Kinetic chain breaking process

The process of oxidation inhibition has been classified into two main categories the kinetic chain breaking process and the initiation prevention mechanisms. 

The hindered phenol antioxidants Formula 4.4 and aromatic amines (e.g., p-phenylene diamines, Formula 4.1) both operate by kinetic chain-breaking processes. They donate a hydrogen to an alkyl peroxide radical and break the free radical chain with reactions such as below. 

The chemical mechanisms involved in the action of antioxidants have been discussed in a number of reviews [8,11-18] and the reader is directed to these and the references they contain for more detailed information. Two complementary antioxidant mechanisms are frequently used synergistically in polyolefins. The first is the kinetic chain-breaking hydrogen donor process, (CB-D) summarised in reaction (3). The relatively stable radicals (A) produced (e.g. phenoxyl from phenols and aminoxyl from aromatic amines) carmot continue the kinetic chain and disappear from the system by coupling with other or the same free radicals. However, it should be noted that this process is stoichiometric and hydroperoxides... 

The classical inhibition of oxidation processes is based on kinetic chain break and deactivation of branching, intermediate products. In the case of TP, chain type of the process is not obvious. The chain break in chemical increments suggests the inertness of residual inhibitor radical. These radicals are active above 200°C. Therefore, classical antioxidants are ineffective at high temperatures. 

Oxidation of solid polymers is radical - chain process with marked branching and square break of kinetic chains, the main branching product being hydroperoxide. Fast - decomposing hydroperoxide is localized in amorphous phase, being more stable in crystals [302]. 

Studies of the kinetic deuterium isotope effects which established the chain-breaking mechanism of antioxidant action by hydrogen donation were carried out in our laboratories by E. T. McDonel, J. C. Crano, and D. N. Vincent. Studies of sulfoxides, sulfenic acids, thiolsulfinates, and their reactions with hydroperoxides which illustrate the chemistry of the processes involved in their activity as preventive antioxidants were done by K. E. Davis, J. V. Webba, E. R. Harrington, and D. M. Kulich. 

Naphthalene excitation by UV-light in glassy cellulose triacetate films induces formation of polymeric free radicals, polymer chain break and addition of naphthalene fractures to macromolecules. In the absence of oxygen, the naphthalene consumption rate increases in direct proportion to UV-light intensity, whereas the rate of polymeric chain breaks varies in proportion to quadratic intensity. The process is initiated by singly excited triplet naphthalene molecules localized in the structural matrix zones, where they are incapable of irradiative deactivation. The features of phototransformation are explained in the framework of the hetero-nanophase kinetic model of the process, taking into account the radiationless translation of triplet excitation energy to naphthalene molecules present in the zones, where sensitization is absent and only triplet state deactivation is performed. 

Because of the precise control of the redox steps by means of the electrode potential and the facile measurement of the kinetics through the current, the electrochemical approach to. S rn I reactions is particularly well suited to assessing the validity of the. S rn I mechanism and identifying the side reactions (termination steps of the chain process). It also allows full kinetic characterization of the reaction sequence. The two key steps of the reaction are the cleavage of the initial anion radical, ArX -, and conversely, formation of the product anion radical, ArNu -. Modeling these reactions as concerted intramolecular electron transfer/bond-breaking and bond-forming processes, respectively, allows the establishment of reactivity-structure relationships as shown in Section 3.5. 

Both statistical and kinetic methods can be used, in principle, for the description of destruction, the process inverse to polymerization. However, the main problem limiting their applicability consists in difficulties of taking into account of all opportunities for breaking bonds inside the polymeric molecules. This problem is solved for chain polymers and some of the simplest kinds of branched polymers, for instance, polymers containing a limited number of units and those with a specified structure, but not for the general case of macromolecules. 

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