Some Changes in Double-Bond Structure During the Vulcanization of Natural Rubber

Near-infrared studies indicate the absence or near-absence of isolated cis and terminal double bonds in rubber vulcanized either with or without accelerators, as well as in the squalene-sulfur reaction product. The use of purified natural rubber indicates a new absorption near 6.1 μ on reaction with sulfur, either with or without accelerators in the rubber system. This new absorption may be due either to a double bond with an attached sulfur atom or to conjugated double-bond systems that do not react readily with maleic anhydride. Ultraviolet spectra of chromatographic fractions show that squalene that has reacted with 15 percent of sulfur contains conjugated dienes, trienes, and possibly dienes, hexaenes, and heptaenes. The conjugated dienes, trienes, and tetraenes are probably present in sufficient proportions to account for a significant portion of the band at 10.4 μ. There is some evidence that the trienes and tetraenes, at least, may be largely all- trans in configuration. Other contributors to the band at 10.4 μ are possibly isolated, trans double bonds and heterocyclic sulfur structures along the chain.