Editing Styrene-butadiene (section)

=== Emulsion polymerization ===
E-SBR produced by emulsion polymerization is initiated by [[free radical]]s.  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 [[hydroperoxide]]s in combination with ferrous salts.  Emulsifying agents include various [[soap]]s. By "capping" the growing organic radicals, mercaptans (e.g. [[dodecylthiol]]), control the molecular weight 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.<ref name=Ullmann/>