Thermal decomposition processes in aromatic polythiocarbonates

The thermal degradation of some polythiocarbonates obtained from bisphenol A bischloroformate and dithiols have been investigated by thermogravimetry and by direct pyrolysis in the mass spectrometer operating both in electron and chemical ionization. Poly(bisphenol A-co-tri-methylenedithiocarbonate) decomposes by an intramolecular exchange process (back-biting) producing trimethylene-1,3-dithiocarbonate and bisphenol A polycarbonate. The latter undergoes further thermal degradation at higher temperature yielding cyclic oligomers. Polymers containing tetramethylene-1,4-dithiocarbonate and hexamethylene-1,6-dithiocarbonate units decompose by the same mechanism, but the elimination of the dithiocarbonate units is not as fast and selective as the previous case. Some bisphenol A units are eliminated in the first thermal degradation stage and a rearrangement reaction producing ether linkage also occurs. Poly(phenylene-1,3-dithiocarbonate) decomposes by CO and COS loss with formation of sulfide and disulfide bridges along the polymer chains, which undergo further thermal degradation by a back-biting process yielding a series of cyclic compounds. The thermal degradation of Poly(bisphenol A-co-phenylenedithiocarbonate) takes place through an interchange reaction producing phenylene-1,3-dithiocarbonate sequences which further decompose as the corresponding polymer. The remaining bisphenol A polycarbonate decomposes at higher temperature producing cyclic carbonates.     

Thermal decomposition kinetics of some aromatic azomonoethers

The non-isothermal kinetic parameters corresponding to the decomposition of 4-[(4-chlorobenzyl)oxy]-4’-nitro-azobenzene were evaluated. The kinetic analysis was performed by means of different multi-heating rates methods: isoconversional (‘model-free’) methods (Flynn–Wall–Ozawa) and invariant kinetic parameters method (IKP) associated with the criterion of the independence of activation parameters on the heating rate. The values of the obtained non-isothermal kinetic parameters are in satisfactory agreement.     

Thermal decomposition kinetics of some aromatic azomonoethers

Thermal analysis of three azomonoether dyes, exhibiting liquid-crystalline properties, was performed in dynamic air atmosphere. Thermal stability studies and the evaluation of the kinetic parameters of each physical or chemical transformations are essential for a full characterization, before attempting accurate thin films’ depositions of such materials used in non-linear optical applications. New synthesized dyes with general formula: where R is a nematogenic group: CN, CF₃ or a highly polarizable group: NO₂ were investigated using TG, DTG, DTA and DSC techniques, under non-isothermal regime. The evolved gases were analyzed by FTIR spectroscopy. The activation energies of the first decomposition step were evaluated for each compound, the obtained results revealing complex mechanisms.     

Thermal decomposition kinetics of some aromatic azomonoethers

Thermal analysis of 4-[(4-chlorobenzyl)oxy]-4′-chloro-azobenzene dye, exhibiting liquid crystalline properties, was performed in dynamic air atmosphere. The compound behavior was investigated using TG, DTG, DTA and DSC techniques, under non-isothermal linear regime. The evolved gases were analyzed by FTIR spectroscopy. Kinetic parameters of the first decomposition step were obtained by means of multi-heating rates methods, such as isoconversioanl methods, IKP method and Perez-Maqueda et al. criterion.     

Thermal decomposition products of a chlorinated aromatic polyamide

Products from the thermal decomposition of a chlorinated aromatic polyamide fabric are described for conditions of pyrolysis and flaming and nonflaming oxidative degradation. Volatile degradation products were identified by gas chromatography-mass spectrometry (GC-MS) and the condensible fraction, by infrared (IR) spectroscopy, high-pressure liquid chromatography (HPLC), and MS. Nonvolatile char was characterized by IR and elemental analysis. Twenty-one compounds were identified as volatile products from pyrolysis at 550°C; the condensible material contained ammonium chloride and at least 22 organic compounds. From volatile compounds produced in flaming oxidative degradation 21 compounds were identified, of which CO, CO₂, and H₂O were prominent. Nonflaming oxidative degradation at 400 and 550°C produced 11 and 21 volatile identifiable compounds, respectively, and results from experiments at the higher temperature compared favorably with results from the flaming experiments. By comparison of data from this work with those from unchlorinated analogs (described in an earlier article), it is concluded that the incorporation of chlorine into the polymeric structure lowers the temperature for the onset of thermal degradation and alters the type and concentration of thermolytic products. The major degradation products can be explained by a mechanism similar to that proposed for aromatic polyamides with the exception of the formation of substantial amounts of ammonium chloride. It is proposed that the latter is formed by an initial acid-catalyzed hydrolysis reaction which is followed by deammoniation or by an intermolecular process that involves an isoimide intermediate.     

