Living Radical Polymerization - ACS Publications

Chapter 1

Controlled/Living Radical Polymerization: State of the Art in 2002 Krzysztof Matyjaszewski Downloaded by 188.214.129.85 on March 21, 2016 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch001

Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, P A 15213

Controlled/Living Radical Polymerization is one of the most rapidly developing areas of polymer science. Atom transfer radical polymerization (ATRP), nitroxide mediated polymerization (NMP) and various degenerate transfer processes, including reversible addition fragmentation transfer (RAFT), enable the preparation of new materials from readily available monomers under undemanding conditions. Some of the relative advantages and limitations of each of these systems and some future challenges are discussed.

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© 2003 American Chemical Society

Matyjaszewski; Advances in Controlled/Living Radical Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Downloaded by 188.214.129.85 on March 21, 2016 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch001

Controlled/Living Radical Polymerization (CLRP) is amongst the most rapidly developing areas of polymer science. Figure 1 illustrates how the number of publications on this topic have increased dramatically over the last decade. The search (using SciFinder Scholar 2002) for terms: controlled radical polymerization, living radical polymerization and both concepts combined, indicates that a nearly equal number of papers have been published using each term and that there is a limited overlap in the use of the terms. The issues related to tenriinology have been discussed in details in a special issue of "J. Polym. Sci., P o l y m Chem. E d . " May 15, 2000, and will not be discussed here. The purpose of graph 1 is just to illustrate the very dynamic (or nearly explosive) development of this field. More than 10 papers per week are presently being published in C L R P ! 600

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Figure 1. Annual number of publication using term controlled radical polymerization (CRP only), living radical polymerization (LRP only) and both term combined, according to SciFinder Scholar 2002, February 1, 2003 Figure 2 illustrates the annual changes in the number of publications in more specific areas of C L R P , namely atom transfer radical polymerization (ATRP), nitroxide mediated polymerization (NMP and stable free radical polymerization, SFRP) and various degenerate transfer processes, including reversible addition fragmentation transfer (RAFT) and catalytic chain transfer. The number of


4 publications also continuously increases each year. The number of papers in the specific area of A T R P nearly matches those using a the more general C L R P term In fact there is an increasing number of papers that do not use the terms C R L P , A T R P , N M P or R A F T in either the abstracts, titles or keywords and therefore can not be identified by computer based searches. This would indicate that for some polymer chemists C L R P is moving from a "specialty" research topic into the realm of commonly applied practice for the synthesis of materials. 600


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Figure 2. Annual number of publication on atom transfer radical polymerization (ATRP), nitroxide mediated polymerization (SFRP&NMP), degenerative transfer systems (RAFT&DT&CCT) and CRP&LRP, according to SciFinder Scholar 2002, February 1, 2003 What are the origins for such a dramatic increase of popularity of C L R P methodology? Conventional free radical polymerization is perhaps the most important commercial process for preparing high molecular weight polymers because it can be applied to the polymerization of many vinyl monomers under mild reaction conditions, and, although requiring the absence of oxygen, is tolerant of water and can be conducted over a wide temperature range (-80 to 250 C). In addition, many monomers can be easily copolymerized via a radical route, leading to the preparation of an infinite number of copolymers with properties dependent on die proportion of the incorporated comonomers. The



5 main limitation of conventional radical systems is the poor control over some of the key elements of engineered macromolecular structures such as molecular weight, polydispersity, end functionality, chain architecture and composition. Well-defined polymers with precisely controlled structural parameters are accessible by ionic living polymerization processes, however, ionic living polymerizations require stringent conditions and are limited to a relatively small number of monomers. It has always been desirable to prepare, by a free radical mechanism, well-defined block and graft copolymers, gradient and periodic copolymers, stars, combs, networks, end-functional polymers and many other materials under mild conditions from a larger range of monomers than available for ionic living polymerizations. This emergent ability to prepare long desired materials is perhaps the main reason why we have witnessed a real explosion of academic and industrial research on C L R P during the last five years, as evidenced by over three thousand papers and hundreds of patents devoted to this area. The industrial interest in C L R P may be explained by a recent estimate from Bob Matheson (DuPont) who projected that C L R P may affect a market of $20 billion/year in such areas as coatings, adhesives, surfactants, dispersants, lubricants, gels, additives, thermoplastic elastomers as well as many electronic and biomedical applications.

Development of Controlled/Living RP

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Figure 3. Areas contributing to development of CLRP Research in C L R P really took off in the mid 90's after utilization of T E M P O as a mediator to styrene polymerization, introduction of A T R P and a subsequent discovery of R A F T / M A D I X systems. Nevertheless, the roots of C L R P extend back over 50 years and are found in synthetic and physical organic



