Promotion of chain growth and suppression of chain transfer by 2-methoxy group in diarylmethyl moiety in pyridine-imine nickel- and palladium-catalyzed ethylene (co)polymerization

Abstract

Diarylmethyl substituents are commonly used to prevent chain transfer in late-transition metal-catalyzed ethylene polymerization. However, this substituent is not effective in inhibiting chain transfer in the pyridine-imine system. In this study, we introduced substituents at the 2-position of the diarylmethyl moiety to synthesize high molecular weight polyethylene in the pyridine-imine system. Using this strategy, we synthesized two pyridine-imine ligands with either ethyl or methoxy at the 2-position diarylmethyl moiety, along with their corresponding nickel and palladium complexes. These complexes were then utilized for ethylene (co)polymerization. The catalysts with OMe exhibited significantly higher polymerization activity (an order of magnitude, 105 vs. 104 g/(mol Ni·h)) compared to the corresponding Et catalysts. In addition, it was found that polyethylene produced from nickel and palladium catalysts with OMe has higher molecular weights than that produced from ethyl nickel and palladium catalysts. The nickel system produces polyethylene with a molecular weight up to an order of magnitude higher (an order of magnitude, 13.0–13.6 vs. 3.1–4.0 kg/mol). Similar results were also observed in the palladium-catalyzed copolymerization of ethylene with MA. Notably, the palladium catalyst with OMe has a significantly higher capacity for MA insertion (three times, 1.68–4.74 vs. 5.00–11.73 mol%) than the ethyl palladium catalyst, despite their similar spatial steric structures. The observed phenomenon may be attributed to the weak interaction between OMe and the metal center. In addition, 13C NMR analysis showed that the palladium-catalyzed polyethylene and E-MA copolymers are hyperbranched polymers, with the polar groups located at the ends of the branches.