Steric and electronic effects of tetradentate nickel(II) and palladium(II) complexes toward the vinyl polymerization of norbornene

Abstract

A series of Ni(II) and Pd(II) complexes bearing N4-type tetradentate ligands [Ni(X1X2-6-Me2bpb) 1] and [Pd(X1X 2-6-Me2bpb) 2]; 6-Me2bpb = N,N'-(o-phenylene) bis(6-methylpyridine-2-carboxamidate) X1 = Cl H or CH3 X2 = NO2 Cl F H CH3 or OCH3) were designed synthesized and characterized to investigate electronic and steric effects of ligand on the norbornene polymerization catalysts. Using modified methylaluminoxanes as an activator the complexes exhibited high catalytic activities for the polymerization of norbornene and the nickel complexes exhibited better catalytic activity the palladium complexes. Ni complex 1a with NO2 group on the benzene ring showed the highest catalytic activity of 4.9 × 106 g of PNBEs/molNi h and molecular weight of 15.28 × 105 g/mol with PDI < 2.30. Complexes with electron-withdrawing groups are more thermally stable (> 100 °C) and tend to afford higher polymerization productivities than the ones having electron-donating groups. Amorphous polynorbornenes were obtained with good solubility in halogenated aromatic solvents. A vinyl addition mechanism has been proposed for the catalytic polymerization.