Natural rubber

Natural rubber is poly (1,4-cis-isoprene), naturally synthesized by the rubber tree, Hevea brasiliensis, present in its milky sap or latex. It is also synthesized industrially by polyaddition of isoprene. The long and flexible molecules (Tg = -73 °C) give elastomeric characteristics to the material, that is usually crosslinked with a curing agent such as elemental sulfur. It partially crystallizes when stretched. Due to the long sequences of double bonds (one per monomeric unit), this rubber has a high reactivity with oxygen, undergoing the peroxidation reactions, yellowing very quickly, and being biodegraded at a relatively high rate. The hydrogen atoms attached to carbon atoms at the alpha position relative to the unsaturations are more reactive, or more labile. Among the main applications are tires and tubes.