Stabilizers chain-breaking antioxidant

Polymer stabilization is another area in which the peroxide-decomposing and chain-breaking antioxidant properties of diorganotellurides has found utUity. Alone or in combination with phenol and phosphate antioxidants, electron-rich dialkylamino-substirnted diaryltellurides and alkylaryltellurides provided greatly enhanced polymer stability for a thermoplastic elastomer and for polypropylene. The effects were unique to the tellurides, with selenides not providing similar protective effects. ... 

Other Physicochemical Aspects Associated to the Chain-Breaking Antioxidant Effect The Stability of the Polyphenol-Derived Free Radicals... 

Pospisil, J. Chain-breaking antioxidants in polymer stabilization. In Developments in Polymer Stabilisation, Scott, G., Ed. Applied Science London, 1979 Vol. 1, 1-37. 

The efficient decomposition of hydroperoxides by a non-radical pathway can greatly increase the stabilizing efficiency of a chain-breaking antioxidant. This generally occurs by an ionic reaction mechanism. Typical additives are sulfur compounds and phosphite esters. These are able to compete with the decomposition reactions (either unimolecular or bimolecular) that produce the reactive alkoxy, hydroxy and peroxy radicals and reduce the peroxide to the alcohol. This is shown in the first reaction in Scheme 1.69 for the behaviour of a triaryl phosphite, P(OAr)3 in reducing ROOH to ROH while itself being oxidized to the phosphate. 

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. 

Since stabilization against photooxidation and metal-catalyzed oxidation are covered elsewhere in this symposium, this discussion is restricted to protection against thermal autoxidation. I will first review the mechanism by which typical chain-breaking antioxidants function and then describe some of our current studies on the way in which certain organic sulfur compounds act as preventive antioxidants. 

J. Pospigil, Chain-Breaking Antioxidants in Pdymer Stabilization, in G. Scott (ed.), Developments in Polymer Stabilization, Applied Science Publishers, London (1979), p. 1. 

Hydroperoxides as intermediate compounds initiate the chain reaction unless stabilizers (InH functioning there as the chain-breaking antioxidants or D as peroxides decomposer) intermpt it (reactions 13 and 14, Scheme 12.5). 

The use of phenolic and amino-based antioxidants (ie, thermal stabilizers) by this approach has been limited because they inhibit the free-radical polymerization process (polymerization inhibitor) leading to lower efficiency. One of the few commercial products produced is based on the polymerizable chain breaking antioxidant (AO 12b, Table 3), designed for NBR rubbers (Chemigum HR 665) that has been shown to offer superior antioxidant performance, especially imder aggressive (hot oil/high temperature) conditions, compared to low molecular mass conventional aromatic amine antioxidants (165). In spite of the successful synthesis and copoljmierization of a large number of reactive antioxidants, there is a lack of major commercial development and production of antioxidant systems... 

Antioxidants are most effective in stabilizing oxidation-prone drug formulations. They have the ability to inhibit or slow down chain reaction oxidative processes at relatively low concentrations. This property of the antioxidant substances is of considerable importance with respect to formulations because of the large number of chemically diverse medicinal agents known to undergo oxidative decomposition. Antioxidants are classified as preventive antioxidants or chain-breaking antioxidants based upon their mechanism. Preventive antioxidants include materials that act to... 

Another group of chain breaking antioxidants of rapidly increasing importance is the so-called hindered-amine stabilizers (HAS). These were originally developed as photostabilizers. The mechanism of their action is discussed below and involves the initial oxidation of the amine to a nitroxide... 

The phenoxy radicals are relatively long-lived and can undergo self-disproportionation, recombination vith alkylperoxy radicals, or isomeric rearrangement followed by recombination. The resulting compounds may have some stabilization activity. Propionate-type hindered phenols constitute a special class in this category during reaction, the phenol is transformed into phenolic cinnamates, which are known to be efficient chain-breaking antioxidants [Eq. (85)]. 

The phosphite efficiency in hydroperoxide reduction decreases in the order phosphites > alkyl phosphites > aryl phosphites > hindered aryl phosphites. Hindered aryl phosphites can also act as chain breaking primary antioxidants when substituted by alkoxy radicals but their activity is lower than that of hindered phenols but in oxidizing media at high temperatures the hydrolysis of aryl phosph(on)ites takes place and produces hydrogen phosph(on)ites and phenols which are effective chain-breaking antioxidants. Multifunctional stabilizers such as those containing HAS-phosph(on)ites moieties show a superior efficiency due to the intramolecular synergism—Table 6 [49]. 

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