Nitroxyl radicals, their stabilizing

Nitroxyl radicals are formed as intermediates in reactions of polymer stabilization by steri-cally hindered amines as light stabilizers (HALS) [30,34,39,59]. The very important peculiarity of nitroxyl radicals as antioxidants of polymer degradation is their ability to participate in cyclic mechanisms of chain termination. This mechanism involves alternation of reactions involving alkyl and peroxyl radicals with regeneration of nitroxyl radical [60 64],... 

There is some evidence to suggest that in the case of bifunctional compounds a proportion of the nitroxyl radicals becomes chemically bonded to the polymer backbone (80MIU507). Such attachments further increase the effectiveness of this class of stabilizer by improving their long term retention by polymeric materials. 

Degradation and Stabilization of Ethylene-Propylene Copolymers and their Blends 201 proofing, nitroxyl radicals are regenerated [158] ... 

Great attention in literature is paid to stabilizers - spatially difficult amines and then-derivatives [10, 70] which act as inhibitors of radical processes of polymer destruction. Difficult piperidines and their nitroxyl radicals are among them ... 

The redox reaction also extends to the participation of hydroperoxides, but their efficient decomposition depends on the formation of a non-radical product such as an alcohol. Another example of a redox couple is found in the behaviour of the nitroxyl radical (R NO )- Depending on the structure of R, these are efficient radical scavengers and a redox couple between the mdical and the hydroxyl amine (R NO /R NOH) is formed (which is analogous to the galvinoxyl radical G-/GH). It is noted that the hindered amine stabilizers (e.g. Tinuvin 770 and the monomeric and polymeric analogues) are ineffective as melt antioxidants, possibly because of reaction with hydroperoxides or their sensitivity to acid. 

This chapter is a comprehensive overview of the progress in the field of generation, chemistry, and application of nitroxyl radicals and their precursors, for example, hindered amines of the 2,2,6,6-tetramethy1-piperidine series. Because of the importance of nitroxyl radicals to polymer stabilization, this application is discussed at length, while the others are touched upon briefly. 

The possibility of stabilizing polymers with hindered nitroxyl radicals is based on their reaction with polymer radicals which are engaged in propagating polymer oxidation. It was established that hindered amines, the precursors of nitroxyl radicals, are also effective photostabilizers and, as such, they are more effective than most of the common photostabilizers for polymers. 

Since UV absorbers cannot completely prevent the initiation reactions above, we felt that better antioxidants were necessary to obtain improved long-term photo-oxidative stability of polyolefins. We reasoned that stable radicals which scavenge radicals formed in the oxidative process might compensate for the imperfections of the absorbers. On the basis of the assumption that the diaryl nitroxyl radicals, rather than their parent diarylamines, are the active antifatigue species in rubber protection, we first attempted to obtain new stable nitroxyl radicals. 

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. 

The formation of nitroxyl radicals NO- is essential for stabilization since the concentration of harmful peroxy radicals falls sharply in their presence [5.19]. 

This cyclic process, along with the fact that HALS are compatible with and relatively nonextractable from the resin, makes them valuable for imparting long-term UV resistance. They effectively delay the degradation of the properties of an exposed polyolefin, and a critical measure of their effectiveness is the speed with which a HALS compound is oxidized into its useful nitroxyl radical form. However, HALS can also react with other stabilizers, flame retardants, or other additives, as discussed in Section 4.4.2. These reactions can lead to color and property changes [1-1, 4-7, 4-12],... 

The ability of these energy quenchers to stabilize polypropylene fibers to weathering permitted the development of many new end-uses, but their capabilities have been surpassed by a new group of stabilizers that contain a hindered piperidine structure. As shown above, these HALS compounds can be very good long-term thermal stabilizers. Hindered piperidines react with hydroperoxides during polypropylene processing to form nitroxyl radicals (II) that arc effective polymer radical traps [134]. These nitroxyl radicals react with polymer free radicals to form the polymeric hydroxylamine (III). 

The principle of spin labeling is very simple - a macromolecule is attached to a stable free radical and the increase in molecular mass of the radical reduces its rate of tumbling, which can be easily monitored by ESR spectroscopy. The most suitable class of free radicals for the purpose are nitroxyls because of their stability and because the anisotropic g and A tensors are sufficiently different in the x-, y-, and -directions to provide high sensitivity to a change in the rate of molecular tumbling. Rates of tumbling can be assessed, usually by computer simulation, by the change from a completely isotropic spectrum (rates... 

Free radicals are neutral or charged particles with one or more uncoupled electrorrs. Unlike usual (short-living) radicals, stable ones (long-living) are characteristic of paramagnetic substances whose chemical particles possess strong delocalized uncoupled electrons and sterically screened reactivity centers. This is the very catrse of the high stability of many classes of nitroxyl radicals of aromatic, fatty-aromatic and heterocyclic series, and ion radicals and their complexes. 

First Generation Monomeric HALS. Several decades ago, physical organic chemists were entranced by the concept of stable free radicals and their potential as radical trapping agents. References on the use of nitroxyls as potential stabilizers for plastics had already been cited in the patent literature (44). 

The above cyclic mechanism has been carefully scrutinized by many workers in the past few years, and, in fact, it has been concluded that it alone cannot fully account for the high photoprotective efficiency of the parent amine molecule. The nitroxyl radical itself is a radical scavenger but is not as effective as hindered phenols in competing with oxygen for alkyl radicals. To account for this deficiency in the cyclic mechanism it has been suggested, and indeed confirmed, by many workers, that hindered piperidine stabilizers and their derived nitroxyl radicals form weakly bonded localized complexes with hydroperoxides in the polymer (Scheme 8). This mechanism raises the local concentration of nitroxyl radicals in regions where alkyl radicals are generated after the... 

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