Which of the following statements about natural rubber is incorrect?

Introduction / Context:
This polymer chemistry question asks about the composition and properties of natural rubber. Natural rubber is an important industrial material and a classic example of a natural polymer. The learner must distinguish between correct statements about its structure and processing and an incorrect statement that actually describes a different synthetic rubber. Knowing that natural rubber is based on cis isoprene and that chloroprene rubber is a separate synthetic material is central to answering this question correctly.

Given Data / Assumptions:
- Four statements describe natural rubber in different ways.
- The task is to identify which statement is false.
- We assume standard polymer chemistry definitions and the distinction between natural rubber and synthetic rubbers such as neoprene.

Concept / Approach:
Natural rubber is obtained from latex sap of rubber trees and is chemically a polymer of 2 methyl 1,3 butadiene, commonly called isoprene. The polymerisation primarily yields the cis isomer, so natural rubber is cis polyisoprene. Because the polymer chains can stretch and return to their original shape, natural rubber is classed as an elastomer. To improve its mechanical properties such as elasticity, durability, and resistance to temperature, natural rubber is vulcanised by heating with sulphur or sulphur based compounds. On the other hand, chloroprene, which is 2 chloro 1,3 butadiene, is the monomer for neoprene, a synthetic rubber. Thus, saying that natural rubber is a polymer of chloroprene is incorrect.

Step-by-Step Solution:
Step 1: Recall that natural rubber is polyisoprene, and more specifically, a polymer of cis isoprene units. This supports statement that it is a polymer formed from cis isoprene units.
Step 2: Recognise that an elastomer is a polymer with elasticity that can be stretched and will return to its original dimensions. Natural rubber clearly behaves this way, so it is correctly described as an elastomer.
Step 3: Understand that natural rubber is often vulcanised by heating with sulphur and accelerators, which forms crosslinks between polymer chains and improves properties such as tensile strength and resistance to deformation.
Step 4: Chloroprene is a different monomer, 2 chloro 1,3 butadiene, and its polymerisation produces neoprene, a synthetic rubber. Natural rubber does not arise from chloroprene.
Step 5: Therefore, the statement that natural rubber is a polymer of chloroprene is incorrect, while the other three statements accurately describe natural rubber.

Verification / Alternative check:
Textbooks and reference materials on polymers list natural rubber as cis polyisoprene obtained from Hevea brasiliensis. They also describe its classification as an elastomer and note that vulcanisation with sulphur is a key process discovered by Charles Goodyear. Neoprene is separately described as a synthetic rubber made from chloroprene. This clear separation confirms that the statement linking natural rubber with chloroprene is wrong.

Why Other Options Are Wrong:
- It is an elastomer: This is true because natural rubber can be stretched and then returns to its original shape due to long chain polymer behaviour.

Why Other Options Are Wrong (continued):
- It is a polymer formed from cis isoprene units: This is correct and matches the chemical structure of natural rubber as cis polyisoprene.
- It is heated with sulphur compounds to improve its properties: This describes vulcanisation, a standard treatment for natural rubber to enhance mechanical strength and elasticity, so it is correct.

Common Pitfalls:
Students sometimes confuse different monomers with similar names such as isoprene, butadiene, and chloroprene. The presence of the term prene in both isoprene and chloroprene may cause mixing of facts, leading to the wrong association of chloroprene with natural rubber. To avoid this, remember that natural rubber is from isoprene, while neoprene is from chloroprene. Keeping a small table linking each monomer with its corresponding rubber type can help in revising these topics.

Final Answer:
Natural rubber is a polymer of chloroprene. is the incorrect statement about natural rubber.