Chain-terminating antioxidant

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. 

Intramolecular hydrogen bond is much stronger than the intermolecular hydrogen bond. The reactivity of such phenolic groups was studied by Pozdeeva et al. [51], Crown phenol A was synthesized, and the reactivity as chain terminating antioxidant in oxidized styrene (323 K) was studied. The chain terminating activity of this crown phenol A was compared with that of ionol. 

Antioxidants act either by radical trapping, interfering in the radical propagation, or by decomposition of the hydroperoxide formed. In this way they inhibit the formation of new radicals. The former are known as chain breaking or chain terminator antioxidants (primary antioxidants). The latter, the hydroperoxide decomposing agents, are also known as preventive antioxidants (secondary antioxidants). 

At present, the most widely used chain-terminating antioxidants that react with peroxide radicals are substituted phenols, aromatic amines, and additives that decompose hydroperoxides into molecular products - sulphides and others. 

Monofunctional, cyclohexylamine is used as a polyamide polymerization chain terminator to control polymer molecular weight. 3,3,5-Trimethylcyclohexylamines ate usehil fuel additives, corrosion inhibitors, and biocides (50). Dicyclohexylamine has direct uses as a solvent for cephalosporin antibiotic production, as a corrosion inhibitor, and as a fuel oil additive, in addition to serving as an organic intermediate. Cycloahphatic tertiary amines are used as urethane catalysts (72). Dimethylcyclohexylarnine (DMCHA) is marketed by Air Products as POLYCAT 8 for pour-in-place rigid insulating foam. Methyldicyclohexylamine is POLYCAT 12 used for flexible slabstock and molded foam. DM CHA is also sold as a fuel oil additive, which acts as an antioxidant. StericaHy hindered secondary cycloahphatic amines, specifically dicyclohexylamine, effectively catalyze polycarbonate polymerization (73). 

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

In the previous section, we have described some of the mechanisms that may lead to the fijrmation of lipid hydroperoxides or peroxyl radicals in lipids. If the peroxyl radical is formed, then this will lead to propagation if no chain-breaking antioxidants are present (Scheme 2.1). However, in many biological situations chain-breaking antioxidants are present, for example, in LDL, and these will terminate the peroxyl radical and are consumed in the process. This will concomitandy increase the size of the peroxide pool in the membrane or lipoprotein. Such peroxides may be metabolized by the glutathione peroxidases in a cellular environment but are probably more stable in the plasma comjxutment. In the next section, the promotion of lipid peroxidation if the lipid peroxides encounter a transition metal will be considered. 

In this reaction scheme, the steady-state concentration of peroxyl radicals will be a direa function of the concentration of the transition metal and lipid peroxide content of the LDL particle, and will increase as the reaction proceeds. Scheme 2.2 is a diagrammatic representation of the redox interactions between copper, lipid hydroperoxides and lipid in the presence of a chain-breaking antioxidant. For the sake of clarity, the reaction involving the regeneration of the oxidized form of copper (Reaction 2.9) has been omitted. The first step is the independent decomposition of the Upid hydroperoxide to form the peroxyl radical. This may be terminated by reaction with an antioxidant, AH, but the lipid peroxide formed will contribute to the peroxide pool. It is evident from this scheme that the efficacy of a chain-breaking antioxidant in this scheme will be highly dependent on the initial size of the peroxide pool. In the section describing the copper-dependent oxidation of LDL (Section 2.6.1), the implications of this idea will be pursued further. 

The importance of vitamin E for maintenance of lipid integrity in vivo is emphasized by the fact that it is the only major lipid-soluble chain-breaking antioxidant found within plasma, red cells and tissue cells. Esterbauer etal. (1991) have shown that the oxidation resistance of LDL increases proportionately with a-tocopherol concentration. In patients with RA, synovial fluid concentrations of a-tocopherol are significantly lower relative to paired serum samples (Fairburn et al., 1992). The low level of vitamin E within the inflamed joint implies it is being consumed via its role in terminating lipid peroxidation and this will be discussed further in Section 3.3. 

Lipid peroxidation (see Fig. 17.2) is a chain reaction that can be attacked in many ways. The chain reaction can be inhibited by use of radical scavengers (chain termination). Initiation of the chain reaction can be blocked by either inhibiting synthesis. of reactive oxygen species (ROS) or by use of antioxidant enzymes like superoxide dismutase (SOD), complexes of SOD and catalase. Finally, agents that chelate iron can remove free iron and thus reduce Flaber-Weiss-mediated iron/oxygen injury. 

Synergism of Chain Termination and Hydroperoxide Decomposing the Antioxidants... 

The a-aminoalkylperoxyl radicals RCH(00 )NHR possess a dual reactivity oxidative (due to the peroxyl group) and reducing (due to the amino group) [5]. As a result, many antioxidants terminate the chains of oxidized amines by the mechanisms of cyclic chain termination (see Chapter 16). 

