Two types of polyamic acid (PAA) precursors were synthesized via combining 3,3',4,4'-Oxydiphthalicdianhydride (ODPA) with 4,4'-thioetherdianiline (TDA) and 4,4'-sulfonyldianiline (SDA), respectively. Thermal gravity analysis (TGA), rigid-body pendulum rheometer experiment (RPR), and Fourier transform infrared (FTIR) spectroscopy were combined to analyze the thermal imidization processes from precursors PAA to polyimides (PI). This procedure provides the new PAA-PI system with new methods for observing the imidization behavior, thermal properties, and finding the optimization of processing conditions. The results of the TGA and FTIR imidization conversion analysis revealed that the PAA-PI systems synthesized with identical dianhydride and different diamines have distinct reaction temperature regions and reaction speed. The inductive effect of withdrawing and releasing group and the rigidity-flexibility of the molecular chain in the structure of molecular backbone chain influence the imidization reaction. We have seen from the RPR test results that the entire imidization reaction in every isothermal testing temperature reaches an equilibrium condition and completes the gradation reaction of each isothermal temperature within 20 min. Glass transition temperatures (T-g) of PIs increase with increasing the conversion rate from PAA into PI and the rigidity of molecular chain. (C) 2008 Wiley Periodicals, Inc.