Animated Solution for Chemistry - Organic Chemistry: Which statement is correct?

Polymers are the unsung heroes of modern chemistry, forming everything from the tires on our cars to the buckets in our homes. In the context of competitive exams like JEE, questions on polymers often test your ability to distinguish between subtle classifications: natural vs. synthetic, homopolymers vs. copolymers, and elastomers vs. thermosetting plastics. Let's embark on a detailed journey to dissect the four statements provided in this problem.

The first statement claims that Buna-N is a natural polymer. To verify this, we must look at its origin. Buna-N, widely known as nitrile rubber, is not harvested from rubber trees. Instead, it is meticulously synthesized in chemical plants.

It is a copolymer, meaning it is formed from two different monomers: $1,3$-butadiene and acrylonitrile. The polymerization process links these two distinct molecules into a long, repeating chain. Because it is entirely man-made, the assertion that it is a natural polymer is fundamentally incorrect.

Moving on to the second statement, we encounter Buna-S, another famous synthetic rubber. The statement describes it as a "synthetic and linear thermosetting polymer." While the "synthetic" part is true, the "thermosetting" classification is a trap.

Buna-S is synthesized from $1,3$-butadiene and styrene. It is an elastomer, which means it possesses elastic properties—it can be stretched and will return to its original shape. Thermosetting polymers, on the other hand, are heavily cross-linked materials like Bakelite that become hard and infusible upon heating. They cannot be remolded. Since Buna-S is an elastomer and not a rigid thermosetting plastic, this statement is also false.

A Solo Act

The third statement introduces Neoprene, claiming it is an "addition copolymer." Let's break down its chemical structure. Neoprene is synthesized via the addition polymerization of a single monomer: chloroprene (chemically known as $2$-chloro-$1,3$-butadiene).

Because the entire polymer chain is built from just one type of repeating unit, Neoprene is strictly classified as a homopolymer. A copolymer, by definition, requires at least two different monomers. Therefore, calling Neoprene a copolymer is a chemical misnomer, rendering the third statement incorrect.

Radical Initiators

Finally, we arrive at the fourth statement: "Synthesis of buna-S needs nascent oxygen." To understand why this is true, we must delve into the mechanism of its creation. Buna-S is formed through free radical addition polymerization.

This type of chain reaction doesn't just start on its own; it requires a chemical "spark" to generate the first free radicals. These radicals then attack the double bonds of the monomers, propagating the chain. Nascent oxygen $[O]$, or various peroxides, serve exactly this purpose. They act as radical initiators, breaking down to form highly reactive species that kickstart the entire polymerization process.

Thus, the synthesis of Buna-S absolutely relies on the presence of an initiator like nascent oxygen, making this the only correct statement among the choices.