The Chemistry of Vulcanization. I. Diphenylmethane as a Model of Rubber Hydrocarbon for Its Reaction with Sulfur

Abstract

Abstract Following the discovery of vulcanization of rubber by Goodyear, a vast number of experiments and speculations have been carried out, but the final clarification of the mechanism of vulcanization and acceleration has remained unsettled. Overemphasis of physical and technological criteria in the investigation of vulcanization may have contributed to the failure to attach importance to the purely chemical investigation. In fact rubber vulcanizates defy chemical investigation because they are insoluble and infusible. However, in 1947 Farmer and collaborators, using olefinic substances such as cyclohexene, dihydromyrcene and squalene as model compounds, proposed a new theory on the vulcanization of rubber by sulfur. According to them α-methylenic hydrogen adjacent to olefinic double bonds is attacked by sulfur, and a hydrocarbon radical is formed. Then this radical couples with the diradical ⋅Sx⋅ and this leads to polysulfides as primary reaction products. Farmer's theory laid the ground work for the chemical investigation of vulcanization but the reaction of olefins with sulfur is too complex to indicate the reaction mechanism. Diphenylmethane (DPM) contains α-methylenic hydrogen (to a benzene ring) and may be expected to react with relative readiness. Moreover the reaction of DPM with molecular oxygen is already reported to produce water, diphenylmethyl hydroperoxide, tetraphenylethane and benzophenone. It is expected then that when DPM reacts with sulfur, it should give the thio analogs of these compounds. Since the reaction and reaction products of sulfur with DPM are considerably less complex than those with olefinic substances, the reaction mechanism should be elucidated by using the simpler hydrocarbon as a model compound. Comparative studies of the reaction with and without accelerator will present the key to the mechanism and theory of acceleration. As fundamental studies of rubber vulcanization as well as acceleration the following are the two objects of this paper. The first is the reaction mechanism of DPM with sulfur at 180° C and the second is the pyrolysis of the primary product (dibenzhydryl polysulfide) to the secondary product (thiobenzophenone) at the same temperature. Investigations on the reaction of DPM with sulfur in the presence of accelerators will be reported in other papers of this series.