How does the presence of double bonds in rubber molecules influence their structure and reactivity?

Explanation

Natural rubber is a linear cis-1, 4-polyisoprene in which double bonds are present between `C_(2) and C_(3)` of isoprene units. The cis configuration about double bonds does not allow the chains to come closer for effective intermolecular attraction due to weak intermolecular attractions. Therefore, the natural rubber has coiled structure and shows elasticity and is non-crystalline.
On the other hand, all trans configurations occur in gutta-percha which is trans-polyisoprene. These zig-zag chains pack more closely in gutta-percha and hence it is highly crystalline and non-elastic and is more hard and brittle than rubber.