What is the structural repeat unit of natural rubber? Explain the process of vulcanization and how it changes the polymer's properties.

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Natural rubber is a natural addition polymer of isoprene, $\text{CH}_2{=}\text{C}(\text{CH}_3)\text{--CH=CH}_2$ , biosynthesised with the residual double bond in the cis configuration at every unit, giving the repeat unit $\big[-\text{CH}_2\text{--C}(\text{CH}_3){=}\text{CH--CH}_2-\big]_n$ (cis-1,4-polyisoprene). The cis geometry keeps the chains kinked and loosely coiled, which is what gives raw rubber its elastic tendency, but in its raw state the chains are held together only by weak van der Waals forces and slide past one another under stress, so raw rubber is soft, tacky, and has poor tensile strength and poor recovery. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set SEM31 markMCQ
$Q. Match Column-I with Column-II and select the correct option: Column-I:$
(i) P-F Resin,
(ii) Neoprene,
(iii) Natural rubber,
(iv) Nylon-6. Column-II:
(a) Chloroprene,
(b) Isoprene,
(c) Caprolactam,
(d) Novolac
(a) (i)-(d), (ii)-(a), (iii)-(b), (iv)-(c)
(b) (i)-(a), (ii)-(b), (iii)-(c), (iv)-(d)
(c) (i)-(b), (ii)-(c), (iii)-(d), (iv)-(a)
(d) (i)-(c), (ii)-(d), (iii)-(a), (iv)-(b)
›Reveal solutionSolution
Match each polymer to its monomer: Phenol-Formaldehyde resin -> Novolac; Neoprene -> Chloroprene; Natural rubber -> Isoprene; Nylon-6 -> Caprolactam. Correct option (a).

Monomer/repeat-unit basis of each:

(i) P-F (phenol-formaldehyde) resin: the linear low-molecular product is Novolac -> (d).

(ii) Neoprene: polymer of chloroprene (2-chloro-1,3-butadiene) -> (a).

(iii) Natural rubber: cis-1,4-polyisoprene, monomer isoprene (2-methyl-1,3-butadiene) -> (b). …
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CBSE 2025Set ANNUAL1 markMCQ
$Q. Vulcanisation of rubber makes it$
(a) soft
(b) hard
(c) less elastic
(d) soluble in water
›Reveal solutionSolution

Vulcanisation cross-links the polymer chains of rubber with sulphur, converting the soft, sticky natural rubber into a harder, more durable material.

Natural rubber (cis-1,4-polyisoprene) on its own is soft, tacky, and has poor mechanical strength, with only weak van der Waals forces holding its long polymer chains together, so it becomes soft and sticky in heat and brittle in cold. Vulcanisation is the process of heating raw rubber with sulphur (and often accelerators), which introduces sulphur cross-links (disulphide/polysulphide bridges) between adjacent polyisoprene chains at their double-bond positions.

…

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CBSE 2024Set ANNUAL1 mark
$Q. What is the purpose of Vulcanisation of rubber?$
›Reveal solutionSolution
Raw natural rubber is soft, sticky, and weak; vulcanisation fixes this by chemically cross-linking its long polyisoprene chains with sulphur bridges.

Raw (unvulcanised) natural rubber is soft, tacky, has poor tensile strength, and its properties change drastically with temperature (becomes sticky when hot, brittle when cold) — making it unsuitable for most practical uses on its own.

Vulcanisation is the process of heating raw rubber with sulphur (and small amounts of additives) at 373–415 K. This forms SULPHUR CROSS-LINKS between adjacent polyisoprene polymer chains, converting the loose network of individual chains into a rigid, interconnected 3D structure.

Purpose/benefits:

Greatly increases tensile strength and elasticity.

Removes the sticky/tacky texture. …
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CBSE 2023Set ANNUAL1 markMCQ
$Q. The element which is added to raw rubber to improve its physical properties is$
(a) Carbon
(b) Sulphur
(c) Nitrogen
(d) Oxygen
›Reveal solutionSolution

Vulcanisation adds sulphur cross-links between rubber's polyisoprene chains, converting soft, tacky raw rubber into a strong, elastic material.

Natural rubber (cis-polyisoprene) is soft, sticky and has poor mechanical strength on its own. When heated with sulphur (typically 3-5%, with accelerators like zinc oxide), sulphur atoms form cross-links (mono-, di-, or polysulphide bridges) between adjacent polymer chains at their residual double bon …

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Q.What is the structural repeat unit of natural rubber? Explain the process of vulcanization and how it changes the polymer's properties.

Concept understanding — Natural Rubber and Vulcanisation

Q.What is the structural repeat unit of natural rubber? Explain the process of vulcanization and how it changes the polymer's properties.

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