Expanding the Scope of RAFT Polymerization ... - ACS Publications

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Expanding the Scope of RAFT Polymerization: Recent Advances and New Horizons Megan R. Hill, R. Nicholas Carmean, and Brent S. Sumerlin* George & Josephine Butler Polymer Research Laboratory, Center for Macromolecular Science & Engineering, Department of Chemistry, University of Florida, PO Box 117200, Gainesville, Florida 32611-7200, United States ABSTRACT: Reversible deactivation radical polymerization (RDRP) has revolutionized modern polymer chemistry over the past two decades, thus laying the groundwork for the synthesis of complex macromolecules and enabling the preparation of previously inaccessible materials. Reversible addition-fragmentation chain transfer (RAFT) polymerization has emerged as one of the most promising techniques because of its functional group tolerance, applicability to a wide range of vinyl monomers, and its nondemanding experimental conditions. However, despite the promise and clearly demonstrated utility of RAFT, limitations of the method sometimes still exist, including the occasional need for extended polymerization times, limited access to high molecular weight polymers, low “livingness” due to unavoidable radical termination events, etc. This Perspective focuses on recent advances that have been specifically designed to address many of these perceived limitations to reinforce the promise of RAFT for the synthesis of complex and well-defined polymers under facile conditions.

1. INTRODUCTION Since the first publication of reversible addition−fragmentation chain transfer (RAFT) polymerization by the pioneering work of Rizzardo, Moad, and Thang in 1998,1 RAFT has emerged as one of the most powerful methods in the field of reversible deactivation radical polymerization (RDRP).2,3 The robust and versatile nature of RAFT has allowed it to become one of the most useful tools in modern polymer synthesis. However, like all other synthetic techniques, RAFT polymerization has perceived limitations, which can include, for example, the need for extended polymerization times, the requirement of an external initiator that leads to unavoidable radical termination events, and a difficulty in producing high molecular weight polymers in a controlled manner. Fortunately, significant effort has been dedicated to expanding the scope of RAFT by directly addressing many of these limitations. This Perspective is meant to highlight many recent developments in this area, with particular attention being paid to advances that have facilitated the RAFT process and/or have provided access to increasingly complex materials. Given the significant recent progress made, RAFT has become one of the most powerful and utilized RDRP techniques to date. Unlike nitroxide-mediated polymerization (NMP)4 and atom transfer radical polymerization (ATRP),5 which rely on reversible termination of propagating radicals, RAFT operates on the principle of degenerative chain transfer.6 To achieve its deactivation−activation equilibrium, RAFT typically utilizes a thiocarbonylthio-containing chain transfer agent (CTA). Because there is no net change in radical concentration during the activation−deactivation process, an external initiator is required, with initiation proceeding as in conventional radical © XXXX American Chemical Society

polymerization. The proposed mechanism of RAFT polymerization is outlined in Figure 1. After initiation, the propagating radical (Pn•) adds to the thiocarbonylthio compound (1) to form the radical intermediate (2), which subsequently fragments to give another thiocarbonylthio group (3) and a new radical (R•) (4). The radical (R•) may then react with the newly formed thiocarbonyl group or reinitiate polymerization by reacting with monomer to form a new propagating radical (Pm•). After the initially added CTA is consumed (i.e., completion of the “initialization period”), 7 the “main equilibrium” of activation−deactivation is established by degenerative chain transfer between propagating (Pm or Pn) and dormant (3/6) chains. Given fast initialization and a rapid equilibrium between active radical-containing chains and dormant thiocarbonylthiocontaining chains, an equal opportunity for growth is provided, which leads to polymers with narrow molecular weight distributions (MWDs). Termination events are inevitable, with the absolute number of dead chains depending primarily on the initiator concentration, the initiator dissociation rate constant (kd), and time. However, because the number of dormant thiocarbonylthio-containing chains is typically much higher than the number of initiator-derived chains, most chains remain “living” after polymerization and can be isolated as stable materials capable of subsequent end-group modifications or chain extension. Put another way, living characteristics are imparted best when the targeted molecular weight of the Received: February 16, 2015 Revised: July 10, 2015

A

DOI: 10.1021/acs.macromol.5b00342 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Reversible addition−fragmentation chain transfer (RAFT) polymerization mechanism.

controlled techniques such as NMP,27 ATRP,28−31 ROP,32−35 and ROMP.36−38 The ability to polymerize reactive monomers is another particular strength of the RAFT technique. The relatively few side reactions that can interfere with the RAFT process allows direct access to well-defined and functional polymer scaffolds capable of postpolymerization modification, often without the need to protect/deprotect the monomers before/after polymer synthesis.39,40 Examples include the polymerization of monomers containing unprotected isocyanates,41 boronic acids,42,43 acid chlorides,44 and acetylenes.45 Additionally, the polymerization of unprotected primary amine-containing monomers, which were previously considered a challenge due to potential aminolysis of the thiocarbonylthio linkages, has now been demonstrated by several groups. For example, McCormick et al. found that maintaining the pH below the pKa of the amine during an aqueous polymerization effectively eliminated aminolysis or hydrolysis of the CTA.46,47 Despite the significant achievements over the past few years, many limitations continue to exist. In this mini-review, we will highlight recent work that has addressed many of the perceived limitations of RAFT including (i) slow polymerization times and difficulty obtaining high molecular weights, (ii) the necessity of conducting radical polymerizations in an oxygenfree environment to avoid retardation or inhibition, (iii) the class specificity of CTAs, which requires careful selection of RAFT agents for each monomer, and (iv) the reliance on an external radical source (i.e., initiator) which leads to inevitable termination and reduces the “livingness” of the technique. We will conclude with a brief outlook of how these perceived limitations can not only be overcome but also be exploited to design increasingly complex materials relevant in diverse fields of polymer science.

