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Polydispersity refers to the degree to which a sample of polymer molecules or other particles varies in size. A perfectly monodisperse sample contains particles of only one size. In contrast, a polydisperse sample contains a distribution of particle sizes. This distribution is often characterized by statistical measures such as the number-average molecular weight and the weight-average molecular weight. The ratio of these two averages, known as the polydispersity index (PDI) or dispersity (Đ), quantifies the breadth of the size distribution. A PDI of 1 indicates a monodisperse sample, while higher values signify greater polydispersity.

The presence of polydispersity is common in many natural and synthetic materials. For polymers, polydispersity arises from the polymerization process itself, where chains grow at different rates and terminate at various points. Different synthesis methods can lead to varying degrees of polydispersity. The physical and mechanical properties of a material are often influenced by its polydispersity. For example, in polymers, a broader size distribution can affect melt viscosity, mechanical strength, and solubility. Understanding and controlling polydispersity is crucial in fields such as materials science, nanotechnology, and pharmaceutical formulation. Techniques like gel permeation chromatography (GPC) or size exclusion chromatography (SEC) are commonly used to determine the molecular weight distribution and calculate the PDI of polymer samples.