In organic chemistry, the polydispersity index, or PDI, is a measure of the distribution of molecular weights in a given polymer sample. The PDI is calculated is the weight-average molecular weight divided by the number average molecular weight. As the polymer chains approach uniform chain length, the PDI approaches unity (1). Typical PDI's vary based on the mechanism of polymerization and can be affected by a variety of reaction conditions. For typical addition polymerization, values of the PDI can range around 10 to 20. For typical step polymerization, values range around 2 to 3. Living polymerization, a special case of addition polymerization, leads to values very close to 1.