Effect of Ancillary Ligands as a Part of Counteranion in Neodymium-Catalyzed Isoprene Polymerization

Organic halides and alkylaluminum chlorides are known as representative additives of lanthanide catalyst systems for the polymerization of conjugated dienes, which enhance the activity and stereospecificity of polymerization. The active species in these polymerization systems are usually considered to be chloroalkyl/dialkyl metal cations, and therefore, the effects of external ligands are unclear. Here we synthesized chloride-bridged bimetallic neodymium borohydride complexes, and their application as isoprene polymerization catalysts by activation with tri(n-octyl)aluminum was performed. Some NMR studies have indicated that the active species in this system are an alkyl neodymium cation and the alkyl/aryl groups on borohydride ligands transferred onto the added alkylaluminum. The alkyl/arylaluminum may exist as a contacting counteranion of the active species, and propagation rate and cis-specificity explains the effect of the counteranion.