Hafnium Amidoquinoline Complexes: Highly Active Olefin Polymerization Catalysts with Ultrahigh Molecular Weight Capacity

The preparation and characterization of a new class of
polyolefin procatalysts is described. Hafnium tribenzyl procatalysts,
supported by amidoquinoline ligands, were prepared in two steps from
commercially available materials. Solid-state structures, determined
by single-crystal X-ray analyses, revealed that all the hafnium complexes
display approximate trigonal-bipyramidal geometry around the metal
center. The complexes were evaluated in an ethylene/1-octene copolymerization
study and were found to be highly active at elevated temperatures
(120 °C). The best catalyst, derived from ((2,6-dimethylphenyl)­(2,4-dimethylquinolin-8-yl)­amino)­tribenzylhafnium
(6d), compares favorably to previously reported systems
supported by bidentate nitrogen-based ligands. In particular, this
catalyst exhibits very high molecular weight capacity and high catalytic
activity, with a moderate 1-octene response. Alkyl substitution at
the carbon ortho to the quinolino nitrogen was found
to be an important factor for improving polymer compositional homogeneity,
as evidenced by a narrowing of the polydispersity index and a single
melting temperature in the resulting copolymer.