 were simultaneously removed with TFA to afford well-defined poly(amidephenol), which was converted to poly(benzoxazole) by heating at 350 1C (Fig. 7). This poly(benzoxazole) is thermally stable and the 10% weight loss temperature (Td10) was 417 1C.32 For the synthesis of defect-free ladder polyamides, we focused on successive construction of two amide linkages between two benzene rings, using the reaction of N-alkylanthranilic acid ester 7 and N-alkylisatoic anhydride 8 as a model. The amide anion of 7 would attack the carbonyl carbon between the benzene ring and the oxygen atom in 8 to form an amide linkage. Decarbonation of the carbamic anion 9 would then provide the amide anion 10, and subsequent intramolecular nucleophilic attack by the amide anion on the ester carbonyl carbon would result in the formation of the second amide linkage, yielding cyclic diamide 11 (Fig. 8). Indeed, when the reaction was carried out with LiHMDS and N,N,N 0 ,N 0 -tetramethylethylenediamine (TMEDA) in THF at 70 1C under reduced pressure, 11 was obtained in high yield. It is noteworthy that polymerization of 7 did not occur at all, and deprotonated 7

Fig. 7

Fig. 8

Successive construction of two amide linkages between two benzene rings.

reacted selectively with 8. Accordingly, it turned out that the amide anion of 7 deactivates the ester moiety at the orthoposition through electron-donating resonance and inductive effects, thus preventing the polymerization of 7.33 Dai and coworkers synthesized polyamides with defined molecular weight and low polydispersity (Mw/Mn = 1.1–1.4) by means of a novel, unconventional condensation polymerization of diisocyanate and dicarboxylic acid in the presence of a small amount of hindered carbodiimide 12 and a carbodiimide catalyst (1,3-dimethyl-3-phospholene oxide (DMPO)) at 180–200 1C, although the mechanism is not based on CGCP (Fig. 9).34 The polymer end groups are derived from the hindered aryl group from 12, and therefore the molecular weight is controlled by the feed ratio of the monomer to 12. The mechanism is complex, but in principle, diisocyanate and 12 are first converted to poly(carbodiimide) with the hindered aryl groups on both ends, and dicarboxylic acids add to the backbone carbodiimides, followed by rearrangement to form an amide linkage and an isocyanate fragment, which is condensed with the terminal isocyanate of poly(carbodiimide) to form a carbodiimide linkage (sequential self-repetitive reaction (SSRR)). This polymerization involves a DMPO-catalyzed trans-carbodiimide disproportionation reaction, which is presumably responsible for

Synthesis of well-defined poly(benzoxazole) by means of chain-growth condensation polymerization of 6.

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Fig. 11 Chain-growth condensation polymerization of 13 bearing the trifluoromethyl group.

Fig. 9 Well-defined polyamide synthesis from diisocyanate and dicarboxylic acid involving hindered carbodiimide 12.

the formation of polyamides with controlled molecular weight and low polydispersity. Well-defined aromatic polyether can also be obtained by means of CGCP based on the change of substituent effects. The phenoxide moiety works as a stronger electron-donating group than does the ether linkage, and the carbon attached to fluorine in the monomer is strongly deactivated, preventing self-condensation of the monomer. Accordingly, phenoxide monomer reacts selectively with the initiator and the polymer end group, resulting in CGCP (Fig. 10).35 Kim and coworkers conducted CGCP of new monomers 13a36 and 13b,37 in which the trifluoromethyl group works as an electron-withdrawing group and increases the solubility of the polymer. Furthermore, preparation of these monomers is easier than the preparation of monomers in Fig. 10 (Fig. 11). In the polymerization of 13a, polyether with Mn E 4000 and Mw/Mn o 1.2 was obtained.

Fig. 10 Polymerization mechanism of chain-growth condensation polymerization leading to aromatic polyether.

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When 4-nitro-3-(trifluoromethyl)benzonitrile was used as an initiator, the polymerization did not proceed in a controlled manner. The cyano group at the para-position of the nitro group as a leaving group is such a strong electron-withdrawing group that transetherification at the ether linkage is likely to occur. The polymerization of 13b was carried out with 2-(trifluoromethyl)-4nitrobenzonitrile as an initiator bearing two electron-withdrawing groups at 70 1C to yield poly(arylene ether azobenzene) with Mn = 2000 and Mw/Mn = 1.09. Attempts to increase the molecular weight above 5000 encountered similar difficulties to those in the case of transetherification. The reactivity of initiators having a single electron-withdrawing group was too low to initiate the polymerization even at 80 1C, at which self-initiated condensation polymerization of 13b occurred. Trichloro(N-trimethylsilyl)phosphoranimine, Cl3P = NSiMe3 (14), undergoes CGCP with PCl5 at ambient temperature to yield well-defined poly(dichlorophosphazene).38,39 The polymerization is initiated by the reaction of 14 and 2 equiv. of PCl5 to generate a counteranion, PCl6, associated with cationic initiator 15, with which 14 successively reacts with the elimination of Me3SiCl, affording the elongated cation (Fig. 12). Manners and coworkers have recently provided new insight into the mechanism by means of studies of model compound chemistry.40 First, the reaction of cationic initiator 15 with 14 is faster at one end, but bidirectional chain growth becomes increasingly dominant as the chains increase in length. Second, the PCl6 counteranion arising in the initiation step is not unreactive but is actually an active participant in the polymerization. This is expected to complicate molecular weight control by providing another source of initiating sites for chain growth, thereby broadening

Fig. 12

Proposed reaction sequence in the PCl5-initiated polymerization of 14.

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the molecular weight distribution. Third, the absence of reaction between phosphazene cations with only internal P–Cl bonds and 14 indicates that the polymer formed is essentially linear. Furthermore, they explored salts consisting of [15]+ with inert, weakly coordinating counteranions to eliminate unwanted effects of PCl6 on the molecular weight distribution of the resulting polyphosphazenes.41 However, those salts did not initiate polymerization, and it turns out that the Cl anion generated at an early stage is not innocuous, but plays a key cooperative role with the living cations in both initiation and chain propagation during the polymerization process. Architectures 1 Block copolymers. Many kinds of block copolymers containing well-defined condensation polymers have been synthesized by CGCP.21,22,42 Recent examples are described here. In the synthesis of poly(N-TEG-p-benzamide)-b-poly(N-octylp-benzamide) with LiHMDS, the phenyl ester monomer 1b should be polymerized first, followed by the methyl ester monomer 1c, because the phenyl ester monomer 1b would be converted to the methyl ester monomer, the polymerization of which was uncontrolled as mentioned above, by transesterification if the methyl ester monomer 1c was polymerized in the first stage of the one-pot copolymerization. In addition, an equivalent amount of LiHMDS was added to each monomer in both the first and second stages to prevent excess of LiHMDS in the first stage from reacting with the terminal phenyl ester moiety of poly1b, inactivating it. Using these procedures, a block copolymer of poly1b and poly1c with low polydispersity was obtained (Fig. 13).24 Block copolymers of aromatic polyamide and vinyl polymer were synthesized by means of CGCP of 1a with a vinyl polymer macroinitiator. However, the block copolymer was contaminated with homopolymer of the polyamide when high-molecular-weight macroinitiators were used. This is probably because the polymer effect of vinyl polymer decreased the efficiency of initiation of the macroinitiator, thereby favoring self-polycondensation of 1a.43

Fig. 13 Block copolymerization of the phenyl ester monomer 1b and the methyl ester monomer 1c.

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To overcome this problem, an aromatic polyamide macroinitiator was used for the polymerization of vinyl monomer. Thus, the polymer end group of the polyamide was converted to a dithiobenzoate moiety, and reversible addition–fragmentation chain transfer (RAFT) polymerization of styrene was carried out in the presence of this polyamide as a macro chain transfer agent to yield a well-defined high-molecular-weight diblock copolymer consisting of aromatic polyamide and polystyrene (Fig. 14(a)).44 An aromatic polyamide macroinitiator for atom transfer radical polymerization (ATRP), which is more easily synthesized than the chain transfer agent for the RAFT polymerization, was also effective for the synthesis of a highmolecular-weight block copolymer of aromatic polyamide and polystyrene (Fig. 14(b)).45 During studies of ATRP of styrene from a polyamide macroinitiator, we discovered styrene-assisted atom transfer radical coupling (ATRC) from methacrylic macroradicals at low temperature, and applied this chemistry to the synthesis of ABA-type triblock polybenzamides.46 Thus, coupling of high-molecular-weight AB-type diblock polybenzamide and 2-bromoisobutyryl-terminated poly(N-octyloxybenzyl-m-benzamide)-b-poly(N-octyl-m-benzamide) afforded ABA-type triblock polybenzamides with high coupling efficiency (>94%). The molecular weight was doubled and a narrow molecular weight distribution (Mw/Mn o 1.18) was maintained. Selective removal of the OOB groups was achieved, resulting in a poly(N-H-mbenzamide) segment (i.e., A block) (Fig. 15). Thermal transitions of the diblock and triblock polybenzamides were examined by differential scanning calorimetry (DSC). In the case of triblock polybenzamides, the Tg value shifted from 45 to 62 1C after removal of the OOB groups; this might be ascribed to a confinement effect of the segments at the extremities via strong intermolecular hydrogen-bonding interactions. We synthesized block copolymers of aromatic polyether (Fig. 10) and vinyl polymer by means of ATRP.47,48 Kim and coworkers also synthesized similar block copolymers by ATRP of vinyl monomers from macroinitiator 16a or 16b derived from poly13a (Fig. 16).36 The ATRP of methyl methacrylate with 16a successfully afforded poly13a-b-poly(methyl methacrylate), but 16a was not suitable for controlled ATRP of styrene, presumably because of poor initiation efficiency at the site adjacent to the amide bond.49,50 On the other hand, 16b afforded both poly13ab-poly(methyl methacrylate) and poly13a-b-polystyrene. Block copolymer 17 of poly13b and polystyrene was similarly synthesized by ATRP of styrene, and the unreacted macroinitiator was separated from the block copolymer by extraction with cyclohexane (Fig. 17).37 The clear cyclohexane solution of 17 micelles became turbid when it was exposed to UV light, but reverted to a clear solution on exposure to visible light. The dipole moment change induced by photoisomerization of azobenzene incorporated along the backbone plays an important role in photoinduced aggregation and segregation of self-assembled 17 micelles in cyclohexane. 2 Star polymers. Star polyamides have been synthesized by both core-first and arm-first methods. We first examined the core-first method by the use of a multifunctional initiator for CGCP of 1a with a prototype base system (N-octyl-N-triethylsilylaniline/CsF/18-crown-6) to afford not only the desired Chem. Commun., 2013, 49, 8281--8310

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Fig. 14

Synthesis of a block copolymer of aromatic polyamide and polystyrene by styrene chain extension from aromatic polyamide via (a) RAFT and (b) ATRP.

Fig. 15

Synthesis of triblock polybenzamides by means of styrene-assisted atom transfer radical coupling.

Fig. 16

ATRP macroinitiators 16 containing aromatic polyether, poly13a.

