Ligand-Based Steric Effects in Ni-Catalyzed Chain-Growth Polymerizations Using Bis(dialkylphosphino)ethanes

Abstract

The role of ligand-based steric effects was investigated in the polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride. Three different Ni(L-L)Cl2 catalysts were synthesized using commercially available bis(dialkylphosphino)ethane ligands with varying steric properties. One of these catalysts (Ni(depe)Cl2) outperformed the others for this polymerization. The polymer characterization data were consistent with a chain-growth mechanism. Rate and spectroscopic studies revealed a rate-limiting reductive elimination for both initiation and propagation with Ni(depe)Cl2. In contrast, less hindered Ni(dmpe)Cl2 and more hindered Ni(dcpe)Cl2 were ineffective polymerization catalysts; NMR spectroscopic studies indicated that competing decomposition and uncontrolled pathways intervene. For other monomers, Ni(depe)Cl2 performed similar to the conventional catalysts. Copolymerization studies revealed that block copolymers could be effectively prepared. Overall, these studies indicate that altering the ligand-based steric properties can have a significant impact on the chain-growth polymerization.