The Polydispersity Index, PDI, is a measure of the width of the molar mass distribution. It is defined as ratio of the weight average molar mass Mw and the number average molar mass Mn, PDI =Mw/Mn.
PDI is 1 for monodisperse substances (no molar mass distribution), it is larger than 1 for the vast majority of all macromolecules. The PDI depends on the polymerization technique applied to produce the material. In praxis the PDI can be very large up to values of 100 and above. Narrow PDIs can only be obtained using controlled/living polymerization techniques.
Please refer to the literature section to learn more.