Control of chain transfer and chain walking by tailored metal–π interactions in polymerization catalysis

Abstract

Chain walking and chain transfer are pivotal processes that govern the microstructure and molecular weight of polyolefins synthesized via late transition metal-catalyzed ethylene (co)polymerization. In this study, we demonstrate that tailored metal–π interactions using heteroatomic dibenzosuberyl substituents can effectively modulate both chain transfer and chain walking in α-diimine nickel- and palladium-catalyzed systems. This modulation leads to significant reductions in the molecular weight and branching density of the resulting polyethylenes and copolymers. To achieve this, we designed, synthesized, and characterized a series of α-diimine Ni(II) and Pd(II) complexes bearing diverse heteroatomic dibenzosuberyl substituents. In nickel-catalyzed ethylene polymerization, the heteroatomic dibenzosuberyl Ni(II) catalysts showed lower catalytic activities and produced polyethylenes with fewer branches (21–48/1000C vs 83–90/1000C) and an order of magnitude lower molecular weight (2.3–6.5 kg/mol vs 54.6–89.1 kg/mol) than the non-heteroatomic dibenzosuberyl Ni(II) catalyst. Comparable trends were observed in palladium-catalyzed ethylene polymerization and ethylene-MA copolymerization, with sulfur-containing substituents exerting more pronounced effects. We propose a mechanism where the weak metal–π interactions between the dibenzosuberyl substituents and the metal center during polymerization catalysis suppress β-H elimination and promote synergistic chain transfer. This provides a rationale for the observed reductions in molecular weight and branching density, offering valuable insights for the rational design of catalysts for tailored polyolefin synthesis.