Chain-breaking acceptors

Oxidizing agents, e.g., quinones, which were shown to be able to retard oxidation [13] can function as antioxidants (via a chain breaking acceptor process, CB—A) if they can compete with oxygen for the alkyl radicals (Scheme 4). In the case of polymers, reaction 4a can... 

Scheme 2 Schematic presentation of the cyclical oxidation process and some of the main reactions/products formed from the propagating radicals. The antioxidant mechanisms interrupting the oxidative cycles are also shown. AO antioxidant, CB-A chain breaking acceptor, CB-D chain breaking donor, PD peroxide decomposer, UVA UV-absorber, MD metal deactivator...

In summary, we have found that the oxidation of these partially hindered amines occurs to give two stable nitroxyl radicals which can function as chain breaking acceptors. The primary radical formed is easily oxidized and thus could also act as a hydrogen atom donor in route to the second stable nitroxyl racical. 

Chain breaking acceptor (CB-A) antioxidants interfere with propagation by oxidizing radical species in a stoichiometric reaction, such as ... 

The primary antioxidants are normally broken down further into the classes of chain-breaking donor (CB-D) and chain-breaking acceptor (CB-A). CB-D additives interact with peroxy radicals, and are by far the commonest class of antioxidant in general use. They are represented by such additives types as hindered phenols and secondary aromatic amines. CB-A additives interact with alkyl radicals but, due to the rapid oxidation of such radicals, these additives are really useful only under low oxygen availability. CB-A types are represented by aromatic nitro and nitroso compounds, and a few speciality stable free radicals. Some transformation products of CB-D antioxidants can also act as CB-A species. 

Eew radical scavengers besides the HALS have been suggested as UV stabilisers for aromatic polyesters. Some time ago, it was noted that certain nitro compounds could be used as UV stabilisers for polyesters, but closer reading of the documents showed this to be limited to certain sulfonated copolyesters [202]. Benzofuran-2-ones have been suggested as UV stabilisers with a good chain breaking -acceptor action, i.e., capable of scavenging alkyl radicals [203, 204, 156], but these are more efficacious as thermal antioxidants and process stabilisers, especially when used in conjunction with hindered phenolic antioxidants. 

Antioxidants interfere with the free-radical oxidative cycle to inhibit or retard the oxidation mechanism. On the basis of the mechanisms by which antioxidants function, they can be classified into two categories primary or chain-breaking antioxidants, and secondary or preventive antioxidants. Chain-breaking antioxidants are of two types chain-breaking donor (CB-D) antioxidants and chain breaking acceptor (CB-A)... 

Chain-breaking acceptor (CB-A) antioxidants, on the other hand, act by oxidizing alkyl radicals in a stoichiometric reaction and hence are only effective under oxygen-deficient conditions (reaction lOd). Antioxidants with structures based on benzofuranone derivatives (lactones) and hydroxylamines, as well as on quinones and stable free radicals, are good examples of CB-A antioxidants (91-96). Hindered amine derivatives [often referred to as hindered amine stabilizers (HAS) eg, AOs 25-27, Table 3 also function by a chain-breaking mechanism and, through their transformation products, are able to trap both R. and ROO in a cyclical regenerative mechanism (50,55,62,94,97-100) for simplified reaction mechanism, see Scheme 11. 

This class of antioxidants acts by removing either alkyl (P ) or alkylperoxyl (POO ) the first by oxidation to an olefin (chain-breaking acceptor, CB-A) and the second by reduction to a hydroperoxide, (chain-breaking donor, CB-D) [20] (see Scheme 1.4). Aromatic amines and hindered phenols fall into the... 

Trapping of radicals by scavengers belongs to the most important methods to prevent autoxidation reactions. Radical scavengers are also referred to as chain breaking acceptors. Further, they are classified as primary antioxidants. 

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