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 polymerization behavior such
as propagation rate and cis-specificity explains
the effect of the counteranion.