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star-shaped poly(p-benzamide), but also a linear poly(p-benzamide) via self-initiated condensation polymerization of the monomer. This contamination with a linear polymer was more liable to occur as compared to the case of CGCP with a monofunctional initiator under the same conditions.51 Accordingly we optimized the polymerization conditions for the synthesis of star polyamides by CGCP with LiHMDS by using the porphyrin-cored tetra-functional initiator 18.52 Since the target star polymer has an absorption at 430 nm due to the porphyrin ring, whereas the linear polymer formed as a by-product does not, it was easy to differentiate the star polymer from the linear polymer by GPC with UV detection and to optimize conditions for the selective This journal is

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Fig. 17

Block copolymer 17 of poly13b and polystyrene obtained by ATRP of styrene from the poly13b macroinitiator.

Fig. 18

Synthesis of star polyamides by the core-first method involving chain-growth condensation polymerization of 1c from tetra-functional initiator 18.

synthesis of the star polymer. It turned out that polymerization of the methyl ester monomer 1c at 10 1C, which was higher than the optimum temperature (10 1C) for the synthesis of linear poly1c with LiHMDS from a monofunctional initiator, yielded the star polyamide with controlled molecular weight and a narrow molecular weight distribution up to the feed ratio of [1c]0/[18]0 of 120 (Fig. 18). Under the optimized conditions, a variety of welldefined tetra-armed star-shaped poly(N-substituted p-benzamide)s, including block poly(p-benzamide)s with different N-substituents, and poly(N-substituted m-benzamide)s, were synthesized by using tetra-functional initiator 18.53 As an arm-first method, macromonomers (MM) with the styryl terminal moiety were synthesized by CGCP of 3-(alkylamino)benzoic acid esters 19 in the presence of phenyl 4-vinylbenzoate as an initiator, and copolymerization with N,N0 -methylenebisacrylamide

Fig. 19

(MBAA) as a divinyl monomer in the presence of 2,2 0 -azobis(isobutyronitrile) (AIBN) at 60 1C yielded the corresponding star polymers (Fig. 19).54 The number of arms per molecule, determined by multi-angle laser light scattering (MALLS), varied in the range of 36–100 depending on the N-alkyl group of 19 and the molecular weight of MM. Blockarm and miktoarm star polymers consisting of poly(N-octyl-m-benzamide) and poly(N-H-m-benzamide) were also synthesized by this method. It is noteworthy that the 1H NMR spectra of the miktoarm star polymer in DMSO at 23 1C showed weak signals of the poly(N-octyl-m-benzamide) moiety, but the intensity increased to the expected level on heating (Fig. 20). This observation indicates that the poly(N-octyl-m-benzamide) moiety, which is insoluble in DMSO, is packed in the star polymers at 23 1C and extended at higher temperatures. It is intriguing that

Synthesis of star polyamides by the arm-first method involving radical copolymerization of MM and MBAA.

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Fig. 20 1H NMR spectra of the miktoarm star polymer in DMSO-d6 at (a) 23 1C and (b) 120 1C.

semi-rigid aromatic polyamide arms in star polymers can dynamically change the molecular geometry in response to

ChemComm thermal stimulation, as in the case of star polymers composed of flexible coil polymers. Star polyethers have been synthesized by the core-first method. AB2 and A2B type miktoarm star copolymers consisting of aromatic polyether arms as the A segment and polystyrene arms as the B segment were synthesized by using orthogonal trifunctional initiators.55 Recently, the (AB)3 type star block copolymer was prepared using ATRP, CGCP, and click reaction (Fig. 21).56 ATRP of styrene was carried out in the presence of 2,4,6-tris(bromomethyl)mesitylene as a trifunctional initiator, and then the terminal bromines of the polymer were transformed to azide groups with NaN3. The azide-terminated polystyrene was used for click reaction with alkyne-terminated aromatic polyether, obtained by CGCP with an initiator bearing an acetylene unit. Excess alkyne-terminated aromatic polyether was removed from the crude product by means of preparative HPLC to yield the (AB)3 type star block copolymer. The morphologies of star copolymers (AB2, A2B and (AB)3) and the linear block copolymer (AB), after they had been annealed under a vacuum at 150 1C for 2 days, were observed by means of transmission electron microscopy (TEM) (Fig. 22). In the case of a sample of AB type diblock copolymer, lamellar morphology consisting of alternating dark PSt and bright aromatic polyether regions was observed (a). On the other hand, samples of AB2, A2B and (AB)3 type star copolymers showed spherical or plate-like morphology (b)–(d). Such unexpected and novel properties might arise from the different solubility and crystallinity of the aromatic polyether segments in

Fig. 21 Synthesis of (AB)3 type star block copolymers consisting of aromatic polyether as the A segment and polystyrene as the B segment by means of ATRP, CGCP, and click reaction.

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Fig. 22 TEM images of CHCl3 cast of (a) AB type diblock copolymer, (b) AB2 type mikto arm star copolymer, (c) A2B type mikto arm star copolymer, and (d) (AB)3 type star block copolymer. The samples were stained with RuO4 prior to TEM measurements.

star copolymers, compared to those of the corresponding linear diblock copolymer. Kim and coworkers synthesized multi-armed poly(ether sulfone)s by means of CGCP of 4-fluoro-40 -hydroxydiphenyl sulfone potassium salt57 with di-, tri-, and tetra-functional initiators at 100 1C (Fig. 23).58 The resulting polymers showed low polydispersity (Mw/Mn o 1.3) and were not contaminated with polymers formed by selfpolycondensation of the monomer. The branched polymers

have higher glass transition temperature (Tg) than linear polymers, whereas the viscosity of the polymers decreases as the number of branches increases. Interestingly, physical gelation of THF solution of these polymers was observed at room temperature, and the gels changed to sols upon heating. The polymers having approximately 5 repeating units per arm formed a gel regardless of the number of arms. It was proposed that gelation is mainly driven by the aromatic amide motif at the initiating site, which forms hydrogen bonds, resulting in the formation of self-assembled nanofibers. Furthermore, a pyrene unit was introduced into the initiator unit of these polymers, and two fluorescence switching modes were observed in different gelation solvents. The THF gel exhibited excimer emission due to dimerization of the pyrene units, whereas excimer emission was quenched after gelation in methylene chloride because of stacking of the pyrene units (Fig. 24).59 Star block copolymers composed of poly(ether sulfone) as a core and hydrophilic polymethacrylate as a shell were also synthesized by the introduction of an ATRP initiator unit at the polymer end group of the tetra-armed poly(ether sulfone) followed by copolymerization of 2-(2-methoxyethoxy)ethyl methacrylate and oligo(ethylene glycol) methacrylate (Fig. 25).60 Dynamic light scattering (DLS) of the star block copolymers in aqueous solution revealed that hydrodynamic diameters of the polymers were decreased significantly by addition of Nile Red due to the transition from polymeric aggregates to unimolecular micelles. Encapsulation of hydrophobic dyes into the hydrophobic core

Fig. 23

Multi-armed poly(ether sulfone)s obtained by chain-growth condensation polymerization.

Fig. 24

Dual-mode fluorescence switching induced by self-assembly of poly(ether sulfone)s containing pyrene and amide moieties in THF and methylene chloride (MC).

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Star block copolymer of poly(ether sulfone) and hydrophilic polymethacrylate obtained by CGCP and ATRP.

of the polymers might interfere with the interaction between the core blocks and fill the core space. Another transition into multimolecular polymer micelles was also observed when an excess of Nile Red was used. 3 Graft copolymers. Conventional graft copolymers containing condensation polymers were produced by polycondensation of AA and BB monomers, one of which carried the side-chain polymer, such as vinyl polymers and polysiloxanes.61–64 Since those reports, CGCP of phosphoranimine has been used to synthesize polymers grafted with well-defined polyphosphazene.65–68 We recently prepared polystyrene grafted with well-defined poly(p-benzamide) and examined its thermal properties.69 Styryl macromonomer 20 containing poly(N-OOB-p-benzamide) was first synthesized by CGCP of methyl 4-(OOB-amino)benzoate 1d with phenyl 4-vinylbenzoate as an initiator. Free radical copolymerization of 20 and styrene in the presence of AIBN at 60 1C gave polystyrene-g-poly(N-OOB-p-benzamide), from which the OOB groups on amide nitrogen were removed with trifluoroacetic acid (TFA) (Fig. 26). The Tg value of polystyrene-g-poly(N-H-p-benzamide) was dramatically increased than that of

Fig. 26

Synthesis of polystyrene grafted with poly(p-benzamide).

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polystyrene (109 1C) to 172 1C in the case of the graft copolymer containing 4.4 mol% of grafted poly(p-benzamide) chains. On the other hand, well-defined aromatic polyamide backbones grafted with conventional polymers were also synthesized. Methyl 4-aminobenzoate bearing polyTHF, which was obtained by quenching the living cationic propagating group of poly(THF) with methyl 4-aminobenzoate, underwent CGCP with LiHMDS to yield a graft copolymer with a well-controlled polyamide main chain and poly(THF) side chains (Fig. 27).70 4 Hyperbranched polymers. Hyperbranched polymers have received considerable attention in recent years due to their unusual properties arising from their unique molecular architecture, but their physical properties depend on polymer topology, molecular weight, and molecular weight distribution, which are not well controlled.71–74 The degree of branching (DB) generally becomes 0.5 according to statistical theory, but polymerization of some monomers gave hyperbranched polymers with DB of 1.75,76 On the other hand, control of the molecular weight and molecular weight distribution of hyperbranched polymers in a manner similar to living polymerization has not been achieved.

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Fig. 27

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Synthesis of aromatic polyamide grafted with polyTHF.

We have found that AB2 monomer 21a undergoes CGCP to yield hyperbranched polyamides (HBPA) with defined molecular weight and low polydispersity. The amide anion of the monomer deactivates both the ester moieties through the inductive effect to suppress self-polymerization (Fig. 28).77 The CGCP of other AB2 monomers with N-ethyl, octyl, and OOB groups afforded the corresponding well-defined HBPAs. HBPA with the N-OOB group was converted to unsubstituted N-H HBPA with low polydispersity by treatment with TFA.78 Since all HBPAs possess well-controlled molecular weight and low polydispersity, we can evaluate precisely the molecular weight dependency of Tg (Fig. 29). The Tg values of HBPAs increased with increasing molecular weight up to 10 000, and the Tg values of N-methyl HBPAs (poly21b and poly21a) leveled off when the Mn value was higher than about 20 000. When we compared the Tg values of N-methyl HBPA and N-ethyl HBPA with similar Mn and the same terminal ester alkyl groups (methyl ester HBPAs: poly21b vs. poly21c; ethyl ester HBPAs: poly21a vs. poly21d), the Tg value of N-methyl HBPAs was about 15 1C higher than that of N-ethyl HBPAs. The Tg value of N-octyl

Fig. 28

HBPA was as low as 30 1C or less, being about 80 1C lower than that of N-ethyl HBPA (poly21e vs. poly21d). On the other hand, when we compared the Tg values of the methyl ester and ethyl ester HBPAs with similar Mn and the same N-alkyl groups (N-methyl HBPAs: poly21b vs. poly21a; N-ethyl HBPAs: poly21c vs. poly21d), the Tg value of the methyl ester HBPAs was about 25 1C higher than that of the ethyl ester HBPAs. These results suggest that the Tg of HBPA is more influenced by the terminal ester alkyl group than by the N-alkyl group. We also compared the Tg of N-ethyl and octyl HBPAs with those of the corresponding linear N-alkyl poly(m-benzamide)s (LPAs). The Tg values of N-ethyl poly(m-benzamide)s are intermediate between the Tg values of N-ethyl HBPA with the methyl ester terminal groups and the ethyl ester counterpart. Meanwhile, the Tg values of N-octyl HBPA and N-octyl poly(m-benzamide) are identical, implying that the Tg values of aromatic polyamides with long N-alkyl groups would not be affected by the polymer topology. Surface segregation of a variety of N-substituted HBPAs in a linear polystyrene matrix after annealing of the blend films was

Chain-growth condensation polymerization of AB2 monomer 21a for the synthesis of hyperbranched polyamide (HBPA).

