radical polymerization

Controlled/living radical polymerization

by Krzysztof Matyjaszewski* and James Spanswick†

Until a little more than a decade ago, controlled/living radical polymerization (CRP) would have been an oxymoron. Full control over all aspects of radical polymerization was deemed well-nigh impossible because radical termination reactions occur at diffusion-controlled rates. However, there are now several procedures for controlling radical polymerization, and corporations are introducing products based on CRP into numerous high-value markets. This review briefly summarizes the evolution of CRP, describes some of the materials that can now be prepared, and highlights some of the commercialization efforts currently underway.

Conventional free radical polymerization (FRP) has many advantages over other polymerization processes. FRP does not require stringent process conditions and can be used for the (co)polymerization of a wide range of vinyl monomers. Nearly 50% of all commercial synthetic polymers are prepared using radical chemistry, providing a spectrum of materials for a range of markets1. However, the major limitation of FRP is poor control over some of the key elements of the process that would allow the preparation of well-defined polymers with controlled molecular weight, polydispersity, composition, chain architecture, and site-specific functionality. CRP provides such control, leading to an unprecedented opportunity in materials design, including the ability to prepare bioconjugates, organic/inorganic composites, and surface-tethered copolymers (Fig. 1). The development of functional polymers with predetermined, well-defined structures allows manufacturers to improve the properties of materials currently in the marketplace and create new markets for materials whose manufacture and processing conditions uniquely meet the targeted properties.

Evolution of CRP Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213-2683, USA *E-mail: [email protected] †E-mail: [email protected]

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In the past decade, the limitations of FRP have been overcome as several procedures for CRP have evolved based on an understanding and integration of chemistry developed over the past 60 years in the fields of organic chemistry, conventional radical polymerization, and living ionic polymerizations (Fig. 2).

ISSN:1369 7021 © Elsevier Ltd 2005

REVIEW FEATURE

A number of CRP methods have been developed and the three most promising are: stable free radical polymerization (SFRP), most commonly nitroxide mediated polymerization (NMP)11,12 but may also include organometallic species13 (Fig. 3, scheme 1); transition-metal-catalyzed atom transfer radical polymerization (ATRP)14,15 (Fig. 3, scheme 2); and degenerative transfer with alkyl iodides16, methacrylate macromonomers17, and dithioesters via reversible additionfragmentation chain transfer (RAFT) polymerization18,19 (Fig. 3, scheme 3). In order to extend the lifetime of the propagating chains, each of these methods relies on establishing a dynamic equilibrium between a low concentration of active propagating chains and a predominant amount of dormant chains that are unable to propagate or terminate. In the case of SFRP or ATRP, the equilibrium is pushed to the left-hand side (deactivated, kdeact), forming an excess of dormant species as a result of the persistent radical effect4. In all radical polymerizations, biradical termination occurs at a rate, Rt, which is dependent

on the concentration of radicals, [P*], where Rt = kt[P*]2. Therefore, at the same polymerization rate (the same [P*]), essentially the same number of chains terminate regardless of being in conventional or CRP systems. However, in the conventional process all chains are terminated, whereas in CRP, as a result of the greater number of growing chains, the terminated chains constitute a small fraction of all the chains (~1-10%). The remaining chains are dormant species, capable of reactivation, functionalization, and chain extension to form block copolymers, etc. Thus, CRP behaves as a ‘living’ system20,21. Additionally, relatively fast initiation, at least as fast as propagation, gives control over molecular weight (the degree of polymerization is defined by the ratio of concentrations of the consumed monomer to the introduced initiator, DPn = ∆[M]/[I]0) and a narrow molecular weight distribution. Significant progress has been reported in each CRP system over the past decade. New nitroxide mediators have been developed that allow polymerization of acrylates22 and new ligands for the various transition metals used in ATRP have increased the activity of catalyst systems 10 000-fold over the initial systems23-25. The amine-based ligands that form the most active catalysts can be readily modified to adjust their solubility and activity26. Increased catalyst activity addresses one of the perceived drawbacks of ATRP – the presence and necessity of removing a transition metal. Additional work has been conducted on catalyst removal27, supported catalysts28, and hybrid catalyst systems29 that reduce the residual metal to less than 5 ppm, making ATRP industrially acceptable. Proceedings from three American Chemical Society (ACS) symposia on CRP have been published, along with several extensive reviews and book chapters, citing many efforts that

Fig. 2 Development of CRP by integration of advances in several fields of chemistry2-10.

