Decomposition of hydroperoxides

Bateman and Hughes [43, 45] found that, in the case of the oxidation of polyolefines, the decomposition of hydroperoxides is initially a reaction of the first order (24), but subsequently, with increasing concentration of hydroperoxide as the oxidation progresses, the less endothermic bimolecular reaction (25) assumes greater importance  

Since the radicals from (24) are probably generated adjacent to one or more hydroperoxide groups [148], a radical-induced decomposition of hydroperoxides similar to that detected in solution [288, 398] is also possible ((26) and (27)). 

The alkoxy radicals formed after the decomposition of hydroxy-peroxides can split by the 3 scission process which plays an important role in backbone scission and in alkyl radical formation at the end of chain ((28) and (29)) [7,125, 405]. 

This reaction results in the formation of an aldehyde group at the end of a chain. The evidence for this mechanism was provided by the observation of luminescence in the oxidation of polyethylene and polypropylene [139,140] and by other methods [59, 110]. 

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Depending on the peroxide class, the rates of decomposition of organic peroxides can be enhanced by specific promoters or activators, which significantly decrease the energy necessary to break the oxygen—oxygen bond. Such accelerated decompositions occur well below the peroxides normal appHcation temperatures and usually result in generation of only one usehil radical, instead of two. An example is the decomposition of hydroperoxides with multivalent metals (M), commonly iron, cobalt, or vanadium ... 

It is virtually impossible to manufacture commercial polymers that do not contain traces of hydroperoxides. The peroxide bond is relatively weak and cleaves homolyticaHy to yield radicals (eqs. 2 and 3). Once oxidation has started, the concentration of hydroperoxides becomes appreciable. The decomposition of hydroperoxides becomes the main source of radical initiators. 

Propagation. Propagation reactions (eqs. 5 and 6) can be repeated many times before termination by conversion of an alkyl or peroxy radical to a nonradical species (7). Homolytic decomposition of hydroperoxides produced by propagation reactions increases the rate of initiation by the production of radicals. 

Autoca.ta.Iysis. The oxidation rate at the start of aging is usually low and increases with time. Radicals, produced by the homolytic decomposition of hydroperoxides and peroxides (eqs. 2—4) accumulated during the propagation and termination steps, initiate new oxidative chain reactions, thereby increasing the oxidation rate. 

Metal-Catalyzed Oxidation. Trace quantities of transition metal ions catalyze the decomposition of hydroperoxides to radical species and greatiy accelerate the rate of oxidation. Most effective are those metal ions that undergo one-electron transfer reactions, eg, copper, iron, cobalt, and manganese ions (9). The metal catalyst is an active hydroperoxide decomposer in both its higher and its lower oxidation states. In the overall reaction, two molecules of hydroperoxide decompose to peroxy and alkoxy radicals (eq. 5). 

According to Figure 3, hydroperoxides are reduced to alcohols, and the sulfide group is oxidized to protonic and Lewis acids by a series of stoichiometric reactions. The sulfinic acid (21), sulfonic acid (23), sulfur trioxide, and sulfuric acid are capable of catalyzing the decomposition of hydroperoxides to nonradical species. 

The early work of Kennerly and Patterson [16] on catalytic decomposition of hydroperoxides by sulphur-containing compounds formed the basis of the preventive (P) mechanism that complements the chain breaking (CB) process. Preventive antioxidants (sometimes referred to as secondary antioxidants), however, interrupt the second oxidative cycle by preventing or inhibiting the generation of free radicals [17]. The most important preventive mechanism is the nonradical hydroperoxide decomposition, PD. Phosphite esters and sulphur-containing compounds, e.g., AO 13-18, Table la are the most important classes of peroxide decomposers. 

Such a reciprocal motion of the kinetic chain (or back reaction) results in the decomposition of hydroperoxide groups without any interruption of kinetic chains and leads to the decrease in hydroperoxy groups. It has... 

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). 

Alkylperoxy radicals are generated by the reactions of carbon-centered radicals with oxygen and in the induced decomposition of hydroperoxides (Scheme 3.82). Their reactions have been reviewed by Howard452 and rate constants for their self reaction and for their reaction with a variety of substrates including various inhibitors have been tabulated.453... 

A very similar rearrangement takes place during the acid-catalysed decomposition of hydroperoxides, RO—OH, where R is a secondary or tertiary carbon atom carrying alkyl or aryl groups. A good example is the decomposition of the hydroperoxide (84) obtained by the air-oxidation of cumene [(l-methylethyl)benzene] this is used on the large scale for the preparation of phenol and acetone ... 

At higher temperature, however, (> 100°C) an accelerated decomposition of hydroperoxide by the hindered amine II was very clearly seen. As Table I shows, the effect is observable in solvents with differing reactivities towards radicals. It is interesting to note that in the experiments made under nitrogen the loss of... 

The complex formation between hydroperoxides and HALS derivatives proposed for the preceding reaction was recently postulated by two different groups of investigators. First, Carlsson determined a complex formation constant for +00H and a nitroxide on the basis of ESR measurements—. Secondly, Sedlar and his coworkers were able to isolate solid HALS-hydroperoxide complexes and characterize them by IR measurements. The accelerated thermal decomposition of hydroperoxides observed by us likewise points to complex formation. It is moreover known that amines accelerate the thermal decomposition of hydroperoxides-. Thus Denisov for example made use of this effect to calculate complex formation constants for tert.-butyl hydroperoxide and pyridineitZ.. 

