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

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 (PPh 3 ), 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 PPh 3 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 /PPh 3 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 PPh 3 ), where the propagating ends were also stable under nitrogen. The observed kinetic coefficients for 2a, 2b, and 2d by 1b /PPh 3 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.