Several benzoxazole and benzothiazole compounds have been prepared and their extended configurations characterized by optical absorption and emission spectroscopy. In general, solutions of these compounds fluoresce strongly and exhibit emission spectral profiles which mirror their respective excitation spectra. One exception to this correlation results from a chromophore with a nonplanar ground state configuration which disrupts the extended π-network, promoting a strong hypsochromic shift of the absorption spectrum. The absorption, excitation, and emission spectra of these compounds also show a strong vibronic progression of ∼1300 cm-1 in accordance with the energy of ring-stretching modes for aromatic frameworks. This excited state molecular distortion is consistent with the ππ* nature of the optical excitation. Also, the energy gap between excitation and emission 0−0 bands of these benzoxazole and benzothiazole compounds and their polymeric forms are strongly influenced by the minimum allowed intermolecular space. In dilute solutions or for structures with bulky substituents, only small energy differences are observed between excitation and fluorescence 0−0 bands. In contrast, solid state samples devoid of side groups exhibit significantly larger energetic displacements accompanied by a pronounced broadening of both excitation and emission spectral profiles. These results suggest that strong intermolecular π-stacking interactions occur for the planar benzoxazoles and benzothiazoles in the solid state. Excited state lifetime decay measurements for PBO model compounds in toluene are monoexponential with essentially identical lifetimes under evacuated and standard pressure conditions. In the solid state, PBO and PBT model compounds exhibit biexponential luminescence decay lifetimes which were also not significantly affected by the presence of O2. Fibers of PBO and PBT revealed three oxygen independent, but wavelength dependent emitting species. The presence of only one emitting species for these benzoxazole compounds in solution, compared with their multiexponential lifetime behavior in the solid state, further supports strong π-interactions between these molecules in the solid state. This molecular configuration permits benzoxazole and benzothiazole compounds to undergo photoinduced electron transfer in the solid state, which in the presence of oxygen leads to the generation of superoxide.

The mechanism of 2-phenylbenzoxazole formation from benzoic acid and o-aminophenol in polyphosphoric acid (PPA) is studied by NMR spectroscopy and chemical analysis. Benzoic acid reacts with PPA to form benzoic-phosphoric anhydride and benzoic-polyphosphoric anhydride. The ratio of mixed anhydride to free carboxylic acid increases dramatically as the P 2 O 5 content of PPA increases, but this ratio is independent of reaction temperature and time. When o-aminophenol dissolves in PPA, part of the hydroxyl group is converted to phosphate ester, and only protonated amine is detected. Benzoic acid, mixed anhydride, and PPA are in dynamic equilibrium, and so are PPA, o-aminophenol, and its phosphate ester. The mixed anhydride and o-aminophenol react to form 2-aminophenyl benzoate as the first reaction intermediate which undergoes rapid acyl migration to generate 2-hydroxybenzanilide. Ring closure of 2-hydroxybenzanilide to form 2-phenylbenzoxazole is acid catalyzed. The reactive components in phosphorus pentoxidemethanesulfonic acid (P 2 O 5 -MSA) which is a convenient alternative to PPA are very similar to those present in PPA. Benzoic acid is also converted into mixed anhydride in P 2 O 5 -MSA.

The mechanism of poly(p-phenylenebenzoxazole) formation from terephthalic acid (TA) and diaminodihydroxybenzene dihydrochloride (DADHB) in polyphosphoric acid (PPA) was studied. The solubility of TA in PPA with 86% P2O5 content was determined to be 0.02% at 100 °C and 0.06% at 140 °C. Dissolved TA existed as three species, TA, α-(4-carboxybenzoyl)-ω-hydroxypoly(oxyphosphinico), and α,α‘-[1,4-phenylenebis(carbonyl)]bis[ω-hydroxypoly(oxyphosphinico)]. DADHB also reacts with PPA, and DADHB, α-(2,4-diamino-5-hydroxyphenyl)-ω-hydroxypoly(oxyphosphinico), and α,α‘-(4,6-diamino-1,3-phenylene)bis[(ω-hydroxypoly(oxyphosphinico)] were found. During the course of polymerization, conversion of DADHB was higher than that of TA. NMR, IR, and mass spectroscopy characterization of PBO oligomer and its product with benzoic-carboxy-13C acid suggested both chain ends were capped with DADHB, which is different from oligomers of conventional AA plus BB step-growth polymers. An unprecedented polymerization mechanism is proposed to account for this unusual oligomer structure and the remarkable fact that high molecular weight PBO was obtained even when 5% excess of one of the monomers, TA, was used. The reactivities of the two functional groups in DADHB toward carboxylic acid to form benzoxazole in PPA were different.

A new class of polyamides can be prepared using a solvent-free polymerization reaction. Bis-(imidazoles) undergo an addition reaction with aromatic polyisocyanates to form polyamides. Since this reaction does not produce any low molecular weight volatile molecules, the reaction is suitable for the synthesis of thermoset polymers. The polyimidazole amides produced using this route have good thermal stability and mechanical properties. The toughness as measured by Izod impact was particularly high for thermoset polymers.