N-substituted cyclic thiourethanes [4(S)-(methoxycarbonyl)-N-benzyl-1,3-oxazolidine-2-thione (BnSL), 4(S)-(methoxycarbonyl)-N-benzoyl-1,3-oxazolidine-2-thione (BzS(L)), and 4(S)-(methoxycarbonyl)N-acetyl-1,3-oxazolidine-2-thione (AcSL)] were synthesized from L-serine methyl ester hydrochloride and were polymerized by using methyl trifluoromethanesulfonate to obtain the corresponding well-defined polythiourethanes. The molecular weight of the polymers can be controlled by the ratio of the monomers to the initiator and the molecular weight distributions are narrow (M-w/M-n < 1.15) similar to the previously reported polymerization of a cyclic thiourethane [4(S)-(methoxycarbonyl)-1,3-oxazolidine-2-thione (S-L)]. The polymerization rates are on the order of S-L > BnSL > BzS(L) > AcSL, which agrees well with the nucleophilicity of the thiocarbonyl moieties of the monomers (S-L, BnSL BzS(L), and AcSL). The Cotton effects in the CD spectra of the polymers from the N-substituted monomers exhibit almost inverse shape with that of poly(S-L) and the specific rotations' signs also inversed, suggesting the idea that poly(S-L) and N-substituted polymers take different high order structures.