Chain breaking, antioxidants

Scott, G. Antioxidants chain breaking mechanisms. In Atmospheric Oxidation and Antioxidants, Scott, G., Ed. Elsevier Applied Science London, 1993 Vol. 1, 121-160. 

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)... 

Scott G. Antioxidants chain breaking mechanisms. In Scott G, editor. Atmospheric oxidation and antioxidants. Amsterdam Elsevier Science Publishers B.V. 1993. p. 121-60. 

Scott G (1993) Antioxidants Chain-breaking Mechanisms in Atmospheric Oxidation and Antioxidants, 2" Edition, Vol. I ed., ed. G. 

Diarylamiaes fuactioa as mbber antioxidants by breaking the peroxidative chain reactions leading to mbber deterioration. Nearly all commercial synthetic mbbers (see Elastomers, synthetic), including neoprene, butyl, styrene—butadiene, and the acrylonitrile—butadiene mbbers, can be protected with about 1—2% of an alkylated diphenylamine. DPA itself is not used as a mbber antioxidant. An objectionable feature of these antioxidants is that they cause discoloration and staining which limits their use to applications where this is not important. 

Chain-breaking antioxidants which interrupt the propagation cycle by reacting with the radicals R and R02, introducing new termination reactions. 

In the past the greatest attention has been paid to the chain-breaking antioxidants and oxidation retarders of general structure AH, which may function in the following ways. 

It will be noticed that with chain-breaking antioxidants the additive will be consumed whilst if we assume that the AO2H molecule will regenerate A radicals the oxidation retarder is not effectively consumed. The difference between the two is illustrated schematically in Figure 7.4. 

Table 7.3 Main types of chain-breaking antioxidants...

Phosphites Tris-(p-nonylphenyl) phosphite (X) No Widely used in conjunction with conventional stabilisers (q.v.) in PVC. Some types appear to be useful heat and light stabilisers in polyolefins. Function primarily as peroxide decomposers rather than chain-breaking antioxidants. 

Materials that promote the decomposition of organic hydroperoxide to form stable products rather than chain-initiating free radicals are known as peroxide decomposers. Amongst the materials that function in this way may be included a number of mercaptans, sulphonic acids, zinc dialkylthiophosphate and zinc dimethyldithiocarbamate. There is also evidence that some of the phenol and aryl amine chain-breaking antioxidants may function in addition by this mechanism. In saturated hydrocarbon polymers diauryl thiodipropionate has achieved a preeminent position as a peroxide decomposer. 

Amongst other materials sometimes used as deactivators are, l,8-bis(salicyli-deneamino)-3,6-dithiaoctane and certain p-phenylenediamine derivatives. It is interesting to note that the last named materials also function as chain-breaking antioxidants and in part as peroxide decomposers. 

In antioxidants, synergism appears to arise either from one antioxidant effectively regenerating another so that the latter does not become consumed or by the two antioxidants functioning by differing mechanisms. The latter is more important and it is easy to see how effective a combination of peroxide decomposer and chain-breaking antioxidant can be. 

The peroxide decomposer will drastically reduce the number of radicals, which can then be more effectively mopped up by the chain-breaking materials. A widely used combination is 4-methyl-2,6,di-t-butylphenol and dilauryl thiodipropionate. It is possible to envisage most powerful combinations where a chain-breaking antioxidant, a regenerating agent, a peroxide decomposer, a metal deactivator and an ultraviolet absorber are all employed together. 

Chain-breaking antioxidant DLTP Carbon black Copper powder Induction period at 140°C (h)... 

The basis to the chain breaking donor (CB—D) mechanism, which was the first antioxidant mechanism to be investigated, was laid down by the late 1940s [10-12]. Many reducing agents, e.g., hindered phenols and aromatic amines, which reduce the ROO to hydroperoxide in a CB—D step have already been empirically selected and used for rubbers and by this time also for the newer plastics industry (e.g., Table la, AO 1-8 and 9-12). The major mechanistic landmarks of the antioxi-... 

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... 

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. 

PD—S) to yield phosphates and alcohols, see Scheme 5 reaction a. Sterically hindered aryl phosphites (e.g., AO 14) have an additional chain breaking activity, i.e. they react with peroxyl and alkoxyl radicals during their function as antioxidants (reactions 5b and 5c) [18]. 

Another approach to safer stabilization is to use a biological antioxidant such as vitamin E (a-tocopherol is the active form of vitamin E, AO-9, Table la). It is essentially a hindered phenol which acts as an effective chain breaking donor antioxidant, donating a hydrogen to ROO to yield a very stable tocopheroxyl radical, a-Tocopherol is a very effective melt stabilizer in polyolefins that offers high protection to the polymer at very low concentration [41], (Table 2). 

Synergism can also arise from cooperative effects between mechanistically different classes of antioxidants, e.g., the chain breaking antioxidants and peroxide decomposers (heterosynergism) [42]. For example, the synergism between hindered phenols (CB—D) and phosphites or sulphides (PD) is particularly important in thermal oxidation (Table 2). Similarly, effective synergism is achieved between metal dithiolates (PD) and UV-ab-sorbers (e.g., UV 531), as well as between HALS and UV-absorbers, (Table 3). 

The main function of vitamin E is as a chain-breaking, free radical trapping antioxidant in cell membranes and plasma lipoproteins. It reacts with the lipid peroxide radicals formed by peroxidation of polyunsaturated fatty acids before they can establish a chain reaction. The tocopheroxyl free radical product is relatively unreactive and ultimately forms nonradical compounds. Commonly, the tocopheroxyl radical is... 

BURTON G w, JOYCE A and INGOLD K u (1983) First proof that vitamin E is the major hpid-soluble chain-breaking antioxidant in human blood plasma , Lancet, 2, 327-8. 

SALAH N, MILLER N, PAGANGA G, TIJBURG L, BOLWELL G P and RICH-EVANS 0 (1 995) Polyphenlic flavanols as scavengers of aqueous phase radicals and as chain-breaking antioxidants , Arch Biochem Biophys, 322, 339-46. 

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