Cis-Trans Isomerization of Natural Rubber under the Influence of Hydrogen Chloride or Ethylaluminum Dichloride

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Abstract
We have established earlier that organo-aluminum compounds, and titanium tetrachloride, bring about the isomerization of the cis-isoprene units in natural rubber into the trans. The isomerization process, as influenced by the above materials, proceeds at appreciable velocity even at room temperature. The present communication reports the results of investigating the cis-trans isomerism of natural rubber as influenced by hydrogen chloride and ethylaluminum dichloride. The isomerizing action of hydrogen chloride upon unsaturated acids and stilbene was dislosed earlier in a series of papers. We investigated the action of dry hydrogen chloride and ethylaluminum dichloride on benzene solutions of natural rubber under the conditions described earlier. Hydrogen chloride was introduced in the form of a saturated benzene solution. In investigating the hydrogen chloride activity, each polymer sample was examined for unsaturation, chlorine content of the polymer chains and microstructure. The latter was characterized with the aid of infrared absorption spectra. As in the previous report, quantitative determination of the cis and trans content was made in the 840 cm−1 region, where the intensity and contour are substantially dissimilar for natural rubber and guttapercha. Furthermore, an additional inspection was made qualitatively in other regions of the IR spectrum (1100–1150 cm−1 and 1300–1330 cm−1), the feasibility of utilizing the 1300–1330 cm−1 region having been reported in a recent publication by Golub, who investigated the isomerization of natural rubber and guttapercha under the influence of selenium at temperatures of 180–220°.

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