Sub-Product

Emulsion-SBR (E-SBR) is the most  widely used rubber in the world, representing 40% of the synthetic rubber market and it is predominantly used for the production of passenger car and light truck tires, as well as truck re-treading compounds. Other applications for E-SBR Rubber include shoe soles, flooring, conveyor belts, industrial hoses, adhesives, and a broad spectrum of other rubber goods^[1]. 

E-SBR is produced by Emulsion Polymerization initiated by Free Radicals. Reaction Vessels are typically charged with the two Monomers, a Free Radical Generator, and a Chain Transfer Agent such as an Alkyl Mercaptan. Radical Initiators include Potassium Persulfate and Hydroperoxides in combination with Ferrous Salts. Emulsifying Agents include various Soaps. By "capping" the growing Organic Radicals, Mercaptans (e.g. Dodecylthiol), control the Molecular Weight, and hence the Viscosity, of the Product. Typically, Polymerizations are allowed to proceed only to ca. 70%, a method called "Short Stopping". In this way, various Additives can be removed from the Polymer.^[2]

References

1. ARLANXEO, EMULSION STYRENE BUTADIEN RUBBER
2. Wikipedia, Styrene butadiene