Some thermally stable second-order nonlinear optical (NLO) polyimides were synthesized. The polyimides were prepared by the ring-opening polyaddition of 4,4'-(hexafluoroisopropylidene) diphthalic anhydride and pyromellitic dianhydride with two aromatic azodiamine derivatives as the NLO chromophores. These chromophores, based on a nitro group connected with azobenzene as the acceptor end of a donor-pi-bridge-acceptor chromophore and a diamine group as the donor end, had specific chemical stability. On the basis of ZERNER'S INDO methods, according to the sum-over-states formula, a program for the calculation of nonlinear second-order optical susceptibilities was devised. The resulting polyimides had high number-average and weight-average molecular weights of up to 26,000 and 53,500, respectively, and a large glass-transition temperature of 248 degreesC. With an in situ poling and temperature ramping technique, the optimal temperatures (T-opt's) for corona poling were obtained for the largest second-order NLO response. The electrooptic coefficient (gamma(33)) of a polyimide at a wavelength of 830 nm was up to 21 pm/V after corona poling under its Top, and the value remained at elevated temperatures (>90.6% was retained at 240 degreesC for >120 h). The thermal stability of the NLO polyimides was studied with UV spectrometry after poling of the films.