Gl. Tullos et al., Thermal conversion of hydroxy-containing imides to benzoxazoles: Polymer and model compound study, MACROMOLEC, 32(11), 1999, pp. 3598-3612
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.