Synthesis of Block Copolymers by Controlled Radical Polymerization

Synthesis of Block Copolymers by Controlled Radical Polymerization 

Novel catalytic systems, initially used for atom transfer radical additions in organic chemistry, have been employed in polymer science and referred to as atom transfer radical polymerization, ATRP [62-65]. Among the different systems developed, two have been widely used. The first involves the use of ruthenium catalysts [e.g. RuCl2(PPh3)2] in the presence of CCI4 as the initiator and aluminum alkoxides as the activators. The second employs the catalytic system CuX/bpy (X = halogen) in the presence of alkyl halides as the initiators. Bpy is a 4,4 -dialkyl-substitutcd bipyridine, which acts as the catalyst s ligand. 

The controlled radical polymerization techniques opened up a new era in polymer synthesis, and further growth and developments are certain. However, the control of the molecular characteristics and the variety of macro-molecular architectures reported by these methods cannot be compared with those obtained by other living polymerization techniques such as anionic polymerization. 

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Chapter 2 by Monge et al. details the synthesis and polymerization of phosphorus-containing (meth)acrylamide monomers. Compared to their (meth)aciylate homologues, this class of monomer is more hydrolytically stable and thus more interesting for a large variety of applications. Nevertheless, these monomers are less studied and most of the results mainly report their photopolymerization in order to develop stable self-etching dental primers. Future research on phosphorus-based (meth)acrylamide monomers is also discussed in this chapter and specifically the synthesis of block copolymers by controlled radical polymerization is investigated. 

Many of the problems in polymer chemistry that some years ago appeared irresolvable are, today, state-of-the-art processes. Examples include the formation of block copolymers by controlled radical polymerization, or the increasingly broad application of transition metal-catalyzed polymerization techniques in aqueous environments. Clearly, polymer synthesis is a highly dynamic art form rather than a mature technological field. ... 

A number of techniques for the preparation of block copolymers have been developed. Living polymerization is an elegant method for the controlled synthesis of block copolymers. However, this technique requires extraordinarily high purity and is limited to ionically polymerizable monomers. The synthesis of block copolymers by a radical reaction is less sensitive toward impurities present in the reaction mixture and is applicable to a great number of monomers. 

Lu, S., Fan, Q.L., Chua, S.J., and Huang, W. (2003) Synthesis of conjugated-ionic block copolymers by controlled radical polymerization. Macromolecules, 36,304-310. 

Howdle reported that a one-pot, simultaneous synthesis of block copolymers by enzymatic ROP and ATRP employing initiator 3, CL and MMA is possible in supercritical C02 (scC02) [17]. scC02 is a unique solvent because it combines gaslike and liquid-like properties most importantly in this case it plasticizes and liquefies polymers very effectively, allowing enhanced mass transport which contributes to more efficient polymerization, particularly important for a supported solid phase enzyme catalyst (see also Chapter 13) [28]. The authors also show that the CL acts as a scC02 co-solvent which was crucial to allow the radical polymerization to remain homogeneous and controlled [18]. The unique ability... 

Steenbock, M., et al. (1996). Synthesis of block copolymers by nitroxyl-controlled radical polymerization. Acta Polym., 47(6/7) 276-279. 

This example demonstrates that free-radical polymerization could be the preferred mechanism for many vinyl monomers since, unlike ionic polymerization, it is tolerant of trace impurities and monomer functionality. However, one of its major drawbacks is the lack of control over the molecular weight distribution due to intrinsic termination reactions. Moreover, the efficiency factor of the initiator decreases by the so-called cage effect, for example by recombination of the primary fi-ee radicals, with increasing molecular weight of the macroinitiator [28]. This normally prevents the synthesis of block copolymers with controlled architectures, narrow molecular weight distributions and well-defined molecular weights. 

