Animated Solution for Chemistry - Organic Chemistry: Among the following, the correct statement(s) about polymers is(are)

The world of polymers is fascinating because tiny changes at the molecular level or in the reaction conditions can lead to drastically different materials. This question from JEE Advanced 2022 is a beautiful test of our understanding of polymer synthesis, monomers, and physical properties. Let's break down each statement to uncover the chemistry behind these everyday materials.

The Rubber Confusion

Imagine a rubber tree oozing out a milky white sap called latex. This natural rubber is a polymer made entirely of isoprene molecules. Chemically, isoprene is $2$-methyl-$1,3$-butadiene. When it polymerizes, it forms a long, coiled chain known as cis-$1,4$-polyisoprene, which gives natural rubber its characteristic elasticity.

Now, what about chloroprene? Chloroprene is chemically $2$-chloro-$1,3$-butadiene. Notice the difference? We replaced a methyl group with a chlorine atom. When chloroprene polymerizes, it forms neoprene. Neoprene is a synthetic rubber, famous for its superior resistance to oils and chemicals compared to natural rubber.

So, the statement claiming that the polymerization of chloroprene gives natural rubber is fundamentally incorrect. Chloroprene gives neoprene, while isoprene gives natural rubber.

The Magic of Teflon

Think about your non-stick cookware. The magic surface that prevents your eggs from sticking is made of Teflon, scientifically known as Polytetrafluoroethylene (PTFE).

How do we make it? We start with the monomer tetrafluoroethene ($C{F}_2=C{F}_2$). To link these molecules together, we heat them under high pressure in the presence of a free radical or a persulphate catalyst. The double bonds break open, and the molecules join hands to form a long, continuous chain: $(-{\text{CF}}_2-{\text{CF}}_2-{)}_n$.

The carbon-fluorine bonds in Teflon are incredibly strong, and the fluorine atoms tightly pack around the carbon backbone, shielding it from chemical attacks. This makes Teflon highly inert and heat-resistant. Statement B perfectly describes this preparation method, making it correct.

The Nature of PVC

Polyvinyl chloride, or PVC, is the material used to make the sturdy white pipes in your home's plumbing system. But what kind of polymer is it?

Polymers are classified based on their intermolecular forces into elastomers, fibers, thermoplastics, and thermosetting polymers. Thermoplastics have intermolecular forces that are intermediate between the weak forces of elastomers and the strong hydrogen bonds of fibers.

Because of these intermediate forces, thermoplastics like PVC have a very useful property: they soften when heated and harden when cooled. This allows them to be easily molded into various shapes, like pipes or raincoats, without any chemical change occurring. Therefore, the statement that PVC is a thermoplastic polymer is absolutely correct.

The Polythene Trap

This is a classic trap set by the examiners! Polythene comes in two main varieties: Low Density Polythene (LDPE) and High Density Polythene (HDPE). The difference lies entirely in how they are made.

When ethene is subjected to extreme conditions—a high temperature of $350-570\text{ K}$ and a massive pressure of $1000-2000\text{ atm}$—in the presence of a peroxide initiator, it undergoes free-radical addition polymerization. The high energy and free radicals cause the growing polymer chains to attack themselves (chain transfer), leading to highly branched structures. Because the chains are branched, they cannot pack closely together. Poor packing means lower density. Thus, these extreme conditions yield Low Density Polythene (LDPE).

To get High Density Polythene (HDPE), we need linear chains that can pack tightly. This is achieved under very mild conditions: a low temperature of $333-343\text{ K}$ and a low pressure of just $6-7\text{ atm}$, using a special Ziegler-Natta catalyst (a mixture of triethylaluminium and titanium tetrachloride).

Statement D claims that the extreme high-pressure conditions yield high density polythene, which is exactly the opposite of the truth! Therefore, Statement D is incorrect.

After carefully dissecting the chemistry behind each option, we find that only statements (B) and (C) hold true. This question serves as a great reminder that in organic chemistry, especially in polymers, the specific reaction conditions and catalysts are just as important as the reactants themselves!