Natural and Synthetic Rubbers

Natural rubber is a genuinely natural polymer, tapped as latex from cuts made in the bark of the rubber tree. Its monomer unit is cis-isoprene (systematically 2-methylbuta-1,3-diene), and natural rubber consists of thousands of these isoprene units linked together in a simple linear chain, all in the cis configuration. On its own, though, natural rubber is neither especially strong nor especially elastic -- it needs to be chemically modified before it becomes the tough, stretchy material we associate with the word "rubber", and that modification is called vulcanization.

The discovery itself was an accident: in 1839, Charles Goodyear accidentally dropped a mixture of natural rubber and sulphur onto a hot stove, and was surprised to find that the rubber to had become strong and elastic. He named the resulting process vulcanization. In practice, natural rubber is mixed with 3-5% sulphur and heated at 100-150 degrees Celsius; this causes the cis-1,4-polyisoprene chains to cross-link with one another through disulphide (-S-S-) bonds. How much sulphur is used directly controls the resulting texture: around 1-3% sulphur gives a soft, stretchy rubber, while 3-10% sulphur gives a somewhat harder but still flexible rubber. …