Matyjaszewski K, Gaynor S and Wang J S (1995) Controlled radical polymerizations The use of alkyl iodides in degenerative transfer. Macromolecules 28 2093-2095. Ameduri B, Boutevin B and Gramain Ph (1997) Synthesis of block copolymers by radical polymerization and telomerization, Adv Polym Sci 127 87-142. Matyjaszewski K (2000) Environmental aspects of controlled radical polymerization, Macromol Symp 152 29-42. 

A few studies have appeared on systems based on persistent nitrogen-centered radicals. Yamada et al.2"1 examined the synthesis of block polymers of S and MMA initiated by derivatives of the triphenylverdazyl radical 115. Klapper and coworkers243 have reported on the use of triazolinyl radicals (e.g. 116 and 117). The triazolinyl radicals have been used to control S, methacrylate and acrylate polymerization and for the synthesis of block copolymers based on these monomers [S,243 245 tBA,243 MMA,243 245 BMA,245 DMAEMA,24 5 TMSEMA,247 (DMAEMA-Wbc/fc-MMA),246 (DMAEMA-Woc -S)246 and (TMSEMA-6/ocfc-S)247]. Reaction conditions in these experiments were similar to those used for NMP. The triazolinyl radicals show no tendency to give disproportionation with methacrylate propagating radicals. Dispcrsitics reported arc typically in the range 1.4-1.8.2"43 246... 

The controlled free-radical miniemulsion polymerization of styrene was performed by Lansalot et al. and Butte et al. in aqueous dispersions using a degenerative transfer process with iodine exchange [91, 92]. An efficiency of 100% was reached. It has also been demonstrated that the synthesis of block copolymers consisting of polystyrene and poly(butyl acrylate) can be easily performed [93]. This allows the synthesis of well-defined polymers with predictable molar mass, narrow molar mass distribution, and complex architecture. 

We are currently exploring new routes to the synthesis of ionomers with controlled architecture, i.e. with control over the amount and location of ionic groups in the polymer backbone. One of our main interests is the synthesis of ion containing block copolymers. The applicability of anionic polymerization in the synthesis of block copolymers and other well defined model systems is well documented (22-24) Not as well appreciated, however, is the blocky nature that certain emulsion copolymerizations may provide. Thus, we have utilized both anionic and free radical emulsion polymerization in the preparation of model ionomers of controlled architecture. In this paper, the synthesis and characteristics of sulfonated and carboxylated block ionomers by both free radical emulsion and anionic polymerization followed by hydrolysis will be discussed. 

Recent advances in polymer synthetic chemistry have allowed the development of elegant and more complicated architectural polymers. This has been driven predominantly by the development of various controlled polymerization methodologies, particularly in the area of free radical polymerization [45-49]. This has equipped the polymer chemist with a rich and abundant synthetic toolbox. In general, these architectural polymers are based on the principle of being able to sequentially add different polymeric blocks with defined molecular weight into a single polymer chain [50]. The synthesis of block copolymers is particularly suited to the combination of two different polymerization techniques. This can be quite easily achieved by the use of a bifunctional initiator and is an elegant synthetic... 

A new strategy has been proposed for the one-step synthesis of block copolymers, based on living/controlled free-radical process. It involves the use of an asymmetric difimctional initiator that is able to start simultaneous polymerization of two comonomers by different polymerization chemistries in such a way that this initiator remains attached to each type of the growing chain (Mecerreyes et al., 1998). The implementation of one-step synthesis is not simple, however. The two catalysts must be tolerant to each other as also to the two comonomers and the reaction temperature must be closely controlled. Living radical polymerization and ROP by coordination and insertion can meet these requirements. 

Zhang, X., and Matyjaszewski, K. (1999). Synthesis of well-defined amphiphilic block copolymers with 2-(dimethylamino)ethyl methacrylate by controlled radical polymerization. Macromolecules, 32(6) 1763-1766. 

Lacroix-Desmazes, P., et al. (2000). Synthesis of poly(chloromethylstyrene-b-styrene) block copolymers by controlled free-radical polymerization. J. Polym. Sci., Part A Polym. Chem., 38(2 ) 3845-3854. 

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