Alkenes with two double bonds per molecule are termed dienes, and the smallest, butadiene, has two structural isomers, as shown at the left. 1,2-Butadiene is a rigid molecule of relatively little use.

1,3-Butadiene can be polymerized to polybutadiene, which is the parent compound for a large class of natural and synthetic rubbers (right and on page 20).

Natural rubber is the all-cis polymer of isoprene, a methyl derivative of 1,3-butadiene. As first obtained from the latex of rubber trees it is soft and elastic, with the polyisoprene molecules tangled and coiled back upon themselves in a random manner.

When rubber is stretched, the molecules straighten out to a considerable extent and become oriented more nearly parallel to one another. The entropy of the molecules decreases. When the tension is released, the long chains relax back to their disordered, higher-entropy state.

Charles Goodyear discovered in 1839 that a small amount of sulfur added to hot rubber makes it stronger and harder.