Substituent effects and activation mechanism of norbornene polymerization catalyzed by three-dimensional geometry α-diimine palladium complexes

Abstract

A series of three-dimensional geometry 9,10-dihydro-9,10-ethanoanthracene-11,12-diimine methyl palladium chloride complexes were synthesized and characterized. These three-dimensional geometry α-diimine palladium complexes exhibited high activities toward the homopolymerization of norbornene and copolymerization of norbornene with 5-norbornene-2-carboxylic acid methyl ester in the presence of B(C₆F₅)₃. It was observed that the palladium–halogen bond of the complexes could be effectively activated by B(C₆F₅)₃. The activation mechanism was clarified by the end group analysis of the polymer, which provided a new pathway for the activation of palladium complexes. Meanwhile, the palladium catalysts with a three-dimensional geometry on the backbone were found to show a good thermal stability and afford a high incorporation of the polar monomer in norbornene polymerization. Moreover, the α-diimine palladium complexes with a large steric hindrance or strongly electron-donating group on the aryl ring of ligands could achieve a higher reactivity.