Ligand pre-inserted α-diimine palladium catalysts and mechanism studies in olefin polymerization

Abstract

Since the pioneering works of Brookhart and co-workers, extensive research efforts have been directed towards the development of high-performance late-transition-metal-based olefin polymerization catalysts. In this contribution, we designed α-diimine ligands bearing an allyl group and the ligand pre-inserted palladium catalysts Pd1-ACN and Pd2-ACN were generated. However, they demonstrated different behaviors due to their different steric hindrance, producing Pd1 and Pd2 intermediates, respectively. The palladium catalysts were utilized in ethylene polymerization and copolymerization. Specifically, Pd2-ACN with its larger steric hindrance exhibited higher activity, affording polyethylene with high molecular weight. Enhanced catalytic performance was also observed in ethylene copolymerization with acrylate-type monomers, demonstrating good polar group tolerance. To further understand this system, DFT calculations and mechanistic studies of the coordination–insertion process were conducted, indicating the formation of the Pd–H catalytically active species during the polymerization process. The pre-inserted α-diimine catalysts in this work demonstrated a unique form in olefin polymerization and provided some new information for catalyst design.