Stable radicals inhibition

As a stable radical, however, NO can also catalyze the recombination of radicals (X,Y) at higher concentrations, eventually inhibiting overall oxidation [45] ... 

Common inhibitors include stable radicals (Section 5.3.1), oxygen (5.3.2), certain monomers (5.3.3), phenols (5.3.4), quinones (5.3.5), phenothiazine (5.3.6), nitro and nitroso-compounds (5.3.7) and certain transition metal salts (5.3.8). Some inhibition constants (kjkp) are provided in Table 5.6. Absolute rate constants (kj) for the reactions of these species with simple carbon-centered radicals arc summarized in Tabic 5.7. 

The kinetics and mechanism of inhibition by stable radicals has been reviewed by Rozantsev el al,lS3 Ideally, for radicals to be useful inhibitors in radical polymerization they should have the following characteristics ... 

The photo-reduction of 2,l,3-benzothiadiazole-4,7-dicarbonitrile (13) by EDTA in the presence of micelles gave a stable radical anion which could be observed by ESR <84CC1324>. The observed 17 line ESR signal was attributed to an overlapping quintet of quintets from a radical with (NA) = 0.255 mT and fl(NB) = 0.075 mT. The radical appears to be protected within the micelle when electron transfer is inhibited. 

Again, for inhibition to be effective there must be destruction of the stable radicals by dimerization or disproportionation. 

Another characteristic feature of chain reactions is inhibition. Interruption of a single chain will prevent the reaction of a large number of substrate molecules hence any substance that diverts radicals will dramatically reduce reaction rate. Inhibitors may themselves be stable radicals or substances (for example, 2,4,6-tri-f-butyl phenol) that can react with radicals to yield stable radicals the requirement that must be met is that the inhibitor react efficiently with radicals and that neither it nor its products be initiators of new chains. 

A termination frequently encountered in many polymerizations results from a chain transfer process. In a radical polymerization such a reaction involves usually a transfer of a hydrogen atom and yields a radical which may or may not initiate further polymerization. The first alternative may be referred to as a proper chain transfer reaction, and such a transferring agent is known as a polymerization modifier. The second alternative is known as an inhibition or retardation of polymerization, the inhibitor or retarder being a substance which forms a stable radical, not sufficiently reactive in respect to the monomer, and therefore unable to initiate further polymerization. 

R-NO are formed. The behavior of propylene is presumably to form the fairly stable radical R—CH2—CH—CH3, which may dimerize. These methods are, for a number of reasons, only partially successful in elucidating the mechanism of the original reaction. In the first instance there is always the possibility that the added inhibitor may play a role in altering the original reaction. This is certainly evidenced in the case of NO in some instances it may even accelerate the reaction.A second difficulty is that the inhibition or capture of free radicals is incomplete, i.e., the radicals may react with other substances either more rapidly or rapidly enough to make the data ambiguous. Finally, there are always the problems of back reactions and of further decomposition of the radical-inhibitor products, found in the case of reactions of CH3CO with V and also for products RNO. These same difficulties appear in the mirror techniques. In brief, while these methods are valuable in certain instances, their use must be circumscribed by a careful consideration of the reaction studied. 

Allylic transfer is also variously named degradative chain transfer, autoinhibition, or allylic termination. The stable radical derived from the monomer by reactions like (6-90) are slow to reinitiate and prone to terminate. Low-molecular-weight products are therefore formed at slow rates and small concentrations of allyl monomers can inhibit or retard the polymerization of more reactive monomers. 

A monomer which forms a stable radical can be used to inhibit the polymerization of another monomer which yields a more reactive radical. Styrene inhibits the polymerization of vinyl acetate, for example. 

Free stable radical quenching (DPPH) Percentage inhibition ECso, concentration to decrease concentration of test free radical by 50% Tecso. tifue to decrease concentration of test free radical by 50%... 