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polymer is smaller than that of a linear polymer of comparable molecular weight. This also leads to surface segregation of a hyperbranched polymer in a matrix polymer. When N-octyl, ethyl, and methyl HBPAs were compared, the extent of segregation was in that order. This is probably because HBPA with a shorter alkyl side chain might form aggregates, leading to an increase in apparent molecular weight. This is entropically unfavorable for segregation at the surface. HBPA bearing a N-tri(ethylene glycol) methyl ester (TEG) chain was soluble in water, and a 0.25 wt% aqueous solution of this HBPA exhibited a lower critical solution temperature (LCST) (Fig. 30).80 Phase separation of HBPA with low molecular weight (Mn = 3810, Mw/Mn = 1.15) gradually occurred between 19 and 35 1C (Fig. 30, curve a), whereas the solubility of HBPA with higher molecular weight (Mn = 12 900, Mw/Mn = 1.11; Mn = 18 600, Mw/Mn = 1.19) sharply altered between 20 and 25 1C (curves c and d). This result indicated that the thermotransition of the HBPA in aqueous solution becomes sharper with increasing molecular weight until the molecular weight exceeds a certain value. The cloud point was 21–23 1C, which is about 30 1C lower than that of the corresponding poly(m-benzamide) with the N-TEG unit. By taking advantage of the character of chain-growth polymerization of AB2 monomer, a well-defined diblock copolymer

Fig. 29

Tg of HBPA and LPA as a function of Mn of HBPA and LPA.

also studied by means of X-ray photoelectron spectroscopy (XPS).79 When two polymers with a similar degree of polymerization are mixed, the lower surface energy component is enriched at the surface. A molecular weight disparity between the components also causes surface segregation of the smaller component. The latter can be explained in terms of a lower conformational or translational entropic penalty for the shorter chain at the surface, in addition to the surface localization of chain end groups. If chain ends have a smaller surface energy than the repeating unit, they act as buoys. Thus, when a hyperbranched polymer is mixed with a linear polymer, the hyperbranched polymer is partitioned to the surface because of the buoys. In addition, the chain dimension of a hyperbranched 8294

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Fig. 30 Transmittance versus temperature curves (500 nm, 0.5 1C min1) obtained for 0.25 wt% aqueous solutions of HBPA bearing a TEG chain (curve a, Mn = 3810, Mw/Mn = 1.15; curve b, Mn = 6760, Mw/Mn = 1.15; curve c, Mn = 12 900, Mw/Mn = 1.11; curve d, Mn = 18 600, Mw/Mn = 1.19).

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Fig. 31

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Synthesis of a block copolymer of linear and hyperbranched polyamides.

of linear and hyperbranched polymers was synthesized.77 Methyl 3-(OOB-amino)benzoate (19b) was polymerized first in the presence of an initiator, LiHMDS, and LiCl to afford a prepolymer. Then 21a was added to the prepolymer in the reaction mixture, and the second polymerization was conducted, affording the linear-hyperbranched block copolymer (Fig. 31). For the synthesis of a block copolymer of HBPA and poly(ethylene glycol) (PEG), CGCP of 21a with a PEG macroinitiator bearing two ester moieties was first attempted.81 However, the obtained copolymer showed a broad molecular weight distribution, probably because PEG coordinates to LiCl, which is a requisite for CGCP of 21a to afford HBPA with low polydispersity. By contrast, condensation of PEG with a carboxyl group at one end and HBPA with a hydroxyl group at the focal point yielded PEG-b-HBPA with defined molecular weight and a narrow molecular weight distribution (Fig. 32). The obtained PEG-b-HBPA was soluble not only in various organic solvents, but also in water. The 1H NMR spectrum of PEG-b-HBPA in CDCl3 showed signals of both the PEG and HBPA units, whereas that in D2O only showed signals of the PEG unit, implying that PEG-b-HBPA forms micelles in water, with HBPA segregated in the core. 5 Helical polymers. In the course of our study of CGCP, we found that poly(p-benzamide)s bearing a chiral tri(ethylene glycol) side chain as an N-substituent adopt a helical conformation with three monomer units per turn in solution.82 Variabletemperature CD studies showed that the CD intensity decreased with the increase of temperature, indicating thermodynamically

Fig. 32

controlled helical character of this polyamide. The helical structure arises from the cis preference of the N-substituted aromatic amide linkage83 and syn arrangement of three consecutive benzene units connected by two amide linkages.84,85 In order to elucidate the helical folding, we studied the cooperativity of the monomer units according to the ‘‘sergeants and soldiers’’86 and ‘‘majority rules’’87 effects by using chiral–achiral random copolymers of poly(p-benzamide)s 22 and 23 and (R)/(S) random copolymer 24.88 The CD spectra of the random copolymers 22–24 were similar in shape, and the intensities changed linearly in proportion to the chiral unit ratio of 22 and 23 and the excess of the (S) unit in 24, indicating the absence of cooperativity between the monomer units along these copolymers (Fig. 33). Polynaphthalenecarboxamide 25a bearing a chiral tri(ethylene glycol) side chain as an N-substituent also adopts a helical conformation. In contrast to N-substituted poly( p-benzamide), the folding of 25a was enhanced by a solvophobic effect and seemed to be completed at 0–15 1C in 70% water–methanol.89 The hydrophobicity of the naphthalene ring of 25 is enough to cause intramolecular self-association of the main chain in aqueous solvents. Furthermore, random copolymers of poly(naphthalenecarboxamide) 26 with chiral and achiral tri(ethylene glycol) side chains showed chiral amplification based on the sergeants and soldiers effect in aqueous solvent. It should be noted that chiral amplification was not observed in chloroform or methanol, in which the cooperativity between the monomer

Synthesis of a block copolymer of poly(ethylene glycol) and hyperbranched polyamide.

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Fig. 33 Plots of CD intensity (261 nm) of (a) 22 and (b) 23 vs. chiral unit ratio, and of (c) 24 vs. excess of (S)-unit. The CD spectra of the copolymers were measured in CHCl3 at 24 1C.

units is weak (Fig. 34).90 Recently, we have found that poly(naphthalenecarboxamide) 25b with a chiral aliphatic side chain adopts a helical structure specifically in cyclohexane by virtue of the solvophobic effect and we observed the sergeants and soldiers effect using random copolymers (Fig. 35).91 These results indicated that the introduction of a naphthalene ring into aromatic polyamide with a chiral aliphatic side chain makes it possible to recognize small differences between the

backbone and side chain in the polymer even in hydrophobic, aliphatic media such as cyclohexane.

CGCP by catalyst transfer Mechanism of catalyst-transfer condensation polymerization We have proposed that CGCP of Grignard bromothiophene monomer 3 with Ni(dppp)Cl2 proceeds according to the catalyst-transfer

Fig. 34 Plots of the Kuhn dissymmetry factor (g = De/e) at 250 nm of 25a and 26 in chloroform (red diamond), methanol (green square) and water/methanol = 7/3 (v/v) (blue circle) against the chiral unit ratio. The dotted lines are shown simply to guide the eye.

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Fig. 35 Helical poly(naphthalenecarboxamide) 25b with a chiral aliphatic side chain.

mechanism illustrated in Fig. 2, in which the Ni catalyst moves unidirectionally to the polymer end. However, Kiriy and coworkers demonstrated that the catalyst is able to walk along the P3HT backbone to the opposite end and polymerization of 3 can be initiated by using Br–C6H4–Ni(dppe)–Br (dppe = 1,3-bis(diphenylphosphino)ethane) as an initiator.92 In this polymerization, both P3HT with a bromophenyl end group and P3HT bearing the phenylene group inside the backbone were obtained (Fig. 36). The content of the product with the internal phenyl ring increased with the increase of the polymerization degree. Furthermore, evaluation of crystallinity and NMR analysis of P3HT obtained with Ni(dppp)Cl2 showed the presence of a single tail-to-tail defect distributed over the whole chain.93 Koeckelberghs and coworkers studied the 1H NMR spectrum of P3HT obtained with Ni(dppp)Cl2 and identified the signals of all possible end-groups.94 Terminal bromine is attached to the tail-to-tail thiophene diad (designated as Br-TT) in the case of unidirectional growth, whereas the bromine is attached to the head-to-tail diad (designated as HT-Br) in the case of bidirectional growth (Fig. 37). The 1H NMR spectrum showed that Br-HT was always present, and the ratio of Br-TT/ (Br-HT + Br-TT) decreased with increasing degree of polymerization, indicating bidirectional growth. Accordingly, it is important to note that one-pot synthesis of block copolymers of A and B with Ni(dppp)Cl2 yields not only AB diblock but also BAB triblock copolymers. For the exclusive synthesis of AB diblock copolymers, external Ar–Ni–Br initiators are necessary (see Initiators section). However, Ni complex 27 containing the benzothiathiazole unit induced unidirectional growth, because Ni(0) dppe is not able to walk over the benzothiathiazole unit to initiate polymerization at the other end of the chain (Fig. 38).95 Kinetic studies were conducted by McNeil and coworkers. They showed that the rate-determining step is dependent on the phosphine ligand of the Ni catalyst, being transmetalation in the polymerization with Ni(dppp)Cl2 (dppp = 1,3-bis(diphenylphosphino)propane)96 and reductive elimination in

Fig. 36

Fig. 37 Unidirectional and bidirectional growth and the consequence for the end groups.

Fig. 38 Unidirectional growth polymerization of 3 with 27 containing the benzothiathiazole unit.

the polymerization with Ni(dppe)Cl2 (dppe = 1,3-bis(diphenylphosphino)ethane).97 This means that an intermediate p-complex in catalyst-transfer polymerization could not be directly observed, because oxidative addition is not the rate-determining step. Accordingly, McNeil and coworkers performed small-molecule competition experiments to determine whether Ni-catalyzed Kumada–Tamao coupling reactions proceed through a p-complex intermediate.98 Ni complex 28 was reacted with a Grignard reagent in the presence of a competitive, more reactive aryl halide 29 (Fig. 39). If intramolecular transfer occurs, complex 30 is formed. On the other hand, if the p-complex does not form at all, then the resulting free Ni(0) should be selectively trapped by 29 to produce 31. The experiment resulted in a 95 : 5 ratio of 30 : 31, consistent with the involvement of an intermediate Ni(0) p-complex. Recently, chain-growth SRN-1 (nucleophilic radical substitutions) type condensation polymerization for the synthesis of polyarenes has been reported by Studer and coworkers.99 This process does not require any transition metal, but represents an intriguing, novel method leading to p-conjugated polymers.

Bidirectional propagation in the polymerization of 3 with Br–C6H4–Ni(dppe)–Br.

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Fig. 39

Competition experiment between intramolecular and intermolecular oxidative addition.

