A novel side-chain-sulfonated aromatic diamine of bis[4-(4-aminophenoxy)-2-(3-sulfobenzoyl)lphenyl sulfone (BAPSBPS) was synthesized. Sulfonated copolyimides were synthesized by random and sequenced block copolymerization of 1,4,5,8-naphthalene tetracarboxylic dianhydride, BAPSBPS and nonsulfonated diamine. They displayed good solubility in common aprotic solvents and high desulfonation temperature of 350 degrees C, suggesting the high stability of sulfonic acid groups. The reduced viscosity was in the range of 0.4-1.8 dl/g at 0.5 g/dl and 35 degrees C. Flexible and tough membranes with reasonably high mechanical strength were prepared. They showed anisotropic membrane swelling with larger swelling in thickness than in plane. They displayed reasonably high proton conductivity (or), taking their lower ion exchanging capacity (IEC) into account. For example, the membrane with IEC of 1.54 mequiv/g showed or values of 81 and 11 mS/cm in water and 70% RH, respectively, at 60 degrees C. (c) 2006 Elsevier Ltd. All rights reserved.