Phosphorus-containing polyurethanes (PU-P) based on bisphenol-AF were synthesized by N-alkylation in a two-step process. First, the polymer was metalated with sodium hydride; then the prepared urethane polyanion was treated with diethyl 2-bromoethyl phosphate. IR spectra exhibited characteristic absorptions at 1303 cm(-1) (P=O stretch), 1015 cm(-1) [GRAPHICS] (CH stretch), 1646 cm(-1) (C=O stretch), 1231 cm(-1) (CNH), and 1053 cm(-1) (CO-O-C stretch). In the H-1-NMR spectrum of the maximum substituted PU-P, the signal of the urethane proton (NH-CO-O) at 9.87 ppm virtually disappeared as expected; new signals appeared at 1.20-1.24 ppm (POCH2CH3) and 3.79 ppm (POCH2CH3). Physical and thermal properties of the N-alkylated polymer were also investigated with differential scanning calorimetry (DSC), solubility, X-ray diffraction, thermogravimetric analysis (TGA), limiting oxygen index (LOI), and reduced viscosity. DSC analysis showed that T-g values were de creased from 81 to 43 degrees C. The reduced viscosities of the PU-Ps were observed in the range of 0.23-0.18 dL g(-1). The results of TGA revealed that the thermal stability was decreased, because the phosphorus moiety in the PUs is readily dissociated thermally. The PU-Ps exhibited enhanced fire resistance because phosphorus simultaneously promoted carbonization of the polymer and inhibited combustion. The X-ray diffraction patterns of the PU-Ps showed that with increasing phosphorus content, the degree of crystallinity of the PUs decreased. N-alkylated PUs are soluble, not only in polar solvents such as N,N-dimethylformamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, and H2SO4, but also in less polar solvents such as phenol, toluene, THF, and trichloroethylene.