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-iPr2C6H3)NC(R1)CHC(2′-R2C6H4)O]Ni(Ph)(PPh3) (4a, R1 = R2 = H; 4b, R1 = H, R2 = Ph; 4c, R1 = H, R2 = Naphth; 4d, R1 = CH3, R2 = Ph; 4e, R1 = CF3, R2 = Ph) have been synthesized and characterized. Molecular structures of 4b and 4e were further confirmed by X-ray crystallographic analysis. Activated with B(C6F5)3, all the complexes are active for the polymerization of ethylene to branched polyethylenes. Ligand structure, i.e., substituents R1 and R2, 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/molNi·h·atm under optimized conditions, which is about 10 times more than unsubstituted complex 4a (14.0 kg PE/molNi·h·atm). Highly branched polyethylene with 103 branches per 1000 carbon atoms has been prepared using catalyst 4e.