Polyisoprene
Natural rubber is polyisoprene [21]. It is produced commercially from the sap of trees called Hevea brasilensis and sometimes referred to as Hevea rubber. These trees yield a latex containing approximately 35% rubber hydrocarbon and 5% nonrubber solids, like proteins, lipids, and inorganic salts. The remaining 60% of the latex is water. The hydrocarbon polymer consists of 97% cis-1,4 units, 1% trans-1,4 units, and 2% 3,4 units, in a head to tail structure. Molecular weights of naturally occurring polyisoprene range from 200,000 to 500,000. A verity of shrubs and small plants, including some weeds, like dandelion and milkweed also contain polyisoprene in their sap. The guayule shrub, which grows in Mexico and in southern United States, is a good potential source of natural rubber. Work is now going on in various places to cultivate this shrub for potential rubber production. An almost all trans-1,4 polymer called gutta-percha is found in the exudations of various trees of the genus Palaquium, Sapotaceae, and Habit. The molecular weights of these polymers range from 42,000 to 100,000. Balata and chicle, also mainly trans-1,4-polyisoprenes, are found in saps of some plants in West Indies, Mexico, and South America. Chapter 9 deals with various reactions of polymers including those of natural and synthetic rubber. That includes vulcanization of rubber. While there are very many commercial applications of the cis isomer, gutta-percha utilization is limited to wire coatings, impregnation of textile belting, and as a component of some varnishes. Its use is limited, because it is considerably harder than natural rubber.
