Living Coordination Polymerization of N-Allenylamides by π-Allylnickel Catalysts

Abstract

The coordination polymerization of N-allenylamides was investigated by π-allylnickel catalysts to elucidate the relationship between the amide structure and the polymerization behavior. The polymerization of N-allenyl-γ-butyrolactam (2a) having a five-membered cyclic amide moiety proceeded smoothly by a π-allylnickel catalyst bearing a trifluoroacetate ligand (1a) in the presence of triphenylphosphine (PPh3), giving a polymer with a narrow molecular weight distribution in high yields. The initiation efficiency of the polymerization was supported quantitatively by measuring the MALDI-TOF mass spectra. The molecular weight of the polymer increased linearly in proportion to the monomer feed ratio ([2a]/[Ni]), supporting a living character of the polymerization. The addition of PPh3 to the initiating system was essential to attain a stable propagating end; otherwise the propagating π-allylnickel species became gradually inactive after the complete monomer conversion. In comparison with 2a, N-allenyl-δ-valerolactam (2b), N-allenyl-ε-caprolactam (2c), and N-allenyl-N-methylacetamide (2d) revealed a lower polymerizability by 1a/PPh3 under the same conditions. However, the living polymerization of 2b−2d occurred smoothly to give corresponding polymers in high yields by modification of the π-allylnickel catalyst (i.e., by using π-allylnickel iodide (1b) in the presence of PPh3), where the propagating ends were also stable under nitrogen. The observed kinetic coefficients for 2a, 2b, and 2d by 1b/PPh3 were estimated as 14.9, 4.21, and 6.54 L·mol-1·h-1, respectively. A plausible polymerization mechanism for the different polymerizability of N-allenylamides (2a−2d) was discussed.