The controlled ring-opening polymerization of the beta-thiolactone derived from N-Boc cysteine was achieved using N-Boc-L-cysteine methyl ester as the initiator in NMP at room temperature. The propagating end is the thiol group, which attacks the carbonyl to open the monomer ring by the C(=O)-S bond scission. A thiol-ene click reaction demonstrated the utility of the thiol group at the propagating terminal. The block copolymer was efficiently produced by the terminal coupling of the polythioester with the norbornene terminated PEG. As another interesting reaction, the polythioester underwent the main chain transformation to polycysteine through the intramolecular S-to-N acyl migration, triggered by the deprotection of the pendant Boc groups. The polythioester from L-cysteine showed Cotton effects between 200 and 300 nm in the circular dichroism (CD) spectrum. Although the CD pattern resembled that produced by the alpha-helix of polypeptide, it was ascribable not to the second structure but to the relative orientation of the thioester and carbamate carbonyls in the repeating unit.