Fig. 40

Chain-growth SRN-1 type condensation polymerization.

Aryl magnesium compound 32, which was readily generated from the symmetrical bisiodide by I–Mg exchange with i-PrMgCl, was polymerized in the presence of 2,2,6,6-tetramethylpiperidineN-oxyl radical (TEMPO) at room temperature to yield poly(m-phenylene) with Mn of 3400–13 700 and Mw/Mn of 1.60–2.20 (Fig. 40). Varying the amount of TEMPO allowed for rough adjustment of the molecular weight of the polymer, but the relationship between the amount of TEMPO and Mn was nonlinear, suggesting that the reaction is not a living process. Initiation occurs by oxidative homocoupling of 32 mediated by TEMPO, followed by elimination of I to generate biaryl radical 33. This reactive radical 33 would then add to the anionic monomer 32 to give the corresponding radical anion via C–C bond formation. Subsequent I elimination, generating the chain-extended aryl radical, completes the chain propagation step. This chain-growth SRN-1 type condensation polymerization was applied to the preparation of poly(p-phenylene) and polynaphthalene. Catalysts We100 and Lucht101 investigated the effects of phosphine ligands of the Ni catalyst on the catalyst-transfer condensation 8298

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polymerization of alkylthiophene Grignard monomers. Ni(dppe)Cl2 and Ni(PPh3)4 gave a polymer with a slightly lower Mn and a slightly broader molecular weight distribution, whereas Ni(PPh3)2Cl2, Ni(dppb)Cl2 (dppb = 1,4-bis(diphenylphosphino)butane), and Ni(dppf)Cl2 (dppf = 1,1 0 -bis(diphenylphosphino)ferrocene) gave polymers with low Mns and broad molecular weight distributions. Among these catalysts, Ni(dppp)Cl2 resulted in a Mn value close to the theoretical value based on the feed ratio of the monomer to the catalyst and the narrowest Mw/Mn ratio. Regarding the polymerization of 4-bromo-2,5-dihexyloxyphenyl Grignard monomer 34102 and N-hexyl-4-bromopyrrole Grignard monomer 35103 Ni(dppe)Cl2 was a better catalyst than Ni(dppp)Cl2, but addition of LiCl in the former polymerization or dppe in the latter polymerization was necessary to suppress the formation of oligomers, resulting in a narrower molecular weight distribution (Mw/Mn o 1.2) (Fig. 41). McNeil and coworkers anticipated that increased electrondonating ability of phosphine ligands would increase the binding affinity of polymers to nickel and minimize competing side reactions. They investigated the Ni-catalyzed polymerization of 34 using a series of bis(dialkylphosphino)ethane ligands with different alkyl substituents.104 As a result, Ni(dmpe)Cl2 This journal is

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Fig. 41

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Monomers that undergo catalyst-transfer condensation polymerization.

(dmpe = bis(dimethylphosphino)ethane) and Ni(dcpe)Cl2 (dcpe = bis(dicyclohexylphosphino)ethane) were inactive in the polymerization at room temperature, and provided only low-molecular oligomers even at 60 1C. Ni(depe)Cl2 (depe = bis(diethylphosphino)ethane) afforded polymers at room temperature, but the polymerization rate was significantly slower than that with Ni(dppp)Cl2. Accordingly, the polymerization was carried out at 60 1C, providing polymers with a narrow molecular weight distribution, as well as a small amount of oligomers, which appeared as a tail in the lower-molecular-weight region of the gel permeation chromatogram (GPC). The effect of addition of LiCl was not examined. To clarify the origin of the small amount of low-molecularweight species, 31P NMR spectroscopic studies were conducted. The predominant species observed during polymerization at 60 1C was complex II, which was the catalyst resting state. This observation and another kinetic study revealed that reductive elimination was rate-limiting for both initiation and propagation. As the monomer concentration decreased with the progress of polymerization, complex I appeared, but was unstable and decomposed over 3 h to afford Ni(depe)Br2. This result is consistent with disproportionation of I: a transarylation

Fig. 42

reaction between 2 equiv. of complex I generates Ni(depe)Br2 and Ni(depe)-(polymer)2, which, after reductive elimination, affords polymer and Ni(0) (Fig. 42). Ni(depe)Cl2 was also an effective catalyst for the polymerization of thiophene monomer 3 and N-alkylpyrrole monomer 35, and block copolymerization of 34 and 3 was also conducted with this catalyst. Small molecule competition experiments (Fig. 39) were also conducted with Ni(depe)Cl2.98 At higher concentrations of competitive agent 29, the intramolecular pathway was still preferred, suggesting that the electron-donating depe ligand promotes the formation and reactivity of the key intermediate p-complex. When the polymerization of 34 was carried out in the presence of 29, the polydispersity of the polymer obtained with Ni(depe)Cl2 was narrower than that of the polymer obtained with Ni(dppp)Cl2 under the same conditions, implying that electron-rich ligands also suppressed these undesired intermolecular reactions during polymerization. Because N-heterocyclic carbenes (NHCs) are stronger s-donors than phosphines, McNeil and coworkers studied polymerization of 3, 34, and fluorene-based monomer 36 with NHC-ligated Pd catalyst 37 (Fig. 43).105 CGCP of 3 and 34 proceeded at room temperature, but the polymerization of 36 did not proceed through a chain-growth pathway. It should be noted that NHCligated Pd catalyst 37 polymerized a regioisomer of 3 (reverse monomer, see Monomers section), which is not polymerized with Ni(dppp)2 under conventional conditions without addition of LiCl.106 Therefore, the regioregularity of P3HT obtained with catalyst 37 was low (80%) at high conversion when Grignard monomers were generated from 2,5-dibromo-3-hexylthiophene with an alkyl Grignard reagent, leading to a mixture of 3 and

Proposed mechanism for catalyst-transfer condensation polymerization of 34 with Ni(depe)Cl2.

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ChemComm poorly controlled.109,110 Recently, Geng and coworkers found that the nickel acetylacetonate (Ni(acac)2)–dppp system mediated living CGCP of fluorene monomers, including 36 (Fig. 44).111 The Mn of poly36 could be controlled in the range of 28 000–62 200 with Mw/Mn of 1.15–1.24 by tuning the amount of the catalyst. Fluorene–fluorene and fluorene–thiophene block copolymers were successfully synthesized.


Fig. 43

Fluorine-based monomer 36 and NHC-ligated Pd catalyst 37.

reverse monomer (approximately 80 : 20). In block copolymerization of 3 and 34, the order of monomer addition was less influential than in the case with Ni(dppp)Cl2,107 but the molecular weight distribution of poly3-b-poly34 was somewhat broader (Mw/Mn = 1.35) than that of poly34-b-poly3 (Mw/Mn = 1.24). This tendency is similar to that observed in the block copolymerization with Ni(dppp)Cl2. Allen, Locklin, and coworkers computed the disproportionation energies for a series of nickel-based initiators containing bidentate phosphino attendant ligands using density functional theory (DFT) at the B3LYP/DZP level.108 Relative disproportion energies computed for complexes with thiophene or 3-methylthiophene were consistent with the reported trend in the polydispersity of P3HT: broader polydispersity in the order of dppp, dppe, and dppb (1,4-diphenylphosphinobutane).100 This is different from the order of bite angle. Compared to Ni-catalyzed CGCP of thiophene monomer 3, phenylene monomer 34, and N-alkylpyrrole monomer 35, the polymerization of fluorene monomer 36 with Ni(dppp)Cl2 was

Fig. 44 Catalyst-transfer condensation polymerization of fluorene monomers with Ni(acac)2/dppp.

Initiators 1 Kumada–Tamao coupling polymerization. In polymerization with the above catalysts, the chain initiators are dimers formed in situ. The bromine of the dimers supports bidirectional growth, which becomes problematic in one-pot synthesis of block copolymers of A and B, resulting in not only AB diblock but also BAB triblock copolymers. Luscombe112 and Kiriy113 have independently developed synthetic methods for effective initiators, ArNi(dppp)X (X = Cl, Br), which yield P3HT with a narrow molecular weight distribution. Both methods use ligand exchange reaction of the primary ArNi(II)X complex with dppp, although Luscombe and Kiriy generated the primary Ni(II) complex by using Ni(PPh3)4 and Et2Ni(2,2 0 -bipyridine), respectively (Fig. 45). A protected functional group could be introduced into the Ar group of the above initiator by means of the Luscombe method.114 An orthogonal difunctional initiator containing nitroxide was synthesized and used for the preparation of a block copolymer of P3HT and polystyrene.115 Di- and trifunctional Ni initiators were also synthesized, in which an extended phenylene spacer was introduced between the core and the o-bromotolyl group. By using these initiators, V- and Y-shaped P3HT were prepared.116 A similar approach was utilized for the synthesis of conjugated polymers grafted with P3HT from o-chlorotolyl side chains.117 Kiriy and coworkers have further developed a convenient method for the synthesis of initiators, because Et2Ni(2,20 -bipyridine) is not commercially available and is highly reactive to moisture and oxygen, making it difficult to handle.118 o-Tolylmagnesium chloride and 3-hexylthiophenyl-2-magnesium chloride were reacted with Ni(dppe)Cl2 and Ni(dppp)Cl2 at 0 1C to yield initiators 38 and 39, respectively. Monotransmetalation is attributed to steric hindrance of the ortho-substituents to reaction of the second Grignard reagent with 38 and 39. The formation of the

Fig. 45

Synthesis of external Ni initiators for catalyst-transfer Kumada–Tamao coupling polymerization.

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Fig. 46

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Convenient synthetic method for external Ni initiators.

bromo derivatives results from the exchange of a chlorine atom of Ni(L)Cl2 with bromide derived from MgBrCl. These initiators polymerize 3 to afford P3HT bearing the aryl groups of the initiator at one end. An initiator carrying an electron-accepting group was also prepared and this group was introduced as the end group of P3HT (Fig. 46). Recently, Seferos and coworkers reported that static light-scattering studies of P3HT obtained with the external catalyst 38 indicated that 100% external initiation holds true only for polymers with a molecular weight of less than 40 000.119 McNeil and coworkers investigated the effect of ligand electronic properties of the initiator (Fig. 47(a)).120 An electronwithdrawing group made the polymerization of 34 faster, whereas an electron-donating group provided polymers with a narrower molecular weight distribution. Comparison of the rate of a model initiation reaction with the rate of polymerization

Fig. 47 (a) Effect of ligand electronic properties of the initiator. (b) Substituent effects of the aryl group on Ni of the catalyst.

