Question

When a saturated paraffin such as polymethylene is chlorinated, chlorine replaces hydrogen at random. Invariably, small amounts of chlorine (10–50 wt% Cl) cause a lowering of softening point. Large amounts (about 70%) raise the softening point. Rationalize on the basis of intermolecular forces

Answer 1

Chlorination of polymethylene with low concentration (10-50 wt% Cl) causes reduction in the softening temperature as chlorination of some sites on the chain occurs. It becomes a mixture of higher amount of underrivatized polymethylene and in it is dispersed chlorinated polymethyene. The chlorinated polymethylene acts as an impurity in the bulk polymethylene system. Polymethylene originally had certain amount of intermolecular forces existing among the chains which reduces upon chlorination. Lesser amount of heat is required to soften the remaining intermolecular forces existing between pure polumethylene chains. Thus lower amount of heat is required to melt of soften the material.

Upon, large doses of Chlorine (about 70%) treatment, most of the positions are chlorinated on the chain and it becomes more even system. The chlorinated polymethylene would have a higher melting point than the pure polymethylene system. Since the bulk mass is now chlorinated chains, it would require greater energy to soften the system with strong intermolecular forces existing between the chlorinated chains of polymethylene.