New unsymmetrical diamine monomer containing both the benzimidazole ring and trifluoromethyl group, 6,4'-diamino-2'-trifluoromethyl-2-phenylbenzimidazole, was prepared from 2-bromo-5-nitro-benzotrifluoride and 4-nitro-1,2-phenylenediamine. The monomer was polymerized with ODPA, BTDA, and 6FDA by using one-pot synthetic method to obtain corresponding polyimides. All the prepared polyimides were soluble in aprotic polar solvents. Incorporation of trifluoromethyl groups unsymmetrically in the rigid polyimides improves their solubility without decreasing their physical properties. The polymers showed high glass transition temperature (T-g=289-352 degrees C), high thermal stability (Td(10) > 500 degrees C), and relatively low coefficient of thermal expansion (CTE = 26.1-46.4 ppm/degrees C) because of their rigid-rod like structure. Also, they showed low refractive indexes (n = 1.46-1.68) and low birefringence (Delta approximate to 0.02) due to the trifluoromethyl pendent groups that interrupt chain packing and increase free volume. (c) 2008 Elsevier Ltd. All rights reserved.