Poly(N-substituted urethane)s with an alkyl or ligo(ethylene oxide) monomethyl ether side chain were synthesized by the reaction operating in the following two-step process: first, by metalation of the starting polymer with potassium tertiary butoxide (t-BuOK) and then by treatment of the obtained urethane polyanion with tosylate in dimethyl sulfoxide. The thermal properties of poly(ethylene oxide) poly(N-substituted urethane) (N-sub PEOPU) were investigated in view of the N-substitution degree and properties of the substituent. The chemical structures were characterized by Fourier transform infrared, H-1 NMR, and C-13 NMR spectroscopies. DSC and thermogravimetric analysis (TGA) were used to investigate the thermal properties of N-Sub PEOPUs. As the degree of N-methylation increased, the glass-transition temperature (T-g) of the N-sub PEOPUs linearly decreased from 6 to -29 degreesC, and the weight-loss temperature of 5% (T-d(5%)) from TGA in air increased from 278 to 360 degreesC. In the fully N-substituted PEOPUs, the behavior of the thermal decomposition of the PEOPU that was processed in two stages was changed to one-step decomposition in the temperature range of 360-440 degreesC. The Tg was shifted to a lower temperature with an increasing length of the substituent in N-sub PEOPU. Improvement of the thermal stability by N-substitution was more significant in N-alkyl PEOPU than in N-ethoxylate PEOPU.