3.17 - Anionic Polymerization of Nonpolar Monomers

Abstract

Anionic polymerization, especially alkyllithium-initiated anionic polymerization in hydrocarbon solution, is the gold standard for living polymerizations. The anionic polymerizations of styrenes and dienes in hydrocarbon solution proceed in the absence of irreversible chain termination and chain transfer. The most useful initiating systems comprise alkyllithium compounds that are very reactive, hydrocarbon-soluble, and are commercially available. Using alkyllithium initiators in hydrocarbon solutions, controlled polymerizations of styrenes and dienes can be effected to produce polymers with high degrees of compositional homogeneity and polydienes with high 1,4-microstructure and low glass transition temperatures. Reactive hydrocarbon monomers include styrenes, 1,3-dienes, 1,3,5-trienes, vinyltrialkylsilanes, and even ethylene (in the presence of N,N,N′,N′-tetramethylethylenediamine). The kinetics of initiation and propagation are complicated by aggregation of both simple and polymeric organolithium compounds and their cross-aggregation. Polymeric organolithium compounds exhibit reduced stability at elevated temperatures and in the presence of chain transfer agents whose pKa values are comparable or lower than the propagating chain ends. Useful chain transfer agents include alkylbenzenes, ammonia, and 1,2-dienes. Retarded alkyllithium-initiated anionic polymerizations in the presence of organomagnesium or organoaluminum compounds can be effected at elevated temperatures and with higher monomer concentrations.