Molecular Mass of Polymers

Why polymer "molecular mass" is always an average

A polymer's physical properties — its strength, flexibility, melting behaviour, and more — are tied closely to its molecular mass, size, and structure. But polymers aren't made molecule-by-molecule to a fixed length: as a chain grows during synthesis, how long it ends up depends on how much monomer happens to be available to it in the reaction mixture at that moment.

Because of this, a real polymer sample is never a collection of identical molecules — it's a mixture of chains of many different lengths. There is no single, exact molecular mass to quote; instead, the molecular mass of a polymer is always reported as an average over this distribution of chain lengths.

• This average molecular mass can be determined by both chemical methods and physical methods. …