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.

Poly(p‐phenylenebenzobisoxazole) (PBO) fiber with polyphenylene sulfide (PPS) pendent groups was made to improve PBO fiber compressive strength by crosslinking. PPS moieties allowed the polymeric network to crosslink at heat‐treatment temperatures at which PBO does not thermally degrade. PBO‐PPS fiber heat‐treated for 30 s at 600°C did not dissolve or break up in methanesulfonic acid. Compressive strength of crosslinked fiber was about 20% better than that of unmodified PBO fiber. In another experiment, 10 mol % of 2,5‐diphenylsulfideterephthalic acid was incorporated into PBO fiber. The side chain of one phenyl sulfide unit was too short to enhance crosslinking, and the fiber had about the same compressive strength as unmodified PBO fiber. © 1995 John Wiley & Sons, Inc.

SynopsisIt is shown that the 13C NMR spectral collapse temperatures T, reported by Axelson and Mandelkern tend to give a constant ratio of T,/T, averaging 1.21 f 0.05 and independent of T, or of polymer structure. It is further shown that T, is not a high-frequency value for T, because this would require TJT, to decline with increasing T,. T,/T, agrees in numerical value with T[l/T,, where Ti/ is the liquid-liquid transition lying above T,. Direct comparison of T, and Tu for four polymers PIB, PnBA, atactic PP, and isotactic PMMA shows very close agreement. The various results suggest, but do not prove, that T, from NMR spectroscopy may be a new, direct measure for TI[. A measured T, of 233K for linear P E is compatible with a Tg near 195 K (233/195 = 1.191, whereas a Tg of 148 K gives the ratio 233/148 = 1.57, which is outside any value shown in tabulated form.frequency of lo3 Hz. Table I assembles the data and these calculations.The most surprising result is the tendency of T,/Tg to be relatively constant from polymer to polymer with an average value of 1.21 (glycerol omitted) and a range from 1.16 to 1.30.The differences (again omitting glycerol) are more erratic at first glance. The

High‐resolution nuclear magnetic resonance spectroscopy was applied to the study of the structure of carbohydrates in deuterium oxide solution. The structural problems considered were the configuration of anomeric protons, the conformation of pyranoside rings, the preferred ring form, the existence and form of complexes, and the mutarotation of carbohydrates in solution. A modification of the Karplus equation was applied to the calculation of the dihedral angles between the C1 and C2 carbon–hydrogen bonds for a number of carbohydrates. A harmonic oscillation of ±15° about the equilibrium dihedral angle was assumed in these calculations. The preferred conformations of several carbohydrates were deduced from the dihedral angles, and an analysis of the diamagnetic susceptibility anisotropy was applied to the predicted conformations in two cases. Observed and calculated shielding differences were determined for each of the ring protons in β‐glucose and β‐mannose. The good agreement obtained from this treatment appears to support the proposed distorted chair and half‐chair conformations for β‐glucose and β‐mannose respectively. Conclusions reached from consideration of NMR spectra were applied to discussions of the stability and structure of several borate complexes and of the preferred ring size of xylose in solution.