page_title: End-group
section_title: NMR Spectroscopy
hierarchical_section_title: End-group / Analysis of Polymers using End Groups / NMR Spectroscopy
caption_attribution_description: English: Example of utility of NMR for end group analysis
context_page_description: End groups are an important aspect of polymer synthesis and characterization. In polymer chemistry, end groups are functionalities or constitutional units that are at the extremity of a macromolecule or oligomer. In polymer synthesis, like condensation polymerization and free-radical types of polymerization, end-groups are commonly used and can be analyzed for example by nuclear magnetic resonance to determine the average length of the polymer. Other methods for characterization of polymers where end-groups are used are mass spectrometry and vibrational spectrometry, like infrared and Raman spectrometry. Not only are these groups important for the analysis of the polymer, but they are also useful for grafting to and from a polymer chain to create a new copolymer. One example of an end group is in the polymer poly diacrylate where the end-groups are circled.
context_section_description: The advantage of NMR for end groups is that it allows for not only the identification of the end group units, but also allows for the quantification of the number-average length of the polymer.   End-group analysis with NMR requires that the polymer be soluble in organic or aqueous solvents.  Additionally, the signal on the end-group must be visible as a distinct spectral frequency, i.e. it must not overlap with other signals. As molecular weight increases, the width of the spectral peaks also increase. As a result of this, methods which rely on resolution of the end-group signal are mostly used for polymers of low molecular weight (roughly less than 20,000 g/mol number-average Molecular weight).  By using the information obtained from the integration of a 1H NMR spectrum, the degree of polymerization (Xn) can be calculated.  With knowledge of the identity of the end groups/repeat unit and the number of protons contained on each, the Xn can then be calculated.  For this example above, once the 1H NMR has been integrated and the values have been normalized to 1, the degree of polymerization is calculated by simply dividing the normalized value for the repeat unit by the number of protons continued in the repeat unit.  For this case, Xn = n = 100/2, and therefore Xn = 50, or there are 50 repeat units in this monomer.
caption: Example of utility of NMR for end group analysis.
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