Review pubs.acs.org/CR

Polymeric Surfactants: Synthesis, Properties, and Links to Applications Patrizio Raffa,†,‡ Diego Armando Zakarias Wever,†,‡ Francesco Picchioni,† and Antonius A. Broekhuis*,† †

Department of Chemical Engineering−Product Technology, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands ‡ Dutch Polymer Institute DPI, P.O. Box 902, 5600 AX Eindhoven, The Netherlands Abbreviations References

1. INTRODUCTION It is well-known that surfactants are molecules able to modify the surface properties of the liquid in which they are introduced (usually water). The simultaneous presence of both hydrophilic and hydrophobic parts in the same molecule has been recognized as the key feature of these systems, which is responsible for their characteristic properties in solution. These include adsorption at interfaces and self-assembly into a variety of micellar aggregates above a critical value of concentration, called critical micelle concentration (CMC). Indeed, the term “solution” in this respect has to be used carefully: it ceases to be correct above CMC, since the surfactant is no longer homogeneously dispersed at a molecular level into the water phase. By structural analogy with their low molecular weight counterparts, macromolecules containing both hydrophilic and hydrophobic parts can be generally referred to as polymeric surfactants. This general definition includes most of the macromolecular systems that in the literature are also described as amphiphilic polymers, micellar polymers, hydrophobically modified water-soluble polymers, associative polymers, and related terms. If the hydrophilic part is charged, they can fall under the categories of polyelectrolytes or polyampholytes (in the latter case, when the hydrophilic part contains simultaneously negative and positive charges). Compared to low-molecularweight surfactants, polymeric ones offer a much higher structural complexity (for example, in the number and distribution of hydrophilic and hydrophobic moieties along the chain), which can result in very different behavior. It is interesting to note that several natural polymers are, in fact, polymeric surfactants.1,2 The most notable example is represented by proteins. Even though they are generally not considered from this point of view, in many cases proteins are found in natural systems as emulsion stabilizers (e.g., casein in milk). The other main class of naturally occurring polymeric surfactants is polysaccharides. Emulsan (a lipopolysaccharide) and chitosan are notable examples.3 It is usually very difficult to isolate polymeric surfactants from natural sources, and their structures and compositions can vary depending on the source; therefore, most of the systems studied and reported in the literature are synthetic.

CONTENTS 1. Introduction 2. Classification of Polymeric Surfactants 2.1. Polysoaps 2.2. Macrosurfactants 2.3. Complex Architectures 3. Synthetic Methods 3.1. Synthesis of Macrosurfactants 3.1.1. Polyacrylic and Methacrylic Acid Blocks 3.1.2. Aromatic Sulfonate Blocks 3.1.3. Polyvinylpyridines and Quaternized Arylamines Blocks 3.1.4. Poly(ethylene glycol)/Poly(ethylene oxide) Blocks 3.1.5. Alkylaminoacrylate Blocks 3.1.6. Other Water-Soluble Acrylate Blocks 3.2. Synthesis of Polysoaps 3.2.1. Polyacrylamides and Related 3.2.2. Hydrophobically Modified Polysaccharides 3.3. Synthesis of Complex Polymeric Surfactants 4. Properties of Polymeric Surfactants 4.1. Surface Properties 4.1.1. Macrosurfactants 4.1.2. Polysoaps 4.2. Rheology of Macrosurfactants 5. Links between Properties and Applications of Polymeric Surfactants 5.1. Emulsion Stabilization 5.2. Coatings 5.3. Oil Recovery 6. Conclusions Author Information Corresponding Author Notes Biographies Acknowledgments © 2015 American Chemical Society

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Figure 1. Schematic representation of the different behavior displayed in solution and at the air/water interface by polysoaps (left) and macrosurfactants (center). Very few data points are available about complex architectures (right).

have provided a really solid basis to better study the effect of composition and molecular architecture on the properties of amphiphilic copolymers, given the possibility to prepare welldefined structures with almost no limitations in geometry and chemistry. In recent decades the synthetic strategies for polymers have reached their maturity. It is now possible to build macromolecular structures with tailored characteristics; molecular weights, compositions, architectures, and molecular weight distributions can be controlled almost without limitations. This allows in turn studying systematically the structure−properties relationships and ultimately selecting the structures more suitable for a desired application. The influence of the different architecture on the properties of amphiphilic polymers has raised interest for decades. It is wellestablished that a polysoap (polymer of an intrinsically amphiphilic monomer or a random copolymer of hydrophilic and hydrophobic monomers) can undergo intramolecular aggregation in water. Block copolymers with hydrophilic and hydrophobic blocks, on the other hand, will mostly give intermolecular aggregation (Figure 1). Also the arrangement at the water/air interface can be sensibly different. From this, completely different properties can be expected and are, in fact, observed. Little is known about the association behavior of amphiphilic polymers characterized by complex architectures. The main purposes of the present review are (1) to provide a general overview of the synthetic strategies adoptable for the synthesis of polymeric surfactants, depending on the desired moleculare architecture; (2) to summarize the available information about the structure−properties relationship, especially concerning surface activity and rheology; and (3) to provide a link between properties and final applications (actual or potential).

As a common feature, polymeric surfactants show interesting association phenomena in selective solvents, which results in peculiar rheological behavior and the formation of self-assembled structures. For these reasons, they have received increasing attention in the last few decades, in particular for their actual and potential applications in several fields including (mini)emulsion polymerizations, coatings, biotechnology, nanotechnology, medicine, pharmacology, cosmetics, agriculture, water purification, electronic, optoelectronic, and enhanced oil recovery.4−31 Also of great interest from the applicative point of view is the possibility of introducing responsive behavior to external parameters (such as pH, temperature, electrolytes concentration, and UV irradiation), which is at the basis of the design of smart materials.32−40 The case of Pluronic systems (block copolymers of PEO and PPO) is paradigmatic of the evolution of polymeric surfactants. First commercialized (by BASF) as industrial detergents, they found afterward important applications in medicine as drug carriers. In recent years, the interest in these polymers increased even more, as they showed potential in treatment of cancer.41 In addition to their great potential for applications, these systems still represent an exciting challenge from a more fundamental point of view. As anticipated, their macromolecular nature affords a range of architectures, length scales, time scales, and levels of interactions much wider than those offered by small amphiphilic molecules. At the same time, such diversity poses great challenges in the characterization and understanding of the solution and surface properties of large amphiphiles. Especially after the development of controlled radical polymerization methods, such as NMP, ATRP, and RAFT (in the late 1990s), the number of available compositions and structures has increased enormously. These synthetic techniques 8505

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Figure 2. Classification of polymeric surfactants

distribution of hydrophobic and hydrophilic parts, which will be discussed in more detail later.46,47 Macrosurfactants, on the other hand, seem to exclusively give intermolecular aggregation in water solutions, even in conditions of extreme dilution.21 The behavior of polysoaps and macrosurfactants in solution is schematically illustrated in Figure 1 and will be discussed in detail in section 4. It is evident that this has great consequences on rheological properties and surface activity of polymeric surfactants. However, it has to be pointed out that the balance between intramolecular and intermolecular aggregation is affected by several factors such as concentration, temperature, pH, and ionic strength. Consequently, the same polymeric surfactant can give both kinds of aggregation depending on the employed conditions (which is the basic concept of the stimuli-responsive behavior). Moreover, from the point of view of molecular structures, several intermediate situations can be found, especially in complex molecular architectures such as multiblock, star block, multibranched, and dendrimeric polymers. In such cases the distinction is not trivial and the aggregation behavior is not easy to infer. Also, gradient amphiphilic copolymers and block copolymers containing a polysoap block represent special cases that cannot be clearly classified. Because the ability to form intermolecular aggregates with tunable structure and stability is undoubtedly the most attractive feature of polymeric surfactants, recently the research has been focused on macrosurfactants. This is especially related to the establishment of controlled radical polymerization methods (NMP, ATRP, and RAFT) as very versatile alternatives to anionic polymerizations for the synthesis of block copolymers. The availability of synthetic procedures, which allow for obtaining complex architectures in a very precise and controlled fashion, has also favored the increase of the interest in these kinds of amphiphilic polymers.17,48−50 For these reasons, they can be considered as a different and new class of polymeric surfactants (Figure 2).

Against this backdrop, particular attention will be given to the application of these materials in enhanced oil recovery. Besides the coupling with the current research activities in our group, EOR provides a paridigmatic example for which all aspects of polymeric surfactant properties (in turn related to the chemical structure) can be conveniently discussed. Furthermore, it provides a relevant topic of investigation (at both academic and industrial levels) also in relation to the current suistainability climate and the efficient use of energy sources.

2. CLASSIFICATION OF POLYMERIC SURFACTANTS In general, polymeric surfactants are divided, from a structural point of view, into two main classes, depending on the relative distribution of hydrophilic and lipophilic parts. Macromolecules constituted by repeating units of intrinsically amphiphilic monomers or oligomers are generally referred to as “polysoaps”, whereas polymers in which there is a neat separation between the two parts are called “macrosurfactants”.4,26,42 Polysoaps are defined as polymers that contain a surfactant-like entity in the polymer repeating unit.43 This situation can be present in polymers of polymerizable surfactants (both homopolymers and random copolymers in which at least one monomer is a surfactant), as well as in statistic or alternating copolymers of hydrophilic and hydrophobic monomers. Also segmented multiblock polymers are preferentially included in the class of polysoaps.4 The second class includes essentially amphiphilic linear, branched, and grafted block copolymers in which there is a clear spatial segregation between hydrophilic and hydrophobic parts.21 Amphiphilic ABA or ABC triblock copolymers usually belong to the class of macrosurfactants.26 It is useful to treat polysoaps and macrosurfactants separately, because their structural differences lead in a quite general way to a completely dissimilar associative behavior in water. At low concentrations in water, polysoaps will give essentially intramolecular hydrophobic association,4 forming unimeric micelles (in fact, this characteristic has also been used as an alternative definition of polysoaps44); intermolecular aggregation starts to appear at higher concentrations.45 The prevalence of intramolecular over intermolecular association is dependent on several structural factors, including the relative amount and

2.1. Polysoaps

As anticipated, we will consider here polymers for which hydrophobic and hydrophilic parts are scattered all over the polymer backbone. The term “polysoap” was coined for these polymers by Strauss, who pioneered the research on their aggregation properties in the 1950s.51−53 As they usually bear 8506

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Figure 3. Structures of complex polymeric surfactants. A, B, and C indicate different monomers, where at least one is hydrophobic and at least one is hydrophilic. Adapted with permission from ref 21. Copyright 2003 Elsevier.

behavior of amphiphilic block copolymers appeared in recent literature.21−24,26,27,37−39,42,54,59−66 Basically, each polymer constituted by a water-insoluble block and a water-soluble (charged or neutral) one can be included in this class. Aside from permanently hydrophobic−hydrophilic block copolymers, an increasing number of polymers with tunable hydrophilicity of one or both blocks has been appearing in the literature. These systems usually show pH- or temperaturedependent micellization and surface activity. When both blocks present switchable hydrophilicity, these polymers are often referred to as “schizophrenic”.20,39,67−71

charged functional groups, these polymeric surfactants are often referred to as hydrophobic or hydrophobically modified polyelectrolytes,54,55 polyampholytes (when both anionic and cationic monomers are present),56,57 or polyzwitterions/ zwitterionic polymers (when both cationic and anion groups are present in the same monomer).58 Most of the polymers utilized or designed for EOR (taken here as an example of a relevant application field) can be included in this class. In this regard, the most popular and studied systems are hydrophobically modified polyacrylamides, polyacrylates, and polysaccharides. A very extensive review by Laschewsky4 gives an overview of all the possible structures and properties of polysoaps that can be found in the literature. Despite being the most important feature of polysoaps, their surface activity has been rarely studied.4

2.3. Complex Architectures

With the synthetic techniques available today, the only limitations in the preparation of polymeric surfactants with complex architectures are the interests and the imagination of the researchers. In Figure 3 the structures of complex polymeric surfactants, as summarized by Riess in 2003, are depicted.21 Despite the large number of synthesized structures, the study of surface properties of such polymers is limited to a few cases, which will be analyzed in the following sections.

2.2. Macrosurfactants

Amphiphilic diblock copolymers are the most important polymers included in the class of macrosurfactants. They have received great interest, especially for their ability to form micellar aggregates exhibiting stimuli-responsive behavior in aqueous solutions, which allows for preparing smart materials for several applications. A number of reviews concerning synthesis, micellization, emulsions stabilization, and stimuli-responsive 8507

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Table 1. List of Amphiphilic Block Copolymers Based on PAA and PMAA

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3. SYNTHETIC METHODS Here we will review the synthetic methods used to prepare amphiphilic polymers whose surface activity in water has been evaluated by direct measurement of surface and/or interface tension, determination of critical micelle concentration (CMC, often referred to as critical aggregation concentration or CAC), and/or study of emulsions and latexes stability and related systems. The most common hydrophobic blocks are based on polystyrene, polyacrylates, polyolefin, or nonwater-soluble polyethers. Hydrophilic blocks are made of either negatively or positively charged monomers, such as vinylic or acrylic monomers bearing sulfonated, carboxylic, or amino groups, or neutral blocks, which are essentially poly(ethylene glycol) (PEG) moieties or hydrosoluble acrylates such as 2-hydroxyethyl methacrylate (HEMA) or PEGylated acrylic monomers. From the late 1990s, controlled living radical polymerization methods such as atom transfer radical polymerization (ATRP),72 reversible addition-fragmentation chain transfer polymerization (RAFT),73 and nitroxide-mediated polymerization (NMP)74,75 are becoming the synthetic approach of choice to produce amphiphilic block copolymers with well-defined structure and very narrow molecular weight distribution. However, there are still some limitations related to functional groups tolerance that often lead to the necessity of using protective or masked groups that have to be subsequently removed or modified via additional postpolymerization transformations. The advantage of these methods is in the excellent control that can be achieved over

composition and molecular weight distribution of the synthesized polymer. Side reactions, such as chain termination by disproportionation or chain transfer processes, are in most cases completely suppressed. Ionic living polymerizationespecially anionic76has always been the preferred way of synthesizing well-defined amphiphilic block copolymers and is still largely used. Conventional free-radical polymerizations are also used, but control over structure and molecular weight distribution is generally poor. As a consequence of the development of versatile controlled/living polymerizations, very few examples of synthesis of amphiphilic block polymers via free radical polymerization are found in the recent literature. The approach for the synthesis of amphiphilic polymers is often dependent on the nature of the hydrophilic block. We will discuss and compare here the most used strategies for the synthesis of amphiphilic copolymers, depending on the nature of the hydrophilic part. 3.1. Synthesis of Macrosurfactants

3.1.1. Polyacrylic and Methacrylic Acid Blocks. Amphiphilic polymers containing a polycarboxylate block show interesting properties. In particular, the hydrophilicity of the polyacid block is strongly dependent on the degree of protonation of the carboxylic moieties and thus on the solution pH. They are more frequently found as hydrophilic partners, but their ability to form micelles in acidic conditions acting as hydrophobic block in water has been also demonstrated.77 8512

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In Table 1 are listed relevant systems based on acrylic and methacrylic acid diblock amphiphilic polymers prepared by ATRP, whose surface properties have been evaluated. Diblock, triblock, and three-arm star 1a (with a quite extended range of molecular weights and low polydispersities) have been prepared via ATRP with a standard CuBr/PMDETA catalytic system in bulk, either starting from the styrenic98 or the acrylic block.97 Several studies have been performed to find the optimal conditions for the polymerization to occur with reasonable rates, high molecular weights, and low dispersities. The CuBr/ PMDETA systems proved to work better for the synthesis of PtBA macroinitiator in a polar solvent (dibenzylether) and in the presence of small amounts of CuBr2 as deactivator.81 The importance of using a brominated initiator has been claimed in the case of 20a,82 given the fact that chlorides are not able to provide fast initiation for ATRP of acrylic monomers.173 However, synthesis of 20a with low polydispersity (1.1 < PDI < 1.3) up to MW = 60 000 has been achieved preparing a PMMA− Cl macroinitiator with a RCl/CuCl/HMTETA system and switching to CuBr/PMDETA for the tBMA polymerization.155 The bromide allows for fast initiation, but the minor liability of the R−Cl bond over the R−Br one allows for better control over molecular weight distribution, by minimizing secondary reactions. CuCl proved to be superior to CuBr in CuX/ PMDETA-catalyzed ATRP of tBMA on a PS−Br macroinitiator, again due to halogen-exchange effects.83 Double hydrophilic block copolymers based on a polymethacrylic acid block also can be prepared by ATRP. These kinds of polymers show useful pH-dependent micellization properties, due to the selective ionization of one of the two blocks at a different solution pH. 21a block copolymers have been prepared with low dispersities in usual ATRP conditions. Hydrolysis of the tBMA block is achieved with HCl/dioxane, without affecting the PDEAEMA block.77 ATRP synthesis of 3 was reported more recently.95,125,127 The copolymers have been prepared starting from a PBA−Br macroinitiator and growing a PtBA block, either in homogeneous conditions95 or in emulsion.127 Hydrolysis conducted in the presence of TFA selectively and quantitatively removes the tertbutyl groups without affecting the ester group in the BA units, contrarily to the HCl/dioxane system.95 Growth of PBA on PtBA−Br macroinitiator is also possible.128Pyrene-labeled 6 and 22 were synthesized in a three-step procedure, by preparing a HO−PCL−Br macroinitiator by means of a Sn(Oct)2-catalyzed ROP, initiated by a difunctional brominated alcohol. Subsequently, tBMA or tBA blocks were grown via ATRP and the products were hydrolyzed. The pyrene unit was added by esterification with a pyrene-containing carboxylic acid just before the hydrolysis step.90 Copolymerization of BA and tBA in ATRP conditions gives a statistical polymer. Both monomers display the same polymerization rate.136 Narrowly dispersed tBA homo- and block copolymer 3a has also been prepared by FeCl2·4H2O(PPh3)2-catalyzed ATRP in acetone.129 Anionic Polymerization. Living anionic polymerization has been extensively used to prepare amphiphilic block copolymers.23,76 Before the development of living radical methods, which require less harsh conditions, anionic polymerization was the most used technique to obtain well-defined polymers with low dispersities. A broader choice of hydrophobic partners is available, including in particular aliphatic olefins, which cannot be polymerized with controlled radical polymerization techniques.23 Relevant examples are listed in Table 1. PIB can be conveniently synthesized by cationic living polymerization. The

