Peroxides and their decomposition

Berger et al. [19] have studied polymerization of styrene initiated by dibenzoyl peroxide (DBP) with the carbonyl groups labelled by the isotope l4C. DBP can react in several ways [20, 21] 

Both R, and R, are sufficienty reactive. Their mutual populations in the initiation reaction will depend on the mean lifetime of the benzoyloxy radical. This is about 10-9 s so that there is sufficient time for the secondary dissociation [22], The macromolecules generated are terminated by both fragments at a ratio depending on the polymerization temperature [19, 23]. 

2fkd and kd grow, whilst lkd/ki, falls with increasing temperature. With increasing temperature, the fraction of chains initiated by the radicals 

Decomposition of DBP in the polymerization of vinyl acetate can also be expressed by eqns. (5) [25]. The medium has a considerable effect on the values of various constants in the presence of a not very reactive monomer such as vinyl acetate the half-time of peroxide breakdown by induced decom- 

Peroxides used to initiate polymerization of ethylene to low-density polyethylene [28] 

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Between 300 and 400°C, the initiation of radicals in mixtures of CsHg and O2 is probably surface-catalyzed, as has already been discussed. The first important propagation steps will be the attack of these radicals on propane, followed by the formation of peroxides and their decomposition. 

Suspension or emulsion processes for the polymerisation of styrene or vinyl chloride can impart to the polymer significant amoimts (up to 0.05%) of proton donating processing chemicals such as potassium and ammonium persulfate or benzyl and lauryl peroxide and their decomposition products, (potassium sulfate, ammonium carbonate, urea and benzoic or lauric acid, respectively), and these are of significance in relation to safety in use for food. 

The result of this change in mechanism is that the major products at high temperatures are olefins and hydrogen peroxide and their secondary decomposition products, which of course include water. The relatively unstable alkyl hydroperoxide produced by the low temperature chain is replaced by the much more stable hydrogen peroxide. The result is that the secondary initiation, responsible for the cool flames, is replaced by a much slower initiation—the second-order decomposition of hydrogen peroxide (Reaction 6). 

Azonitriles are not susceptible to radical-induced decompositions (56) and their decomposition rates are not usually affected by other components of the environment. Cage recombination of the alkyl radicals occurs when azo initiators are used, and results in the formation of toxic tetrasubstituted succinonitrile derivatives (56). This can be a significant drawback to the use of azo initiators. In contrast to some organic peroxides, azonitrile decomposition rates show only minor solvent effects (54—56) and are not affected by transition metals, acids, bases, and many other contaminants. Thus azonitrile decomposition rates are predictable. Azonitriles can be used as thermal initiators for curing resins that contain a variety of extraneous materials since cure rates are not affected. In addition to curing of resins, azonitriles are used for polymerization of commercial vinyl monomers. 

Metal salt dyes cannot be removed with these preparations because of the catalytic decomposition of hydrogen peroxide and their resistance to reducing agents. 

Firstly, I will discuss recent evidence supporting the hypothesis that free radicals contribute to important chronic diseases in man and exert an important life-shortening effect. Secondly, I will review data on the toxicity of lipid hydroperoxides and their decomposition products, since lipid hydroperoxides can be a source of free radicals in vivo. And lastly, I will review a system under study in our laboratory in which quantitative data on lipid peroxidation and antioxidants is being obtained using linoleic acid in SDS micelles. 

The difficulties associated with this system are the decomposition of hydrogen peroxide and the resultant evolution of large amounts of gas which make the separation of the solid precipitates from liquid solution difficult. In addition,Np(VI),Pu-(VI) and Am(VI) ions are reduced to Np(V), Pu(IV) or Am(III) ion by hydrogen peroxide, and their precipitates with cobalt(III) complex ion cannot be obtained. Consequently,the peroxide system is very selective for the separation and the recovery of U(VI) ion. 

One approach to determination of whole-body lipid peroxidation has been measurement of exhaled hydrocarbons by GLC, especially ethane. Hydrocarbon gases are, however, minor end-products of peroxidation and their formation depends on the decomposition of peroxide. Recent studies have demonstrated that isoprostane is a good biomarker of lipid peroxidation in the human body. Isoprostanes are specific products arising from the peroxidation of unsaturated fatty acid residues in lipids and detection of them and their metabolites in urine is a useful assay of whole-body lipid peroxidation. Isoprostanes can be accurately and sensitively measured by mass spec-trometric techniques. 

A special feature of the autoignition of many organic solvent vapours, including those from hydrocarbons, alcohols, ethers, aldehydes and acids is their ability to form cool flames at temperatures well below their autoignition temperature as measured in the standard apparatus [10]. A cool flame is a form of incomplete combustion, usually involving the formation of an unstable peroxide and its decomposition to an aldehyde. Cool flames emit a pale blue light which is visible only in the dark and on their own produce a relatively modest ca. 20-50 K) temperature rise, hence their name. The main hazard with cool flames lies in their potential for transition into true combustion, and that their products are often less stable and more reactive than the original compound. 

Syntheses, structures, and chemistry of various peroxides were described thoroughly in the literature [5]. Here will only be mentioned some properties of peroxides and their performance as they pertain to initiations of polymerizations. Decompositions of peroxides, such as the azo compounds, are also temperature dependent [6]. This means that the rates increase with temperature. The rates are also influenced by the surrounding medium, such as the solvents that imprison or cage the produced pairs of free radicals. Before undergoing a net translational diffusion out of the cage, one... 

