A series of CH3COCo(CO)3L complexes (1, L = PCy3; 2, L = PMe2Ph; 3, L = PPh3; 4, L = P(para-F-Ph)3; 5, L = P(meta-F-Ph)3; and 6, L = P(ortho-tolyl)3) were studied as precatalyst for the title polymerization. The Co–P bond length primarily responds to the cone angle of the phosphine ligand (6 > 1 > 2 ≈ 3 ≈ 4 ≈ 5), while the back-donation to the axial acetyl ligand and the equatorial CO ligand depends on the electron-donating ability of the phosphine and increases in the order 1 > 6 > 2 > 3 > 4 > 5. The equilibrium constant for CH3COCo(CO)3L + CO ↔ CH3COCo(CO)4 + L depends on the electron-donating ability of the phosphine ligand except for 6 and follows the order 6 ≫ 5 > 4 > 3 > 2 > 1. The catalytic activity follows the order 6 > 5 > 4 > 3 > 1 > 2. The activity difference cannot be explained solely by the above equilibrium and is consistent with the competition for the acyl site by the phosphine as nucleophile against aziridine. The production of the β-lactam byproduct is attributed to catalyst decomposition, which is accelerated to the basicity/nucleophilicity of the phosphine ligand.