We report a new synthetic methodology for poly(ester amide)s by anionic ring-opening copolymerization of N -sulfonyl aziridines and cyclic anhydrides. Phosphazenes organocatalysts have been found to promote a highly-active, controlled, and selective, alternating copolymerization in the absence of any competitive side reaction (zwitterionic and transacylation). Mechanistic studies have shown first-order dependence of the copolymerization rate on N -sulfonyl aziridines and phosphazenes, and zero-order on cyclic anhydrides. This one-pot methodology leads not only to homopolymers but also to poly(ester amide)-based block copolymers. Two catalytic cycles involving ring-opening alternating copolymerization of N -sulfonyl aziridines with cyclic anhydrides and ring-opening polymerization of N -sulfonyl aziridines have been proposed to explain the one-pot synthesis of poly(ester amide)-based homo- and block copolymers.