To extend the upper operating temperature range of melt processable PIs, two series of end-capped copolyimides based on hydroquinone diphthalic anhydride (HQDPA) isomers were designed by varying the contents of appropriate diamines. Thus, copolyimides PI-(0a similar to 20a) based on 3,3'-HQDPA were prepared by varying the ratio of rigid 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) and flexible 4,4'-oxydianiline (ODA). The resulting copolyimides demonstrated good properties, such as excellent solubility, high T-g ranging from 328 to 354 degrees C as determined by dynamic mechanical analyzer (DMA), besides, exhibited good thermal stability based on 5% weight loss temperature (T-5%) of 502-514 degrees C and low melt viscosity in the range of 291-935 Pa.s. Most importantly, PI-20a showed a good combination of low melt viscosity simultaneously with significantly high T-g (328 degrees C). Furthermore, this system was modified by copolymerization with different feed ratio of 4,4'-HQDPA isomer and produced another series PI-(20a similar to 20e), that exhibited high T-g in the range of 315-330 degrees C with T-5% of 502-543 degrees C and minimum complex viscosity in the range of 291-479 Pa.s. All PIs form tough and flexible films that have remarkable tensile strength and moduli in the range of 93-116 MPa and 3.7-4.7 GPa, respectively. (C) 2017 Elsevier Ltd. All rights reserved.