End group analysis

If we know the skeletal structure of a polymer (i.e., how many end groups there are per molecule of polymer) and the end groups are in some way distinguishable from other chemical moieties on the polymer, then end group analysis can be a powerful tool to allow us to “count” the number of molecules in a sample and hence solve for $M_n$.Alternatively, if we know $M_n$ from some other method, then end group analysis can actually help us figure out how many end groups there were on average per molecule. There are a variety of analytical methods that could facilitate quantification of the end groups, such as spectroscopic methods, elemental analysis, and titration. We will not be going into depth regarding spectroscopic analysis, but in this lesson we will go over examples for which acid-base titrations and elemental analysis can be used.

Figure 11.03: Example polymers for end group analysis

Source: Lauren Zarzar

End group analysis would be like counting the stars on these polymers (6) and knowing there are 2 end groups per molecule – we can use that information to figure out that there are 3 polymer molecules.

There are several requirements that must be met in order for end group analysis to be useful:

• There has to be some way to quantify the end group functionality in the first place, usually by titration, elemental analysis, or spectroscopy.
• Other functional groups on the polymer or in the solution can’t interfere with measurement of your target end groups.
• The concentration of end groups has to be sufficient to get an accurate measurement (so may not be good for very high molecular weight samples).
• You have to know how many end groups there are per molecule (to find molar mass), OR you know the molar mass, and want to know the number of end groups per molecule.