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

Natural rubber is cis-1,4-polyisoprene, i.e., a polymer of isoprene units joined so that a double bond remains at every fourth carbon along the chain, all in the cis configuration.

Effect on structure: the cis arrangement about each double bond prevents the chain from lying in a fully extended, closely-packed zig-zag the way a trans (all-trans as in gutta-percha) chain can. Instead, the cis double bonds introduce kinks/bends, so the chains adopt a loosely coiled, entangled shape held together only by weak van der Waals forces. This loose, coiled packing is exactly what allows the material to be stretched extensively (uncoiling) and to be soft/tacky in its raw form. …

Separate the answer into structural consequence (cis double bonds → coiled chain shape → stretchability) and reactivity consequence (double bonds …

• Only discussing vulcanisation reactivity and forgetting the structural (cis-shape) reason for natural elasticity before vulcanisation.
• Not specifying that the double bonds are in the cis configuration, which is what distinguishes true rubber's elasticity from gutta-percha's rigidity. …