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revealed that initiation is approximately 20 times slower than propagation, despite the fact that they exhibit the same ratedetermining reductive elimination and have similar catalyst resting states. Substituent effects in the aryl group on Ni of the catalysts were also studied in the polymerization reaction of 34 (Fig. 47(b)).121 The reductive elimination rate constant decreased in the order of NMe2 > OMe > CF3 > F. The fastest initiating catalyst (R = NMe2) yielded polymers with the lowest polydispersity whereas the slowest initiating catalyst (R = F) yielded polymers with the highest polydispersity. 2 Suzuki–Miyaura coupling polymerization. Suzuki–Miyaura coupling polymerization also proceeds via a catalyst-transfer mechanism with the external initiator t-Bu3PPd(Ph)Br.122 Welldefined polyfluorene,123,124 poly(p-phenylene),125 and P3HT126 are obtained, and the phenyl group of the initiator is incorporated at the polymer end. Bao and coworkers introduced a functional group on the aryl group of the Pd initiator. The polymerization of thiophene monomer 40 was carried out with this initiator, followed by end-capping with azide-functionalized boronic ester to afford hetero end-functionalized polythiophene with low polydispersity (Fig. 48).127 The azide end-group reacts with DNA via ‘‘click chemistry’’ to form a polythiophene–DNA hybrid structure. Huck and coworkers synthesized a variety of Pd initiators with the t-Bu3P ligand (Fig. 49) and employed a similar approach to obtain hetero end-functionalized polyfluorene.128 The obtained polyfluorene with a donor group at one end and an acceptor group at the other exhibited charge transfer and energy transfer through the polyfluorene backbone. The above Pd initiators are prepared by the reaction of Pd(t-Bu3P)2 with the corresponding aryl bromide and then isolated. The yield is dependent on the functional group on the aryl bromide. Hu and coworkers recently demonstrated that Pd2(dba)3 (dba = dibenzalacetone)/t-Bu3P with ArI for in situ generation of ArPd(t-Bu3P)I is an efficient initiator for Chem. Commun., 2013, 49, 8281--8310

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Fig. 48

Fig. 49

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Synthesis of hetero end-functionalized polythiophene by using functionalized Pd initiator and end-capping with functionalized boronic ester.

A variety of t-Bu3PPd(Ar)Br initiators.

catalyst-transfer Suzuki–Miyaura coupling polymerization.129 Iodobenzene with various substituents (Cl, Br, F, OMe, CO2Et) at the para position can be used for in situ-generated Pd initiators, and well-defined polyfluorenes with these functional groups at one end were obtained. Bromobenzene instead of iodobenzene was less effective, resulting in a broader molecular weight distribution. Well-defined poly(m-phenylene) and poly(p-phenylene) were also obtained with this in situ initiator system. 3 Negishi coupling polymerization. McCullough and co-workers reported CGCP of 2-bromo-3-hexyl-5-cholorozincio monomer with Ni(dppp)Cl2, in which a catalyst-transfer mechanism is presumably involved.130 Koeckelberghs and coworkers recently found that CGCP of the similar zinc monomer 41 with a Pd initiator 42 having Ruphos phosphine ligand proceeded not via the catalyst-transfer mechanism, but through the substituent effect, as mentioned earlier in this review.131 Thus, the polymerization

is initiated by the reaction of 42 and 41, and Pd(0)[Ruphos] dissociates from the growing active center into the solution. However, the electron-donating organozinc of 41 deactivates the C– Br bond for oxidative addition from Pd(0)[Ruphos], which instead undergoes oxidative reinsertion into the C–Br bond at the end of a polymer chain (Fig. 50). This mechanism was proposed on the basis of the fact that the molecular weight was decreased in proportion to the amount of 2-bromo-3-hexylthiophene, which has a nondeactivated C–Br bond. If the catalyst remains complexed with the propagating polymer chain, the amount of 2-bromo-3-hexylthiophene should have no effect. Reaction of fluorene monomer also proceeded in a chain-growth manner when the monomer was slowly added to a solution of the Pd[Ruphos] initiator. Block copolymers P3HT-b-polyfluorene and polyfluorene-b-P3HT were successfully synthesized using this protocol. The order of monomer addition for this copolymerization was unimportant; the polydispersity was almost the same in either case. Metalation of monomer We prepared Grignard thiophene monomer 3 from 2-bromo3-hexyl-5-iodothiophene via iodine–magnesium exchange reaction with i-PrMgCl or t-BuMgCl at 0 1C for 1 h. However, when 2,5-dibromo-3-hexylthiophene was treated with t-BuMgCl at room temperature, it took 20 h to generate Grignard monomer (mixture of 3 and reverse monomer). If the Grignard monomer formation was incomplete and thus unreacted t-BuMgCl remained, P3HT with H/H end groups was generated by the reaction of t-BuMgCl with the polymer end group.132 LiCl was shown to accelerate the Grignard monomer formation, but negatively affected the regioregularity to a small extent because the reverse monomer was polymerized in the presence of LiCl (see Monomers section).

Fig. 50

Chain-growth condensation polymerization of 41 with 42 through substituent effect.

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Grignard fluorene monomer 36 is prepared by treatment of the corresponding bromoiodo precursor with i-PrMgCl in the presence of equimolar LiCl at 20 1C for 1 h (conversion = 98%).109 For the complete and clean conversion of the precursor to 36, a more reactive turbo Grignard reagent Bu2i-PrMgCl133 was needed to react with the precursor at 40 1C for 45 min.131 The dibromo precursor also afforded 36 with i-PrMgCl in the presence of equimolar LiCl at room temperature for 24 h (conversion = 85%). The use of LiOtBu as an additive to i-PrMgCl resulted in the formation of 36 in 90% yield in 8 h, but the bis(chloromagnesio) compound was formed in 7% yield. Bu3MgLi gave similar results, although 36 was formed in 90% yield in 4 h.134 Higashihara et al. reported preparation of a zincate thiophene monomer 43a by treatment of 2-bromo-3-hexyl-5-iodothiophene with t-Bu4ZnLi2 (ref. 135) at 0 1C.136 The polymerization of 43 with Ni(dppe)Cl2 at room temperature was sluggish, but polymerization at 60 1C yielded P3HT with low polydispersity. The feature of this polymerization is tolerance to protonic impurities; as-received THF can be used as a reaction solvent. Poly[3-(6-hydroxyhexyl)thiophene] was directly obtained by the polymerization of hydroxyhexylthiophene zincate monomer 43b (Fig. 51). Dehydrohalogenative polymerization of aryl halides (so-called direct arylation) is an attractive polymerization method, because it does not require preparation of organometallic monomers; waste is reduced and monomer synthesis is easier. Ozawa and coworkers developed condensation polymerization of 2-bromo3-hexylthiophene with Hermann’s catalyst 44 and tris(2-dimethylaminophenyl)phosphine as catalyst precursors at 125 1C, giving P3HT with a relatively narrow polydispersity (Mw/Mn = 1.60) and high regioregularity (98%) in almost quantitative yield (99%) (Fig. 52).137 When the polymerization was carried out in the presence of functionalized aryl bromides (iodides) as capping agents, those aryl groups were introduced at the end of P3HT with high selectivity (86–98%).138 The polymerization proceeds via a two-stage process. Before the monomer is consumed, competitive formation of end-capped Ar-P3HT and non-capped P3HT occurs, and the molecular weight increases linearly with monomer conversion. After the monomer is consumed, the resulting Ar-P3HT and P3HT are coupled with each other to afford Ar-P3HT with a higher molecular weight. Mori and coworkers investigated polymerization of 2-chloro3-substituted thiophenes.139 Grignard monomer was generated in situ by using a stoichiometric amount of magnesium amide,

Fig. 51

Fig. 52 Direct arylation polymerization of 2-bromo-3-hexylthiophene for the synthesis of P3HT.

TMPMgCl 3 LiCl (chloromagnesium 2,2,6,6-tetramethylpiperidide lithium chloride salt), or a combination of a Grignard reagent and a catalytic amount of secondary amine. Ni(dppe)Cl2 was not effective for this polymerization, whereas a nickel catalyst 45 bearing NHC as a ligand induced polymerization with good control of molecular weight and molecular weight distribution (Fig. 53). End-functionalization of polymers McCullough and coworkers introduced functional groups on one or both ends of P3HT by using Grignard reagents.140 Allyl, ethynyl, and vinyl Grignard reagents afford monofunctionalized polythiophenes, whereas aryl and alkyl Grignard reagents yield difunctionalized polythiophenes. Interestingly, quenching with a dimethylsilyl-protected aniline group provides exclusively the monofunctional product.140,141 End group functionality is 80–99%. For introduction of a terminal bromoalkyl group into P3HT, Higashihara and coworkers used 2-(4-bromobutyl)-5-(chloromagnesio)thiophene, which reacted with both ends.142 Recently, Pickel and coworkers investigated the effects of additives and reaction conditions on end group functionalization of P3HT.143 They first comprehensively analyzed all of the products obtained by quenching polymerization with tolylmagnesium chloride at room temperature. The relative concentration of each product was determined by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDITOF MS) from the monoisotopic peak height for each distribution. As a result, P3HT was found to contain 85% tolyl/tolyl, 9% H/tolyl, and 2% Br/tolyl ends. When the functionalization was done in the presence of LiCl and styrene at 0 1C, monofunctional P3HT became the major product (72%).

Catalyst-transfer condensation polymerization of zincate monomers 43 with Ni(dppe)Cl2.

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Fig. 53

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Chain-growth condensation polymerization of 2-chloro-3-substituted thiophenes.

New monomers and new polymerization methods 1 Thiophene monomers. Various substituted thiophene monomers undergo CGCP.144 Herein we describe the reversed thiophene monomer 46, which is a regioisomer of 3. Luscombe and coworkers reported that 46 was not polymerized with Ni(dppp)Cl2.106 This is ascribed to steric hindrance of the hexyl group of 46, interfering with the second transmetalation on the Ni catalyst. This accounts for the formation of a highly regioregular P3HT even under conditions where different regioisomers of the monomer (3 and 46) are formed from 2,5-dibromo3-hexylthiophene with alkyl Grignard reagent.145 Geng146 and Catala147 independently reported that LiCl promoted the polymerization of 46 with Ni(dppp)Cl2 (Fig. 54). The polymerization exhibited living characteristics, but initiation was much slower than propagation, resulting in a large polydispersity and higher Mn than the theoretical value based on the feed ratio of 46 to the catalyst. Catala conducted a kinetic study of the polymerization of both 3 and 46 with Ni(dppp)Cl2 in the presence of LiCl. The polymerization rate constant for 3 in the presence of 4 equivalents of LiCl to monomer showed a constant value 20 times higher than that in the absence of LiCl. For the reverse monomer 46, a similar propagation rate was observed when 4 equivalents of LiCl were added. The addition of LiCl enhances the reactivity of the Grignard species probably by deaggregation and formation of a new complex, but it also modifies the nickel center through halogen exchange reaction. 2 Acceptor monomers. Catalyst-transfer condensation polymerization has been limited to the polymerization of donor monomers for the synthesis of p-type p-conjugated polymers. The catalyst-transfer condensation polymerization of acceptor monomers faces the following difficulties: (1) some electronwithdrawing groups, such as the carbonyl group, in acceptor

Fig. 54

Catalyst-transfer condensation polymerization of reverse monomer 46.

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monomers cannot tolerate the conditions used for formation of the Grignard monomer; (2) the solubility of n-type p-conjugated polymers is generally lower than that of p-type ones, because acceptor aromatics have stronger p–p stacking interactions than donor aromatics do; (3) the weaker p-donation of the n-type polymer backbone to zero-valent metal catalyst may not sufficiently assist intramolecular catalyst transfer. We set out to explore the catalyst-transfer condensation polymerization of pyridine monomers as the acceptor monomer. Kumada–Tamao coupling polymerization of 2-bromo-5-chloromagnesio-3-(2-(2-methoxyethoxy)ethoxy)pyridine 47 with a Ni catalyst was first carried out.148 However, the polymer was precipitated during the polymerization (Fig. 55(a)), although the obtained polymer was soluble in dichloromethane and chloroform. To increase the solubility of polypyridine, we changed the polymerization position of this monomer unit from the 2,5-position (para type) to the 3,5-position (meta type) while retaining the same side chain. The polymerization of meta type monomer 48

Fig. 55

Polymerization of pyridine monomers 47 and 48 with Ni(dppp)Cl2.

