Synthesis of aromatic polythioamide-oxothioxoquinazolines from bis(1,3,4-thiadiazoline-5-thione) and aromatic bis-o-amino esters

Aromatic polythioamide-oxothioxoquinazolines were synthesized by the polycondensation of 2,2′-(m-phenylene)bis-1,3,4-thiadiazoline-5-thione with aromatic bis-o-amino esters. The polymerizations were carried out at 160°C in acidic media such as m-cresol, sulfolane, and polyphosphoric acid to produce polymers with reduced viscosities up to 0.5 dL/g. These polymers were soluble in polar aprotic solvents like N-methyl-2-pyrrolidone and some acidic media including m-cresol. The polythioamide-oxothioxoquinazolines showed relatively good thermal stability with 10% weight loss at 344–394°C in air.     

Chemistry of thienopyridines. XXXIII. Synthetic routes to 5- and 7-substituted thieno[3,2-b]pyridines from the N-oxide

Thieno[3,2-b]pyridine ( 1 ) is oxidized to N-oxide 1a by means of m-chloroperoxybenzoic acid (83%). Compound 1a forms adducts with hydrogen chloride and picric acid and gives ring substitution alpha or gamma to the heteronitrogen atom. Thus, 1a plus nitric and sulfuric acids produces the 7-nitro-N-oxide 1m (63%), or plus phosphorus oxychloride gives a mixture of 5-chloro and 7-chloro ( 1j ) derivatives of 1 . Compound 1m is convertible into a variety of other derivatives of 1 , viz. 7-chloro-N-oxide, 1j , 7-bromo-N-oxide, 7-nitro and 7-amino. 5-Cyano- 1 , formed from 1a , is, in turn, transformed into a methyl imidate (93%), cyclic amidines, and a 5-tetrazolyl- 1 (91%). These results confirm the prediction that 1a , thieno[2,3-b]pyridine-4-oxide and quinoline 1-oxide should exhibit closely similar (i.e. analogous) chemical reactions.     

Alkylated polypyrazines

The syntheses of four new monomers and two new polyaromatic pyrazines are described. The monomers; bis-p,p′-(octanoyl)diphenyl ether (Ia), bis-p,p′-(hexadecanoyl)diphenyl ether (Ib), bis-p,p′-(α-bromooctanoyl)diphenyl ether (IIa), and bis-p,p′-(α-bromohexadecanoyl)diphenyl ether (IIb), were produced by Friedel-Crafts acylation of diphenyl ether with the corresponding acyl chloride and subsequent α-bromination. Prepolymers were synthesized by the condensation of (IIa) and (IIb) with ammonia in N,N-dimethylformamide (DMF), and polymers were prepared by subsequent melt condensation of the prepolymer to produce poly[2,5-(oxydiphenylene)-3,6-(dihexyl)pyrazine] (IIIa), and poly[2,5-(oxydiphenylene)-3,6-(ditetradecyl)pyrazine] (IIIb). Polymer IIIa was thermally (stable at >400°C while polymer IIIb was a tacky substance). The inherent viscosity of IIIa produced by 12 hr of melt condensation was 0.30 dl/g in formic acid. Additional heating in excess of 24 hr gave a slightly soluble polymer. The inherent viscosity of IIIb produced by 40 hr of melt condensation was 0.37 dl/g in formic acid.     

Polyaromatic pyrazines: Synthesis and thermogravimetric analysis

The syntheses of five polyaromatic pyrazine polymers are described. These polymers were synthesized by the condensation of bis-α-haloaromatic ketones with ammonia in N,N-dimethylacetamide (DMAc) solvent in the presence of air or peroxides. The condensation of bis-p-(α-bromoacetyl)benzene (IIIa), bis-p,p′-(α-chloroacetyl)biphenyl (IIIb) bis-p,p′-(α-chloroacetyl)diphenyl ether (IIIc), bis-p,p′-(α-chloroacetyl)diphenylmethane (IIId), and α,α′-dibenzoyl-α,α′-dibromo-p-xylene (V) under these reaction conditions gave poly[2,5-(1,4-phenylene)pyrazine] (IVa), poly[2,5-(4,4′-biphenylene)-pyrazine] (IVb), poly[2,5-(4,4′-oxydiphenylene)pyrazine] (IVc), poly[2,5-(4,4′-methylenediphenylene)pyrazine] (IVd), and poly[2,5-(1,4-phenylene)-3,6-diphenylpyrazine] (VI), respectively. Thermogravimetric analysis (TGA) of these polymers showed them to be thermally stable up to the temperature range of 450–550°C in air for short periods of time. The inherent viscosities of these polymers ranged from 0.18 to 1.30.     

