Which of the following is not a linear polymer?

Bakelite

The correct answer is option 1. Bakelite.

Let us look at each of the given options:

1. Bakelite: Bakelite is a thermosetting polymer formed from the reaction between phenol and formaldehyde under basic conditions. It undergoes extensive cross-linking during polymerization, resulting in a three-dimensional network structure. Therefore, Bakelite is not a linear polymer; it is a cross-linked polymer.

2. Cellulose: Cellulose is a linear polymer consisting of repeating glucose units linked together by β(1→4) glycosidic bonds. It forms the structural component of plant cell walls and is the most abundant organic polymer on Earth.

3. Polyester: Polyesters are a class of synthetic polymers that can have both linear and branched structures. However, many commercially important polyesters, such as polyethylene terephthalate (PET), have a linear structure.

4. Polythene (Polyethylene): Polyethylene is a linear polymer composed of repeating ethylene monomer units \((-CH_2-CH_2-)\) linked together by single bonds. It is one of the most widely used plastics and can have various molecular weights and degrees of branching, but the most common form is linear polyethylene.

Bakelite is not a linear polymer; it is a cross-linked polymer. Cellulose, polyester, and polythene are examples of linear polymers. Therefore, the correct answer is Bakelite.