Autoxidation chain termination

During the polymeriza tion process the normal head-to-tad free-radical reaction of vinyl chloride deviates from the normal path and results in sites of lower chemical stabiUty or defect sites along some of the polymer chains. These defect sites are small in number and are formed by autoxidation, chain termination, or chain-branching reactions. Heat stabilizer technology has grown from efforts to either chemically prevent or repair these defect sites. Partial stmctures (3—6) are typical of the defect sites found in PVC homopolymers (2—5). 

Antioxidants markedly retard the rate of autoxidation throughout the useful life of the polymer. Chain-terminating antioxidants have a reactive —NH or —OH functional group and include compounds such as secondary aryl amines or hindered phenols. They function by transfer of hydrogen to free radicals, principally to peroxy radicals. Butylated hydroxytoluene is a widely used example. 

The autoxidation of aldehydes, and of other organic compounds, may be lessened considerably by very careful purification—removal of existing peroxides, trace metal ions, etc.—but much more readily and effectively by the addition of suitable radical inhibitors, referred to in this context as anti-oxidants. The best of these are phenols and aromatic amines which have a readily abstractable H atom, the resultant radical is of relatively low reactivity, being able to act as a good chain terminator (by reaction with another radical) but only as a poor initiator (by reaction with a new substrate molecule). 

The duration of the inhibition period of a chain-breaking inhibitor of autoxidation is proportional to its efficiency. Indeed, with an increasing rate of chain termination, the rates of hydroperoxide formation and, hence, chain initiation decrease, which results in the lengthening of the induction period (this problem will be considered in a more detailed manner later). It should be noted that when initiated oxidation occurs as a straight chain reaction, the induction period depends on the concentration of the inhibitor, its inhibitory capacity, and the rate of initiation, but does not depend on the inhibitor efficiency. 

Phosphites can react not only with hydroperoxides but also with alkoxyl and peroxyl radicals [9,14,17,23,24], which explains their susceptibility to a chain-like autoxidation and, on the other hand, their ability to terminate chains. In neutral solvents, alkyl phosphites can be oxidized by dioxygen in the presence of an initiator (e.g., light) by the chain mechanism. Chains may reach 104 in length. The rate of oxygen consumption is proportional to v 1/2, thus indicating a bimolecular mechanism of chain termination. The scheme of the reaction... 

By using the differential form of the copolymer composition equation (26, 28) the products of oxidation of mixtures at low conversions permit comparison of rates of chain propagation in autoxidations of various compounds, essentially free from effects of chain initiation, chain termination, and over-all rates. 

Table II. Rate Constants and Kinetic Parameters for Chain Termination in Autoxidation of Hydrocarbons as Determined with the Rotating Sector (25, 26, 27, 28) (Neat Hydrocarbon or Hydrocarbon Diluted with Chlorobenzene)...

There is excellent agreement between the decay constants obtained by ceric ion oxidation of secondary hydroperoxides and the rate constants for chain termination in hydrocarbon autoxidation determined by the rotating sector. The agreement suggests that secondary peroxy radicals do not undergo many nonterminating interactions, so that most self-reactions of secondary peroxy radicals must be chain terminating. 

The ready formation of benzylic hydroperoxides is used in industrial oxidations, as in the synthesis of propylene oxide and phenol (see Sections 9.5.2 and 9.5.4, respectively). In contrast with autoxidation of alkenes, where various secondary processes may follow, autoxidation of arenes is less complicated. Chain termination of 99 may lead to an alcohol and aldehyde [Eq. (9.151)], and the rapid autoxidation of the latter may produce the corresponding carboxylic acid [Eq. (9.152)] ... 

Termination of the autoxidation chain process occurs as peroxyl radicals couple to yield non-radical products. This reaction takes place through an unstable tetroxide intermediate. Primary and secondary tetroxides decompose rapidly by the Russell termination mechanism to yield three non-radical products via a six-membered cyclic transition state (Fig. 95). The decomposition yields the corresponding alcohol, carbonyl compound, and molecular oxygen (often in the higher energy singlet oxygen state) three... 

In chain mechanisms, radical-radical combination and disproportionation reactions are seen only in the termination part of the mechanism. The one exception is the combination of Cb with a free radical, one of the propagation steps in the autoxidation chain mechanism. 

The intervention of nitrate anions in the meehanism of S(IV) autoxidation depends on the concentration of these anions. At relatively low nitrate concentrations, the rate of scavenging 804 radieal anions attains low values, comparable with the rate of chain termination. In this instanee two speeific cases may be distinguished (Table 2) ... 

Termination of the radical chain reaction As the reaction proceeds, autoxidation is followed by an autoretardation stage, resulting in a standstill before the hydrocarbon is completely consumed. This autotermination is called the chain termination reaction and dominates in this final phase of the oxidation process such that degradation comes to a halt. Termination may be effected by the combination of radical species such as peroxy radicals to yield ketones and alcohols. Reaction sequence (4.14) ... 

The addition of mobilizing additives, such as mineral oil, may effectively enhance the reaction of the chain termination, decreasing the number of radical sites, and significantly reducing pol)oner chain autoxidation. For example, the use of mineral oil as mobilizing additives [5] for two formulations of PP, with the same degree of crystallinity, leads to an increase of the chain termination by a factor of four. Consequently the use of a mobilizer additionally increases the radiation stability of PP, where the irradiation resistance depends on the level of mobilizer added. 

By analogy with other oxidations mediated by the Co/NHPI catalyst studied by Ishii and coworkers [123, 124], Eq. (5.16) probably involves a free-radical mechanism. We attribute the promoting effect of NHPI to its ability to efficiently scavenge alkylperoxy radicals, suppressing the rate of termination by combination of these radicals. The resulting PINO radical subsequently abstracts a hydrogen atom from the a-C-H bond of the alcohol to propagate the autoxidation chain (Eqs. (5.17)-(5.19)). 

Antioxidants (see Section 11.2.2) are substances that can react with free radicals of the autoxidation chain, especially with peroxyl radicals (Figure 3.66). The reaction creates hydroperoxides or other non-radical Hpid products. The antioxidant is transformed to the form of a free radical, which, however, is fairly stable, so it is unable to continue in the autoxidation reaction. The role of the antioxidant thus lies in shortening the autoxidation chain and increasing the rate of termination reactions. During the reaction the antioxidant is consumed. When aU of the antioxidant has been consumed, the autoxidation reaction proceeds as if no antioxidant was present. Antioxidants therefore cannot completely stop the autoxidation reaction they just slow this reaction down, ideally to the initial reaction rate. 

The kinetic parameters for reaction of the thiocarbamides with cumylperoxide radicals and cumyl hydroperoxide are also consistent with M-benzyl-A[ -(3-thiethanyl) thiocarbamide being faster in both oxidation chain termination (reacting with cumylperoxide radicals) and in catalytic decomposition of cumyl hydroperoxide. It was also shown that A -benzyl-V-(3-thiethanyl) thiocarbamide inhibits cumene autoxidation better than the other thiocarbamide derivatives when used in lower concentrations than the latter. 

Further quantitative investigations on autoxidation kinetics have led to a more detailed picture of the initiation and of chain termination steps [6, 7]. Details of the steps leading to chemiluminescence are as follows. 

Under steady state conditions the formation of radicals starting an autoxidation chain process is equal to their disappearence in the chain terminating step [67]. 

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