Weight average molecular weight or Mw is more closely proportional with a polymer’s physical properties, in comparison to its number average molecular weight or Mn. With that said, if you divide Mw by Mn you would be calculating the polydispersity index which also has meaning. The polydispersity index gives you a measurement of how monodisperse the size of the chains of polymers exist. The polymer will never have a polydispersity index less than 1 because Mw is always greater than Mn.