A Dual Steric Enhancement Strategy in α-Diimine Nickel and Palladium Catalysts for Ethylene Polymerization and Copolymerization

Abstract

Generally, steric tuning of the ortho positions of an aryl group and of the ligand backbone are the two major strategies to modify the ligand structure of an α-diimine system in the olefin polymerization field. In this study, a series of unsymmetrical dual steric α-diimine palladium and nickel complexes bearing bulky axial diarylmethyl moieties and an equatorial bulky dibenzobarrelene backbone were considered. The nickel complexes exhibited high activities (ca. 106 g/(mol h)) in ethylene polymerization even at 90 °C, yielding very high molecular weight (up to 1.27 million g/mol) polyethylenes with particularly high branching densities (up to 126/1000C). Interestingly, the resultant polyethylene products from Ni1 and Ni3–Ni5 with methyl substituents on the less bulky side showed very high branching densities even at high pressure and low temperature (104–121/1000C at 30 °C and 6 atm), and the branching density is hardly affected by the reaction temperature. Correspondingly, the palladium complexes Pd1 and Pd3–Pd5 displayed moderate activities (ca. 105 g/(mol h)), yielding highly branched polyethylene with a high molecular weight (up to 272 kg/mol) in ethylene polymerization. In comparison, complex Pd2 showed lower activities (ca. 104 g/(mol h)), generating lower branched polyethylene with a much lower molecular weight (1.7–3.2 kg/mol). Moreover, these palladium complexes exhibited low copolymerization activities and gave E-MA copolymers of low to moderate molecular weights (1–2 kg/mol with Pd2; 6–47 kg/mol with Pd1 and Pd3–Pd5) with average incorporation ratios (1.3–3.8 mol %). The bulky axial diarylmethyl substituents and equatorial bulky dibenzobarrelene backbone with a smaller steric o-methyl group on the less bulky side constitute a unique ligand environment conducive to chain walking in the nickel-catalyzed ethylene polymerization system.