Transformations of 3-methyl-2-azafluorene involving the methyl and methylene groups

The condensation of 3-methyl-2-azafluorene with benzaldehyde at the methyl group, as a result of which the cis and trans forms of 3-styryl-2-azafluorene are formed, proceeds without catalysts. The subsequent condensation with benzaldehyde takes place in the presence of potassium ethoxide and leads to 3-styryl-9-benzylidene-2-azafluorene. Treatment of azafluorene with phenoxyacetyl chloride in the presence of triethylamine yielded 3-methyl-9-(-hydroxy--phenoxyethylidene)-2-azafluorene. On the basis of the spectral data it was concluded that the latter exists in the form of a mixture of the enol form and the zwitterionic form. 3-Methyl-9-(-phenyl--cinnamoyloxyallylidene)-2-azafluorene was obtained by acylation of azafluorene with cinnamoyl chloride under the same conditions. The PMR and IR spectral data are presented.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 79–81, January, 1978.     

Thermal behavior of aromatic polyamic acids and polyimides containing oxadiazole rings

Two aromatic polyimides and the corresponding poly(amic acid)s, with oxadiazole and para/meta phenoxyphenylene rings in the backbone, were synthesized and the structure — thermal properties correlation was followed by dynamic mechanical analysis. Concerning the poly(amic acid)s, the glass transition domain was emphasized only for the compound with meta-oriented rings because the process of imidization takes place with increasing temperature. A multiplex experiment was performed to calculate the activation energy of the transition localized under 200°C. Consecutive heating-cooling-heating cycles were accomplished. All phenomena are discussed by cross-examination of the storage modulus (E′), loss modulus (E″) and loss factor tanδ variation with temperature.     

Thermal decomposition and stability of fatty acid methyl esters in supercritical methanol

In recent years, non-catalytic supercritical processes for biodiesel production have been proposed as alternative environmentally friendly technologies. However, conditions of high temperature and pressure that occur while biodiesel is in supercritical fluid can cause fuel degradation, resulting in low yield. In this study, we performed the thermal decomposition of fatty acid methyl esters (FAMEs) in supercritical methanol at temperatures ranging from 325 °C to 420 °C and pressure of 23 MPa to investigate the degradation characteristics and thermal stability of biodiesel. The primary reactions we observed were isomerization, hydrogenation, and pyrolysis of FAMEs. The main pathway of degradation was deduced by analyzing the contents of degradation products. We found that if FAME has shorter chain length or is more saturated, it has higher thermal stability in supercritical methanol. All FAMEs remained stable at 325 °C or below. Based on these results, we recommend that transesterification reactions in supercritical methanol should be carried out below 325 °C (at 23 MPa) and 20 min, the temperature at which thermal decomposition of FAMEs begins to occur, to optimize high-yield biodiesel production.     

Molecular electrostatic potentials. Negative potentials associated with some methyl and methylene groups

It is shown that both CH₃ and CH₂ groups can give rise to negative electrostatic potentials. Several examples are presented and discussed, including both molecules which do and which do not show these effects.     

Thermal decomposition of some linear trinuclear schiff base complexes with acetate bridges

Ni(II)–M(II)–Ni(II) nuclear structured complexes were prepared from N,N’-bis(salicylidene)-1,3-propanediamine (LH2) and its derivatives N,N’-bis(salicylidene)-2,2’-dimethyl-1,3-propanediamine (LDMH₂) and N,N’-bis(salicylidene)-2-hydroxy-1,3-propanediamine (LOH3), where M represents one of the following metal ions; Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II). Two different μ-bridges are found between the metal nucleus of the complexes. The phenolic oxygens and acetate ions tend to form μ-bridges between the terminal Ni(II) ions and central metal(II) ion. The coordinatively bonded DMF molecules, in the complexes, were observed to abandon the structure between 160–180°C. Further heating resulted primarily in the thermal decomposition of the complexes above 310°C, whereas metal oxide residue mixtures were observed above 650°C.     

Thermal decomposition of methyl nitrite in shock waves studied by laser probing

The unimolecular decomposition of methyl nitrite in the temperature range 680–955 K and pressure range 0.64 to 2.0 atm has been studied in shock-tube experiments employing real-time absorption of CW CO laser radiation by the NO product. Computer kinetic modeling using a set of 23 reactions shows that NO product is relatively unreactive. Its initial rate of production can be used to yield directly the unimolecular rate constant, which in the fall-off region, can be represented by the second-order rate coefficient in the Arrhenius form: A RRKM model calculation, assuming a loose CH₃ONO^≠ complex with two degrees of free internal rotation, gives good agreement with the experimental rate constants.     

