What is the polydispersity index?

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The polydispersity index (abbreviated as IP; correctly but less frequently used: dispersity, IUPAC symbol: Đ, pronounced D-stroke) of a polymer or biopolymer sample, e.g. a chitin or a chitosan sample, is a measure of its molecular weight distribution. It is an important parameter describing any polymer sample as such samples are always mixtures of molecules of different chain lengths or degrees of polymerization. The value of Đ can be 1 or larger, with a Đ of 1 indicating a (theoretical) sample where all molecules have the same molecular weight. In reality, polymer samples always have a polydispersity index above 1, high quality chitosan samples have Đ values below 2, sometimes even below 1.5.