For most practical purposes, one usually characterizes
the MW of a polymer by measuring Mn and Mw.
• Mn is biased toward the lower-MW-fractions.
• Mw is biased toward the higher-MW-fractions.
The ratio of the two, Mw/Mn, is the polydispersity in a polymer.
For monodisperse polymer,
the ratio would be unity (1) but not for actual polymers.