Induced decomposition of peroxides

Usually, such a situation occurs at induced decomposition of peroxides and hydroperoxides of organic substances. 

The maximum value of G for the particular solution used is that given for the H2 yield in Fig. 3-18. During reactor operation the radiation-induced decomposition of peroxide is negligible, and the rate of decomposition is essentially the thermal rate given by Eq. (3 17). At the steady state the peroxide concentration is given by... 

The so-called induced decomposition of peroxides is another side reaction leading to a diminfehed radical yield. In the case of acylperoxides, primary formed radicals may attack the carbonylic oxygen atom of diacyl peroxides, leading to the formation of a carbon-centered radical ... 

Both reactions are strongly catalysed by the fluorescer, but in the case of phthaloyl peroxide this is most likely to be the result of the well known induced decomposition of peroxides. This view is supported by the extensive oxidation of the fluorescer, in contrast to their truly catalytic behaviour in the CIEEL mechanism. 

Thermal or photo-induced decompositions of dialkyl peroxides in the presence of suitable substrates yield various products. For example, with nitric oxides, alkyl nitrites or nitrates are formed and, with carbon monoxide, Z fZ-alkyl esters are obtained (44) ... 

Thermally induced homolytic decomposition of peroxides and hydroperoxides to free radicals (eqs. 2—4) increases the rate of oxidation. Decomposition to nonradical species removes hydroperoxides as potential sources of oxidation initiators. Most peroxide decomposers are derived from divalent sulfur and trivalent phosphoms. 

The second major discovery regarding the use of MTO as an epoxidation catalyst came in 1996, when Sharpless and coworkers reported on the use of substoichio-metric amounts of pyridine as a co-catalyst in the system [103]. A change of solvent from tert-butanol to dichloromethane and the introduction of 12 mol% of pyridine even allowed the synthesis of very sensitive epoxides with aqueous hydrogen peroxide as the terminal oxidant. A significant rate acceleration was also observed for the epoxidation reaction performed in the presence of pyridine. This discovery was the first example of an efficient MTO-based system for epoxidation under neutral to basic conditions. Under these conditions the detrimental acid-induced decomposition of the epoxide is effectively avoided. With this novel system, a variety of... 

Nitro- and nitroso-compounds,170171 amines, and thiols induce the decomposition of diacyl peroxides in what may be written as an overall redox reaction. Certain monomers have been reported to cause induced decomposition of BPO. These include AN,172 A -vinylcarbazole,17,177 Ar-vinylimidazole178 and NVP.177... 

The chemistry of the di-/-butyl and cumyl peroxides is relatively uncomplicated by induced or ionic decomposition mechanisms. However, induced decomposition of di-/-butyl peroxide has been observed in primary or secondary alcohols31" "14 (Scheme 3.37) and primary or secondary amines.312 The reaction... 

Oxygen-centered radicals are arguably the most common of initiator-derived species generated during initiation of polymerization and many studies have dealt with these species. The class includes alkoxy, hydroxy and aeyloxy radicals and tire sulfate radical anion (formed as primary radicals by homolysis of peroxides or hyponitrites) and alkylperoxy radicals (produced by the interaction of carbon-centered radicals with molecular oxygen or by the induced decomposition of hydroperoxides). 

Some limitations of the method arise due to side reactions involving the nitroxide. However, such problems can usually be avoided by the correct choice of nitroxide and reaction conditions. Nitroxides, while stable in the presence of most monomers, may act as oxidants or rcductants under suitable reaction conditions.516 The induced decomposition of certain initiators (e.g. diacyl peroxides) can be a problem (Scheme 3.94).166 177 There is some evidence that nitroxides may disproportionate with alkoxy radicals bearing a-hydrogens,123 Side reactions with thiols have also been identified.4 18... 

Induced decomposition of hydrogen peroxide can be interpreted as follows. The HO2 radical formed in reaction (91) reduces OSO4, which will be re-oxidised... 

Induced reactions involving hydrogen peroxide can be observed with hydrogen peroxide derivatives, as well. For instance, the reaction between cumene hydroperoxide and iron(IT), in the absence of oxygen, results in a considerable induced decomposition of the peroxy compound, while, in the presence of oxygen, a marked oxidation of iron(II) takes place s . 

Initiators are introduced into the reactant, as a rule, in very small amounts. The initiator produces free radicals, most of which react with the reactant or solvent or recombine with other free radicals. Radicals formed from the initiator or reactant react with the initiator very negligibly. However, systems (initiator reactant) are known where free radicals induce the chain decomposition of initiators [4,13-15]. Nozaki and Bartlett [16,17] were the first to provide evidence for the induced decomposition of benzoyl peroxide in different solvents. They found that the empirical rate constant of benzoyl peroxide decomposition increases with an increase in the peroxide concentration in a solution. The dependence of the rate of peroxide decomposition on its concentration was found to be... 