polymer is substantially lower than that which would be formed in the absence of a RAFT agent (i.e., during conventional radical polymerization) and when the number of polymer molecules with RAFT agent-derived chain ends exceeds the number formed as a consequence of termination.6 Experimentally, RAFT is carried out under conditions that are nearly identical to those of conventional radical polymerization, with the primary exception being the presence of the CTA. Therefore, it is no surprise that many of the strengths of RAFT are derived from the inherent versatility of conventional radical polymerization. These strengths include a large array of polymerizable monomers (e.g., (meth)acrylates, styrenics, (meth)acrylamides, vinyl esters, dienes), a wide range of polymerization solvents and temperatures, and a high tolerance to diverse functional groups (e.g., −OH, −NR3, −COOH, −CONR2).8 The control afforded during RAFT expands the possible chain architectures9 and molecular weight control10 that can be achieved under typical radical polymerization conditions, provided the thiocarbonylthio moiety is preserved. Importantly, this enhanced synthetic utility does not require the use of metal catalysts or high polymerization temperatures, which makes RAFT particularly useful for many biological applications.11 The tolerance of RAFT to a wide variety of polymerization conditions has considerably contributed to its role as one of the most useful RDRP methods. For example, RAFT polymerizations have been successfully conducted over a wide range of temperatures, from −15 to 180 °C,12−14 and in a variety of solvents, including water, and under biologically friendly conditions.15−17 Additionally, the synthesis of neutral, anionic, cationic, or zwitterionic water-soluble polymers has significantly contributed to the advancement of biological and green polymer chemistry.11,18 Notably, McCormick and co-workers19−22 have worked extensively to establish both homogeneous and heterogeneous aqueous RAFT as a viable means to prepare water-soluble, amphiphilic, and stimuli-responsive materials. Recent work in the field of polymerization-induced self-assembly (PISA) using RAFT has facilitated the in situ synthesis and self-assembly of amphiphilic block copolymers into well-defined nanostructures.23−26 Moreover, the compatibility of RAFT in various polymerization environments has enabled the synthesis of tandem polymerizations with other

2. ACHIEVING HIGH MOLECULAR WEIGHTS AND ACCELERATING POLYMERIZATION Conventional radical polymerization is commonly employed to produce high molecular weight (MW) materials; however, in many applications, material function may be diminished by chain branching, microstructure (e.g., head-to-head units), and broad molecular weight distributions that result from the uncontrolled nature of conventional radical polymerization. In B


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CTA (cumyl dithiobenzoate), the vinyl group of MMA, and the propagating MMA radical preferentially resided in the polar domain, with the resulting microenvironment being similar to microemulsion polymerization. This work was consistent with the increased kp of MMA in ionic solvents and provided an interesting platform for continued polymerizations in these media. Polymerization under extremely high pressures has also been shown to increase polymerization rate, often without broadening of the MWD. High pressures accelerate bimolecular bond forming reactions, which increases kp, while reducing the rate of diffusion-controlled reactions, such as termination.59 A decade ago, Rzayev et al.53 reported the production of ultrahighmolecular-weight poly(MMA) at elevated pressures from 5 to 9 kbar in the presence of a dithiobenzoate. At this high pressure, the polymerization was extremely well controlled yielding polymers with an average MW of 1.25 × 106 g/mol and an Mn/ Mw of 1.03, with near-quantitative monomer conversion. Interestingly, at 9 kbar the Mn/Mw broadened to 1.38, and the polymerization rate was retarded. Vana also outlined the controlled polymerization of a low-kp monomer, styrene, at 2.5 kbar, and although MW > 106 g/mol was not targeted, the polymerization rate was significantly enhanced.52 Microwave heating during RAFT polymerization is another method that has shown promise for the preparation of highMW polymers. Microwave irradiation has been a widely employed technique for small molecule synthesis60 and has more recently been exploited to accelerate RAFT polymerization.61,62 Although the direct influence of microwave heating has been the topic of debate,63−66 this method has clearly been shown to result in high polymerization rates of both polar and nonpolar monomers. Our group demonstrated the rates of polymerization of DMA, N-isopropylacrylamide (NIPAM), vinyl acetate, vinyl benzoate, and vinyl pivalate by RAFT were significantly higher than those observed under conventional heating conditions.61,67 Despite this acceleration, the resulting polymers retained their thiocarbonylthio end groups, as evidenced by efficient extension during subsequent block copolymerizations. In all cases, narrow molecular weight distributions were observed, and the agreement between theoretical and experimental molecular weights was excellent. Rate enhancement was ultimately attributed to thermal effects that were not easily duplicated under conventional heating conditions. Lastly, because it is generally necessary to use monomers with high propagation rate coefficients (e.g., acrylates, acrylamides) to achieve ultrahigh molecular weight without a high occurrence of side reactions and bimolecular termination, there still exists a difficulty in synthesizing high-MW polymers of low-kp monomers with good control and narrow MWD. Recently, however, Davis et al. employed emulsion polymerization techniques with macro-CTA/macrostabilizers to synthesize ultrahigh-molecular-weight polystyrene diblock copolymers with a Mn up to 106 g/mol and MWD < 1.4.68 While this method provides a route for high-molecular-weight styrenic block copolymers, a general method for the preparation of high MW homo- and copolymers for low-kp monomers is still needed.

