Fundamentals of Atom Transfer Radical ... - ACS Publications

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George B. Kauffman California State University Fresno, CA 93740

Fundamentals of Atom Transfer Radical Polymerization Veerle M. C. Coessens and Krzysztof Matyjaszewski* Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 *[email protected]

Today's market increasingly demands sophisticated materials for advanced technologies and high-value applications, such as nanocomposites, optoelectronic, or biomedical materials. Therefore, the demand for well-defined polymers with very specific molecular architecture and properties increases. Until recently, these kinds of polymers could only be prepared via living ionic polymerization, a process that requires very stringent experimental conditions. Recently, atom transfer radical polymerization (ATRP), a controlled/living radical polymerization (CRP) process, has emerged as a viable alternative. ATRP allows the synthesis of polymeric structures that are well-defined in terms of composition and molecular architecture. By virtue of its simplicity, versatility and scope to make polymers with site-specific functionality and novel architecture, ATRP has become the most extensively studied CRP technique. Controlled/Living Radical Polymerization For many years, controlled/living radical polymerizations (CRP) were considered impossible, primarily because of the specific, intrinsic characteristics of radicals. The radical propagation reaction is fast and radicals terminate at diffusion-controlled rates. Chain-extension is not possible, so well-defined block copolymers cannot be prepared via conventional radical polymerization (1). However, over the last 10 years, thousands of scientific publications on controlled/living radical polymerizations appeared. The key to success lies in masking the radical in the form of a dormant species. A dormant species cannot terminate but can be intermittently activated to form a radical that, after addition of a few monomer units, returns into its dormant state. An equilibrium between dormant and growing species is established and, as the equilibrium is very much shifted toward the dormant state, the proportion of terminated chains is minimized. Moreover, all chains grow steadily at approximately the same rate, therein enabling a predictable molecular weight (MW) and a narrow molecular weight distribution (MWD) to be achieved. Also, chain-extension and end-functionalization are possible (1). Among various CRP techniques, the most widely used ones are atom transfer radical polymerization (ATRP), stable freeradical polymerization, mostly nitroxide-mediated, and degenerative transfer polymerizations, mainly reversible additionfragmentation chain transfer (RAFT) polymerization. This article will further focus on ATRP. ATRP versus Other Living Polymerizations Well-defined polymers were first made by living anionic polymerization (2). In this case, all chains are instantaneously initiated and grow simultaneously while chain-breaking reactions, 916

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such as termination and transfer, are absent. The result is uniform polymers with controlled MW and narrow MWD. Composition, topology, and functionality of polymer chains can be precisely controlled and designed. However, the range of monomers that can be polymerized is limited and copolymerization is challenging because of large differences in reactivity of monomers. Some drawbacks of these ionic processes include their sensitivity to moisture, carbon dioxide, traces of acid or base, and impurities. Meanwhile, several other controlled/living processes were developed (2). So, over the years, the challenge has always been to make the controlled/living polymerization process more user-friendly, to relax process conditions, and extend the monomer choice. These challenges are exactly met by ATRP. The main advantage of a radical polymerization process is its tolerance to various functional groups. Also, the polymerization can be run in the presence of water and even in either biphasic or homogeneous aqueous media. ATRP versus the Conventional Radical Polymerization The conventional radical polymerization is a chain polymerization that quickly produces high MW polymers (Scheme 1): the initiator (In-In) continuously and slowly generates radicals that react with monomers, forming the first growing species (P1*). Propagation (with rate constant of propagation, kp) is fast, much faster than initiation and termination. As radicals terminate at diffusion-controlled rates (in Scheme 1, only radical recombination with rate constant kt is shown, disproportionation is also possible), the steady radical concentration, established by balancing the rates of initiation and termination, is very low (from ppb to ppm). A propagating radical reacts with a monomer approximately every 1 ms and chains terminate after about 1 s. During the polymerization, the overall percent conversion of monomer to polymer increases with reaction time but the MW of the polymers does not. The outcome is a high MW polymer, with a broad MWD (high polydispersity defined as a ratio of weight to number average MW, Mw/Mn) (3). With the free radical polymerization technique, it is impossible to prepare well-defined polymers. Because the lifespan of the propagating chain is so short, functionalization, synthesis of block copolymers, or polymers with controlled architecture is no option. In ATRP, the radical is intermittently masked as a dormant species. The initiator is an alkyl halide or pseudohalide, which interacts with an activator, a low-oxidation state transition metal complex, to form a propagating radical. The radical is quickly deactivated by a higher-oxidation state transition metal complex (deactivator) and the dormant chain is created. The most efficient redox-active transition metal complex has been a copper complex with various nitrogen-based ligands. Equilibrium between