Fig. 3 The three main CRP methods.

Fig. 1 Examples of molecular structures attained through CRP.

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Scope of CRP

moved the field forward1,12,25,30-35. A fourth ACS symposium will be held in August 2005. Over 7000 papers have been published on CRP since 1995. Fig. 4 shows the results of a SciFinder Scholar search using the following terms: ‘controlled radical polymn’ or ‘living radical polymn’ (CRP & LRP); ‘ATRP’ or ‘atom transfer (radical) polymn’ (ATRP, this does not include terms like metal-mediated or metalcatalyzed radical polymerization); ‘NMP’ or ‘SFRP’ or ‘nitroxide mediated polymn’ or ‘stable free polymn’ (SFRP & NMP); and ‘RAFT’ or ‘reversible addition transfer’ or ‘degenerative transfer’ or ‘catalytic chain transfer’ (RAFT & DT & CCT). The latter two terms were refined with the term ‘radical polymn’ since they coincide with other popular names such as ‘N-methylpyrrolidone’ or ‘raft-associated proteins’. In 2004, there were 305 papers published using the term ‘controlled radical polymn’, 321 papers with the term ‘living radical polymn’ (combined 518 papers), and 669 papers using the term ‘ATRP’. However, searching for ‘CRP or LRP or ATRP’ gave 1028 papers. Thus, it seems that ATRP is currently the most often used methodology, as reflected in this article. It must also be noted that many CRP techniques are used for the synthesis of specific materials and authors have progressively stopped using any of the above terms in either title, abstract, or keywords, so the actual number of publications incorporating CRP as a critical aspect is much larger than shown in Fig. 4. The publication activity does not just reflect academic interest in these new synthetic tools, since over 500 patents on CRP have been issued. The driving force behind this significant industrial research effort is an anticipated $20 billion/year market for products made by CRP36.

CRP may be used to form new (co)polymers, including, for example, gradient37, block33, tri- and multi-arm star copolymers38, site-specific functional polymers39, and graft copolymers with controlled graft density and graft distribution40,41. In addition, inorganic materials and natural products can be linked to synthetic polymers to form nanocomposites and copolymers tethered to surfaces (Fig. 1)42,43. The capabilities of CRP differ from earlier ionic living polymerization processes (Fig. 2) as a result of the adaptability of radical polymerization processes to different polymerization systems and the tolerance of CRP to functional groups on the monomer unit. CRP can be run in the presence of water44, ionic liquids45,46, and supercritical CO247. CRP processes have been developed for biphasic44,48-54 and homogeneous aqueous media55,56, and for the direct polymerization of acidic, basic, and ionic monomers to form homopolymers57-59 and block copolymers60,61. Polymers with complex architectures, including brushshaped macromolecules (Fig. 5)62,63, stars, and dendrimers64-67 have been prepared using multifunctional initiators. A preliminary study of the physical properties of molecularbrush copolymers, formed when essentially every unit along a polymer backbone initiates a ‘grafting from’ reaction, indicates that a new state of matter is achieved when the brushes are lightly crosslinked69,70. The polymeric materials are one thousand times softer than a typical elastomer (G' ~103 Pa versus 106 Pa) and called supersoft elastomers. When CRP initiators are attached to particles or flat surfaces, the ‘grafting from’ reaction forms nanocomposites43,71,72 and materials with responsive surfaces73.

Fig. 4 Number of publications on different CRP methods according to a SciFinder Scholar search on January 15, 2005.

Fig. 5 Atomic force microscopy (AFM) image of a poly(n-butyl acrylate) molecular brush on a mica substrate68. (Courtesy of S. Sheiko, The University of North Carolina at Chapel Hill.)

