Suppression of chain transfer via a restricted rotation effect of dibenzosuberyl substituents in polymerization catalysis

Abstract

The introduction of bulky ortho-steric substituents into α-diimine Ni(II) and Pd(II) species has been often used as an effective strategy to retard chain transfer in olefin polymerization. In this work, we further demonstrate that the “restricted-rotation” effect originating from dibenzosuberyl substituents is also capable of suppressing the chain transfer and increasing the molecular weight of the polymer in ethylene (co)polymerization with α-diimine Ni(II) and Pd(II) species. Herein, a series of acenaphthene-based α-diimine ligands bearing merely one diarylmethyl or dibenzosuberyl moiety at the ortho-positions of N-aryl rings and the corresponding Ni(II) and Pd(II) complexes were synthesized and characterized. These Ni(II) complexes exhibited very high activities (up to 6.32 × 10⁶ g mol⁻¹ h⁻¹) in ethylene polymerization even at 80 °C, generating high-molecular-weight polyethylenes with varying branching densities. The resultant polyethylene products showed excellent mechanical properties and elastic recovery (SR up to 76%). Correspondingly, the palladium complexes displayed moderate activities, generating highly branched polyethylene with moderate molecular weight (ca. 10⁴ g mol⁻¹) in ethylene polymerization. Moreover, these Pd(II) complexes also displayed moderate copolymerization activity and generated moderate molecular weight polar functional copolymers (11.9–28.5 kg mol⁻¹) with low to moderate incorporation ratios (0.50–1.29 mol%). Compared with diarylmethyl-substituted species, the α-diimine Ni(II) and Pd(II) complexes containing rotationally restricted dibenzosuberyl substituents possess a superior ability to retard the chain transfer in the ethylene (co)polymerization process, resulting in higher molecular weight polyethylenes and copolymers.