Substituent Effect in Ligand Modification for Efficient Ethylene (Co)polymerization with α-Diimine Nickel Catalysts

Abstract

With the development of various catalyst backbones, introducing different substituents on the ligands has become an efficient modification strategy. In this work, methyl, methoxy and fluorine substituents were introduced at the ortho-, meta- and para-positions of the diarylmethyl group on the ligand (Ni1–Ni9) to investigate the effects of substituent identity and position. In ethylene homopolymerization, ortho-substituted catalysts showed increased steric hindrance, inhibiting the insertion of ethylene and reducing the rate of chain transfer. The ortho-methoxy catalyst (Ni6) produced polymer with high activity (10.6×10⁶ g·mol⁻¹·h⁻¹) and a high molecular weight (54.9×10⁴ g/mol), which suggests the presence of weak attractive interactions between the adjacent methoxy substituent and metal center. The ortho-fluorinated catalyst Ni9 produced polyethylene with high molecular weight and low branching density, likely due to an ortho-fluorine-associated chain-ligand interaction. Fluorinated catalysts also exhibited better catalytic performance at 80 °C. In copolymerization with methyl 10-undecenoate, methoxy-substituted catalysts showed higher activity and polar monomer incorporation, whereas fluorinated catalysts exhibited lower catalytic activity and comonomer incorporation. These results indicate that both substituent position and electronic properties are important for tuning α-diimine nickel catalysts in ethylene polymerization and copolymerization.

Keywords

Nickel catalyst / Ethylene polymerization / Substituent effect / Copolymerization

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