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.