Natural rubber may be considered as a linear polymer of isoprene ($2$-methyl-$1, 3$-butadiene) and is also called as cis - $1, 4$ - polyisoprene.

To improve the physical properties, a process of vulcanisation is carried out. This process consists of heating a mixture of raw rubber with sulphur and an appropriate additive at a temperature range between $373 \mathrm{K}$ to $415 \mathrm{K}$. 

In the manufacture of tyre rubber, $5\%$ of sulphur is used as a cross linking agent. The probable structures of vulcanised rubber molecules are depicted below: