A highly thermally and temporally stable, second-order, nonlinear-optical polyimide with an X-type chromophore (2-{4-[4,5-di(4-nitrophenyl)imidazolyl]phenyl}-4,5-di(4-aminophenyl)imid azole) was synthesized by the Michael addition reaction. The structure of the prepolymer was characterized with Fourier transform infrared (FTIR). Its thermal stability and curing behavior were studied with differential scanning calorimetry, thermogravimetric analysis, and FTIR. After the Curing, the glass-transition temperature of the polyimide greatly increased. The FTIR results suggested that addition and/or crosslinking reactions were carried Out during the thermal Curing of the prepolymer. A poling polyimide film was achieved, and the thermal and temporal stability of the poling-induced orientation were evaluated with a multistep corona-poling technique at an elevated temperature and with in situ second harmonic generation (SHG) measurements. The temporal orientation of the poled polyimide film was ox,er 1000 h at 150 degrees C; it retained 85% of the initial second-order, nonlinear-optical coefficient. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 3758-3762, 2008