ROLE OF BIMETALLIC ACTIVE-CENTERS IN ANIONIC-POLYMERIZATION OF HYDROCARBON MONOMERS

Polymerization of hydrocarbon monomers initiated by organometallic com pounds of alkali and alkaline earth metals modified by derivatives of other metals of Groups I-III is reviewed. Various mixed initiators, su ch 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 a nd also the structure and molecular weight of polymer formed. Active c entres in all the systems are complexes containing two or more metal a toms linked by a bridge bond; the addition of a monomer occurs by an i nsertion 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.