Nonradical degradation mechanisms

There are two ways in which stabilizers can function to retard autoxidation and the resultant degradation of polymers. Preventive antioxidants reduce the rate of initiation, e.g., by converting hydroperoxide to nonradical products. Chain-breaking antioxidants terminate the kinetic chain by reacting with the chain-propagating free radicals. Both mechanisms are discussed and illustrated. Current studies on the role of certain organic sulfur compounds as preventive antioxidants are also described. Sulfenic acids, RSOH, from the decomposition of sulfoxides have been reported to exhibit both prooxidant effects and chain-breaking antioxidant activity in addition to their preventive antioxidant activity as peroxide decomposers. 

Although the thermal degradation of PLLA proceeds mainly by nonradical mechanisms, previous studies by McNeill and Leiper [10, 31] have suggested that degradation in the presence of a free radical inhibitor proceeds much more slowly than in the case of a pure polymer. Tsuji et al. [37] also... 

The basic objective in the selection of antioxidants is to obtain the best stabilization with no effect on optimum values of technical properties of polymers. The stabilizing effect on oxidative degradation is based on protection against the formation of active free radicals and on nonradical decomposition of hydroperoxides. Optimum stabilization is obtained by the combination of different compounds that function with different mechanisms as metallic ion deactivating agents, UV absorbers, and so on. 

Poly((] )-3-hydroxybutyrate) (PHB), poly(E-lactide) (PLEA) and poly(e-caprolactone) (PCL) are biodegradable polyesters used in practical applications mainly because they combine remarkable physico-mechanical performances with biodegradability, compostability and compatibility with different forms of waste disposal a.439. As shown in Scheme 17(a), thermal degradation of PHB has been suggested to occur almost exclusively by a nonradical random chain scission reaction (c/s-elimination) involving a six-membered ring transition state a.l50] 852663 832485. ... 

UV studies of PC samples at different extents of dehydrochlorination have revealed that the predominant absorption are characteristic of trienes. This is to be expected if random loss of HCl occurs by the nonradical mechanism shown in Scheme 26. PC does not accelerate PMMA degradation in PC/PMMA blends,which is also consistent with a nonradical dehydrochlorination. 

Polyoxymethylene (6) as prepared has OH ends the polymer is of low stability and degrades to formaldehyde in 100% yield from about 100 °C. The stability is improved by acetylating the chain ends, which indicates that end initiation of depolymerization plays an important part, but the product of degradation of the end-capped polymer is the same. The formation of formaldehyde has been explained by a nonradical mechanism as shown in Scheme 36. A similar six-centre transition state may be envisaged, leading to the release of two molecules of HCHO at each step. 

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