Photodegradation of aramids. II. Irradiation in air

A series of isomeric fully aromatic polyamides (aramids) were photodegraded in the presence of oxygen. Films and fibers of these aramids gave carboxylic acids as the major products when measured by infrared spectroscopy and potentiometric titration. These acids probably resulted from the oxygen interception of the radicals generated by photocleavage of the amide bonds. In contrast to results found upon irradiation in the absence of oxygen, carboxylic acid formation was accompanied by a rapid loss in molecular weight, and a decrease in useful mechanical properties. Quantum yields for carboxylic acid formation were ≤5.5 × 10^?5 mole/einstein and decreased along the aramid series roughly in agreement with increases in T_g. The photo-Fries rearrangement product was observed in aramid fibers irradiated in air, whereas no rearrangement product was seen in films irradiated in air.     

Preparation and properties of new aromatic polyamides from bis(4-aminophenyl) phenylphosphine and aromatic dicarboxylic acids

A new triphenylphosphine-type monomer, bis(4-aminophenyl) phenylphosphine, was synthesized starting from p-bromoaniline and dichlorophenylphosphine. The aromatic polyamides (aramids) containing triphenyphosphine unit in the polymer backbone was prepared by the polycondensation of this diamine with various aromatic diacid chlorides using a low-temperature solution method in N,N-dimethylacetamide (DMAc). The aramids having inherent viscosities of 0.4–0.7 dL/g were obtained in quantitative yields. The polymers were amorphous and soluble in various organic solvents such as DMAc, N-methylpyrrolidone, dimethyl sulfoxide, pyridine, and m-cresol. Transparent, tough, and flexible films were obtained by casting from the DMAc solutions. The glass transition temperatures of the aramids were in the range of 265–310°C, and the 10% weight loss temperatures were above 400°C in air. © 1993 John Wiley & Sons, Inc.     

Preparation and properties of disilane-containing photodegradable aromatic polyamides from bis(p-aminophenyl)tetramethyldisilane and aromatic diacid chlorides

1,2-Bis(p-aminophenyl)tetramethyldisilane was synthesized from 1,2-dichlorotetramethyldisilane and 4-[N,N-bis(trimethylsilyl)amino]phenyllithium. The diamine was reacted with various aromatic diacid chlorides giving disilane-containing aromatic polyamides (aramids), whose inherent viscosities were between 0.27 and 0.70 dL/g, depending on the diacid chlorides used. The aramids had glass transition temperatures between 194 and 255°C. No weight loss was observed below 350°C. Some of the polymers were found to be semicrystalline based on their x-ray diffractograms. The aramid films showed a strong ultraviolet (UV) absorption at 287 nm, which decreased during irradiation with UV light, suggesting that cleavage of the silicon-silicon bond in the aramid backbone occurs. A decrease in the inherent viscosity and molecular weight of the soluble aramid derived from phenylindanedicarbonyl chloride was also observed by irradiation with UV light.     

Synthesis and properties of aramids and polyarylates having perfluoro-substituents on the benzene ring

A new series of 16 aramids and 16 polyarylates having perfluoro-substituents on the benzene ring was prepared by a low temperature solution or an interfacial polycondensation. The effects of fluorine substituents on the structure and properties of polymers were examined. Fluorinated aramids exhibited higher crystallinity, while fluorinated polyarylates show lower crystallinity. The melting point (T_m) of aramids decreased with fluorine substitution, whereas T_m of polyarylates from fluorinated aromatic diols was higher than that of those from unfluorinated ones. The temperature of 10% weight loss and the residue at 900°C decreased with fluorine substitution except for the aramids from fluorinated diamines. Solubility and contact angle also increased with fluorine substitution. Some polyarylates were found to exhibit an optical anisotropy.     

