Unusual Superior Activity of the First Generation Grubbs Catalyst in Cascade Olefin Metathesis Polymerization

Recently, we reported a new cascade ring-opening/closing metathesis polymerization of monomers containing two cyclopentene moieties. Several Ru catalysts were tested, but the best polymerization results were unexpectedly obtained using the first-generation Grubbs catalyst (G1). This was puzzling since the second- and third-generation Grubbs catalysts are well-known for their higher activities compared to G1. In order to explain the unique and superior activity of G1, we conducted a series of kinetics experiments for the polymerization of 3,3'-oxydicyclopent-1-ene, a representative monomer of this cascade polymerization, as well as the competition polymerization with cycloheptene using the various Grubbs catalysts. Based on our results, we propose a model in which the differences in the steric hindrance between the different ligands and the monomer determine the selectivity of the catalyst approach to the monomer and, therefore, the extent to which the productive pathway leads to successful cascade polymerization. In short, G1 with the smaller ligand showed a high preference for the productive pathway.