Mechanism of Oxidative Degradation

The thermal stability of polyethylene (PE) decreases sharply in the presence of oxygen. This effect is clearly observed at elevated temperatures, which promote rapid development of the oxidative processes that essentially decrease the mechanical properties of PE. Thus, low-pressure PE (0.93 g/cm ) completely loses its mechanical strength after exposure at 100 C for 48 hours in air the impact viscosity of such a material is only 7% of its initial value [1]. 

The oxidation process is complex not only from the chemical, but also from the physicochemical viewpoint. This is due to the uneven course of the process in the bulk of the PE. It has been shown that the amorphous zones of PE, being more accessible to oxygen molecules than the densely packed crystalline regions, are oxidised first. The amorphous zones of PE act as a buffer which protects the crystalline regions from attack during thermal degradation. Thus, the increase in crystallinity results in an increase in sensitivity of the mechanical properties of ethylene to oxidation (though its oxidation rate is lower) [2]. 

Studies on the mechanism of polyalkene oxidation have shown that hydroperoxides are its primary products, which then decompose to form other oxygen-containing compounds. The oxidation of polyalkenes is described by a radical-chain scheme as follows. Initiation may occur with the formation of radicals R or RO -  

In the case of the developed oxidation process, initiation occurs owing to the decomposition of hydroperoxides with the formation of radicals by monomolecular or bimolecular reactions [4]  

The radicals formed attack the C-H groups of polyalkenes to produce hydroperoxides  

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In contrast to the extensive work of the pure thermal degradation of polymers, less fundamental chemical information is available on the mechanism of oxidative degradation of polymeric materials. As another point of... 

Scheme 12. Tentative mechanism of oxidative degradation ofp-ferrocenylani-line (147).

Microscopic Mechanisms of Oxidative Degradation and Its Inhibition at a Copper-Polyethylene Interface... 

Fig. 14.7 Mechanism of oxidative degradation of full-carbon backbone polymeric materials, a General scheme including also the ultimate stage of biodegradation, b Specific transition metal salts able to promote oxidation followed by degradation of full-carbon backbone polymers in a tandem fashion action...

Figure 1.3 Free-radical mechanism of oxidative degradation of polymers...

Ochoa s attention was next concentrated upon one of the central issues of that period the mechanisms of oxidative degradation of pyruvic acid to CO2 and H2O. In the previous decades, Torsten Thunberg had discovered the oxidative capacity of tissue extracts for a number of dicarboxylic acids. Albert Szent-Gyorgyi had observed the catalytic effect of succinic, fumaric, malic and oxaloacetic acids in the respiratory process and related these observations to the metabolism of carbohydrates. The oxidative process implied the formation of CO2 and hydrogen, the latter being eventually accepted by respiratory oxygen to form water with the participation of cytochrome. 

Formed ketonic (CO) groups play a very important role in further mechanisms of oxidative degradation of polymers (cf. section 3.2.1). 

The mechanism of this degradation has received considerable attention, and for some species the reaction is equivalent to a nonenzymatic Baeyer-Villager reaction, producing first the 17j8-acetate. This functionality can then be hydrolyzed and oxidized to the ketone and may undergo a second Baeyer-Villager reaction to produce a lactone ... 

Simulation programs for the ESR line shapes of peroxy radicals for specific models of dynamics have been developed for the study of oxidative degradation of polymers due to ionizing radiation [66]. The motional mechanism of the peroxy radicals, ROO, was deduced by simulation of the temperature dependence of the spectra, and a correlation between dynamics and reactivity has been established. In general, peroxy radicals at the chain ends are less stable and more reactive. This approach has been extended to protiated polymers, for instance polyethylene and polypropylene (PP) [67],... 

The mechanism of PIP degradation appeared to be principally different. PIP has double bonds and oxidizes through intramolecular peroxyl radical addition to the double bond with formation of peroxide bridges. 

The studied inhibitors differ in their ability to retard degradation as well as oxidation of PIB. There is no similarity in their activity to retard oxidation and destruction. The following mechanism of polymer degradation was proposed for PIB [85] ... 

The influence of substituents on the rates of degradation of arylazo reactive dyes based on H acid, caused by the action of hydrogen peroxide in aqueous solution and on cellulose, has been investigated [43]. The results suggested that the oxidative mechanism involves attack of the dissociated form of the o-hydroxyazo grouping by the perhydroxyl radical ion [ OOH]. The mechanism of oxidation of sulphonated amino- and hydroxyarylazo dyes in sodium percarbonate solution at pH 10.6 and various temperatures has also been examined. The initial rate and apparent activation energy of these reactions were determined. The ketohydrazone form of such dyes is more susceptible to attack than the hydroxyazo tautomer [44]. 

The mechanism of metabolic degradation of indol-3-ylacetic acid (39) is a matter of debate. A possible route demonstrated in vitro includes oxidative decarboxylation to skatolyl hydroperoxide (40), catalyzed by horseradish peroxidase isoenzyme C (HRP-C), followed by rearrangement to 3-(hydroxymethyl)oxindole (41), as shown in equation 12 . 

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