12226 IUPAC Gold Book - dispersity 10.1351/goldbook.12226 12226 current 1 Ratio of \(M_{\rm{m}}\) to \(M_{\rm{n}}\), or the ratio of \(X_{\rm{m}}\) to \(X_{\rm{n}}\) for a homopolymer or an alternating copolymer of sufficiently large molar mass, such that the effects of the distinct structures of the end-groups of the constituent macromolecules can be neglected, giving \(X_{\rm{n}}\) directly proportional to \(M_{\rm{n}}\), \(X_{\rm{m}}\) directly proportional to \(M_{\rm{m}}\) and \(Ð\!_{\rm{M}} = Ð\!_{\rm{X}} = Ð\). Dispersity is a measure of the dispersion (or spread) of a molar mass, molecular weight, relative molar mass, relative molecular mass, or degree-of-polymerisation distribution function. For a uniform polymer, \(Ð = 1\); for a polymer of sufficiently high \(X_{\rm{n}}\) having a Poisson distribution of molar masses, molecular weights, relative-molar-masses, or relative-molecular-masses, \(Ð \cong 1\); and for a polymer of sufficiently high \(X_{\rm{n}}\) having a most probable distribution of molar masses, molecular weights, relative-molar-masses, or relative-molecular-masses, \(Ð \cong 2\). For a copolymer that is not an alternating copolymer \(X_{\rm{n}}\) cannot be considered to be directly proportional to \(M_{\rm{n}}\), nor \(X_{\rm{w}}\) directly proportional to \(M_{\rm{w}}\). It is then necessary to state whether \(Ð\!_{\rm{M}}\) or \(Ð\!_{\rm{X}}\) is being used. molar mass https://goldbook.iupac.org//terms/view/12214 PAC, 2015, 87, 71. 'Definitions of terms relating to individual macromolecules, macromolecular assemblies, polymer solutions, and amorphous bulk polymers (IUPAC Recommendations 2014)' on page 89 (https://doi.org/10.1515/pac-2013-0201) https://goldbook.iupac.org/terms/view/12226/html https://goldbook.iupac.org/terms/view/12226/json https://goldbook.iupac.org/terms/view/12226/plain Citation: 'dispersity' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.12226 The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using. 2026-07-14T22:59:36+00:00