Electronic effects of amine-imine nickel and palladium catalysts on ethylene (co)polymerization

Abstract

A series of bis-(aryl)-amine-imine nickel and palladium complexes with electron-donating (OMe), H, and -withdrawing (F and Br) groups at the para-aryl position were synthesized and characterized to probe the ligand electronic effects on homo- and copolymerization of ethylene. The influences of electronic perturbations on complex synthesis, polymerization property, and polymer microstructure were investigated systematically. The electronic effects of para-substituents were verified by Hammett constants and topographic steric maps of nickel and palladium complexes. Electron-rich ligands had stronger complexation ability to the nickel and palladium metal, thereby facilitating the synthesis of nickel and palladium complexes. Ethylene polymerizations at different temperatures showed that electron-deficient amine-imine nickel and palladium catalysts were more active than electron-rich analogues at low temperatures while the reverse trend was observed at high temperatures. The electronic effects of para-substituents affected polyethylene molecular weight, but had little influence on the branching density of highly branched polyethylenes. Copolymerization of ethylene and methyl acrylate (MA) using palladium catalysts demonstrated that the electron-donating group promoted copolymerization performance, and was favorable for direct insertion of MA to produce main-chain MA unit. Electronic effects of amine-imine nickel and palladium catalysts on ethylene polymerization were comprehensively summarized to electronic perturbations on the metal electrophilicity and the thermal tolerance of catalyst.