Ring-opening polymerizations of alpha-amino acid N -carboxyanhydrides (NCAs) initiated with amines typically form polypeptides with uncontrolled molecular weights and broad molecular weight distributions. However, we found that hexamethyldisilazane (HMDS)mediated controlled NCA polymerizations gave polypeptides with predictable molecular weights and narrow molecular weight distributions. Using MS, NMR, and FT-IR, we demonstrated that the initiation step involved the cleavage of the N-Si bond of HMDS and the formation of a trimethylsilyl carbamate (TMS-CBM) terminal group. Polypeptide chains were propagated through the migration of TMS of the TMS-CBM end group to the incoming monomer and formed a new TMS-CBM terminal group. This organosilicon reagent mediated NCA polymerization offers a metal-free strategy for the convenient synthesis of homo- or block polypeptides with predictable molecular weights and narrow molecular weight distributions.