Preparation and properties of polyamides from 4-phenoxy- and 4-phenylthio-m-phenylenediamine and both aromatic and aliphatic diacids

New soluble aramids having pendant phenoxy and phenylthio groups were prepared in high molecular weights by the polycondensation of aromatic diacids with 4-phenoxy-m-phenylenediamine and 4-phenylthio-m-phenylenediamine, respectively. Glass transition temperatures (T_g) of these aramids were in the range 195–255°C, where T_gs of phenoxy pendant aramids were higher than those of phenylthio substituted aramids. These properties were compared with those of the parent aramids derived from m-phenylenediamine and aromatic diacids. Aromatic-aliphatic polyamides were also prepared by the reaction of these three diamines with aliphatic diacids having 4–10 methylene groups and were characterized in detail.     

STUDY ON PYROLYSIS OF POLYPHENYLSILSESQUIOXANE

X-ray photoelectron spectroscopy and Raman spectroscopy were used to determine the chemical change ofpolyphenylsilsesquioxane (PPSQ) during pyrolysis in flowing nitrogen. Two temperature ranges were found for pyrolysedPPSQ below and above 600℃, respectively. The former is related to the rearrangement of PPSQ backbone and the latterreflects that most of backbone structure of PPSQ might be broken down and unorganized. Carbon formed in carbonization ofPPSQ sample pyrolysed at 900℃ should be sp~3 bonded carbon with crystallite size effects or defects.     

Fragmentation–Rearrangement of Peptide Backbones Mediated by the Air Pollutant NO2.

The fragmentation–rearrangement of peptide backbones mediated by nitrogen dioxide, NO₂^., was explored using di‐, tri‐, and tetrapeptides 8 – 18 as model systems. The reaction, which is initiated through nonradical N‐nitrosation of the peptide bond, shortens the peptide chain by the expulsion of one amino acid moiety with simultaneous fusion of the remaining molecular termini through formation of a new peptide bond. The relative rate of the fragmentation–rearrangement depends on the nature of the amino acids and decreases with increasing steric bulk at the α carbon in the order Gly>Ala>Val. Peptides that possessed consecutive aromatic side chains only gave products that resulted from nitrosation of the sterically less congested N‐terminal amide. Such backbone fragmentation–rearrangement occurs under physiologically relevant conditions and could be an important reaction pathway for peptides, in which sections without readily oxidizable side chains are exposed to the air pollutant NO₂^.. In addition to NO₂^.‐induced radical oxidation processes, this outcome shows that ionic reaction pathways, in particular nitrosation, should be factored in when assessing NO₂^. reactivity in biological systems.     

Phenyl group rotation in polystyrene and the nature of twofold anisotropic internal rotation

A review of the experimental evidence for phenyl group rotation in various polystyrenes and improvements in the interpretation of spin relaxation data for dilute solutions of randomly coiled polymers led to a new calculation of the correlation time for phenyl group rotation from nuclear magnetic resonance (NMR) data. Both ¹³C and ¹⁹F NMR data were reconsidered by using a model which includes motional modulation of the dipole–dipole interaction by overall rotatory diffusion, backbone rearrangements, and internal anisotropic rotation. The choice of a twofold potential to characterize local resistance to internal phenyl group rotation strongly influences the estimate of the rate of phenyl group rotation. The magnetic resonance data are found to be consistent with a correlation time for phenyl group rotation which is one to six times longer than the average correlation time for backbone rearrangement.     

Preparation of silicon containing polymers. II.. Effect of structure on the thermal stability of organosilicon aramids

Two silicon-containing acid dichlorides, bis(4-chlorocarbonylphenyl)dimethylsilane and bis(4-chlorocarbonylphenyl)diphenylsilane, were synthesized and reacted with 1,3-phenylene diamine, 1,4-phenylene diamine, 4,4′-diaminodiphenyl, 4,4′-diaminodiphenyl methane 4,4′-diaminodiphenyl ether, and 4,4′-diaminodiphenyl sulfone in the preparation of 12 structurally different high molecular weight aromatic polyamides. A low-temperature interfacial polycondensation technique was used. Most of the polyamides formed tough, transparent, flexible films and were characterized by solubility, solution viscosity, infrared spectroscopy (IR), and glass transition temperature (T_g). The thermal behavior of these aramids was studied by dynamic thermogravimetry. The effect of diamine and acid dichloride structure on the aramids properties is also discussed.     