Synthesis and characterization of thianthrene-containing poly(benzoxazole)s

New thioether- and thianthrene-containing poly(benzoxazole)s (PBOs) were synthesized from 4,4′-thiobis[3-chlorobenzoic acid] and thianthrene-2,7- and -2,8-dicarbonyl chlorides with commercially available bis-o-aminophenols. Polymers were prepared via solution polycondensation in poly(phosphoric acid) at 90–200°C. Transparent PBO films were cast directly from polymerization mixtures or m-cresol. The films were flexible and tough. Non-fluorinated PBOs were soluble only in strong acids and AlCl₃/NO₂R systems by forming complexes with the benzoxazole heterocycle Glass transition temperatures ranged from 298–450°C, and thermogravimetric analysis showed good thermal stabilities in both air and nitrogen atmospheres. © 1995 John Wiley & Sons, Inc.     

Polyimides derived from 1,2-bis(4-aminophenoxy)propane

The diamine 1,2-bis(4-aminophenoxy)propane, containing the flexible 1,4-dioxa-2-methylbutyl-ene unit, was synthesized. Polyimidization was carried out with 5,5′-[2,2,2-trifluoro-(trifluoromethyl)ethylidine]bis-1,3-isobenzofurandione in m-cresol employing toluene as azeotroping agent to yield a polyimide that was soluble in a variety of solvents and had an inherent viscosity of 0.84 dL/g in N,N-dimethylformamide. Poly(amic acid) formation with pyromellitic dianhydride, 4,4′-carbonyldiphthalic anhydride, and 5,5′-[ethanediylbis(oxy)]-bis-1,3-isobenzofurandione was carried out in N,N-dimethylformamide with imidization completed by heating at 160°C for 24 h under vacuum. All of the polyimides exhibited a 5% weight loss in air and in helium by 420°C.     

Synthesis of an s-butyldibenz[a,h]acridine. Alkyl migration in the bernthsen reaction

The Bernthsen reaction between N-1-naphthyl-2-naphthylamine and 2-methylbutanoic acid and its anhydride at 200–230° for seven hours gives a low yield of 12- or 13-s-butyldibenz[a,h]acridine, instead of the expected 14-isomer. The parent molecule dibenz[a,h]acridine results from the same reaction conducted at 270° for thirteen hours. It is suggested that alkyl migration may have occurred in some other cases where the Bernthsen reaction was reported to yield 14-alkyldibenz[a,h]acridines.     

The insertion and extrusion of heterosulfur bridges. XV. S-bridging of 2,2′-binaphthyl and 1-(2-naphthyl)cyclohexene. Studies on hydrodehalogenation during the reaction

Regioselectivity occurs in the sulfur-bridging reactions of 2,2′-binaphthyl (1) and 1-(2-naphthyl)cyclohexene (7) by means of hydrogen sulfide and a chromia-alumina-magnesia catalyst (designated I) in a flow apparatus at 550°. Thus, 1 gives a higher yield (6.1%) of dinaphtho[1,2–6:2′,l'-d]thiophene from 1,1′-bridging than of dinaphtho[1,2-b:2′,3′-d]thiophene (3.4%) from l,3′-bridging. No product expected from 3,3′-bridging was identified. Substrate 7 undergoes both dehydrogenation and bridging to yield 2-phenylnaphthalene (8%), benzo[b]naphtho[2,1-d]thiophene (9%) from alpha bridging, and benzo[b]naphtho[2,3-d]thiophene (3%) from beta bridging into the naphthalene ring. Exploratory studies showed that either sulfided catalyst I or a sulfided molybdenum( VI ) oxide-alumina-cobalt( II ) oxide catalyst ( II ) effects hydrodehalogenation of various monohalo- and polyhaloarenes (where halo, X, is chloro or bromo) at 450–550°. In the biphenyl, phenanthrene, naphthalene, and pyrene systems, halogen was lost either under sulfur-bridging conditions or under hydrogenolysis conditions, i.e. with methanol as a reactant. For every substrate the parent arene was isolated or identified as a reaction product. In selected experiments, acid HX was also identified in the effluent. Use of hydrogen sulfide as a reactant led to formation of dibenzothiophene and phenanthro[4,5-bcd]thiophene as main products in the biphenyl and phenanthrene systems, respectively; while use of methanol as a reactant gave small amounts of methyl bromide (for X = Br) and methylarenes.     

