The ring-opening polymerization of N-carboxy anhydrides (NCA) of gamma-benzyl-L-glutamate and beta-benzyl-L-aspartate was studied in the presence of an ammonium chloride-functionalized poly(ethylene oxide) macroinitiator, which possibly prevents side reactions such as NCA deprotonation. Although polymerization initiated by such macro-initiators was found to be quite slow, well-defined conjugates of poly(ethylene oxide)-block-poly(gamma-benzyl-L-glutamate) and poly(ethlene oxide)-block-poly(beta-benzyl-L-aspartate) with polydispersity indexes as low as 1.05 were prepared. Moreover, the presence of ammonium chloride chain ends significantly prevented end-group cyclization of poly(gamma-benzyl-L-glutamate) after polmerization.