Fluorine Effect on α-Imino-ketone- and Phenoxyiminato Nickel-Catalyzed Ethylene Homo- and Copolymerization

The synthesis of ultrahigh molecular weight polyethylene (UHMWPE) using late transition metal catalysts is highly challenging. In this work, 3,5-bis(trifluoromethyl)phenyl and m-xylyl groups were selected to introduce at the ortho position of 4-(trifluoromethyl)aniline and 4-methylaniline. These amines were condensed with 2,3-butandione and 3-substituted salicylaldehyde and subsequently with nickel precursors to afford cationic keto-imine-based Ni1 and Ni2, as well as 3-substituted neutral phenoxyiminato Ni3 and Ni4 catalysts. The properties of these nickel complexes in ethylene polymerization and copolymerization were studied in detail. Ni2 with a keto-imine framework was found to be highly active in ethylene polymerization with an activity of up to 6.9 × 106 g·mol–1·h–1, whereas the fluorinated Ni1 generated UHMWPE with Mn of up to 2.79 × 106 g·mol–1, along with very low branch density and high Tm values. These nickel catalysts also showed good activity and incorporation of long-chain special polar monomers. It is believed that the higher steric hindrance of fluorine atoms around the metal center may involve suppressing the chain transfer process.