Syntheses, structure and ethylene polymerization behavior of beta-diiminato titanium complexes

A series of new titanium complexes bearing beta-diiminato ligands [(Ph)NC(R-1)CHC(R-2)N(Ph)](2)TiCl2 (4a: R-1 = R-2 = CH3; 4b: R-1 = R-2 = CF3; 4c: R-1 = Ph, R-2 = CH3; 4d: R-1 = Ph, R-2 = CF3) has been synthesized and characterized. X-ray crystal structures reveal that complexes 4a and 4c adopt distorted octahedral geometry around the titanium center. With modified methylaluminoxane (MMAO) as a cocatalyst, complexes 4a-d are active catalysts for ethylene polymerization, and produce high molecular weight polyethylenes. Catalyst activities and the molecular weights of polymers are considerably influenced by the steric and electronic effects of substituents on the catalyst backbone under the same polymerization condition. With the strong electron-withdrawing groups (CF3) at R-1 or/and R-2 position, complexes 4b and 4d show higher activities than complexes 4a and 4c, respectively.