Polythene, polyvinyls and polystyrene soften on heating and harden on cooling. Such polymers are called thermoplastic polymers. These polymers are linear array of slightly branched long chain molecules.

These possess intermolecular forces whose strength lies between strength of intermolecular forces of elastomers and fibres while bakelite, urea formaldehyde resin are thermosetting polymers which on heating undergo extensive cross linking in moulds and again become infusible.