Congestion of steric and electronic factors in α-diimine nickel precatalysts for high-temperature ethylene polymerization in accessing direct PE elastomer

Abstract

In α-diimine-nickel catalysts mediated ethylene polymerization, the synergistic impact of steric and electronic factors plays an instrumental role in fine-tuning catalytic activity, polymer molecular weights, microstructure, as well as mechanical and elastic properties. In this study, a new series of nonsymmetrical bis(arylimino)-acenaphthene-nickel complexes was prepared and investigated for ethylene polymerization. These complexes feature both bulky N-2,6-dibenzhydryl-3,4,5-fluorophenyl imine and smaller N-2,6-di(R)-4-(R1)phenyl imine units [Ni2Me (R, = Me, R1 = H), Ni2Et (R, = Et, R1 = H) and Ni2iPr (R, = iPr, R1 = H), Ni3Me (R, R1 = Me), Ni2Et,Me (R, = Et, R1 = Me)]. The incorporation of both steric congestion and strong electronic withdrawal groups into the catalyst structure resulted in significantly improved catalytic performance: high thermal stability and exceptional activity were observed at both ambient temperature (21.24 × 106 at 30 °C) and industrially relevant temperatures (1.65 × 106 at 100 °C). Polymer molecular weights and branching degree were tunable by adjusting the ligand structure and reaction conditions. This correlation extended to mechanical and elastic properties, demonstrating high tensile strength at low temperatures and good elastic recovery at high reaction temperatures, consistent with typical polyolefin elastomers.