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 pyridylami-dohafnium olefin polymerization catalysts have been optimized for living behavior, isoselectivity, and activity of propylene polymerization (D = 1.2, [m(4)] = 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-C-aryl 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 C-13 labeling NMR studies, a different mechanism of stereocontrol involving a direct ligand-monomer interaction was confirmed.