text: Synthetic polymers invariably consist of a mixture of macromolecular species with different degrees of polymerization and therefore of different molecular weights. There are different types of average  polymer molecular weight, which can be measured in different experiments. The two most important are the number average (X) and the weight average (X).
  The number-average degree of polymerization is a weighted mean of the degrees of polymerization of polymer species, weighted by the mole fractions (or the number of molecules) of the species. It is typically determined by measurements of the osmotic pressure of the polymer.
  The weight-average degree of polymerization is a weighted mean of the degrees of polymerization, weighted by the weight fractions (or the overall weight of the molecules) of the species. It is typically determined by measurements of Rayleigh light scattering by the polymer.