Oxidation reactions of polymers

Uncontrolled oxidation of rubber is detrimental to its physical properties. Oxidation reactions take place readily at unsaturated groups in polymers and are often referred to collectively as epoxidation however, oxidation under controlled conditions can lead to useful products such as the epoxidized natural rubber introduced by the Malaysian Rubber Producers Association (Schults etal., 1983 Cunneen and Porter, 1965 Ceresa, 1965 Avery and Watson, 1956). Natural rubber in the latex form is treated with hydrogen peroxide dissolved in acetic acid. This gives 50% epoxidized natural rubber. This rubber shows very interesting physical properties and excellent carbon black dispersion. Similarly, nonaqueous epoxidizations of synthetic polyisoprene can be achieved 

A polymer (PH) or other organic compounds can react with a molecular oxygen as follows  

If the polymer contains double bonds the original adduct degrades into the products as follows  

Abstracting hydrogen, it reacts with another polymer (PH) generating hydroperoxides 

the homolytic POOH splitting off leads to the formation of the (P ) intermediate which can cause generation of other degradation products 

Oxidative degradation can be initiated photochemically, or it can be catalysed by metals. This type of reaction can be exemplified by the catalytic effect of zinc oxide on the degradation of peroxides, brought about by UV radiation 

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Gugumus F. Re-examination of the thermal oxidation reactions of polymers 2. Thermal oxidation of polyethylene. Polym Degrad Stab 2002 76(2) 329-40. 

VI. Other Addition Reactions to Double Bonds Vn. Oxidation Reactions of Polymers Vin. Functionalization of Polymers IX. Miscellaneous Chemical Reactions of Polymers X. Block and Graft Copolymerization References... 

This section deals not only with oxidative reactions of polymers at room temperature, but also with thermo-oxidative degradation at elevated temperatures. Although this type of degradation resembles photo-oxidative degradation, for the purpose of maintaining clarity, the latter is discussed separately in the next section. 

The oxidation mechanism for all oxidation reactions of polymers passes initially through a stage of hydroperoxide formation. Oxidation can be represented by reactions as indicated belowl5 (or by variations thereof). 

Oxidative degradation of polymers is initiated by radicals (R ) generated in the polymer by heat or mechanical shear during processing or by exposure to UV light. These radicals, in turn, react with 02 to form peroxy and hydroperoxide radicals that promote radical reactions. 

Measuring the relative oxidative stability of polymers is important. Measurements can be used to determine dependencies on structural and molecular weight/weight distribution or the effectiveness of an antioxidant, or to perhaps assess the amount present in a polymer sample, etc. The preferred and commonest method consists in raising the sample temperature to a predetermined level, while in an inert atmosphere, then switching the atmosphere to air or oxygen. The time to the onset of exothermic reaction is measured. 

This reaction is very exothermic (A// —180 to —200kJ mol-1) and, therefore, seems to be very probable from the thermochemical point of estimation. The pre-exponential factor is expected to be low due to the concentration of the energy on three bonds at the moment of TS formation (see Chapter 3). To demonstrate that this reaction is responsible for the oxidative destruction of polymers, PP and PE were oxidized in chlorobenzene with an initiator and analyzed for the rates of oxidation, destruction (viscosimetrically), and double bond formation (by the reaction with ozone) [131]. It was found that (i) polymer degradation and formation of double bonds occur concurrently with oxidation (ii) the rates of all three processes are proportional to v 1/2, (iii) independent of p02, and (iv) vs = vdbf in PE and vs = 1.6vdbf in PP (vdbf is the rate of double bond formation). Thus, the rates of destruction and formation of double bonds, as well as the kinetic parameters of these reactions, are close, which corroborates with the proposed mechanism of polymer destruction. Therefore, the rate of peroxyl macromolecules degradation obeys the kinetic equation ... 

At elevated temperatures, the oxidation of PE occurs as a quasistationary process with the kinetic equilibrium between the formation and decay of hydroperoxyl groups (see Chapter 4). The ratio of the two discussed reactions of polymer degradation is ... 

