| Abstract: | Four structurally different bisimidobenzoxazole monomers were synthesized, based on the reaction of two isomeric diaminobenzoxazoles, viz., 5-amino-2-(p-aminophenyl) benzoxazole and 5-amino-2-(m-aminophenyl) benzoxazole with maleic and citraconic anhydrides. The diamines were synthesized by a new route. The imides and the amic intermediates were characterized by elemental analysis, IR, NMR, and mass spectra. The imides could be thermally polymerized to crosslinked brittle polybisimidobenzoxazoles. The citraconimides polymerized at a lower temperature than the maleimide. Thermal stability of the cured resins was evaluated by TGA and was correlated to the structure of the polyimide. The polybismaleimidobenzoxazoles were stable up to about 500°C in N2, leaving 50–60% anaerobic char yield at 800°C, whereas polybiscitraconimidobenzoxazoles were stable up to 420°C. Comparison of the thermal behavior of similar polyimides based on oxydianiline revealed that incorporation of benzoxazole structure enhances the decomposition temperature, lowers the rate of decomposition, and enhances the anaerobic char yield at high temperature. Addition of diamines as chain-extending agents decreased the thermal stability of the resins without any change in the anaerobic char yield. |
