A uniform polymer (often referred to as a monodisperse polymer) is composed of molecules of the same mass. Nearly all natural polymers are uniform. Synthetic near-uniform polymer chains can be made by processes such as anionic polymerization, a method using an anionic catalyst to produce chains that are similar in length. This technique is also known as living polymerization. It is used commercially for the production of block copolymers. Uniform collections can be easily created through the use of template-based synthesis, a common method of synthesis in nanotechnology.
A polymer material is denoted by the term disperse, or non-uniform, if its chain lengths vary over a wide range of molecular masses. This is characteristic of man-made polymers. Natural organic matter produced by the decomposition of plants and wood debris in soils (humic substances) also has a pronounced polydispersed character. It is the case of humic acids and fulvic acids, natural polyelectrolyte substances having respectively higher and lower molecular weights. Another interpretation of dispersity is explained in the article Dynamic light scattering (cumulant method subheading). In this sense, the dispersity values are in the range from 0 to 1.
The dispersity (Đ), also known as the polydispersity index (PDI) or heterogeneity index, is a measure of the distribution of molecular mass in a given polymer sample. Đ (PDI) of a polymer is calculated:
where  is the weight average molecular weight and  is the number average molecular weight.  is more sensitive to molecules of low molecular mass, while  is more sensitive to molecules of high molecular mass. The dispersity indicates the distribution of individual molecular masses in a batch of polymers. Đ has a value equal to or greater than 1, but as the polymer chains approach uniform chain length, Đ approaches unity (1). For some natural polymers Đ is almost taken as unity.