The synthesis of block copolymers containing acrylic or methacrylic acid is challenging because of the noncompatibility of the free carboxylic group with most of the controlled polymerization strategies (with RAFT being the only notable exception).78−80 The general approach in those cases in which the acid is not directly polymerizable consists of utilizing the corresponding tert-butyl ester as monomer. This can be easily converted into the free acid by postpolymerization reactions, such as thermolysis, catalytic cleavage, or hydrolysis in both acidic and basic conditions. This strategy also circumvents the problem of finding a proper solvent for both the hydrophilic and the hydrophobic partners, which is normally not trivial. The precursor of choice for these systems is poly(tert-butyl (meth)acrylate), which can be easily hydrolyzed into poly(meth)acrylic acid by the deprotection reaction with acids. Typical conditions are hydrochloric acid in refluxing dioxane/ water,77,81−85 trifluoroacetic acid in dichloromethane,86−90 and p-toluenesulfonic acid in refluxing toluene.91−94 The hydrolysis is usually complete and very selective, especially in the case of TFA, which is conducted in mild conditions, whereas for the HCl/ dioxane system a partial hydrolysis of the other sensitive groups seems to occur, for example, the butyl ester in the case of PBAcontaining copolymers.95 Although less frequently, the tert-butyl group can also be eliminated by thermal decomposition.96 A list of relevant examples of block copolymers containing PAA or PMAA is provided in Table 1. Atom Transfer Radical Polymerization. Atom transfer radical polymerization (ATRP) is increasingly used for the synthesis of amphiphilic polymer containing a carboxylic acid as the repeating unit in the hydrophilic block. As anticipated, the direct ATRP of a free-carboxyl group containing monomer has been proven not successful because of poisoning effects on the copper (II) species formed in the reaction media.97 Ashford and co-workers successfully performed the direct ATRP of sodium acrylate in water 169 using an oligomeric PEO initiator and the CuBr/2,2bipyridine system, obtaining low-molecular-weight polymers (up to 7400 Da) with low polydispersities (1.20−1.30). The blocking efficiency of the systems has not been investigated. Surprisingly, this strategy has received little attention, and its use seems to be restricted to the functionalization of various surfaces with poly(acrylic acid) brushes.170,171 To our knowledge, the synthesis of amphiphilic block copolymers by direct ATRP of sodium acrylate has not been reported yet, probably due to the difficulties in finding a proper solvent for both the hydrophobic and the hydrophilic partners. The most common approach remains the synthesis of block copolymer containing polyacrylic or polymethacrylic acid as hydrophilic block, starting from the corresponding tert-butyl ester, which can be readily polymerized via ATRP. The resulting polyester can be quantitatively converted to the corresponding polyacid, by thermolysis or by hydrolysis, by the already mentioned procedures such as HCl in dioxane/water mixture or TFA in CH2Cl2. Matyjaszewski’s group first reported the successful preparation of amphiphilic block copolymers (diblock, star, ABA, and ABC triblock) by ATRP following this approach, utilizing either tert-butyl acrylate (tBA)81,172 or tert-butyl methacrylate (tBMA).151 The polymerization of tBA displays a good living character up to relatively high molecular weight (50 KDa) and good blocking efficiency when poly(tert-butyl acrylate) (PtBA) is used as macroinitiator. Both addition of small amounts of Cu(II) and use of polar solvents (which enhance the solubility of the deactivator) decrease the reaction rate and afford polymers with narrower MWD. 8513

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copolymers, including 3 and 29, was reported for the first time in 1999.79 3 was prepared by direct RAFT polymerization of acrylic acid (AA) to completion, followed by polymerization of BA in a one-pot procedure.130,131 To achieve high conversions, the chain transfer agent (CTA) has to be carefully selected. Indeed, when dithiobenzoate esters are used as CTAs, conversion rarely exceeded 30%. This behavior has been explained by the stabilizing effects of the phenyl group on the intermediate radical formed during the addition-fragmentation step or, alternatively, by the formation of a terminating adduct between the intermediate radical and a propagating radical. Xanthates and trithiocarbonates proved to be better CTAs for RAFT of AA in alcohol or water/alcohol mixtures.175 However, the one-pot procedure is only possible for short PAA chains (20−50 units), because transfer to the solvent occurs to a significant degree at high conversions when the [AA]/[CTA] ratio is high.130 The possibility of a RAFT synthesis of 3 in emulsion conditions in water, with a good control over the dispersity, has been demonstrated.132 An amphiphilic RAFT agent and controlled feeding of the second monomer (starvefeed) are required to avoid droplet nucleation.176 Preparation of 1 112 and 5 copolymers by RAFT/MADIX has been reported.117,137−139 Respectively, 1b and 5a were first synthesized in aqueous emulsion and then hydrolyzed in basic conditions (NaOH at 90−95 °C). Small amounts of AA are added for the synthesis of the second PEA block (for polymerization stability). Nevertheless, the dispersity indices are always greater than 2. The RAFT/MADIX can also be performed by synthesizing a PBA macroinitiator (1K−3K Da) in ethanol and then growing the PAA (3K−12K Da) block in the same solvent in a one-pot synthesis. This was achieved through the use of xanthate 2-mercaptopropionic acid methyl ester oethyl dithiocarbonate (Rhodixan A1, Rhodia) as the RAFT agent.133−135 With the same protocol, 10 was successfully synthesized.142,143 1 was synthesized by RAFT in dioxane with 2[(dodecylsulfanyl)carbonothioyl]sulfanyl propanoic acid. The conversion reached 40% for AA and 10% for styrene.116 Short PAA10-b-PS1−10 was synthesized with the same procedure.117,118 1, 19, and 30 can be synthesized in either water or dioxane using in all cases 4-cyano-4-thiothiopropylsulfanyl pentanoic acid (CTPPA) as a control agent and 4,4′-azobis-4-cyanopentanoic acid (ACPA) as the initiator.78 This is, to the best of our knowledge, the first report of RAFT synthesis of PMAA in water. A PAA macroinitiator was prepared with 2-dodecylsulfanylthiocarbonylsulfanyl-2-methyl propanoic acid as RAFT agent and AIBN in THF (61 units). PS block (132−704 units) was grown in MeOH in a second step, after purification of the PAA block, with a RAFT dispersion polymerization.119 Nitroxide-Mediated Polymerization. Nitroxide-mediated polymerization (NMP) is one of the first and conceptually simplest controlled radical polymerization techniques. It only involves the reversible deactivation of the propagating polymer radical through reaction with a stable radical generated in situ by the nitroxide.74 Even though in some aspects it has been surpassed by the more sophisticated ATRP and RAFT, NMP still attracts interest because of its broad range of applicability, the limited amount of additives required (basically, only the nitroxide) with respect of the other controlled polymerizations, and the relatively mild conditions. Scope and limitations of NMP have been reviewed recently.75 Charleux and co-workers have reported the first direct nitroxide-mediated polymerization of acrylic acid in the presence of SG1/DEPN (1-(diethoxyphosphinyl)-2,2-dimethylpropyl-

capped PIB block can therefore be converted in an anionic macroinitiator. Thiophene has been found to be a suitable capping agent, which is more successful compared to furan (known to lead to unwanted secondary reactions).159 Reaction of thiophene-terminated PIB with n-BuLi provides an efficient macroinitiator for anionic polymerization of tBMA. The polyisobutylene hydrophobic block is usually synthesized by a cationic procedure using BCl3/TiCl4 catalyst. The polymer is capped with DPOMe, and the following block of poly(tert-butyl methacrylate) is prepared by polymerization in anionic conditions, using Na/K alloy in dry tetrahydrofuran (THF) to create the active species.121 The blocking efficiency is higher than 95%, and the dispersity is very low (PDI = 1.06−1.13). Double hydrophilic 25 was synthesized by anionic polymerization of the protected monomers (4-tert-butoxystyrene (tBOS) followed by tBMA), and the product was completely hydrolyzed with HCl in water/dioxane, in standard conditions.161 Capping of the first block with DPE and the addition of LiCl are required to ensure a well-defined character of the second tBMA block. Eisenberg’s group studied extensively 1 copolymers of various compositions prepared by anionic living polymerization using sec-butyllithium as initiator.91−94,100−111,174 LiCl and α-methylstyrene were used to prevent side reactions during polymerization of tBA.91,174 The hydrolysis of the tBA block was performed with p-toluenesulfonic acid (pTSA) in refluxing toluene. Block copolymers with a broad range of compositions and low dispersities have been successfully prepared, allowing a systematic and extensive study of their properties. Polymers with long PS chains (170−2300 units) and short PAA blocks (8−360 units),94,103,110 as well as short PS (6−110 units) attached to long PAA (270−2360 units),100,102 have been synthesized. A triblock 18a has also been prepared by an analogous procedure.108 According to one of the first reports on anionic synthesis of 19,93 the polystyrene block has to be capped with 1,1diphenylethylene to avoid side reactions. As initiator, secbutyllithium, in combination with α-methylstyrene and LiCl, is effective in avoiding broad molecular weight distributions.154 In the synthesis of a PMAA-based amphiphilic polymer containing a fluorinated block (26), the tert-butyl group was removed by thermolysis. Subsequently, the formed anhydrides were hydrolyzed in alkaline conditions.96 PDMAEMA block can be efficiently prepared by anionic polymerization, provided that (diphenylmethyl)lithium is used as initiator (n-BuLi gives side products) and LiCl is added to maintain a narrow MWD. The living character of the PDMAEMA block has also been demonstrated. However, the synthesis of the diblock 21a was performed starting from a living PtBMA block and was used successfully as a macroinitiator in the polymerization of DMAEMA.162 2 block copolymers, with narrow dispersities, were synthesized by anionic polymerization through initiation with (diphenylhexyl)lithium in the presence of LiCl, followed by sequential addition of tert-butyl acrylate (tBA) and methyl methacrylate (MMA).124 The polymers can also be endfunctionalized by reaction of the living anion with the bromomethyl derivative of a chromophore. Reversible Addition-Fragmentation Chain Transfer Radical Polymerization. Reversible addition-fragmentation chain transfer (RAFT) radical polymerization techniques for the synthesis of water-soluble and responsive amphiphilic polymers has recently extensively been reviewed by McCormick and coworkers, in particular from a mechanistic point of view and for what concerns its application for the synthesis of stimuliresponsive systems.13,38,66RAFT synthesis of various block 8514

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Table 2. Amphiphilic Block Copolymers Containing an Aromatic Sulfonate Block

acrylate, demonstrating the ability of the homopolymer to initiate the polymerization of a second block.178 1 was successfully synthesized via NMP of a PS macroinitiator and direct polymerization of AA.113 The same group reported NMP of block 1 and 3 latexes with short AA block (14−38 units) and long hydrophobic block, by using a nitroxide-terminated

1,1-dimethylethyl nitroxide) and dioxane as solvent to avoid transfer reaction.177 Moreover, a short poly(acrylic acid) with a narrow molecular weight distribution, prepared by SG1 nitroxide-mediated controlled free-radical polymerization, was recently used for chain extension with styrene and n-butyl 8515

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by this procedure can be used as a macroinitiator for the growth of a hydrophobic block such as PS in emulsion183 to afford 36, polyvinylnaphthalene in homogeneous conditions using dimethylsulfoxide (DMSO) as solvent188 (37), or other styrenic-type blocks of various hydrophilicities184 (38−42). DEPN has been used more recently for the synthesis of 36, starting from a PS macroinitiator and adding the PSSNa block in three steps, passing through the neopentyl and the trimethylsilyl derivatives of SS.185,186 Reversible Addition-Fragmentation Chain Transfer Radical Polymerization. Since the very first report of RAFT polymerization, 201 the possibility to prepare PSSNa by this procedure has been demonstrated. However, few examples concerning RAFT of PSS containing amphiphilic block copolymers have been reported. A tunable double hydrophilic copolymer 42 has been prepared and studied by Mitsukami et al.189 The efficiency has shown to be highly dependent on the nature of the CTA for the polymerization of SS in aqueous media: while 4- cyanopentanoic acid dithiobenzoate-mediated RAFT yielded a poly(sodium 4styrene sulfonate) homopolymer with a narrow molecular weight distribution (Mw/Mn = 1.17) and a molecular weight close to the theoretical value, carboxymethyl dithiobenzoate-mediated RAFT yielded a homopolymer with a broad molecular weight distribution and average molecular weight significantly higher than the theoretical value. Interestingly, the conversions are almost identical for the two polymerizations. Recently, RAFT has been used to prepare PSS-TOA, which can be subsequently used as a macro RAFT agent for a second block of PS of various lengths. The controlled nature of both polymerization steps has been emphasized.187 This system can be easily converted to the corresponding Na salt by ion exchange. The use of the TOA salt of styrene sulfonate is required to solubilize the monomer and perform the polymerization in a nonpolar organic solvent. Polystyrene Sulfonation. Sulfonated polystyrene block copolymers have also been prepared by the postpolymerization sulfonation of polystyrene. This represents the oldest approach to the synthesis of PSS-containing block copolymers. However, it is difficult to achieve 100% sulfonation of the PS block. Side reactions can occur during the postpolymerization sulfonation, giving rise to intramolecular sulfone linkages,202 and it is not always possible to selectively sulfonate the PS block. For example, polydienes are more reactive under polystyrene sulfonation conditions and can be selectively sulfonated in the presence of polystyrene using a sulfur trioxide/1,4 dioxane complex.203 Nowadays, other procedures involving polymerization of sulfonated styrene are preferred. Diblock 43 and triblock 44 of various compositions can be prepared by sequential anionic polymerization of tBS and styrene and subsequent sulfonation.190,191 The sulfonation, usually carried out with SO3 complexed in an opportune solvent (triethyl phosphate in 1,2-dichloroethane), is selective toward the styrene and gives good conversions (80−100%). With an analogous procedure, which also involves the catalytic hydrogenation of the polyolefinic block prior to the sulfonation step, 45 has been prepared.192,193 If H2SO4/P2O5 is used, the degree of sulfonation is high but side reactions are observed that lead to a broadening of the MWD. Acetyl sulfate is less effective but more selective. However, H2SO4/P2O5 was used more recently in the preparation of 46, giving polymers with an average to good degree of sulfonation (0.68−0.92) and narrow distributions.194−196 3.1.3. Polyvinylpyridines and Quaternized Arylamines Blocks. Polyamine blocks also include polymerized vinyl-

