A new synthesis route was developed to produce the diamine monomer containing benzimidazole units, 6,4-diamino-2-phenylbenzimidazole (BIA), which was easily prepared by the reduction of 6,4-dinitro-2-phenylbenzimidazole synthesized via a reaction between 4-nitro-1,2-phenylenediamine and 4-nitrobenzoyl chloride. The composition and structure of the products were confirmed by Fourier transform infrared, NMR, and elemental analysis. A series of polyimides were prepared by the polycondensation of BIA and various dehydrates, pyromellitic dianhydride, 3,3,4,4-biphenyl tetracarboxylic dianhydride, 4,4-oxydiphthalic anhydride, and benzophenone tetracarboxylic dianhydride via a traditional two-step procedure. The polyimides exhibit excellent thermal stability up to 540 degrees C and high glass transition temperature (Tg) in the range of 382409 degrees C. The polyimide films are tough and flexible and possess good mechanical properties of tensile strength of 222232 MPa and modulus of 3.15.6 GPa without stretching. The polyimides are amorphous state in wide-angle X-ray diffraction patterns, whereas small-angle X-ray diffraction patterns show the existence of molecular aggregation. The rigid-rod structure and intermacromolecular hydrogen bond are likely responsible for the excellent properties of the polyimides. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013