Because of the high reactivity, N-carboxyanhydride (NCA) can be initiated by the thiol group. On the contrary, N-thiocarboxyanhydride (NTA) is more stable and is able to tolerate it. Herein, we apply cysteamine as a regioselective initiator for ring-opening polymerization (ROP) of N-substituted glycine N-thiocarboxyanhydride (NNTA) to synthesize well-defined thiol-capped polypeptoids. ROPs of sarcosine NTA (Sar-NTA) and N-ethylglycine NTA (NEG-NTA) are well controlled when [M]/[I] <= 100 with high yields (>87.5%) producing polypeptoids with designable molecular weights and low polydispersity indices (<1.2). All the polypeptoid chains contain a thiol end group, which is confirmed by NMR analyses, MALDI-ToF MS spectra, and Ellman's assay. Through radical-mediated thiol-ene reaction with styrene, all the thiol chain ends are transferred to oligostyrene, revealing the convenience of further modification. Benefiting from the thiol-ene click chemistry, thiol-capped polysarcosine (PSar) and poly(N-ethylglycine) (PNEG) are promising candidates to replace PEG for their nontoxicity and biocompatibility.