E-SBR and S-SBR grades compete, but usually, S-SBR grades are applied to high-performance tire applications. While beneficially standardized ESBR grades supplied by several producers can be relatively exchanged, while the replacement of ESBR by SSBR for high-performance tires requires tire compound re-formulation and often new processing equipment, and thus significant investment.

So, one of the main efforts of the S-SBR producers has been the search for a better processability, which is achieved through modification of the polymer macrostructure (ie MWD and LCB). With this respect, the use of coupling agents, such as for example SiCl4 and SnCl4, primarily in batch-wise anionic polymerization provided an option to broaden the molecular weight distribution (MWD) and to form star-shaped polymer macromolecules.

Automotive companies have continuously developed vehicles with superior performance and durability (increased mileage). This has resulted in increasingly stringent tire performance specifications. These have been partly met by redesigning the structure components (ie tread patterns and reformulating compounds).

Due to their low resistance, high grip and abrasion resistance properties in tires, tire manufacturers have found no alternative to S-SBR grades for highly specified, high-performance tire components. The need for reducing fuel consumption led to the definition of rolling resistance specifications of the tire tread material. In the U.S. the CAFE (Corporate Average Fuel Economy) regulations are increasingly demanding high fuel efficiency and excellent grip & wear properties. Aforementioned performances specifications cannot be achieved with E-SBR, boosting the marked extension on S-SBR.

The production of emulsion and solution SBR can be carried out by means of two process configurations, batch and continuous process configuration. Typically, the E SBR process is continuous, while S-SBR facilities exist which are based on the continuous and batch operation.

The batch process allows a better and easier “fine-tuning” of the reaction conditions and, therefore, of the polymer characteristics. Solution SBR is made through anionic “living” polymerization usually initiated by alkyl lithium compounds, especially n- or sec.-butyl lithium. Alkyl lithium compounds are selected (a) due to their solubility in many solvents including technical relevant hydrocarbon solvents, such as n-hexane and cyclohexane, and (b) due to their relatively high base strength which is required to initiate polymerization.

Quantitative monomer conversion can be considered a beneficial feature of a “living” anionic solution polymerization. Accordingly, there is usually no need to isolate and recycle residual monomer which remained in the polymerization solvent upon termination of the polymerization process. In contrast, technical emulsion polymerization production processes require monomer recovery. Reportedly only about 70 % of the monomers are currently converted into polymers, including oil comprising ESBR versions.

There is a broad range of possible S-SBR micro- and macro structures. Therefore, only a more generalized procedure for SSBR production is presented below. The procedure refers to a copolymer comprising 25% styrene, while the vinyl content may vary from 10 to 60% depending on the kind and concentration of the randomizing Lewis acid (also called randomized agent).