PAA macroinitiator in a water emulsion and surfactant-free conditions at alkaline pH.114,115 The same procedure was used to prepare 11.144 Also diblock, triblock, and star amphiphilic 1 copolymers has been prepared by SG1-mediated NMP of a PS macroinitiator and chain extension with PtBA, followed by hydrolysis.89 In this case, a small amount of styrene is added in the second polymerization step to decrease the propagation rate, which results in incorporation of some styrenic units in the PtBA block. 12 has also been prepared by NMP, through SG1/DEPN-mediated polymerization of tBA and chain extension with isoprene, followed by hydrogenation with sulfanyl hydrazide and deprotection in acidic conditions.145 Group Transfer Polymerization. Group transfer polymerization (GTP) has been limitedly used for the synthesis of acrylic acid-based amphiphilic polymers. As for ATRP, protection of the carboxylic group is required, because of catalyst deactivation by means of the free acid. Most of the reports of amphipihlic polymers prepared by GTP appeared in the literature in the 1990s. Nowadays ATRP and RAFT, which provide better control over molecular weight distribution and architecture, have almost completely replaced GTP for the preparation of acrylic acid-containing block copolymers. Patrickios and co-workers164 synthesized random, diblock, and triblock copolymers containing DMAEMA (27) and MAA by GTP, using trimethylsilyl (TMS)- or tetrahydropyranyl (THP)-protected acrylic acid as precursors of the PMAA block (27b and 27c). Similarly, Armes and co-workers prepared 31168 and 27165,166 using tBMA and THPMA, respectively, as the starting monomer for the acid block. 20 was also synthesized by GTP of 20a and subsequent hydrolysis.156 3.1.2. Aromatic Sulfonate Blocks. Well-defined polymeric surfactants containing an aromatic sulfonate block have been prepared by controlled polymerization methods. Relevant examples are summarized in Table 2. Atom Transfer Radical Polymerization. Analogously to acrylic acid-based copolymers, poly(styrene sulfonate) blocks can be prepared by ATRP of protected styrene sulfonate (e.g., ethyl ester), followed by thermolysis or hydrolysis, usually in basic conditions. 34 has been recently synthesized by ATRP of the corresponding neopentyl ester, followed by thermolysis at 150 °C.179 A PSSNa macromonomer has been successfully prepared by ATRP of styrene sulfonate ethyl ester, which was subsequently hydrolyzed with NaOH in dimethylformamide (DMF)/water or KOH in DMF.197 Ethyl and dodecyl esters of styrene sulfonic acid brushes have also been grown by ATRP.198 However, direct ATRP of sodium styrene sulfonate has also been reported199,200 and very recently has been successfully used to synthesize 35 with different HLBs.180,181 The ATRP of SSNa in water is not a true living process, because of the competitive coordination of solvent and ligand to the copper species.200 Nevertheless, the polymerization with CuBr/bpy catalytic system and a water-soluble initiator proceeds very fast and, initially, with a bimodal distribution. However, the final dispersity is not high (1.2−1.6). Addition of Cu(II) and the use of water/methanol mixtures as solvent gives better results in terms of dispersity indices. A halogen-exchange effect has been proposed also for this system.199 Nitroxide-Mediated Polymerization. Sodium styrene sulfonate can be directly polymerized by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated polymerization in a water/ ethylene glycol mixture.182 High molecular weights (up to 900 KDa) and low dispersities can be obtained. The PSSNa prepared 8516

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Table 3. P2VP- and P4VP-based Amphiphilic Block Copolymers

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pyridine or styrene derivatives that contain amino groups attached to the phenyl ring. The amino group can also be present as a quaternary ammonium salt. The latter class of compounds is usually derived by postsynthetic quaternization of the corresponding polyamine precursor (Table 3). Direct measurements of surface activity of such systems are very rare, but there are several studies concerning formation of micelles and stabilization of particles in water solution. The most studied systems of this kind are 47 and 49 and their derivatives. There are also reports of polyvinylpyrrolidone (PVP)-based block copolymer with different hydrophobic blocks (Table 3). The aggregation behavior of these polymers is obviously pHdependent. The PVP ring is often quaternized after synthesis to get a permanently charged quaternary ammonium block. TEMPO-mediated NMP of 4VP has been found to proceed in a controlled manner,208,221 and the resulting polymer has been used successfully as a macroinitiator for hydrophobic blocks such as styrene208 and dimethylacrylamide (DMA).218 The use of DEPN instead of TEMPO allowed the synthesis of 53 with longer DMA blocks.219 The polymerization of 4VP requires high temperatures (>125 °C) and gives slightly broader MWD compared to those obtained with styrenic monomers208 or anionic methods.221 Also in these cases, subsequent quaternization of the 4VP ring with various groups has been achieved by reacting the polymer with alkyl bromides in boiling EtOH.218 Direct polymerization of quaternized 4VP cannot be carried out because of the instability of the ammonium salt in the polymerization conditions. 218 Once the possibility to polymerize 4VP via ATRP was disclosed,217 block copolymers have been prepared in this way, such as 53217 and 49.206 However, because of the competitive complexation of copper with the pyridine ring, which deactivates the catalyst, a ligand with a very high binding constant for copper, such as Me6TREN, is required.206,217 Also RAFT has been used with good results to make di- and triblock copolymer based on P4VP and styrene.207Vinylbenzyl chloride can be block copolymerized by controlled radical polymerization mediated by TEMPO, giving reactive polymers with narrow MWDs.220 This approach has been used to prepare diblock PS-b-PVBC 54, which has been further modified by reaction with a secondary amine to give permanently positively charged blocks.220,222,223 The resulting MWDs are higher than those usually obtained for styrene. This behavior has been explained in terms of side transfer reactions between the capping agent and the chloromethyl group.222 To avoid broadening of the MWD by side reactions, other authors prepared similar systems via RAFT polymerization of styrene, followed by VBC and subsequent reaction with tertiary amines.224 In this case, better results in terms of MWD have been obtained. It is important to remark that, for polymerization of VBC, other controlled methods such as cationic, anionic, or ATRP are precluded because of interactions with the chloromethylenic group.224 For these reasons, Patra et al.225 proposed the synthesis of VBC-containing diblock copolymers via reversible iodine transfer polymerization (RITP, Figure 4). N,N-dimethylvinylbenzylamine and the corresponding quaternary ammonium salt can be directly polymerized and block copolymerized by RAFT, as demonstrated by Mitsukami et al.189,226 3.1.4. Poly(ethylene glycol)/Poly(ethylene oxide) Blocks. Amphiphilic copolymers based on poly(ethylene oxide) (PEO) moieties represent by far the first and most studied class of polymeric surfactants. Many di- and triblock

Figure 4. Representation of RITP mechanism. Reproduced with permission from ref 225. Copyright 2010 Elsevier.

copolymers of PEO and PPO with a variety of compositions and molecular weights have been present in the market as industrial surfactants for a long time (e.g., Pluronic by BASF). Several reviews concerning their properties have appeared in the last two decades.5,6,8,9,21−23,122 PEG/PEO polymers are synthetized mainly by two methods: condensation of ethylene glycol (EG) or ring-opening anionic polymerization of ethylene oxide. Indeed, it is common to refer to PEG when the polymer is synthesized by the former way and to PEO if the polymer derives from the latter. The synthesis of PEG (or PEO) is very well established, and today, systems characterized by a broad range of molecular weight and different terminal groups, as well as some of their corresponding block copolymers (e.g., Pluronic series), are commercially available. For this reason, the preparation of new PEG-based amphiphilic copolymers is normally accomplished by addition of the hydrophobic block on a preformed, commercially available PEG macroinitiator. Some examples of amphiphilic copolymers synthesized starting from a PEG macroinitiator are listed in Table 4. Depending on the polymerization method, modification of the terminal group of the PEG moiety might be necessary prior to starting the synthesis of the second block. Hydroxy-terminated PEG has been used directly as a macroinitiator of anionic polymerization (for example, of acrylic monomers227) or ringopening polymerization for the synthesis of PEG-b-PLA228). The terminal hydroxyl group can be easily modified to obtain the proper initiator for a selected kind of polymerization (dithiocarboxylate for RAFT or halogen for ATRP). 4Cyanopentanoic acid dithiobenzoate-terminated PEG has been prepared and used as a macroinitiator for RAFT polymerization of N-acryloyl-2,2-dimethyl-1,3-oxazolidine;229 halogen-terminated PEG has been used as an ATRP macroinitiator for amphiphilic block copolymers such as 56,230 57,231 58, 59 and 60,232 61,233 and a series of amphiphilic or double hydrophilic block copolymers 65−71.234 PEG has also been used as precursor of a thiolate for subsequent anionic polymerization of propylene sulfide.235,236 However, especially in the case of nonlinear or multiblock structures, sequential anionic polymerization is often used. 237 PEO-b-PS (58),237,238 72, 73,235,236 74,239 and 75240 have been prepared in this way. HEUR. Hydrophobically modified polyurethanes (HEUR) have been extensively studied in fundamental research aimed at understanding the aggregation behavior of associating polymers. Here again we focused on the systems of which either the CMC or the CAC have been measured. The polymers (see Table 5) can be prepared by a two-step or single-step reaction.241 In the two-step method, an excess (a large excess is used to suppress the dispersity index (PDI) of the product) of the diisocyanate is reacted with poly(ethylene oxide) (PEO) to produce an isocyanate-functionalized polymer. This polymer is subsequently functionalized with hydrophobes by the 8518

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Table 4. PEG-Containing Macrosurfactants

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Table 5. Ethoxylated Polyurethanes: Structures of the Studied Systems

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Table 6. Amphiphilic Block Copolymers Derived from Alkylaminoacrylates

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Table 6. continued

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Table 6. continued

reaction with aliphatic alcohols. The single-step method involves the reaction of PEO with a monoisocyanate containing a hydrophobic group. The PDIs of the synthesized polymers are close to the PDIs of the parent PEO polymer. 3.1.5. Alkylaminoacrylate Blocks. Several amino derivatives of acrylic acid have been employed for the synthesis of amphiphilic block copolymers. The most studied systems are shown in Table 6. These kinds of block copolymers have attracted much interest because of their pH-dependent hydrophilicity and for the possibility to easily change the characteristics of the pendant amino group by subsequent postsynthetic functionalizations. For example, they can be straightforwardly quaternized to give permanently positively charged or zwitterionic blocks (see Table 6). The most employed alkylaminoacrylates for the preparation of amphiphilic polymers are DMAEMA and DEAEMA. Armes’ group, who pioneered the investigations of this kind of system,

prepared several amphiphilic block copolymers containing a PDMAEMA moiety by GTP and ATRP polymerization.39,67−69,254−258,266,270,276,278,279 ATRP proved to be more versatile than GTP, allowing the synthesis of block polymers with comonomers other than acrylates.280 In addition, in GTP-based synthesis, significant amounts of homopolymer can be found, which makes the purification steps problematic.254 Homopolymer of DMAEMA can be prepared by ATRP either in organic solvent280 or in water.281 The kinetic plots show fast initiation and negligible terminations. It has been reported that DMAEMA and other alkylaminoacrylates can undergo unwanted transesterification with methanol at room temperature if it is used as solvent.282 The problem can be easily circumvented by using the more sterically hindered isopropanol, by increasing the reaction temperature, or by using water as cosolvent, which sensibly increases the polymerization rate.262,282 8523

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Table 7. Amphiphilic Block Copolymers Based on Miscellaneous Acrylates

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Table 7. continued

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Table 7. continued

In the synthesis of amphiphilic block copolymers, the situation is more complicated. The PMMA−Br macroinitiator shows poor blocking efficiency, due to elimination of HBr from the tertiary terminal carbon, so the more stable PMMA−Cl has to be used instead.263 On the other hand, initiation from PMA is relatively slow compared to the propagation of PDMAEMA (halogen is on a secondary carbon), so in this case the brominated macroinitiator is used. The system works quite well, provided that CuCl is used as catalyst, taking advantage of the halogen exchange. A PS macroinitiator gives the broadest MWD.263

Well-controlled ATRP of PDEAEMA initiator and 100 has also been carried out successfully with p-TsCl/CuCl/HMTETA in water/methanolic solutions.274,283 Similarly, an aldehydeterminated 100 was synthesized by ATRP.275 Problems of slow initiation have been found also in the ATRP of 99.273 Also in this case, halogen exchange provides better results. PDMAEMA-containing block copolymers can also be made by living anionic polymerization157,261 and TEMPOmediated free-radical polymerization.267 The latter method has been used for the synthesis of 94, but the polymers obtained have 8526

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for acrylic monomers.303 In addition, the dithiophenyl acetate residue should minimize retardation effects in the polymerization.316 The synthesis of poly(butyl acrylate) exhibited a much shorter induction period than reported for the classical dithiobenzoates as chain transfer agents.303,304 Several examples of PEGMA-based amphiphilic copolymers have also appeared in recent literature.307−313,317 These kinds of polymers have received great interest in the past few years.318 They can be considered analogous to PEG-based copolymers, resembling them from a chemical point of view, but with the advantage of being polymerizable via mild controlled radical methods. Also, they give access to more complex graft/comb/ brush structures, with the possibility to tune their characteristics by changing the length of the pendant oxyethylene group. In most cases, the polymers are prepared by ATRP (see Table 7). Concerning copper(I)-mediated ATRP in the presence of oxyethylene groups, it has been shown that polymerization of poly(ethylene glycol)methyl methacrylate monomer (MeOPEGMA, molecular weight = 480 g mol−1) can be conducted in toluene. The polymerization is mediated by copper(I) bromide/ N-(n-propyl)-2-pyridylmethanimine as the catalyst and is fast relative to benzyl methacrylate (BzMA) under similar conditions. This high rate of polymerization is ascribed to complexation of the oxyethylene groups with copper in a dynamic equilibrium with the pyridyl methanimine ligand complexation, resulting in a more active catalyst.312,319 When the difference in hydrophilicity of the two partners is too different (like in the case of strong electrolytes), the polymerization of a neutral precursor of the hydrophilic monomer and subsequent modification is the usual approach. For example, in the synthesis of a block copolymer of poly(diethyldisilacyclobutane) (PDESB) and poly(sulfopropylmethacrylate) 106, a nonionic diblock copolymer PDESB-PtBMA was synthesized first; this was converted to a weak polyelectrolyte block copolymer PDESB-PMAA, and then the weak acid groups were changed to a strong acid group by reaction of propanesultone.292 An analogous method had been utilized before for the synthesis of 107.289,290 A slightly different procedure was used for the postsynthetic esterification of the hydroxyl group of HEMA with a sulfonated benzoic acid, to obtain 135.306 Reports of controlled block copolymerization of acrylamide and N-isopropylacrylamide were absent in the literature until very recently.314,315

broad MWDs and the reaction failed in the attempt to prepare the homopolymer of DMAEMA.267 Homopolymerization and block copolymerization of DMAEMA by RAFT has been investigated,284 but to the best of our knowledge no synthesis of block copolymers with a permanently hydrophobic block has been reported yet, although several examples of block copolymers responsive to pH and/or temperature can be found.38,71,285,286 Quaternization. PDMAEMA blocks can be quite easily quaternized in several ways. Betainization with propane-1,3sultone, in order to obtain zwitterionic derivatives, has been performed.255,260,276 The reaction is selective for DMAEMA in presence of DEAEMA and DPAEMA blocks when performed in mild conditions.276 Quaternary alkyl amines can be obtained by coupling with alkyl halogenides, such as MeI,266 ethyl, pentyl, heptyl, n-propyl, n-butyl and tert-butyl bromides,261 and octyl bromide,273 with nearly quantitative yields. 3.1.6. Other Water-Soluble Acrylate Blocks. Several other acrylic monomers have been successfully used as hydrophilic partners in amphiphilic block copolymers for various purposes, usually by means of living controlled radical or anionic methods (see Table 7). In some cases, surface/interface activities of the resulting polymers have been evaluated (a list of recent examples is provided in Table 7). From a synthesis point of view, each system represents a particular case. Nevertheless, especially in the last few years, ATRP seems to be the preferred way to prepare well-defined amphiphilic block copolymers based on acrylates. Regardless of the polymerization method, the critical factor is usually the choice of the solvent. Common solvents for hydrophilic and hydrophobic monomers can be found in some cases,303,304 such as THF,301 dioxane,303 DMF,309 DMA,303 NMP,303 methanol, and isopropanol,294 either alone or in combination with water. In most cases, mixtures of water and alcohols are able to dissolve both the hydrophilic and the hydrophobic monomers (or macroinitiators). In a quite general way (and in particular in all the cases examined here), ATRP is faster in water than in alcoholic media, but the process seems to be not truly controlled, leading to broad MWDs. Alcohols slow down the polymerization and provide a better living character, which results in better blocking efficiency and narrower distributions.293,295,311 Generally speaking, for ATRP in water/alcohol mixtures, the rate increases with the polarity of the solvent, which can be tuned by varying the water/alcohol ratio or using alcohols of different polarities (commonly, isopropanol and methanol).294 Because of the better control, even if the polymerizations in some cases can be conducted in pure water, a mixture is often preferred.311 For example, in the preparation of 2-methacryloyloxyethyl phosphorylcholine (MPC) homopolymers by ATRP, it has been observed that the monomers polymerize even in the absence of any ATRP initiator in aqueous solution at 20 °C, due to impurities in the water phase that are able to start free radical polymerization.295 In methanol the rate of ATRP is much slower, but fortunately autopolymerization of MPC does not occur in this solvent.294,295 Homo-, di-, and triblock (co)polymers based on the hydrophilic MPC have been prepared in methanol and water−methanol mixtures.296−300 Garnier and Laschewsky performed the synthesis of various acrylic neutral, cationic, and anionic amphiphilic block copolymers, characterized by increasing polarity of the hydrophilic group, by the RAFT/MADIX procedure.303,304 Benzyldithiophenyl acetate was employed as the RAFT agent, as it is known that dithioesters and trithioesters are good RAFT agents

