(i) Natural rubber is mixed with 3-5% sulphur and heated at 100-150°C causes cross-linking of the cis-1,4-polyisoprene chains through disulphide (-S-S-) bonds.
(ii) The physical properties of rubber can be altered by controlling the amount of sulphur that is used for vulcanization.
(iii) In sulphur rubber, made with about 1 to 3% sulphur is soft and stretchy.When 3 to 10% sulphur is used the resultant rubber is somewhat harder but flexible.

Anti fertility drugs are synthetic hormones that suppresses ovulation and fertilisation. They are used in birth control pills.
eg: Synthetic Oestrogen : Ethynylestradiol, Menstranol,
Synthetic Progestrone : Norethindrone, Norethynodrel etc.

(i) Thermoplastic: They become soft on heating and hard on cooling. They can be remolded
E.g. polythene, PVC, polystrene
(ii) Thermo setting : Do not be come soft on heating set to an infusible mass upon heating.
E.g. bakelite, melamine, formaldehyde

1. The materials that are readily decomposed by microorganisms in the environment are called biodegradable.
2. Natural polymers degrade on their own after certain period of time but the synthetic polymers do not.
3. It leads to serious environmental pollution. One of the solution to this problem is to produce biodegradable polymers which can be broken down by soil micro organism.
Examples:
(i) Polyhydroxy butyrate (PHB)
(ii) Polyhydroxy butyrate-co-A- hydroxyl valerate (PHBV)
(iii) Polyglycolic acid (PGA), Polylactic acid (PLA)
(iv) Poly ( E caprolactone) (PCL)
(v) Biodegradable polymers are used in medical field such as surgical sutures, plasma substitute etc...
4. these polymers are decomposed by enzyme action and are either metabolized or excreted from the body.