Autoxidation cycle

Co(HI) and Mn(ffl) salts are used as initiators for the autoxidation of methylaromatics to carboxylic acids. The metal "radicals" assist in making the first organic radicals which will subsequently enter into the common catalytic autoxidation cycle for RH molecules ... 

The second general method for making polymer materials photo-chemically degradable is to mix a radical initiator into the polymer. Once carbon-based radicals have formed, the chains degrade by the autoxidation cycle (Scheme 2). Numerous radical initiators have been investigated, and a partial list includes metal dithiocarbamates,8 metal oxides (e.g., Ti02, ZnO, CuO), metal chlorides (e.g., LiCl, FeCl3), M(acac) complexes, M (stearate), complexes, benzophenone, quinones, and peroxides.1,2... 

These initial steps are highly exothermic, which accelerates the autoxidation cycle to an explosive rate to give CO, CO2, and H2O as the stable products. Within biological matrices the autoxidation and peroxidation of lipids and fats from foodstuffs are important examples of oxygen radical... 

With 1,2-diphenylhydrazine (PhNHNHPh), the azobenzene anion radical is rapidly oxidized by dioxygen to azobenzene, which also is true for the phenazine and lumi-flavin anion radicals. Hence, 02 acts as an initiator for the autoxidation of these compounds (equation 153). For 1,2-diphenylhydrazine, turnover numbers in excess of 200 substrate molecules per 02 have been observed. The 1,2-diphenylhydrazine autoxidation cycle can be initiated by HO , which indicates that 02 is formed in the HO initiated process. Superoxide ion also initiates the autoxidation of dihydrophenazine, which is a model for dihydroflavin. For example, the addition of 1 mM (Me4N)02 in DMF to 10 mM H2Phen in an 02-saturated DMF solution results in the complete oxidation of the substrate (about 80% recovered as phenazine) and the production of 9 -10 mM HOOH. 

Propagation in such autoxidation cycles by 3O2 requires that the ROO-intermediate be a sufficiently strong radical to break a C-H bond of the substrate. Allylic, aldehydic, and benzylic C-H bonds are examples that meet this limitation. 

Autoxidatlon mechanisms. A simplified visual representation of the self-propagating autoxidation cycles of a polymer dotted lines indicate the points in the mechanism where various AOs interfere with the... 

The primary photolytic reactions postulated are not themselves self-propagating, but provide the variety of radical species involved in subsequent oxidation and chain scission reactions characteristic of PET. The extent of oxidation reaction will depend in part on the balance of scission reactions in the initial process. Once established, the autoxidation cycle has a very long kinetic chain length, and will result in a rapid build-up of hydroperoxide in the polyester. 

Several more complex species may also result from addition of H- to the polymer, especially on the aromatic rings. The presence of oxygen will lead to an autoxidation cycle to be established. The relative importance of chain scission versus crosslinking in PET will be very dependent on conditions, e.g., humidity, oxygen levels, crystallinity, dosage, dosage rate and temperature. 

As may be deduced from the discussions undertaken in Chapter 3, the most damaging reactions within the autoxidation cycle posited for polymers are initiation and chain branching, and propagation. These are also the reactions where it may be possible to utilise specific chemical compounds to interfere with the processes involved. 

Each polymer has different degradation pattern and here shows representative degradation pattern of polyolefin, the so-called autoxidation cycle (Figure 6.1). 

Photo-initiated oxidation accompanied by scission With PE and PP, light-initiated oxidation is the predominant reaction, and extensive main-chain scission, sometimes accompanied by cross-linking, takes place with rapid loss of mechanical integrity. The autoxidation cycle associated with photo-initiated scission is discussed in detail in Eigure 6.3. 

Benzofuranones. in 1997, a fundamentally new type of chemistry was introduced, which not only inhibits the autoxidation cycle, but attempts to shut it down as soon as it starts (20). The exceptional stabilizer activity of this class of benzofuranones is due to the ready formation of a stable benzofuranyl radical by donation of the weakly bonded benzylic hydrogen atom (see Fig. 4). 

Fig. 19.1 Polymer autoxidation cycle (Data from Van Beusichem and Ruberto 2005)...

It is obvious that although of great importance, primary antioxidants are not the complete answer to limiting autoxidation in polyethylene. In practice, they are commonly used in combination with other types of antioxidants, such as hydroperoxide decomposers, which inhibit other reactions of the autoxidation cycle. 

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