Preparation and properties of aromatic polyamides and copolyamides from phenylindanedicarboxylic acid and aromatic diamines

Aromatic polyamides (aramids) having inherent viscosities of 0.5–1.10 dL/g were prepared by the direct polycondensation of 1,1,3-trimethyl-3-(4-carboxyphenyl)indane-5-carboxylic acid with various aromatic diamines using triphenyl phosphite and pyridine as the condensing agents. Copolyamides were also prepared by a similar procedure from a mixture of the phenylindane diacid, terephthalic acid, and p-phenylenediamine. Almost all of the aramids were soluble in a variety of solvents such as N-methyl-2-pyrrolidone, pyridine, and m-cresol, and afforded transparent and tough films by the solution casting. These aramids and copolyamides had glass transition temperatures in the range of 290–355°C, and started to lose weight at 340°C in air.     

Preparation and properties of aromatic polyamides from 5-t-butylisophthalic acid and various aromatic diamines

Aromatic polyamides (aramids) having pendant t-butyl group were synthesized by the direct polycondensation of 5-t-butylisophthalic acid with various aromatic diamines in N-methyl-2-pyrrolidone (NMP) using triphenyl phosphite and pyridine as condensing agents. The aramids having inherent viscosities of 0.6–2.4 dL/g were obtained in quantitative yields. These polymers were readily soluble in various solvents such as NMP,N,N-dimethylacetamide, dimethyl sulfoxide, and pyridine, and gave transparent, tough and flexible films by casting from the NMP solutions. The aramids had glass transition temperatures between 250 and 330°C, and started to lose weight around 350°C, with 10% weight loss being recorded at about 450°C in air.     

Application of liquid chromatography–tandem mass spectrometry for the characterization of galactosylated and tagatosylated β-lactoglobulin peptides derived from in vitro gastrointestinal digestion

This article describes a comprehensive characterization of bovine β-lactoglobulin peptides glycated with an aldohexose (galactose) or a ketohexose (tagatose), derived from in vitro gastrointestinal digestion, by liquid chromatography coupled to positive electrospray ion trap tandem mass spectrometry. In addition to the dissociation pathway previously described for aldohexoses-derived Amadori compounds, i.e. formation of the pyrylium ([M+H]⁺-54) and furylium ions ([M+H]⁺-84) via the oxonium ion ([M+H]⁺-18), another and more direct fragmentation route involving the formation of the imminium ion ([M+H]⁺-150) is also reported following extensive glycation rates of β-lactoglobulin with both carbohydrates. These results indicated that the analysis of digested proteins by LC-ESI-MS² on a three-dimensional ion trap monitoring neutral losses is an efficient and direct method to identify peptides glycated not only through the Amadori rearrangement but also via the Heyns rearrangement. Nevertheless, as the predominating MS² fragmentation pattern of the glycated peptides is derived from the sugar moiety, the sequence-informative b- and y-ions resulting from peptide backbone cleavage were undetected. To overcome this drawback, and taking advantage of multi-stage fragmentation capabilities of ion traps, the indicative Amadori and Heyns-derived imminium ions were successfully used in MS³ analyses to identify the peptide backbone, as well as the specific glycation site. In addition, further MS⁴ analyses were needed to carry out the characterization of doubly glycated peptides.     

