Polydispersity Index (PDI)

Polydispersity index (PDI)

Unlike a small molecule, a polymer has no single molar mass — it is a mixture of chains of varying sizes, formed during polymerization. Molar mass is therefore described by averages, and the two most important ones are the number average (M_n, the plain arithmetic mean, which counts every chain equally) and the weight average (M_w, which gives more weight to the longer chains because it weights by mass). The polydispersity index is the ratio between them: PDI = M_w ÷ M_n, and it measures how broad the size distribution is. Since M_w ≥ M_n always holds, the PDI is always ≥ 1. A PDI of 1 means a monodisperse polymer — every chain the same length — an ideal but rare situation, reached only through special techniques such as living polymerization (controlled anionic) or in biological proteins. Commercial polymers typically show a PDI of 2 to 20, depending on the mechanism: free-radical polymerization gives a PDI near 2; processes with chain transfer and branching give high PDIs. The PDI matters because it drives how the material behaves: very short chains act as plasticizers (they cut strength), very long chains raise melt viscosity and make processing harder; a broad distribution may improve processability while compromising strength. Controlling the PDI means controlling the uniformity of the product. The full distribution is measured by gel permeation chromatography (GPC/SEC). Enter M_w and M_n.