RDRP, it is generally expected that livingness is diminished when high-MW polymers are targeted due to longer reaction times and increased termination and side reactions. However, carefully chosen RAFT techniques have allowed for the preparation of ultrahigh-molecular-weight polymers (∼106 g/ mol), often with narrow molecular weight distributions (MWDs). The discussion below highlights many of these recent results. The synthesis of polymers with high MW and narrow MWD hinges on the consideration and/or manipulation of fundamental kinetic principles. Specifically, high propagation rate constants (kp) and low termination rate constants (kt) favor the formation of polymers with a high degree of polymerization (DP).48−50 Expressed another way, the kp/kt ratio largely determines the maximum obtainable molecular weight during polymerization.48 Prudent selection of reaction conditions (i.e., solvent,48−51 temperature,48 concentration, and pressure52,53) may potentially increase the rate of propagation without significantly increasing the rate of termination. For example, it has been proposed that increased monomer concentration increases the Arrhenius pre-exponential factor, thereby accelerating the rate of polymerization. This can potentially be rationalized by the greater number of monomer molecules in radical microenvironments which may lower the activation entropy of forced bond rotation during the formation of the sp3-hybridized carbon−carbon backbone from sp2 vinyl monomers.54−56 Furthermore, polar solvents (e.g., water) are known to stabilize the transition state of many propagating radicals, lowering activation energy and increasing the rate of propagation.56 Through careful control of the reaction conditions, Destarac et al. prepared acrylamido polymers up to 106 g/mol with narrow MWDs (90%) with narrow MWDs being maintained.57 More recently, Liscene and Thurecht studied the fundamental influence of ionic liquid solvents on RAFT polymerization of MMA,51 noting that ionic liquids are inherently viscous, and as expected, polymerization rate increased with viscosity, though the apparent increase was not linear, suggesting a more complex influence on kp. Previously, it had been shown that MMA monomer and oligomers will selectively partition into different regions of ionic liquids,58 and ROESY NMR spectroscopy was employed to determine spatial mapping of both the CTA and propagating species during the polymerization of MMA. Using this method, Liscene and Thurecht found that during polymerization the aromatic groups of the

3. DEOXYGENATION It is well-known that oxygen sensitivity is a key limitation of radical polymerization. While low concentrations of oxygen or other inhibitors are commonly overcome by increasing the C


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is dependent on the catalyst loading. Therefore, polymerizations cannot be run in open air, but rather must be carried out in closed vessels, albeit with no prior deoxygenation. Using this approach, Boyer and co-workers reported the first example of block copolymer synthesis by RAFT without intermediate purification or degassing of monomer between each chain extension.72 Further work with PET-RAFT has been explored using various organo-photocatalysts in place of the Ir and Ru complexes.75,76 It was found that eosin Y, in particular, showed good control over many functional methacrylate monomers (i.e., glycidyl methacrylate, hydroxylethyl methacrylate, dimethylaminoethyl methacrylate, pentafluorophenyl methacrylate, methacrylic acid) in various solvents (water, acetonitrile, dimethylformamide, dimethyl sulfoxide).75 Interestingly, when polymerizations proceeded in the presence of oxygen (without prior deoxygenation), the rate of propagation significantly decreased unless triethylamine (TEA) was added. Boyer et al. noted that the increase in propagation rate with the addition of TEA may be attributed to the reducing abilities of TEA, which acts as a sacrificial agent in the photoredox cycle, donating an electron to eosin Y. Despite the significant achievements of PET-RAFT, the need for a closed vessel and the long inhibition time still provide challenges. Recently, work by Stevens et al. developed “EnzRAFT” by utilizing glucose oxidase (GOx), an enzyme known to consume oxygen and generate hydrogen peroxide in the presence of glucose, to actively consume oxygen and produce well-defined polymers via RAFT in an open vessel (Figure 3).77 Because of the high activity of GOx, only low concentrations of enzyme were required (1−4 μM) to fully consume oxygen in the system and to achieve similar kinetics to traditionally argondegassed, closed-vessel systems (Figure 3b). Such low concentrations also means that hydrogen peroxide production