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ABC block copolymers that undergo self-organization have been prepared. The functionality in each segment can be tailored for the final application, such as delivery of drugs to a specific site within the body74 or delivery of an Fe nanoparticle for degradation of environmental pollutants (e.g. chlorinated solvents)75. The reactivity ratios for comonomers in CRP and FRP are essentially identical because the propagating species are radicals76-78, but this does not mean that the two processes produce identical products. Inherent differences in the relative rates of consumption of the two different monomers in a CRP copolymerization result in the formation of compositionally homogeneous products, where composition changes along each polymer chain, as opposed to forming a mixture of compositions in an FRP79. This type of copolymer has been called a gradient copolymer (Fig. 1) as a result of this gradual change in composition along each and every polymer chain. The gradient, or shape of the average change in composition along the chain, can be controlled by taking into consideration the relative reactivity ratios of the comonomers, which ultimately determine the instantaneous concentration of the comonomers in the reaction medium as the reaction progresses37. The latter can be further controlled by continuous or sequential addition of one or more comonomers. Indeed, it is possible to go to an extreme gradient and prepare one-pot, one-step, block copolymers by careful consideration of the molar ratio of monomers initially added to a CRP80. However, in a single-mechanism sequential copolymerization, successful preparation of block copolymers depends on the cross-propagation kinetics81-83. This is also the case when using two different polymerization mechanisms to prepare individual polymer segments and so expand the range of monomers that can be incorporated into the final copolymer35,84,85. A similar freedom of choice exists in the preparation of graft copolymers using CRP. Graft copolymers have been synthesized using ‘grafting from’ various backbones, including commercially available polymers such as polyvinyl chloride, polyethylene, and polyisobutylene86-88, or ‘grafting through’ desired macromonomers such as polyolefins89 or polysiloxanes41. Research groups are beginning to examine how the physical properties of a material can be controlled by this increased ability to fine tune the composition, structure, and functionality of polymers. For example, Pakula et al.41,82 have examined the effect of a gradient of B monomer in the A

Fig. 6 Effect of structure on the tensile properties of compression molded films of poly(methyl methacrylate)-graft-poly(dimethylsiloxane)41. (Adapted with permission from41. © 2003 American Chemical Society.)

blocks of an ABA block copolymer formed in a continuous sequential copolymerization. They find that minor changes in the composition of the A block have a significant effect on the modulus and behavior under stress82. The group has also examined the effect of varying the distribution of grafts along a copolymer backbone in a series of graft copolymers prepared by different polymerization techniques41. Fig. 6 shows how differences in the reactivity ratios of the monomer and macromonomer for different polymerization processes result in a variation in the distribution of the grafts along the copolymer backbone and the resultant physical properties. Differences in graft distribution in the RAFT and ATRP processes are amplified when a macroinitiator is employed to improve compatibility of the macromonomer and copolymer in the ATRP process. Fig. 7 is an AFM image of a four-armed star brush copolymer; both macroinitiator preparation and ‘grafting

Fig. 7 AFM image of four-arm star molecular brushes with poly(n-butyl acrylate) side chains on a mica surface38,65. (Courtesy of S. Sheiko, The University of North Carolina at Chapel Hill.)

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from’ reaction used ATRP. Fig. 7 shows that, while CRP processes do control the polymerization process (the polydispersity index for the star is 1.04), some side reactions are unavoidable. Stars with only three arms can be seen along with some linear chains38,65. The effect of such imperfections on the overall properties of materials prepared by CRP is currently being investigated.

