Dispersity

A sample of objects that have an inconsistent size, shape and mass distribution is called non-uniform.

[citation needed] IUPAC has deprecated the use of the term polydispersity index, having replaced it with the term dispersity, represented by the symbol Đ (pronounced D-stroke[4]) which can refer to either molecular mass or degree of polymerization.

[citation needed] A polymer material is denoted by the term disperse, or non-uniform, if its chain lengths vary over a wide range of molecular masses.

[7] Natural organic matter produced by the decomposition of plants and wood debris in soils (humic substances) also has a pronounced polydispersed character.

Another interpretation of dispersity is explained in the article Dynamic light scattering (cumulant method subheading).

Typical dispersities 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.

The reactor polymerization reactions take place in can also affect the dispersity of the resulting polymer.

[9] With respect to batch and plug flow reactors (PFRs), the dispersities for the different polymerization methods are the same.

In conventional bulk free radical polymerization, the dispersity is often controlled by the proportion of chains that terminate via combination or disproportionation.

As a result, reactor type does not affect dispersity for free radical polymerization reactions in any noticeable amount as long as conversion is low.

In batch reactors or PFRs, well-controlled anionic polymerization can result in almost uniform polymer.

To achieve any high molecular weight polymer, the fractional conversion must exceed 0.99, and the dispersity of this reaction mechanism in a batch or PFR is 2.0.

[9] Thus, for the similar reasons as anionic polymerization, the dispersity for heterogeneous CSTRs lies between that of a batch and a homogeneous CSTR.