(i) Elastomers are the polymers in which the intermolecular forces of attraction between the polymer chains are the weakest (van der Waals forces of attraction), eg, neoprene. (ii) Fibres are the polymers in which the intermolecular forces of attraction are the strongest (H-bonding or dipole-dipole interactions). eg, terylene. (iii) Thermoplastics are the polymers in which intermolecular forces of attraction are in between those of elastomers and fibres, eg, polystyrene. (iv) Thermosetting polymers are the semi-fluid substances with low molecular masses which when heated in a mould, undergo a permanent change in chemical composition to give a hard, infusible and insoluble mass. eg, bakelite.