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with Ni(dppp)Cl2 in the presence of LiCl showed chain-growth polymerization behavior (Fig. 55(b)).149 On the basis of analysis of the polymer end groups by means of MALDI-TOF MS and model reactions, this polymerization was concluded to proceed via a catalyst-transfer polymerization mechanism. Since poly48 is a nonconjugated polymer, we investigated polymerization of another para-type monomer 49 with Ni(dppp)Cl2 for the synthesis of well-defined n-type p-conjugated polymers.150 A soluble polymer was obtained, but the molecular weight distribution was broad (Mn = 10 300, Mw/Mn = 4.35). MALDITOF MS showed that the Br/Br-ended polymer was the major product from the early stage to the final stage and that the polymer with Br/H was a minor product. This result indicated that disproportionation proceeded from the initial stage of polymerization. Thus, the polymerization of 49 with Ni(dppp)Cl2 proceeds essentially via an intramolecular catalyst-transfer polymerization mechanism to afford a polymer with Br/H ends after hydrolysis of the polymer–Ni–Br complex, but disproportionation reaction continually occurs to afford polymer with Br/Br ends, as well as Ni(II) and Ni(0) complexes (Fig. 56). The tendency for occurrence of disproportionation in the polymerization of pyridine monomer 49 can presumably be attributed to coordination of the nitrogen in the pyridine–Ni–Br end to the Ni in another pyridine–Ni–Br end, as shown in complex X in Fig. 56. Suzuki–Miyaura coupling polymerization of a pinacol boronate monomer, having the same substituted pyridine structure, with o-tolylPd(tBu3P)Br was also accompanied by disproportionation. 3 Biaryl and triaryl monomers. Alternating copolymers of donor and acceptor arenes have been shown to outperform P3HT when incorporated into solar cells, because the optical

Fig. 56

Proposed mechanism of polymerization of 49 with Ni(dppp)Cl2.

Fig. 57

Chain-growth condensation polymerization of 50.

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bandgaps are reduced and hence a broader solar spectrum can be absorbed. These polymers are generally synthesized by metal-catalyzed step-growth polycondensation.151 Huck and Kiriy investigated the chain-growth polymerization of AB type monomers consisting of strong acceptor arene and weak donor arene. Solubilizing side chains are introduced into the donor unit of the monomers. AB type monomer 50 composed of fluorene boronic acid ester and bromobenzothiadiazole was synthesized, and Suzuki–Miyaura coupling polymerization with ArPd(tBu3P)Br (Ar = Ph and pyrenyl) was conducted in the presence of CsF and crown ether.152 The molecular weight of the obtained polymer increased with reaction time (Mn = 3300–7400), and the molecular weight distribution remained narrow (Mw/Mn o 1.27), indicating a chain-growth polymerization mechanism. Furthermore, MALDI-TOF MS showed two series of peaks corresponding to the polymer with Ar/Br and Ar/H ends. The presence of an aryl group on every polymer chain strongly supports the involvement of chain-growth polymerization from the Pd complex initiator (Fig. 57). Symmetrical dibromo monomer 51, consisting of thiophene, naphthalenediimide, and thiophene, was also investigated.153 The first attempt to form a Grignard monomer from 51 for Kumada–Tamao coupling polymerization failed. Activated Zn was next reacted with 51 for the generation of an organozinc monomer. Remarkably, however, acidic work-up of the prepared 51/Zn complex resulted in recovery of 51, but not the hydrized monobromo compound. Electron paramagnetic resonance measurements revealed that the 51/Zn complex was a radical anion; single electron transfer had occurred from Zn to the electron-deficient 51. This radical anion was polymerized

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Feature Article with Ni(dppe)Br2 or PhNi(dppe)Br at room temperature. The polymerization behavior showed a chain-growth polymerization mechanism: the molecular weight increased with increasing feed ratio of the monomer to the Ni catalyst, with retention of low polydispersity (Mn = 25 000–104 000, Mw/Mn = 1.3–1.7), and the phenyl group was introduced into the polymer end group when PhNi(dppe)Br was used (Fig. 58). The following mechanism has been proposed on the basis of NMR studies and model reactions (Fig. 59).154 When Ni(dppe)Br2 is used as a catalyst, it is reduced by 51/Zn (Br–Ar–Br/Zn) in a two-electron transfer process to give the coordination-unsaturated Ni(0) complex, Ni(dppe). The latter complex associates with the monomer residue Br–Ar–Br to give (Z2-Br–Ar–Br)(dppe)Ni0, which rearranges via oxidative addition into Br–Ar–Ni(dppe)–Br. The latter species acts as an initiator in the polymerization of 51/Zn.

ChemComm The reduction of Ni(dppe)Br2 and the formation of Ni(0) complexes were confirmed by model experiments. The chain propagation starts with a single-electron redox process between the chain end, (Ar)n–Ni(dppe)–Br (or the initiator Br–Ar–Ni(dppe)–Br), and the monomer molecule, Br–Ar–Br/Zn. Presumably, the reduction leads to a Ni(I) species, (Ar)n–Ni(dppe), which oxidatively adds to the C–Br bond of the anion-radical monomer followed by elimination of the bromide anion, leading to (Ar)n–Ni(dppe)–Ar–Br. Such a mechanism is analogous to the mechanism of reductive coupling of aryl halides, investigated by Amatore and Jutand.155 Thus formed (Ar)n–Ni(dppe)–Ar–Br rearranges via reductive elimination followed by intramolecular oxidative addition to afford (Ar)n+1–Ni(dppe)–Br. The Ni(0) catalyst resting intermediates were observed by 31P NMR spectroscopy during this Ni-catalyzed polymerization of 51/Zn.

Fig. 58

Chain-growth condensation polymerization of 51.

Fig. 59

Proposed mechanism of catalyst-transfer condensation polymerization of 51/Zn with Ni(dppe)Br2.

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Fig. 60 Catalyst-transfer condensation polymerization of biaryl monomer 52 with Ni(dppp)Cl2.

Furthermore, DFT calculations predict remarkable stability of the naphthalene diimide-based Ni(0) complexes. Bielawski and coworkers conducted Kumada–Tamao coupling polymerization of a biaryl monomer 52 composed of thiophene and phenylene (Fig. 60).156 When the polymerization was carried out with Ni(dppp)Cl2 in the presence of LiCl, poly(thiophene-alt-pphenylene) with controlled molecular weight (Mn = 6400–39 000) and narrow polydispersity (Mw/Mn r 1.33) was obtained. MALDITOF MS showed that the major population of the polymer contained H/Br end groups, as would be expected for efficient catalyst transfer during polymerization with minimal occurrence of chain termination. The block copolymer of poly52 and P3HT was also obtained both by addition of thiophene monomer 3 to poly52 and by addition of 52 to P3HT. However, the former method yielded a copolymer with a narrower molecular weight distribution (Mw/Mn = 1.33) than did the latter method (Mw/Mn = 1.70).

Fig. 61 Catalyst-transfer Stille coupling polymerization of 53 for the synthesis of poly(p-phenyleneethynylene).

Fig. 62

4 Miscellaneous polymerization methods. Stille coupling polymerization is another candidate for catalyst-transfer condensation polymerization. The first report, by Bielawski and coworkers, described the polymerization of a stannylated 4-iodophenyleneacetylene 53 with t-Bu3PPd(Ph)Br to afford poly(p-phenyleneethynylene) (PPE) (Fig. 61).157 A key feature of this polymerization is that both CuI and additional Ph3P are necessary to obtain a high-molecular-weight polymer. Under optimized conditions, the molecular weight of the polymer increased linearly with monomer consumption, and was controlled by adjusting the initial monomer-to-catalyst ratio. The chain-growth nature of the polymerization reaction was utilized to produce well-defined block copolymers of PPE and poly(fluorenylethynylene), as well as to grow PPE brushes from silica nanoparticles via a surfaceinitiated polymerization process. Swager and coworkers reported Lewis acid-promoted CGCP of 2-chloroalkylenedioxythiophenes 54 (Fig. 62).158 A number of Lewis acids were screened, and polymerization with SnCl4 in ortho-dichlorobenzene at 120 1C gave the best performance. The polymerization does not involve a catalyst-transfer mechanism, but rather proceeds through a cationic chain-growth mechanism, in which 54 reacts with the polymer terminal cation. They proposed that HCl elimination from the repeat adduct unit takes place during work-up. By taking advantage of the cationic polymer end, thiophene–thiophene block copolymers and thiophene–vinyl ether block copolymers were synthesized. Okamoto and coworkers discovered a novel chain-growth [2+2+2] cycloaddition polymerization of yne–diyne monomers with a Co catalyst, although this polymerization is not classified as a condensation polymerization.159 Yne–diyne 55 polymerized with dipimp/CoCl26H2O/Zn [dipimp = 2-(2,6-diisopropylphenyl)iminomethylpyridine] in the presence of an electron-deficient alkyne such as dimethyl acetylenedicarboxylate as an activator of the catalyst at 50 1C to afford a polymer containing benzene rings in the backbone, with a narrow molecular weight distribution

Lewis acid-promoted condensation polymerization of chlorothiophene 54.

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Fig. 63

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Chain-growth [2+2+2] cycloaddition polymerization of yne–diyne monomer 55.

(Mw/Mn = 1.22–1.28). The increase in Mn was proportional to the monomer conversion. They proposed that this chain-growth polymerization involves a catalyst-transfer mechanism (Fig. 63). The cobalt catalyst may be intramolecularly transferred from the [4+2] cycloaddition intermediate to the diyne termini simultaneously with its reductive elimination when the ligand (dipimp) no longer participates in the propagation process. At the initiation stage, concurrent addition of the reaction components is essential to attain controlled polymerization. Thus, the addition of a highly reactive electron-deficient alkyne, which undergoes cyclotrimerization much more quickly than the monomer(s) employed, might be effective for the rapid generation of an active catalyst.

Conclusions

and applied research focusing on the synthesis of well-defined donor–acceptor low band gap polymers for photovoltaics are likely to be key areas of investigation.

