Polymer Fractionation and Molecular Weight

The measurement of the molecular weight is a task undertaken in its own right by polymer chemists, and is concerned with the development of new polymers and process control in the case of existing polymers. Additionally, however, it is necessary to separate a polymer not into unique molecules each with a particular molecular weight, but into a series of narrower MWD fractions. This is required in order to obtain a more detailed picture of the polymer composition and structure, and these separated fractions may be required for further analysis by a wide range of techniques. 

In the simplest case it is required simply to separate, for example, the total gel fraction (i.e., crosslinked material which is insoluble in organic solvents) of a polymer from the total soluble fraction (non-crosslinked material). An example would be the separation of polystyrene (PS) into its gel and soluble fractions. In a more complicated case it may be required to carry out a separation of the original polymer into a series of fractions each with a narrower MWD than the parent polymer. These methods were formerly based on fractionation techniques, gradient elution, or precipitation. 

Nowadays, such fractionations are carried out by a variety of techniques such as gel permeation chromatography (GPC)/size exclusion chromatography (SEC), high- 

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