Inhibitors stable" radicals

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

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 efficiency of these inhibitors may depend on reaction conditions. For example the reaction of radicals with stable radicals (e.g. nitroxides) may be reversible at elevated temperatures (Section 7.5.3) triphenylmethyl may initiate polymerizations (Section 7.5.2). A further complication is that the products may be capable of undergoing further radical chemistry. In the case of DPPH (22) this is attributed to the fact that the product is an aromatic nitro-compound (Section 5.3.7). Certain adducts may undergo induced decomposition to form a stable radical which can then scavenge further. 

In the paper published in 1900, he reported that hexaphenylethane (2) existed in an equilibrium mixture with 1. In 1968, the structure of the dimer of 1 was corrected to be l-diphenylmethylene-4-triphenylmethyl-2,5-cyclohexadiene 3, not 2 [38]. Since Gomberg s discovery, a number of stable radicals have been synthesized and characterized, e.g., triarylmethyls, phenoxyls, diphenylpicryl-hydrazyl and its analogs, and nitroxides [39-43]. The radical 1 is stable, if oxygen, iodine, and other materials which react easily with it are absent. Such stable radicals scarcely initiate vinyl polymerization, but they easily combine with reactive (short-lived) propagating radicals to form non-paramagnetic compounds. Thus, these stable radicals have been used as radical scavengers or polymerization inhibitors in radical polymerization. 

Stable radicals such as 1 are commonly used as the radical trapping agents and inhibitors or modifiers for polymerization. In the reaction of 1 with vinyl monomers, such as St, VAc, and BD, the adducts 20 are isolated (Eq. 23) ... 

Widely encountered are the reactions, which produce OH, OAlk unstable radicals from reactants OH, OAlk and rather stable anion-radicals from substrates, say, quinones. The radicals are revealed by radical interceptors and the anion-radicals are disclosed with the help of inhibitors (oxidizers). Radical interceptors is considered in Section 4.3.6.3 here attention is drawn to inhibitors. 

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. 

Radical active centres are unstable formations with mean lifetimes mostly of the order of 0.1-10 s. Because of their reactivity, the concentration of the centres cannot be increased much above 10 7 mol dm-3. Only the most sensitive of modern ESR instruments are able to detect and measure their signal quantitatively [66]. Therefore in those cases in which it is necessary, some authors allow these radicals to react with suitable inhibitors to yield stable radicals that can be followed by ESR [67]. 

A radical reacting with a molecule must produce a particle with an unpaired electron. For the reacting molecule to be called an inhibitor, these secondary radicals should have negligible tendency to propagation. They can have various fates. They either dimerize or react with a further radical that is able to propagate. The initiation rate can be determined by means of an ideal inhibitor. The most important inhibitors are some quinones, nitro and nitroso aromatics, polycyclic aromatic hydrocarbons, some metal chlorides, and stable radicals (e.g. I,l-diphenyl-2-picrylhydrazyl-DPPH, etc.). 

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. 

The ability of nitroxide stable radicals to react with carbon-centered radicals and to act as radical inhibitors [5a] has been known since the beginning of the 1980s, when Solomon and co-workers showed that the ability of nitroxides to react reversibly with growing polymer chains can be used to produce low-DP polymers [5b]. However, it was only in the 1990s with the work of Georges and co-workers [5c,d] that this novel polymerization technique, and in general LRP, received the attention it deserved. 

For some examples of radical initiators, see Chapter 39. Radical inhibitors are usually stable radicals such as those on p. 000. 

Tinuvine 770-bis (2, 2, 6, 6 - tetramenthylpiperidile - 4) sebacate, proposed by the firm CIBA [71], and some polymer addititives, including fragments of piperidine cycle are used in industry. As a rule, stabilizers, containing stable radical in their structure, are also inhibitors of thermooxidative destruction. 

Thus, information on modifying additives of polyfunctional action being used at present is represented mainly by patent works. Inorganic and organic compounds such as black, ultraviolet absorbers on the basis of benzophenone derivatives, inhibitors of radical processes-piperidines and stable nitroxyl radicals and so on - mainly colourless compounds, are recommended as modifying additives. However, problems of the effect of modifiers on photo- and thermal destruction of PETP are not completely interpreted in literature. Besides, different researchers come to different conclusions. 

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. 

Two examples of radical inhibitors that are present in biological systems are vitamin C and vitamin E. Like hydroquinone, they form relatively stable radicals. Vitamin C (also called ascorbic acid) is a water-soluble compound that traps radicals formed in the aqueous environment of the cell and in blood plasma. Vitamin E (also called a-tocopherol) is a water-insoluble (hence fat-soluble) compound that traps radicals formed in nonpolar membranes. Why one vitamin functions in aqueous environments and the other in nonaqueous environments should be apparent from their structures and electrostatic potential maps, which show that vitamin C is a relatively polar compound, whereas vitamin E is nonpolar. 

These radicals (DPPII, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5.3.1) and as reversible lennination agents (in living radical polymerization - Section 9.3). 

Stable radicals can show selectivity for particular radicals. For example, nitroxides do not trap oxygcn-ccntcrcd radicals yet react with carbon-centered radicals by coupling at or near diffusion controlled rates.This capability was utilized by Rizzardo and Solomon " to develop a technique for characterizing radical reactions and has been extensively used in the examination of initiation of radical polymerization (Section 3.5.2.4). In contrast DPPH, while an efficient inhibitor, shows little selectivity and its reaction with radicals is complex. ... 

The thiazetine (49) is in thermal equilibrium with the ring-opened heterodiene (50), and on exposure to Sb2Te3 at elevated temperature gives the novel A -l,2,4-thiatellurazoline system (51). A number of 1,2-thiazetine 5-oxides (52) have been prepared by thermolysis of diazo-ketones in the presence of A-suIphinyl-amines and from the reaction of 2-acyl-acetamides and thionyl chloride in the presence of base/ l,2-Thiazet-2-yls (53), a new class of stable radicals, are obtained by photolysis and thermolysis of (54) and sulphur amides, for example (55). They are useful as catalysts, regulators, or inhibitors in radical-initiated reactions. ... 

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. 

Allgrl-substituted phenols are the most widespread inhibitors. The phenoxyl radicals corresponding to them are readily produced by oxidizing the phenols with lead dioxide [3], in photolysis, y and /3 radiolysis, as well as in their reaction with active radicals produced in the thermal or catalytic decomposition of organic peroxides and hydroperoxides [4]. The stability of the radicals formed is determined by the structure of the initial phenol. The most stable radicals are strongly shielded phenols. Thus, 2,4,6-tri-tert-butylphenol (I), when oxidized by Pb02, forms phenoxyl radicals, the EPR spectmm of which is presented in Fig. 34a, in almost 100% yield ... 

As was indicated in Chapter I, stable radicals of inhibitors, the structure and mechanism of the formation of which were analyzed, are produced under conditions simulating the oxidation process. The question of whether the same radicals are formed in oxidation processes and of the rates of their formation and consumption merits still greater attention, since this will make it possible to characterize the mechanism and rates of the investigated processes. 

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