Notes and references 1 2 3 4 5 6 7 8 9 10

We have reviewed recent developments in the areas of CGCP utilizing the substituent effect and CGCP via catalyst transfer. The former approach has afforded many architectures containing aromatic polyamides and aromatic polyethers. Future research efforts should be directed toward the development of materials with novel properties arising from synergy between aromatic polymers and conventional polymers. For example, block copolymers of aromatic and conventional polymers can serve as compatibility-enhancing agents for immiscible polymer blends of engineering plastics and conventional coil polymers. The latter approach is generally at an earlier stage of development, although the mechanism, catalysts, and initiators of Ni- and Pd-catalyzed coupling polymerizations leading to poly(alkylthiophene)s and poly(p-phenylene)s have been extensively investigated. Other well-defined p-conjugated polymers, such as polyfluorenes, n-type polymers, and alternating aryl polymers, have also been synthesized by means of catalysttransfer condensation polymerization. For the immediate future, basic studies of catalyst transfer on p-conjugated polymers 8308

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G. Odian, Principles of Polymerization, Wiley, 4th edn, 2004. W. H. Carothers, Trans. Faraday Soc., 1936, 32, 39–53. P. J. Flory, J. Am. Chem. Soc., 1936, 58, 1877–1885. P. J. Flory, Chem. Rev., 1946, 39, 137–197. R. W. Lenz, C. E. Handlovits and H. A. Smith, J. Polym. Sci., 1962, 58, 351–367. A. B. Newton and J. B. Rose, Polymer, 1972, 13, 465–474. J. R. Lovering, J. H. Ridd, D. G. Parker and J. B. Rose, J. Chem. Soc., Perkin Trans. 2, 1988, 1735–1738. D. B. Hibbert, J. P. B. Sandall, J. R. Lovering, J. H. Ridd and T. I. Yousaf, J. Chem. Soc., Perkin Trans. 2, 1988, 1739–1742. D. R. Robello, A. Ulman and E. J. Urankar, Macromolecules, 1993, 26, 6718–6721. W. Risse, W. Heitz, D. Freitag and L. Bottenbruch, Makromol. Chem., 1985, 186, 1835–1853. W. Koch, W. Risse and W. Heitz, Makromol. Chem., Suppl., 1985, 12, 105–123. V. Percec and J. H. Wang, J. Polym. Sci., Part A: Polym. Chem., 1991, 29, 63–82. V. Percec and T. D. Shaffer, J. Polym. Sci., Part C: Polym. Lett., 1986, 24, 439–446. V. Percec and J. H. Wang, Polym. Bull., 1990, 24, 493–500. J. H. Wang and V. Percec, Polym. Bull., 1991, 25, 33–40. T. Yokozawa, T. Asai, R. Sugi, S. Ishigooka and S. Hiraoka, J. Am. Chem. Soc., 2000, 122, 8313–8314. A. Yokoyama, R. Miyakoshi and T. Yokozawa, Macromolecules, 2004, 37, 1169–1171. R. Miyakoshi, A. Yokoyama and T. Yokozawa, Macromol. Rapid Commun., 2004, 25, 1663–1666. M. C. Iovu, E. E. Sheina, R. R. Gil and R. D. McCullough, Macromolecules, 2005, 38, 8649–8656. R. Miyakoshi, A. Yokoyama and T. Yokozawa, J. Am. Chem. Soc., 2005, 127, 17542–17547. T. Yokozawa and A. Yokoyama, Chem. Rev., 2009, 109, 5595–5619. T. Yokozawa, in Polymer Science: A Comprehensive Reference, ed. ¨ller, Elsevier B.V., Amsterdam, 2012, K. Matyjaszewski and M. Mo pp. 115–139. T. Yokozawa, D. Muroya, R. Sugi and A. Yokoyama, Macromol. Rapid Commun., 2005, 26, 979–981. This journal is

c


View Article Online


ChemComm

Feature Article

24 K. Yoshino, K. Hachiman, A. Yokoyama and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 1357–1363. 25 K. Mikami, H. Daikuhara, J. Kasama, A. Yokoyama and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2011, 49, 3020–3029. 26 R. Sugi, A. Yokoyama, T. Furuyama, M. Uchiyama and T. Yokozawa, J. Am. Chem. Soc., 2005, 127, 10172–10173. 27 T. Ohishi, R. Sugi, A. Yokoyama and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2006, 44, 4990–5003. 28 R. Sugi, T. Ohishi, A. Yokoyama and T. Yokozawa, Macromol. Rapid Commun., 2006, 27, 716–721. 29 T. Ohishi, T. Suzuki, T. Niiyama, K. Mikami, A. Yokoyama, K. Katagiri, I. Azumaya and T. Yokozawa, Tetrahedron Lett., 2011, 52, 7067–7070. 30 K. Takagi, S. Sugimoto, R. Yamakado and K. Nobuke, J. Org. Chem., 2011, 76, 2471–2478. 31 F. Campbell and A. J. Wilson, Tetrahedron Lett., 2009, 50, 2236–2238. 32 T. Niiyama, A. Yokoyama and T. Yokozawa, Polym. Prepr. Jpn., 2006, 55, 283. 33 A. Yokoyama, M. Karasawa, M. Taniguchi and T. Yokozawa, Chem. Lett., 2013, 641–642. 34 A.-L. Chen, K.-L. Wei, R.-J. Jeng, J.-J. Lin and S. A. Dai, Macromolecules, 2011, 44, 46–59. 35 T. Yokozawa, Y. Suzuki and S. Hiraoka, J. Am. Chem. Soc., 2001, 123, 9902–9903. 36 Y. J. Kim, M. Seo and S. Y. Kim, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 1049–1057. 37 J. Heo, Y. J. Kim, M. Seo, S. Shin and S. Y. Kim, Chem. Commun., 2012, 48, 3351–3353. 38 C. H. Honeyman, I. Manners, C. T. Morrissey and H. R. Allcock, J. Am. Chem. Soc., 1995, 117, 7035–7036. 39 H. R. Allcock, C. A. Crane, C. T. Morrissey, J. M. Nelson, S. D. Reeves, C. H. Honeyman and I. Manners, Macromolecules, 1996, 29, 7740–7747. 40 V. Blackstone, A. J. Lough, M. Murray and I. Manners, J. Am. Chem. Soc., 2009, 131, 3658–3667. 41 V. Blackstone, S. Pfirrmann, H. Helten, A. Staubitz, A. Presa Soto, G. R. Whittell and I. Manners, J. Am. Chem. Soc., 2012, 134, 15293–15296. `res and M. Destarac, Polymer, 2012, 53, 42 A. Sandeau, S. Mazie 5601–5618. 43 S. Kim, Y. Kakuda, A. Yokoyama and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 3129–3133. 44 T. Masukawa, A. Yokoyama and T. Yokozawa, Macromol. Rapid Commun., 2009, 30, 1413–1418. 45 C.-F. Huang, A. Yokoyama and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 2948–2954. 46 C.-F. Huang, Y. Ohta, A. Yokoyama and T. Yokozawa, Macromolecules, 2011, 44, 4140–4148. 47 N. Ajioka, Y. Suzuki, A. Yokoyama and T. Yokozawa, Macromolecules, 2007, 40, 5294–5300. 48 N. Ajioka, A. Yokoyama and T. Yokozawa, Macromol. Rapid Commun., 2008, 29, 665–671. 49 Y. Li, Y. Tang, R. Narain, A. L. Lewis and S. P. Armes, Langmuir, 2005, 21, 9946–9954. 50 D. J. Adams and I. Young, J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 6082–6090. 51 R. Sugi, Y. Hitaka, A. Yokoyama and T. Yokozawa, Macromolecules, 2005, 38, 5526–5531. 52 K. Yoshino, A. Yokoyama and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 6328–6332. 53 K. Yoshino, A. Yokoyama and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2011, 49, 986–994. 54 T. Ohishi, T. Masukawa, S. Fujii, A. Yokoyama and T. Yokozawa, Macromolecules, 2010, 43, 3206–3214. 55 Y. Yamazaki, N. Ajioka, A. Yokoyama and T. Yokozawa, Macromolecules, 2009, 42, 606–611. 56 Y. Yamazaki, A. Yokoyama and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2012, 50, 3648–3655. 57 T. Yokozawa, T. Taniguchi, Y. Suzuki and A. Yokoyama, J. Polym. Sci., Part A: Polym. Chem., 2002, 40, 3460–3464. 58 J. Park, M. Seo, H. Choi and S. Y. Kim, Polym. Chem., 2011, 2, 1174–1179. 59 J. Park, J. Kim, M. Seo, J. Lee and S. Y. Kim, Chem. Commun., 2012, 48, 10556–10558. 60 J. Park, M. Moon, M. Seo, H. Choi and S. Y. Kim, Macromolecules, 2010, 43, 8304–8313. This journal is

c


61 Y. Yamashita, Y. Chujo, H. Kobayashi and Y. Kawakami, Polym. Bull., 1981, 5, 361–366. 62 Y. Nagase, S. Mori, M. Egawa and K. Matsui, Die Makromolekulare Chemie, 1990, 191, 2413–2421. 63 M. Sato, M. Yanahara, T. Kobayashi and K.-I. Mukaida, Die Makromolekulare Chemie, 1992, 193, 991–999. 64 M. Sato, T. Mangyo and K.-I. Mukaida, Macromol. Chem. Phys., 1995, 196, 1791–1799. 65 R. Prange, S. D. Reeves and H. R. Allcock, Macromolecules, 2000, 33, 5763–5765. 66 H. R. Allcock, C. R. de Denus, R. Prange and W. R. Laredo, Macromolecules, 2001, 34, 2757–2765. 67 H. R. Allcock, E. S. Powell, A. E. Maher and E. B. Berda, Macromolecules, 2004, 37, 5824–5829. 68 A. A. Stefopoulos, C. L. Chochos, G. Bokias and J. K. Kallitsis, Langmuir, 2008, 24, 11103–11110. 69 Y. Ohta, T. Shirakura, A. Yokoyama and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 1887–1892. 70 R. Sugi, D. Tate and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 2725–2729. 71 Y. H. Kim, J. Polym. Sci., Part A: Polym. Chem., 1998, 36, 1685–1698. 72 B. Voit, J. Polym. Sci., Part A: Polym. Chem., 2000, 38, 2505–2525. 73 M. Jikei and M.-a. Kakimoto, J. Polym. Sci., Part A: Polym. Chem., 2004, 42, 1293–1309. 74 C. Gao and D. Yan, Prog. Polym. Sci., 2004, 29, 183–275. 75 B. I. Voit and A. Lederer, Chem. Rev., 2009, 109, 5924–5973. 76 Y. Segawa, T. Higashihara and M. Ueda, Polym. Chem., 2013, 4, 1746–1759. 77 Y. Ohta, S. Fujii, A. Yokoyama, T. Furuyama, M. Uchiyama and T. Yokozawa, Angew. Chem., Int. Ed., 2009, 48, 5942–5945. 78 Y. Ohta, Y. Kamijyo, S. Fujii, A. Yokoyama and T. Yokozawa, Macromolecules, 2011, 44, 5112–5122. 79 T. Hirai, L. Huan, Y. Ohta, T. Yokozawa and K. Tanaka, Chem. Lett., 2011, 366–367. 80 Y. Ohta, Y. Kamijyo, A. Yokoyama and T. Yokozawa, Polymers, 2012, 4, 1170–1182. 81 Y. Ohta, T. Kanou, A. Yokoyama and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 3762–3766. 82 A. Tanatani, A. Yokoyama, I. Azumaya, Y. Takakura, C. Mitsui, M. Shiro, M. Uchiyama, A. Muranaka, N. Kobayashi and T. Yokozawa, J. Am. Chem. Soc., 2005, 127, 8553–8561. 83 A. Itai, Y. Toriumi, N. Tomioka, H. Kagechika, I. Azumaya and K. Shudo, Tetrahedron Lett., 1989, 30, 6177–6180. 84 K. Yamaguchi, G. Matsumura, H. Kagechika, I. Azumaya, Y. Ito, A. Itai and K. Shudo, J. Am. Chem. Soc., 1991, 113, 5474–5475. 85 I. Azumaya, H. Kagechika, K. Yamaguchi and K. Shudo, Tetrahedron, 1995, 51, 5277–5290. 86 M. M. Green, M. P. Reidy, R. D. Johnson, G. Darling, D. J. O’Leary and G. Willson, J. Am. Chem. Soc., 1989, 111, 6452–6454. 87 M. M. Green, B. A. Garetz, B. Munoz, H. Chang, S. Hoke and R. G. Cooks, J. Am. Chem. Soc., 1995, 117, 4181–4182. 88 A. Yokoyama, Y. Inagaki, T. Ono and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2011, 49, 1387–1395. 89 K. Mikami, A. Tanatani, A. Yokoyama and T. Yokozawa, Macromolecules, 2009, 42, 3849–3851. 90 K. Mikami, H. Daikuhara, Y. Inagaki, A. Yokoyama and T. Yokozawa, Macromolecules, 2011, 44, 3185–3188. 91 K. Mikami and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 739–742. 92 R. Tkachov, V. Senkovskyy, H. Komber, J.-U. Sommer and A. Kiriy, J. Am. Chem. Soc., 2010, 132, 7803–7810. 93 P. Kohn, S. Huettner, H. Komber, V. Senkovskyy, R. Tkachov, A. Kiriy, R. H. Friend, U. Steiner, W. T. S. Huck, J.-U. Sommer and M. Sommer, J. Am. Chem. Soc., 2012, 134, 4790–4805. 94 M. Verswyvel, F. Monnaie and G. Koeckelberghs, Macromolecules, 2011, 44, 9489–9498. 95 H. Komber, V. Senkovskyy, R. Tkachov, K. Johnson, A. Kiriy, W. T. S. Huck and M. Sommer, Macromolecules, 2011, 44, 9164–9172. 96 E. L. Lanni and A. J. McNeil, Macromolecules, 2010, 43, 8039–8044. 97 E. L. Lanni and A. J. McNeil, J. Am. Chem. Soc., 2009, 131, 16573–16579. 98 Z. J. Bryan and A. J. McNeil, Chem. Sci., 2013, 4, 1620–1624. 99 S. Murarka and A. Studer, Angew. Chem., Int. Ed., 2012, 51, 12362–12366. 100 R. Miyakoshi, A. Yokoyama and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 753–765. Chem. Commun., 2013, 49, 8281--8310

