Several, novel polyimide-polyurethane-urea-imide (PI-PUI) copolymers with different polyimide (PI) contents were prepared by an in situ interpenetrating reaction between polyurethane-urea-imide (PUI) precursor and poly(amide acid). Changing the ratio of the PUI precursor and poly(amide acid) resulted in a change of the properties of the PI-PUT copolymers from plastic to elastomer. Fourier transform infrared spectra revealed the characteristic absorption bands of PI-PUI copolymers; differential scanning calorimetry (DSC) analyses revealed that the glass-transition temperature of the soft-segment-rich phase decreased as the PI content increased in PI-PUIs, meaning that the soft segments (i.e., PEG) were more incompatible and had better crystallizability with a high-PI-segment content in PI-PUI. Wide-angle X-ray diffraction curves exhibited more ordered structure within the disordered PI-PUI copolymer's state with an increasing PI content, which further agreed with the DSC results. Scanning electron micrographs clearly showed that the molecular chains in PI-PUI with a high-PI-content packed, ordered lamellar structure. Thermogravimetric curves indicated that the heat resistance of PI-PUI was better than pure PUI. The introduction of the PI component into PUI by an in situ interpenetrating reaction method is an effective way to improve the thermal stability and solvent resistance of PUI. (C) 2003 Wiley Periodicals, Inc.