Ligand steric effects on α-diimine nickel catalyzed ethylene and 1-hexene polymerization

Abstract

A series of ortho-dibenzhydryl or ortho-sec-phenethyl substituted α-diimine nickel complexes with systematically varied ligand steric effects were used as precatalysts for the polymerization of ethylene and 1-hexene upon activation with Et₂AlCl. The effects of ligand sterics and polymerization temperature on the catalytic activity, molecular weight and polymer microstructure were evaluated in detail. In ethylene polymerization, it is possible to tune the catalytic activities [(0.98–2.41) × 10⁶ g (mol Ni h)⁻¹], polymer molecular weights [M_n: (1.8–13.1) × 10⁵ g mol⁻¹] and branching densities (55–108/1000C) over a very wide range. The molecular weights and branching structure depended on the nickel complexes as well as the polymerization temperature, and the polymer branching densities were decreased with increasing ligand steric effects and decreasing polymerization temperature. Polymerization of 1-hexene with ortho-dibenzhydryl substituted nickel complexes resulted in branched polymers (106–140/1000C) with high molecular weights [M_n: (0.64–3.88) × 10⁴ g mol⁻¹] and narrow molecular weight distribution (M_w/M_n = 1.16–1.52, 40–80 °C). The increasing steric hindrance of the catalyst leads to enhanced 2,1-insertion of 1-hexene and the chain-walking reaction.