A number of reports on the thermal decomposition of peroxides have been published. The thermal decompositions of f-butyl peroxyacetate and f-butyl peroxypivalate, of HCOH and a kinetic study of the acid-induced decomposition of di-f-butyl peroxide in n-heptane at high temperatures and pressures have been reported. Thermolysis of substituted f-butyl (2-phenylprop-2-yl) peroxides gave acetophenone as the major product, formed via fragmentation of intermediate alkoxy radicals RCH2C(Ph)(Me)0. A study of the thermolysis mechanism of di-f-butyl and di-f-amyl peroxide by ESR and spin-trapping techniques has been reported. The di-f-amyloxy radical has been trapped for the first time. jS-Scission reaction is much faster in di-f-amyloxyl radicals than in r-butoxyl radicals. The radicals derived from di-f-butyl peroxide are more reactive towards hydrogen abstraction from toluene than those derived from di-f-amyl peroxide. 

A study of the anion-induced decomposition of 2-ethoxycarbonyl prop-2-enyl peroxides has established that epoxides so formed arise (Scheme 4) through (i) addition of the nucleophile to the acrylic unsaturated bond and (ii) intramolecular anionic... 

We have recently described a calibration procedure for the determination of excitation quantum yields on commercial fluorimeters, utilizing the luminol standard , and have thereby determined singlet excitation quantum yields for the peroxyoxalate reaction with bis(2,4,6-trichlorophenyl) oxalate (TCPO), hydrogen peroxide and imidazole, using various activators . The same calibration method has been utilized to determine the singlet quantum yields obtained in the induced decomposition of protected phenoxyl-substituted 1,2-dioxetanes 6 and and compared them to the well-investigated... 

Experimental evidence of the involvement of a biradical intermediate in the decomposition of 3,3-dimethyl-l,2-dioxetane (10) has been obtained by radical trapping with 1,4-cyclohexadiene (CHD). Decomposition of 10 in neat CHD was shown to result in the formation of the expected 1,4-dioxy biradical trapping product, 2-methyl-1,2-propanediol (11) ° . However, more recently, it has been shown that the previously observed trapping product 11 was formed by induced decomposition of the dioxetane, initiated by the attack of the C—C double bond of the diene on the strained 0—0 bond of the cyclic peroxide (Scheme 9)"°. 

Part of the high yield of formaldehyde may arise from the thermal-induced decomposition of hydrogen peroxide which is known (25) to occur in anhydrous methanol. 

With sensitizers, initiation stops when the source of radiation is turned off, which is followed by a rapid decay of the polymerization process. When a conventional initiator, such as dibenzoyl peroxide, is also present, the process is more rapid than when the sensitizer is used by itself. It also seems to continue after the radiation source has been discontinued. It is presumed that ultraviolet (UV)-induced decomposition of the peroxide becomes involved in the process. By this method, polymerizations may be carried out at temperatures well below those normally used with thermal initiators such as organic peroxides. 

For low radiation doses, peroxides accumulate almost linearly with dose. However, after a certain dose has been reached, their concentration tends to level off. This conclusion can be derived from the observed change in the rate of graft copolymerization initiated by polymers subjected to increasing doses of preirradiation in air. Figure 2 illustrates this effect in the case of grafting acrylonitrile onto polyethylene (2). The drop in the yield of peroxide production presumably results from the efficient radiation-induced decomposition of these peroxides. Peroxides are known to decompose under free radical attack, and selective destruction of peroxides under irradiation has been established experimentally (8). This decomposition can become autocatalytic, and sometimes the concentration of peroxides may reach a maximum at a certain dose and decrease on further irradiation. Such an effect was observed in the case of poly (vinyl chloride). Figure 3 shows the influence of preirradiation dose on the grafting ratio obtained with poly (vinyl chlo-... 

The most commonly used water-soluble initiator is the potassium, ammonium, or sodium salt of peroxodisulfates. Redox initiators (Fe2+salt/peroxodisul-fate, etc.) are used for polymerization at low temperatures. Oil-soluble initiators, such as azo compounds, benzoyl peroxides, etc., are also used in emulsion polymerization. They are, however, less efficient than water-soluble peroxodisulfates. This results from the immobilization of oil-soluble initiator in polymer matrix, the cage effect, the induced decomposition of initiator in the particle interior, and the deactivation of radicals during des orption/re-entry events [14, 15]. 

Thermal unimolecular decomposition of perfluorodiacyl peroxides seems to be less prone to cage-recombination, with only 5 % of coupling remaining when such decomposition is carried out in the presence of an excess of a radical scavenger such as CCl3Br [68]. Of course, donor-induced decomposition of diacyl peroxides leads to clean chain processes with virtually no radical recombination being observed [66]. 

This scheme explaining the copolymerization observed has not yet been explained in kinetic aspects. Indeed, if decomposition rate of diacetyl peroxide equals 10-5 s-1 at 60 °C [120], this value does not follow the classic evolution of rate vs temperature (according to Arrhenius law). The authors suggest induced decompositions of these peroxides by CH3 radicals existing in the medium, and also by macroradicals coming from growing chains during... 

In the literature (e.g. refer to [16]), one can find analogous schemes of induced decomposition of hydrogen peroxide, the simplest representative of peroxy compounds. Nevertheless, mechanical translation of the concept of induced initiator decomposition to H202 dissociation induces conclusions contradicting the notion of chemical induction and conjugated processes. 

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