The tendency of olefins to absorb molecular oxygen increases rapidly with increase in unsaturation. In monoolefins absorption is usually very slow unless it is actively promoted by irradiation, by the use of temperatures above room-temperature, or by chemical catalysts; in dienes and trienes it usually becomes considerable in diffused daylight; and in higher polyenes it tends to occur unpreventibly and excessively. The strong tendency towards oxidation shown by polyenes appears to be shared by unconjugated and conjugated substances alike, although the precise courses of reaction are probably somewhat different in the two cases. In the present paper attention is focussed mainly on monoolefinic and unconjugated polyolefinic substances.
In the previous publication, of Farmer, Tristram and Bolland, it was stated that methods were required to estimate the various forms in which oxygen occurs united to the rubber molecule, and a method for the estimation of hydroxyl oxygen was discussed. The determination of peroxidic oxygen in oxidized rubber presents two distinct problems. It is necessary to employ a method which, first, is satisfactory in the presence of unsaturation and, secondly, ensures effective contact between the long rubber chains and the analytical reagent. These requirements are both satisfied by the modification described below of the colorimetric method of Young, Vogt and Nieuwland. Before describing the modified technique, some mention must be made of the earlier methods and of the errors which arise when they are applied to the determination of peroxidic oxygen in unsaturated compounds. All the earlier methods for the determination of peroxidic oxygen depend on the oxidation of potassium iodide and titration of the liberated iodine. This method is open to the objection that the liberated iodine immediately attacks the double bonds remaining in the oxidized material. This difficulty was recognized by Stephens, who, using a method in which the peroxidic oxygen was reduced with stannous chloride, succeeded in estimating about 80 per cent of the total peroxide present in cyclohexene peroxide. Yule and Wilson estimated the peroxide in cracked gasoline by shaking with an acid solution of ferrous thiocyanate and back-titrating the resulting ferric salt with titanous chloride. The results were later contested by Young, Vogt and Nieuwland, who described the colorimetric method which has been adopted, with some modification, in the present work.
Peroxides have repeatedly been shown to appear during the slow oxidation (perishing) of rubber in air, and during both its milling in air and the passage of air or oxygen through its solutions: yet in none of the recorded experiments has the production of any very substantial proportion of peroxide been demonstrated. This arises largely because, under many conditions of autoöxidation, no very considerable proportion of peroxide is present in the oxidation product at any stage, but in part because no convenient and reasonably accurate method of determining the peroxide content in so insoluble a substance as rubber has been available. It must inevitably happen in a long-chain molecule of polymer-homologous type containing on the average about 5,000 autoxidisable olefinic units that the progressive entrance of a heterogeneous element will yield a large variety of closely similar products if attack is fairly evenly distributed over the chain, but when, as in the case of oxygen, the heterogeneous element enters the hydrocarbon as a diatomic molecule, the atoms in which can ultimately separate and become attached to the carbon chain in a variety of ways, and causes, as the result of its progressive incorporation, scission of the chain at an ever-increasing number of the olefinic units, then the potential range of diversity of the products becomes enormously increased.
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