Q.From the cis-polyisoprene structure of natural rubber explain the low strength of van der Waals forces in it.
Explain how vulcanization of natural rubber improves its elasticity ? (Hint : consider the intermolecular links.)

Step 1. In cis-polyisoprene, the cis geometry at each retained double bond puts a kink/bend in the chain at every repeat unit, so the overall chain cannot lie flat or pack tightly against its neighbours -- it coils loosely instead.

Step 2. Because the chains are kept apart by this coiled, irregular shape, the only forces acting between neighbouring chains are weak, short-range van der Waals forces -- there is comparatively little chain-to-chain contact area for these forces to act over, so their net strength is low.

Step 3. This weak, loose packing is exactly what lets natural rubber stretch easily under an applied force -- but without any crosslinks, the chains can also permanently slide past one another, so unvulcanized rubber does not fully recover its shape and stays soft/tacky. …

Explain van der Waals weakness as a direct consequence of the cis geometry's loose chain packing, then explain vulcanization's crosslinks as …

• Explaining vulcanization only as 'making rubber harder' without connecting it to the specific mechanism of crosslinks restoring …