Natural and synthetic rubber have different properties because they contain different constituent molecules. Natural rubber is extracted as sap from the rubber tree, Hevea brasilensis, and heated to make a finished product. Synthetic rubber is usually made from polymers found in petroleum that are extracted during distillation.
The process by which natural rubber is prepared for use is called vulcanization, and it entails heating the rubber in the presence of sulfur. Sometimes, an additive such as carbon black is added to stiffen the rubber for heavy-duty uses such as automobile tires. Synthetic rubber is a direct polymer of isoprene, which is extracted from oil or natural gas.
Unlike natural rubber, a variety of synthetic rubber types have been developed to meet certain specialty needs. Thiokol, for example, is used mainly for sealants. The neoprene group of synthetics, which are derived from the polymerization of chloroprene, are very stable relative to natural rubber and most other synthetics, and so are used for multiple applications such as wetsuits, laptop sleeves and durable medical devices. Neoprene's low electrical conductivity also makes it useful as an insulator in electrical wiring. Because natural rubber's molecules form unsaturated double bonds, it is too reactive with other chemicals to be widely used for such applications.