3.2. Synthesis of Polysoaps

The general strategy for the synthesis of polysoaps is exhaustively covered in the seminal review of Laschewsky, to which the reader is referred.4 Although this work was published in 1995, there are no significant new developments in the approach used: the synthesis of polysoaps is normally not very challenging, being performed mostly via very well established free radical polymerization or polycondensation methods.4 In most cases, no particular attention is paid to the precise control of the macromolecular structure, but in the recent literature there are few exceptions in which the copolymers are prepared via controlled radical processes such as NMP320,321 or RAFT.322,323 The most common kinds of polysoaps for which surface properties have been investigated are based on amphiphilic derivates of acrylamide or acrylic acid and on hydrophibically modified polysaccharides. Other systems include homopolymers of polymerizable surfactants320,324 or random copolymers of hydrophobic and hydrophilic monomers.325 The hydrophobic monomer can also be pH-responsive. 326 8527

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Table 8. Acrylamide-based Polymeric Surfactants: Structures of Studied Systems

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Table 8. continued

3.2.1. Polyacrylamides and Related. Almost all of the literature references on hydrophobically modified polyacrylamides (HMPAMs) included in the review by Wever et al.327 consider the thickening capability of the polymers in aqueous solution. However, here we focus on the interfacial properties of these polymeric materials. Although most of the publications do not discuss the interfacial properties of the materials, their critical association concentration (CAC) and/or critical micelle concentration (CMC) do give information on their ability to perform as interfacial agents. Most of the acrylamide-based polymeric surfactants (Table 8) are synthesized via free radical polymerization. The differences lie in the conditions employed during the synthesis. For example, polymers 142 and 143 (random copolymers) are both prepared in dimethylformamide (DMF) through free radical polymerization,328,329 while polymer 144 (random copolymer) is prepared in water.43 Another method widely used is the micellar copolymerization technique.330,331 Here the reaction medium is water and the hydrophobic monomer is dissolved using surfactants (the most commonly used surfactants are sodium dodecyl sulfate (SDS) and cationic hexadecyltrimethylammonium bromide (CTAB)). The drawback of the micellar copolymerization method is the purification step that is necessary to remove the surfactants. This can be avoided by employing a method, closely related to the

micellar technique, involving micelle-forming polymerizable surfactant monomers.59−61,332 However, to identify suitable surfactant monomers for the desired structure remains a challenge. A relatively new method is the template polymerization. This method involves the use of a template onto which the comonomer is polymerized. The structure of the polymer is determined by the template. The advantage of this method over the micellar copolymerization is the ability to introduce longer hydrophobic blocks.42,333 3.2.2. Hydrophobically Modified Polysaccharides. Polysaccharides are suitable starting materials for the preparation of polysoaps, due to their broad availability and the presence of easily functionalizable hydroxyl groups that can be used to introduce hydrophobic moieties in the hydrophilic backbone. As for polyacrylamide, these systems are often used as thickeners for water solutions,327 but also their surface/interface properties have been the subject of many studies for applications in emulsion stabilization and similar. The backbone is usually constituted by hydroxyethyl-357−360 or carboxymethyl 361 cellulose, dextran,362−365 carboxymethylpullulan,366,367 inulin,368,369 pectin,370 and chitosan.371 The hydrophobic groups can be introduced via reaction of the hydroxyl groups with organic halogenide (substitution), epoxides (addition), or esters (transesterification). Other more sophisticated methods are seldom used and only for particular systems such as PLA-grafted 8529

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Figure 5. Structures of polysaccharide-derived polysoaps

dextran. 372 The products have a random distribution of the hydrophobic groups. In most cases the reaction is performed in water or water/isopropanol mixtures and in the presence of NaOH as the base.357,359,364 Hydroxyethyl cellulose (167) has been variously functionalized with hydrophobic groups via reaction of the hydroxyl group with an organic halogenide357 or an epoxide360 in basic conditions, using water or mixtures of water and isopropanol as solvent. The degree of substitution can reach values as high as 25 weight %, but above 10 weight % of substitution the polymer is no longer water-soluble.357 Carboxymethyl cellulose (168) has been hydrophobically modified by microwave-assisted reaction of its sodium salt with fatty esters from rapeseed oil in DMF/water or DMF/TSA mixtures. Slightly higher degrees of substitution were obtained when Na2CO3 was used as catalyst.361 Pullulan (169) has been converted to carboxymethylpullulan by reaction with sodium chloroacetate in water/IPA mixture and in the presence of NaOH. The counterion was then exchanged with tetrabutylammonium by neutralization and reaction with TBAOH, and the following step of functionalization was performed by reaction with an alkyl bromide in DMSO.366 A similar approach (substitution with TBAOH and reaction with

the electrophile in DMSO) was also used for the functionalization of dextran (170) with hydrophobic epoxides362,363 and for pectin (171) with alkyl bromides.370 In the case of dextran, in this way it is possible to obtain higher degrees of substitution compared to the NaOH/water system.362 Structures are depicted in Figure 5. 3.3. Synthesis of Complex Polymeric Surfactants

Despite the large number of prepared polymeric surfactants with complex architectures, not many of these systems have been characterized in terms of surface activity. The general synthetic strategies for the synthesis of copolymer with complex architectures have been reviewed several times in recent years.16,21−23,48,76,373−375 Here we will focus on the synthesis of those polymeric surfactants whose surface activity in water has been actually evaluated and related systems. Most of the complex polymeric surfactants studied for their surface properties are represented by comb-like, hyperbranched, and dendrimeric copolymers, but there are also some examples of gemini, miktoarm, and star copolymers (Table 9). In one of the early studies of surface properties of gemini polymeric surfactants, 376 a miktoarm PS(PEO)2 copolymer 172 was prepared via two rather complicated alternative procedures involving several protection−deprotection steps, anionic poly8530

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Today, especially thanks to the availability of controlled radical polymerization methods and “click” chemistry, the preparation of geminior more in general miktoarmamphiphilic copolymers can be achieved with simpler methodologies.23,374 For example, various PSn(PEO)m heteroarm copolymers have been made by combinations of ring-opening polymerization of EO and ATRP of styrene using opportune multifunctional crosslinkers.377,378 Another strategy based on ATRP has been used by the same group to synthesize block 3- and 4-arm star of PS-b-PEO 176 and 177 (Figure 6). The PEO blocks were made by anionic ringopening polymerization after modification of the terminal bromides with ethanolamine. 379 An interesting example of the synthesis of a PMMA-PEO-PS μ-star miktoarm polymer 178 based exclusively on ATRP was recently reported.380 Because of the high deactivation constant and the low propagation rate of styrene with the CuBr/ PMDETA catalyst at room temperature, the authors were able to incorporate only one styrene-terminated PEO unit on the macroinitiator (Figure 7). Star block copolymers constituted by vinylic monomers both as hydrophilic and hydrophobic partner are generally prepared by anionic polymerization or ATRP. ATRP has been used to prepare gemini381 and star block PS-b-PAA.99 98 The latter can also be made by NMP.89 A 3-arm block star PBMAPDMAEMA268 and a star PMMA-b-PDMAEMA261 (also quaternized on the nitrogen) were made via ATRP. The synthesis and study of a complex PS-PAA made by ATRP, which can be considered as a joining link between block star and dendrimeric polymer, has been recently reported.385,386 A schematic representation of the structures prepared is reported in Figure 8. The synthesis is based on the use of multihalogenated calixarenes (Figure 9a) for the preparation of the PS core with 2− 8 arms followed by a two-step chemical modification of the terminal bromide into a difunctional ATRP initiator for the growth of tBMA chains (Figure 9b). A dendrimeric PS-PAA has been also synthesized again via ATRP, starting from a poly(propylene imine) dendrimer with 64 NH2 functionalities. The polymerization involved, as usual (see section 3.1.1), ATRP of styrene followed by ATRP of tBMA and hydrolysis.387 However, from their studies on the behavior of the prepared polymers, the authors concluded that only a minor fraction of the 64 active sites was actually initiated, leading to polymers with a low (not specified) number of branches. On the other hand, an initiator with 8 sites provided an 8-arm star copolymer. Dendritic PPO-PEO polymers 184 with different arm lengths have been prepared starting from a third-generation poly(amido amine) (PAMAM) core by base-catalyzed ringopening polymerization of PO followed by EO.383 An analogous system, but with diblock PPO-PEO (182) or triblock PPO-PEOPPO (181) arms, was prepared by the same approach starting from tetraethylenepentamine (TEPA). 382 This copolymer is constituted by a low number of arms (7), so it can be considered more like a star rather than a dendrimer. Star amphiphilic polymers with a higher number of arms can be conveniently prepared by an “arm first” methodology. A series of block, heteroarm, and random star amphiphilic copolymers with 20−80 arms based on HEGMA (methoxy hexaethylene glycol methacrylate) and BzMA were prepared with a GTP procedure, using a diacrylate (ethylene glycol dimethacrylate, EGDMA) for the formation of the core, as schematized in Figure 10.388

Table 9. Structure and Composition of Typical Polymeric Surfactants with Complex Molecular Architecture

merization of styrene and ring-opening polymerization of ethylene oxide. 8531

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Figure 6. Synthesis of 3- and 4-arm star PS-b-PEO polymeric surfactants. Redrawn with permission from ref 379. Copyright 2003 Americal Chemical Society.

PMMA-PAA copolymer 198 (Figure 11).389 First, methyl methoxyacrylate (MOMA) was polymerized using CuBr/ PMDETA as the catalytic system. The resulting polymer was subsequently acylated with α-bromoisobutyryl bromide to give a multifunctional initiator for the growth of PMMA-grafted chains and hydrolyzed to convert the MOMA units into AA units (Figure 11). Starting from a tetrafunctional initiator, a stargrafted copolymer has been made by the same procedure.390 The synthetic approach to complex structures is very dependent on the nature of the monomeric groups that one wants to incorporate. For example, an hyperbranched ionene (a polymer containing quaternary nitrogen atoms in the back-

Figure 7. Schematic synthesis of a PMMA-PEO-PS μ-star miktoarm polymer via ATRP. Redrawn with permission from ref 380. Copyright 2009 Elsevier.

More complex architectures are also possible. An approach completely based on ATRP has been used to synthesize a graft

Figure 8. Schematic representation of dendrimeric PS-PAA copolymers. Adapted with permission from ref 385. Copyright 2005 American Chemical Society. 8532

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Figure 9. Synthesis of dendrimeric PS-PAA copolymers: (a) structure of PS core and (b) modification of end groups of PS macroinitiator for the growth of PtBMA arms.

modification of CL with a PEO chain followed by ring-opening copolymerization (Figure 14).395 A highly branched PNIPAM with terminal peptidic groups has been made with a variant of the Frechet’s self-condensing vinyl polymerization396 involving a combination of free radical polymerization (for the backbone) and RAFT (for the branches).397 The highly branched polymer was made in one step by copolymerizing NIPAM and a vinyl-functionalized RAFT initiator (see Figure 15). The same approach has been used for amphiphilic highly branched acrylates, but with ATRP as the polymerization method (Figure 16).398 A combination of ATRP and oxyanionic polymerization has been used for the synthesis of comb-like amphiphilic copolymers based on MMA, HEMA, and DMAEMA (Figure 17).399 For the first step of this synthesis, the authors preferred to use a silyl derivative of HEMA, in order to decrease the difference in polarity with MMA and achieve better control over the polymerization. 400 An amphiphilic block-comb terpolymer containing an hydrophobic block of tBMA and an hydrophilic block containing DMAEMA and a PEG-grafted acrylate was entirely made by RAFT polymerization (Figure 18).401 The obtained polymer 204 has been further shell cross-linked via the nitrogen atoms using 1,2-bis(2-iodoethoxy)ethane as cross-linking agent.

Figure 10. Schematic synthesis of HEGMA, BzMA, and EGDMA star copolymers. Adapted with permission from ref 388. Copyright 2005 Elsevier.

4. PROPERTIES OF POLYMERIC SURFACTANTS 4.1. Surface Properties

391

bone ) has been prepared by condensation of AB2 type monomers represented in Figure 12.392 The exact structure and molecular weight of these polymers are not very well defined. “Click” chemistry can represent a very versatile way to comblike copolymers. For example, a block-graft copolymer of NIPAM and PCL 199 with various pendant groups has been synthesized by ring-opening polymerization of α-Cl-CL on a PNIPAM macroninitiator. The chlorine was then converted into azide, and various alkynes have been attached to it via a click reaction (Figure 13).393 Click chemistry offers a very versatile tool for the synthesis of libraries of polymeric surfactants with various complex architectures, as exemplified in a recent work of Ren et al.394 Other copolymers based on PCL have been prepared by previous

Polymeric surfactants, because of the presence of hydrophilic and hydrophobic moieties, have been widely studied for their selfassembling properties,373 yielding in general a huge variety of possible micelles and similar structures.113,115,118,224,255,301,387,402−405 Aggregation of polymeric structures into micelles or vesicles represents a crucial topic when considering applications of these materials in the biological/biomedical world.406−408 The general idea in this case is to use such aggregates as loci for the dispersion/ solubilization of different chemicals and drugs.8,36,304 The subject of micellization behavior for polymeric materials has been extensively studied in the past 20 years and has already been reviewed.21,58,409,410 As a consequence, in the present work, the focus will mainly lay in the underlying factors for this behavior 8533

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Figure 11. Schematic synthesis of copolymer 198.

and, mostly, on the relationship between the latter and the enduse properties of polymeric surfactants. We start by noticing how the aggregation provides a variety of structures and morphologies,119,411 including superaggregates and Janus particles.412 The latter seems to indicate that the driving force for micellization is in the case of polymeric surfactant remarkably strong and able to overcome other counteracting forces, e.g., steric hindrance. Indeed, this is shown by the tendency to aggregation also for hyperbranched macromolecular architectures, as in the case of 201 P(NIPAM),397 for which steric hindrance constitutes indeed the main factor preventing micellization. The relatively high molecular weight of polymeric surfactants, as opposed to low molecular weight ones, allows a fine-tuning of the micelle’s properties. In this respect the dependence of the aggregate structures on temperature,413 pH,77,90,95,254,414−419 external

Figure 12. AB2 type monomer for condensation synthesis of multibranched polymeric surfactants.

Figure 13. Schematic synthesis of polymers 199. Redrawn with permission from ref 393. Copyright 2011 Wiley. 8534

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Figure 14. Schematic synthesis of polymers 200. Redrawn with pernmission from ref 395. Copyright 2006 American Chemical Society.

Indeed, Garnier et al.,303 when working on several different copolymers of butyl acrylate 129−134 (PBA, hydrophilic block of constant length), clearly observed how the block length and not its chemical structure mainly determine the kind of aggregate and mostly its size. In this case the studied hydrophilic blocks comprised hydrophilic acrylic derivatives, including one (positively) charged one. On the other hand, Zhang et al.,306 when working on copolymers with the same hydrophobic block (PBA) but a significantly different hydrophilic one 135, were able to suggest exactly the opposite. Indeed, they observed that the aggregation behavior, as shown by the CMC values, as well as the surface activity of these copolymers are predominantly determined by the chemical structure of the hydrophilic block rather than by its length. The CMC value is often taken as representative for the aggregation as well as the surface activity behavior of a given polymeric surfactant. The CMC is in most cases clearly dependent on block length and structure. Indeed the change in Gibbs energy for micellization (ΔGmic) is usually expressed as100

Figure 15. Schematic synthesis of polymers 201.

ΔGmic = −RT ln(CMC)

with R being the ideal gas constant and T being the absolute temperature. ΔGmic is in turn a function of the chemical structure as well as block length, with the dependence becoming explicit when making allowance for group contribution theories, often used for the calculation of ΔGmic values. 4.1.1. Macrosurfactants. CMC of Diblocks. Theoretical treatments of CMC for both small molecules and macrosurfactants predict that the CMC decreases exponentially with the length of the hydrophobic group (or block), while the dependence from the hydrophilic block length is much less pronounced.425 Experimentally, it has been shown that CMC ≈ exp(−cN)where c is a parameter dependent on the Flory− Huggins interaction parameter and N is the length of hydrophobic group (or block)only for low N. For higher N

Figure 16. Schematic synthesis of polymers 202. Adapted with permission from ref 398. Copyright 2002 American Chemical Society.

shear field,420 and salt concentration70,390,421 have been reported already and extensively studied. The kind of aggregate can be molecularly controlled, in the case of block copolymers, by a correct design of the blocks’ chemical structure,102,236,390,422 length,91,92,229,423 and topology.88,424 In this respect an absolute ranking of these two factors, and even less a general quantification of both, is still lacking in the open literature.

