Free radicals from peroxides

Polymerization reactions. There are two broad types of polymerization reactions, those which involve a termination step and those which do not. An example that involves a termination step is free-radical polymerization of an alkene molecule. The polymerization requires a free radical from an initiator compound such as a peroxide. The initiator breaks down to form a free radical (e.g., CH3 or OH), which attaches to a molecule of alkene and in so doing generates another free radical. Consider the polymerization of vinyl chloride from a free-radical initiator R. An initiation step first occurs ... 

Figure 15.11 Possible scheme for the formation of free radicals from the metabolism of dopamine. Normally hydrogen peroxide formed from the deamination of DA is detoxified to H2O along with the production of oxidised glutathione (GSSG) from its reduced form (GSH), by glutathione peroxidase. This reaction is restricted in the brain, however, because of low levels of the peroxidase. By contrast the formation of the reactive OH-radical (toxification) is enhanced in the substantia nigra because of its high levels of active iron and the low concentration of transferin to bind it. This potential toxic process could be enhanced by extra DA formed from levodopa in the therapy of PD (see Olanow 1993 and Olanow et al. 1998)...

Since the analogous peroxides usually decompose by a free radical mechanism, it is noteworthy that this hydroxamic acid is not sensitive to the action of free radicals from anisoyl peroxide. A radical chain mechanism like that shown below can therefore be ruled out for this compound. 

Initiation of a chain reaction involves the formation of free radicals from an initiator. Benzoyl peroxide is a commonly used initiator. 

Kensler, T.W., Egner, P.A., Swauger, J.E., Taffe, B.G. Zweier. J.L. (1988) Formation of free radicals from benzoyl peroxide in murine keratinocytes (Abstract No. 599). Proc. Am. Assoc. Cancer Res.. 29, 150... 

Thermal decomposition of peroxides initially forms oxygen-centered free radicals from the oxygen—oxygen bond homolysis. These radicals are reactive intermediates generally having very short lifetimes, i.e., half-life... 

The peroxide-free radicals capture a hydrogen ion from another fat molecule, forming a hydroperoxide and another fatty free radical ... 

Initiation normally requires molecules with weak bonds to undergo homolytic cleavage to produce free radicals. Since bond homolysis even of weak bonds is endothermic, energy in the form of heat (A) or light (hv) is usually required in die initiation phase. However, some type of initiation is required to get any free-radical reaction to proceed. That is, you must first produce free radicals from closed-shell molecules in order to get free-radical reactions to occur. Benzoyl peroxide contains a weak 0-0 bond that undergoes thermal cleavage and decarboxylation (probably a concerted process) to produce phenyl radicals which can initiate free-radical chain reactions. 

Formation of free radicals from a catalyst (initiator), such as a peroxide. 

Shi X, Dalai NS, Kasprzak KS. 1993. Generation of free radicals from hydrogen peroxide and lipid hydroperoxides in the presence of Cr(III). Arch Biochem Biophys 302(1) 294-291. 

Fig. 3. Autoxidation of polyunsaturated fatty acids in phospholipid membranes. Addition of oxygen to lipid free radicals is extremely fast. It yields peroxyl radicals ROO which will tend to capture labile hydrogen atoms of neighbouring polyunsaturated lipids. Accidentally produced free radicals will therefore initiate a chain reaction of lipid peroxidation which will propagate along membranes. This process can result in several dozen propagation steps before it is stopped by a termination reaction. Examples of such termination reactions are the recombination of peroxyl radicals and the formation of a stable free radical from a free radical scavenger (scavH). Termination through recombination of low steady-state concentration of alkyl radicals is unlikely in aerobic medium.

Salo, D. C., Lin, S. W., Pacifici, R. E., and Davies, K. J., Superoxide dismutase is preferentially degraded by a proteolytic system from red blood cells following oxidative modification by hydrogen peroxide. Free Radicals Biol. Med. 5, 335—339 (1988). 

FIGURE 3-22 Generation of free radicals from benzoyl peroxide followed by reaction with the first monomer (initiation). 

Free radicals react most efficiently with other free radicals, and the chemicals that have relatively stable free radicals, such as 2,2-diphenyl-1-pricylhydrazyl (DPPH), could be used, in principle, to quantitatively estimate the amount of free radicals (DPPH method). The peroxide formed on the surface that derived from free radical could be quantitatively analyzed by the determination of iodine liberated from KI solution, which could be used to calculate the amount of the original free radical from that peroxide was derived (iodine method). 

Phenolics are one of the major groups of non-essential dietary components that have been associated with the inhibition of atherosclerosis and cancer. The bioactivity of phenolics may be related to their antioxidant behavior, which is attributed to their ability to chelate metals, inhibit lipoxygenase, and scavenge free radicals. However, phenolics can also function as prooxidants by chelating metals in a manner that maintains or increases their catalytic activity (Fig. 6). Also, poly-phenolics reduce metals, thereby increasing their ability to form free radicals from peroxide. 

Many halogenated hydrocarbons exert toxicity in their native form whereas others are more toxic and exert effects from the intermediates and metabolites formed in the biotransformation for excretion or other purposes. A special group of intermediates or by-products that are formed are different types of very reactive macromolecule and tissue-harming free radicals from carbon, oxygen and chlorine and organic radicals such as CHj", as well as hydrogen peroxide, epoxides and other electrophilic compounds emanating from the oxidative metabolism. 

The peroxide free radical is very unstable and quickly acquires an electron by a radical mechanism via, for instance, the abstraction of hydrogen from an adjacent polymer molecnle, thereby propagating further oxidation ... 

Di-tert-butyl peroxide Free-radical addition to carbon-carbon double bonds Thioethers from ethylene derivatives... 

Oxidative degradation of polymers typically follows a free-radical mechanism involving crosslinking and/or chain scission initiated by free radicals from peroxides formed during the initial oxidation step [1-11]. Enhanced stability has been achieved by the use of additives which are frequently called antioxidants or heat stabilizers. One approach employed to reduce the oxidation of polyolefins like PE and PP is to terminate the chain reaction by introducing an antioxidant with a greater affinity than a polyolefin for the peroxy radical RO. Such antioxidants (AH) function by reacting with RO2 to form a relatively inactive radical A, i.e.,... 

The possible mechanism of grafting maleated sulfur pre-vulcanized NR (M-SPNR) with cellulose fibre by using benzyl peroxide (BPO) and K2S2O8 at 80 °C is shown in Figure 13.21. Free radicals from BPO activate the carbon-carbon double bonds of MA, which then react with carbon arbon double bonds of SPNR molecules activated by K2S2O8 to form the M-SPNR. When the M-SPNR reacts with cellulose fibre, the M-SPNR-g-cellulose fibre is... 

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