Synthetic Applications of Living Anionic Polymerization

One of the unique and important synthetic applications of living polymerizations is the synthesis of block copolymers by sequential monomer addition [225-228, 192]. The ability to prepare block copolymers is a direct consequence of the stability of the carbanionic chain ends on the laboratory time scale when all of the monomer has been consumed. Since a living polymerization and the ability to prepare well-defined block copolymers require the absence (or reduction to a negligible level) of chain termination and chain transfer reactions, monomer purity, and the absence of side reactions with the monomer are necessary 

2 Block Copolymer Synthesis by Two-Step Sequential Monomer Addition and Coupling In order to avoid the problems associated with the three-step sequential monomer addition process, a two-step sequential monomer addition process followed by addition of a difunctional 

3 Block Copolymers by Difunctional Initiation and Two-Step Sequential Monomer Addition This process requires a dilithium initiator, so that the center block (D) is 

LiCH2CH =CHCH2-F butadiene -I- butadiene -CH2CH = CHCH2U 

There have been two general approaches that have been used to increase the efficiency of linking reactions of polymeric organolithium compounds with multifunctional silyl halides. The first procedure is to add a few units of butadiene to either the poly(styryl)lithium or poly(isoprenyl)lithium chain ends to effectively convert them to the corresponding less sterically hindered poly(butadienyl)lithium chain ends. For example, after crossover to butadienyllithium chain ends, the yield of four-armed star polyisoprene with silicon tetrachloride was essentially quantitative in cyclohexane [255]. The second method is to utilize a polychlorosilane compound in which the silyl halide units are more separated to reduce the steric repulsions in the linked product. 

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