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Vol. 87 No. 9 September 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed1002256 Published on Web 07/08/2010

Chemistry for Everyone

Scheme 1. Major Elementary Reactions in Radical Polymerization

Scheme 3. Examples of Monomers (Co)Polymerized by ATRP

Scheme 4. Illustration of ARGET and ICAR ATRP

Scheme 2. Equilibrium between Active and Dormant Species in ATRP, Catalyzed by Cu/2,20 -bipyridine Complex

dormant and growing species is strongly shifted to the side of dormant species (4, 5) (see Scheme 2). All polymer chains grow at the same pace; therefore, welldefined MW polymers with a narrow MWD can be achieved. Kinetic plots show a linear progression of MW as a function of conversion, a clear demonstration of a controlled process. Grown chains can also be reactivated upon addition of a second monomer; therefore well-defined block copolymers are within reach. ATRP opens the possibility of precise control of molecular architecture (composition, topology, and functionality), which leads to new materials with interesting applications, as discussed below. The ATRP Components Monomers ATRP is a versatile process and many monomers such as styrenes, acrylates, methacrylates, acrylamides, acrylonitrile, and others have been successfully polymerized (Scheme 3). ATRP is tolerant to many functional polar groups; monomers polymerized include those with hydroxy and amino groups. Some groups interfering with the catalyst system (such as acids) can be blocked and unprotected after the polymerization process. Initiator In traditional ATRP, the initiator is an alkyl halide (or pseudohalide), either of low or high molar mass, or the alkyl halide tethered to the surface of inorganic particles, flat wafers, fibers, or even biomolecules (6, 7). To create functional moiety at the polymer chain, initiators that include a functional group such as an alcohol, ester, or epoxide and others can be used. Alternatively, halogens from the chain end can be displaced by functional nucleophiles. To create star polymers or hyperbranched polymers, multifunctional initiators or so-called inimers (molecules containing monomer and initiator) are used (8). Catalyst In ATRP, the commonly used redox-active transition metal complex (Mt m /L) is based on copper but other metals are

r 2010 American Chemical Society and Division of Chemical Education, Inc.

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explored as well. The complexing ligand L is, for example, derivative of 2,20 -bipyridyne (cf. Scheme 2), PMDETA (N,N, N0 ,N00 ,N00 -pentamethyldiethylenetriamine) or Me6TREN tris[2-(dimethylamino)ethyl]amine. The optimal ligand selection depends on the monomer, targeted molecular weight, and reaction conditions. Parameters such as solubility, stability of the metal complex, its redox potential, and affinity toward halides have to be considered (9-11). Mechanistically, ATRP is based on an inner-sphere electron transfer process, which involves a reversible homolytic (pseudo)halogen transfer between a dormant species, (P n -Br in Scheme 2) and a transition metal complex in the lower oxidation state (Mtm/L, such as Cu(I)/bpy2 in Scheme 2), resulting in the formation of propagating radicals (Pn*) and the metal complex in the higher oxidation state with a coordinated halide ligand (e.g., X-Mtmþ1/L). During the course of the polymerization, the equilibrium is strongly shifted toward the dormant species (ka , kda). One drawback of the first ATRP systems was the relatively large amount of catalyst used, typically on the order of 0.1-1 mol % relative to the monomer. The final products often contained a significant amount of residual metal. Various strategies were introduced to remove the catalyst from the final product or to carry out the reactions at lower catalyst concentration. Recent Developments in Catalyst Design To make ATRP more industrially acceptable, more efficient and “greener” catalyst systems were developed (12-14). A first strategy resulted in ARGET (activators regenerated by electron transfer) ATRP. In ARGET, a very small amount of active catalyst is used, and the deactivator formed owing to radical termination is constantly converted to activator via a redox process. Environmentally friendly reducing agents are used to continuously regenerate the metal complex in the lower oxidation state. The reducing agents include ascorbic acid, sugars, amines, tin(II) octoate, or metals such as Cu or Fe.