Reaction of 2, 6-dimethylthiapyrone with compounds containing active methyl or methylene groups

The reaction of 2, 6-dimethylthiapyrone with compounds containing active methyl or methylene groups is investigated. Position 4 in the thiapyrone ring is found to react. If the components with active methylene groups are malonodinitrile or cyclic diketomethylene compounds, the methylene groups in the resultant 2, 6-dimethylthiapyrane compounds are still reactive towards electrophilic reagents, a property which is utilized for synthesis of nonionic polymethylene dyes containing thiapyrane rings.     

Aromatic reactivity : LVIII. Base-cleavage of bonds between trimethylsilyl groups and polynuclear aromatic rings

The relative rates of cleavage of aryl-SiMe₃ compounds in a mixture of 1.0 M potassium hydroxide (1 vol) and DMSO (6 vol) at 70° have been found to be as follows: (aryl = ) Ph, 1.0; 1-naphthyl, 12.5; 2-naphthyl, 4.7; 9-phenanthryl, 51; 1-pyrenyl, 71. The order of reactivity is the same as that observed for hydrogen-exchange of the corresponding hydrocarbons in cyclohexylamine containing lithium cyclohexylamide, and it seems that the stability of the aryl carbanion is the dominant influence in both cases.     

C-H stretching vibrations of methyl, methylene and methine groups at the vapor/alcohol (N = 1-8) interfaces

In IR and Raman spectral studies, the congestion of the vibrational modes in the C-H stretching region between 2800 and 3000 cm(-1) has complicated spectral assignment, conformational analysis, and structural and dynamics studies, even with quite a few of the simplest molecules. To resolve these issues, polarized spectra measurement on a well aligned sample is generally required. Because the liquid interface is generally ordered and molecularly thin, and sum frequency generation vibrational spectroscopy (SFG-VS) is an intrinsically coherent polarization spectroscopy, SFG-VS can be used for discerning details in vibrational spectra of the interfacial molecules. Here we show that, from systematic molecular symmetry and SFG-VS polarization analysis, a set of polarization selection rules could be developed for explicit assignment of the SFG vibrational spectra of the C-H stretching modes. These polarization selection rules helped assignment of the SFG-VS spectra of vapor/alcohol (n = 1-8) interfaces with unprecedented details. Previous approach on assignment of these spectra relied on IR and Raman spectral assignment, and they were not able to give such detailed assignment of the SFG vibrational spectra. Sometimes inappropriate assignment was made, and consequently misleading conclusions on interfacial structure, conformation and even dynamics were reached. With these polarization rules in addition to knowledge from IR and Raman studies, new structural information and understanding of the molecular interactions at these interfaces were obtained, and some new spectral features for the C-H stretching modes were also identified. Generally speaking, these new features can be applied to IR and Raman spectroscopic studies in the condensed phase. Therefore, the advancement on vibrational spectra assignment may find broad applications in the related fields using IR and Raman as vibrational spectroscopic tools.     

The rate of methyl radical decomposition at high temperatures and pressures

Rate constants are calculated for CH₃ (+ Ar) ? CH₂ + H (+ Ar) at the limiting low-pressure, the limiting high-pressure, as well as the intermediate fall-off ranges. The results show that published experimental rate constants for methyl dissociation correspond to the fall-off region close to the low-pressure limit. At the low-pressure limit the activation energy is less than the bond dissociation energy, in agreement with experimental results. Forward and backward rate coefficients at the high-pressure limit are compared with other theoretical calculations. More theoretical and experimental work is necessary to understand the reverse reaction and its competing reactions, as well as the decomposition channel leading to CH + H₂. © 1994 John Wiley & Sons, Inc.     

Separation of the mineral oil aromatic hydrocarbons of three and more aromatic rings from those of one or two aromatic rings

An analytical method was developed for the quantitation of the mineral oil aromatic hydrocarbons in cosmetic raw materials separating those of one or two aromatic rings from those of three and more aromatic rings. Normal phase high performance liquid chromatography was used with donor‐acceptor complex chromatography. The composition of both fractions and the quantities of respective compounds were determined by comprehensive two dimensional gas chromatography with time of flight mass spectrometry and by liquid chromatography coupled to gas chromatography with flame ionization detection.