Correlating electronic properties of a nickel polymerisation catalyst with the branching density of a polyethylene

Abstract

This work investigates how the electronic properties of a nickel ethylene polymerisation catalyst can be used to exert control on key polymerisation steps namely chain walking and chain propagation and in turn affect branching and polymer molecular weight. Specifically, a series of sterically enhanced 2-(imino)-4-R-pyridine–nickel complexes (R = OMe Ni1, Me Ni2, H Ni3, Br Ni4, CF₃Ni5, NO₂Ni6) that differ only in the electronic properties of the para-R substituent have been prepared and fully characterised. Under activation with EASC (ethyl aluminium sesquichloride), all nickel catalysts exhibited high activities for ethylene polymerisation and produced branched polyethylenes (BPE: branching density range: 61–99/1000 Cs) with a range of molecular weights (4.89–15.4 kg mol⁻¹); the relative order of catalytic activity being: Ni5 (CF₃) > Ni4 (Br) > Ni1 (OMe) > Ni6 (NO₂) > Ni3 (H) > Ni2 (Me). Additionally, cyclovaltametry measurements performed on Ni1–Ni6 have been used to measure half-wave potentials (E_1/2) that can serve as a quantitative descriptor of the electronic effect. Moreover, good correlations exist between E_1/2 and i) the degree of branching and ii) the polymer molecular weight. Importantly, this study highlights the potential of E_1/2 as a predictive tool for rational design of catalysts to make tailored BPE's.