Argon pyrolysis

Unlike polycondensation polymers, polymers of addition polymerization such as polyethylene and polypropylene when depolymerized in inert atmosphere (39) or in supercritical water (37) do not convert to just the monomer, but a homologous series of oligomers (alkanes and alkenes). Compared to pyrolysis in argon, for polyethylene, the portion of the lighter products increases in supercritical water depolymerizations conducted at 693 K and water densities of 0.13 and 0.42 g/cm. The 1-alkene to n-alkane ratio also increases in supercritical water and with density. These are shown in Figure 11. These results are attributed to the fact that in argon pyrolysis, the reaction proceed in the molten state of the polymer, whereas in supercritical water, some of degradation products... 

Acetylene traditionally has been made from coal (coke) via the calcium carbide process. However, laboratory and bench-scale experiments have demonstrated the technical feasibiUty of producing the acetylene by the direct pyrolysis of coal. Researchers in Great Britain (24,28), India (25), and Japan (27) reported appreciable yields of acetylene from the pyrolysis of coal in a hydrogen-enhanced argon plasma. In subsequent work (29), it was shown that the yields could be dramatically increased through the use of a pure hydrogen plasma. 

Thermal and Photochemical Reactions. Unsubstituted ethyleneimine has astonishing thermal stabihty. The reaction of ethyleneimine diluted with argon proceeds to give a mixture of unidentified compounds only at temperatures above 400°C (339). In a flow pyrolysis system under pressures of <1.33 kPa (<10 mm Hg) on quartz wool, isomerization to give /V-methylenemethylamine and ethylideneimine was observed only ia the temperature range 510—535°C. Higher temperatures result ia fragmentation (340). 

UV irradiation (A>300nm) of an argon matrix containing tetra-fluoromethane led to the formation of difluorocarbene CF2 (Milligan and Jacox, 1968a). It was shown that the IR spectrum of this species contains three bands at 1222 (i i), 1102 (v ), and 668 (i 2)cm . Some time later difluorocarbene was stabilized in a neon matrix at 4.2 K from the gas phase after vacuum flash pyrolysis (1300°C) of perfluoroethene (Snelson, 1970b). In this case the IR bands of CF2 differed from those in an argon matrix by less than 2 cm . ... 

An informative IR spectrum of the t-butyl radical, containing 18 bands, has been recorded after freezing of the products of vacuum pyrolysis of azoisobutane [110] and 2-nitrosoisobutane [111] in an argon matrix at 10 K (Pacansky and Chang, 1981). This spectrum is in agreement with a pyramidal structure of the radical (CH3)3C (symmetry C3v) which has elongated CH bonds in positions trans to the radical centre. On the basis of experimental vibrational frequencies and ab initio calculations of the radical geometry the enthalpy value [// (300)] of its formation has been calculated as 44 kJ moP. ... 

An IR spectroscopic study of the radicals CF3, C2F5, C3F7 and i-C3F7 has been carried out. These radicals were formed as products of vacuum pyrolysis in a platinum reactor of the respective fluorinated iodoalkanes and were stabilized in argon matrices at 10-12 K (Snelson, 1970b Butler and Snelson, 1980a,b,c) as shown in (6). 

Perfluoroallyl radical, C3F5, was obtained by vacuum pyrolysis (850-950°C, 10 Torr) of 1,5-perfluorohexadiene or of 3-iodopentafluoro-propylene (14) and was studied by pyrolytic mass spectrometry (Kagrama-nov et al., 1983b) and by IR spectroscopy in an argon matrix (Mal tsev et al., 1986). 

Finally, the cyclopentadienyl radical, C5H5, was obtained by vacuum pyrolysis (970°C, 10 Torr) of bis(cyclopentadienyl)nickel (18) and it was frozen into an argon matrix at 12 K (Nefedov, 1991a,b Korolev and... 

The first successful stabilization of a silene, Me2Si=CH2 [117], in an argon matrix was achieved in experiments on the vacuum pyrolysis of 1,1-dimethylsilacyclobutane [118] (Mal tsev et al., 1976b). The IR spectra of the silene [117] and some of its deuterated analogues have been recorded... 

Matrix IR spectra of various silenes are important analytical features and allow detection of these intermediates in very complex reaction mixtures. Thus, the vibrational frequencies of Me2Si=CH2 were used in the study of the pyrolysis mechanism of allyltrimethylsilane [120] (Mal tsev et al., 1983). It was found that two pathways occur simultaneously for this reaction (Scheme 6). On the one hand, thermal destruction of the silane [120] results in formation of propylene and silene [117] (retroene reaction) on the other hand, homolytic cleavage of the Si—C bond leads to the generation of free allyl and trimethylsilyl radicals. While both the silene [117] and allyl radical [115] were stabilized and detected in the argon matrix, the radical SiMc3 was unstable under the pyrolysis conditions and decomposed to form low-molecular products. 

A matrix isolation IR study of cyclic siladienes was more successful (Khabashesku et al., 1992). At first, unstable l-silacyclopenta-2,4-diene [128] was generated by vacuum pyrolysis (800°C 10 -10 Torr) of 5-silaspiro[4.4]nona-2,7-diene [129] or pyrolysis and photolysis (A = 248 nm) of l,l-diazido-l-silacyclopenta-2,4-diene [130] it has been studied by UV and IR spectroscopy in an argon matrix at 12 K. The UV band at Amax = 278 nm and nine IR bands (including two sp Si-H stretching vibrations at 2175 and 2144 cm ) have been recorded in matrix spectra of [128]. Reversible photochemical interconversion of [128] with silacy-... 

Pyrolysis of the white solids obtained in these KH-catalyze< dehydrocyclodimerization reactions (under argon from 50-950°C) produced black ceramic residues, with the exception of the 1 1 THF ammonolysis-derived solid which left a brown residue. The ceramic yields were excellent (all greater than or equal to 82%, with the highest being 88%). 

These polymers may be used in the preparation of quite pure silicon nitride if the pyrolysis is carried out in a stream of ammonia (a reactive gas) rather than under nitrogen or argon. The ammonia reacts with the... 

Assuming a cooler temperature for helium, this means the amount of heat energy available for pyrolysis is less, which would produce less hydrocarbon products. This ultimately would lead to a relatively smaller SL quenching compared to that observed in argon saturated solutions. The relative change in bubble temperature would also be less in helium saturated solutions due to the lower amount of hydrocarbon products generated. Solubility differences would re-enforce this. 

Table I. Ceramics Obtained in the Pyrolysis of the Methylpolysilane/(T]5-C5H5)2MMe2 Reaction Product Polymers to 1500 °C in a Stream of Argon...

Polysilazane fibers are rendered infusible by humidity or in the absence of oxygen by ammonia. The final step of producing ceramic fibers is the pyrolysis. The cured fibers are heated at 1200 -1300°C in argon, nitrogen, or in vacuo, and SiC- or SiC/SijN fibers with a diameter of around 15 /xm are obtained. Heating up silicon-polymers, whether polysilanes or polysilazanes, results in the evolution of CH4 and H2. 

While infrared characterization of several silenes has been carried out, it has not always been possible to assign the observed bands unequivocally. Table II summarizes the available infrared data. The infrared spectra of reactive silenes, generated by the pyrolysis of suitable precursors, have most often been recorded in an argon matrix at 10 K, whereas the infrared... 

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