Effects of Labile Ligands and Substituents in Nickel Enolate Catalysts on Ethylene/Acrylate Copolymerization Activity: A DFT Study

Abstract

In this study, density functional theory (DFT) was employed to investigate the copolymerization mechanism of ethylene with tert-butyl acrylate (tBA) catalyzed by neutral nickel enolate complexes featuring distinct substituents. It is computationally found that [2,6-(PhO)₂C₆H₃]₂PCHC(Ph)O-Ni (A) is more active than [2,6-(MeO)₂C₆H₃]₂PCHC(Ph)O-Ni (B), which is in line with experimental observation. Based on this agreement, it is demonstrated that the ethylene insertion into the tBA chain-end is the rate-determining step and the activity discrepancy between A and B is closely associated with the electronic effects of the substituents rather than the difference in sterics between the PhO group in A and the MeO in B. The natural population analysis (NPA) indicates that the phenoxy substituent can more effectively increase the positive charge on the Ni center, thereby enhancing its copolymerization activity. The influence of the labile ligand (L) (L = pyridine, PEt₃ and PPh₃) on the copolymerization activity of the more active A has been further investigated. The coordination strength of the labile ligand was found to significantly influence the catalytic performance. Specifically, a weaker coordinating labile ligand facilitates the ligand exchange between L and monomer (tBA and ethylene) and enhances the efficiency of chain propagation. These mechanistic insights are helpful for the molecular design of copolymerization catalysts with high performance.