Thermally stable unsymmetrical 2,3-diiminobutane-nickel pre-catalysts for synthesis of high molecular weight polyethylene elastomers

Abstract

Steric effects significantly impact catalytic performance of α-diimine nickel pre-catalyst toward ethylene polymerization. In this contribution, a family of five new unsymmetrical 2,3-diiminobutane nickel complexes bearing sterically encumbered ortho- and para-benzhydryl groups was prepared and employed as pre-catalysts for the catalysis of ethylene polymerization. When used with an excess of an alkyl aluminium compound as a co-catalyst, specifically with optimal co-catalysts like Et2AlCl or MMAO, these complexes achieved remarkable activities (typically 106–107 g mol−1 h−1) and produced highly branched polyethylene with high molecular weights (in the level of 105 g/mol) and narrow molecular weight distributions (about 1.6) across all reaction temperatures. Of notable significance, the Ni3 complex exhibited high thermal stability within a broad temperature range, spanning from 30 to 100 °C, and even performed efficiently at 100 °C with an activity of 1.7 × 106 g mol−1 h−1. Moreover, polyethylene branching content increased from 141 to 220 per 1000C with rising temperature. The mechanical tests revealed ultimate tensile strength of 4.69 MPa, high strain at break (773 %), high Young modulus (5.45 MPa), high toughness (2455.28 MJ/m3) and strain recovery up to 76 % after 10 cycles, highlighting characteristics of typically thermoplastic polyolefin elastomers.