8309

View Article Online


Feature Article 101 Y. X. Mao, Y. Wang and B. L. Lucht, J. Polym. Sci., Part A: Polym. Chem., 2004, 42, 5538–5547. 102 R. Miyakoshi, K. Shimono, A. Yokoyama and T. Yokozawa, J. Am. Chem. Soc., 2006, 128, 16012–16013. 103 A. Yokoyama, A. Kato, R. Miyakoshi and T. Yokozawa, Macromolecules, 2008, 41, 7271–7273. 104 E. L. Lanni, J. R. Locke, C. M. Gleave and A. J. McNeil, Macromolecules, 2011, 44, 5136–5145. 105 Z. J. Bryan, M. L. Smith and A. J. McNeil, Macromol. Rapid Commun., 2012, 33, 842–847. 106 S. D. Boyd, A. K. Y. Jen and C. K. Luscombe, Macromolecules, 2009, 42, 9387–9389. 107 R. Miyakoshi, A. Yokoyama and T. Yokozawa, Chem. Lett., 2008, 1022–1023. 108 J. A. Bilbrey, S. K. Sontag, N. E. Huddleston, W. D. Allen and J. Locklin, ACS Macro Lett., 2012, 1, 995–1000. 109 L. Huang, S. P. Wu, Y. Qu, Y. H. Geng and F. S. Wang, Macromolecules, 2008, 41, 8944–8947. 110 A. E. Javier, S. R. Varshney and R. D. McCullough, Macromolecules, 2010, 43, 3233–3237. 111 A. Sui, X. Shi, S. Wu, H. Tian, Y. Geng and F. Wang, Macromolecules, 2012, 45, 5436–5443. 112 H. A. Bronstein and C. K. Luscombe, J. Am. Chem. Soc., 2009, 131, 12894–12895. 113 V. Senkovskyy, R. Tkachov, T. Beryozkina, H. Komber, U. Oertel, M. Horecha, V. Bocharova, M. Stamm, S. A. Gevorgyan, F. C. Krebs and A. Kiriy, J. Am. Chem. Soc., 2009, 131, 16445–16453. 114 A. Smeets, K. Van den Bergh, J. De Winter, P. Gerbaux, T. Verbiest and G. Koeckelberghs, Macromolecules, 2009, 42, 7638–7641. 115 E. Kaul, V. Senkovskyy, R. Tkachov, V. Bocharova, H. Komber, M. Stamm and A. Kiriy, Macromolecules, 2010, 43, 77–81. 116 M. Yuan, K. Okamoto, H. A. Bronstein and C. K. Luscombe, ACS Macro Lett., 2012, 1, 392–395. 117 J. Wang, C. Lu, T. Mizobe, M. Ueda, W.-C. Chen and T. Higashihara, Macromolecules, 2013, 46, 1783–1793. 118 V. Senkovskyy, M. Sommer, R. Tkachov, H. Komber, W. T. S. Huck and A. Kiriy, Macromolecules, 2010, 43, 10157–10161. 119 M. Wong, J. Hollinger, L. M. Kozycz, T. M. McCormick, Y. Lu, D. C. Burns and D. S. Seferos, ACS Macro Lett., 2012, 1, 1266–1269. 120 S. R. Lee, Z. J. Bryan, A. M. Wagner and A. J. McNeil, Chem. Sci., 2012, 3, 1562–1566. 121 S. R. Lee, J. W. G. Bloom, S. E. Wheeler and A. J. McNeil, Dalton Trans., 2013, 42, 4218–4222. 122 J. P. Stambuli, C. D. Incarvito, M. Buhl and J. F. Hartwig, J. Am. Chem. Soc., 2004, 126, 1184–1194. 123 A. Yokoyama, H. Suzuki, Y. Kubota, K. Ohuchi, H. Higashimura and T. Yokozawa, J. Am. Chem. Soc., 2007, 129, 7236–7237. 124 A. Gutacker, C.-Y. Lin, L. Ying, T.-Q. Nguyen, U. Scherf and G. C. Bazan, Macromolecules, 2012, 45, 4441–4446. 125 T. Yokozawa, H. Kohno, Y. Ohta and A. Yokoyama, Macromolecules, 2010, 43, 7095–7100. 126 T. Yokozawa, R. Suzuki, M. Nojima, Y. Ohta and A. Yokoyama, Macromol. Rapid Commun., 2011, 32, 801–806. 127 J. K. Lee, S. Ko and Z. Bao, Macromol. Rapid Commun., 2012, 33, 938–942. 128 E. Elmalem, F. Biedermann, K. Johnson, R. H. Friend and W. T. S. Huck, J. Am. Chem. Soc., 2012, 134, 17769–17777. 129 H.-H. Zhang, C.-H. Xing and Q.-S. Hu, J. Am. Chem. Soc., 2012, 134, 13156–13159.

8310

Chem. Commun., 2013, 49, 8281--8310

ChemComm 130 E. E. Sheina, J. S. Liu, M. C. Iovu, D. W. Laird and R. D. McCullough, Macromolecules, 2004, 37, 3526–3528. 131 M. Verswyvel, P. Verstappen, L. De Cremer, T. Verbiest and G. Koeckelberghs, J. Polym. Sci., Part A: Polym. Chem., 2011, 49, 5339–5349. 132 R. H. Lohwasser and M. Thelakkat, Macromolecules, 2011, 44, 3388–3397. 133 K. Kitagawa, A. Inoue, H. Shinokubo and K. Oshima, Angew. Chem., Int. Ed., 2000, 39, 2481–2483. 134 M. C. Stefan, A. E. Javier, I. Osaka and R. D. McCullough, Macromolecules, 2009, 42, 30–32. 135 M. Uchiyama, T. Furuyama, M. Kobayashi, Y. Matsumoto and K. Tanaka, J. Am. Chem. Soc., 2006, 128, 8404–8405. 136 T. Higashihara, E. Goto and M. Ueda, ACS Macro Lett., 2012, 1, 167–170. 137 Q. Wang, R. Takita, Y. Kikuzaki and F. Ozawa, J. Am. Chem. Soc., 2010, 132, 11420–11421. 138 Q. Wang, M. Wakioka and F. Ozawa, Macromol. Rapid Commun., 2012, 33, 1203–1207. 139 S. Tamba, K. Shono, A. Sugie and A. Mori, J. Am. Chem. Soc., 2011, 133, 9700–9703. 140 M. Jeffries-El, G. Sauve and R. D. McCullough, Macromolecules, 2005, 38, 10346–10352. 141 A. Takahashi, Y. Rho, T. Higashihara, B. Ahn, M. Ree and M. Ueda, Macromolecules, 2010, 43, 4843–4852. 142 H. Fujita, T. Michinobu, M. Tokita, M. Ueda and T. Higashihara, Macromolecules, 2012, 45, 9643–9656. 143 W. M. Kochemba, S. M. Kilbey and D. L. Pickel, J. Polym. Sci., Part A: Polym. Chem., 2012, 50, 2762–2769. 144 T. Yokozawa, in Conjugated Polymer Synthesis, ed. Y. Chujo, WileyVCH, Weinheim, 2010, pp. 35–58. 145 R. D. McCullough, R. D. Lowe, M. Jayaraman and D. L. Anderson, J. Org. Chem., 1993, 58, 904–912. 146 S. Wu, L. Huang, H. Tian, Y. Geng and F. Wang, Macromolecules, 2011, 44, 7558–7567. 147 J. P. Lamps and J. M. Catala, Macromolecules, 2011, 44, 7962–7968. 148 Y. Nanashima, A. Yokoyama and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2012, 50, 1054–1061. 149 Y. Nanashima, A. Yokoyama and T. Yokozawa, Macromolecules, 2012, 45, 2609–2613. 150 Y. Nanashima, R. Shibata, R. Miyakoshi, A. Yokoyama and T. Yokozawa, J. Polym. Sci., Part A: Polym. Chem., 2012, 50, 3628–3640. 151 H. Zhou, L. Yang and W. You, Macromolecules, 2012, 45, 607–632. 152 E. Elmalem, A. Kiriy and W. T. S. Huck, Macromolecules, 2011, 44, 9057–9061. 153 V. Senkovskyy, R. Tkachov, H. Komber, M. Sommer, M. Heuken, B. Voit, W. T. S. Huck, V. Kataev, A. Petr and A. Kiriy, J. Am. Chem. Soc., 2011, 133, 19966–19970. 154 V. Senkovskyy, R. Tkachov, H. Komber, A. John, J.-U. Sommer and A. Kiriy, Macromolecules, 2012, 45, 7770–7777. 155 C. Amatore and A. Jutand, Organometallics, 1988, 7, 2203–2214. 156 R. J. Ono, S. Kang and C. W. Bielawski, Macromolecules, 2012, 45, 2321–2326. 157 S. Kang, R. J. Ono and C. W. Bielawski, J. Am. Chem. Soc., 2013, 135, 4984–4987. 158 B. Bonillo and T. M. Swager, J. Am. Chem. Soc., 2012, 134, 18916–18919. 159 Y. Sugiyama, R. Kato, T. Sakurada and S. Okamoto, J. Am. Chem. Soc., 2011, 133, 9712–9715.

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