Oxygen centered radicals

All laromatics. The aromatic ring is fairly inert toward attack by oxygen-centered radicals. Aromatic acids consisting of carboxyl groups substituted on aromatic rings are good candidates for production by LPO of alkylaromatics since thek k /k ratios are low. TerephthaUc acid [100-21 -0]... 

The ultimate fate of the oxygen-centered radicals generated from alkyl hydroperoxides depends on the decomposition environment. In vinyl monomers, hydroperoxides can be used as efficient sources of free radicals because vinyl monomers generally are efficient radical scavengers which effectively suppress induced decomposition. When induced decomposition occurs, the hydroperoxide is decomposed with no net increase of radicals in the system (see eqs. 8, 9, and 10). Hydroperoxides usually are not effective free-radical initiators since radical-induced decompositions significantly decrease the efficiency of radical generation. Thermal decomposition-rate studies in dilute solutions show that alkyl hydroperoxides have 10-h HLTs of 133—172°C. 

As with other hydroperoxides, hydroxyaLkyl hydroperoxides are decomposed by transition-metal ions in an electron-transfer process. This is tme even for those hydroxyaLkyl hydroperoxides that only exist in equiUbrium. For example, those hydroperoxides from cycHc ketones (R, R = alkylene) form an oxygen-centered radical initially which then undergoes ring-opening -scission forming an intermediate carboxyalkyl radical (124) ... 

Generation of radicals by redox reactions has also been applied for synthesizing block copolymers. As was mentioned in Section II. D. (see Scheme 23), Ce(IV) is able to form radical sites in hydroxyl-terminated compounds. Thus, Erim et al. [116] produced a hydroxyl-terminated poly(acrylamid) by thermal polymerization using 4,4-azobis(4-cyano pentanol). The polymer formed was in a second step treated with ceric (IV) ammonium nitrate, hence generating oxygen centered radicals capable of starting a second free radical polymeriza-... 

Oxygen-centered radicals are arguably the most common of initiator-derived species generated during initiation of polymerization and many studies have dealt with these species. The class includes alkoxy, hydroxy and aeyloxy radicals and tire sulfate radical anion (formed as primary radicals by homolysis of peroxides or hyponitrites) and alkylperoxy radicals (produced by the interaction of carbon-centered radicals with molecular oxygen or by the induced decomposition of hydroperoxides). 

There is an excellent, if non critical, compilation of absolute and relative rate data for reactions of oxygen-centered radicals covering the literature through 1982389 and for 1982-1992.39 1 Selected data from these and other sources are summarized in Table 3.7 and Table 3.8. The reactions of oxygen-centered radicals and their use in organic synthesis has been recently reviewed by Hartung el uIS )]... 

The pathways whereby oxygen-centered radicals interact with monomers show marked dependence on the structure of the radical (Table 3.8). For example, with MMA the proportion of tail addition varies from 66% for f-butoxy to 99% for isopropoxycarbonyloxy radical. The reactions of oxygen-centered radicals are discussed in detail in the following sections. 

Table 3.8 Specificity Observed in the Reactions of Oxygen-Centered Radicals with Various Monomers at 60 °C...

Because of the importance of hydroperoxy radicals in autoxidation processes, their reactions with hydrocarbons arc well known. However, reactions with monomers have not been widely studied. Absolute rate constants for addition to common monomers are in the range 0.09-3 M"1 s"1 at 40 °C. These are substantially lower than kL for other oxygen-centered radicals (Table 3.7). 454... 

Watanabe et al,25-5 52s applied AMS dimer (116) as a radical trap to examine the reactions of oxygen-centered radicals (e.g. r-butoxy, cumyloxy, benzoyloxy). AMS dimer (116) is an addition fragmentation chain transfer agent (see 6.2.3.4) and reacts as shown in Scheme 3,96. The reaction products are macromonomers and may potentially react further. The reactivity of oxygen centered radicals towards 116 appears to be similar to that of S.2 1 Cumyl radicals are formed as a byproduct of trapping and are said to decay mainly by combination and disproportionation. 

In contrast to phenols (Section 5,3.4), phenothiazine (39) is reported to be an excellent scavenger of both carbon-centered and oxygen-centered radicals by hydrogen atom transfer and is also used to stabilize monomers in storage.198... 

Most polymerizations in this section can be categorized as stable (Tree) radical-mediated polymerizations (sometimes abbreviated as SFRMP). In the following discussion systems have been classed according to the type of stable radical involved, which usually correlates with the type of bond homolyzed in the activation process. Those described include systems where the stable radical is a sulfur-ccntered radical (Section 9.3.2), a selenium-centered radical (Section 9.3.3), a carbon-centered radical (Sections 9.3.4 and 9.3.5), an oxygen-centered radical (Sections 9.3.6, 9.3.7), or a nitrogcn-ccntcrcd radical (Section 9.3.8). Wc also consider polymerization mediated by cobalt complexes (Section 9.3.9) and certain monomers (Section 9.3.5). 

Other Oxygen-Centered Radical-Mediated Polymerizations... 

Correa and Waters50 also proposed a mechanistic scheme where the key step of the overall reaction involves the recombination of sulfonyl radicals to form an intermediate with an O—S bond, the decomposition of which yields a sulfinyl radical (ArSO ) and an oxygen-centered radical ArS020 Later this suggestion was further strengthened on the basis of ESR studies51 and thanks to the elucidation of the electronic structure of sulfonyl radicals. However, it seems to the author that the available literature data point to an overall mechanism for equation 17 more complex that the one suggested50 (cf., for... 

Nitrogen- and oxygen-centered radicals offer some specific features. The rate constant of the reaction of dialkylaminyl radicals with (TMSlsSiFl is close to the analogous reaction of secondary alkyl radicals. In the reactions of... 

For this reason we have concentrated on the study of ketone photolysis in the presence of oxygen and the interaction of the oxygen-centered radicals arising in this reaction with certain tetramethylpiperidine derivatives. ... 

Formation of a Charge Transfer Complex between the nitroxide and one of the oxygen-centered radicals c or d. In the next step the complex is assumed to react with the remaining peroxyradical to form the products actually found, with release of the additive (reactions (13)/(14) or (13 )/(14 )) ... 

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