Deciphering Ligand Roles in Nickel-Catalyzed Polymerization of Ethylene: A DFT Study

Catalysts play a key role in determining the microstructure of polyethylene, which is crucial for optimizing polyethylene’s properties and applications. Understanding the mechanistic characteristics of different catalysts is fundamental to achieving microstructure control. In this study, the mechanism of polymerization of ethylene by two representative types of nickel/salicylaldimine and nickel/NHC (N-heterocyclic carbene) was studied theoretically, to understand roles of the ligand on the catalytic properties such as linearity and molecular weight of polyethylene, by systematic investigation of the entire mechanistic processes, including propagation, ethylene coordination, β-H elimination, chain transfer, etc. The calculation results indicate that nickel/NHC catalysts, benefiting from the strong trans effect of NHC, facilitate easier ethylene insertion compared with nickel/salicylaldimine catalysts. Both the steric hindrance on the ligands and the strong trans effect of NHC result in the high free-energy barrier for β-H elimination, inhibiting branching and producing linear polyethylene. However, the empty p orbital of the NHC is susceptible to alkyl or hydride attack, potentially causing catalyst deactivation; embedding the NHC ligand within a rigid backbone helps mitigate this deactivation. Overall, selecting nickel/NHC catalysts within a rigid backbone with appropriate steric effects promises to achieve superior performance in synthesizing linear polyethylene and may offer unique advantages in copolymerization. These findings offer new insights into the mechanisms of different catalysts for controlling the polyethylene microstructure and may guide the design of more effective catalyst systems for this crucial polymer.