 The "Mutual" recipe for styrene-butadiene rubber was based on the Buna-S recipe, developed by I. G. Farben in the 1930s.The Buna-S recipe, however, produced a very tough, high molecular weight rubber that required heat processing to break it down and make it processable on standard rubber mills.Researchers at Standard Oil Development Company and the U. S. Rubber Company discovered that addition of a mercaptan "modifier" to the recipe not only produced a lower molecular weight and more tractable rubber, but it also increased the polymerization rate.