Effect of ligand electronic properties on precatalyst initiation and propagation in Ni-catalyzed cross-coupling polymerizations

Abstract

The role of ligand-based electronic effects was investigated in the Ni-catalyzed polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride. The catalyst with the most electron-donating ligand outperformed the other catalysts by providing polymers with narrower molecular weight distributions. This result is attributed to both a suppression of competing reaction pathways (e.g., chain transfer and termination) as well as a relative acceleration of precatalyst initiation compared to propagation. Further studies revealed that, for all three catalysts, precatalyst initiation is slower than propagation, despite the fact that they exhibit the same rate-determining steps (i.e., reductive elimination) and have similar catalyst resting states. These results suggest that better control over the polymer molecular weight, end-functionality and sequence can be obtained with electron-rich catalysts, such as those described herein.