A polymeric substance obtained from the sap of the tree Hevea brasiliensis. Crude natural rubber is obtained by coagulating and drying the sap (latex), and is then modified by vulcanization and compounding with fillers. It is a polymer of isoprene containing the unit –CH2C(CH3):CHCH2–. Various synthetic rubbers can also be made. See neoprene; nitrile rubber; silicones.