Functional aramids: Aromatic polyamides with reactive azido and amino groups in the pendant structure

The structure of the high‐performance aromatic polyamides, also known as aramids, was modified to render functional polymers by the introduction of primary amine and azide functional groups in the main polymer chain. These materials were prepared as parent polymers for future use in the simple and inexpensive preparation of other high‐performance materials by the chemical derivatization of the primary amine and azide groups using their well‐known and broad reactivity. The potential of the parent aramids was exemplified with the preparation of four novel functional aramids containing the fluorescent dansyl group, the anion and cation receptors urea and triazole with free primary alcohol functional groups, and the iminophosphorane ligand for forthcoming synthetic processes. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1469–1477     

Progress in the chemistry of functional aramids properties

High-performance polymers based on amide aromatic rings are known as wholly aromatic polyamides or aramids. The arrangement and admirable properties of aramids are built on the basis of amide linkage and rigid aromaticity. Aramids are attractive because of their extraordinary bond strengths and very high stiffness. Synthetic aromatic polymeric chains provide increased mechanical resistance and thermal softening compared to aliphatic aramids. In addition, aramids exhibit high thermal stability, low creep, and good optical activity with fluorescence. Hence, aramids are found in advanced arenas for engineering thermoplastics such as transport applications, electroactive materials, films, bullet-proof body armor, smart materials, protective clothing, fibers, in nanocomposites as asbestos alternatives, cutting edge complexes in arming, high-temperature lining material, in space engineering, and more. The objective of this review is to make the field of aramids functionality more accessible to the materials science community, that is, scientists, academicians, and engineers.     

Novel aromatic polyamides bearing pendent diphenylamino or carbazolyl groups

Two new diamines, 2,4‐diaminotriphenylamine ( 3 ) and N‐(2,4‐diaminophenyl)carbazole ( 4 ), were synthesized via the cesium fluoride‐mediated aromatic substitution reactions of 1‐fluoro‐2,4‐dinitrobenzene with diphenylamine and carbazole, followed by palladium‐catalyzed hydrazine reduction. Amorphous and soluble aramids having pendent diphenylamino and carbazolyl groups were prepared by the phosphorylation polycondensation of aromatic dicarboxylic acids with diamines 3 and 4 , respectively. The aramids derived from diamine 3 had sufficiently high molecular weights to permit the casting of flexible and tough films. They exhibited excellent mechanical properties and moderately high softening temperatures in the 221–298 °C range. However, the reactions of diamine 4 with aromatic diacids gave relatively lower molecular weights products that could not afford flexible films. For a comparative purpose, the parent aramids derived from m‐phenylenediamine and aromatic diacids were also prepared and characterized. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3302–3313, 2004     

Mechanism and structural aspects of thermal Curtius rearrangement. Quantum chemical study

The electronic and geometric structures of formyl, acetyl, and benzoyl azides were studied and fragments of the potential surfaces for the thermal Curtius rearrangement of these azides into the corresponding isocyanates were calculated by density functional theory at the PBE/TZ2P level. Acyl azides adopt two stable, conformations syn and anti, with respect to the C-N bond. The syn conformers are more stable than their anti analogs. The activation energies of the syn-anti isomerization in the series under study are 9.4, 7.0, and 9.2 kcal mol⁻¹, respectively, and the activation energies of the reverse reaction are 8.5, 6.1, and 2.5 kcal mol⁻¹. The rearrangement of syn-acyl azides is a one-step process, in which elimination of N₂ occurs synchronously with the rearrangement of atoms and bonds to form isocyanates. The activation energies of the rearrangements of syn-HC(O)N₃, syn-MeC(O)N₃, and syn-PhC(O)N₃ are 28.0, 32.9, and 34.5 kcal mol⁻¹, respectively. The rearrangement of the anti conformers of the above-mentioned azides involves the formation of singlet acylnitrene. The activation energies of the latter process are 34.6, 32.9, and 32.3 kcal mol⁻¹, respectively. The activation energies of the rearrangement of acylnitrenes into isocyanates are 20.9, 18.9, and 13.6 kcal mol⁻¹, respectively. The energy characteristics of the process and the structural data for the starting compounds, final products, and transition states provide evidence that the thermal Curtius rearrangement occurs predominantly by a concerted mechanism. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2200–2209, October, 2005.     