Academic research on CRP applications While the initial drive for the development of CRP was the preparation of polymers with novel architectures and functionality, the current focus of research also involves preparing and fabricating materials for specific applications. Electronics, surface modification, and bioapplications are at the forefront of these efforts. Since the phase-separation behavior of segmented copolymers can be readily modified, the resulting control over the morphology of thin films cast from materials prepared by CRP reactions provides opportunities to develop materials for electronic applications. Different approaches are being examined at Carnegie Mellon University (CMU). One method is the formation of well-defined, carbon-based structures by pyrolysis of polyacrylonitrile block copolymers prepared by different CRP processes (Fig. 8)90-92. The phaseseparated structures can be stabilized prior to pyrolysis, forming carbon nanostructures that are being evaluated for use as field-effect transistors (FETs). Another approach toward making FETs is the preparation of segmented copolymers with conjugated polymer blocks that self assemble in a variety of new conducting morphologies93. This is expected to generate processible materials with better mechanical properties and tunable electrical properties, both a result of the original composition of the copolymers. Elsewhere, Armes et al. at the University of Sheffield, UK94, in conjunction with Biocompatibles International, has developed several bioresponsive materials95, including a fully reversible pH-sensitive biocompatible triblock copolymer that forms a solution at low pH and a micellar gel network at pH 8. The biocompatible gels can be loaded with cardiovascular drugs for slow diffusional or fast triggered delivery of the pharmaceutical. Biointeractive materials are also being addressed by the groups of Nolte and van Hest at the University of Nijmegen, the Netherlands96. They are working with Encapson on the synthesis of novel block copolymers specifically for encapsulating enzymes and other

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Fig. 8 Tapping mode AFM phase image of a thin film formed by zone casting a poly(butyl acrylate)-b-(polyacrylonitrile) block copolymer with 17.5% polyacrylonitrile. (Casting and image courtesy of the Kowalewski group, CMU.)

bioactive compounds. Maynard and coworkers at the University of California, Los Angeles are using CRP as a component of a multidisciplinary strategy to develop materials that provide rapid access to well-defined bioconjugates, which also act as building blocks for nanotechnology applications. They have developed soluble polymers that bind to and release active proteins in a controlled manner to enhance wound healing, blood-vessel growth, and other therapeutically important processes97. In the same field, biocompatible peptide-derived, shellcrosslinked nanoparticles are being examined by Wooley et al. at the University of Washington in St Louis74,98. Russell at the University of Pittsburgh has prepared permanent nonleaching antimicrobial surfaces by depositing an initiator for ATRP on surfaces, including glass and paper, and growing copolymers that include monomers with tertiary amino groups from the surface99. After quaternization of the tethered polymer chains, this surface provides permanent antimicrobial activity. Indeed, extensive research on the modification of surfaces and the preparation of responsive nanocomposite materials is being conducted in a number of academic laboratories using all types of CRP42,71,100-104.

Corporate CRP research Despite this academic interest in applications, commercial corporations are going to drive focused application research. Target applications, based on an analysis of patent applications, include the components of coatings, adhesives, nonionic surfactants, dispersants, polar thermoplastic elastomers, bulk performance materials, membranes, personal

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care products, detergents, double hydrophilic block copolymers for crystal engineering and drug delivery systems, gels and hydrogels, lubricants and additives, surface modifiers, hybrids with natural and inorganic polymers, various bio- and electronic materials, etc. One of the earliest adopters of CRP was DuPont Performance Coatings, which now prepares several commercial components of paints, coatings, and inks using CRP. Cumulative commercial production of materials made with various CRP techniques now totals several million pounds and is slowly but steadily growing, year by year. The most commercially important polymer architectures for DuPont and other corporations are block copolymers; di- and tri-block structures have been commercialized in multiple applications. DuPont maintains a very active research and development effort in the general area of CRP and has licensable intellectual property covering catalytic chain transfer and a variety of addition-fragmentation chain transfer agents. The company expects to see new products based on these synthetic techniques in the next few years. The self-organizing ability of block copolymers has been exploited by IBM to create Si memory chips using new nanoscale manufacturing techniques105, and this method will be extended to provide a general approach to surface modification and nanoscopic templates106. Ciba Specialty Chemicals, Degussa, PPG, and Kaneka have all been members of the ATRP and CRP Consortia at CMU107, which provide access to CMU intellectual property. The companies discussed the status of their research on CRPbased preparation of products at the 227th ACS National Meeting in 2004108-111. Ciba has focused on the preparation of amphiphilic graft copolymers by copolymerization of macromonomers with other monomers using both ATRP and NMP to give well-defined comb-copolymers108. Its first CRPbased products are acrylic block copolymers, commercialized last year through EFKA, which offer superior rheological performance and improved stabilization of pigment dispersions in coating applications112. This effort received the Ciba Specialty Chemicals R&D Award for 2004. RohMax Oil Additives, a subsidiary of Degussa, discussed commercially feasible and economically acceptable conditions for ATRP preparation of additives based on long chain poly(alkyl methacrylates) that are suitable for use as components of lubricating oils. Degussa has also developed the commercial capability to prepare block

