Because virtually all polymers are mixtures of many large molecules, one must resort to averages to describe molecular weight. Among many possible ways of reporting averages, three are commonly used: the number average, weight average, and z-average molecular weights. The weight average is probably the most useful of the three, because it fairly accounts for the contributions of different sized chains to the overall behavior of the polymer, and correlates best with most of the physical properties of interest.
The ratio of Mw to Mn is known as the polydispersity index (PDI), and provides a rough indication of the breadth of the distribution. The PDI approaches 1.0 (the lower limit) for special polymers with very narrow MW distributions, but, for typical commercial polymers, is typically greater than 2 (occasionally much greater). Here is a typical MW distribution curve, measured by Size Exclusion Chromatography (SEC):
Many polymer properties of interest (Tg, modulus, tensile strength, etc.) follow a peculiar pattern with increasing MW. Small molecules have small values, then there is a sharp rise in properties as the chains grow to intermediate size (oligomers), and then the properties level off as the chains become long enough to be true polymers.
However, a few properties important for polymer processing, like melt viscosity and solution viscosity, increase monotonically with MW. This means that the goal of polymer synthesis is not to make the largest possible molecules, but rather, to make molecules large enough to get onto the plateau region. Increasing the MW beyond this does not improve the physical properties much, but makes processing more difficult.
A few properties are dictated by the repeat units alone, and therefore these are not changed much by MW. Examples: color, dielectric constant, and refractive index.
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