Synergistic influence of methoxy and benzhydryl groups in α-diimine nickel precatalysts for facile synthesizing UHMW PE elastomers

Abstract

Concerted effect of steric and electronic substituents is a powerful strategy to enhance the catalytic performance of α-diiminonickel precatalysts for ethylene polymerization, but less investigated so far. In this study, the synthesis of a series of unsymmetrical α-diiminonickel complexes, featuring the sterically crowded 2,6-dibenzhydryl-3,4,5-trimethoxyphenylimine and 2,4,6-(alkyl)phenylimine units, is described. Some of these complexes, including ligand and abnormal organic compound are presented in X-ray structures. All the nickel complexes, extensively studied with two alkylating reagents (Me2AlCl and MMAO), demonstrated excellent catalytic performance, with activity level in the range of 106–107 g mol−1 h−1 across a broad temperature range (30 °C–90 °C). Of significant note, concerted effect of three OMe substituents and two benzhydryl groups present at the same imine unit, significantly improved chain propagation and thereby facilitated the synthesis of ultra-high molecular weight (up to 1.2 × 106 g mol−1) polyethylene with high branching degree (up to 205/1000C) and narrow polymer molecular weight distributions (Ð = < 1.80). Moreover, the obtained polyethylene, especially the ultra-high MW samples, exhibited excellent material properties (up to σb = 7.58 MPa with εb = 1025 % and E = 5.32 MPa), along with elastic recovery rates of 59–65 %. These properties of obtained polyethylene highlight characteristics of typical thermoplastic polyolefin elastomers.