copolymers and remove traces of catalysts from the products110. PPG indicated that materials prepared by ATRP offer many benefits over those prepared by other polymerization processes, including the ability to control the polymer molecular weight and achieve a narrow molecular weight distribution109. PPG also noted that another substantial benefit of ATRP is the ability to manipulate the composition, functionality, and architecture of (co)polymers. This permits the formation of complicated structures, such as block, gradient, comb, and star copolymers, which are being evaluated as components of various coating materials. Kaneka announced that it currently has a large pilot unit producing commercial samples (Fig. 9) and is constructing a full-scale plant to produce reactive telechelic materials using ATRP111. Products include a range of moisture-curable and addition-curable polyacrylates directed at sealant and adhesive markets. The main advantages over current products are high heat, oil, and ultraviolet resistance. One of the advantages that accrue from use of environmentally stable materials is their nonstaining characteristics. The benefits are seen in the lack of surface contamination on artificial marble attached to the exterior of buildings by sealants prepared using ATRP (compare Fig. 10a with Fig. 10b). The noncontamination properties of similar products also allow retention of the self-cleaning properties of TiO2-treated glass currently being introduced by PPG for use in offices and residential buildings113. The heat and oil resistance of the materials prepared by ATRP also provide ideal materials for the formation of liquid-based gaskets for use in various engines.

Fig. 9 Kaneka ATRP pilot plant in Kashima, Japan. (Courtesy of Y. Nakagawa, Kaneka.)

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(b)

(a)

Fig. 10 Tiles attached with (a) acrylate-based sealant prepared by ATRP; and (b) silicone-based sealant, showing effect of contamination. (Courtesy of Y. Nakagawa, Kaneka.)

Arkema, formerly AtoFina, and Dionex are among other members of CRP Consortium at CMU considering commercialization of products based on CRP. Arkema has developed a novel class of stable free radical mediator (SG-1)22 for CRP of acrylate monomers and claims they are suitable for preparing high solids coating resins with controlled rheology114. The company is planning on introducing block copolymers based on acrylate and methacrylate monomers as toughening agents. Dionex uses ATRP to nanoengineer the stationary phase of chromatographic columns. An example is the preparation using a ‘grafting from’ technique of columns for separation of bioactive materials. Dionex has produced a high-resolution, immobilized metal affinity chromatography (IMAC) column capable not only of peptide and protein enrichment, but also of the separation of components within the classes in the same run. This is accomplished by grafting a hydrophilic layer from the particle surface, reacting the polymer grafts with chelating groups, and then inducing chain collapse by introducing Cu ions for intramolecular coordination crosslinking to form tethered metal-polymer composite nanoparticles (Fig. 11). The tethered nanocomposite particles interact with the eluants, causing separation of proteins that differ by only one methyl substituent.

Fig. 11 Schematic of the stationary phase of an IMAC chromatographic column.

Summary Clearly the field of CRP has developed since the seminal paper on SFRP of styrene by Georges in 199311. Significant advances in every CRP procedure have provided synthetic chemists with the ability to prepare materials that were impossible to manufacture a decade ago. Controlled synthesis and processing allows the properties of materials to be characterized and readily manipulated on the nanoscale. As a result, numerous corporations in a broad range of markets are preparing a spectrum of new materials for external evaluation. They now have the capability to tailor the properties of their products to their customers’ needs. MT

Acknowledgments The authors wish to thank the members of both the ATRP and CRP Consortia at CMU, the US National Science Foundation and Environmental Protection Agency for funding the fundamental work on ATRP, and the collaboration of numerous scientists throughout the world, including in particular T. Kowalewski, M. Moeller, T. Pakula, and S. Sheiko.

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