Living radical polymerization normal

For this book, we have decided to entitle this chapter Living Radical Polymerization and use the term throughout, it is a chapter describing various approaches to living radical polymerization. We do not intend to imply that termination is absent from all or, indeed, any of the polymerizations described, only that the polymerizations display at least some of the observable characteristics normally associated with living polymerization. 

Scheme 8.4 Synthetic approach to block copolymers using sequential normal/living radical polymerization...

In 1985, Rizzardo et al.27 filed a patent for the use of alkoxyamines (Scheme 12) as regulating initiators for the living radical polymerization and block copolymerization of vinyl monomers. R is a group that upon dissociation (Scheme 10) forms a radical that adds to the monomer. The mechanism was disclosed shortly thereafter and involves the reversible dissociations shown in Scheme 11, with the nitroxide radical taking the role of X.28 In a later simulation, the group also revealed the reason for the remarkable absence of the usual terminations and rediscovered the principles of the persistent radical effect 29 As chains undergo termination transient radicals are removed from the system and the concentration of persistent species builds . Further, the authors noted correctly that, in contrast to normal radical polymer-... 

Li, I. Q., et al. (1997). Block copolymer preparation using sequential normal/living radical polymerization techniques. Macromolecules, 50(18) 5195-5199. 

As well as functional monomer polymerization and PPM, one-pot polymerization has also been used to obtain a highly functionalized polymer. One-pot polymerization combines polymerization with other compatible reactions to achieve the target new polymer in the same reactor. In contrast to functional monomer polymerization, one-pot polymerization can form the functional monomer in situ and thus avoid the purification step necessary with functional monomer synthesis. This approach also avoids hindrance from the polymer backbone, leading to a higher modification yield compared with the normal PPM approach. With less time and cost, the desired polymer can be achieved and functionalized with a high yield in one pot. Some reactions such as the CuAAC click reaction and enzymatic transesterification have been used for construction of a one-pot polymerization system with controlled/living radical polymerization approaches to achieve new functional polymers [119-125]. 

Catalysts of the Ziegler type have been used widely in the anionic polymerization of 1-olefins, diolefins, and a few polar monomers which can proceed by an anionic mechanism. Polar monomers normally deactivate the system and cannot be copolymerized with olefins. However, it has been found that the living chains from an anionic polymerization can be converted to free radicals in the presence of peroxides to form block polymers with vinyl and acrylic monomers. Vinylpyridines, acrylic esters, acrylonitrile, and styrene are converted to block polymers in good yield. Binary and ternary mixtures of 4-vinylpyridine, acrylonitrile, and styrene, are particularly effective. Peroxides are effective at temperatures well below those normally required for free radical polymerizations. A tentative mechanism for the reaction is given. 

It is obvious from the above discussion that under the correct conditions and with the appropriate mediating nitroxide free radical, living polymerization conditions can be achieved. On the basis of this realization, numerous groups have demonstrated that the degree of structural control normally associated with more traditional living processes, such as anionic procedures, can be equally applied to nitroxide-mediated living free radical polymerizations. 

The tacticity of the random copolymers prepared by living free radical polymerization is also found to have the same sequence distribution and tacticity as those prepared by normal free-radical methods. Besides low polydispersities, another advantage of living free radical polymerization is that the chain ends can be controlled to a degree previously only obtainable with more demanding techniques. In one example, a pyrene-ended styrene-methyl methacrylate copolymer (VI) was prepared by living free radical polymerization using the unimolecular initiator (V), which in turn was prepared by esteri cation of pyrene-1-butyryl chloride with (IV) in the presence of 4-(dimethylamino)pyridine (see Fig. 11.9). 

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