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In organic chemistry, the polydispersity index (PDI), is a measure of the distribution of molecular weights in a given polymer sample. The PDI calculated is the weight average molecular weight divided by the number average molecular weight. It indicates the distribution of individual molecular weights in a batch of polymers. The PDI has a value always greater than 1, but as the polymer chains approach uniform chain length, the PDI approaches unity (1).
Effect of Polymerization Mechanism on PDI
Typical PDI's vary based on the mechanism of polymerization and can be affected by a variety of reaction conditions. In synthetic polymers, it can vary greatly due to reactant ratio, how close the polymerization went to completion, etc. For typical addition polymerization, values of the PDI can range around 10 to 20. For typical step polymerization, values range around 2 to 3—Carother's equation limits PDI to values of 2 and above. Living polymerization, a special case of addition polymerization, leads to values very close to 1. Such is the case also in biological polymers, where the PDI can be very close or equal to 1, indicating only one length of polymer is present.