Highly Active Neutral Nickel(II) Catalysts for Ethylene Polymerization Bearing Modified β-Ketoiminato Ligands

A series of novel neutral nickel complexes 4a − e bearing modified β-ketoiminato ligands [(2,6- i Pr 2 C 6 H 3 )N═C(R 1 )CHC(2′-R 2 C 6 H 4 )O]Ni(Ph)(PPh 3 ) ( 4a, R 1 = R 2 = H; 4b, R 1 = H, R 2 = Ph; 4c, R 1 = H, R 2 = Naphth; 4d, R 1 = CH 3, R 2 = Ph; 4e, R 1 = CF 3, R 2 = Ph) have been synthesized and characterized. Molecular structures of 4b and 4e were further confirmed by X-ray crystallographic analysis. Activated with B(C 6 F 5 ) 3, all the complexes are active for the polymerization of ethylene to branched polyethylenes. Ligand structure, i.e., substituents R 1 and R 2, greatly influences not only catalytic activity but also the molecular weight and branch content of the polyethylene produced. The phenyl-substituted complex 4b exhibits the highest activity of up to 145 kg PE/mol Ni ·h·atm under optimized conditions, which is about 10 times more than unsubstituted complex 4a (14.0 kg PE/mol Ni ·h·atm). Highly branched polyethylene with 103 branches per 1000 carbon atoms has been prepared using catalyst 4e .