A ‘One-Pot’ Anellation Method for the Transformation of Heptalene-4,5-dicarboxylates into Benzo[a]heptalenes

It has been found that dimethyl heptalene-4,5-dicarboxylates, when treated with 4 mol-equiv. of lithiated N,N-dialkylamino methyl sulfones or methyl phenyl sulfone, followed by 4 mol-equiv. of BuLi in THF in the temperature range of ?78 to 20°, give rise to the formation of 3-[(N,N-dialkylamino)sulfonyl]- or 3-(phenylsul-fonyl)benzo[a]heptalene-2,4-diols of. (cf. Scheme 4, and Tables 2 and 3). Accompanying products are 2,4-bis{[(N,N-dialkylamino)sulfonyl]methyl}- or 2,4-bis[(phenylsulfonyl)methyl]-4,10a-dihydro-3H-heptaleno[1,10-bc]furan-3-carboxylates as mixtures of diastereoisomers of. cf. Scheme 4, and (Tables 2 and 3) which are the result of a Michael addition reaction of the lithiated methyl sulfones at C(3) of the heptalene-4,5-dicarboxylates, followed by (sulfonyl)methylation of the methoxycarbonyl group at C(5) and cyclization of. (cf. Scheme 5). It is assumed that the benzo[a]heptalene formation is due to (sulfonyl)methylation of both methoxycarbonyl groups of the heptalene-4,5-dicarboxylates of. (cf. Schemes 6 and 8). The resulting bis-enolates 35 are deprotonated further. The thus formed tris-anions 36 can then cyclize to corresponding tris-anions 37 of cyclopenta[d]heptalenes which, after loss of N,N-dialkylamido sulfite or phenyl sulfinate, undergo a ring-enlargement reaction by 1,2-C migration finally leading to the observed benzo[a]heptalenes of. (cf. Schemes 8 and 9). The structures of the new product types have been finally established by X-ray crystal-structure analyses (cf. Figs. 1 and 2 as well as Exper. Part).     

Conducting polymers. I. Electrical conductivity of poly(bis-p-phenylenediaminosulphoxide)

Poly(bis-p-phenylenediaminosulphoxide) was prepared by Michael addition of p-bis-N-sulphinylphenylenediamine with p-phenylenediamine at 150°C. Thermal and electrical behaviors of the polymer have been studied. The polymer is found to have increased conductivity possibly due to the participation of lone pairs of electrons on nitrogen and sulphur atoms with σ bond of the macrochain. Thermogravimetric analysis indicates that the polymer is fairly stable than other conducting polymers up to 200°C. The activation energy of the polymer was measured and found to be 13 kcal mol^?1.     

CHLORINATION OF AROMATIC HALIDES WITH CHLOROSULFONIC ACID

Abstract

Chlorosulfonic acid–iodine mixture has been shown to chlorinate a number of aromatic halides under mild conditions. In reaction with p-dichlorobenzene, the maximum yield (82%) of hexachlorobenzene required 5 mol of chlorosulfonic acid and 2.5 mol of iodine. The yield of product increased with the amount of iodine present. A mechanism of chlorination is proposed involving iodine-catalysed homolytic decomposition of the intermediate sulfonyl chlorides followed by heterolytic chlorination by the evolved iodine monochloride.

The reaction of o-, m-, and p-dichlorobenzenes with chlorosulfonic acid has been investigated. o-Dichlorobenzene at 100° gave a good yield (85%) of 3,4,3′,4′-tetrachlorodiphenylsulfone although m and p-dichlorobenzenes gave only the expected sulfonyl chlorides. This difference arises from the lack of steric hindrance in the p-position of o-dichlorobenzene leading to facile sulfone formation.

This was confirmed by the observation that 3,4-dichlorobenzenesulfonyl chloride undergoes the Friedel–Crafts reaction with o-dichlorobenzene to give 3,4,3′,4′-tetrachlorodiphenylsulfone (60%), but m- and p-dichlorobenzenes did not give any appreciable amounts of the corresponding sulfones under identical conditions.     

