Explain why some polymers do not melt when heated.

Polymers that do not melt when heated are called thermosetting polymers. Their structure explains this property. Unlike thermoplastics, which consist of individual chains, thermosetting polymers have a rigid, 3D network structure. This is because there are strong covalent bonds, called **cross-links**, that form between the long polymer chains. These cross-links hold the chains firmly in place and prevent them from sliding past each other when heated. A great deal of heat energy would be needed to break these strong covalent bonds, and by the time the temperature is high enough, the polymer itself starts to decompose and char rather than melt.