The phenomenon of hydroperoxide decomposition under the action of NO was studied using macromolecular peroxides and their low-molecular analogues [65, 66]. It has been shown [66] that NO-induced decomposition of polypropylene (PP) hydroperoxides leads to the formation of low-molecular hydrocarbons, which is indicative of the degradation of macromolecules. The primary stage of the decomposition is represented by the reaction [65, 66] ... 

Polymer hydroperoxides are active participants in degradation processes. The reactions of nitrogen oxides with these particles are of interest to understand the mechanism of the influence of pollutants on polymer stability in the course of the oxidation process. The phenomenon of hydroperoxide decomposition under the action of NO was discussed long ago using both macromolecular peroxides and their low molecular weight analogues [34]. Some authors assumed that the primary stage of peroxide decomposition can be represented by the reaction [35] ... 

The decomposition of perfluorodibenzoyl peroxide occurs at 60—80 C by a homolytic mechanism, resulting in the formation of a pentafluoroben-zyl structural unit at the polymer terminus (Scheme 4.4) [31, 32]. Thus, poly(perfluoro-2-methylene-l,3-dioxolanes) are generally thermally stable, and their decomposition temperature (Tj) in air is >300 C. 

The properties of organic peroxides used of the crosslinking of elastomers are discussed from a safety point of view, with particular emphasis on its stability and decomposition effects. Differences between pure peroxides and peroxide formulations are shown. Classification of organic peroxides and their hazard types is briefly discussed, and general requirements for storage... 

However, because of the high temperature nature of this class of peroxides (10-h half-life temperatures of 133—172°C) and their extreme sensitivities to radical-induced decompositions and transition-metal activation, hydroperoxides have very limited utiUty as thermal initiators. The oxygen—hydrogen bond in hydroperoxides is weak (368-377 kJ/mol (88.0-90.1 kcal/mol) BDE) andis susceptible to attack by higher energy radicals ... 

Peroxophosphoric Acids and Their Salts. In its usual impure form (H PO is the main contaminant), peroxomonophosphoric acid [13598-52-2] (5), is a viscous, coloress Hquid. The three ionization constants for peroxomonophosphoric acid are pifj = 1.1, P-A2 = 5.5, and pK (peroxide proton) = 12.8 (44). Oxidations comparable to those of peroxomonosulfuric acid, H2SO, occur in acid solutions of ca pH 2, but at higher pH values, H PO becomes less reactive as an oxidant and more unstable with respect to decomposition (44). The stmcture of H PO is probably similar to that of... 

Laboratory studies have generally focused on the diisopropyl, dicyclohexyl and di-t-butyl derivatives. These and the. s-butyl and 2-cthylhcxyl derivatives arc commercially available.189 The rates of decomposition of the peroxydicarbonates show significant dependence on the reaction medium and their concentration. This dependence is, however, less marked than for the diacyl peroxides (36) (see 3.3.1.1.4). Induced decomposition may involve a mechanism analogous to that described for diacyl peroxides. However, a more important mechanism for primary and secondary peroxydicarbonates involves abstraction of an cx-hydrogen (Scheme 3.31).190... 

It was found that the value of F, is markedly increased by ions which are effective catalysts of oxidation reactions of peroxydisulphate. These are silver(I) copper(n), and iron(III). Cobalt(II) and nickel(II) ions, although they are good catalysts for the decomposition of hydrogen peroxide, exert their effect merely as inert electrolytes in the induced reaction. Therefore it can be concluded that, in this process, activation of the rather less reactive 8203 is more important than that of hydrogen peroxide . ... 

The procedure described for the preparation of l-(m-nitro-phenyl)-3,3-dimethyltriazene is the method of Elks and Hey,2 and the preparation of m-nitrobiphenyl is also a modification of their procedure. The other principal methods for the preparation of m-nitrobiphenyl are the decomposition of N-nitroso-w-nitroacetanilide in benzene 3 and the decomposition of alkaline m-nitrobenzenediazohydroxide in benzene.4 Other methods that have been reported include the decomposition of potassium ire-nitrobenzenediazotate in benzene with acetyl chloride,6 the decomposition of m-nitrobenzoyl peroxide in boiling benzene,6 the decomposition of benzenediazonium borofluoride in nitrobenzene 7 at 70°, and the reduction of 4-(3 -nitrophenyl)-benzenediazonium acid sulfate in boiling ethanol.8... 

Certain metal salts effectively reduce the photoactivity of titanium dioxide pigments. Combination of these salts with an appropriate antioxidant and/or ultraviolet stabilizer provided highly efficient stabilization of polypropylene. The deactivation/ stabilization performance of the metal salts is adequately explained on the basis of their decomposition of hydrogen peroxide at the pigment surface and by annihilation of positive holes in the pigment crystal lattice. 

The set of the rate constants k determined for experimental runs of Figure 15 and their comparison with the rate constants of hydroperoxide decomposition determined by other methods may be seen in Table 3. When we take into account that PPs of different origin were examined, the agreement seems quite satisfactory. This agreement is valid for faster decomposing peroxides, which are the species determining the resulting rate of oxidation [49]. 

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