Natural rubber is a polymer found in the sap of the rubber tree, Hevea brasilensis, originally native to Brazil. The rubber tree grows throughout the tropics and is cultivated in plantations, primarily in Southeast Asia; Malaysia and Indonesia are the most significant sources. The sap is collected, and on exposure to air and mild heat gives natural rubber. Chemically, this is a polymer of 2-methyl-1,3-butadiene (isoprene), CH2=C(CH3)-CH=CH2. The polymerization reaction is: