Mechanism, hydroperoxides thioethers

The secondary antioxidants are usually sulfur compounds (mostly thioethers and esters of thiodipropionic acid) or trimesters of phosphorous acid (phosphates). A remedy for many types of discoloration in plastics is often the use of a phosphite or a thioether. Both have the ability to react with hydroperoxides, as those formed in Reaction 1.72, to yield noruadical products, following heterolytic mechanisms. The reaction of phosphites to phosphates as an example ... 

Preventive antioxidants (sometimes referred to as secondary antioxidants), on the other hand, interrupt the second oxidative cycle by preventing or inhibiting the generation of free radicals. The most important preventive mechanism is the non-radical hydroperoxide decomposition, PD. Phosphite esters and sulphur-containing compounds, e.g. AO 12-18 in Table 1, are the most important classes of peroxide decomposers. The simple trialkyl phosphites (e.g. Table 1, AO 12) decompose hydroperoxides stoichiometrically (PD-S) to yield phosphates and alcohols, see reaction 4. Sulphur compounds, e.g. thioethers and esters of thiodipropionic acid and metal dithiolates (Table 1, AO 15-18, 31, 32), decompose hydroperoxides catalytically (PD-C) whereby one antioxidant molecule destroys several hydroperoxides through the intermediacy of sulphur acids, see reaction 5. References 1 and 2 give detailed discussion on antioxidant mechanisms. 

Based on the accumulated experimental evidence we can summarize by saying that HALS protect polymers by a combined mechanism where the principal activities are free radical scavenging, hydroperoxide deactivation, and formation of charge transfer complexes with oxygen. The stabilization action of HALS can be adversely affected by some other substances, like organic thioether costabilizers, halogen flame retardants, or an acidic environment. Several N-substituted HALS derivatives have... 

One of the reasons that thioethers are effective stabilizers is that their oxidation products act as longterm heat stabilizers [103-105]. They can be even more effective hydroperoxide decomposers than the original compounds [ 106]. A proposed mechanism of action of thioethers is shown in Scheme 18.15 [107]. 

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