Figure 17. Schematic synthesis of polymers 203. 8535

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Figure 18. Schematic synthesis of polymers 204.

Figure 19. Dependence of CMC from hydrophobic block length. Adapted with permission from ref 425. Copyright 2012 American Chemical Society.

Figure 20. Relationship between partition coefficient of pyrene and number of propylene units (left) and CMC (right) for Pluronic systems. Adapted with permission from ref 427. Copyright 2000 American Chemical Society.

relationship between logP and logCMC has been observed (Figure 20b), suggesting that micellization and solubilization of hydrophobes in Pluronic systems are favored in a similar fashion by common factors, particularly, the size of the hydrophobic block.427 The general principle that CMC decreases with the hydrophobicity of the insoluble block has been established on the basis of several different systems. Lee and Wu393 prepared different copolymers of NIPAM 197 (considered here the hydrophilic monomer) with a series of caprolactone derivatives. The

the dependence becomes weaker and CMC scales with exp(−cN1/3). This behavior seems to be universal, as shown by the data collected for several systems425 (Figure 19). A theoretical model has been proposed to explain this dependence, but it is based on an unusual aggregation mechanism.426 On the basis of simple interfacial arguments, other authors found that the CMC should scale with exp(−cN2/3).425 A good correlation between partition coefficient of pyrene in Pluronic polymers and the number of PO units has been found for a large number of systems (Figure 20a). Also, a linear 8536

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corresponding CMC values (Table 10) display a wide range of variability depending not only on the length of hydrophobic block but also on its nature.

Table 11. CMC Values for Star-(block)-Copolymers of Ethylene (EO) and Propylene (PO) Oxides 182 and 183; Adapted with Permission from Ref 382; Copyright 2009 Elsevier)

Table 10. CMC Values for Copolymers of NIPAM and CL Derivatives 199 (Adapted with Permission from Ref 393; Copyright 2011 Wiley) copolymer

NIPAM

CL

CL substituted units

CMC (mg/L)

199a

10 10 10 36 36 10 10 36 36 10

13 26 33 21 26 8 26 21 28 31

10 16 26 21 25 8 25 19 27 22

5.01 3.31 2.04 9.77 3.98 3.63 2.16 5.86 4.07 7.15

199b

199c

copolymer

CMC (mg/L)

182 [(PO)36(EO)100]7 183 [(PO)36(EO)100(PO)36]7 183 [(PO)56(EO)100(PO)56]7

306 202 118

polyelectrolytes130 and also for hyperbranched systems, like 184.383 The authors attribute the observed effect to hydrophobic interactions present in solution. By increasing the hydrophobic character of the copolymer (either by changing the chemical structure of the block or its length), the chains display a more favorable tendency to aggregate at the CMC, thus resulting in relatively lower values. It is worthwhile remarking how such hydrophobic interactions might dominate over electrostatic repulsion as well as steric hindrance in determining the aggregation behavior. Despite this general principle, exceptions are often reported, when complex architectures are involved. Peng et al.389 studied graft copolymers of methyl methacrylate and acrylic acid (MMA/ AA) 198 at different molar ratios from 1.5 to 11.0 mol % AA. They did not observe relevant changes (i.e., the observed differences are within the experimental error for the measurement) in the CMC values, which varied from 3.39 × 10−7 to 5.62 × 10−7 g/mL in the above-mentioned interval. On the other hand, polymers having roughly the same monomer composition and molar mass, and thus the same HLB, but different architecture can show different CMCs and surface tension values. Gibanel et al.376 studied copolymers of PS with PEO, namely, a linear diblock and a gemini-like one (172). CMC and surface activity in water solution are completely different (Figure 22). In this case the higher CMC for the gemini

In this case the more hydrophobic the PCL derivative (i.e., benzoate vs acetate vs tetra-o-acetyl-β-D-glucopyranoside), the lower is the CMC. This observation broadens the generality of the previous concept. As already mentioned, the dependence of CMC on the hydrophilic block length is predicted to be less pronounced. This aspect is usually not extensively investigated, but a systematic study of PS-b-PAA block copolymers 1102 showed that the principle holds at least for this system (Figure 21).

Figure 21. Dependence of CMC on length of hydrophilic block for PSb-PAA copolymers 1 with PS block length of 11 (▲) and 23 (●) units. Reproduced with permission from ref 102. Copyright 1995 American Chemical Society.

Figure 22. Surface tension vs log(concentration) variation for the (●) PS-b-PEO2 gemini-type 170 and (▲) PS-b-PEO linear macromonomer surfactants. Reproduced with permission from ref 376. Copyright 2001 American Chemical Society.

CMC of Complex Architectures. The same trend in the dependence of CMC on hydrophobic block length also has been observed for nonlinear systems by Gong et al.382 When studying star-(block)-copolymers of ethylene and propylene oxides 182 and 183, they observed a clear dependence of the CMC on the relative block length (Table 11), with the CMC decreasing with the relative length of the hydrophobic block. In this case it is also worth noticing the change in CMC as a function of the kind of block copolymer used, with the triblock displaying in general lower values than the diblock one. The same trend, i.e., decrease of CMC with more prominent hydrophobic character, has been detected for hydrophobically modified

derivative is attributed to the chain topology and in particular to the higher steric hindrance preventing aggregation of the macromolecules. Surface Activity. The discussion so far has been focused on the CMC values. It must be mentioned how it has been reported often to be independent of the absolute molecular weight of the block copolymer (provided that the composition remains the same).382,428 On the other hand, precisely such lack of dependence can be used to illustrate the fact that the CMC 8537

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values are often insufficient when trying to study the surface activity of polymeric surfactants. It is worth noticing that the determination of CMC for macrosurfactants, especially for those known to give “frozen” micelles, has been questioned.134 Given the very low CMC values usually found for macrosurfactant, it has been proposed that the technique of pyrene fluorescence (often used in these cases) could give an artifact: the partition of pyrene between micelles and water could be in favor of the water just because of the large difference in concentration and not because of the dissociation of micelles. Dependence of Surface Activity on Molecular Weight. It is possible to observe that in general polymeric surfactant, when dissolved in water, causes a decrease in the corresponding surface tension. This is normally attributed to a specific orientation of these copolymers at the surface with hydrophobic block pointing at the air side of the surface. However, the macromolecular nature of the surfactant has direct consequences for the nature of this behavior. Indeed, for the same copolymers mentioned above,428 displaying the same CMC as a function of the average molecular weight, a clear dependence of the surface activity on the same parameter (average molecular weight) is clearly detected (Figure 23).

Figure 24. Concentration dependence of the surface tension curves of aqueous solutions of 35 with different PMMA block length: no PMMA (■); PMMA13 (⧫); PMMA25 (●); PMMA45 (▲); PMMA60 (*); random PMMA21 (○); and random PMMA53 (Δ). Reproduced with permission from ref 181. Copyright 2011 Elsevier.

Dependence of Surface Activity on Chemical Structure. The surface activity of a polymeric surfactant is sensitive to minor changes in the block chemical structure. Antoun et al.261 were able to illustrate this principle in a very elegant way. They studied copolymers of MMA with 2-(dimethylamino)ethyl methacrylate 89, with the latter group being used as such or after quaternization with ethyl (89b), pentyl (89e), or heptyl (89f) groups. When comparing the corresponding surface tension in water solution, relevant differences could be observed (Figure 25).

Figure 23. Critical micelle concentrations of PEO-PPO-PEO macrosurfactants determined from the measurement of surface tensions: Mn = 1100 g/mol (■); Mn = 2000 g/mol (●); Mn = 2800 g/mol (▲). Reproduced with permission from ref 428. Copyright 2002 Wiley.

Dependence of Surface Activity on Monomer Distribution. Furthermore, the kind of copolymer (i.e., block vs random) has a prominent influence on the corresponding surface activity. When considering181 copolymers of sulfonated polystyrene (in the form of their Na salt) and MMA 35, the surface tension curves of the corresponding water solutions (Figure 24) are a clear function of this parameter. It is clear how the block copolymers (closed symbols) display in general higher surface activity than the random ones (open symbols) at comparable molecular weights. This has also been demonstrated by Liu et al.248 using polymers 85−87. Polymer 87 (with a grafting ratio of 0.04, n = 8) has a lower CAC compared to polymer 85 (with a grafting ratio of 1.02, n = 8). Even with the higher grafting ratio (more hydrophobes), the block-like distribution present in polymer 87 leads to a lower CAC compared to the random distribution of the hydrophobes in polymer 85.

Figure 25. Concentration dependence of the surface tension of aqueous solutions of the PMMA-b-PDMAEMA 15/85 diblock 89 nonquaternized (Q0) and quaternized by different alkyl bromides: 89b (Q2), 89e (Q5), and 89f (Q7). All the solutions are added with NaCl (0.1 M). Reproduced with permission from ref 261. Copyright 2001 Elsevier.

In particular, quaternization by ethyl groups results in the higher CMC and lowest surface activity. This can be explained by a change of the conformation of the chains at the water−air interface. Upon quaternization electrostatic repulsion is generated, thus leading to a low degree of packing of the chains at the interface. The general principle is that a decrease in the charge density of the hydrophilic block results in lower CMC. The differences with 89e and 89f are due to the formation of 8538

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AA136 or DMAEMA432) is randomly incorporated up to 50 mol %. 4.1.2. Polysoaps. PAM-Derived Polysoaps. Two studies investigated the association behavior of polymer 158 (R1 = NIPAM, R2 = CH3) using fluorescence spectroscopy. Zhang et al.345 demonstrated that the association degree (indicative for surface activity) increases with the incorporation rate of the hydrophobic group, and Li et al.433 observed a decrease in CAC from 3.1 to 0.8 g/L as the incorporation rate of the hydrophobic group increased from 0.06 to 0.88 mol %. One improvement for the fluorescence studies is the fluorocarbon-substituted pyrene, suggested by Li et al.433 The fluorocarbon-modified pyrene is more informative for hydrophobic association given its better affinity for the microdomains of the fluorocarbon-modified amphiphilics. For polymer 158 (R1 = OH, R2 = CH3), the decrease of the I1/I3 ratio is more pronounced as the incorporation rate of the hydrophobic monomer is increased.55 Similarly, the CAC of polymer 158 (R1 = NIPAM, R2 = H) decreases from 4.5 × 10−4 to 1.3 × 10−4 g/mL as the incorporation rate of the hydrophobic monomer is increased from 0.113 to 0.227 mol %.346 The interfacial tension decreases upon increasing the incorporation rate, from 2 to 3 mol %, of the hydrophobic monomer in polymer 148 (R1 = 2, R2 = H).65 However, the surface tension does not change significantly when the incorporation rate is increased. The same trend is observed for polymer 148 (R1 = 1, R2 = H) when the incorporation rate is increased from 3 to 5 mol %.65,434 However, the comparison is not completely sound given the fact that the molecular weights of both polymers (148 [R1 = 1, R2 = H] and [R1 = 2, R1 = H]) are not provided. Polymer 148 (R1 = 3, R2 = H) displays a much lower IFT than polymers 148 [R1 = 1, R2 = H] and [R1 = 2, R2 = H]; however, the comparison is not justified given the differences in polymer composition, molecular weight, and polymer concentration at which the IFTs were measured. Polymer 148 (R1 = 3, R2 = H) is prepared using the micellar copolymerization technique,336 which yields a block-like distribution of the hydrophobic monomers. Nevertheless, polymer 148 (R1 = 3, R2 = H) is pivotal in that it was used to illustrate the relaxation processes below and above the CAC.435 Below the CAC, two relaxation processes exist, one (fast) related to the exchange of hydrophobic microdomains between the proximal and distal regions in the interface and another one (slow) related to the conformational changes of the polymer backbone in the interface (oil[octane]−water).435 Above the CAC only one relaxation process exists, which is believed to be the associations formed by hydrophobic macrodomains.435 In polymer 149 the hydrophobic groups are randomly distributed as blocks (between 25 and 28 units). To determine the onset of association, i.e., the CAC, nonradiative energy transfer (NRET) studies are usually performed. Smith and McCormick339 determined the CAC for polymer 149 (m = 8) to be 0.1 g/dL (1000 ppm). A decrease in the CAC is observed when the length of the hydrophobes is increased, i.e., the CAC drops to 0.04 g/dL (400 ppm) when m = 10 in polymer 149.338 A similar trend was observed for polymer 162, where the CAC decreases from 0.05 (polymer 162, m = 7) to 0.025 wt % (polymer 162, m = 11) with the increase in length of the hydrophobe.353 One can argue that the CAC can be lowered even further by using longer hydrophobes; however, this will lead to insolubility as demonstrated by increasing the length of the hydrophobes (polymer 149, m > 12).338 The water solubility of polymers 144 and 145 is dependent on the incorporation rate of the hydrophobic (DEmMA) unit.

hydrophobic aggregates (of the alkyl groups) rendering the situation more difficult. Indeed, such interaction, even at relatively low concentration of the responsible groups, might prevent migration of the polymeric chains toward the interface.64 This interplay between electrostatic interactions and hydrophobic ones is quite general and observed also in other systems. Fischer et al.218 prepared copolymers of dimethylacrylamide (DMA) with N-hexadecyl-4-vinylpyridinium bromide 53a. The corresponding surface tension measurements (in a mixture of solvents) are a clear function of the DMA block length as well as its relative percentage, with an exact interpretation being hindered by the use of a three-component mixture as solvent. Another consequence of the macromolecular nature of polymeric surfactants resides in anomalous micellization behavior as well as in the corresponding surface activity. Indeed, many systems are actually characterized by a so-called “schizophrenic behaviour”,68,69,71,278,279 i.e., the presence of normal micelles, inverse ones, and molecularly soluble species as a function of slight changes in external parameters such as temperature or pH (Figure 26).

Figure 26. Schematic schizophrenic behavior. Adapted with permission from ref 67. Copyright 2001 Wiley.

In this case the block structure and length are considered to be crucial in determining the copolymer solubility and thus its behavior in water.429 To conclude the discussion on surface activity and micellization of macrosurfactants, it is worth noticing that the fundamental research in the field is still active. The micellization dynamic and the mechanisms lying beneath are not yet completely elucidated, and they might sensibly differ from what is observed for low-molecular-weight surfactants.430 For example, for some charged polymeric surfactants, the surface activity decreases upon addition of salts, contrarily to what happens for their low-molecular-weight counterparts.431 Several experiments show that in most cases the surface activity of macrosurfactants is detectable only when the hydrophobic block is sufficiently short (below 20 units in case of 34, for example). More in general, it seems that, to display a clearly measurable surface activity, the hydrophobilicity of the insoluble block has to be moderate. This happens not only for short blocks of highly hydrophobic monomers96,191,254,255,258,262,301,431 but also for long blocks of moderately hydrophobic monomers (such as DEGA)117 or hydrophobic blocks in which a hydrophilic monomer (such as 8539

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CAC decreases with increasing molecular weight.340 However, surprisingly the block length and the incorporation rate of the hydrophobic groups do not decrease the CAC.340 Polymer 148 (R1 = 3, R2 = COOH) has a CAC value of 0.5 g/dL, which is comparable to polymer 149 (m = 8).337 The position of the hydrophobic groups is also an important parameter that affects the CAC. This has been demonstrated using polymers 151 through 157. Although the CAC was not directly measured, the concentrations (Cη) at which the solution viscosity departs from the values of the unmodified polymers were measured. The theory of Einstein stipulates that the solution viscosity (of a dispersion) in the dilute regime is determined by the size of the particles (or aggregates). The solution viscosity of a semidilute solution is determined by the size of the particles and their interaction with one another. The size of polymer particles is dependent on the molecular weight and on their conformation. With the presence of hydrophobic groups, the polymer coil can be smaller than the unmodified polymer at the same molecular weight due to intramolecular hydrophobic associations. This leads to a lowering of the critical overlap concentration for the hydrophobically modified polymers. Therefore, for polymers 151−157, the measured Cη is dependent on the critical overlap concentration (C*) and the CAC. Since aggregation occurs below the C*, the Cη is higher than the corresponding CAC. This has been demonstrated for associative polymers based on poly(ethylene oxide) (PEO).245,248,253 The Cη decreases upon increasing the length of the hydrophobic blocks in the telechelic polymer 152.343 The Cη of polymer 151 (n = 7) is higher than that of polymer 152 (n = 15).341,342 The authors341 hypothesized that the length of the hydrophobe is more important than hydrophobicity for hydrophobic association to arise. However, when comparing the multisticker [151 (n = 7) compared to 154 (m = 15)] and combined polymers [155 (n = 7) compared to 156 (m = 15)], the exact opposite is observed.341 The Cη of the combined polymer 155 (n = 7) is the lowest compared to its telechelic [151 (n = 7)] and multisticker [153 (n = 7)] analogues.341 The placement of the hydrophobic groups at the ends leads to a lower Cη compared to randomly distributed (multisticker) along the backbone or combined as demonstrated by comparing polymer 152 (m = 11 and 15) with polymer 153 (n = 5 and 7) and polymer 157 (n = 5, m = 11),343,344 which can be seen as evidence that the telechelic polymers are better surface-active agents than multisticker ones. However, this comparison is not justified given the difference in length of the hydrophobic groups (C16 and C12 compared to diC8 and diC6). A later study compared polymers 151, 153, and 154 (all with n = 7), and the Cη of the multisticker polymer was the lowest, suggesting that multisticker polymers are more surface-active that telechelic ones. However, the lengths of the hydrophobic groups (although held constant for the comparison) are just above the length below which no hydrophobic associations arise.342,436 Therefore, it can be concluded that to obtain a good surface-active agent the placement of the hydrophobic groups is dependent on its nature (length and chemistry). It was demonstrated by Yahaya et al.64 that the surface tension of the aqueous polymeric solution containing polymer 148 (random copolymer) decreases as the concentration of the polymer increases. The interfacial tension, using n-decane−aqueous polymeric solutions, also displayed a decrease with increasing polymer concentration. A small decrease in the interfacial tension was also observed with increasing salt (NaCl) concentration, up to 9 wt %. The same

