Radicals inhibiting effect

The presence of stable free radicals in the final polycondensate is supported by the observation that traces of (11) have a strong inhibiting effect on the thermal polymerization of a number of vinyl monomers. Radical polymerization was inhibited to a larger extent by a furfural resin than by typical polymerization inhibitors (34). Thermal degradative methods have been used to study the stmcture of furfural resinifted to an insoluble and infusible state, leading to proposed stmctural features (35). 

Many researchers have correlated the overall decomposition as an nxh. order reaction, with most paraffins following the first order and most olefins following a higher order. In general, isoparaffin rate constants are lower than normal paraffin rate constants. The rate constants are somewhat dependent on conversion due to inhibition effects that is, the rate constant often decreases with increasing conversion, and the order of conversion is not affected. This has been explained by considering the formation of aHyl radicals (38). To predict the product distribution, yields are often correlated as a function of conversion or other severity parameters (39). 

Thus the activity of the methyl groups in this reaction decreases in the series C-4 > C-5 > C-3. This may be considered as evidence of the inhibiting effect of the nitrogen hetero atom on the radical substitution in methyl groups at C-3 and, to a lesser extent, at C-5 (compare the effect of the heterocyclic nitrogen in the pyridine and azole series ) and of a similar effect of the electron-accepting substituents in the 4-position on the methyl group at C-5. 

The presence of stable free radicals in the resin was further suggested by the strong inhibiting effect of traces of this product on the thermal polymerization of styrene. 

It is to be expected from chemical evidence that the replacement of hydrogen by an aliphatic radical would have some further inhibiting effect on the carboxyl resonance. It is found, however, that to within the experimental error of about 0.2 v.e. the resonance energy is the same for methyl and ethyl esters as for carboxylic acids. 

The induction period observed at the very beginning of the irradiation is due to the well known inhibition effect of oxygen on these radical-induced reactions. Once it is over, after the, v10 ms needed to consume essentially all of the oxygen dissolved in the liquid film (19), the polymerization starts rapidly to reach 75 % conversion within 0.08 s. Further UV exposure leads only to a slow increase in the cure, mainly because of mobility restrictions in the rigid matrix, so that there still remains about 15 % of acrylic unsaturation in coatings heavily irradiated for 0.4 s. 

It is commonly known that vinyltin compounds undergo no free-radical polymerization and do not readily copolymerize with various vinyl monomers which can be attributed to their inhibiting effect towards radical reactions48,79. ... 

Another situation is observed when salts or transition metal complexes are added to an alcohol (primary or secondary) or alkylamine subjected to oxidation in this case, a prolonged retardation of the initiated oxidation occurs, owing to repeated chain termination. This was discovered for the first time in the study of cyclohexanol oxidation in the presence of copper salt [49]. Copper and manganese ions also exert an inhibiting effect on the initiated oxidation of 1,2-cyclohexadiene [12], aliphatic amines [19], and 1,2-disubstituted ethenes [13]. This is accounted for, first, by the dual redox nature of the peroxyl radicals H02, >C(0H)02 and >C(NHR)02 , and, second, for the ability of ions and complexes of transition metals to accept and release an electron when they are in an higher- and lower-valence state. 

Acceptors of alkyl radicals are known to be very weak inhibitors of liquid-phase hydrocarbon oxidation because they compete with dioxygen, which reacts very rapidly with alkyl radicals. The situation dramatically changes in polymers where an alkyl radical acceptor effectively terminates the chains [3,49], The study of the inhibiting action of p-benzoquinone [50], nitroxyl radicals [51-53], and nitro compounds [54] in oxidizing PP showed that these alkyl radical acceptors effectively retard the oxidation of the solid polymer at concentrations ( 10-3 mol L 1) at which they have no retarding effect on liquid hydrocarbon oxidation. It was proved from experiments on initiated PP oxidation at different p02 that these inhibitors terminate chains by the reaction with alkyl macroradicals. The general scheme of such inhibitors action on chain oxidation includes the following steps ... 

The catalytic reduction of the radicals, particularly the O atom, by sulfur compounds will generally reduce the rates of reactions converting atmospheric nitrogen to NO by the thermal mechanism. However, experiments do not permit explicit conclusions [21], For example, Wendt and Ekmann [46] showed that high concentrations of S02 and H2S have an inhibiting effect on thermal NO in premixed methene-air flames, while deSoete [47] showed the opposite effect. To resolve this conflict, Wendt el al. [48] studied the influence of fuel-sulfur on fuel-NO in rich flames, whereupon they found both enhancement and inhibition. 

Alfassi, Z. B., and Benson, S. W., A simple empirical method for the estimation of activation energies in radical molecule metathesis reactions, Int. J. Chem. Kinetics S, 879 (1973). Allara, D. L., and Edelson, D., A computational analysis of a chemical switch mechanism. Catalysis-inhibition effects in a copper surface-catalyzed oxidation, J. Phys. Chem. 81, 2443 (1977). 

The superoxide ion or its protonated form (hydroperoxy radical HO2 ) is produced in the system (Shugalei and Tselinskii 1993, 1994). Hydroquinone, which is known to interact effectively with superoxide ion (Afanas ev and Polozova 1978), exerts fairly strong inhibiting effect on the reaction. Addition of potassium ferricyanide to the system has, in contrast, an accelerating effect. The cause of the effect consists of transformation of the following type ... 

Oxygen radical inhibition. Seed oil, administered to rats at a concentration of 8% of diet, produced equivocal effect on macrophages. Capsaicin or curcumin enhanced the effecH . ... 

Figure 16.14 represents a summary of the most important studies performed in the field of free radical scavenging effects. G115 inhibits lipid peroxidation in rats. It protects the rabbit pulmonary artery from free radical injury and the rat heart from ischemia reperfusion injury. Figure 16.15 illustrates the various diseases that are due to an excessive production of free radicals, such as atheroclerosis, diabetes, rheumatoid arthritis, and aging. 

Moderate amounts of formaldehyde exerted an inhibiting effect by increasing the induction period in pentane and hexane oxidation (13). The higher aldehydes and formaldehyde appear in approximately equivalent amounts. Decomposition of alkoxy radicals, RCH20 —> R + CH20, is considered the source of formaldehyde. The effect of added formaldehyde is shown in Table I. 

This is in marked contrast to the higher aldehydes which oxidize much more readily, at temperatures as low as 100° C.—e.g., acetaldehyde (55). The mechanism by which formaldehyde reduces the induction period in hydrocarbon oxidation below 300° C. is not evident. An inhibiting effect is explicable on the basis of removal of free radicals, as Lewis and von Elbe (32) have pointed out. This could occur by CH20 -f R — RH -f HCO. HCO is then oxidized to the relatively inert CH03 which diffuses to the wall and is destroyed. Above 300° C. formaldehyde is oxidized more rapidly, giving rise to free radicals, and it is not surprising to find that the induction period in some hydrocarbon oxidations is shortened. 

Dipyridamole has also been shown to have antioxidant effects (19). Antioxidants act to remove harmful reactive-oxygen species and protect low-density lipoproteins (LDL) from oxidation oxidized LDL plays a key role in the development and propagation of atherosclerosis. The antioxidant effects of dipyridamole may be both direct (by scavenging oxygen and hydroxyl radicals, inhibiting lipid peroxidation and oxidative modification of LDL) (20-22) and indirect (via adenosine, which reduces superoxide anion generation). Dipyridamole has been shown to be a more effective anioxidant than ascorbic acid, alpha-tocopherol, or probucol (22). 

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