bank of the Rhine. The Ferrara, Italy, synthetic rubber factory (near a river bridge) was bombed August 23, 1944. Three other synthetic rubber facilities were at Ludwigshafen/Oppau (15,000), Hanover/Limmer (reclamation, 20,000), and Leverkusen (5,000). A synthetic rubber plant at Oświęcim, in Nazi-occupied Poland, was under construction on March 5, 1944 operated by IG Farben and supplied with slave labor, by the SS, from the associated camp Auschwitz III (Monowitz). Types The most prevalent synthetic rubber is styrene-butadiene rubbers (SBR) derived from the copolymerization of styrene and 1,3-butadiene. Other synthetic rubbers include: polyisoprene, prepared by polymerization of isoprene chloroprene, prepared by polymerization of 2-chlorobutadiene nitrile rubber made from cyanobutadiene or 2-propenenitrile and butadiene Many variations of these can be prepared with mixtures of monomers and with various catalysts that allow for control of stereochemistry. Polyisobutylene or butyl rubber is commonly used in tyre inner tubes or linings owing to its resistance to diffusion of air through the lining. It is however, a much less resilient material than cis-polybutadiene which is frequently used in tyre sidewalls to minimize energy losses and hence heat build-up. Indeed, it is so resilient that it is used in super balls. An elastomer widely used for external sheet such as roof coverings is Hypalon or chlorosulphonated polyethylene. Synthetic rubbers like EPR can also be used for electrical insulation. Silicone rubber Silicone rubber is also a synthetic elastomer composed of silicone polymers. Silicone rubbers are widely used in industry, and there are multiple formulations. Silicone rubbers are often one- or two-part polymers, and may contain fillers to improve properties or reduce cost. Silicone rubber is generally non-reactive, stable, and resistant to extreme environments and temperatures. Natural vs. synthetic rubber Natural rubber, coming from latex of Hevea brasiliensis, is mainly poly-cis-isoprene. Synthetic rubber, like other polymers, is made from various petroleum-based monomers. Some synthetic rubbers are less sensitive to ozone cracking than NR. Natural rubber is sensitive owing to the double bonds in its chain structure, but some synthetic rubbers do not possess these bonds and so are more resistant to ozone cracking. Examples include Viton rubber, EPDM and butyl rubber. A new class of synthetic rubber is the thermoplastic elastomers which can be moulded easily unlike conventional NR vulcanized rubber. Their structure is stabilized by cross-linking by crystallites in the case of polyurethanes or by amorphous domains in the case of SBS block copolymers. References Rubber Organic polymers