Above an incorporation rate of 30 mol %, the polymers are not water-soluble.334 The CMC of polymer 144 (m = 2) decreases upon increasing the incorporation rate of the hydrophobic unit. A similar behavior is observed for polymer 145 where m = 6 and 25. However, for polymers 144 (m = 2) and 145 (m = 6 and 25), a minimum in the CMC is reached at incorporation rates of 7 and 10 mol %, respectively.329,334 Polymer 165 has a CMC of 0.6 g/L (determined via the I1/I3 ratio), which decreases with increasing incorporation of the hydrophobic groups (although the CMC of the polymer containing the highest amount of hydrophobe is provided).54 A decrease in the CMC of the copolymers of NaAMPS/DEmMA (polymer 145) is observed when the number of ethylene oxide (EO) units in the random polymers is increased.62 In addition it was demonstrated that the aggregation number (Nagg) of the C12Em (EO-containing) moieties is similar (somewhat larger) to the values observed for free C12Em surfactants where the number of EO units is 6 or 25.328,329,334 Similar results were obtained when NaAA (instead of NaAMPS, polymer 144) was used as the charged moiety.62,329 For both polymers (144 and 145), it was demonstrated that the CMCs of the polymers are between 10 and 100 times smaller than the values for the discrete (DEmMA) surfactants.328 The CMC of polymer 146 decreases as the incorporation rate of the hydrophobic moieties increases, which is in line with small surfactants.63 The effect of the chain length (number of EO units) on the CMC was not determined, although smaller micelles are formed with longer side chains (more EO units).63 Polymer 163 displays the same trend, a decrease in CAC with an increase in incorporation rate of the hydrophobic groups, as investigated using fluorescence spectroscopy.354 Although the exact CAC was not determined, the ratio between the intensities of the first and third vibronic peaks (I1/I3 ratio) is lower for polymer 163 (x = 1, y = 25, z = 0.1) compared to polymer 163 (x = 0.5, y = 25, z = 0.1). In addition the inflection point of the curve (I1/I3 ratio vs polymer concentration) occurs at a higher polymer concentration for the latter polymer. The effect of small differences in the chemistry of the surfactant monomers is demonstrated with polymer 147. The authors335 determined the CMC of the free surfactant monomers (prior to incorporation into the copolymer) and observed that the presence of an extra C2H5 group (R1 = C2H5) reduces the CMC from 0.77 to 0.56 mM. Nevertheless, it remains difficult to comment on the effect of the extra C2H5 group on the CMC of the polymer without the actual measurement. Although Noda et al.328 demonstrated that the CMCs of copolymers are between 10 and 100 times lower than the corresponding free surfactants, the interactions between the extra C2H5 group with the polymer backbone can give rise to an unexpected behavior. Zhang et al.345 demonstrated that the presence of an alkyl group can influence the interfacial properties significantly. The decrease of the intensity ratio of excimer to monomer (I550/I375) in fluorescence experiments indicates the formation of aggregates. The decrease of I550/I375 occurs at a much lower polymer concentration for polymer 158 (R1 = NIPAM, R2 = CH3) compared to polymer 158 (R1 = NIPAM, R2 = H). The location of a charge on the hydrophobic moiety also affects the surface activity of the polymer. This was demonstrated by Morimoto et al.350 with polymer 158; the amphiphilic with the charge closest to the backbone (polymer 160, R = 1) displays the lowest CMC of the two polymers. The molecular weight of the polymer also plays a crucial role for the value of CAC, with higher molecular weights displaying the lowest CAC. This was demonstrated with polymer 150 (random distribution of hydrophobic blocks) where the 8540

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Although the studies243,244 did not measure the differences in CAC, the same trend is expected when comparing polymer 78 with 79. The combination of the hexadecyl end group and the isophorone diurethane (IPDU) linking group has a higher hydrophobicity compared to the hexadecyl group only. Similar results were obtained when polymer 80 (x = 304) was compared with polymer 81 (x = 304). The subtle difference in the linking group has a pronounced effect on the CAC. Polymer 80 (x = 304) possessing ether linkages has a CAC of 8.5 × 10−5 M, whereas polymer 81 (urethane linkage, x = 304) has a CAC of 2.5 × 10−5 M.246 Even polymers functionalized at only one end display the same difference. The CMC of polymer 82 (R = C8F17, x = 120) is 20 times lower than the CMC of polymer 83.252 The molecular weights of the polymers also affect the CMC in aqueous solution, as demonstrated with one-ended fluorocarbon polymers 83. Increasing the molecular weight by a factor of 2 (from 5 to 10 kDa) results in an increase of the CMC from 1.5 to 7.0 × 10−6 M.251 The same trend is observed for the polymer series 80 (x = 227, 454, or 795). The CAC increases from 1.5 to 10 g/L upon increasing the molecular weight of the polymer from 10 to 35 kDa.245 The CAC of polymer 81 (n = 12) increases from 2 × 10−3 to 7 × 10−3 g/mL as the molecular weight increases from 12 to 20 kDa.248 In addition, it is expected that the PDI of the polymers also affects the CAC. The low-PDI polymers can associate more readily than polydisperse polymers and thus display the lowest CAC values.242 Alami et al.249 compared the micellization behavior of polymer 81 (x = 454) to that of two different low-molecular-weight surfactants (ionic SDS and nonionic C12E6) using fluorescence spectroscopy. The I1 /I3 ratio decreases over a narrow concentration range for the two lower-molecular-weight surfactants, while for polymer 81 (x = 454) it decreases over a much wider concentration range. Therefore, it is difficult to assign a CMC for the polymer. Nevertheless, the authors249 define a CMC close to the inflection point of the curve. The CMCs of SDS and C12E6 are both also close to the inflection point of the corresponding curve. The CMC of the polymer, 7.5 × 10−4 M, lies between those of the ionic and nonionic surfactants. Similar results, a decrease of I1/I3 over a wide concentration range, were obtained when comparing polymers 85−87. The CAC was defined close to the aforementioned inflection point of the curve, with the values decreasing when n (length of the hydrophobes, polymers 85−87) is increased (i.e., the hydrophobicity). Increasing the grafting density also leads to a decrease in the CAC.248 Also noteworthy is the fact that the concentration range over which the decrease of the I1/I3 is observed increases with an increase in n.248 In this case relatively lower molecular weight seems to result in better surface activity (i.e., lower surface tension of the corresponding water solution). Polysaccharides. Because of the large availability and the relative simplicity of the functionalization procedures, several naturally occurring water-soluble polysaccharides have been used as starting materials for the synthesis of polymeric surfactants. As described in section 3.2.2, the addition of the hydrophobic groups is generally carried out in conditions that afford polymers with a random distribution of the modifying moieties. The resulting hydrophobically modified polysaccharides can be included in most cases in the class of polysoaps. Many polymeric surfactants have been reported to have good emulsification and dispersion abilities but give only a moderate reduction of surface and interfacial tension.437 Hydrophobically modified polysaccharides are not an exception.The literature on hydrophobically modified dextrans with molecular weights up to 100 000 Da and

results (in terms of the trend) were obtained for polymer 148 (R1 = 2, R2 = H) (random copolymer).65 However, the decrease in surface tension and interfacial tension (in the absence of salt) is slightly more pronounced (lower values), indicating a more surface-active agent. In the presence of salt, the exact opposite behavior is observed where polymer 148 (R1 = 1, R2 = H) has a lower interfacial tension. The surface activity of an aqueous solution containing polymer 166 depends on the incorporation rate of the 3-(N,N-diallyl-N-methylammonium)propane sulfonate (DAMAPS) unit and on the pH of the solution. When the incorporation rate of the DAMAPS is zero, the surface tension of the solution decreases from 74 (pure water) to 55 mN/m2 as the polymer concentration is increased to 1 g/dL at a pH of 4.60. When the polymer concentration is increased to 0.65 g/dL at a pH of 9.71, the surface tension decreased to 33 mN/m2. According to the authors,356 as the ionization degree is decreased, the effective hydrophobic content of the polymer increases and thus hydrophobic interactions arise. Similar results are obtained when the incorporation rate of the DAMAPS unit is 44 mol %. However the decrease in surface tension with the increase in pH is not as pronounced. The difference is attributed to the high hydrophilicity of the groups even at low ionization degrees.356 The interfacial properties of polymer 161 are slightly dependent on the presence of electrolyte (NaCl).351 The I1/I3 ratio reaches the hydrophobic limit at a lower polymer concentration in the presence of electrolyte (0.10 instead of 0.15 wt %). This indicates that the hydrophobic associations arise at a lower polymer concentration in salt solutions. Feng et al.353 noticed that the addition of NaCl leads to a reduction in the I1/I3 ratio of an aqueous solution containing polymer 162 (m = 7). Similar results were obtained with polymer 164. Both studies353,355 attributed the results to the stronger hydrophobic interactions caused by the presence of salt. The surface tension of an aqueous solution containing polymer 161 decreases as the concentration increases and displays a minimum (in the lowconcentration region) at a concentration of 0.15 wt %. The lowest IFT is observed at the same polymer concentration (in an n-decane−water solution).351 The dynamic surface properties of polymer 159 have been investigated by Kopperud and Hansen.347 They demonstrated that all the amphiphilic polymers investigated display surface activity. However, the adsorption rate of the polymers to the surface layer is low. The authors347 hypothesized that the adsorption is controlled by unfolding and reconfiguration of the bulk polymer and the penetration of the surface layer by individual hydrophobic blocks. A decrease in the adsorption rate is observed when the molecular weight is increased and when the block length of the hydrophobes (maintaining a constant hydrophobe content) is increased. However, higher hydrophobe contents (maintaining a constant hydrophobe length) lead to higher adsorption rates. HEUR. The association behavior of polymer 77 was investigated using fluorescence spectroscopy. The emission intensity increases when hydrophobic associations arise, indicating the formation of micellar aggregations. Increasing the length of the hydrophobes, i.e., the hydrophobicity, leads to aggregation being formed at lower concentrations.242 This was demonstrated with polymer 84 where the CAC decreases from 5 × 10−2 to 1 × 10−4 g/mL when an octadecyl is used as the hydrophobic group instead of dodecyl.248 The linking group also affects the CAC in that the more hydrophobic (H12MDI [a] is more hydrophobic than HDI [b]) it is, the lower is the CAC.242 8541

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most studied systems are AB diblock and ABA or ABC triblock (telechelic) copolymer in which the hydrophilic block B is constituted by PEO or PAA and the hydrophobic blocks are constituted by polystyrene or aliphatic alkyl chains.442−444 These kinds of polymers form viscoelastic solutions that present a transition to gels at a certain critical concentration, depending on the composition and the architecture of the polymer. Polymers containing a polyelectrolyte chain such as PAA are of particular interest because the presence of Columbic interactions leads to highly stretched conformations in solution, which causes a dramatic increase in moduli and viscosity at relatively low concentrations and an unusual rheological behavior. Moreover, the charge density and the extent of ionic interactions are dependent on pH and ionic strength, and consequently, the rheology can be tuned by acting on these parameters. A deep study of the rheology of telechelic ABA and ABC copolymers with a charged PAA or PMAA hydrophilic middle block has been performed by Tsitsilianis and coworkers.147−150,445 In this case the two associative blocks can assemble either into the same core, forming “flower-like” micelles, or into different cores, forming bridges and thus a gel via formation of transient networks (Figure 28), depending on the concentration and the compatibility of the hydrophobic blocks.150 A PS-PAA-PnBA (PnBA = poly(n-butyl acrylate)) triblock copolymer 16 shows a similar viscosity profile to the analogous PS-PAA-PS system 15 (Figure 29) but a lot higher absolute values of viscosity and yield stress at the same concentration.150 According to the authors, the incompatibility between the PS and the PnBA blocks would favor the intermolecular aggregation over the intermolecular one, explaining the observed behavior. When the hydrophilic block is constituted by a charged polyelectrolyte, it is expected to have an extended conformation in water due to repulsive electrostatic interactions. As a consequence, 15 exhibits a gel concentration about 1 order of magnitude lower than that of the latter PEO derivative in which the middle block has comparable length. 148 A viscosity/ concentration profile similar to that of 15 was observed for a MMA-b-DMAEMA-b-MMA telechelic copolymer 91 but, remarkably, direct observation of the micellar structure by means of atomic force microscopy (AFM) microscopy reveals the absence of “loops”. This has been attributed to the shortness of the DMEAMA middle block.265 Diblock copolymer aggregates in solution are normally constituted by a dense rigid core surrounded by a hydrophilic corona. The rheological properties of these polymers are determined by the interactions between the coronas, but little is known about the interaction mechanism.446 Anyway, at sufficiently high concentrations the micelles are jammed and the solution stops flowing, forming a viscoelastic gel.444 A relatively short PS20-PAA85 polymer 1 has been studied by Korobko et al.446 It exhibits a Newtonian plateau at low shear in a large interval of concentrations and shear thinning at high concentrations. Also in this case, the zero shear viscosity increases dramatically with polymer concentration, from 1−4 orders of magnitude greater than water. Addition of salt (1 M KBr) yields solutions with a zero shear viscosity comparable to that of water. This behavior is explained in terms of chain collapse due to shielding effects, with a consequent weaker interaction between the micelles.443,446 If hydrophobic groups are present in the polyelectrolyte block they can act as “stickers”, favoring the gelation of the system (Figure 30).443 PS-b-(AA-co-EA) copolymers show a decrease in

with a degree of substitution (DS) up to 0.2, as summarized by Rotureau et al.,362 shows that there is little influence of the nature of the hydrophobic group and the DS on the CMC, which is generally in the range of 1−2 g/L (10−5−10−6 mol/L). On the other hand, the surface activity above the CMC decreases sensibly by increasing the DS. Similar or slightly lower values and the same trend have been observed for other studied polysaccharides such as hydroxyethylcellulose,357−360 carboxymethylcellulose, 361 carboxymethylpullulan,366,367 and pectin;370 therefore, it seems to be quite general. Comparable values of CMC (expressed in mol/L) have been found also for hydrophobically modified pectin with a low molecular weight (∼4000 Da) 368 and for a dextran functionalized with relatively long PLA-insoluble chains, which can be considered more like a comb polymer. 372 Regardless of the structure, the decrease of surface tension is generally moderate, with typical values being around 45−55 mN/m and in any case not less than 30 mN/m. However, stable O/W emulsions are formed for many of the studied systems.361,363−365,368,438−441 The rheological properties of these polymeric surfactants, which show gelation properties,327 seem to play an important role in the emulsion stability.371,438 To our knowledge, no systematic study concerning the dependence of surface activity on the molecular weight of the polymer has been reported yet. The adsorption of hydrophobically modified polysaccharides at the air/water as well as at the oil/water interface is generally slow, so a gradual decrease with time of the surface/interface activity to an equilibrium value is often observed with these systems. Dynamic surface activity measurements have been carried out in some cases.358,362,371 A higher degree of hydrophobic substitution361 or the presence of a charged substituent362 seem to accelerate the adsorption process. The adsorption of the polymeric surfactant is considered to be a diffusion-controlled process at short times, with negligible desorption and no interaction between adsorbed and adsorbing macromolecules, while for longer times the process is better described as a Langmuir isotherm.362 Some proposed mechanisms also take into account a barrier effect or reorientation/rearrangement of the polymer at the surface.358,362 In the case of a hydrophobically modified chitosan, various stages of adsorption and folding/unfolding of the polymer have been proposed to explain the presence of lag times in the dynamic surface tension and some rheological data (Figure 27). 371 4.2. Rheology of Macrosurfactants

Despite the large amount of publications concerning synthesis and self-assembly of amphiphilic copolymers, the rheology of these systems has not been extensively investigated so far. The

Figure 27. Schematic representation of different stages of the alkylated chitosan adsorption. Adapted with permission from ref 371. Copyright 2005 Elsevier. 8542

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Figure 28. Schematic representation of the gelation mechanism of 16/water system. Reproduced with permission from ref 150. Copyright 2005 American Chemical Society.

