Unexpected Effect of Catalyst’s Structural Symmetry on the Branching Microstructure of Polyethylene in Late Transition Metal Polymerization Catalysis

In this study, several α-diimine nickel complexes were prepared by merging a bulky dibenzobarrelene backbone and bulky dibenzhydrylanilines. The single-crystal X-ray diffraction analysis confirmed that the complex Ni1 exists in an anti-conformation, possessing a quasi-centrosymmetry. These nickel complexes displayed high activities (above 106 g mol–1·h–1) in ethylene polymerization, even at 90 °C, providing polyethylenes with very high molecular weights (well above 500 kg/mol). Unexpectedly, slightly branched semi-crystalline polyethylenes (11-34/1000C) with high melting points were obtained with these quasi-centrosymmetric nickel catalysts, contrary to those common bulky α-diimine nickel complexes that often render highly branched polyethylenes. The resultant slightly branched semi-crystalline polyethylenes have good mechanical properties, possessing moderate to high stress at break values and high strain at break values. Ethylene–methyl undecenoate copolymerization using these nickel complexes also gave access to slightly branched high-molecular-weight polar functionalized copolymers. The experimental and density functional theory calculation results suggested that the distribution of the steric hindrance in the as-prepared nickel complexes, namely, the catalyst’s structural symmetry, might be more favorable to the chain propagation relative to the branch formation, which was not observed in α-diimine nickel polymerization systems before.