Relationship between the stereospecificity of lanthanide catalysts and thestructures of active sites and dienes, the nature of a cocatalyst, and preparation conditions

Several types of active sites can be formed during the polymerization of di enes in the presence of lanthanide catalytic systems. These sites differ in the nearest environment of a lanthanide atom (the numbers of chlorine and carbon atoms) and in the number of lanthanide-carbon bonds involved in chai n growth. Quantum-chemical calculation shows that the Is-allyl binding of t he terminal unit of a growing polymeric chain with the lanthanide atom of a n active site may be the reason for the cis-stereospecificity of the site. Therefore, the active sites containing electron-acceptor chlorine atoms tha t favor pi -allyl binding can form cis-polydienes. Depending on the conditi ons of catalytic system preparation, primarily on the nature of an organic compound of a nontransition metal, one or another set of active sites is fo rmed that can affect its stereospecificity. The quantitative analysis of ki netic data for diene polymerization shows that the anti-syn isomerization o f terminal units of growing polymeric chains made a certain contribution to the formation of trans-1,4 units in the presence of cis-regulating active sites.