6 chemistry (Kharsch, Minisci, Curran, Belus, Fischer), in controlled or living ionic reactions (Szwarc, Kennedy, Penczek, Mueller) in addition to the many attempts to control rates and functionalities in conventional radical polymerization (Bamford, Starks, Boutevin), as illustrated in Figure 3. As already mentioned, A T R P , N M P and R A F T are currently the three most commonly used methods for C L R P but there are several others, such as iniferters, other degenerative transfer systems, including alkyl iodides, oligomers with methacrylate functionality, various types of non-nitroxide stable free radicals, and other transition metal mediators, which are also very efficient under specific conditions. It seems that each of the three major systems (and perhaps others) have some relative advantages and limitations, depending on the monomers used, the particular synthetic targets, and additional requirements concerning functionality, purity, process such as bulk, solution or biphasic, and perhaps the cost of the final product. Figure 4 attempts to illustrate some areas in which A T R P , N M P or R A F T may be easier / simpler / more precise / less expensive or more versatile. It must be recognized that this qualitiative picture should change with developments of new control agents and improvement in polymerization catalysts, conditions and perhaps development of entirely new systems. LMW

HMW Figure 4. Relative advantages and limitations of ATRP, NMP and RAFT as applied to the synthesis of low (LMW) and high molecular weight polymers (HMW), range of polymerizable monomers (Mon Range), block copolymers (Blocks), end-functional polymers (End Funct), hybrids (Hybrids ), aqueous systems (Water) and some environmental issues (Env).



7 Current research in C L R P encompasses a range of mechanistic, synthetic and materials aspects. Perhaps most interesting may be preparation of new materials with enhanced control of molecular weight, structural uniformity, and microstructure as well as topology, composition and functionality. In addition, the ease of mechanistic transformation between C L R P and various other systems based on ionic, coordination and even step-growth systems allows incorporation of any synthetic polymeric material into a C L R P . Another very exciting topic is the preparation of new hybrid materials involving inorganics, ceramics and natural products and well-defined vinyl polymers synthesized by C L R P methods. Figure 5, below, illustrates some of the many possible structures which can be prepared by copolymerization (also including macromonomers) and leads to the question "what is the effect the chain architecture on properties?" The correlation between molecular structure and macroscopic properties of welldefined polymers prepared by C R L P has just started and may become the most challenging and rewarding area for future research since it may lead to rational design of new commercially important materials. We can perhaps also speculate about which other reseach areas will be developing in a future. This may be related to current trends, needs and expectations.

Figure 5. Examples of controlled topology and composition in CLRP copolymerization. We now realize that none o f the C L R P processes are true living polymerization procedures and that tenriination reactions, which limit efficiency of blocking reactions and chain end functionalities, can never be completely avoided, although they can be minimized. However, the effect of differing levels


8 of tennination and other side reactions on the properties of the (copolymers obtained in a specific reaction remains to be established. There are a few other important issues for C L R P that have not yet been adequately addressed and they may be easier to resolve i f first approached in conventional radical systems. C L R P will always benefit from developments in conventional macromolecular and small molar mass radical systems. For example: • A n increase of kp/k ratios will increase the selectivity of chain propagation and allow one to carry out C L R P at a much faster rate. The precise effect of chain length and viscosity on k and k, should also be established. They might be selectively affected in compartmentalized systems, e.g. (micro/rnini)emulsions, zeolites or other inclusion complexes. • A n examination of radical systems has shown that complexation with solvents, Lewis acids or other additives can affect not only the above rate constants but can also influence their chemo- and stereoselectivities. C L R P methodology can exploit these concepts and perhaps enhance their effects because of the slower controlled chain growth operating under the various exchange reactions. The slower rate of polymerization could also increase the precision for some template systems. • Homo- and copolymerization of non-polar olefins by radical means is very challenging and enhanced chemoselectivity and perhaps stereoselectivity in radical based olefin polymerization could enable production materials that would compete with metallocene and ZieglerNatta systems. On the other hand, synthesis of block/grafts copolymers of polyolefins with well defined segments of polyacrylates and other polar monomers prepared by C L R P can generate a new class of additives for commodity polyolefins. t


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Nevertheless, complete mechanistic understanding and optimization of the current processes and the aforementioned structure-property correlation remain perhaps most important items for immediate C L R P research: • Complete mechanistic understanding, including structure-reactivity correlation for all C L R P systems. This will help define, and plausibly expand, the scope of each method with respect to range of polymerizable monomers, preservation of end-functionalities and more specifically to a reduction in the amount of A T R P catalyst required for a reaction. It may also, perhaps, enable control of microstructure in terms of tacticity and sequence distribution. • Expanding control to encompass polymerization in various media (emulsion, suspension, gas-phase), surface polymerization, designing



9 more robust and perhaps continuous processes, and general process optimization, catalyst recycling, etc. • Construction of a comprehensive structure-property correlation which will allow preparation materials with new properties for targeted applications. This will require synthesis of many new materials with controlled, and systematically varied molecular weight, polydispersity, and a broad spectrum of chain architectures, perhaps even including microstructure. Continuation and expansion of fundamental kinetic, mechanistic and characterization studies is needed to solve these challenging problems. This will also require efficient collaborations of synthetic polymer chemists with theoreticians, organic chemists, inorganic/coordination chemists, kineticists, physical organic chemists, polymer physical chemists, physicists and engineers. We hope some of these goals will be reached in a near future and results will be presented at the next A C S Symposium on Controlled/Living Radical Polymerization.


Living Radical Polymerization - ACS Publications

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