Animated Solution for Chemistry - Organic Chemistry: Which one of the following polymers is not obtained by condensation polymerisation?

Polymers are the giant molecules that make up almost everything around us, from the plastic in our pens to the DNA in our cells. But how do these giant chains form? They are built by linking together smaller building blocks called monomers. The way these monomers hold hands determines the type of polymerisation.

There are two primary ways to build a polymer: Addition Polymerisation and Condensation Polymerisation.

Addition: The Handshake Analogy

Imagine a group of people holding hands to form a long chain. If they simply grab each other's hands without dropping anything, that is Addition Polymerisation. In the chemical world, this usually happens when monomers with double or triple bonds break those extra bonds to link up with their neighbors. Nothing is lost; everything is added.

Now, imagine that every time two people hold hands, they have to drop a glove. That is Condensation Polymerisation. When monomers link up, they release a small molecule as a byproduct—most commonly water ($H_{2}O$), ammonia ($N{H}_3$), or hydrogen chloride ($HCl$). This happens because the monomers have reactive functional groups (like $-OH$, $-COOH$, or $-N{H}_2$) at their ends.

Let's interrogate the options provided in the question to see how they are made:

1. Nylon 6, 6: This is a classic condensation polymer. It is made by reacting adipic acid (which has two $-COOH$ groups) with hexamethylenediamine (which has two $-N{H}_2$ groups). When they link, they form an amide bond and release a molecule of water.

2. Bakelite: This is the tough plastic used in old telephones and electrical switches. It is formed by the condensation reaction between phenol and formaldehyde. As the complex 3D network forms, water molecules are eliminated.

3. Nylon 6: This one can be slightly tricky. It is made from a single monomer called caprolactam. When heated with water, the caprolactam ring opens up to form an amino acid, which then undergoes condensation polymerisation, releasing water to form the long polyamide chain.

Buna-N

This brings us to Buna-N, also known as nitrile rubber. It is a synthetic rubber used to make oil-resistant gloves and hoses. Buna-N is formed by the copolymerisation of two different monomers: 1,3-butadiene ($C{H}_2=CH-CH=C{H}_2$) and acrylonitrile ($C{H}_2=CH-CN$).

Notice the structure of these monomers? They are packed with double bonds. During the reaction (often initiated by a peroxide catalyst), these double bonds break open, allowing the molecules to link together into a continuous chain:

Absolutely nothing is dropped or lost in this process. Every single atom from the monomers ends up in the final polymer chain. Therefore, Buna-N is a pure addition polymer, making it the correct answer to our question!