Alternating Ring-Opening Metathesis Polymerization Promoted by Ruthenium Catalysts Bearing Unsymmetrical NHC Ligands

In this paper, Grubbs- and Hoveyda–Grubbs-type olefin metathesis catalysts featuring N-cyclopentyl/N’-mesityl backbone-substituted N-heterocyclic carbene (NHC) ligands were synthesized. Their propensity to promote the alternating ring-opening copolymerization (ROMP) of norbornene (NBE) with cyclooctene (COE) or cyclopentene (CPE) was evaluated compared that shown by analogous N-cyclohexyl complexes. High degrees chemoselectivity achieved in both copolymerizations. The presence N-cyclopentyl substituent allowed for achievement up 98% 97% diads NBE-COE NBE-CPE copolymers, respectively, at low comonomer ratios. Density functional theory (DFT) studies showed sterical electronic effects NHC influence catalyst selectivity.