Nitroxide Formation

A very early indication that C-nitroso-compounds are particularly susceptible to radical attack was the methyl affinity value of 105 determined for nitrosobenzene (Heilman el al., 1957). This figure reflects the reactivity of nitrosobenzene, relative to that of benzene, towards attack by methyl radicals. It was apparently several years before this reactivity was linked with nitroxide formation... 

The exact mechanisms for the conversion of hindered amine to nitroxide are not known in these coatings. According to the treatment above, the initial rate of formation of nitroxide should be proportional to [Y00 ]. From the scheme of equation 5, the overall rate of photooxidation should also be proportional to [Y00 ]. Even though there appears to be a qualitative correlation between the initial nitroxide formation rate and the rate of degradation, the initial formation rate is hardly proportional to either K or Ke. Apparently nitroxide formation from HALS- is more complex than indicated by the simple scheme above. Compared to the excellent correlation found between photodegradation rates and photoinitiation rates in these coatings, no single parameter in the... 

The latter pathway is very vague in the 1,4-PD series. No nitroxide corresponding to N,N -diphenyl-l,4-PD (DPPD) was detected in the oxidized substrate by Adamic and co-workers (2. It has been explained as a consequence of the quick oxidation of the initially formed monoaminyl into BQDI (4) without nitroxide formation. An ESR signal characteristic of nitroxide corresponding to N-isopropyl-N -phenyl-1,4-phenylenediamine (IPPD) was detected in an oxygen deficient system (9), i.e. under rubber fatiguing conditions. It is therefore possible that the mononitroxide is an intermediate formed... 

Reaction of mesomeric C-radicals 42b and 42c with ROO is a competitive process to the nitroxide formation and accounts for coloured benzoquinone monoimines (BQMI), e.g. 1 -phenyl-1,4-benzoquinone-4-imine (57, R = H) or 1,2-benzoquinone imine 58, (R = ter/-octyl or an aralkyl) [5]. QI 58 transforms further into 3,7-di-te/7-octyl-1 //-phenoxazone-1 (59) or 2-(4 -tert-... 

In addition to the study of the effect of nitroxides on the reaction rate (see below), Ingold and his coworkers [24] have also turned their attention to the possibility of nitroxide formation from different amines in the context of the possibility of complex formation [7,8], reactions (9) and (10), in the systems. ESR studies have been used to show that, for... 

Typical representatives of primary antioxidants are secondary aromatic amines and sterically hindered phenols both classes are widely used in the protection of polymers (Figs. 11.5 and 11.6). Aromatic amines act as H donors by forming aminyl radicals, followed by coupling reactions and/or nitroxide formation and further coupling reactions. 

Attack of methyl radical on oxygen of the azoxy structure (eq. 3) is also plausible but is considerably less exothermic and has a higher activation energy than aminyl nitroxide formation (eq. 2). 

Performed experiments proved the heterogeneous character of oxidation of the PP plaques containing HAS by profiling nitroxide formation using ESRI method. 

Figure 8 - Nitroxide formation from 2,2,6,6-tetramethyl piperidine-4-one...

Besides interaction between stabilizers with other additives, different types of stabilizers can react with each other too. It is known that combinations of HA(L) S and suffiir-containing stabilizers can show an antagonism [128]. This antagonism was ascribed to a reduced nitroxide formation rate due to the decomposition of... 

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