Pyrolysis color

Tetracyanoethylene is colorless but forms intensely colored complexes with olefins or aromatic hydrocarbons, eg, benzene solutions are yellow, xylene solutions are orange, and mesitylene solutions are red. The colors arise from complexes of a Lewis acid—base type, with partial transfer of a TT-electron from the aromatic hydrocarbon to TCNE (8). TCNE is conveniendy prepared in the laboratory from malononitrile [109-77-3] (1) by debromination of dibromoma1 ononitrile [1855-23-0] (2) with copper powder (9). The debromination can also be done by pyrolysis at ca 500°C (10). 

Another process for silicon carbide fibers, developed by Verbeek and Winter of Bayer AG [45], also is based on polymeric precursors which contain [SiCH2] units, although linear polysilmethylenes are not involved. The pyrolysis of tetramethylsilane at 700°C, with provision for recycling of unconverted (CHg Si and lower boiling products, gave a polycarbosilane resin, yellow to red-brown in color, which was soluble in aromatic and in chlorinated hydrocarbons. Such resins could be melt-spun but required a cure-step to render them infusible before they were pyrolyzed to ceramic... 

The color of the polymer can also be affected by inappropriate reaction conditions in the polymerization process, such as temperature, residence time, deposits of degraded polymer or the presence of oxygen. Degradation of polyesters and the generation of chromophores are thermally effected [29b, 29c, 39], The mechanism of thermal decomposition is based on the pyrolysis of esters and the formation of unsaturated compounds, which can then polymerize into colored products. It can be assumed that the discoloration takes place via polymerization of the vinyl ester end groups or by further reaction of AA to polyene aldehydes. 

Reaction of the dicyanoindenone 231 with pyrazole or 1,2,3-triazole diazonium salts gives deeply colored amino-triazines 232 and 233, which cyclize upon pyrolysis to give the pentacyclic compounds 234 and 235 containing the pyrido[2,3-f]-l,2,4-triazine nucleus (Scheme 36) <2003JRM1161>. 

The parent azocine (91) was isolated at —190 °C from flash vacuum pyrolysis of diazabas-ketene (90) (71JA3817). The compound, which must be handled in KOH-coated glassware, decomposes at -50 °C to colored tarry material. Characterization was by mass spectrum (m/e 107), NMR spectrum (see 91) and conversion with potassium in liquid ammonia to a dianion, which on quenching and hydrogenation gave azocane in low yield. 

Perhaps one of the best known syntheses of a heterocyclic polymer via the modification method is the generation of nitrogen-containing ladder polymers by pyrolysis of polyacrylonitrile) (77MI11109). The thermolysis is known to take place in discrete steps. The first step in the sequence, which can take place with explosive violence if the heating rate is not sufficiently slow, occurs at about 150 °C and can be detected by the onset of intense color formation. The product of this reaction (Scheme 101) is the cyclic tetrahydropyridine ladder structure (209). The next step, which is conducted in the presence of air at ca. 250 °C, involves the thermooxidation of polymer (209) to form what is best described as terpolymer (210) containing dihydropyridine, pyridone and pyridine units. 

Benzo[6]thiophene is best stored in amber-colored bottles or tin cans in the presence of certain acid catalysts it polymerizes.397 On pyrolysis (see also Section V,C) it undergoes self-condensation less readily than indole.398 On exposure to daylight or mercury vapor lamp... 

The use of organic components increases the discrimination in soils that are otherwise similar. For example, soils that have identical Munsell color values could be discriminated by subtractive FTIR. A new ancillary method using thermal gravimetric analysis in addition to IR analysis on samples prior to and after pyrolysis has been applied to soils and could give additional valuable information for the discrimination of soils (Thermo electron corporation application note 50862). 

Selenoketenes formed by pyrolysis of 4-alkyl-l,2,3-selenadiazoles in the vapour phase are isolated at -196 °C as highly colored materials which dimerize rapidly. 

The anhydrous oxides, ZnO and CdO, are formed on pyrolysis of the nitrates or carbonates, or by roasting the metals in air. Both oxides can be sublimed at very high temperatures. Both ZnO and CdO tend to have lattice defects leading to coloration. For ZnO the pure white compound turns yellow on heating (deficit of oxygen), while CdO may be brown or even black, depending on how much it has been heated. 

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