concentration of initiator in conventional radical polymerizations, RDRP typically requires initial oxygen-free conditions to achieve precise control over molecular weight. Therefore, many RDRP techniques require stringent deoxygenation procedures prior to polymerization, such as freeze−pump− thaw or purging with nitrogen or argon gas. This intolerance of RAFT and many other RDRP methods to oxygen limits applicability in industrial settings and presents significant challenges. Developments in copper-catalyzed radical polymerization over the past decade, particularly ARGET-ATRP, SARAATRP,5 and SET-LRP69,70 have led to the use of reducing agents to preserve chain-end functionality in the presence of small amounts of oxygen. Until recently, less work had focused on RAFT polymerization in the presence of oxygen. Because the fidelity of end groups in RAFT is dependent on the ratio of CTA to initiator, it can be assumed that every radical introduced to the reaction will eventually terminate or react with oxygen, producing a dead chain devoid of the RAFT end group. Thus, the absence of oxygen is particularly important to achieve highly controlled polymerizations and to preserve livingness. Inspired by the work of Hawker and co-workers on the development of a visible-light-mediated living radical polymerization with an Ir-based photoredox catalyst and an alkyl bromide,71 Boyer et al. recently introduced photoinduced electron-transfer (PET) RAFT.72 PET-RAFT relies on a photoredox catalyst (intitially using fac-[Ir(ppy)3] or Ru(bpy)3Cl2) to reduce thiocarbonylthio CTAs and produce radicals to initiate the polymerization, via an iniferter-like mechanism (Figure 2).72,73 PET-RAFT has been shown to

Figure 2. Proposed mechanism of a photoinduced electron transfer (PET) RAFT polymerization using fac-[Ir(ppy)3] as a photocatalyst to reduce a thiocarbonylthio CTA. This method has proven useful for the controlled polymerization of a wide variety of monomers in the presence of oxygen. Reproduced with permission from ref 72.

control a large range of monomer families (e.g., (meth)acrylates, styrene, vinyl ester, and (meth)acrylamides), with careful CTA selection and catalyst loading in both organic and aqueous solutions.72,74 Because of the strong reductive properties of fac-[Ir(ppy)3] or Ru(bpy)3Cl2, oxygen can be reduced into superoxide, an inactive species, via single-electron reduction. Although the mechanism for the reduction of oxygen remains somewhat elusive, PET-RAFT was shown to produce well-defined polymers without prior deoxygenation. However, a long inhibition period (3−24 h) precedes the polymerization for samples that have not been deoxygenated, and a slightly lower apparent propagation rate is sometimes observed as compared to the deoxygenated samples.72,73 This inhibition time is attributed to the reduction of oxygen by the catalyst and

Figure 3. (a) Scheme for “Enz-RAFT” polymerizations with chain transfer agent (CTA) and glucose oxidase (GOx), an enzyme used to remove oxygen from the solution, which allows the polymerization to proceed in an open vessel to produce well-defined polymers. (b) Pseudo-first-order kinetic plots with and without GOx and radical source 2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-044). (c) SEC RI traces for each block in a tetrablock polymer synthesized by three successive chain extensions. Reproduced with permission from ref 77. D


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favorable.81 Therefore, proper matching of monomer and CTA is one of the critical steps for ensuring a well-controlled RAFT polymerization and is often seen as a limitation of the RAFT process. Furthermore, construction of block copolymers is currently limited to poly(MAM)-block-poly(LAM), where the poly(MAM) block must be synthesized first due to the poor homolytic leaving abilities of the poly(LAM) block. The development of a single RAFT agent capable of polymerizing monomers of disparate reactivity would greatly enhance the utility of RAFT. Recent efforts in designing CTAs with versatile reactivity have shown promise in this respect. Dithiocarbamates and xanthates can offer modest control of both MAMs and LAMs for the synthesis of MAM-block-LAM copolymers. For example, Destarac employed xanthates to prepare PDMA-b-poly(vinyldiene fluoride), 83 PDMA-bPVAc,84 PDMA-b-PDADMAC,85 and polyacrylamides-bPNVP.86 Destarac also described the systematic design of dithiocarbamates to control both MAMs, like St and ethyl acrylate, as well as LAMs, (i.e., VAc).87 Recently, Shipp has reported the use of a single dithiocarbamate to prepare welldefined PSt-b-PVAc,88 despite the fact that highly reactive PVAc radicals might be expected to otherwise undergo degradative chain transfer in the presence of compounds with benzylic hydrogens.89 The free radical polymerization of ethylene typically results in high MWDs and branching via backbiting, and the unstabilized nature of the monomer has made its polymerization difficult to control through RAFT. However, D’Agosto and co-workers identified mild reaction conditions that resulted in relatively well-defined polyethylene by using two similar xanthates (Mn = 2000 g/mol, MWD = 1.9).90 The polymerization required 200 bar of pressure, which made kinetic sampling difficult. Therefore, the authors compared discrete reactions quenched at predetermined intervals (every hour for 7 h) and observed an increase in molecular weight as a function of yield. The near-linear increase in molecular weight supports the claim of a living polymerization, though irreversible degradation of the CTA resulted in increased termination, evident during subsequent chain extension of a low-MW macro-CTA. Importantly, this approach allows functional groups to be inserted into the polyethylene chain through copolymerization of VAc. Addition of 5 or 25 vol % VAc in the polymerization mixture resulted in incorporation of 2 or 9 mol % of the monomer, respectively, in the final copolymer. Another possibility to further tune CTA reactivity includes the incorporation of less common heteroatoms into the design of the RAFT agents. Work in this field has shown that different types of bonds (e.g., weaker C−Se bonds in place of C−S bonds) may alter CTA properties and lead to more compatible RAFT agents. Selenium has a similar electronic structure to sulfur, and the development of Se-based RAFT agents has received considerable interest over the past few years.91−93 Recently, progress in obtaining controlled polymerizations with narrow MWDs have been obtained using Se-substituted carbonates,94 N,N-dimethyldiselenocarbamates,95 and selenobenzoate.96 Notably, Zeng et al. reported a Se-RAFT agent with the ability to control both methacrylate and VAc,97 and Destarac showed their N,N-dimethylselenocarbamate could efficiently polymerize the MAM-b-LAM poly(tert-butyl acrylate)-b-poly(vinyl acetate) with relatively good control (MWD = 1.42).95 Phosphorus-98 and metallophosphorus-based99,100 RAFT agents (M(CO)5PPh2CS2R, where M = W, Cr, Mo, and R = CH2CN, CH(CH3)C6H5) have also been shown

