Q.The monomer unit of natural rubber is ______.

What is Natural Rubber?

Imagine a long, tangled telephone cord. When you pull it, the coils straighten out and the cord stretches. When you let go, the coils snap back into their original jumbled shape. That is the essence of rubber's elasticity — and it all comes down to the shape of the molecules inside.

Natural rubber is a polymer, which means it is a giant molecule made by linking thousands of smaller repeating units together. The repeating unit here is isoprene (2-methyl-1,3-butadiene). The rubber tree (Hevea brasiliensis) produces a milky white fluid called latex, which is essentially a suspension of these polymer chains in water. When you collect the latex and coagulate it (usually with acid), you get raw natural rubber.

The key to rubber's stretchiness lies in the geometry of the polymer chain. The isoprene units join in a specific way — called cis-1,4 addition. This "cis" arrangement means that the bulky methyl groups (the -CH₃ side branches) all lie on the same side of the double bond in the polymer backbone.

Because the chains cannot crystallise or align neatly, they exist in a random, coiled state. When you apply a tensile force, these coils straighten out — the chains slide past each other and align in the direction of the pull. When you release the force, the chains return to their original coiled, disordered state. That is the molecular origin of elasticity.

The Precise Statement

Natural rubber is a natural addition polymer obtained from the latex of the rubber tree. Its repeating unit is isoprene (2-methyl-1,3-butadiene), and the polymerisation occurs via 1,4-addition in the cis configuration. The resulting polymer is cis-1,4-polyisoprene.

The elasticity of natural rubber arises from the coiled, disordered conformation of its polymer chains, which is a direct consequence of the cis geometry at each double bond. Under stress, the chains uncoil and align; upon release, they recoil.

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