It was in 1924 when Christiansen [3] put forward the conception of the chain mechanism of oxidation and explained the action of antioxidants via chain termination by the antioxidant [3]. Three years later, Backstrom and coworkers [4—6] experimentally proved the chain mechanism of benzaldehyde oxidation (see Chapter 1) and the mechanism of antioxidant (hydroquinone) action via chain termination. The systematic study of the oxidation kinetics of esters of nonsaturated acids was performed by Bolland and ten Have [7,8], They observed in the kinetic experiments that substrates are oxidized by the chain mechanism with chain propagation via the cycle of reactions (see Chapter 2). 

The retarding action of antioxidants (InH), such as phenols and aromatic amines, was proved to be the result of chain termination by accepting the peroxyl radicals. 

Cyclic chain termination by antioxidants. Oxidation of some substances, such as alcohols or aliphatic amines, gives rise to peroxyl radicals of multiple (oxidative and reductive) activity (see Chapters 7 and 9). In the systems containing such substances, antioxidants are regenerated in the reactions of chain termination. In other words, chain termination occurs as a catalytic cyclic process. The number of chain termination events depends on the proportion between the rates of inhibitor consumption and regeneration reactions. Multiple chain termination may take place, for instance, in polymers. Inhibitors of multiple chain termination are aromatic amines, nitroxyl radicals, and variable-valence metal compounds. 

Mechanism IV Inhibited Chain Oxidation when In Propagates the Chain by Reaction with RH If the radicals In formed from an antioxidant are active toward RH, the chain termination is limited by reactions (8) and (9) rather than by reaction (7). Inhibited oxidation also involves the following reaction [18,23,31,32,38] ... 

SYNERGISM OF CHAIN TERMINATION AND HYDROPEROXIDE DECOMPOSING THE ANTIOXIDANTS... 

The induction period is measured experimentally at the constant sum of concentrations of two antioxidants, namely, Co = [S]o + [InH]0 = const. Theoretically this problem was analyzed in [9] for different mechanisms of chain termination by the peroxyl radical acceptor InH (see Chapter 14). It was supposed that antioxidant S breaks ROOH catalytically and, hence, is not consumed. The induction period was defined as t = (/[InH /v, where vV2 is the rate of InH consumption at its concentration equal to 0.5[InH]o. The results of calculations are presented in Table 18.1. 

Nitroxyl radicals are formed as intermediates in reactions of polymer stabilization by steri-cally hindered amines as light stabilizers (HALS) [30,34,39,59]. The very important peculiarity of nitroxyl radicals as antioxidants of polymer degradation is their ability to participate in cyclic mechanisms of chain termination. This mechanism involves alternation of reactions involving alkyl and peroxyl radicals with regeneration of nitroxyl radical [60 64],... 

In addition to this reaction, quinones and other alkyl radical acceptors retard polymer oxidation by the reaction with alkyl radicals (see earlier). As a result, effectiveness of these inhibitors increases with the formation of hydroperoxide groups in PP. In addition, the inhibiting capacity of these antioxidants grows with hydroperoxide accumulation. The results illustrating the efficiency of the antioxidants with cyclic chain termination mechanisms in PP containing hydroperoxide groups is presented in Table 19.12. The polyatomic phenols producing quinones also possess the ability to terminate several chains. 

Such a dependence was interpreted within the scope of the model of chain oxidation with diffusionally controlled chain termination on the surface of solid antioxidant (for example, Mo or MoS2). According to the Smolukhovsky equation, the diffusion velocity of radical R02 at the distance //2 is v = 0.2DkS13, where S is the surface of the solid inhibitor and k is the coefficient of proportionality between the surface and number n of the solid particles (S=k x ). The function F for such diffusionally controlled chain termination is the following ... 

It is considerably more difficult to inhibit oxidation in the gas phase than in the liquid phase. At the high temperatures of gas-phase oxidations the rates of the chain-propagating and branching reactions are increased to a greater extent than the rates of the chain-terminating reactions. Initiation by surfaces can also constitute a serious problem. The majority of liquid-phase antioxidants which are effective at high temperatures are too involatile to be useful in the gas phase. However, inhibition can be achieved with aliphatic amines, which are generally rather ineffective inhibitors of low temperature liquid-phase oxidations. The mechanisms by which the different types of antioxidants inhibit oxidation are briefly described below. 

Chain-breaking antioxidants which interfere with the normal propagation processes may react with peroxy radicals, R02 or, more rarely, with the carbon radical, R. The antioxidant may react with the propagating radical by addition, by hydrogen transfer, or by electron transfer. The chain can be terminated directly, but more commonly a new radical is formed, which either continues the chain at a reduced rate or terminates a second chain. 

Reactions Limited by Rotational Diffusion in Polymer Matrix Antioxidants Reacting with Peroxyl Radicals Antioxidants Reacting with Alkyl Radicals Cyclic Chain Termination in Oxidized Polymers... 

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