Editing Styrene-butadiene (section)

== Types ==
SBR is derived from two [[monomer]]s, [[styrene]] and [[butadiene]].  The mixture of these two monomers is polymerized by two processes: from solution (S-SBR) or as an emulsion (E-SBR).<ref>[https://web.archive.org/web/20060311034046/http://www.iisrp.com/WebPolymers/12SolutionSBR.pdf International Institute of Synthetic rubber Producers, Inc. (IISRP)] article on S-SBR (retrieved 2011-12-02)</ref> E-SBR is more widely used.

=== 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/>

=== Solution polymerization ===
Solution-SBR is produced by an anionic polymerization process. Polymerization is initiated by [[alkyl lithium compound]]s.  Water and oxygen are strictly excluded. The process is homogeneous (all components are dissolved), which provides greater control over the process, allowing tailoring of the polymer. The organolithium compound adds to one of the monomers <!--which?-->, generating a [[carbanion]] that then adds to another monomer, and so on. For tire manufacture, S-SBR is increasingly favored because it offers improved  wet grip and reduced rolling resistance, which translate to greater safety and better fuel economy, respectively.<ref name=Ullmann2>H.-D.Brandt et al. "Rubber, 5. Solution Rubbers" in Ullmann's Encyclopedia of Industrial Chemistry, 2012, Wiley-VCH, Weinheim.  {{doi|10.1002/14356007.o23_o02}}</ref>