Enhanced Suppression of Chain Transfer in Ethylene (Co)Polymerization via Synergistic Axial Substituent Effects in Pyridine-Imine Ni(II) and Pd(II) Catalysts

Abstract

Controlling chain-transfer reactions represents a fundamental challenge in pyridine-imine nickel and palladium catalyzed ethylene (co)polymerization. We present a breakthrough dualaxial-substituent strategy that synergistically suppresses chain transfer, enabling efficient production of high-molecular-weight polyethylenes and polar functionalized copolymers. A family of well-defined nickel and palladium complexes featuring both 8-benzhydryl and 2diarylmethyl naphthylpyridine-iminate ligands were synthesized and thoroughly characterized.The nickel catalysts, activated by diethylaluminum chloride, showed moderate activities (~10⁵ g mol⁻¹ h⁻¹) while producing branched polyethylene with high molecular weights (Mn up to 246.4 kg/mol) -an order of magnitude higher than single-substituent control systems.Remarkably, analogous palladium catalysts generated hyperbranched polyethylenes with M up to 43.8 kg/mol. These systems further achieved outstanding methyl acrylate incorporation (up to 13 mol%) in copolymerizations while maintaining practical molecular weights (4.1-8.4 kg/mol). Mechanistic studies reveal that cooperative shielding of both axial sites by the dual substituents prevents displacement of active intermediates, while the flexible 8benzhydrylnaphthyl moiety enhances activity 5-7-fold compared to rigid analogues by facilitating ethylene coordination. This work establishes new design principles for chaintransfer suppression in late-transition-metal polymerization catalysis.