5.0 Natural Rubber

5.1 Vulcanization: Cross-Linking of Rubber

It is isolated from a white fluid, called latex, that exudes from cuts in the bark of Hevea brasiliensis, the South American rubber tree. Many other plants secrete this polymer, as well. The name rubber was first used by Joseph Priestly, who used the crude material to “rub out” errors in his pencil writing. Natural rubber is soft and sticky. An enterprising Scotsman named Charles Macintosh found that rubber makes a good waterproof coating for raincoats. Natural rubber is not strong or elastic, however, so its uses were limited to waterproofing cloth and other strong materials.

Like many other plant products, natural rubber is a terpene composed of isoprene units. If we imagine lining up many molecules of isoprene in the $s-$ cis conformation, and moving pairs of electrons as shown below, we would produce a structure similar to natural rubber. This polymer results from $1,4-$addition to each isoprene molecule, with all the double bonds in the cis configuration. Another name for natural rubber is cis$,1-4-$polyisoprene.

The cis double bonds in natural rubber force it to assume a kinky conformation that maybe stretched and still return to its shorter, kinked structure when released. Unfortunately, when we pull on a mass of natural rubber, the chains slide by each other and the material pulls apart. This is why natural rubber is not suitable for uses requiring strength or durability.