Figure 29. Double-logarithmic plots of shear viscosity at (γ = 1 s−1) for PS-PAA-PS (left) and zero shear viscosity for PS-PAA-PnBA (right) versus concentration. Adapted with permission from refs 148 and 150. Copyright 2000 and 2005 American Chemical Society.

in the literature where the dependence of the rheological properties from the molecular architecture of block polyelectrolytes is the subject of the study. Rheology of star PAA polymers capped with short PS ends also has been studied.99,447 Also these polymers form elastic gels in water at a relatively low concentration (∼2.2 wt %).99 As for other PAA-based copolymers, a drop in viscosity is observed upon addition of salt (0.1−0.9 M NaCl), but the system still presents the characteristic of a gel, showing that the polyelectrolyte screening upon salt addition shrunk the PAA chains but did not affect the hydrophobic association.99 The authors compared their findings with other results published by other groups for analogous linear systems. Elastic moduli of the studied star polymers are comparable with those of diblock PS-PAA 1 and triblock PSPAA-PS 15 copolymers, but the gelation concentration is sensibly higher. This difference can be attributed to the different topology, as starlike polymers are likely to have a smaller hydrodynamic radius than a corresponding linear one, but the longer length of the hydrophobic blocks and the higher

Figure 30. PS-PAA micelles associated via ethyl acrylate stickers in the PAA block. Reproduced with permission from ref 443. Copyright 2006 American Chemical Society.

the gelation concentration and an increase in viscosity and elastic moduli by increasing the amount of EA groups scattered along the AA block.443 Raffa et al. investigated the rheology of a diblock 18, a triblock 33, and a four-arm block star 180 PS-b-PMAA copolymer prepared by ATRP.85 This work is one of the very few examples 8543

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The close relationship between hydrophobic interactions and viscosity of the corresponding water solution has been recently reviewed.327 For the present paper it suffices to underline the general trend, i.e., an increase in the absolute viscosity as a function of the salt concentration, as the one measured for copolymers of acrylamide and styrene of the polysoap type 161 (Figure 32).351

molecular weight of the linear polymer make the two systems difficult to compare.99 When a linear PS-PAA-PS 15 copolymer with comparable PS end blocks length and PS/PAA ratio is considered, the star polymer results in more effective network formation and gel with higher elasticity.447 The solutions of polymers with longer PS ends form gels characterized by short linear viscoelastic region, more pronounced shear thinning, and less marked gel softening with temperature. This behavior is rationalized by the authors considering that longer PS chains lead to stronger hydrophobic association. 447 Besides application as emulsion stabilizers, polymeric surfactants have been also claimed in other fields. In particular, the presence of hydrophobic blocks renders these materials suitable as viscosity modifiers.131 In this case the same hydrophobic interactions responsible for the surface activity and micellization behavior are also the cause for the increased viscosity. Colinet et al.,448 when studying graft copolymers of CL onto alginates, pointed out the fact that these two effects are actually coupled with each other simply because of a common origin. Indeed, when looking at viscosity as a function of the grafting efficiency (Figure 31a), a clear trend is detectable for both PCL average molecular weights.

Figure 32. Viscosity of a 2% solution of 161 as a function of NaCl concentration at three different shear rates. Reproduced with permission from ref 351. Copyright 1999 Wiley.

As the salt concentration increases, hydrophobic interactions become stronger, which renders the apparent viscosity higher than in the case where no salt is present. The same kind of parallel dependence (of aggregation and viscosity) can be found as a function of the polymer topology.96 Besides direct correlation, also conceptual discrepancies are observed. Indeed, Eghbali et al.125 prepared copolymers of butyl acrylate and acryclic acid 3. They measured surface tension and viscosity of the corresponding water solutions as a function of the pH. The latter has a direct influence on the ionization of the −COOH groups (of the AA block) and thus on the ionization degree (α), defined as the fraction of −COOH groups ionized at a given pH. While the surface tension did not change as a function of the pH, the solution viscosity shows the presence of a maximum (Figure 33). The authors were able to explain this behavior by assuming that, at about α = 0.5, corresponding to the maximum position in Figure 33, the polymeric chains are already fully stretched in water solution and that a further increase in the amount of NaOH (and thus in pH and α values) does not result in any conformational (and thus viscosity) change. A maximum in viscosity, this time as a function of temperature (Figure 34), also has been observed for copolymers of caprolactone (CL) and ethylene oxide (EO).449 Such a maximum is attributed in this case to a sol−gel−sol transition, which can take place at relatively high concentration. It is also interesting to notice how an increase in the CL block length, going from (a) to (c), results in a lower onset for the maximum. This suggests once more the close relationship

Figure 31. Intrinsic viscosity (a) and critical overlap concentration (CCr) (b) for PCL-grafted alginates. Data reproduced with permission from ref 448. Copyright 2009 Elsevier.

In both cases the decrease of [η] with the grafting efficiency can be attributed to the screening of the electrostatic repulsion of the alginate precursor as well as to the increased chance of hydrophobic interactions with the increasing number of available PCL chains. Indeed, the observed trend is practically mirrored by the one of the critical overlap concentration (CCr) as a function of the same grafting efficiency. In a semidilute regime, as the one presently used, such concentration is indicative of the CMC and is also a function of the interaction forces present in the system. The underlying concept here is that the same two factors promoting association (i.e., screening of the electrostatic repulsion between the backbone as well as favored hydrophobic interactions) are also responsible for the decreased viscosity. 8544

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Application of polymeric surfactants in gene and drug delivery is becoming very popular. These and related applications are based on the dependence of the hydrophilicity of some moieties on external parameters (such as temperature, pH, and ionic strength). This can induce the controlled formation and disruption of micelles when the polymeric aggregate experiences some change in such parameters when introduced in a living system and in particular conditions. This topic is extensively covered in many recent reviews,13,35,40,71 and it will not be further discussed here. The importance of polymeric surfactants for many applications can be seen in the fact that in many cases both rheology and surface properties play a relevant role in this respect. In turn, these features are directly related to the chemical composition and the structure of the polymer. We will analyze these aspects in more detail for what concerns emulsion stabilization, coating, and oil recovery, categories which include most of the known industrial applications of polymeric surfactants. Even if this aspect will not be treated here, it is important to note that the stimuli-responsivity of some of these polymers allows the preparation of smart emulsifier, smart coatings, and so on, greatly improving the applicative possibilities for these classes of materials.35

Figure 33. Zero shear viscosity of a 0.5% solution of 3 as a function of α. Reproduced with permission from ref 125. Copyright 2006 American Chemical Society.

5.1. Emulsion Stabilization

One of the main applications of polymeric surfactant is the use as stabilizers in emulsion polymerization.16,19,22,97,116,123,192,450 In this context they are believed to act as stabilizers (probably via a steric repulsion mechanism) against coalescence122,183 and Ostwald ripening.259 They also decrease the interfacial tension between the two phases.451,452 Moreover, their chemical composition determines the emulsion structure according to the Bancroft rule in a way comparable to classic low-molecularweight surfactants.388 However, a main difference with their lowmolecular-weight counterparts is the fact that the behavior of the polymeric surfactant can hardly be predicted by the hydrophilic− lipophilic balance (HLB).453 For the polymeric surfactants to be active in their action, they have to be adsorbed at the interface between water and the apolar medium. In this respect the adsorption process is clearly influenced by the same factors outlined above for the surface tension at the water/air interface, i.e., pH,117,125,191,239,277,454,455 block structure and length,325,456,457 concentration,428 and topology.96 The block length is in many cases crucial in determining the adsorption at the interface. Indeed, when considering polymeric compounds, one has to consider the folding of the chains at the relevant side of the interface. Gref et al.228 illustrated this for copolymers of P(EG-b-LA) (Figure 35). When increasing the relative PLA (hydrophobic) block length with respect to the PEG (hydrophilic) one, the former chains assume a more sparse conformation at the interface, thus factually limiting the adsorption at the interface. The situation is even more complicated if one takes into account the influence of the chain topology (and related steric hindrance) on the adsorption.89,378,379,458 In particular the difference between block and graft copolymers often results in different chain topologies at the interface and thus different stabilization mechanisms via steric repulsion. Exerowa et al.459 illustrated this (Figure 36), although by using different materials. The difference in chain topology at the interface has a clear importance in determining the stability of the emulsion. Indeed, the CAC (critical aggregation concentration) is again a function

Figure 34. Viscosity vs temperature curves for different diblock copolymers at 20 wt % concentration: (a) E17C24, (b) E17C21, and (c) E17C16. Reproduced with permission from ref 449. Copyright 2006 Wiley.

between hydrophobic interactions (or associative behavior) and the rheological properties.

5. LINKS BETWEEN PROPERTIES AND APPLICATIONS OF POLYMERIC SURFACTANTS As already mentioned in the Introduction, the applications of polymeric surfactants are widespread. They have been extensively discussed in books and reviews; thus, the main purpose of this section will be to outline the correlation between physical properties of the polymeric surfactants and their applications. For a detailed description of the polymer actually used for a given application, the reader will be referred to previous literature. The most studied systems from the point of view of the applications are block copolymers (di- and triblocks, mostly), where the hydrophilic moiety is constituted by PEO and the hydrophobic one is PPO, PBO, or, more rarely, styrene. These kinds of polymeric surfactants (which include the commercially available Pluronic and Synperonic series) have been extensively employed as dispersants and an emulsion stabilizer and can be found in formulations for detergents, dyestuffs, paints, coatings, agrochemicals, ceramics, printing inks, etc. 8545

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Figure 37. Adsorption at the interface for block and graft copolymers. Adapted with permission from ref 395. Copyright 2006 American Chemical Society.

A classical example of structure−properties relationship is given by the use of Pluronic polymers as emulsifiers: triblock PEO-b-PPO-b-PEO are more suitable for O/W emulsion, whereas for W/O emulsions PPO-b-PEO-b-PPO are preferred. Moreover, systematic studies proved that there are optimums in molecular weight and HLB, which affect important parameters like interfacial tension, partition coefficient, thickness of the adsorbed layer, and steric stabilization.18,29 Also rheology plays a relevant role in emulsion stabilization. Because of the differences in density, the dispersed phase can migrate under the action of gravity to the top (creaming) or the bottom (sedimentation) of the dispersing phase. This phenomenon can be described by the Stokes equation ( eq 1),

Figure 35. Scheme illustrating the hydrophobic PLA block length on the density of filling of surfactant molecules at the oil/water interface. Reproduced with permission from ref 228. Copyright 1998 Elsevier.

v=

2Δρgr 2 9η0

(1)

where v is the droplet velocity, Δρ is the difference in density, g is the gravity acceleration, r is the droplet radius, and η0 is the viscosity of the medium. From the equation it can be seen that the segregation can be steered by increasing the medium phase viscosity (water). A polymeric surfactant acting at the same time as a solution thickener and as a surface-active system represents a successful choice for this kind of application. The same principles apply to emulsion and or dispersion polymerization. The use of polymeric surfactant in emulsion polymerization is also well-established and has been reviewed, also recently.16,18,19,22,122 The compatibility between the hydrophobic block and the forming latex particles seem to play a crucial role in the quality of the formed material.307

Figure 36. Schematic representation of conformation of triblock (left) and graft (right) copolymeric surfactants at oil/water interface. The situation when the oil droplets approach to a separation distance h that is smaller than twice the adsorbed layer thickness δ is illustrated also. Adapted with permission from ref 459. Copyright 2009 Elsevier.

5.2. Coatings

Polymeric surfactants often have been envisaged as suitable materials for applications in the coatings and paints industry.4,7,16,18,21,33−35,37,48,460 However, the analysis of the literature shows that the claimed use of polymeric surfactants for industrial coatings and paints formulations is in most cases only potential and rarely actual. Again, the interest of using polymeric surfactants for such applications originates from the importance of both rheology and surface properties of the required materials. Important phenomena for coating such as coalescence, wetting, sagging, and leveling are governed by viscosity, yield stress, nonNewtonian behavior, elasticity, and surface tension. For example, leveling of a sinusoidal surface has been modeled, and the leveling speed tv has been expressed as

of chemical composition, topology, and (in this case) not of the molecular weight (F108 = 14.000 EO128PO54EO128, P104 = 5.900 EO31PO54EO31). In many cases graft copolymers have been considered as more effective than block ones (of comparable average molecular weight).395 This has been attributed (Figure 37) to a different adsorption of the copolymer at the interface. Such dependence of the surface activity with the chain topology is quite general if one excludes the special case of polysoaps.392 The reason for such a discrepancy from the general behavior is not discussed in the corresponding paper, even if it is worth remarking on the fundamental difference in chemical structure between block copolymers and polysoaps as a possible reason.

tv =

16π 4h3γ ⎛ at ⎞ ln⎜ ⎟ 3λ 3η ⎝ a0 ⎠

(2)

where h is the average thickness of the coating layer, λ is the wavelength, at and a0 are, respectively, the final and initial 8546

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amplitudes, γ is the surface tension, and η is the viscosity at low shear rate (the zero-shear viscosity is often used).461 It can be noted here that, to achieve good leveling, high surface tension and low viscosity are optimal. On the other hand, good coalencence is given by low surface tension and sagging is prevented by high viscosity. Thus, it is not easy to predict theoretically the best system to be used for a desired coating.

Ca =

ΔP·k γ

(3)

where ΔP is the gradient pressure, γ is the interfacial tension between the two immiscible phases (often also indicated with IFT), and k is a constant. According to Darcy’s law, the equation can be rewritten as v ·ηc Ca = γ (4)

5.3. Oil Recovery

The potential of amphiphilic polymers for enhanced oil recovery (EOR) has been recognized, mainly because of their ability to form shear-dependent transient association in water with subsequent solution thickening at high salt concentrations and in extensional flow.7,57,462 Surprisingly, less attention has been paid to emulsification and surface properties of such polymers, which is also a crucial parameter for EOR. In fact, such polymeric systems are usually employed in combination with surfactants and alkaline compoundsthe so-called surfactant−polymer (SP) and alkaline−surfactant−polymer (ASP) formulationsin which the polymer is added only to increase the viscosity of the water, employed as a displacing fluid, while the other components act as emulsifiers. Polymers proposed as solution thickeners have been reviewed recently.327 Chemical methods for EOR consist of the injection of a displacing fluid in oil reservoirs, in order to mobilize the crude oil adsorbed in the porous rocks. The displacing fluid is generally constituted by a dilute water solution containing an inorganic base, a surfactant, a water-soluble polymer, or combinations of these three components.463,464 The scope of the base (generally a hydroxide as NaOH or a carbonate) is the same as the surfactant, which is to decrease the interfacial tension between water and oil phases. Indeed, the base reacts with the organic acid components of the crude oil, forming a surfactant in situ.464 Polymer flooding for EOR has been extensively studied.327 The mechanism of enhanced recovery is generally explained in terms of decreasing the mobility difference between injected and in-place reservoir fluids to reduce channeling effects. The displacing phase should have mobility equal to or lower than the mobility of the oil phase.56,465 When the water/oil mobility ratio (M) is 1 or slightly less, the displacement of the oil by the water phase will occur in a piston-like fashion. By contrast, if M is greater than 1, the more mobile water phase will finger through the oil, causing a breakthrough and poor recovery. Because the mobility is inversely proportional to the viscosity, the polymer should act as an effective viscosifier for the aqueous phase. The main features of such polymers are very high molecular weights, resistance to mechanical degradation in shear, and complete solubility in water. In surfactant flooding, the displacing water solution needs to be able to create a stable emulsion with the oil in flow conditions. Theoretically, it is well-established that the process of emulsion formation in laminar flow is governed by the balance between the viscous force acting on a drop in a laminar flow field and the Laplace pressure.466,467 The dimensionless ratio between the viscous-to-capillary forces is called the Capillary number (Ca). This parameter has been related to the residual oil saturation in porous media, and thus it is very important for EOR applications.468,469 The Capillary number can be defined for several purposes. For EOR it is convenient to base the calculation on the force balance on a drop of oil squeezed through a pore throat.470 In this case we have

where ηc is the viscosity of the continuous phase and v is the Darcy velocity. Other formulations of Ca have been proposed that take into account parameters such as porosity, relative permeability, and contact angle, but this form is the most used because of its simplicity. Typical Ca in waterflooding are on the order of magnitude of 10−8−10−7 It is generally assumed that for a substantial increase in oil production the Ca values should be increased by 2−3 orders of magnitude, meaning that surfactants should be able to give ultralow values of γ (γ ≤ 10−2 mN/m).471−473 The use of polymeric surfactants could represent a favorable option because in principle a decrease in interfacial tension and an increase in viscosity are expected to occur at the same time, with a subsequent positive effect on the Ca. Moreover, this approach should also have the advantage of avoiding the segregation into two phases that can occur in a flow stream for conventional polymer−surfactant mixtures.469,474,475 The choice of a suitable system on the basis of the above considerations is not trivial. For example, several polymeric surfactants show very low or even completely absent surface activity.117,133,143,185,194,205,287 This lack of surface activity is usually explained with a very slow equilibration of polymeric surfactant micelles (the so-called “frozen” micelles) that prevents the possibility of the macromolecules to migrate to the interfaces. As clearly documented by literature, the structure and the composition of amphiphilic polymers are fundamental to determine surface properties and rheology of their aqueous solutions (vide supra). Moreover, the physical properties and behavior can be greatly affected by pH, temperature, presence of dissolved salts, and kind of flow. This review will be focused on the synthesis and the properties of polymeric surfactants from the point of view of their capability to increase the viscosity of the water phase and to effectively lower the interfacial tension at the water/oil interface. These systems are, at least in principle, more suitable for potential EOR applications. The idea to use polymeric surfactants for EOR, which can in principle act either as solution thickeners or as surfactants, is not new. Indeed, most of the systems successfully employed or proposed as solution thickeners based on hydrophobically modified water-soluble polymers (recently very extensively reviewed327) can in principle also act as surfactants, even if the surface activity of such systems is usually not considered in the studies. To improve interfacial activity and ability of polymer to solubilize and emulsify crude oil, the surfactant-containing mixtures, such as SP or ASP flooding systems, were developed. However, due to the different properties, such mixtures often separate into two phases in a flow stream. Other problems can be attributed to the attraction of surfactant to the rock−water interface, which can result in the loss of surfactant to reservoir rock surface by adsorption,476 or to the incompatibility between surfactant and polymer, resulting in the decline of polymer properties such as aggregation, adsorption, and diffusion performance in porous medium.477 Moreover, even if ASP 8547