is negligible, as evidence by retention of the RAFT end groups confirmed by UV−vis spectroscopy and subsequent chain extension. Because this oxygen consumption route is completely decoupled from the polymerization mechanism, only very low concentrations of initiator need to be used, and no evidence of inhibition was noted for these polymerizations.

4. MONOMER CLASS SPECIFICITY OF RAFT AGENTS Although the ability of RAFT to polymerize a wide range of monomers has contributed to its success, careful selection of the RAFT agent for a given monomer is needed to achieve a well-controlled polymerization. The inherent difference in reactivity of propagating radicals of various monomers requires careful selection of CTA to ensure good polymerization control. Most monomers can be generally divided into two groups based on their reactivity: “more-activated monomers” (MAMs) and “less-activated monomers” (LAMs). MAMs are typically those with vinyl groups conjugated by a carbonyl group or an aromatic ring (e.g., (meth)acrylates, (meth)acrylamides, and styrenics (St)), while LAMs typically contain a saturated carbon or oxygen/nitrogen lone pair adjacent to the vinyl group (e.g., vinyl esters (VAc), ethylene, N-vinylpyrrolidone (NVP), and diallyldimethylammonium chloride (DADMAC)). Several reviews have provided guidance for selecting the ideal RAFT agents for most monomers.78−80 The reactivity of a CTA (ZC(S)SR) during RAFT is affected by both the Z and R groups. The Z group governs the general reactivity of the CS bond toward radical addition and affects the lifetime/fate of the resulting intermediate radical, while the R group acts as a homolytic leaving group and can reinitiate radical polymerization during the initialization period described in Figure 1. In terms of molecular orbital theory, the Z group will affect the energy level of the lowest unoccupied molecular orbital (LUMO) of the CTA and determine if radical addition is favored.81 The electron-withdrawing groups in MAMs lead to lower energy singly occupied molecular orbitals (SOMO), while LAMs generally have higher energy SOMOs. Thus, in general, two distinct classes of RAFT agents are necessary for each monomer family. For example, if the intermediate radical that results from addition of the propagating chain end to the thiocarbonyl is considerably more stable than either of the radicals that would result upon fragmentation, the polymerization will be retarded. Conversely, because propagating chain ends with terminal MAM units are less reactive in radical addition, a more active RAFT agent is required for good control while a deactivated CTA, with a higher energy LUMO, may not actively participate during the polymerization, which would lead to uncontrolled polymerization. Therefore, MAMs are typically polymerized with a trithiocarbonate or dithiobenzoate RAFT agent to achieve control because the SOMO of the monomer and the LUMO of the CTA have similar energy levels. This affords favorable addition and fragmentation during the degenerative transfer process.82 A LAM leads to a highly reactive propagating radical that must be matched with a less active CTA, such as a dithiocarbamate or xanthate, as LAMs make for poor homolytic leaving groups, and inhibition or retardation becomes likely with more active RAFT agents. Furthermore, the lone pair of electrons adjacent to the CS bond, from oxygen or nitrogen in xanthates and dithiocarbmates, respectively, reduce the CS double-bond character. This conjugation also increases the energy of the LUMO and makes radical addition less E


