Nonstoichiometric synthesis of poly(4,4 '-oxydiphenylene pyromelliteimide) was examined by using reaction-induced crystallization of oligomers during solution polymerization. Polymerizations of pyromellitic dianhydride (PMDA) and 4,4 '-oxydianiline (ODA) were carried out in Barrel Therm 400 at 330 degrees C at various molar ratios of PMDA and ODA in feed Q). Although high molecular weight polymer was usually prepared only under stoichiometric condition by a conventional solution polymerization, it was obtained as the crystals at chi of 0.5-2.0. The polymerization at chi of 0.77 afforded the highest polymer having M-w of 2.81 X 10(5), which was higher than that prepared by the solution polymerization. The oligomers were precipitated to form the crystals due to the low miscibility into the solvent. The precipitated oligomers were end-capped by excess monomers, but the end-capping groups were eliminated by transimidization reaction when they were registered into the crystals. The transimidization reaction also occurred in the crystals, leading to the increase in molecular weight. Since the nucleophilicity of amino end group was higher, the transimidization reaction proceeded more efficiently with eliminating ODA, and thereby the higher molecular weight polyimides were given in the range of chi < 1.0 than the range of chi > 1.0. The polymerization by the reaction-induced crystallization of oligomers was unsusceptible to the stoichiometry.