Natural rubber, found in latex and used in tyres and elastic materials, is a polymer formed by the repeated addition of which monomer?

Introduction / Context:
Natural rubber is an important polymer obtained from the latex of rubber trees. It has been used for many years to manufacture tyres, elastic bands and numerous flexible products. In polymer chemistry, natural rubber is described in terms of the small unsaturated hydrocarbon molecules (monomers) that join together to form the long chains. Knowing the correct monomer helps you understand addition polymerisation in organic chemistry and distinguishes natural rubber from synthetic rubbers based on other monomers.

Given Data / Assumptions:

• The question is about natural rubber, not synthetic variants.
• The options include butadiene, 2-methyl-1,3-butadiene, ethylene and isoprene.
• We assume standard IUPAC naming: isoprene is another name for 2-methyl-1,3-butadiene.
• The task is to match natural rubber with its correct monomer unit.

Concept / Approach:
Natural rubber is cis-1,4-polyisoprene, meaning it is made of many isoprene units joined together in a particular orientation. Isoprene is the common name for 2-methyl-1,3-butadiene. While butadiene itself can form synthetic rubbers such as polybutadiene, and ethylene forms polyethylene, natural rubber specifically arises from the polymerisation of isoprene units in biological systems. Therefore, the correct answer must either be isoprene or its IUPAC description, which both refer to 2-methyl-1,3-butadiene.

Step-by-Step Solution:
Step 1: Recall the structural formula of isoprene: CH2=C(CH3)-CH=CH2, which corresponds to 2-methyl-1,3-butadiene.Step 2: Recognise that natural rubber consists mainly of cis-1,4-polyisoprene chains.Step 3: Compare the options. Butadiene is CH2=CH-CH=CH2 and is used in synthetic rubbers but is not the monomer of natural rubber.Step 4: Ethylene, CH2=CH2, polymerises to form polyethylene, not rubber.Step 5: Both 2-methyl-1,3-butadiene and isoprene refer to the same molecule, but the standard textbook statement is that natural rubber is a polymer of isoprene.Step 6: Select isoprene as the correct monomer for natural rubber.

Verification / Alternative check:
Chemistry textbooks and exam guides typically summarise natural rubber in a short statement such as "Natural rubber is cis-1,4-polyisoprene". They often show the repeating unit as a segment derived from isoprene. Synthetic rubbers like styrene butadiene rubber (SBR) and polybutadiene are clearly distinguished from this natural material. Cross checking with these standard descriptions confirms that the monomer for natural rubber is isoprene, not plain butadiene or ethylene.

Why Other Options Are Wrong:
Option a, butadiene, is an important monomer for synthetic rubbers but does not describe natural rubber. Option b, 2-methyl-1,3-butadiene, is chemically the same as isoprene but the question and standard phrasing specifically highlight isoprene by name; using the common name emphasises the link that students are expected to remember. Option c, ethylene, is the monomer of polyethylene and is unrelated to natural rubber. Therefore, these options either point to different polymers or use less common naming in this context.

Common Pitfalls:
A frequent confusion arises because the names butadiene and isoprene sound similar and both are conjugated dienes. Students sometimes wrongly assume that plain butadiene must be the monomer of natural rubber. Another pitfall is to memorise that rubber comes from "a diene" without remembering which specific diene. To avoid this, associate the word "isoprene" directly with natural rubber and remember the phrase "cis-1,4-polyisoprene" as a key descriptor.

Final Answer:
Natural rubber is a polymer of the monomer isoprene (2-methyl-1,3-butadiene).