Thermooxidative reactions

Because of its extended polyconjugated framework, polymer (210) exhibits semiconducting properties both by itself and in the presence of additives. Perhaps a more remarkable property, however, is that polymer (210) does not burn when exposed to a flame. Thus, by employing this thermooxidative reaction, woven or knitted poly(acrylonitrile) fibers can be transformed into fire-proof materials. Polymer (210) can be pyrolyzed still further at temperatures generally in excess of 1000 °C to expel all heteroatoms and generate carbonized or graphitized fibers. These fibers find application where an inert, extremely high temperature, e.g. up to 3000 °C, material is required. 

The recorded chemiluminescence originated from only a thin surface film. The thickness of this film depends on the extent of self-absorption of the emitted radiation and remains unknown at this time. The pseudo-first-order rate of thermo-oxidative reactions responsible for the chemiluminescence is not limited by oxygen concentration. The applied stress decreases the activation energy for thermooxidative reactions, resulting in the observed chemiluminescence increase. As stress-activated bonds in the surface film react, what can be called surface stress relaxation occurs resulting in the observed SCL decrease. 

Figure 12.6 shows the kinetic curves for the thermooxidation of propylene copol5miers with 1-pentene. It is seen that the induction periods for all samples are close. Atlliattime, the reaction rate corresponding to the slope of the kinetic curves for samples copolymers with a low content of 1-pentene unit (0.3 and 1.5%) compared to the IPP slightly increases. Further increasing of the 1-pentene content in copolymers leads to decreasing of the thermooxidation reaction rate (Table 12.4). 

Thermal, Thermooxidative, and Photooxidative Degradation. Polymers of a-olefins have at least one tertiary C-H bond in each monomer unit of polymer chains. As a result, these polymers are susceptible to both thermal and thermooxidative degradation. Reactivity in degradation reactions is especially significant in the case of polyolefins with branched alkyl side groups. For example, thermal decomposition of... 

Since the utility of these materials is improved by the incorporation of these reactive functionalities without severely decreasing other favorable properties such as thermooxidative stability and solvent resistance the chemistry of the isoimide isomerization and acetylene crosslinking reactions is of considerable interest. Previous work in our laboratory has shown that these materials, when loaded with metal powders, provide a convenient and effective method of optimizing the electrical conductance and thermal stability of aluminum conductor joints. 

Replacing the hydrogen in 68 with a phenyl group leads to the secondary acetylenic monomer 70. It was believed that this disubstituted acetylene would suppress the reaction of acetylene with itself and insure that there was an acetylene functionality available for reaction with the o-quinodimethane at 200 °G The DSC of 68 showed the presence of a single exothermic peak at 263 °C which the authors felt was adequate evidence for the occurrence of a Diels-Alder reaction between the acetylene and benzocyclobutene. Unfortunately they did not report on any control experiments such as that between diphenylacetylene and simple benzocyclobutene hydrocarbon or a monofunctional benzocyclobutene in order to isolate the low molecular weight cycloaddition product for subsequent characterization. The resulting homopolymer of 68 had a Tg of 274 °C and also had the best thermooxidative stability of all of the acetylenic benzocyclobutenes studied (84% weight retention after 200 h at 343 °C in air). 

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. 

No definitive evidence has appeared that identifies the source of the color generated during thermooxidation (95). However, two laboratories have postulated that the reactions leading to the formation of the color chromophores are aldol-type reactions, either via the reaction of aldehydes diiecdy (96)... 

Polymers of a-olefins are susceptible to thermal and thermooxidative degradation. Reactivity in degradation reactions is especially significant in the case of polyolefins with branched alkyl side groups,... 

If the carboxylic acids on the cellulosic chain are not the major cause of the thermooxidative decay of old cellulosic textiles, one must consider the carbonyl species, particularly the aldehydes on the C2 and Q) carbons. Nikitin (14) noted that the primary autoxidation process is a reaction of molecular oxygen with aldehyde groups, which initiates a chain reaction resulting in more profound changes and decomposition of the molecule . Thus, reduction of the aldehyde groups should lead to improved stability of degraded cellulose. 

Production and use of PVC occur in the presence of air, i. e. in the presence of oxygen. Therefore, it is surprising that the mechanistic details of thermooxidative degradation of PVC are still not fiilly revealed. The major reactions of this process are shown in Scheme 1. As indicated in this Scheme, thermal dehydrochlorination yields HCl and simultaneously sequences of conjugated double bonds (polyenes) in the chain. The reactive polyenes lead to peroxides in a reaction with oxygen followed by the formation of radicals. Subsequent chain reactions result in additional initiation of HCl loss and further oxidative processes (/, 8). 

Polysiloxanes exhibit exceptional properties over an extremely wide range of temperatures because of their unique combination of high thermal stability and low-temperature fiexibility. However, polysiloxanes cannot completely satisfy the needs for high-temperature elastomers that will perform in extreme thermooxidative environments for extended periods. This deficiency originates from the susceptibility of their backbone chains, which are composed completely of polarized siloxyl units, to degradation by ionic reactions when these materials are exposed to temperatures above 200-250 C. At and above such temperatures, polysiloxanes are degraded by... 

It has been noted lately [14] that within the limits of one class of stabilizers change of their structure, introduction of substituents weakly change their reaction ability in solid polymer. Obtained by us results do not agree with this supposition, as HC-1 and HC-2 have different effect on thermal and thermooxidative stability of PETP - fibres. 

And so, the work on mechanisms of autooxidation at the British Rubber Producers Association, the early work on the synthesis and reaction of stable free radicals, the recognition of the rale of stable free radicals in polymer stabilization, the discovery of stable triacetonamine-N-oxyl, and the search for practical candidates for commercialization, have led to the development of hindered amine stabilizers, a new class of polymer stabilizers. They are effective in many polymers against photodegradation and also are effective against thermooxidation in some polymers. The structures of the current commercially available products for polymer stabilization may be seen in Figure 7. These compounds are effective in meeting the stabilizer requirements in many commercial polymers however, others are under development to satisfy requirements not being met by them. 

This volume is including information about thermal and thermooxidative degradation of polyolefine nanocomposites, modeling of catalytic complexes in the oxidation reactions, modeling the kinetics of moisture adsorption by natural and synthetic polymers, new trends, achievements and developments on the effects of beam radiation, structural behaviour of composite materials, comparative evaluation of antioxidants properties, synthesis, properties and application of polymeric composites and nanocomposites, photodegradation and light stabilization of polymers, wear resistant composite polymeric materials, some macrokinetic phenomena, transport phenomena in polymer matrix, liquid crystals, flammability of polymeric materials and new flame retardants. 

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