Question

Rubber is a natural polymer and possess elastic properties. It is also termed as elastomer and has a variety of uses. It is manufactured from rubber latex which is a colloidal dispersion of rubber in water. This latex is obtained from the bark of rubber tree and is found in India, Srilanka, Indonesia, Malaysia and South America. Natural rubber becomes soft at high temperature $(>335K)$ and brittle at low temperatures $(<283K)$ and shows high water absorption capacity. To improve upon these physical properties, a process of vulcanisation is carried out.
Consider the following structures

Which of the above given structures is/are the probable structure(s) of vulcanised rubber molecules?

• A. Only I
• B. Both I and II
• C. Only II
• D. None of these

Answer 1

Answer: (B)

Natural rubber is cis-polyisoprene. It is obtained by polymerisation of isoprene units at $1,4$ positions. In rubber molecule, double bonds are located between $C_2$ and $C_3$ of each isoprene unit. These cis-double bonds do not allow the polymer chain to come closer.
Therefore, only weak van der Waals' force are present. Since, the chains are not linear, they can be stretched just like springs and exhibit elastic properties.