A thermosetting polymer is a type of polymer that, once cured, becomes an infusible and insoluble material. This means it cannot be remelted or reshaped upon heating. The curing process involves a chemical reaction that forms strong cross-links between the polymer chains, creating a rigid, three-dimensional network structure.

Now, let's evaluate the options:

Nylon 6: This is a thermoplastic polymer. Thermoplastics soften when heated and harden when cooled, and this process is reversible. Nylon 6 can be melted and reshaped multiple times.

Bakelite: This is a classic example of a thermosetting polymer. It is made from phenol and formaldehyde. Once it is molded and cured under heat and pressure, it forms a highly cross-linked structure that is hard, rigid, and cannot be softened by heat.

Buna–N: This is a synthetic rubber (nitrile rubber), which is an elastomer. Elastomers are polymers with elastic properties. While some cross-linking is involved in vulcanization to improve its properties, it is not typically classified as a rigid thermoset like Bakelite. It is more flexible and can undergo significant deformation.

PVC (Polyvinyl Chloride): This is a thermoplastic polymer. It can be softened by heating and hardened by cooling repeatedly. It is widely used in pipes, cables, and other applications where reshaping might be needed.

Therefore, the correct answer is Bakelite.

Related Topics and Formulae

Types of Polymers:

Thermoplastics: Linear or slightly branched chains, soften on heating, harden on cooling. Examples: Polyethylene, PVC, Nylon.
Thermosetting Polymers: Highly cross-linked, infusible and insoluble after curing. Examples: Bakelite, Melamine, Epoxy resin.
Elastomers: Polymers with elastic properties, can be stretched and return to original shape. Examples: Natural rubber, Buna-S, Buna-N.

Polymerization:

Formation of polymers from monomers. For Bakelite, it involves a condensation polymerization reaction between phenol and formaldehyde, leading to cross-linking.