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Scheme 5. Some Examples of Controlled Macromolecular Architectures Prepared by ATRP

The second strategy is ICAR (initiators for continuous activator regeneration) ATRP, whereby a radical initiator is used to continuously regenerate activator from the deactivator (formed during radical termination) (see Scheme 4). Media ATRP can be performed in bulk, in many polar and nonpolar organic solvents, in CO2, or in water. Water-soluble monomers (both neutral and ionic) can be polymerized by ATRP in protic (aqueous) media, provided that some basic rules for catalyst selection are obeyed. Range of temperatures span from subambient to over 130 °C. ATRP can also be run at room temperature. Typically, no pressure is required, although at higher pressure less termination and better control are obtained (15). Materials Made by ATRP, and Their Applications ATRP allows the synthesis of (co)polymers with targeted composition, controlled molecular architecture, predetermined MW, and narrow MWD. The chain topologies include statistical, gradient, and segmented (block or graft) copolymers, and for each of these cases, the chain architecture can be varied in a controlled manner, to include combs, brushes, multi-arm stars, and dendritic macromolecules with controlled degrees of branching (16) (see Scheme 5).

Figure 1. Atomic force microscopy image of the molecular brush prepared by ATRP. The backbone consists of poly(2-hydroxyethyl methacrylate) with DP = 400; the densely grafted side chains are poly(n-butyl acrylate) with DP = 40.

Block and Segmented Copolymers

In copolymerization, any changes in instantaneous copolymer composition as a function of monomer conversion in CRP are recorded along the backbone of each individual chain (17). Therefore, the chains not only have nearly identical molecular weight, but also the same compositional gradient along the backbone. Such gradient copolymers are not readily accessible by other polymerization techniques. Gradient polymers have unusually broad ranges of glass transition temperature values and have potential applications as vibration and noise dampening materials, and are also efficient surfactants for polymer blends.

Blocks or segments of different composition are incompatible and thus phase-separate, resulting in nanophase separation. Several applications are possible depending on the morphology, domain size, and the properties (glass transition temperature, solubility, mechanical strength, surface energy, etc.) of the polymers forming each segment, including adhesives, dispersants, surfactants, thermoplastic elastomers, and others. Another example is a diblock copolymer with a hydrophilic segment and tertiary amine groups in the second segment. In basic aqueous solutions, the copolymer forms micelles; upon acidification, the amino groups are protonated and become water-soluble, resulting in dissociation of the aggregates. This kind of block copolymer with responsive segments, whether it is 918


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toward pH, light, or temperature, is being intensely studied for potential for drug-delivery systems. Gradient Copolymers

Hyperbranched and Star Polymers Hyperbranched and star molecules have multiple functionalities per polymeric molecule and they have a lower viscosity


