Title: molar-mass dispersity
Long Title: IUPAC Gold Book - molar-mass dispersity
DOI: 10.1351/goldbook.12224
Status: current

Definition
Ratio of the mass-average molar mass, \(M_{\rm{m}}\), molecular weight, relative molar mass, or relative molecular mass, \(M_{\rm{r,m}}\), to the number-average molar mass, \(M_{\rm{n}}\), molecular weight, relative molar mass, or relative molecular mass, \(M_{\rm{r,n}}\). \[Ð\!_{\rm{M}} = M_{\rm{m}}/M_{\rm{n}} = M_{\rm{r,m}}/M_{\rm{r,n}}\]

Note
Use of the term "polydispersity index", or other terms involving the word "polydispersity", for \(M_{\rm{m}}/M_{\rm{n}}\), \(M_{\rm{r,m}}/M_{\rm{r,n}}\) is strongly discouraged.

Related Term
- mass-average molar mass: https://goldbook.iupac.org//terms/view/12217

Source
- PAC, 2015, 87, 71. 'Definitions of terms relating to individual macromolecules, macromolecular assemblies, polymer solutions, and amorphous bulk polymers (IUPAC Recommendations 2014)' on page 88 (https://doi.org/10.1515/pac-2013-0201)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/12224/html
- json: https://goldbook.iupac.org/terms/view/12224/json
- xml: https://goldbook.iupac.org/terms/view/12224/xml

Citation: Citation: 'molar-mass 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.12224
License: 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.
Collection: If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org .
Disclaimer: 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.
Accessed: 2026-04-18T10:05:42+00:00