Secondary metathesis with Grubbs catalysts in the 1,4-polybutadiene system

The Grubbs catalysts show low cis:trans selectivity in the primary metathesis of 1,5-cyclooctadiene (COD). However, secondary metathesis of 1,4-polybutadiene, which is a much slower reaction, transforms the mixture into thermodynamical equilibrium with considerable formation of trans,trans,trans-1,5,9-cyclododecatriene (CDT). Only the NHC (N-heterocyclic carbene) substituted catalysts were found to be efficient in the polymerization of trans,trans,trans-1,5,9-cyclododecatriene to obtain 1,4-polybutadiene. For the polymerization of cyclooctene (CO), NHC based catalysts show an initial higher trans content, while the phosphine catalysts show more cis content. This initial cis:trans ratio is a fingerprint of the active species and can be used to determine mechanistic pathways.