Natural rubber is a polymer of ....................

Answer

**Step-by-Step Solution:**

1. **Identify the Monomer of Natural Rubber:**
Natural rubber is a polymer, and to determine its composition, we need to identify its monomer. The monomer unit of natural rubber is isoprene.

2. **Chemical Structure of Isoprene:**
The chemical structure of isoprene can be represented as:
\[ \text{H}_2\text{C}=\text{C}(\text{CH}_3)-\text{C}=\text{CH}_2 \]
This shows that isoprene has a double bond and a branched structure.

3. **Polymerization Process:**
When isoprene molecules undergo polymerization, they link together to form a long chain. This process involves the breaking of the double bonds in isoprene, allowing the molecules to connect.

4. **Formation of Polyisoprene:**
The polymer formed from isoprene is called polyisoprene. The repeating unit in the polymer chain is derived from the isoprene monomer.

5. **Natural Rubber as Polyisoprene:**
The specific form of polyisoprene that is commonly known as natural rubber is cis-polyisoprene. This means that the configuration of the double bonds in the polymer chain is in the cis form, which gives natural rubber its unique properties.

6. **Conclusion:**
Therefore, the answer to the question "Natural rubber is a polymer of ...................." is:
**Isoprene.**

**Final Answer:**
Natural rubber is a polymer of isoprene.

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