Reaction of yttrium tris[bis(trimethylsilyl)amide] [(TMSN)3Y] with equivalent L-alanine N-carboxyanhydride (ALA NCA) yields yttrium a-isocyanato carboxylate (II), yttrium ketenyl carbamate (III), and hexamethyldisilazane (V). The products indicate that 4-CH group of ALA NCA monomer is deprotonated in addition to 3-NH group, which has been neglected in NCA chemistry for decades. This result proves the acidity of 4-CH in NCA and provides the first direct evidence for racemization phenomenon of NCA in strong base in microscopic aspect. Rare earth tris[bis(trimethylsilyl)amide] (TMSN)3Ln (Ln = Sc, Y, La, Dy, and Lu) compounds are high efficient catalysts for ring-opening polymerizations of NCAs. Polypeptides can be produced in quantitative yields with narrow molecular weight distributions below 1.3, and block copolypeptides can be facilely prepared by sequential addition method. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012