Polymers
Polymers are a class of materials in which the building unit is of high molecular weight. Many of those in common use are based on carbon chemistry with covalently bonded carbon atoms forming a backbone for the macromolecules with molecular weights of more than one million. The diagram shows the three broad classifications of polymeric

materials. The thermoplastics have long chain molecules interacting largly through van der Waals forces. Thermosetting materials form rigid three dimensional networks that are co-valently bonded, and which de-compose before melting. Elastomers form from linear molecules that are crosslinked. The degree of cross-linking determines the rigidity of the material. Elastomers may be deformed elastically by as much as 300% and return to their original shape when the forces are removed.

From: Askeland, "The Science of Engineering Materials," PWS (1994)