Poly(amide imide) copolymers were synthesized with different molar ratios of 4,4-diphenylmethane diisocyanate, two types of aromatic dianhydrides (pyromellitic dianhydride (PMDA) and 3,3',4,4'-sulfonyl diphthalic anhydride (DSDA)), and a diacid, which was derived from 3,3'-dinitrobenzidine and isophthaloyl chloride in a previous work. In this study, the copolymers were further reacted with a reducing agent, and the nitro groups in the copolymers were hydrogenated into amine groups. Then, the amine-group-containing poly(amide imide) copolymers were cyclized at 180degreesC to form the poly(benzimidazole imide amide) copolymers in poly(phosphoric acid), which acted as a cyclizing agent. The resultant copolymers were soluble in sulfuric acid and poly(phosphoric acid) at room temperature and in sulfolane or N-methyl-2-pyrrolidone under heating to 100degreesC with 5% lithium chloride. According to wide-angle X-ray diffraction, all the copolymers were amorphous. According to thermal analysis, the glass-transition temperature ranged from 270 to 322degreesC, and the 10% weight-loss temperature ranged from 460 to 541degreesC in nitrogen and from 441 to 529degreesC in air. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 378-386, 2004.