Hydroperoxide complexes, HALS

The complex formation between hydroperoxides and HALS derivatives proposed for the preceding reaction was recently postulated by two different groups of investigators. First, Carlsson determined a complex formation constant for +00H and a nitroxide on the basis of ESR measurements—. Secondly, Sedlar and his coworkers were able to isolate solid HALS-hydroperoxide complexes and characterize them by IR measurements. The accelerated thermal decomposition of hydroperoxides observed by us likewise points to complex formation. It is moreover known that amines accelerate the thermal decomposition of hydroperoxides-. Thus Denisov for example made use of this effect to calculate complex formation constants for tert.-butyl hydroperoxide and pyridineitZ.. 

In sum, the results described have led us to postulate the following possible mechanism as explanation of the observed retardation of hydroperoxide formation by TMP derivatives The HALS studied form a complex with the hydroperoxides which is much more efficiently broken down by peroxy and/or alkoxy radicals - with formation of harmless products - than hydroperoxides alone (reaction (26)). The result is a lowering of the rate of formation of hydroperoxides. 

Transition Metal Complexation It is well known [84] that transition metals can initiate the photo-oxidation of polymers. Fairgrieve and McCallum [85] showed that HALS is capable in complexing transition metals. They postulated that the mechanism of action of HALS is related to this complexation, which could prevent the decomposition of hydroperoxides into initiating radicals. Moreover, in this case an increased concentration of HALS in the oxidizing zones can be expected. 

HALS operate by a combination of different mechanisms, including the trapping of free radicals, the deactivation of hydroperoxides, and the formation of charge transfer complexes with oxygen. 

Keywords sunlight, heat, hydroperoxides, autoxidation, weathering, photochemical degradation, processing, phosphites, sulphur compounds, metal thiolates, nickel complexes, UV absorbers, HALS, metal deactivators, hindered phenols. 

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... 

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