This study describes the first synthesis of a functional and activatable initiator for the living ring-opening polymerization of N-carboxyanhydrides. Well-defined functionalized polysarcosines bearing this initiator can be activated via oxidation (using m-CPBA) and utilized for post-polymerization modification with different -SH and -NH2 nucleophiles of low and high molecular weight nucleophiles, including bovine serum albumin as a model protein. Successful functionalization was demonstrated by NMR spectroscopy, GPC, FPLC, SDS-PAGE and MALDI-ToF spectrometry. Even though there are limitations due to occurring hydrolysis of the functionalized group during storage and imperfect conjugation, we believe that the introduced method is an exciting tool for the post-polymerization bioconjugation of polypeptides and polypeptoids and could also be extended to other classes of polymers.