Polyimides derived from bis-N-hydroxyimides. II. Synthesis and properties of polyimide-esters

Polycondensation of bis-N-hydroxyimides, N,N′ dihydroxypyromellitic diimide, and N,N′ -dihydroxybenzophenonetetracarboxylic diimide with dicarboxylic acid chlorides was carried out in dimethylacetamide in the presence of triethylamine to produce novel polyimide-esters. The resulting polymers had inherent viscosities up to 0.27 dl/g. These polyimide-esters and model compounds exhibited high reactivity toward nucleophiles such as aniline and n-butylamine, which brought about rapid reductions in the viscosity of the polymers. These polymers were fairly resistant to organic solvents but soluble in m-cresol. Thermal stability oft he polyimide-esters was evaluated by thermogravimetry and their good heat-resistant properties were confirmed.     

Syntheses and properties of organosoluble polyimides based on 5,5′‐bis[4‐(4‐aminophenoxy)phenyl]‐4, 7‐methanohexahydroindan

A series of organosoluble aromatic polyimides (PIs) was synthesized from 5,5′‐bis[4‐(4‐aminophenoxy)phenyl]‐4,7‐methanohexahydroindan (3) and commercial available aromatic dianhydrides such as 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA), 4,4′‐oxydiphthalic anhydride (ODPA), 4,4′‐sulfonyl diphthalic anhydride (SDPA), or 2,2′‐bis(3,4‐dicarboxyphenyl) hexafluoropropanic dianhydride (6FDA). PIs (III_c–f), which were synthesized by direct polymerization in m‐cresol, had inherent viscosities of 0.83–1.05 dL/g. These polymers could easily be dissolved in N,N′‐dimethylacetamide (DMAc), N‐methyl‐2‐pyrrolidone (NMP), N,N‐dimethylformamide (DMF), pyridine, m‐cresol, and dichloromethane. Whereas copolymerization was proceeded with equivalent molar ratios of pyromellitic dianhydride (PMDA)/6FDA, 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (BTDA)/6FDA, or BTDA/SDPA, or ½ for PMDA/SDPA, copolyimides (co‐PIs), derived from 3 and mixed dianhydrides, were soluble in NMP. All the soluble PIs could form transparent, flexible, and tough films, and they showed amorphous characteristics. These films had tensile strengths of 88–111 MPa, elongations at break of 5–10% and initial moduli of 2.01–2.67 GPa. The glass transition temperatures of these polymers were in the range of 252–311°C. Except for III_e, the 10% weight loss temperatures (T_d) of PIs were above 500°C, and the amount of carbonized residues of the PIs at 800°C in nitrogen atmosphere were above 50%. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1681–1691, 1999     

Polycondensed heterocycles. IV. synthesis of 1,4-dioxo-2,3,3a,4-tetrahydro-1H-pyrrolo[2,1-c][1,4]benzothiazine

A method for the synthesis of the title compound 3 consisted of an intramolecular cyclization in a stannic chloride catalyzed Friedel-Crafts reaction of N-(2-methylthiophenyl)-5-oxoproline chloride 10 , prepared by chlorination of the corresponding acid 9 obtained by hydrolysis of its ethyl ester 8 . Condensation of 2-methylthioaniline 4 with diethyl bromomalonate 5 afforded diethyl 2-methylthioanilinomalonate 6 which gave 8 either directly by reaction with ethyl acrylate or by alkylation with ethyl β-bromopropionate or ethyl acrylate and cyclization of resulting triethyl 2-(2-methylthio)anilino-2-carboxyglutarate 7 . This method was not convenient because of the poor yield of 3 (14%). On the other hand, cyclization of N-(2-mercaptophenyl)-5-oxoproline 14 with DCC and DMAP provided 3 in 45% yield. Oxidation with m-CPBA of the esters 11 and 8 , demethylation via the Pummerer rearrangement of the respective sulphoxides 12 and 17 with TFAA and oxidation with iodine of resulting N-(2-mercap-tophenyl)-5-oxoproline esters 13 and 18 gave the corresponding disulphides 16 and 19 . Hydrolysis of these latter compounds and reduction of the resulting bis[2-[2-(hydroxycarbonyl)-5-oxo-1-pyrrolidinyl]phenyl] disulphide 15 with sodium dithionite afforded the required 14 . Deprotection of t-butyl ester 13 with TFA at 55° to obtain 14 led to 3 in 42% yield. Finally the Pummerer rearrangement of N-(2-methylsulphinylphenyl)-5-oxo-proline 20 yielded the mixture of 14 and 15 .     

