Q.Natural rubber has

Natural rubber is tapped as latex from the rubber tree; its monomer is cis-isoprene (2-methylbuta-1,3-diene), linked linearly, all-cis. Alone it is neither strong nor elastic -- it needs vulcanization, discovered by accident in 1839 when Charles Goodyear dropped natural rubber and sulphur onto a hot stove. In practice, rubber is mixed with 3-5% sulphur and heated at 100-150 degC, cross-linking the cis-1,4-polyisoprene chains via disulphide (-S-S-) bonds -- 1-3% sulphur gives a soft, stretchy rubber, 3-10% gives a harder, still-flexible one. …