Which of the following polymer is NOT a thermoplastic?

Neoprene is the correct answer because it is not a thermoplastic. Neoprene is polychloroprene, a synthetic rubber (elastomer) made by polymerising chloroprene. Its useful properties come from cross-linking, done by vulcanisation, which locks the chains into a permanent network. Once cross-linked, it cannot be softened and reshaped by heating, which is the defining feature of a thermoset.

Now the other options:

• Teflon (PTFE) is treated as a thermoplastic in standard classifications, though its very high melt viscosity makes conventional melt processing difficult, so it is usually sintered.
• Polystyrene is a classic thermoplastic. It softens on heating and sets on cooling, which is why it is moulded into packaging and cups.
• Polythene (polyethylene) is a thermoplastic used in carry bags, bottles and pipes.

Concept: thermoplastics have linear or branched chains held by weak intermolecular forces, so they can be remelted and recycled. Thermosets form 3D cross-linked networks. For Mains, this links to plastic waste management, since thermoplastics are recyclable while thermosets are not.

Trap: students reason that a soft, rubbery material must be a thermoplastic. Neoprene is a conventional vulcanised rubber, not a thermoplastic elastomer.