Thermal degradation/oxidation scheme

PEG can be severely degraded in air. Its melting point and heat of fusion are reduced by as much as 13 °C and 32 kJ kg"1, respectively [81]. The thermal degradation of PEG in air follows a random chain scission oxidation mechanism, and could be suppressed by addition of an antioxidant, 2, 2,-methylene-bis (4-methyl-6-tert-butylphenol) (MBMTBP), due to the reaction of MBMTBP with ROO radicals formed in the propagation step [79]. Low-molecular-weight esters including formic esters are produced as the main products of the thermal degradation of PEG (Scheme 3.17) [80]. 

It might be assumed that, as condensed-phase flame retardants function by modifying the normal thermal degradation processes of polymers, they would also function as thermal stabilizers and that thermal antioxidant stabilizers would show flame-retardant properties. However, these statements are rarely the case, and to understand why, it is necessary to compare the mechanistic aspects of flame retardance as discussed earlier with those of thermal degradation and thermal oxidation as well, briefly alluded earlier, and in the case of the latter, the Bolland and Gee mechanism,17 in Scheme 2.1. 

The fractions obtained in these schemes are defined operationally or procedurally. The amount and type of asphaltenes in an asphalt are, for instance, defined by the solvent used for precipitating them. Fractional separation of asphalt does not provide well-defined chemical components. The materials separated should only be defined in terms of the particular test procedure (Fig. 15.5). However, these fractions are generated by thermal degradation or by oxidative degradation and are not considered to be naturally occurring constituents of asphalt. The test method for determining the toluene-insoluble constituents of tar and pitch (ASTM D-4072, ASTM D-4312) can be used to determine the amount of carbenes and carboids in asphalt. 

Methylcobalamin (2 c) can be isolated from microorganisms. Its largely covalent Co—CH3 bond undergoes all three possible types of reactions, namely homolysis, carbonium-ion transfer, and carbanion transfer (Scheme 1), thus including reduction and oxidation of cobalt, respectively. Thermal degradation of cobalamin preferentially yields methane and ethane as radical-type reaction products (cf. 

The thiazine (42) can be photo-oxidized in the presence of methylene blue. The major photo product (50) is derived via a singlet oxygen ene reaction <88JPR(330)79). Thiazines such as (42) are relatively stable to heat, but (42) can be thermally degraded at 180°C with copper bronze to yield the pyrrole (51) (Scheme 10) <92MI 606-01). 

An experimental investigation directly on polymers in order to find out the degradation products is difficult to realize. However, both the study of degradation of model compounds like butylene dibenzoate [5-7], and the general mechanism of thermo-oxidation of organic molecules (scheme 2), can be very helpful to propose a specific mechanism for the thermo-oxidative degradation of PBT (scheme 6). In order to simplify the discussion, the contribution of thermal degradation has not been considered. 

Carbon-chain commodity and engineering polymers (PH) suffer dining all phases of their lifetime in the earth atmosphere from oxidative degradation, either thermal or phototriggered. Mechanism of oxidation was studied in detail [1,2]. The oversimplified free radieal oxidation scheme involves initiation (1), propagation (2 and 3) and termination steps (4) followed by reactions of primary intermediates and produets. 

The complexity of the chemical structure of heterocyclic polymers, including PPO, which are very strong, thermally stable polymers, makes the study of their thermal and thermal-oxidative degradation difficnlt. The schemes suggested for their thermal degradation are in many cases only hypothetical, however, the available experimental data make it possible to delineate the major factors determining the thermal stability of these polymers [44-46]. 

A comparative study has been conducted into the thermal and thermooxidative degradation of PET and PBT polymer films and their model compounds, ethylene dibenzoate and butylene dibenzoate, in an oxygen atmosphere at 160 °C 832485. On the basis of the compounds identified by GC-MS, a mechanism was proposed for the degradation of the model compounds that involves oxidation at the a-methylene carbon with the formation of unstable peroxides and carboxylic acids a.l53. From the studies performed under N2 at 160 °C, it was concluded that benzoic acid and esters are products of the thermal degradation as illustrated in Schemes 15 and 16, while benzoic and aliphatic acids, anhydride and alcohols were due to thermooxidative degradation. 

The degradation process has a free radical mechanism. It is initiated by free radicals P that appear due to, for example, hydroperoxide decomposition induced thermally or by trace amounts of metal ions present in the polysaccharide. One cannot exclude even direct interaction of the polysaccharide with oxygen in its ground triplet state with biradical character. Hydroperoxidic and/or peracid moieties are easily formed by oxidation of semiacetal chain end groups. The sequence of reactions on carbon 6 of polysaccharide structural unit that ultimately may lead to chemiluminescence is shown in Scheme 11. 

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