A series of CH₃C(O)Co(CO)₃L complexes (L = PCy₃, PMe₂Ph, PPh₃, P(p-F-Ph)₃, P(m-F-Ph)₃ and P(o-toly)₃) are synthesized, characterized and studied as precatalyst for the COP of N-butylaziridine.  The Co-P bond length primarily responds to the cone angle of the phosphine ligand, while the back-donation to the axial acetyl ligand and the equatorial CO ligand depends on the electron-donating ability of the phosphine.  Using the optimized catalyst CH₃C(O)Co(CO)₃P(o-Tol)_3, the synthesis of amphiphilic block polymers is realized by sequential addition of different N-alkyl-aziridine under 1000 psi of CO in moderately polar solvent.  GCMS and in situ ATR-IR are used for monitoring reaction progress.  The block polymers are characterized using NMR, DSC and GPC techniques.  The key for the block copolymer synthesis is the timely addition of the subsequent monomer after the consumption of the previous monomer.