Combined Experimental and Theoretical Approach for Living and Isoselective Propylene Polymerization

The controlled polymerization of propylene with living catalysts in an isoselective fashion with high activities is a continuing challenge for catalysis, with at least one of these properties suffering in most existing systems. Through experimental and theoretical studies, the pyridylamidohafnium olefin polymerization catalysts have been optimized for living behavior, isoselectivity, and activity of propylene polymerization (Đ = 1.2, [m4] = 91%, TOF = 25 000 h–1). Substitution of the bridgehead position with geminal dimethyl substituents forces the stereoselective elements of the catalyst into closer proximity with the active site through a “buttressing effect” while simultaneously preserving symmetry in the catalyst after activation and the first monomer insertion into the Hf–Caryl bond of the ligand. Propagation was shown to be favored over termination through beta-hydrogen transfer to the monomer through a combination of theoretical modeling and experimental monitoring of the reaction which showed a linear increase in molecular weight as a function of polymer yield. Furthermore, through computational and 13C labeling NMR studies, a different mechanism of stereocontrol involving a direct ligand–monomer interaction was confirmed.