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flooding has proven to successfully increase oil recovery in the field, the presence of the strong alkali has detrimental effects on polymer performance, and in many cases additional polymer is required to achieve the desired viscosity.474 Nevertheless, it is believed that the alkali positively affects the process by reacting with the organic acids present in the crude oil to form an in situ surfactant and reduces the surfactant adsorption on the sand surface, due to electrostatic repulsion between negatively charged systems. A polymeric surfactant that combines the high viscosity of a polymer with the interfacial property of a conventional surfactant could reduce the tension at water/oil interfaces and enhance the viscosity of the aqueous solution simultaneously.478 Such an approach has already been applied in some fields but, because it is often observed that polymeric surfactants have only a slight ability to decrease the IFT, relatively few uses of polymeric surfactants in EOR have been reported. However, the common belief that ultralow IFT (order of magnitude of 10−3 mN/m) values are needed for good performance in EOR has been questioned recently.479 Some very recent SP flooding experiments480 showed that in some cases there is not a straightforward correlation between lowering the IFT and recovery, but there is rather an optimum IFT, which is higher than ultralow values. Several papers37,64,96,99,117,146,153,199,218,351,424,460,481−484 propose various amphiphilic polymers as systems for EOR. Despite this, only a few studies concern experiments performed to prove the effectiveness of such systems for the claimed application. We could identify few papers in which viscosity, surface properties, and salt effects are taken into account for the evaluation of a given polymeric surfactant in EOR performance. As anticipated, even if their surface and interface properties received little or no attention, hydrophobically modified watersoluble polymers, extensively used as solution thickeners in EOR, can also act as polymeric surfactants. For example, the introduction of 1% of hydrophobic acrylates in a polyacrylamide strongly increases the stability of water/oil emulsions.354 An analogous polymer is capable of stabilizing water/n-heptane and water/oil mixtures, even in the presence of NaCl and NaOH in a simulation of ASP flooding formulation.477 PAM containing 2−3 wt % N-phenethylacrylamide units distributed in a blocky way shows a decrease of surface tension as well as a decrease in interfacial tension between water and ndecane. Interestingly, the polymer containing the lowest amount of hydrophobic groups is more effective in reducing the surface tension but is less effective in decreasing the interfacial tension. This has been explained by the authors with the consideration that, for higher amounts of hydrophobic groups, the polymer’s ability to form intramolecular aggregates increases as well, determining a major stabilization in solution and consequently a minor adsorption at the interface. The reverse behavior in IFT measurements has been explained by the good solubilization ability of the hydrophobe by n-decane, which provides the hydrophobes with a more favored environment than the air phase and the hydrophobic aggregates. As a result, the hydrophobes adsorb at the interface as much as they can in the absence of competition by hydrophobic aggregation. Selfaggregation also helps to explain the reduced surface activity at high polymer concentrations.65 The nature of the hydrophobic groups seems to play a crucial role in interfacial activity. A multiblock copolymer of AM with styrene,351 phenylAM,434 and benzylAM64 showed different surface behavior. Apparently, the higher the conformational freedom of the monomer (PhenethylAM > benzylAM >

phenylAM > sty), the lower is the interfacial activity. This can be explained in terms of rearrangements at the interfaces: the longer hydrophobic spacer gives more freedom to the hydrophobes to extend away from the aqueous phase without causing a significant change in the conformation of the copolymer.65 All these AM-based polymers showed interesting salt effects: viscosity and interfacial activity in the presence of added NaCl are unaffected or in some cases improved. Their properties make these polymers of potential interest for oil recovery.

6. CONCLUSIONS Polymeric surfactants, i.e., macromolecular chemicals displaying the presence of hydrophilic and hydrophobic moieties, represent a relatively new class of chemicals characterized by a wide range of application fields. These range from classical (e.g., as stabilizers for emulsions) to relatively new ones (e.g., as thickeners for EOR). The reason for such versatility resides basically in the fact that in many aspects polymeric surfactants closely resemble their low-molecular-weight counterparts. For example, they still behave as surfactants in that they lower the interfacial tension between water and an apolar phase. However, the polymeric character of these materials confers to them additional properties. In this respect the steric stabilization of dispersed systems, the possibility of finely tuning the aggregation behavior as a function of external stimuli, and the use of topology to modulate the surface activity represent just paradigmatic examples of these advantages. Many scientific challenges remain open for a full understanding of the behavior of these materials and in particular in the structure−property relationship. The HLB theory, allowing reliable prediction for low-molecular-weight surfactants, in general does not apply to polymeric materials. Along the same line, the relationship between the aggregation behavior and the rheological one is in many cases strongly dependent on the studied system, thus hindering the establishment of general principles. Furthermore, theoretical aspects bridging the “gap” between different chemical structures are still lacking. This makes it almost impossible at the moment to choose a priori a given system for a given application. Application of polymeric surfactant has been so far hampered by the laborious and costly synthetic methods. Although the situation is in this respect still difficult to predict, the use of modern copolymerization techniques (such as ATRP and RAFT) described in the present work in a detailed way allow for envisaging a bright future for these materials. From an application perspective and particularly in the general field of EOR, several points are worth noticing. A clear, generally valid and undisputed proof of the added values of polymeric surfactant for EOR should be provided. In this respect one should be able to correctly identify the correct reference conditions. In the first instance, the alternative process (e.g., polymer or polymer−surfactant flooding) should be identified. Second, the use of the corresponding reference system (i.e., polymer or polymer and surfactant) should be correctly defined. The use of an alternative, although simpler chemical solution (e.g., glycerin solutions for polymer flooding)485 severely restricts, in our opinion, the generality of the obtained results. To provide a general proof (vide supra), one should be able to prepare polymeric materials in which the thickening effect (mostly related to the average molecular weight and its distribution) and the surface properties can be independently varied. To the best of our knowledge, the only option available for the product designer in this context is constituted by 8548

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controlled radical polymerization methods, which, however, have been limited so far (especially for water-soluble polymers with different topologies) to mostly academic studies. The application of these relatively new synthetic methods should then be studied in terms of scaling possibilities. A clear and generally valid relationship between the chemical structure of the polymer and its properties in water solution (i.e., viscosity and surface tension) should be established. This is essential for the design of new polymeric materials with tailored properties linked to different oil well situations. Such a relationship should also take into account the peculiar associative behavior of macromolecular surfactant as opposed to their lowmolecular-weight counterparts. Indeed, the macromolecular association is not only related to the thickening effect but also, and perhaps most importantly, to the surface activity. The lack of such activity for some of the reported structures constitutes a clear distinction point (with respect to low-molecular-weight surfactants) and is indicative of a peculiar force balance in the corresponding water solutions. This should constitute the starting point of a (predictive) model, yet to be investigated.

Diego Armando Zakarias Wever ([email protected]) was born in Oranjestad, Aruba, in 1984. He studied chemistry and chemical engineering at the University of Groningen. He received his M.Sc. Degree in 2009 in the field of Chemical Engineering−Product Technology. He then started Ph.D. research on new polymers for enhanced oil recovery under guidance from Prof. A. A. Broekhuis and

AUTHOR INFORMATION

Prof. F. Picchioni, and he received his Ph.D. degree in 2013. As of

Corresponding Author

October 2013 he is employed at Shell Global Solution International, and

*E-mail: [email protected]. Phone: +31 (0)50 363 4918. Fax: +31 (0)50 363 4479.

he is working in the field of enhanced oil recovery.

Notes

The authors declare no competing financial interest. Biographies

Francesco Picchioni ([email protected]) was born in Terni, Italy, in 1971. He studied chemistry at the University of Pisa (Italy). He received his M.Sc. degree in 1996 in the field of Polymer Chemistry. He then Patrizio Raffa (p.raff[email protected]) was born in Pescara, Italy, in 1978. He obtained his Master’s degree in organic chemistry at the University of Pisa and Scuola Normale Superiore of Pisa (Italy) in 2004 with a thesis on organosilicon compounds. He completed his Ph.D. in chemistry at Scuola Normale Superiore of Pisa in 2008, working on transition metal nanoparticles. After postdoctoral experiences in heterogeneous catalysis at Ircelyon and polymer chemistry at the University of Pisa, he moved to the University of Groningen in 2011, where he is currently employed as

started Ph.D. research on thermoplastic elastomers under the guidance of Prof. F. Ciardelli and Prof. M. Aglietto (both at the University of Pisa), and he received his Ph.D. degree in 2000. After a postdoc (2000−2003) at the Technical University of Eindhoven (The Netherlands), he joined the University of Groningen (The Netherlands) firstly as assistant professor and then as associate one. As of November 2013 he is

a postodoctoral fellow in the group of Prof. F. Picchioni and Prof. A. A. Broekhuis. His present research includes amphiphilic polymers for industrial applications.

Professor of Chemical Product Technology at the University of Groningen. 8549

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LGA = lactide-co-glycolide MADIX = macromolecular design via interchange of xanthates Me6TREN = tris(2-aminoethyl)amine MOMA = methyl methoxyacrylate MPC = 2-methacryloyloxyethyl phosphorylcholine MWD = molecular weight distribution NaAMPS = sodium 2-acrylamido-2-methylpropane sulfonate NMP = nitroxide-mediated polymerization NRET = nonradiative energy transfer O/W = oil-in-water (emulsion) P(Q)DMAEMA = poly(quaternized 2-dimethylaminoethyl methacrylate) P2VP = poly(2-vinylpyridine) P4VP = poly(4-vinylpyridine) PAA = poly(acrylic acid) PBA = poly(butyl acrylate) PBD = polybutadiene PBO = poly(butylene oxide) PBzA = poly(benzyl acrylate) PBzMA = poly(benzyl methacrylate) PCL = poly(caprolactone) PDEAEMA = poly(2-diethylaminoethyl methacrylate) PDEGA = poly(diethylene glycol) PDI = polydispersity index PDMA = poly(dimethylacrylamide) PDMAEMA = poly(2-dimethylaminoethyl methacrylate) PEA = poly(ethyl acrylate) PEB = poly(ethylene-co-butene) PEGMA/PEOMA/PEOnMA = poly(ethylene glycol methacrylate) PEMA = poly(ethyl methacrylate) PEO/PEG = poly(ethylene oxide)/poly(ethylene glycol) PEtSB = poly(diethyldisilacyclobutane) PFMA = 2-(N-methylperfluorobutanesulfonamido)ethyl methacrylate) PFOMA = poly(fluoroctyl methacrylate) PGMA = poly(glycidyl methacrylate) PhIP = poly(hydrogenated isoprene) PHMA = poly(n-hexyl methacrylate) PHOS = poly(4-hydroxystyrene) PI = polyisoprene PIB = polyisobutene PIP = polyisoprene PLA = poly(lactic acid) PMA = poly(methyl acrylate) PMAA = poly(methacrylic acid) PMDETA = N,N,N′,N′,N″-pentamethyldiethylenetriamine PMMA = poly(methyl methacrylate) PnBA = poly(n-butyl acrylate) PNFHMA = poly(nonafluorohexyl methacrylate) PNIPAM = poly(N-isopropylacrylamide) PnOMA = poly(n-octyl methacrylate) POFPMA = poly(2,2,3,3,4,4,5,5-octafluoropentyl methacrylate) PPO/PPG = poly(propylene oxide)/poly(propylene glycol) PPS = poly(isopropylsulfide) PS = polystyrene PSF5 = poly(pentafluorostyrene) PSGMA = poly(sulfonated glycidyl methacrylate) PSSNa = poly(sodium stryrene sulfonate) PtBA = poly(tert-butyl acrylate) PtBMA = poly(tert-butyl methacrylate) PtBOS = poly(4-tert-butoxystyrene)

Antonius (Ton) A. Broekhuis obtained his Ph.D. degree from the Radboud University in Nijmegen, in 1979. From 1979 to 2003 he held different research and management appointments in Shell Chemicals in Amsterdam, Houston, and London. His work encompassed basic and applied research directed at process and product innovation, as well as at new product development. In 2003 he joined the University of Groningen to shape and develop the new chair dedicated to “Chemical Product Design and Engineering” (CPD&E) within the chemical engineering program. From 2004−2011 he was chairman of the Section of the European Federation of Chemical Engineers on CPD&E.

ACKNOWLEDGMENTS This work is part of the Research Program of the Dutch Polymer Institute DPI, Eindhoven, The Netherlands, project no. 716. ABBREVIATIONS ACPA = 4,4′-azobis-4-cyanopentanoic acid AIBN = azobisisobutyronitrile AM = acrylamide ASP = alkaline−surfactant−polymer ATRP = atom transfer radical polymerization bpy = bipyridine Ca = capillary number CAC = critical aggregation concentration CMC = critical micellar concentration CTA = chain tranfer agent CTAB = hexadecyltrimethylammonium bromide CTPPA = 4-cyano-4-thiothiopropylsulfanyl pentanoic acid DAMAPS = 3-(N,N-diallyl-N-methylammonium)propane sulfonate DEmMA = dodecyl ethylene glycol methacrylate DEPN = N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) nitroxide DPE = diphenylethylene DS = degree of substitution EGDMA = ethylene glycol dimethacrylate EOR = enhanced oil recovery GTP = group transfer polymerization H12MDI = bis(4-isocyanatocyclohexyl)methane HDI = hexamethylene diisocyanate HEGMA = methoxy hexaethylene glycol methacrylate HEMA = 2-hydroxyethyl methacrylate HLB = hydrophilic−lipophilic balance HMPAM = hydrophobically modified polyacrylamide HMTETA = 1,1,4,7,10,10-hexamethyltriethylenetetramine IFT = interfacial tension IPDU = isophorone diurethane LDA = lithium diisopropylamide 8550

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PtBS = poly(tert-butylstyrene) PTHPMA = poly(tetrahydropyranyl methacrylate) PTMSMA = poly(trimethylsilyl methacrylate) PTPFCBPMA = poly(p-(2-(p-tolyloxy)perfluorocyclobutoxy)phenyl methacrylate) PVA = poly(vinyl alcohol) PVBC = poly(vinyl benzylchloride) RAFT = reversible addition-fragmentation chain transfer polymerization RITP = reversible iodine transfer polymerization ROP = ring-opening polymerization SDS = sodium dodecyl sulfate SP = surfactant−polymer TBAB = tetrabutylammonium bromide TBABB = tetrabutylammonium bibenzoate TBAOH = tetrabutylammonium hydroxide tBVBA = tert-butyl vinylbenzoic acid TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl TFA = trifluoroacetic acid TMAEMA = 2-trimethylammonioethyl methacrylate TOA = trioctylamine TSA = p-toluene sulfonic acid W/O = water-in-oil (emulsion)

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