dispersities

Dispersity refers to the breadth of a distribution of a property in a sample. In polymer chemistry, it most often describes the molecular weight distribution of a polymer. It is quantified by the dispersity index Đ = Mw/Mn, the ratio of weight-average molecular weight to number-average molecular weight. Mn is the average chain length counted for each molecule once, while Mw weights each molecule by its mass, so Đ reflects how widely chain lengths vary. A perfectly monodisperse sample would have Đ = 1.0; real polymers typically have Đ > 1, with values around 1.05–1.2 for controlled/living polymerizations and larger values for conventional radical polymerizations.

Dispersity affects physical properties such as viscosity, crystallinity, melting temperature, and mechanical behavior. It is a key parameter for process design and material performance. Dispersity can be influenced by the polymerization method, initiator efficiency, chain transfer, and termination events. Narrow dispersity is sought in advanced materials like block copolymers and high-performance thermoplastics, while broader distributions may be acceptable or advantageous for certain applications.

Measurement is typically done by gel permeation chromatography (GPC, also called SEC), which provides Mn and Mw by comparing to polymer standards. Absolute molecular weights can be obtained with multi-angle light scattering or mass spectrometry. In colloidal and nanoparticle science, dispersity also describes the breadth of particle-size distributions; here the term polydispersity index (PDI) from dynamic light scattering or other methods is commonly used.

Related concepts include molecular weight distribution and the use of Đ in polymer design.

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