Thermal conversion of hydroxy-containing imides to benzoxazoles: Polymer and model compound study

A series of hydroxy-containing polyimides, containing pendent hydroxyl grou ps ortho to the heterocyclic imide nitrogen, were prepared via the solution condensation of aromatic dianhydrides with bisaminophenols. The polyimides were found to undergo thermal conversion to polybenzoxazoles upon heating between 350 and 500 degrees C under nitrogen or vacuum. The thermal convers ion was accompanied by loss of carbon dioxide. No other volatile byproducts were detected by IR or NMR. Structures were confirmed by IR, solid-state N MR, and elemental analysis. Polybenzoxazole films, obtained by the thermoly sis of solvent-cast poly(amic acid) or polyimide solutions, displayed excel lent solvent resistance and good thermal stability. Insolubility of the pol ybenzoxazoles in all solvents tested indicates possible cross-linking. No c rystallinity was observed by X-ray diffraction. Due to the insolubility of the resulting polybenzoxazoles, model compound reactions were also investig ated. In the vapor phase at 400 degrees C, N-(2-hydroxyphenyl)phthalimide u nderwent intramolecular thermal conversion to 2-phenylbenzoxazole in 83% yi eld, emulating the polyimide reactions. Thermal conversion of N-(2-hydroxyp henyl)phthalimide in the melt at 400 degrees C resulted in the formation of intermolecular reaction products. The intermolecular reactions may be a so urce of possible cross-links, which would explain the insolubility of the p olybenzoxazoles.