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.