Quantitative Analysis of the Steric Effect in Phosphinophenolate Nickel Catalysts for Ethylene Polymerization.

Ethylene polymerization catalysts play a pivotal role in the modern polyolefin industry. Modulating the steric effect of complexes is an effective strategy for controlling catalytic activity and the microstructure of polymers in ethylene polymerization. Phosphinophenolate Ni-(II) complexes represent a significant subclass that has been extensively studied in terms of catalyst design and ethylene polymerization performance. Nevertheless, quantitative studies of the influence of the steric effect on the catalytic performance of the complexes remain relatively limited. In this work, a series of phosphinophenolate Ni-(II) complexes bearing alkylphenyl substituents on the phosphorus side were synthesized and employed for ethylene (co)-polymerization to investigate the impact of the steric effect on ethylene polymerization. Analysis of polymerization and catalyst structure outcomes revealed a strong correlation between the key polymerization parameters (catalytic activity, polymer molecular weight, and the incorporation ratio of the polar monomer) and Sterimol B5 parameters of the substituents on the phosphorus atom, with an R2 of approximately 0.8. Furthermore, during the copolymerization of ethylene and tert-butyl acrylate, the increase in the volume of the phosphorus side substituents boosted the molecular weight of the obtained copolymers from 4.0 × 103 to 6.9 × 104, in spite of the incorporation ratio decreasing from 2.9% to 1.1%. This work contributes a systematic study of the steric effect in the phosphinophenolate Ni-(II) complex and valuable insights into the rational design of catalysts for synthesizing functionalized polyolefins.