Piperidinoxyl Radical

Transformation of Piperidinyl to Piperidinoxyl Radical In the above-mentioned radical scavenging mechanisms, the nitroxide is the key intermediate. Several mechanisms have been suggested for the formation of the nitroxide from the parent amine. Sedlar et al. [76] proposed a reaction of a hydroperoxide with the amine to form an alkyloxyamine, which reacts with a peroxy radical to form a nitroxide (Scheme 17.15). 

The hindered-amine stabilizers readily form nitroxyl compounds. Denisov (10,11) has found facile formation in polypropylene. Rozantsev (12,13) indicates that the mechanisms by which these sterically hindered amines act is via the generation of stable piperidinoxyl radicals, and he proposes their formation. 

ESR studies of free radicals formed under UV-irradiation were reported for hindered piperidine photostabilisers and antioxidants [820]. Kelen et al. [819] reported an ESR study of hindered piperidine derivatives in a chalk filled PP matrix in the presence of other additives (frganox 1010, Tmuvin 770/622), with particular emphasis on concentration changes of Ai-oxyl radicals and interaction between a HALS compound and a hindered phenol. Other additives present in the polymer influence the concentration of the /V-oxyl radicals. Lattimer et al. [821] studied oxidation of the partially hindered bicyclic amine 3,3-dialkyldecahydroquinoxalin-2-ones (excellent UV stabiliser and thermal antioxidant) with w-chloroperbenzoic acid by means of ESR and reported some extremely stable radical derivatives (over 231 days of stability). ESR was also used to measure the piperidinoxyl radical concentration, and hence the HALS content in LDPE/(Chimassorb 944, Tmuvin 622) agricultural film during use. Evidence was reported for polymer-bound radicals [117]. 

In an attempt to elucidate the mechanism, the reaction was performed in the presence of a radical trap (TEMPO=2,2,6,6-tetramethyl-l-piperidinoxyl), a reversible electron acceptor (dinitrobenzene), or a radical sensitizer (dimethoxybenzene). In all cases the proportion of coupled product was increased, suggesting that both radical and polar mechanisms contribute to the outcome of this reaction. 

If R = R = H, R" = OCHj, then radical VII is oxidised by bromine. In the case of R = R = R" = OCHj, radical Vll is readily oxidised (even by iodine). The basic products of the vigorous reaction of radical 1 with ozone are nitro-t-butane and oxygen [27]. The oxidation of alcohols into carbonyl compounds can be carried out via the interaction of piperidinoxyl with copper (11) [28] ... 

By the same method of living radical polymerisation, a series of block copolymers of poly(ethylene oxide-styrene) with narrow polydispersity were synthesised by the following two-step approach [96]. Initially, living anionic polymerisation of ethylene oxide with sodium-4-oxy-2,2,6,6-tetramethyl-l-piperidinoxyl as initiator yields polyethylene oxide with ARs at the chain end ... 

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