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5. HIGH LIVINGNESS AND NUMBER OF BLOCKS It has generally been accepted that the need for an external initiator in RAFT leads to lower overall livingness as compared to other RDRP techniques. Livingness, or the fraction of polymer chains that retain the active end group at a particular monomer conversion, is particularly important in the synthesis of block copolymers. While the ability to prepare block copolymers is one of the most significant benefits of RDRP, higher-order block copolymers (e.g., tetra-, penta-, hexablocks, etc.) had rarely been reported until recently. The lack of progress in this area is most likely due to the exhaustive conditions of block copolymer synthesis, which traditionally requires stopping the polymerization well before full conversion to ensure chain end retention, followed by thorough purification of the macro-CTA/macroinitiator to remove unreacted monomer before polymerization of the next block. New methodologies in polymerizations that involve Cu(0) have emerged over the past few years that allow the one-pot sequential addition of monomers with essentially full monomer conversion being reached for each block.105−107 Eliminating the need for purification after the synthesis of each block significantly facilitates the preparation of high-order multiblock copolymers. This technique has been successfully used for the synthesis of acrylate multiblock copolymers up to 12 blocks in length with narrow MWDs (Mw/Mn = 1.2).69 However, extension of these metal-catalyzed techniques to other monomer classes has proven difficult.69,108 One of the challenges in preparing higher order block copolymers by RAFT is that chain ends are inevitably lost during the synthesis of each block as a result of unavoidable termination reactions. Because the activation−deactivation process in degenerative transfer systems does not lead to a change in the overall number of radicals, the number of chains that undergo termination directly corresponds to the number of radicals derived from the decomposition of the initiator. Perrier and Vana previously reported a method for removing terminated, nonliving chains by immobilizing RAFT agents to solid supports via their Z groups.109 In this manner, only CTAcontaining (i.e., living) chains remain bound to the support after polymerization, while all terminated (i.e., dead) chains remained in solution and could be removed by simple filtration. Although this approach can lead to nearly quantitative chain end functionality, it is not readily scalable and would still require purification after the synthesis of each block. Recently, Perrier and co-workers developed a highly living RAFT process by exploiting the inherent “disadvantage” of the need of an external initiator in degenerative chain-transfer systems.49,110 Because the number of terminating chains is proportional to the initiator concentration, it is possible to conduct RAFT polymerizations to full monomer conversion (>99%) while maintaining high livingness, provided the initiator concentration is minimized. In this manner, it should become possible to prepare well-defined high-order multiblock copolymers without the need for purification after the synthesis of each block. As explained by Perrier et al.,49 the theoretical livingness (i.e., number fraction of living chains) can be calculated with eq 1, where L is the livingness, f is the initiation efficiency, fc is the the coupling factor of chains by radical−radical coupling, and [CTA]0 and [I]0 are the initial concentrations of CTA and initiator. The factor of 2 arises from each initiator decomposition event producing two radicals.

to serve as efficient mediators in the polymerization of St and acrylates. Thang, Moad, and Rizzardo have reported “switchable” CTAs that have a thiocarbonyl moiety with reactivity that can be modulated via protonation or deprotonation of a basic nitrogen atom contained within the Z group (Figure 4).82 In

Figure 4. Acidic or basic conditions influence the reactivity of “switchable” CTAs. Acidic conditions stabilize the intermediate radical, allowing for control of MAMs (e.g., styrenics and acrylates). Basic conditions deprotonate the pyridyl ring, allowing for control of LAMs (e.g., vinyl acetate). From ref 82.

the presence of a strong acid, the Z group of these switchable CTAs is protonated, thereby altering the reactivity of both the CS and the intermediate radical that results after addition of the propagating radical, which in turn allows controlled polymerization of MAMs. Alternatively, under basic conditions, the CTA is deprotonated, which reduces the reactivity of the CS and promotes faster fragmentation of the intermediate radical, both of which favor control during the RAFT of LAMs. Importantly, switching the CTA reactivity of a macroCTA may allow the synthesis of, for example, well-defined MAM-blockLAM copolymers. While these “switchable” transfer agents rely on acid/base chemistry that may restrict their applicability, this approach is a promising step toward broadening CTA versatility and identifying a universal CTA capable of controlling the polymerization of both MAMs and LAMs. Kamigaito et al. have recently reported another type of “switchable” transfer agent that allowed interconvertible polymerization of acrylates and vinyl ethers by both living cationic or radical polymerization, using a Lewis acid or azo initiator, respectively.101,102 It is also worth noting that ab initio investigations have suggested the use of fluorine as the stabilizing Z group can afford CTAs that may also be capable of successfully mediating the polymerization of monomers with disparate reactivity.103,104 However, to date, this possibility has been only rarely explored, perhaps as a result of the rather challenging synthesis required to obtain CTAs with fluorine Z groups. Although outstanding efforts have been made in the synthesis of previously inaccessible block copolymers, the general restriction remains that block copolymers of MAMs and LAMS require the MAM block be prepared first. Polymerization techniques that overcome this regimented blocking order would further expand the scope of RAFT to prepare sophisticated block copolymer architectures, especially multiblocks. F


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Figure 5. (a) Synthesis of icosablock copolymer PNAM10-(PNAM3-b-PDEA3-b-PNAM3-b-PDMA3)4-b-PNAM3-b-PDEA3-b-PNAM3, (b) GPC traces of each block in icosablock synthesis, and (c) Number average molecular weight of each block in icosablock with the black line representing theoretical molecular weight, the full squares representing experimental molecular weight from SEC and the empty squares representing Mw/Mn from SEC.

