Zirconium and hafnium Salalen complexes in isospecific polymerisation of propylene

Abstract

The activity of dibenzylzirconium and dibenzylhafnium Salalen complexes in polymerisation of propylene with MAO as a cocatalyst is described. Three Salalen ligand precursors combining a bulky alkyl group (1-adamantyl) on the imine-side phenol and electron withdrawing halo groups of different sizes on the amine-side phenol were explored. All metal complexes were obtained as single diastereomers. An X-ray crystallographic structure of a hafnium complex of an additional ligand carrying the combination of tert-butyl and chloro substituted phenolates, 4-Hf, revealed a fac–mer wrapping of the Salalen ligand around the metal centre. All complexes led to active catalysts in propylene polymerisation and to isotactic polypropylene of high regioregularity. The zirconium complexes led to polypropylene having molecular weights of M_w = 132 000–200 000 and isotacticities of [mmmm] = 65.7–75.0%. The hafnium complexes led to polypropylene of higher molecular weights of M_w = 375 000–520 000 and higher stereoregularities of [mmmm] = 80.6–89.3%, the highest isotacticity obtained with 3-Hf.