N-Substituted glycine N-thiocarboxyanhydrides (NNTAs) are promising cyclic monomers to synthesize polypeptoids with the advantages of easier preparation and higher stability during purification and storage than N-substituted glycine N-carboxyan-hydrides (NNCAs). NNTAs were commonly considered too stable to polymerize for their low reactivity. In this contribution, we report controlled polymerizations of N-ethylglycine NTA (NEG-NTA) and sarcosine NTA (Sar-NTA) using primary amines as initiator under proper polymerization conditions. The controllability has been fully supported by H-1 NMR end group analyses, MALDI-ToF mass spectra, kinetic data, block copolymerizations by sequential monomer addition, and low polydispersities (1.14-1.17) of polypeptoids. Variation of the [NNTA]/[initiator] ratio allows well control of the molar mass, and degrees of polymerization (DPs) up to 287 can be reached for poly(N-ethylglycine) or DPs up to 262 for polysarcosine. NNTAs exhibit excellent activity and they are potential to synthesize polypeptoids with controllable polymerization. (C) 2016 Wiley Periodicals, Inc.