Cis-Trans Isomerization in Natural Polyisoprenes

The cis and trans isomers of many simple olefins and conjugated polyolefins can be interconverted by the action of heat, light, and various catalysts, and in many such systems the changes in geometric configuration can be readily followed and the isomers separated and identified. Previous attempts to isomerize natural rubber and gutta-percha by treatment with ultraviolet light and various chemicals gave no detectable cis-trans isomerization, and in general configurational changes in unconjugated polyolefins have been unknown until recently when Golub converted the cis-1,4 units in polybutadiene into the corresponding trans units. This isomerization was achieved by irradiation of the cis-polymer with ultraviolet light in the presence of various organic bromides and sulfur compounds, but apparently this method did not isomerize natural rubber. In attempts to retard the rate of crystallization of natural rubber at moderately low temperatures—i.e., −10° to −40° C—by the attachment of side groups which would interfere with the molecular packing, it was observed that very small amounts of thiol acids were remarkably effective. This suggested that structural changes other than simply the attachment of side groups were occurring in the polyisoprene molecule. Recently it was found that thiol acids could interconvert the cis and trans forms of a simple trialkyl ethylene—e.g., 3-methylpent-2-ene—and subsequently, the isomerization of natural rubber and gutta-percha by these reagents was also observed. Following this, natural rubber, gutta-percha, squalene and cis and trans forms of 3-methylpent-2-ene have also been isomerized by treatment with sulfur dioxide and allied compounds; this work, including an investigation of the physical properties of the vulcanizates obtained from some of the isomerized polyisoprenes, is described in the present paper.