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

In secondary metathesis of 1,5-cyclooctadiene (COD) the thermodynamic equilibrium is reached with an abundant trans, trans, trans-1,5,9-cyclododecatriene (CDT) production, since secondary metathesis is a slower process than primary metathesis. Low cis:trans selectivity is achieved with Grubbs catalysts in primary metathesis, whereas a trans content in polymer increases significantly (transformation of cis into trans in similar to 75 percent) in secondary metathesis. Because the cis: trans ratio found in initial stage of polymerization is a fingerprint of the active species, this is an easy tool which can help to study the reaction mechanism. The formation of 1,4-polybutadiene through polymerization of CD T is only possible with the NHC (N-heterocyclic carbene) substituted catalysts. In case of unsymmetrical NHC-based alkylidene Grubbs catalysts, it appears that the bulkier the moiety on the N in the NHC ligand, the more secondary metathesis takes place.