Five rare earth complexes are first introduced to catalyze ring opening polymerizations (ROPs) of ?-benzyl-L-glutamate N-carboxyanhydride (BLG NCA) and L-alanine NCA (ALA NCA) including rare earth isopropoxide (RE(OiPr)3), rare earth tris(2,6-di-tert-butyl-4-methylphenolate) (RE(OAr)3), rare earth tris(borohydride) (RE(BH4)3(THF)3), rare earth tris[bis(trimethylsilyl)amide] (RE(NTMS)3), and rare earth trifluoromethanesulfonate. The first four catalysts exhibit high activities in ROPs producing polypeptides with quantitative yields (>90%) and moderate molecular weight (MW) distributions ranging from 1.2 to 1.6. In RE(BH4)3(THF)3 and RE(NTMS)3 catalytic systems, MWs of the produced polypeptides can be controlled by feeding ratios of monomer to catalyst, which is in contrast to the systems of RE(OiPr)3 and RE(OAr)3 with little controllability over the MWs. End groups of the polypeptides are analyzed by MALDI-TOF MS and polymerization mechanisms are proposed accordingly. With ligands of significant steric hindrance in RE(OiPr)3 and RE(OAr)3, deprotonation of 3-NH of NCA is the only initiation mode producing a N-rare earth metallated NCA (i) responsible for further chain growth, resulting in a-carboxylic-?-aminotelechelic polypeptides after termination. In the case of RE(BH4)3(THF)3 with small ligands, another initiation mode at 5-CO position of NCA takes place simultaneously, resulting in a-hydroxyl-?-aminotelechelic polypeptides. In RE(NTMS)3 system, the protonated ligand hexamethyldisilazane (HMDS) initiates the polymerization and produces a-amide-?-aminotelechelic polypeptides. (c) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012