Chemistry of thienopyridines. XLII. Three novel compounds derived from thienopyridine N-oxides

Extension of the Reissert-Henze reaction to treatment of thieno[2,3-b]pyridine 7-oxide with potassium thiocyanate and benzoyl chloride in water-methylene chloride gives a 2% yield of bis(6-thieno[2,3-b]pyridyl) disulfide. Peroxidation of 5-ethylthieno[2,3-b]pyridine ( 4 ) forms the 7-oxide 5 (53%), converted to a monopicrate 5a . Picrate 5a undergoes N-deoxygenation to 4 -picrate on drying at 78° in vacuo, but shows the expected additive mass spectrum of 5 (thermally stable) and picric acid. Nucleophilic displacement of chloride ion from 7-chlorothieno[3,2-b]pyridine (derived, in turn, from thieno[3,2-b]pyridine 4 -oxide) by the anion from ethyl cyanoacetate gives 7-(1-cyano-1-ethoxycarbonyl)methylene-4,7-dihydrothieno[3,2-b]pyridine (82%), stable in this iminodienic tautomeric form.     

Synthesis,Crystal Structure and Thermal Decomposition Process of Complex [Sm(BA)3phen]2

A binuclear samarium(III) complex with benzoic acid and 1,10‐phenanthroline, [Sm(BA)₃phen]₂ was synthesized and characterized by elemental analysis, UV, IR and TG‐DTG techniques. The structure of the title complex was established by single crystal X‐ray diffraction. The crystal is triclinic, space group P1 with a = 10.8216(11) Å, b = 11.9129(13) Å, c = 12.425(2) Å, α = 105.007(2)°, β = 93.652(2)°, γ = 113.2630(10)°, Z = 1, D_c = 1.650 mg·m^?3, F(000) = 690. The carboxylate groups are bonded to the samarium ion in three modes: bidentate chelating, bidentate bridging, and tridentate chelating‐bridging. Each Sm³⁺ ion is coordinated to one bidentate chelating carboxylate group, two bidentate bridging and two tridentate chelating‐bridging carboxylate groups, as well as one 1,10‐phenanthroline molecule, forming a nine‐coordinate metal ion. Based on thermal analysis, the thermal decomposition process of [Sm(BA)₃phen]₂ has been derived.     

Organosoluble optically transparent poly(amide-imide)s based on 2,2-bis [<Emphasis Type="Italic">N</Emphasis>-(3-carboxyphenyl)phthalimidyl]- hexafluoropropane and 2,2-bis [<Emphasis Type="Italic">N</Emphasis>-(4-carboxyphenyl)phthalimidyl]- hexafluoropropane and various aromatic diamines

 Two diimide-dicarboxylic acids, 2,2-bis[N-(4-carboxyphenyl)phthalimidyl]hexafluoropropane (p-I) and 2,2,-bis[N-(3-carboxyphenyl)phthalimidyl]hexafluoropropane (m-I), were prepared by azeotropic condensation of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride and p-aminobenzoic acid or m-aminobenzoic acid at a 1:2 molar ratio in N,N-dimethylacetamide/toluene. Two series of organosoluble and colorless poly(amide–imide)s were synthesized from diimide–diacid p-I or m-I with ten kinds of aromatic diamines by direct polycondensation using triphenyl phosphite and pyridine as condensing agents. The thin films cast from N,N-dimethylacetamide were measured by UV–vis spectroscopy and Macbeth color-eye colorimetery, the cutoff wavelengths of almost all the films were below 400 nm (361–389 nm) and the values of the parameter b* were between 15.31 and 34.72; these polymers are much lighter in color than other analogous polymers. Almost all the polymer were soluble in N-methyl-2-pyrrolodone, N,N-dimethylacetamide, N,N-dimethylformamide and dimethyl sulfoxide, and some polymers could dissolve in less polar solvents, such as dioxane and tetrahydrofuran, etc. The cast films exhibited yield strengths of 95–131 MPa, tensile strengths ranging from 92 to 130 MPa, elongations at break from 9 to 27%, and initial moduli from 2.1 to 3.3 GPa. The poly(amide–imide)s had glass-transition temperatures between 259 and 328°C and 10%-weight-loss temperatures above 510 °C in nitrogen and air, indicating excellent thermal stability. Received: 25 April 2001 Accepted: 27 June 2001     

