ORGANOPHOSPHORUS ANTIOXIDANTS - ACTION MECHANISMS AND NEW TRENDS

Phosphite and phosphonite esters can act as antioxidants by three basi c mechanisms depending on their structure, the nature of the substrate to be stabilized and the reaction conditions. All phosph(on)ites are hydroperoxide-decomposing secondary antioxidants. Their efficiency in hydroperoxide reduction decreases in the order phosphonites > alkylpho sphites > arylphosphites > hindered arylphosphites. Five-membered cycl ic phosphites are capable of decomposing hydroperoxides catalytically due to the formation of acidic hydrogen phosphates by hydrolysis and p eroxidalysis in the course of reaction. Hindered aryl phosphites can a ct as chain-breaking primary antioxidants being substituted by alkoxyl radicals and releasing hindered aryloxyl radicals which terminate the radical chain oxidation. At ambient temperatures. the chain-breaking antioxidant activity of aryl phosphites is lower than that of hindered phenols, because the rate of their reaction with peroxyl radicals and their stoichiometric inhibition factors are lower than those of pheno ls. In oxidizing media at medium temperatures. however, hydrolysis of aryl phosph(on)ites takes place giving hydrogen phosph(on)ites and phe nols which are effective chain-breaking antioxidants. 2,2,6,6-Tetramet hyl- and 1,2.2,6,6-Pentamerhylpiperidinyl phosphites and phosphonites (HALS-phosph(on)ites) surpass many common phosphites, phenols and HALS compounds as stabililizers in the thermo- and photo-oxidation of poly mers. Their superior effciency is probably due to an intramolecular sy nergistic action of the HALS and the phosph(on)ite moieties of their m olecules.