Polyisoprene

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Polyisoprene is a polymer where isoprene is the repeating unit. The name most commonly means the cis-1,4-polyisoprene isomer, which is the main chemical structure of natural rubber.

Isoprene is an asymmetric molecule with a conjugated system of double bonds. Polymerisation can happen in four different ways. If the two end carbons connect to the bigger molecule, it makes 1,4-polyisoprene. This polymer has a double bond in the middle with two rotational isomers: cis, where the bonds to the rest of the molecule are on the same side of the bond, and trans, where they are on opposite sides. If the first two carbons connect to the bigger molecule, it makes 1,2-polyisoprene; if the last two carbons connect, it is 3,4-polyisoprene.

These four polymers have different properties. The cis form of 1,4-polyisoprene is an elastomer, and the main part of natural rubber. The trans isomer makes a hard thermoplastic that is used in golf balls and dentistry. The difference between the two types of 1,4-polyisoprene is because the trans isomer makes crystals at room temperature, and the cis isomer does not.