Polydispersity index

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In organic chemistry, the polydispersity index (PDI), is a measure of the distribution of molecular mass 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 masses 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). For some natural polymers PDI is almost taken as unity. The PDI from polymerization is often denoted as:

$\ PDI = M_w/M_n$

A polymer material is denoted by the term polydisperse if its chain lengths vary over a wide range of molecular masses. This is characteristic of man-made polymers.[1]. Natural organic matter produced by the decomposition of plants and wood debris in soils (humic substances) has also a pronounced polydispersed character. It is the case of humic acids and fulvic acids, natural polyelectrolyte substances having respectively higher and lower molecular weights.

[edit] 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, most probable values of the PDI are around 2 —Carothers' equation limits PDI to values of 2 and below.

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.