Thermal Conversion of Hydroxy-Containing Imides to Benzoxazoles:  Polymer and Model Compound Study

Abstract

A series of hydroxy-containing polyimides, containing pendent hydroxyl groups 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 °C under nitrogen or vacuum. The thermal conversion was accompanied by loss of carbon dioxide. No other volatile byproducts were detected by IR or NMR. Structures were confirmed by IR, solid-state NMR, and elemental analysis. Polybenzoxazole films, obtained by the thermolysis of solvent-cast poly(amic acid) or polyimide solutions, displayed excellent solvent resistance and good thermal stability. Insolubility of the polybenzoxazoles in all solvents tested indicates possible cross-linking. No crystallinity was observed by X-ray diffraction. Due to the insolubility of the resulting polybenzoxazoles, model compound reactions were also investigated. In the vapor phase at 400 °C, N-(2-hydroxyphenyl)phthalimide underwent intramolecular thermal conversion to 2-phenylbenzoxazole in 83% yield, emulating the polyimide reactions. Thermal conversion of N-(2-hydroxyphenyl)phthalimide in the melt at 400 °C resulted in the formation of intermolecular reaction products. The intermolecular reactions may be a source of possible cross-links, which would explain the insolubility of the polybenzoxazoles.

Two-step thermal conversion from poly(amic acid) to polybenzoxazole via polyimide: Their thermal and mechanical properties

“…At 550°C, PBO (IV) was prepared by the thermal conversion of PI (II) via the PBO precursor (III). 19,20 Characterization Elemental analyses were performed by the Analytical Laboratory of the Korea Institute of Chemical Technology; see Table I. The values in parentheses are those calculated for the chemical formulas.…”

“…Mathias et al 19,20 reported that a series of hydroxy-containing PIs, containing pendant hydroxyl groups ortho to the heterocyclic imide nitrogen, were prepared by solution condensation of aromatic dianhydrides with bisaminophenols. PBOs were synthesized from PIs with a carboxy-benzoxazole intermediate.…”

Smart Synthesis of High-Performance Thermosets Based on ortho-Amide–Imide Functional Benzoxazines

“…Aromatic PAIs possess balanced characteristics between polyamides and polyimides such as high thermal stability and good mechanical properties together with ease of processability (Imai, 1996). Besides, Mathias and others reported the possible thermal conversion of orthohydroxypolyimides into PBOs with loss of carbon dioxide at temperature around 400°C (Tullos et al, 1999;Park et al, 2007). The above features inspired the idea to incorporate amide and imide groups into one benzoxazine compound, which can be developed as a precursor for PBOs with better processability.…”

Gas separation membranes made through thermal rearrangement of ortho-methoxypolyimides

“…The generally accepted mechanism for the imide-to-polybenzoxazole thermal rearrangement involves the decarboxylation of the ortho-hydroxypolyimide at very high temperature (well over 350 °C) through the previous formation of a carboxybenzoxazole, with further elimination of a CO2 molecule [24]. Additionally, other functionalized polyimides have been studied as precursors, particularly polyimides containing ortho-ester groups [25].…”