The Influence of Catalysts in the Reaction of Sulfur on Unsaturated Organic Substances. I. A Study of Vulcanization at Elevated Temperature
Authors: Schwarzkopf Otto 
Published: February 2011

Abstract
From these results it might be thought that in the presence of potassium xanthate or hexamethylenetetramine there is formed a compound of sulfur and cinnamate containing one atom of sulfur per molecule of the ester. This compound would not be identical to that found by Baumann and Fromm, which was mentioned in Section 1. It has been seen that it is formed at a higher temperature and that it contains two atoms of sulfur per molecule of the ester. We attempted to isolate the compound containing one atom of sulfur, the existence of which we suspected. To this end, we distilled under reduced pressure (13 mm.) the product extracted with acetone. However, at 143° C. the ethyl cinnamate distilled almost quantitatively, whereas the sulfur remained in the residue from the distillation. Moreover, we observed a very intense Tyndall effect in the acetone solution. Several reasons for these phenomena suggest themselves: 1. The compound formed is stable only in equilibrium with its components. By distillation the cinnamate is eliminated, equilibrium is destroyed, and the compound is quantitatively decomposed during distillation. 2. Upon the addition of acetone, with a view to extracting the substance, the compound is destroyed and the sulfur which is precipitated in the colloidal state is dissolved with ethyl cinnamate in the acetone. In the acetone solution, therefore, the cinnamate and colloidal sulfur are found in stoichiometric proportion. This would explain both the equivalence which was found and the colloidal aspect of the acetone solution. 3. A definite chemical compound is not formed, purely colloidal natural phenomena are involved, and the stoichiometric relations must he due purely to chance. We shall not pay any particular attention at this time to experiments which would lead to a choice among these explanations. They will be the object of a future work. Nevertheless it seems of interest to us to note the parallelism between the vulcanization of rubber and the sulfuration of a well-defined unsaturated compound, and particularly the action of catalysts of vulcanization. Finally another important similarity may be pointed out. In the case of vulcanized rubber, a definite compound has not been isolated. It is likewise known that sulfur, which alone is insoluble in acetone, passes over in large quantity when extractions are made with this solvent. The question has always arisen whether this is a case of phenomena of a colloidal nature or of the formation of chemical compounds. We shall continue our work with a study directed particularly to simple cases where the starting compounds are well-known ones. Our efforts will be especially directed to the existence or non-existence of definite compounds. We have pointed out in this first article the difficulties before us. We shall then continue with studies of isoprene and methylbutadiene. It is conceivable that by the progressive polymerization of these products the complex case of rubber will finally be reached.