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 dual-axial-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 2-diarylmethyl naphthylpyridine-iminate ligands were synthesized and thoroughly characterized. The nickel catalysts, activated by diethylaluminum chloride, showed high activities (∼10⁶ g mol⁻¹ h⁻¹) while producing branched polyethylene with high molecular weights (M_n up to 246.4 kg mol⁻¹) – an order of magnitude higher than single-substituent control systems. Remarkably, analogous palladium catalysts generated hyperbranched polyethylenes with M_n up to 43.8 kg mol⁻¹. 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⁻¹). Mechanistic studies reveal that cooperative shielding of both axial sites by the dual substituents prevents displacement of active intermediates, while the flexible 8-benzhydrylnaphthyl moiety enhances activity 5–7-fold compared to rigid analogues by facilitating ethylene coordination. This work establishes new design principles for chain-transfer suppression in late-transition-metal polymerization catalysis.