Chain Scission in the Oxidation of Polyisoprene

Abstract

Abstract The composition of the mixture of volatile products formed during the oxidation in air of natural rubber or synthetic cis-polyisoprene in the raw state was investigated in order to obtain further information on the mechanism of chain scission. An important observation was made: in order to obtain, by means of gas chromatography, the peaks of all the constituents, a completely anhydrous solution of the trapped mixture must be used. If the water liberated by the oxidized polyisoprene is present, the peaks of some constituents do not appear. By removing this difficulty, four new less volatile compounds which could result from chain scission were detected on heating extracted polyisoprenes which do not contain protective agents. The concentrations of these four compounds are similar to those of levulinaldehyde, methacrolein, or methyl vinyl ketone, and the structures which were identified correspond to a γ-substituted γ-butyrolactone, a primary γ-ketol, and probably a tertiary γ-aldol and a secondary γ-ketol. These components could arise from a chain scission between two isoprene units, due to a β-scission process of an alkoxy radical. Natural rubber or synthetic polyisoprene which contain protective agents give off mainly the lactone and the tertiary γ-aldol, but levulinaldehyde is produced in small amounts, and methacrolein and methyl vinyl ketone are absent.