Polymer-reactive antioxidant reactions antioxidants

Reactions of Polymer-Reactive Antioxidants. Three different modifications of this approach have been reported. 

Another polymer reactive antioxidant which can be combined with rubber during vulcanization involves the 1,3 addition reaction of nitrones to the double bond in rubbers, reaction 4 (11). 

Reactions of Antioxidants with Preformed Functional Groups. The reactive chlorine in epichlorohydrin polymers is a specific though typical example of this approach (11). A more general reaction is the epoxidation of the double bonds in rubbers and subsequent reaction of the epoxide group with an amine antioxidant (reaction 1)... 

Reactions of Reactive Antioxidants with Polymers by Normal Chemical Procedures. Grafting of vinyl antioxidants e.g., VI, into rubbers has been used to produce modified rubber latices (26). Even simple... 

Unwanted reactivity with host polymer. Conversely, deliberate reactions can be indnced to attach antioxidants to the polymer chains, or to bnild them into chains. 

Reaction of conventional antioxidants with functionalised polymers. Many unsaturated rubbers can be made reactive toward conventional antioxidants by chemical modification. W ys in which this can be... 

Reaction of reactive antioxidants with conventional polymers. 

It is apparent that photo-oxidation of polymers is a complex and very important component of photodegradation of such materials. Detailed discussion of mechanism and the effects of these reactions is not part of this review. However, prevention of breakdown has a very obvious economic impact and a few generalizations on stabilization of polymers are relevant before the detailed discussion which follows in Volume 6, Chapter 19. Given that radiation produces free radicals, which become involved in a chain reaction, then any substance which preferentially absorbs the harmful radiation (a screen) or any substance which can efficiently remove radicals via non-reactive products (an antioxidant) will operate as a stabilizer. However, the planning and chemical design of stabilizers is not easy and Volume 6, Chapter 19 gives an excellent survey of just how complicated the process of stabilization can be. 

The above approach of mechanochemically initiated addition of reactive antioxidants on different polymers, such as rubbers and unsaturated thermoplastics such as ABS is illustrated here for thiol-containing antioxidants. For example, using thiol compounds (37) and (38) as the reactive antioxidants, Kharasch-type addition of the thiol function to the polymer double bond takes place during melt processing to give bound antioxidant adduct (see reaction 7) the polymer becomes much more substantive under aggressive environments. 

Polyolefins such as polyethylene and polypropylene contain only C—C and C—H bonds and may be considered as high molecular weight paraffins. Like the simpler paraffins they are somewhat inert and their major chemical reaction is substitution, e.g. halogenation. In addition the branched polyethylenes and the higher polyolefins contain tertiary carbon atoms which are reactive sites for oxidation. Because of this it is necessary to add antioxidants to stabilise the polymers against oxidation Some polyolefins may be cross-linked by peroxides. 

The proximity of the methyl group to the double bond in natural rubber results in the polymer being more reactive at both the double bond and at the a-methylenic position than polybutadiene, SBR and, particularly, polychlor-oprene. Consequently natural rubber is more subject to oxidation, and as in this case (c.f. polybutadiene and SBR) this leads to chain scission the rubber becomes softer and weaker. As already stated the oxidation reaction is considerably affected by the type of vulcanisation as well as by the use of antioxidants. 

The di- and monoalkyltin compounds are considered to be effective as stabilizers because they (i) inhibit the onset of the dehydrochlorination reaction by exchanging their anionic groups, X, with the reactive, allylic chlorine atoms in the polymer (ii) react with, and thereby scavenge, the hydrogen chloride that is produced and that would otherwise induce further elimination (jii) produce the compound HX, which may also help to inhibit other undesirable side reactions and iiv) prevent breakdown of the polymer initiated by atmospheric oxidation, i.e., by acting as antioxidants. 

Antioxidants are species that accept the reactive byproducts of oxidation reactions. They are typically hindered amines or phenols that accept radicals, inactivating them and preventing further effects of oxidation. The level of antioxidant used in a polymeric item depends on the expected lifetime of the final part, the environment in which the part will be used, and the susceptibility of the polymer to oxidation. Figure 9.7 shows two common antioxidants used in polyolefins. 

Interaction of acceptors of reactive free radicals and compounds that suppress the transfer reaction of an inhibitor radical with the substrate as it occurs in a system comprising antioxidants and polymer chain with conjugated system of double C = C bonds. 

Antioxidants act so as to interrupt this chain reaction. Primary antioxidants, such as hindered phenol type antioxidants, function by reacting with free radical sites on the polymer chain. The free radical source is reduced because the reactive chain radical is eliminated and the antioxidant radical produced is stabilised by internal resonance. Secondary antioxidants decompose the hydroperoxide into harmless non-radical products. Where acidic decomposition products can themselves promote degradation, acid scavengers function by deactivating them. 

A second mechanism in the. aging of CTPB propellants also exists and proceeds concurrently with the reactions proposed above. It consists of an attack at the reactive points of unsaturation in the backbone polymer, which causes additional crosslinking and hence an increase in propellant modulus, particularly at the surface. The exposed surface of CTPB propellants changes, as indicated by an increase in hardness. Heavy metal ions are particularly harmful, and it was found that an increase from 10 to 80 p.p.m. of iron caused a significant increase in surface hardening by catalytic attack on the double bonds. Antioxidants in general provide sufficient protection for polymer storage. In CTPB propellants the antioxidant selected to protect the double bond is very important. Amine-type antioxidants have provided better surface stability than phenolic compounds. 

The use of a reactive di- or poly-functional comonomer (non-antioxidant) which can co-graft with a monofunctional polymerisable antioxidant on polymers can improve the grafting efficiency from as low as 10-40% to an excess of 80-90%. This strategy, however, presents immense challenges due to the presence of more than one polymerisable group in the comonomer which could lead to additional undesirable (competing) side reactions com-... 

Attachment of B ansformation Products of Stabilizers. Up-to-date knowledge dealing with the chemistry of transformation products of phenolic [6, 15, 17, 20] and aromatic aminic [16, 43, 230] antioxidants and photoantioxidants based on hindered piperidines [10] indicates the possibility of attaching compounds having structures of quinone imine or quinone methide, or of radical species like cyclohexadienonyl, phenoxyl, aminyl or nitroxide to polymeric backbones. These reactions proceed mostly via reactivity of macroalkyl radicals derived fi-om stabilized polymers. Various compounds modelling this reactivity have been isolated [19, 230]. These results are of importance mainly for the explanation of mechanisms of antioxidant activity [6, 22, 24]. 

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