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
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.