Sterics versus electronics: Imine/phosphine-oxide-based nickel catalysts for ethylene polymerization and copolymerization

Abstract

A series of imine/phosphine-oxide ligands was synthesized and characterized. These ligands are easily prepared in a modular fashion, with positions that are independently tunable. The corresponding nickel allyl complexes were synthesized and serve as single-component catalysts for the homopolymerization of ethylene. It is generally believed that sterically bulky substituents lead to high polymer molecular weights during olefin polymerization. In this system, however, catalysts bearing the sterically bulky dibenzhydryl substituent afforded polyethylenes with much lower molecular weights than those bearing the sterically unhindered isopropyl substituent. Based on control experiments, theoretical calculations, and literature evidence, we propose that electronic effects override steric effects and play dominant roles in this nickel system. Efficient copolymerizations of ethylene with methyl 10-undecenoate and 6-chloro-1-hexene were also facilitated by this system without the need for any cocatalyst or protecting agent.