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than their linear counterparts with the same molecular weight. Therefore, they are of special interest for applications ranging from lubricants, additives, viscosity modifiers, to drug delivery formulations (18). Molecular Brushes Densely grafted polymers resemble bottlebrush structures. If side chains are short and soft (for example poly(n-butyl acrylate) with molecular weight below 10,000), then the brush has supersoft properties with moduli in the range of 1 kPa, such as hydrogels (19) (see Figure 1). However, in contrast to hydrogels that become rigid and brittle after water evaporates, brushes never harden because they are diluted by their covalently attached side chains, which act as internal diluents. Potential applications of brushes range from medical supplies, to cosmetic products, to packaging of delicate mechanical parts. Hybrid Polymers Alkyl halides can be attached to complex structures such as natural products, organic or inorganic particles, and surfaces (6, 8). Therefore, materials such as nanocomposites, bioconjugates, and biodegradable functional macromolecules are accessible. Conclusions ATRP is a controlled radical polymerization technique that allows the synthesis of well-defined polymers. Because of its relative simplicity and versatility, polymers for different application fields can be prepared. ATRP is so robust and simple that it has been already introduced to undergraduate laboratories (20, 21). New ATRP techniques have been also adopted for organic synthesis (22). Literature Cited 1. Matyjaszewski, K.; Davis, T. P., Eds. Handbook of Radical Polymerization; Wiley: Hoboken, NJ, 2002.


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2. Mueller, A. H. E.; Matyjaszewski, K. Controlled and Living Polymerizations: From Mechanisms to Materials; Wiley-VCH: Weinheim, 2009. 3. Goto, A.; Fukuda, T. Prog. Polym. Sci. 2004, 29, 329–385. 4. Wang, J.-S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614–5615. 5. Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921–2990. 6. Lutz, J.-F.; Boerner, H. G. Prog. Polym. Sci. 2008, 33, 1–39. 7. Oh, J. K.; Drumright, R.; Siegwart, D. J.; Matyjaszewski, K. Prog. Polym. Sci. 2008, 33, 448–477. 8. Peleshanko, S.; Tsukruk, V. V. Prog. Polym. Sci. 2008, 33, 523–580. 9. Pintauer, T.; Matyjaszewski, K. Coord. Chem. Rev. 2005, 249, 1155–1184. 10. Qiu, J.; Matyjaszewski, K.; Thouin, L.; Amatore, C. Macromol. Chem. Phys. 2000, 201, 1625–1631. 11. Matyjaszewski, K.; Paik, H. J.; Zhou, P.; Diamanti, S. J. Macromolecules 2001, 34, 5125–5131. 12. Tsarevsky, N. V.; Matyjaszewski, K. Chem. Rev. 2007, 107, 2270–2299. 13. Jakubowski, W.; Matyjaszewski, K. Angew. Chem., Int. Ed. 2006, 45, 4482. 14. Matyjaszewski, K.; Jakubowski, W.; Min, K.; Tang, W.; Huang, J.; Braunecker, W. A.; Tsarevsky, N. V. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15309–15314. 15. Kwiatkowski, P.; Jurczak, J.; Pietrasik, J.; Jakubowski, W.; Mueller, L.; Matyjaszewski, K. Macromolecules 2008, 41, 1067–1069. 16. Matyjaszewski, K.; Tsarevsky, N. V. Nature Chem. 2009, 1, 276–288. 17. Matyjaszewski, K.; Ziegler, M. J.; Arehart, S. V.; Greszta, D.; Pakula, T. J. Phys. Org. Chem. 2000, 13, 775–786. 18. Gao, H.; Matyjaszewski, K. Prog. Polym. Sci. 2009, 34, 317–350. 19. Sheiko, S. S.; Sumerlin, B. S.; Matyjaszewski, K. Prog. Polym. Sci. 2008, 33, 759–785. 20. Beers, K. L.; Woodworth, B.; Matyjaszewski, K. J. Chem. Educ. 2001, 78, 544–547. 21. Matyjaszewski, K.; Beers, K. L.; Woodworth, B.; Metzner, Z. J. Chem. Educ. 2001, 78, 547–550. 22. Pintauer, T.; Matyjaszewski, K. Chem. Soc. Rev. 2008, 37, 1087–1097.


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Fundamentals of Atom Transfer Radical ... - ACS Publications

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