Ultra-low-molecular-weight PE wax using a cyclohexyl-fused bis(imino)pyridine-cobalt catalyst functionalized with a remote para-phenyl group

Abstract

With the goal to produce high quality polyethylene wax, five distinct bis(cyclohexyl)-fused bis(imino)pyridine-cobalt complexes, Co1–Co5, bearing a remote para-phenyl group have been synthesized and employed as precatalysts in ethylene polymerization. All complexes, under activation with methylaluminoxane (MAO) or modified MAO (MMAO), exhibited extremely high catalytic activity reaching up to 25.36 × 10⁶ g (PE) mol⁻¹ (Co) h⁻¹ for Co1/MAO, levels that surpass that found for most related cobalt analogues. In terms of the polymer, ultra-low molecular weight (M_w < 1.00 kg mol⁻¹) linear polyethylene of narrow polydispersity was obtained with the M_w remaining almost constant (range: 0.500–0.600 kg mol⁻¹) on varying the temperature, Al/Co molar ratios and reaction time, reflecting the excellent control displayed by this cobalt catalyst. Moreover, high selectivity for vinyl chain-ends was a common feature of the polymers (range: 73–97%) that could be exploited in the form of post-functionalization via epoxidation. Furthermore, a comparison of the number-average molecular weight (M_n) determined through NMR spectroscopy and GPC analysis has been studied, highlighting the merit of using NMR spectroscopy over GPC for measuring polymer molecular weight of less than 1.00 kg mol⁻¹.