The formation of alternating copolymers through the polymerization of pairs of monomers, one of which is the donor and the other the acceptor of an electron, is well known. We shall mention only a few studies out of a great number of those recently published. First, those dealing with the nature of active centers in such systems will be examined. When radical initiators are used, e.g., benzoyl peroxide as in17), and the reaction is inhibited with different radical polymerization inhibitors, such as stable radicals like 2,2,6,6-tetramethylpiperidine 1-oxide, quinones, fluorene etc., questions concerning the nature of active centers can be regarded as solved. 

An inhibitor is used to completely stop the conversion of monomer to polymer produced by accidental initiation during storage. To induce the inhibition, some stable radicals are mixed with the monomer. Such radicals are incapable for initiation the polymerization, but they are very effective in combining with any propagating radical. Diphenylpicryl-hydrazyl and tetramethylpiperidinyloxy (TEMPO) are two examples of radicals used to inhibit the radical polymerization. The chemical reactions of the inhibition produced by these compounds are shown in Scheme 4.8. 

However, H. C. Brown discovered that the reaction was completely inhibited by just 5% of the stable radical galvinoxyl (shown on p. 975), known to be an efficient scavenger for radicals. But where were the radicals coming from Further experiments showed that small amounts of oxygen were needed to make the reaction work. As you saw in Chapter 3, oxygen is a triplet diradical and displaces alkyl radicals from the trialkyl borane. This reaction looks at first like an 5 2 and is called an Sh2 (second order homolytic displacement), but in reality the oxygen adds to the empty p orbital of planar trigonal boron to release an alkyl radical and start the chain reaction. 

FORMATION OF STABLE RADICALS OF INHIBITIONS DURING OXIDATION PROCESSES... 

Characterizing the mechanism of the action of amines, the overwhelming majority of tiie authors believe that their basic role reduces to the termination of kinetic oxidation chains on account of reaction (3). Actually, stable radicals (CjH5)2NO have been detected by the method of electron paramagnetic resonance as a result of the interaction with peroxide radicals, formed in the liquid-phase oxidation of a hydrocarbon inhibited with diphenylamine [30]. The interaction of certain secondary amines (Ar2NH) with peroxide radicals, prepared by oxidative radiolysis. 

Another commonly used stable radical is 1,3,5-triphenylverdazyl 19 (446, 447). It is less thermally stable than TEMPO. Both TEMPO and the verdazyl radical do not react with oxygen-centered radicals or oxygen. If an initiator generates an oxygen-centered radical the nitroxide will capture the carbon-centered radical that is generated via the first addition step involving a monomer. Galvinoxyl 20 and l,3-bisdiphenylene-2-phenylallyl (or Koelsch s) radical 21 can also be used as inhibitors. Diphenylpicrylhydrazyl 22 is used much less frequently because of its complicated reaction mode of inhibition (448). 

Neelson et al. [138] investigated the emulsion polymerization of vinyl chloride in the presence of inhibitors. The used p-benzoquinone and a stable radical 2,2, 6,6 -tetramethylpyperidine-JV-oxide at concentrations 1 x 10 mol dm After the consumption of inhibitor, the conversion vs. time curve was the same shape as that without inhibitor. In some experiments the inhibition period varies with the emulsifier and initiator concentration. The inhibitor efficiency decreased with increasing concentration of emulsifier. The solubilization of inhibitor is expected to decrease the amount of inhibitor available for reactions with radicals. For this reason, the inhibitor acts more efficiently at the low emulsifier concentration as it was reported in Ref. [139]. 

Inhibition (Section 3.04.4.2) the reaction of a propagating radical with another species (Z , Scheme 45) to give a dead polymer chain. Z is usually of low molecular weight. Examples of inhibitors are stable radicals (e.g., nitroxides, oxygen), nonradical spedes that react to give stable radicals (e.g., phenols, quinones, nitroso compounds) and transition metal salts. 

The relatively stable radicals (A) produced (e.g. phenoxyl from phenols and amin-oxyl from aromatic amines) cannot continue the kinetic chain and disappear from the system by coupling with other or the same free radicals. It should be noted that this process is stoichiometric and hydroperoxides are produced in each inhibiting step (reaction 10). 

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