Question

Natural rubber is a polymer of isoprene. During polymerisation:

• A. 1,4 addition takes place
• B. 1,2 addition takes place
• C. 1,3 addition takes place
• D. both double bonds convert into single bonds

Hint:

Natural rubber = cis-1,4-polyisoprene (1,4-addition polymerization of isoprene).

Answer 1

The Correct Option is A

Concept: Natural rubber is cis-1,4-polyisoprene, formed by the polymerization of isoprene (2-methyl-1,3-butadiene).

Step 1: Structure of isoprene. Isoprene has the structure: \[ CH_2 = C(CH_3) - CH = CH_2 \] It is a conjugated diene with double bonds at positions 1 and 3.

Step 2: Type of polymerization. During polymerization, isoprene undergoes 1,4-addition. The double bonds at positions 1 and 4 (numbering from one end) open up, and monomers link together through carbons 1 and 4. One double bond remains in each monomer unit (between carbons 2 and 3).

Step 3: Structure of natural rubber. \[ [-CH_2-C(CH_3)=CH-CH_2-]_n \] This is cis-1,4-polyisoprene.

Step 4: Why not other options?
• (B) 1,2 addition — would give a different polymer structure (not natural rubber).
• (C) 1,3 addition — does not occur in diene polymerization.
• (D) both double bonds convert into single bonds — false, one double bond remains in each monomer unit.