The ring-opening (co)polymerization (ROP) of N -carboxyanhydride (NCA) monomers bearing unprotected/reactive side groups is rare and challenging. Here, we report the ROP of a d-penicillamine NCA (Pen-NCA) monomer for the synthesis of tertiary thiol-functionalized (co)polypeptides. Through judicious selection of reaction solvents and the use of benzoic acid as an additive in the ROP, the intramolecular isomerization side reactions of Pen-NCA are suppressed, generating homo- and copolypeptides with improved yield, high molecular weight, and narrow molecular weight distributions. Successful postpolymerization modifications of the d-Pen-containing copolypeptides on the tertiary thiols are achieved with high efficiency through thiol-Michael, S N 2, and nitrosylation reactions. This work provides an efficient protection-free approach to generating functional polypeptides and creates a fundamental understanding for Pen-NCA chemistry.