Deciphering\nLigand Roles in Nickel-Catalyzed Polymerization\nof Ethylene: A DFT Study

Catalysts\nplay a key role in determining the microstructure of\npolyethylene, which is crucial for optimizing polyethylene’s\nproperties and applications. Understanding the mechanistic characteristics\nof different catalysts is fundamental to achieving microstructure\ncontrol. In this study, the mechanism of polymerization of ethylene\nby two representative types of nickel/salicylaldimine and nickel/NHC\n(N-heterocyclic carbene) was studied theoretically, to understand\nroles of the ligand on the catalytic properties such as linearity\nand molecular weight of polyethylene, by systematic investigation\nof the entire mechanistic processes, including propagation, ethylene\ncoordination, β-H elimination, chain transfer, etc. The calculation\nresults indicate that nickel/NHC catalysts, benefiting from the strong <i>trans</i> effect of NHC, facilitate easier ethylene insertion\ncompared with nickel/salicylaldimine catalysts. Both the steric hindrance\non the ligands and the strong <i>trans</i> effect of NHC\nresult in the high free-energy barrier for β-H elimination,\ninhibiting branching and producing linear polyethylene. However, the\nempty p orbital of the NHC is susceptible to alkyl or hydride attack,\npotentially causing catalyst deactivation; embedding the NHC ligand\nwithin a rigid backbone helps mitigate this deactivation. Overall,\nselecting nickel/NHC catalysts within a rigid backbone with appropriate\nsteric effects promises to achieve superior performance in synthesizing\nlinear polyethylene and may offer unique advantages in copolymerization.\nThese findings offer new insights into the mechanisms of different\ncatalysts for controlling the polyethylene microstructure and may\nguide the design of more effective catalyst systems for this crucial\npolymer.