Phosphoric Acid-Catalyzed Enantioselective Synthesis of Axially Chiral Anthrone-based Compounds

Anthrones and analogues are structural cores shared by diverse pharmacologically active natural and synthetic compounds. The sp²-rich nature imposes inherent obstruction to introduce stereogenic element onto the tricyclic aromatic backbone. In our pursuit to expand the chemical space of axial chirality, a novel type of axially chiral anthrone-derived skeleton was discovered. This work establishes oxime ether as suitable functionality to furnish axial chirality on symmetric anthrone skeletons through stereoselective condensation of the carbonyl entity with long-range chirality control. The enantioenriched anthrones could be elaborated into dibenzo-fused seven-membered N-heterocycles containing well-defined stereogenic center via Beckmann rearrangement with axial-to-point chirality conversion.     

Induced-Fit Recognition of CCG Trinucleotide Repeats by a Nickel–Chromomycin Complex Resulting in Large-Scale DNA Deformation

Small-molecule compounds targeting trinucleotide repeats in DNA have considerable potential as therapeutic or diagnostic agents against many neurological diseases. Ni^II(Chro)₂ (Chro=chromomycin A3) binds specifically to the minor groove of (CCG)_n repeats in duplex DNA, with unique fluorescence features that may serve as a probe for disease detection. Crystallographic studies revealed that the specificity originates from the large-scale spatial rearrangement of the DNA structure, including extrusion of consecutive bases and backbone distortions, with a sharp bending of the duplex accompanied by conformational changes in the Ni^II chelate itself. The DNA deformation of CCG repeats upon binding forms a GGCC tetranucleotide tract, which is recognized by Ni^II(Chro)₂. The extruded cytosine and last guanine nucleotides form water-mediated hydrogen bonds, which aid in ligand recognition. The recognition can be accounted for by the classic induced-fit paradigm.     

Thermal chemistry of poly(aryl ether phthalazine)s and the synthesis of poly(aryl ether quinazoline)s

Poly(aryl ether phthalazine)s were found to undergo an exothermic reaction at a temperature range of 360–440°C. We elucidated the origin of the exothermic reaction and the physiochemical phenomena associated with it, based on thermal analyses, model compound studies, and ¹³C solid-state NMR studies. At elevated temperatures, polymers containing a diphenylphthalazine moiety 4 underwent extensive thermal crosslinking reactions as a result of a nitrogen elimination reaction of the phthalazine moiety. However, polymers containing the tetraphenyl or hexaphenyl phthalazine moiety 5 and 6 were found to undergo principally a backbone rearrangement reaction, in which the phthalazine moiety rearranged to a quinazoline. Utilizing this efficient thermal rearrangement of polyphenylated phthalazines, we have prepared a novel activated difluoride, 2,4-bis(4-fluorophenyl)-,5,6,7,8-tetraphenylquinazoline 9d, which underwent high-temperature solution polycondensation with BPA to give the quinazoline containing poly(aryl ether) 14. Polymer 14 is amorphous, has a glass transition temperature of 264°C, and has high thermooxidative stability with 5% weight loss being recorded at 514°C in nitrogen. © 1996 John Wiley & Sons, Inc.     

Unsaturated carboxylic acid dienolates. Michael addition to enones through tandem 1,2-addition - oxy-cope rearrangement.

Michael addition of the dienolate derived from 2-butenoic acid to 1,3-diphenylpro-penone occurs through a 1,2-addition followed by a ³,3-sigmatropic rearrangement. α,γ -re-gioselectivity found for Michael addition to other styryl ketones depends on the steric parameter of the substituents at the carbonyl group, in agreement with the same tandem addition rearrangement mechanism.