Nitroxide regeneration

The nitroxide concentration rises rapidly to a maximum then slowly decays. The slow decay of the nitroxide concentration In the hindered amine doped coatings together with the high rate of Initiation demonstrates the Importance of nitroxide regeneration as a stabilization mechanism. The effectiveness of the hindered amine stabilizers Is a function of the kinetic chain length and of the lifetime of the stabilizer In the coating. The results found here are contrasted with degradation and stabilization studies In other polymers such as polypropylene and polyethylene. 

Based on the results of the degradation of unstabilized PP and the effectivity of HALS stabilizers in different model systems it is postulated that HALSs are mainly effective in systems in which peracids can be formed [50]. According to Step et al. [63,64] this is because in the nitroxide regeneration mechanism the peracyl radical plays a key role (Scheme 17.13). 

In all the nitroxide regeneration mechanisms the nitroxide reacts with an alkyl radical, this reaction is in competition with the extremely fast reaction between an alkyl radical and oxygen (see Scheme 17.14)... 

Reactivity with ROO was also used for explanation of the nitroxide regeneration from 65 and, consequently, of the high stoichiometric factor of DPA in autoxida-tion of hydrocarbons [66]. Thermolysis of NOR 65 at temperatures exceeding 100 °C was proposed as another process regenerating the antioxidant activity [65]. Hydroxylamines 66, strong CB AO, are formed. NOH 66 are reoxidized to 56 either by scavenging ROO (Scheme 7) or by oxidation with ROOH via CTC pN +OH, HO, RO ]. 

Results of experiments performed with oxidized PP doped with secondary HAS Tinuvin 770 show high concentration of nitroxides in the vicinity of both surfaces due to the DLO. This was observed after continuous exposure to radiation in the Weather-Ometer on the irradiated (front) and non-irradiated (back) surfaces as well as on the both surfaces of the samples e qjosed thermally in hot air oven. Very low concentration of nitroxides was present inside of the samples. The concentration profiles are of characteristic U-shape and indicate preferential surface oxidation of PP, with a specific response to thermal and photochemical stress (Fig. 2). The assumed complex HAS mechanism is thus more explicitly confirmed in thick samples than by monitoring nitroxide concentration in PP films. It is consistent with surface consumption of oxygen in thick plaques and lower availability of oxygenated products in the depth of the PP matrix necessary for a direct development of nitroxides from HAS as well as for nitroxide regeneration from O-alkylhydroxylamine >NOP within the regenerative cycle (7). 

Step et al. [118,119] concluded that peracyl radicals play a key role in the nitroxide regeneration mechanism. 

Eor antioxidant activity, the reaction of aminyl radicals with peroxy radicals is very beneficial. The nitroxyl radicals formed in this reaction are extremely effective oxidation inhibitors. Nitroxides function by trapping chain-propagating alkyl radicals to give hydroxylamine ethers. These ethers, in turn, quench chain propagating peroxy radicals and in the process regenerate the original nitroxides. The cycHc nature of this process accounts for the superlative antioxidant activity of nitroxides (see Antioxidants). Thus, antioxidant activity improves with an increase in stabiUty of the aminyl and nitroxyl radicals. Consequendy, commercial DPA antioxidants are alkylated in the ortho and para positions to prevent undesirable coupling reactions. 

Oxidations Using Oxoammonium Ions. Another oxidation procedure uses an oxoammonium ion, usually derived from the stable nitroxide tetramethylpiperidine nitroxide, TEMPO, as the active reagent.31 It is regenerated in a catalytic cycle using hypochlorite ion32 or NCS33 as the stoichiometric oxidant. These reactions involve an intermediate adduct of the alcohol and the oxoammonium ion. 

The fact that the nitroxyl radical emerges from the reaction practically unchanged points to a mechanism in which there is specific regeneration of nitroxide involving all the radicals present. 

This process is very important in the HALS stabilization mechanism. It should, however, be considered only as a minority pathway in diarylamines just because of the lower stability of the corresponding diarylnitroxides. This results in the participation of the latter in side reactions leading to antioxidant ineffective species, e.g. benzoquinone (BQ) and nitrobenzene ( 7 ). Transformation of diarylnitroxide into a mixture of diarylamine and N-aryl-1,4-benzoquinone monoimine-N-oxide (4. 8) seems therefore to be a more probable pathway regenerating partly amine antioxidant than the hydroxylamine/nitroxide cyclical process (Scheme 2). 

The importance of the cyclical regeneration of nitroxide in the aromatic series seems to be therefore questionable N,N -di-substituted PD, most probably not involved in this cycle, are generally more efficient chain-breaking antioxidants than both diphenylamine and N-phenyl-1-naphthylamine, potentially partly involved in the nitoxide cycle. It may, therefore, be supposed that the high antioxidant activity of PD more probably accounts for the positive cooperative effects of PD with its principal oxidative transformation product, BQDI. 

Besides the formation of NO from the parent NH or NCH3, the nitroxides arising in light-induced and dark regeneration steps from the diamagnetic... 

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