Nucleotides. Part LXXII

The synthesis of various N‐methylated nucleosides (m⁶A, m³C, m⁴C, m³U) is described. These minor nucleosides can be obtained by simple methylation with diazomethane of [2‐(4‐nitrophenyl)ethoxy]carbonyl(npeoc)‐protected nucleosides. These methylated compounds are easily further derivatized to fit into the scheme of the [2‐(dansyl)ethoxy]carbonyl (dnseoc) approach for RNA synthesis (dansyl=[5‐(dimethylamino)naphthalen‐1‐yl]sulfonyl). Various oligoribonucleotides containing N⁶‐methyladenosine were synthesized, underlining the usefulness of the dnseoc approach, especially for the synthesis of natural tRNA‐derived oligoribonucleotide sequences.     

Synthesis and properties of aromatic polyamides based on non-, methyl-, and phenyl-substituted 4,4′-bis(1,4-phenylenedioxy)dibenzoic acids

4,4′-(1,4-Phenylenedioxy)dibenzoic acid (3), 4,4′-(2,5-tolylenedioxy)dibenzoic acid (Me-3), and 4,4′-(2,5-biphenylenedioxy)dibenzoic acid (Ph-3) were prepared by the nucleophilic substitution reaction of p-fluorobenzonitrile with hydroquinone, methylhydroquinone, and phenylhydroquinone, respectively, followed by alkaline hydrolysis. Several aromatic polyamides having inherent viscosities of 0.66–1.34 dL/g were directly prepared by a Yamazaki phosphorylation polyamidation technique from dicarboxylic acids 3, Me-3, and Ph-3, respectively, with aromatic diamines using triphenyl phosphite and pyridine as condensing agents. The solubility of methyl- or phenyl-substituted polyamides was remarkably enhanced when compared to that of nonsubstituted analogues. Most of the substituted polyamides revealed an amorphous nature and were readily soluble in a variety of organic solvents including N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide, dimethyl sulfoxide, and m-cresol. Transparent, flexible, and tough films of these polymers could be cast from the DMAc or NMP solutions. These films had tensile strength of 60–100 MPa, elongation to break of 6–11%, and tensile modulus of 1.68–2.25 GPa. The glass transition temperatures (T_g) of most polyamides could be determined by differential scanning calorimetry (DSC) and were in the range of 200–232°C. Thermogravimetric analyses established that these polymers were fairly stable up to 450°C, and the 10% weight loss temperatures were recorded in the range of 458–535°C in nitrogen and 468–528°C in air atmosphere. In general, the phenyl-substituted polyamides exhibited relatively higher T_g, thermal stability, and solubility. © 1996 John Wiley & Sons, Inc.     

A study of benzobisoxazole and benzobisthiazole compounds and polymers under hydrolytic conditions

The stability of benzobisoxazole and benzobisthiazole compounds and polymers under hydrolytic conditions was studied. 2,6-Bis(4-tert-butylphenyl)benzo[1,2-d;4,5-d′]bisoxazole (1) dissolved in acetonitrile containing sulfuric acid and water at 80°C is stable. A suspension of 2,6-bis[4-(2-benzoxazoyl)phenyl]benzo[1,2-d;5,4-d′]bisoxazole (2) in 0.2 N H₂SO₄ or 0.2 N NaOH solution at 100°C for 21 days is stable. The intrinsic viscosity of a poly(p-phenylene)benzobisoxazole (PBO) fiber sample soaked in 0.2 N H₂SO₄, water with 1 wt % polyphosphoric acid (PPA), or 0.2 N NaOH remained the same. Under very severe hydrolytic conditions such as dissolution of compound 2 or PBO in PPA or methanesulfonic acid with residual water followed by coagulation in water, benzobisoxazole underwent bond cleavage to generate carboxylic acid and o-aminophenol functional groups. This is in contrast to an earlier hypothesis that the decrease in intrinsic viscosity under these conditions was due to chain association. Poly(p-phenylene)benzobisthiazole (PBT) also underwent bond cleavage under these very severe conditions, which are unlikely to be encountered in normal applications. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2637–2643, 1999