Q.The critical degree of polymerization is low for nylon 6 while high for polythene. Explain.

A bulk polymer sample is never made of chains of one exact length -- it is a complex mixture of molecules spanning a range of molecular masses, so 'the' molecular mass reported for a polymer is really an average over all its constituent chains. This average depends directly on the degree of polymerization (DP), defined as the number of monomer units strung together in a single polymer molecule. Because most mechanical properties of a polymer depend on its molecular mass, there are two undesirable extremes: too low a molecular mass gives a brittle, mechanically weak material, while an excessively high molecular mass gives a tough but practically unmanageable material to process. A useful polymer must therefore exceed some minimum molecular mass -- its critical degree of polymerization -- to display the properties its application needs, which is exactly why real polymerization reactions are deliberately controlled/stopped once that stage is reached, rather than left to run indefinitely. Crucially, this critical DP is not a fixed number for all polymers: it depends on how strong the intermolecular forces between chains are. Polymers held t …