We describe here the first example of trimethylsilyl sulfide (S-TMS) mediated controlled ring-opening polymerization (ROP) of a-amino acid N-carboxyanhydrides (NCAs). We show that phenyl trimethylsilyl sulfide (PhS-TMS), an inexpensive and commercially available compound, mediates rapid ROP of a broad scope of NCA monomers, produces functional poly(amino acids) (PAAs) with controllable molecular weights (MWs), narrow polydispersity index (PDI), arid an in situ generated phenyl thioester group at the C-terminus (PAA-SPhs). PhS-TMS offers more rapid chain initiation than previously reported hexamethyldisilazane (HMDS) initiator, ensuring a: living polymerization with better control. Mechanistic studies suggest that a reactive trimethylsilyl carbamate (TMSC) was generated during the chain initiation and continued to regulate the chain propagation through a TMS transfer process. Considering the versatility of NCAs, and the potential of leveraging the C-terminal phenyl thioester for native chemical ligation (NCL), we believe this method may offer a powerful platform enabling the rapid generation of functional PAAs and their C-terminal conjugates for numerous biological applications.