Synthesis of lightly branched ultrahigh-molecular-weight polyethylene using cationic benzocyclohexyl nickel catalysts

Abstract

Generally, it is usually difficult to obtain lightly branched polyethylene with a very high molecular weight using bulky α-diimine Ni(II) catalysts. In this study, a family of benzocycloalkyl α-diimine Ni(II) complexes with different backbones and axial dibenzhydryl substituents were synthesized to overcome this problem. In ethylene polymerization, these Ni(II) complexes exhibited exceptionally high activities (level of 10⁷ g mol⁻¹ h⁻¹), and lightly branched (25–43/1000C) polyethylenes with very high molecular weights (M_n up to 1846 kg mol⁻¹) were obtained. All the yielded polyethylenes showed good mechanical properties (high stress-at-break and strain-at-break values). Interestingly, lightly branched ultrahigh-molecular-weight (1.05–1.85 million Da) polyethylenes were accessible with these cationic benzocyclohexyl Ni(II) complexes due to the very high chain propagation rate. Additionally, in the copolymerizations of ethylene and methyl 10-undecenoate (MU) catalyzed by these Ni(II) complexes, high yields, moderate molecular weights, low branching densities and low incorporation ratios (0.46–1.63 mol%) were observed under the given conditions.