Ligand electronic effects on late transition metal polymerization catalysts

A series of bis-(aryl)-alpha-diimine ligands were synthesized bearing a range of electron-donating and -withdrawing substituents to systematically investigate the ligand electronic effects on late transition metal olefin polymerization catalysts. Their palladium(II) complexes were prepared and characterized. Electronic perturbations were verified by analysis of the H-1 and C-13 NMR chemical shifts of the corresponding methyl chloride complexes and the CO stretching frequencies of the corresponding cationic carbonyl complexes, which were found to correlate strongly with the Hammet substituent constant (sigma(p)) of the substituent on the ligand. The palladium(II) complexes of the functionalized alpha-diimine ligands were employed in the polymerization of ethylene and the copolymerization of ethylene with methyl acrylate. For ethylene homopolymerization, higher molecular weight was obtained with catalysts bearing more strongly electron-donating ligands. It was observed for the first time that the ligand electronic structure of the catalysts had a significant effect on topology of the polyethylene formed. More dendritic polyethylene was obtained with catalysts bearing more strongly electron-withdrawing ligands. This provides a fundamentally different approach (catalyst approach) to control polyolefin branching topology, which complements our previous strategy of controlling polymer topology by polymerization conditions. For the copolymerization of ethylene with methyl acrylate, strong correlations were observed in the incorporation ratio of methyl acrylate; catalysts bearing strongly electron-donating ligands afforded copolymers with higher incorporation of the polar comonomer. These catalysts also exhibited far greater activity in the presence of polar monomers than catalysts bearing more weakly donating ligands possessing withdrawing substituents, which were deactivated completely when methyl acrylate concentrations were sufficiently high.