Galvinoxyl radical stability

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. 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

Free radicals can also be stabilized kinetically by steric shielding of the radical centers. Such free radicals are called persistent. Examples of persistent free radicals include perchlorotrityl, galvinoxyl, and the radical derived from BHT (butylated hydroxytoluene, 2,6-di-tert-butyl-4-methylphenol), an antioxidant that is used as a food preservative. 

In the structure shown, galvinoxyl has the radical on the lower oxygen. It has another resonance structure, equivalent to this one, which has the radical located on the upper oxygen. In addition, it has a number of resonance structures which have the radical located on the carbons ortho and para to the two oxygens, and on the carbon connecting the two rings. Because it has considerable resonance stabilization, and the positions where the radical electron density is located are sterically hindered, the radical is relatively unreactive. 

When steric effects are combined with stabilizing effects such as delocalization, radicals that are very "stable" can be prepared. For example, both galvinoxyl and diphenylpicrylhydrazyl (DPPH) are commercially available free radicals that can be handled with no special precautions (see margin). 

Transformation (oxidation) products of antioxidants formed during melt processing may exert either anti- or pro-oxidant effects the extent of their contribution determines the overall effectiveness of the antioxidant. For example, in BHT, peroxydienones, PxD (see reactions 8) lead to pro-oxidant effects, due to the presence of the labile peroxide bonds, whereas quinonoid oxidation products, SQ and G (reactions 6 and 7), are antioxidants and are more effective than BHT as melt stabilizers for PP. The quinones are effective CB-A antioxidants and those which are stable in their oxidized and reduced forms (e.g. galvinoxyl, G, and its reduced form, hydrogalvinoxyl, HG) may deactivate both alkyl (via CB-A mechanism) and alkylperoxyl (via CB-D mechanism) radicals in a redox reaction (reaction 7). 

Both nitroxyls and their derived hydroxylamines are good UV stabilizers. The overall high efficiency of nitroxyl radicals as UV stabilizers in polyalkenes (see Figure 6) is, therefore, due to a complimentarity of the donor and acceptor antioxidant mechanisms. Scheme 13 summarizes the regenerative cycle involving nitroxyl radicals formed from hindered piperidines and emphasizes the redox antioxidant function (cf Scheme 8 for a comparison with galvinoxyl) of the CB-A/CB-D combination during photooxidation of PP. 

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