Synthesis of high molecular weight polybenzoxazoles in polyphosphoric acid and investigation of their hydrolytic stability under acidic conditions

Studies on the synthesis of high molecular weight polybenzoxazoles (PBOs) in polyphosphoric acid (PPA) and investigations of their hydrolytic stability in aqueous methanesulfonic acid are described. The polymerization of PBOs based upon 2,2′-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (6FAP) and 4,4′-oxydibenzoic acid (ODBA) was investigated as a function of the percentage of P₂O₅ content in PPA, polymerization temperature, percentage monomer (total weight of monomers/weight of monomers plus PPA), and reaction time. It was found that the percentage of P ₂O₅ content was the most important polymerization variable and that PPA with more than 85% P₂O₅ content was required to achieve high molecular weight PBOs. Hydrolytic stability of PBOs prepared from selecting 6FAP and either ODBA, isophthalic acid (IA), or terephthalic acid (TA) was investigated by solution viscosity measurements, ¹HNMR and FTIR spectroscopy. The hydrolysis experiments were conducted in aqueous methanesulfonic acid solutions at room temperature, 50, and 80°C. The 6FAP/ODBA PBO showed improved stability over time with respect to the 6FAP-IA and 6FAP-TA PBOs, which may be due to the decreased electrophilicity of the carbon atom of the oxazole ring. But, the inherent viscosities of all the PBOs decreased over time and ¹H-NMR and FTIR spectroscopic evidence for chain scission was obtained. Benzoxazole model compounds, 2,2′-Bis(2- phenylbenzoxazol-6-yl)hexafluoropropane, 2,2′-bis[2-(4-phenoxyphenyl) benzoxazol-6-yl]hexafluoropropane, were synthesized from selecting 6FAP and ether 4-phenoxybenzoic acid or benzoic acid in N-cyclohexylpyrrolidinone. Nuclear magnetic resonance spectroscopic data provided direct evidence of hydrolysis of oxazole functional groups.