L=

ponsive behavior, demonstrating the potential useful properties of high-order multiblock copolymers. To synthesize an even higher order block copolymer, further optimization was required. The authors thus utilized VA-044, an initiator that decomposes faster than AIBN to polymerize acrylamide and acrylate monomers in water. This work resulted in the first reported icosablock copolymer (20 blocks), PNAM 10 -b-(PNAM 3 -b-PDEA 3 -b-PNAM 3 -b-PDMA 3 ) 4 -bPNAM3-b-PDEA3-b-PNAM3 with an Mw/Mn = 1.36 and theoretical livingness of nearly 94% (Figure 5). It was noted that the process allowed the reaction to be stopped after five successive blocks and stored in the refrigerator overnight, with the polymerization being restarted over 4 consecutive days to reach 20 blocks.110 Further reports by Perrier and co-workers have carefully analyzed the experimental conditions of the process to develop guidelines for conducting such high-living RAFT polymerizations that reach near-quantitative monomer conversion.49 For example, when decablock copolymers were synthesized under nonoptimized (AIBN/dioxane/65 °C) and optimized (ACVA/water/70 °C) conditions, the nonoptimized polymerization led to a total theoretical livingness of 81−86%, while the optimized conditions resulted in 91−94% living polymer after the synthesis of 10 blocks. For each block synthesis, monomer concentration was kept as high as possible while maintaining a workable solution viscosity (2−3 M, depending on the monomer), and the [CTA]0/[initiator]0 concentration was kept as high as possible without significantly affecting polymerization time. The concentration of initiator employed for the synthesis of the initial block was higher than that used for subsequent blocks, most likely a result of the induction period observed during the first cycle. After the first block, [CTA]0/[initiator]0 ratios as high as 800 were used for the second block, which led to a livingness of ∼99%, according to eq 1. The addition of more monomer and solvent for subsequent blocks diluted the solution and required increased amounts of initiator. Additionally, polymerization time could be

[CTA]0 [CTA]0 + 2f [I]0 (1 − e

−kdt

(

) 1−

fc 2

)

(1)

It is apparent from eq 1 that higher [CTA]0/[I]0 ratios will lead to higher livingness. While lowering the amount of initiator seems simple enough, reduced initiator concentration also leads to decreased polymerization rate. Typically, the initiator concentration is chosen to provide a balance between an acceptable rate of polymerization and an acceptable level of dead chains (e.g., ∼10%).6 This level of nonextendable chains would clearly be disadvantageous in the synthesis of high-order multiblock copolymers, where the number of dead chains is amplified during each subsequent blocking step. However, slow polymerization times can be avoided by optimizing other polymerization rate parameters, for example, increasing the monomer concentration, utilizing monomers with inherently high kp, choosing a solvent that facilitates high propagation rates, employing initiators with fast decomposition rates, or using high polymerization temperatures. Perrier and co-workers have demonstrated the power of this approach by preparing multiblock acrylamide (high kp) copolymers in dioxane at 65 °C utilizing AIBN as an initiator and a trithiocarbonate as the CTA.110 Near-quantitative monomer conversion (>99%) was achieved in 24 h for each block, producing multiblock copolymers up to a dodecablock, PDMA10-b-(PNAM10-b-PDEA10-b-PNIPAM10-b-PDMA10)2-bPNAM10-b-PDEA10-b-PNIPAM (DMA = N,N-dimethylacrylamide, NAM = 4-acrylomorpholine, DEA = N,N-diethylacrylamide, NIPAM = N-isopropylacrylamide) with Mw/Mn = 1.6 and a theoretical livingness of 90% (using eq 1). Similar conditions produced a functional acrylamide−acrylate hexablock copolymer PBA25-b-PNAM5-b-P(NAM7-stat-HEA3)PNAM5-b-P(NAM7-stat-AA3)-b-PNIPAM19 (BA = butyl acrylate, HEA = 2-hydroxyethyl acrylate, AA = acrylic acid) with an Mw/Mn = 1.36 and theoretical livingness of nearly 95%. The hexablock copolymer showed self-assembly and thermoresG


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Macromolecules

techniques outlined throughout this mini-review have brought RAFT polymerization to the forefront of polymeric sequence control.111 The development of more “universal” CTAs and enhanced blocking techniques should improve sequential monomer polymerization. However, there are still challenges that remain in applying RAFT to the synthesis of new block copolymers. For example, blocking sequence is still restricted to MAM-b-LAM, as crossover from a poly(LAM)-based macroCTA to a MAM is complicated by preferential fragmentation of the MAM segment from the intermediate radical formed during crossover to the second block. While this limitation can be overcome during the synthesis of simple diblock copolymers by reversing the blocking sequence, synthesis of more complex block copolymers (e.g., poly(MAM)-b-poly(LAM)-b-poly(MAM)) remains difficult. Additionally, the ability to obtain high livingness and blocking efficiency are most applicable to monomers and conditions that lead to a high kp. Nearquantitative conversion of lower kp monomers, such as styrene, has yet to be achieved. Additionally, there remains an opportunity for exploiting spectroscopic methods (e.g., FTIR, UV−vis, NMR) for rapidly identifying the energetic characteristics of new CTAs that enable their efficient implementation during RAFT. Furthermore, efforts to improve the shelf life of many RAFT agents by mimimizing their susceptibility to hydrolysis or oxidation would also be beneficial. We hope the examples outlined above inspire continued refinement of the RAFT polymerization process. The field has shown tremendous promise within recent years, and we look forward to the continued advancements that inevitably will follow.

