Two novel platinum complexes, [bis(diphenylphosphino)ethane][N-((1S,2R)-2-hydroxo-1,2-diphenylethyl)-4 -methyl-benzenesulfonamidato]platinum(II) [(dppe)Pt(MBS-O), 2a] and [bis(diphenylphosphino)ethane] [N-((IS,2R)-2-amido-1,2-diphenylethyl)-4-methylbenzenesulfonamidato]plat inum [(dppe)Pt(MBS-NH), 2b] were synthesized and structurally characterized. Experimental results show that compound 2b is an efficient initiator toward ring opening polymerization of amino acid N-carboxyanhydride (NCA) yielding polypeptides with narrow PDIs (1.07-1.19). By comparing the activity of complex 2a with that of 2b, the mechanism for polymerization of NCA initiated by an amido-sulfonamidate is proposed.