Systematic studies on dibenzhydryl and pentiptycenyl substituted pyridine-imine nickel(ii) mediated ethylene polymerization

Abstract

As the analogues of classical α-diimine nickel catalysts, pyridine-imine nickel catalysts are of great interest for olefin polymerization to produce low molecular weight and branched polyethylenes. In this contribution, pyridine-imine nickel complexes Ni1–Ni4 bearing dibenzhydryl- and pentiptycenyl-N-aryl substituents and H- and Me-imine backbones were synthesized and systematically studied for ethylene polymerization. X-ray diffraction studies revealed that Ni1, Ni2 and Ni4 adopted a monoligated/binuclear structure, while Ni3 was found to adopt a monoligated/mononuclear structure, which differed from the bisligated/mononuclear mode reported previously. Upon activation with aluminum reagents such as Et₂AlCl, MAO or MMAO, all these nickel complexes displayed very high activities (up to 14 530 kg mol⁻¹ h⁻¹) for ethylene polymerization. Branched (12–69/1000C) polyethylenes with low molecular weights (M_w: 0.7–22.1 kDa) were obtained with internal double bonds as the predominant unsaturated groups. The influences of the catalyst structure, type and amount of cocatalyst, time, temperature, pressure, and polar additive on the catalytic performances were thoroughly investigated.