Thermally stable C2-symmetric α-diimine nickel precatalysts for ethylene polymerization: semicrystalline to amorphous PE with high tensile and elastic properties†

Abstract

In α-diimine nickel catalyst-mediated ethylene polymerization, adjusting catalytic parameters such as steric and electronic factors, as well as spectator ligands, offers an intriguing approach for tailoring the thermal and physical properties of the resulting products. This study explores two sets of C₂-symmetric α-diimine nickel complexes—nickel bromide and nickel chloride—where ortho-steric and electronic substituents, as well as nickel halide, were varied to regulate simultaneously chain walking, chain transfer, and the properties of the polymers produced. These complexes were activated in situ with Et₂AlCl, resulting in exceptionally high catalytic activities (in the level of 10⁶–10⁷ g (PE) mol⁻¹ (Ni) h⁻¹) under all reaction conditions. Nickel bromide complexes, with higher ortho-steric hindrance, exhibited superior catalytic activity compared to their less hindered counterparts, whereas the reverse was observed for complexes containing chloride. Increased steric hindrance in both sets of complexes facilitated higher polymer molecular weights and promoted chain walking reactions at lower reaction temperature (40 °C), while the effect became less pronounced at higher temperature (100 °C). However, the electron-withdrawing effect of ortho-substituents hindered the rate of monomer insertion, chain propagation, and chain walking reactions, leading to the synthesis of semi-crystalline polyethylene with an exceptionally high melt temperature of 134.6 °C and a high crystallinity of up to 31.9%. Most importantly, nickel bromide complexes demonstrated significantly higher activity compared to their chloride counterparts, while the latter yielded polymers with higher molecular weights and increased melt temperatures. These high molecular weights, coupled with controlled branching degrees, resulted in polyethylenes with excellent tensile strength (up to 13.9 MPa) and excellent elastic properties (up to 81%), making them suitable for a broad range of applications.