Thermal Stability. Pyrolysis Reactions

570 °C. Thermal degradation of polyazines of this type occurs with marked exothermal effects, accompanied by a practically complete evolution of nitrogen as N2 from the polymer. 

The interruption of conjugation by heteroatoms40,41 259-) leads to a higher heat stability for the particular reason that in these cases the possibility of azo-azine rearrangement is excluded. 

Thermal stability of polyazines of the third group, i.e., polymers with conjugated sections separated by aliphatic blocks, is determined by the thermal stability of the latter. The degradation process takes place gradually, and it is not accompanied by any considerable heat effects41,117-.  

We have studied the effect of the stereoregularity of the initial PAN on the pyrolysis of products formed by cyclization of the polymer238,239,241). A com- 

Together with the decarboxylation, dehydration of PPA takes place on heating. Examination of the decarboxylation of copolymers of acrylic and propynoic adds having blocks in their structure has revealed two interesting phenomena261 262.  

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This reaction provides a third indication of the usefulness of a radiofrequency discharge in the synthesis of compounds of low thermal stability. The more-stable (CFaljTej had been prepared by the interaction of CFj radicals, formed in the pyrolysis of (CF3)2CO, with a tellurium mirror (19). The less-stable (CFsljTe was not, however, observed in that experiment. 

In 1989, Nefedov and coworkers have reinvestigated the thermolysis of the above-mentioned allyloxysilane derivatives 16-18 and of 2,2,6-trimethyl-2-silapyrane (21) using vacuum pyrolysis and matrix isolation techniques23. IR spectroscopic studies on the products isolated in the matrices enabled them to probe directly the intermediacy of 10 in these reactions and to discuss its thermal stability. Only in the case of allyloxydimethylsilane (17) did they find direct spectroscopic evidence for the formation of 10 by observation of its most intense band at 798 cm-1 in the matrix IR spectrum of the pyrolysis products. In all other cases silanone 10 was not detected and it was assumed that it is thermally unstable, undergoing fragmentation into SiO and CH3 radicals as shown in Schemes 7, 8 and 9 (the species actually observed in the matrix are indicated). In this paper, Nefedov and coworkers have reaffirmed the thermal and kinetic stability of dimethylsilanone 10 in the gas phase, which they had previously described19. 

The carbonyl halides, COCIF, COClj and COFj, were co-produced in small quantities during the air-oxidation of CCl FCClFj (CFC-113) [1325]. The concentrations of each of these products increases from about 600-650 "C, and reach a maximum (COCIF 2600, COClj 1300, COFj 800 p.p.m.) at around 700 - 800 C. Thereafter, the concentrations of each of these components decrease as the pyrolysis temperature increases further (the thermal stability decreasing with fluorine content). The concentration of COCIF diminishes to zero at about 1000 "C. The stoicheiometry of the oxidation reactions in which COCIF is formed is as follows [1325] ... 

A number of phosphate and thiophosphate esters are of limited thermal stability and undergo highly exothemiic self-accelerating decomposition reactions which may be further catalysed by impurities. The potential hazards can be reduced by appropriate thermal control measures. An example is the substitution of hot water at 60 C for pressurised steam to melt a solid phosphate ester, which on adiabatic calorimetric examination was found to have a time to maximum decomposition rate of 6 h at 110° but 11 h at lOO C [2]. The combined use of vapour phase pyrolysis to decompose various phosphoms esters, and of GLC and mass spectrometry to analyse the pyrolysis products, allowed a thermal degradation scheme to be developed for phosphorus esters [3]. Individually indexed compounds are ... 

The relatively low thermal stability of the acetylene precursors inspired the search for a more stable, masked ethynyl group that can be quantitatively converted into acetylenes in the gas phase of the pyrolysis apparatus. Presently, the state of the art consists in the substitution of ethynyl groups by chloroethenyl substituents [54b -f, 55,56]). The latter show a higher thermal stability and are conveniently available from acetyl derivatives by reaction with PC15 or from tri-methylsilyl (TMS)-substituted acetylenes by treatment with hydrochloric acid in glacial acetic acid (see Scheme 8). 

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