Scheme 18.4 Photo-oxidation reactions of PECT [11], Reprinted from Polymer, 41, Grossetete,T., Rivaton, A., Gardette, J.-L., Hoyle, C. E.,Ziemer, M., Fagerburg, D. R. and Clauberg, H., Photochemical degradation of poly(ethylene terephtha-late)-modified copolymer, 3541-3554, Copyright (2000), with permission from Elsevier Science...

The observed catalytic effect of the crown ether appears to be dependent on the nucleophile employed in both polymerization and corresponding model reactions. Not surprisingly, it appears that the stronger the nucleophile employed, the smaller the catalytic influence of the crown ether. For example, with potassium thiophen-oxide yields of polymer or model products were almost quantitative with or without catalyst. By contrast, the reaction of PFB with potassium phthalimide, a considerably weaker nucleophile, affords 6 in 50% with catalyst and in 2-3% without catalyst under identical conditions. However, it may be that this qualitative difference in rates is, in fact, an artifact of different solubilities of the crown complexed nucleophiles in the organic liquid phase. A careful kinetic study of nucleophilicity in catalyzed versus non-catalyzed reactions study is presently underway. 

Any chemical reaction, whether involving the main chain or side groups, results in a change of composition of one or more groups and consequently in the IR spectrum. This makes it possible to study oxidation, thermal degradation, cyclization, grafting, and other reactions of polymers [2,4]. Evaluation of both qualitative and quantitative changes, as well as determination of kinetic constants of the reaction, is possible [2]. 

The first example of catalysis by a polymer-metal complex was presented by Lautsch et al. u3 Metalloporphyrin was linked to a poly(phenylalanine) chain by a peptide bond. The catalytic properties of this polymer-Fe(III)porphyrin complex were compared with Fe(III)porphyrin in the oxidative reaction of phenylenedi-amine. The catalytic activity of the polymer complex was twice as large as that for the corresponding analog. 

These redistribution reactions of polymer molecules with other polymer molecules as well as with monomer, continue throughout the polymerization and should result in randomization of the polymer. Inasmuch as dimethylphenol is among the most reactive and diphenylphenol the least reactive of the phenols which have been oxidized successfully to linear high polymers, it appears likely that oxidation of any mixture of phenols will yield random copolymers. 

Photolysis of atmospheric pollutants by solar radiation results in an increase of ozone concentration in certain urban areas and is the cause of a sequence of oxidation reactions with polymers. Ozone reacts with practically all organic materials especially with alkenes. The rate of its reaction with alkene is several orders of magnitude higher than that with alkane. The ratio of the rate constants of ozone with ethene/ethane is 1.5 x 105, with propene/propane 1.6 x 106, and with butene- 1/butane 1.1 x 106, at room temperature [5],... 

In initiation of oxidation, the important role may also be played by reactive bonds in a polymer. With polyethylene which is the most simple structurally, such reactive sites may be small concentrations of double bonds or more numerous sites of branching of a main chain to side alkyls. Investigation of the oxidation reaction of different types of polyethylene has. however, revealed that the degree of polyethylene branching from 0 to 20 branches for each 1000 carbon atoms of the main chain does not affect the induction period of oxidation [13]. 

In the oxidation reaction of the anionic reactant, [Fe(II)-EDTA] with poly(4-vinylpyridine)-Co(III)en2 complex(XIII), the coulombic binding between the reactant and the polymer is considered (122-124). 

PospKil J (1981) Photo-oxidation reactions of phenolic antioxidants, In Developments in polymer photochemistry-2, Allen N S (Ed), Applied Science Publishers London, pp 53-133. 

Figure 6 (a) Decay ofIR absorption atl984 cm 1 with exponential curve fit (b) plots ofkobs v.v. [Mel] for oxidative addition reactions of [Polymer][Rh(CO)2I2] and Bu4N[Rh(CO)2I2] (25 °C)... 

Kcal/mol, and that the rate of oxydation the radicals is independent of oxygen pressure over a wide range of oxj en pressures. These two facts seem to support our picture that the mechano-radicals are on the fre surfece, because small concentration of ox3 n at low pressure is enou fm oxidation reaction of radicals only when all the radicals are on the surface and also such small value of the activation energy is presumably too small for the diffusion of oxygen into solid polymers. 

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