TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The organic solvent is polar organic solvent. The acid is carboxylic acid such as monocarboxylic acid or dicarboxylic acid, or sulfonic acid. The monocarboxylic acid is benzoic acid, hydroxybenzoic acid, methylbenzoic acid, or aminobenzoic acid. The dicarboxylic acid is phthalic acid, isophthalic acid, or terephthalic acid. The sulfonic acid is aminosulfonic acid such as taurine or sulfanilic acid. Preferred Process: The flame resistant polymer is obtained by heating acrylonitrile series polymer as precursor polymer, in polar organic solvent in presence of acid, acid anhydride or acid chloride. Preferred Composition: The total additional amount of acid, acid anhydride and acid chloride is 0.05-7 weight parts with respect to 10 weight parts of acrylonitrile series polymer. Preferred Use: A flame resistant fiber is formed by shaping dispersion containing flame resistant polymer. A carbon fiber is formed by carbonizing flame resistant fiber. EXAMPLE - Acrylonitrile homopolymer (in wt.parts) (10), ortho-nitro toluene (8), benzoic acid (3), and monoethanolamine (3.5) were dispersed in dimethylsulfoxide (75.5), reaction mixture was heated at 150 degrees C for 8 hours, cooled to 30 degrees C, and obtained flame resistant polymer was dispersed in dimethyl sulfonyl, to obtain dispersion containing flame resistant polymer. A film of thickness of 141 mu m was formed using dispersion containing flame resistant polymer which had tensile strength of 3.15 MPa per cross-sectional area. A flame resistant fiber was formed using dispersion containing flame resistant polymer which had excellent fineness of 1 dtex, intensity of 2.3 g/dtex, elongation of 18%, specific gravity of 1.3, and excellent flame resistance and thread-breaking resistance. A carbon fiber was formed by carbonization of flame resistant fiber at 300-1400 degrees C under nitrogen atmosphere. The obtained carbon fiber had intensity of 3600 MPa, elasticity modulus of 230 GPa, and specific gravity of 1.78.