Thermoplastic Polymers: They are linear or slightly branched long chain molecules capable of repeatedly softening on heating and hardening on cooling. They possess intermolecular forces of attraction intermediate between elastomers and fibers. Some common thermoplastics are polythene, polystyrene, polyvinyl, etc.

Thermosetting Polymers: They are cross-linked or heavily branched molecules, which on heating undergo extensive cross-linking in moulds and again become infusible. Some common examples are Bakelite, urea-formaldehyde resins, etc.