Nitroxyl radicals light stabilizing activity

The mode of action of hindered piperidine compounds continues to be rather controversial. A recent study of the effect of different N-substituents on the light stabilizing activity of a particular hindered piperidine molecule in polypropylene showed that the nitroxyl radical —O was the most effective... 

As shown in Table I, these stable radicals showed strikingly higher light stabilization activity in polypropylene than that of the UV absorber tested. We felt that their activity was related to their radical scavenging ability. This hypothesis is supported by the observation that the coupled products (32) and (33) were obtained by the reaction of the nitroxyl radicals (2) and (27), respectively, with a C-radical derived from AIBN (10). The radical scavenging ability of the stable nitroxyl radicals is now well known to play a major role in the mechanism of light stabilization by hindered amine compounds (13). 

Elimination of the yellowing introduced by the stable nitroxyl radicals was essential for commercial development of these excellent stabilizers. Since phenolic antioxidants are necessary for the thermal stabilization of polymers during processing, we turned our attention to ways in which unfavorable interactions between the hindered phenols and the stabilizing nitroxyl radicals could be avoided. In this section we describe the discovery of the light-stabilizing activity of hindered amine compounds, an improved synthetic method for these compounds, the synthesis of a number of derivatives, and the evaluation of their stabilizing activity. 

These results suggest that hindered amine compounds could be converted to stable nitroxyl radicals through the corresponding amino radicals in polymers that is, they are the precursors of the stable radicals. In fact a key compound, 2,2,6,6-tetramethyl-4-oxopiperi-dine (42), showed high light-stabilizing activity in polypropylene, comparable to that of the nitroxyl radicals, lending support to this interpretation. 

At the very beginning it was found that HALS do not absorb UV light, and so they cannot be classified as UV-absorbers or quenchers of excited states. The first mechanism proposed to explain HALS stabilization activity is known as the Denisov cycle. This mechanism attributes the key role in stabilization to HALS transformation products (nitroxyl radicals and hydroxylamino ethers), and it fits the generally accepted pathway of hydrocarbon degradation through alkyl- and alkylperoxy-radicals (Figure 3) ... 

The most recent class of light stabilizer is the Hindered Amine Light Stabilizer (HALS). These materials have been shown to function as radical traps, thus interrupting the radical chain degradation mechanism. The cyclic stabilization mechanism proposed for HALS involves multiple regeneration of the active nitroxyl stabilizer. The surprising performance of HALS at relatively low concentrations supports this non-sacrificial mechanism. 

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