Polymers are classified in four classes on basis of molecular forces. These classes are

(i) Elastomers: in these polymers, weak intermolecular forces mainly hold chains. These weak links permit polymers to be stretched.

(ii) Fibres: they possess high tensile strength and high modulus. They have strong intermolecular forces like hydrogen bonding. These strong forces also lead to close packing of chains and thus impart crystalline nature. The examples are polyamides, polyesters, etc.

(iii) 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.

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