Natural rubber is made up of long chains of the molecule isoprene with a few cross links between the chains. These kinked, looping polymers are intertwined in un-stretched rubber. When these polymers are subjected to a stretching force, the tangled polymer opens up because of the flexible bonds between the molecules and becomes elongated. When the applied force is released, the chemical bonds in the chains try to come back to their original state. This accounts for rubber’s elasticity.
