Free radical reactions oxidative degradation

Fig. 15. Free radical induced oxidative degradation reactions.

The second set of chemical reactions summarizes the chelant, free-radical-induced oxidative degradation chemistry and is considerably more complex ... 

Once free radicals are produced on the polymer chain by some reaction, oxidative degradation takes place in the presence of air by the following mechanism ... 

Ascorbate is most likely required by hydroxylases to maintain iron or copper at the active enzyme site in the reduced form, since it is necessary for hydroxylation. The semidehydroascorbate radical is not very reactive (Bielski and Richter, 1975 Rose, 1989). It decays by disproportionation to ascorbate and dehydroascorbate (the latter subsequently degrades to oxalic acid and L-threonic acid), rather than acting as a reactive free radical. Reaction of ascorbic acid with (OH) is rapid and diffusion-dependent (K 7.2 x 10 -1.3 x lO o M- s- ) (Cabelli and Bielski, 1983). (O2) oxidizes ascorbic acid with a rate constant of 10 -10 M s (Bielski et al., 1985). Besides direct scavenging of radicals, ascorbic acid is known to have a number of physiological effects (Padh,... 

In polymers, the oxidative degradation is caused by the free radical reactions that lead to the formation of the peroxy radicals, viz. (R-22). The role of the peroxy radicals has been elucidated by several groups, e.g., by the luminescence and electron spin resonance methods, ESR [Partridge, 1972 Dole, 1973b Kadir et al., 1989 Kashiwabara and Seguchi, 1992 Williams, 1992 Jipa et al.,... 

Hydrolysis is the principal degradation mechanism for the condensation polymers. From the point of view of chemistry, the equilibrium molecular weight of these polymers is determined by the H O concentration at given temperature, T. However, owing to the moisture absorption from the air, the reaction equilibrium is shifted toward depolymerization. The rate of hydrolytic depolymerization depends on the moisture content, T and the presence of catalyst. Since these polymers are also subject to free-radical and oxidative processes (that lead to formation of unsaturations, hence the... 

Phospholipase A2 provides the major repair mechanism for membrane lipids damaged by oxidative free radical reactions. Arachidonic acid, which is a polyunsaturated fatty acid, can be peroxidatively cleaved in free radical reactions to malondialdehyde and other products. Phospholipase Aj recognizes the distortion of membrane structure caused by the partially degraded fatty acid and removes it. Acyltransferases then add back a new arachidonic acid molecule. 

Furans. Isomerization of allent (o give furan derivatives is catalyzed by Free-radical reactions. Silver n constitute an oxidizing system that dej reactive intermediates to radical acceptc acrylonitrile initiates synthetically useb oxidative degradation by (NHj).S.O, isocyanates in a biphasic system (11 exa... 

Free-radical reactions. Silver nitrate, sodium persulfate, and iron(III) nitrate constitute an oxidizing system that degrades carboxylic acids to radicals. Adding these reactive intermediates to radical acceptors such as methyl vinyl ketone, acrylic esters, and acrylonitrile initiates synthetically useful processes. Monoamides of oxalic acid undergo oxidative degradation by (NH )2SjOg in the presence of AgNOj-Cu(OAc)2 to afford isocyanates in a biphasic system (11 examples, 45-87%). ... 

Two reactions can occur during wear oxidative degradation as a result of frictional heating in the contact zone and mechanochemical degradation initiated by shear-induced rupture of chemical bonds. Present evidence favors the latter process. For example, in the absence of oxygen, the wear of cis-polyisoprene changes to resemble that of c/x-polybutadiene, whereas the wear of poly(ethylene-co-propylene) is unaltered (Gent and Pulford, 1983). These results are in accord with the response of these materials to free radical reactions. 

---