Role of bimetallic active centres in anionic polymerization of hydrocarbon monomers

Polymerization of hydrocarbon monomers initiated by organometallic compounds of alkali and alkaline earth metals modified by derivatives of other metals of Groups I-III is reviewed. Various mixed initiators, such as RLi-RONa, RLi-RNa, R₂Ba-R₃Al, RNa-R₃Al, are discussed in details. It is shown that the presence of a second metal greatly affects the kinetics of elementary steps of chain propagation and chain transfer and also the structure and molecular weight of polymer formed. Active centres in all the systems are complexes containing two or more metal atoms linked by a bridge bond; the addition of a monomer occurs by an insertion either into the “usual” metal-carbon bond which is external with respect to the polymetal nucleus of the complex or directly into Mt¹-R-Mt² bridge bond.