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

The introduction of bulky ortho-steric substituents into alpha-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 alpha-diimine Ni(ii) and Pd(ii) species. Herein, a series of acenaphthene-based alpha-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 x 10(6) g mol(-1) h(-1)) in ethylene polymerization even at 80 degrees 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. 10(4) 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 alpha-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.