thumb|a uniform (monodisperse) collection thumb|a non-uniform (polydisperse) collection in physical and organic chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. a collection of objects is called uniform if the objects have the same size, shape, or mass. a sample of objects that have an inconsistent size, shape and mass distribution is called non-uniform. the objects can be in any form of chemical dispersion, such as particles in a colloid, droplets in a cloud, crystals in a rock, or polymer molecules in a solvent. polymers can possess a distribution of molecular mass; particles often possess a wide distribution of size, surface area and mass; and thin films can possess a varied distribution of film thickness. iupac has deprecated the use of the term polydispersity index having replaced it with the term dispersity, represented by the symbol Ä (pronounced d-stroke) which can refer to either molecular mass or degree of polymerization. it can be calculated using the equation Äm = mw/mn, where mw is the weight-average molar mass and mn is the number-average molar mass. it can also be calculated according to degree of polymerization, where Äx = xw/xn, where xw is the weight-average degree of polymerization and xn is the number-average degree of polymerization. in certain limiting cases where Äm = Äx, it is simply referred to as Ä. iupac has also deprecated the terms monodisperse, which is considered to be self-contradictory, and polydisperse, which is considered redundant, preferring the terms uniform and non-uniform instead.stepto, r. f. t.; gilbert, r. g.; hess, m.; jenkins, a. d.; jones, r. g.; kratochvÃ­l p. (2009). "dispersity in polymer science" pure appl. chem. 81 (2): 351â353. doi:10.1351/pac-rec-08-05-02. ==overview==  a monodisperse, or uniform, polymer is composed of molecules of the same mass.{{cite web |url=http://goldbook.iupac.org/m04012.html |title= monodisperse polymer (see: un
Copolymers Polymer_chemistry