Occurrence and production
Natural rubber is a polymer of isoprene — most often cis-1,4-polyisoprene — with a molecular weight of 100,000 to 1,000,000. Typically, a few percent of other materials, such as proteins, fatty acids, resins, and inorganic materials are found in high quality natural rubber. Some natural rubber sources called gutta percha are composed of trans-1,4-polyisoprene, a structural isomer that has similar, but not identical properties.[1]
Isoprene is produced and emitted by many species of trees into the atmosphere (major producers are oak trees). There, isoprene is converted by free radicals (like the hydroxyl (OH) radical) into epoxides, which mix into water droplets and help create aerosols and haze.[2] [3] This appears to be a mechanism that trees use to overcome the overheating of leaves caused by the Sun and also a way to fight against free radicals and especially against ozone.[4]
Isoprene was first isolated by thermal decomposition of natural rubber.[5] It is most readily available industrially as a byproduct of the thermal cracking of naphtha or oil, as a side product in the production of ethylene. About 20M kg are produced annually.[1] About 95% of isoprene production is used to produce cis-1,4-polyisoprene—a synthetic version of natural rubber.
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