decreased significantly by utilizing initiators with shorter halflives, such as VA-044, allowing preparation of multiblock copolymers with high DP (e.g., a pentablock copolymer with average DP of 100 per block with final Mw/Mn < 1.3 and 92− 94% living chains).49 Utilizing the same type of approach, Boyer et al. demonstrated that PET-RAFT could accomplish similar highorder multiblock copolymers given the very low concentrations of photocatalyst used for the technique (ppm of photocatalyst to monomer). A decablock P(MA)10 was prepared with high MW, good control (Mn ∼ 82 000 g/mol, Mw/Mn = 1.40), and full monomer conversion of each block. Notably, no additional catalyst was needed for each block.72 Subsequent blocks thus required only the sequential addition of monomers after reaching full monomer conversion during each step, as the photocatalysts ( fac-[Ir(ppy)3]) does not decompose like thermal initiators. The MWD of the multiblock copolymers remained narrow even after 10 chain extensions, although it was noted that low molecular weight tailing was observed after 4−5 cycles. While a gradual loss of chain end functionality was apparent after each cycle, livingness was still greater than 65% after 10 chain extensions.

6. CONCLUSIONS AND FUTURE OUTLOOK Increased fundamental understanding of the RAFT mechanism has greatly expanded the versatility of the approach for both the synthesis of new materials and the preparation of previously accessible materials in a more efficient, faster, or convenient manner. In many cases, perceived limitations of RAFT have been addressed through creative manipulation of reaction conditions, the synthesis of new CTAs, or by the addition of supplemental reagents that provide light sensitivity, oxygen tolerance, etc. Previously, RAFT was considered most suited to the synthesis of low to moderate molecular weight polymers, but recent advances have facilitated access to molecular weights well in excess of 106 g/mol. Higher pressures, lower temperatures, ionic solvents, and microwave irradiation have been shown to increase kp, and in some cases lower kt, both of which promote high chain lengths. While RAFT and most RDRP methods are known to be sensitive to oxygen, the development of techniques like PET-RAFT and Enz-RAFT has allowed the synthesis of well-defined polymers and multiblock copolymers at ambient temperatures without the need for strict deoxygenated conditions, allowing the possibility of increasingly complex or industrial-sized syntheses. The monomer class specificity of many RAFT agents is becoming less rigid as advanced CTA design and synthesis improves. Moreover, polymerizations leading to near-quantitative livingness can be achieved by reducing the concentration of initiator and employing monomers with high kp. Perhaps most importantly, the retention of chain-end functionality allows multiblock copolymers to be prepared with more blocks than previously reported by any RDRP method. Each new development in the field expands the toolbox used to prepare novel and highly complex polymeric materials. Currently, one of the most appealing challenges in modern chemistry is the ability to mimic the high sequence specificity of biopolymers such as proteins and nucleic acids.111−113 Particularly, the use of RDRP is of interest as it allows the ability to produce appreciable amounts of material under mild reaction conditions. While the radical nature of RDRP makes systematic monomer addition inherently difficult, creative

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]fl.edu (B.S.S.). Notes

The authors declare no competing financial interest. Biographies

Megan R. Hill earned a B.S. in Chemistry from California Polytechnic State University in San Luis Obispo, CA, in 2012 under the supervision of Prof. Philip Costanzo. Megan is currently an NSF Graduate Research Fellow pursuing a Ph.D. under the guidance of Prof. Brent S. Sumerlin at the University of Florida. Her research focuses on the synthesis of biodegradable and stimuli-responsive polymers. H


Macromolecules

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R. Nicholas Carmean received his B.S. in biochemistry from Virginia Polytechnic Institute and State University in 2013, where he conducted undergraduate research with Prof. Timothy E. Long. He is currently working toward his doctorate in chemistry under the supervision of Prof. Brent S. Sumerlin. His current research interests focus on developing and expanding controlled polymerization techniques.

Brent Sumerlin graduated with a B.S. from North Carolina State University in 1998 and received his Ph.D. in 2003 at the University of Southern Mississippi under the direction of Dr. Charles McCormick. He continued his work as a Visiting Assistant Professor/Postdoctoral Research Associate in the group of Krzysztof Matyjaszewski at Carnegie Mellon University from 2003−2005. In 2005 he joined the Department of Chemistry at Southern Methodist University as an Assistant Professor, and in 2009 he was promoted to Associate Professor with tenure. In the fall of 2012, Prof. Sumerlin joined the George & Josephine Butler Polymer Research Laboratory and the Center for Macromolecular Science & Engineering within the Department of Chemistry at the University of Florida. He is a Fellow of the Royal Society of Chemistry and was been named a Kavli Fellow (Frontiers of Science, National Academies of Sciences). Prof. Sumerlin has won a number of awards, including the Alfred P. Sloan Research Fellowship, NSF CAREER Award, ACS Leadership Development Award, Journal of Polymer Science Innovation Award, and the Biomacromolecules/Macromolecules Young Investigator Award.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation (DMR-1410223) and the National Science Foundation Graduate Research Fellowship under Grant DGE1315138 (M.R.H.) Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research (53225-ND7). I


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