Polyisoprene natural and synthetic rubber

Polyisoprene is a plant-polymer, extracted of hevea sap. It can also be produced commercially. 

Hevea sap is a suspension of 30% of latex in a serum. By adding an acid, globular clusters of polyisoprene linear chains are obtained, and the average number of links per chain is 300. The chemical structure of polyisoprene is 

This double bond introduces a stereoisomerism the unit may have two different forms  

Cis-polyisoprene crytallizes partially between 0° and 35 °C and the glass transition temperature Tf of the amorphous fraction corresponds to - 35 °C. On the other hand, trans-polyisoprene crystallizes in two forms, the a-form which melts at 65 °C and the metastable /J-form, obtained by quick cooling down, which melts at 56 °C. 

The major discovery concerning polyisoprene is vulcanization, i.e. the creation of bridges between chains by means of sulphur atoms which saturate double bonds actually, this goes back to 1839, when Goodyear observed the stability of this new material. Vulcanization is a series of reactions which can be shown schematically as follows 

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