Some traces of metal and metal ions may initiate the decomposition of hydroperoxides even at room temperature. Traces of metal ions are present in almost all polymers and they may affect considerably the polymer oxidation and its subsequent degradation. The sequence of efficiency of metal ions to enhance degradation depends on its valence state and the type of its ligand, but may be postulated as follows ... 

The decompositions of hydroperoxides (reactions 4 and 5) that occur as a uni-or bimolecular process are the most important reactions leading to the oxidative degradation (reactions 4 and 5). The bimolecular reaction (reaction 5) takes place some time after the unimolecular initiation (reaction 4) provided that a sufficiently high concentration of hydroperoxides accumulates. In the case of oxidation in a condensed system of a solid polymer with restricted diffusional mobility of respective segments, where hydroperoxides are spread around the initial initiation site, the predominating mode of initiation of free radical oxidation is bimolecular decomposition of hydroperoxides. 

The rate constant k, which involves the rate constant of bimolecular decomposition of hydroperoxides khi from the Scheme 2 may thus be determined, so that we find the value of [d(f/Imax)/df], which is the slope of chemiluminescence-time record at its inflexion point. This ratio is function of k as follows ... 

Table 3 The rate constants k determined from chemiluminescence experiments of Figure 15 and Equation (14) and the rate constants k of decomposition of hydroperoxides from literature... 

Kinetics based on the idea of spreading is formally based on the model of development of an infectious disease among human population [59,60]. The formalism of chemical kinetics, however, should be treated with a care as a similar equation can be derived from the homogeneous model assuming bimolecular decomposition of hydroperoxides as an initiating event. 

Assuming that the reaction occurs under homogeneous conditions, it appears quite clear that in the case of sufficiently high length of kinetic chains, the primary appearance of free radicals by the rate w (Scheme 2) is very soon replaced by the decomposition of hydroperoxides and the process has the character of a "closed loop" [68]. 

If we presume that oxidation of polymer PH takes place according to the Scheme 2 of Bolland-Gee mechanism, in which reactions 2, 3, 5, 6, 7, 8, 9 and 10 predominate, involving initiation, propagation and termination of the free radical process. The production of primary radicals by the reaction 1 is governed rapidly by decomposition of hydroperoxides. From the shape of kinetic runs it may be assumed that decomposition of hydroperoxides has the character of a bimole-cular reaction (reaction 5) even in a very short time after the start of experiment. 

G. Geuskens, D. Baeyens Volant, G. Delaunois, Q. Lu Vinh, W. Piret, and C. David, Photo oxidation of Polymers II. The Sensitized Decomposition of Hydroperoxides as the Main Path for Initiation of the Photo oxidation of Polystyrene Irradiated at 253.7 nm, Eur. Polym. J., 14, 299 303 (1978). 

Zinc dithiophosphates act as anti-oxidants by promoting the decomposition of hydroperoxides. The mechanism of this reaction is complicated involving hydroperoxides and peroxy radicals192,193 and is also affected by the other additives present in the lubricant oil.194 However the first step is thought to be a rapid initial reaction of the zinc dithiophosphate and hydroperoxide to give a basic compound [Zn4(/i4-0)(S2P(0R)2)6] (Equation 88 Figure 9).141... 

Competition between Homolytic and Heterolytic Catalytic Decompositions of Hydroperoxides... 

Salts and complexes of transition metals accelerate hydrocarbon oxidation due to the catalytic decomposition of hydroperoxides to free radicals (see Chapter 10). 

Due to the unimolecular and bimolecular homolytic decomposition of hydroperoxide, such as... 

In addition to this reaction, many other reactions of hydroperoxide decay occur in solution and they will be discussed later. This is the reason why the unimolecular decomposition of hydroperoxides was studied preferentially in the gas phase. The rate constants of the unimolecular decomposition of some hydroperoxides in the gas phase and in solution are presented in Table 4.11. The decay of 1,1-dimethylethyl hydroperoxide in solution occurs more rapidly. This demonstrates the interaction of ROOH with the solvent. 

Rate Constants of the Unimolecular Decomposition of Hydroperoxides and Peracids... 

The parameters used in the IPM are presented in Table 4.16 and Table 4.17. In these tables, the additional parameters for the reactions of hydroperoxides with molecules and free radicals are given. The reactions of hydroperoxide with free radicals are important for the chain processes of the decomposition of hydroperoxides (see later). The results of the calculation of rate constants of various hydroperoxide reactions are collected in Tables 4.18-4.21. The comparison of the calculated values with the experimental values helped to introduce a few corrections in the traditional view on the bimolecular reactions of hydroperoxides. 

The chain decomposition of hydroperoxides was proved and studied for hydroperoxides produced by the oxidation of polyesters such as dicaprilate of diethylene glycol and tetra-valerate of erythritol [140], The retarding action of phenolic antioxidant on the decay of hydroperoxides was observed. The initial rate of hydroperoxide decomposition was found to depend on the hydroperoxide concentration in accordance with the kinetic equation typical for the induced chain decomposition. 

The chain decomposition of hydroperoxide was proved for the hydroperoxide of dimethy-lacetamide [141]. The rate of chain decay vv = kind[ROOH]03/2. The values of the rate constant kind in dimethylacetamide were found to be the following [141,142] ... 

The primary and secondary alcohols induce the chain decomposition of hydroperoxides with the participation of ketyl radicals that possess high reducing activity [143-145]. 

The rate of chain decomposition of hydroperoxide v = (kp/v/2kd)[ROOH]i i1/2. Dioxygen reacts with ketyl radicals with the formation of hydroxyperoxyl radicals. The latter is decomposed into ketone and H02 . Hydroperoxyl radicals also possesses reducing activity and induce the chain decomposition of hydroperoxide [121,143]. 

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