In an earlier paper (1) it has been reported that by a judicious choice of the solvent molecule subjected to attack by a free radical the relative percentages of cleavage2 and of disproportionation experienced by an alkyl free radical or by an alkoxy free radical can be varied. When the solvent molecules in these reactions contained readily cleft hydrogen atoms the cleavage reaction predominated, while a solvent molecule with only difficultly cleft hydrogens caused these free radicals to react to a lesser extent by cleavage and to a greater extent by disproportionation. This evidence supports the theory of the existence of a so-called "energy-factor" (1) in these reactions involving free radicals.The present paper is the preliminary report of a study the purpose of which is to determine the effect that the introduction of the carbomethoxy group3 into a solvent molecule in positions of close proximity to the site of the cleavage attack by a free radical has upon the cleavage tendencies of the methyl and the ethoxy free radical with regard to that solvent molecule. The introduction of this group into said position has little or no effect on the percentage of cleavage exhibited by the methyl free radical, while the proximity of these groups to the preferred seat of attack seems to decrease the percentage of cleavage exhibited by the ethoxy free radical, to increase its percentage disproportionation, and in selected cases, even to reverse the seat of the cleavage attack. It is suggested that these carbomethoxy groups in the solvent molecule exert a possible repulsive effect upon the approaching free radical which repulsion is distinctly greater when this free radical is ethoxy than when it is methyl. This phenomenon involving the contribution of such repulsion to the over-all activation energy required for cleavage by a free radical at a given position in the solvent molecule has been termed the "repulsion factor" (1). In the preceding paper (1) the sol-1 This material has been presented as paper number 84 before the Organic Division of the American Chemical Society a t the Spring meeting, Los Angeles, March 19, 1953. 2 Some referees prefer the term hydrogen abstraction to cleavage. We have for this reason used this term elsewhere (1). Both of these terms apply t o reactions involving the attack of the free radical (here *CHs and *OC&) upon an hydrogen-containing solvent molecule withdrawing therefrom a single hydrogen atom and imprisoning it as part of the resulting molecule (CHa and CZHSOH). This leaves behind a "residual free radical" derived from the solvent molecule. Depending on the nature of this residual free radical, the temperature] the nature of the solvent, and the nature of the parent peroxide molecule, it may react in several ways. In most of the cases here reported the residual free radical dimerizes. In other cases it has apparently disproportionated. J Other groups found t o exhibit a similar effect are chlorine and carboxy. Studies involving these groups will be reported in a later publication. Fl...

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It is generally agreed that oxidation is the controlling factor in the decay of rubber compounds. Measurements of the decay of any physical properties—commonly tensile strength—can be made in a convenient time only if the decay is greatly accelerated, and there is always a grave doubt about the equality of the acceleration for different materials. There is also a difficulty in selecting a universally suitable aging period, since the decay of the physical properties is not linear. A direct measure of the rate of oxidation is, in our view, more fundamental and less equivocal. It can, moreover, be made in a reasonable time at a temperature not far removed from service temperatures. Again, whereas tensile tests require several samples of each point in the timecurve to give acceptable mean values, here a single sample suffices for the whole test, and this sample can be simply prepared from a specimen of any form by rasping. The merits and simplicity of the method should commend it to rubber technologists.

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