What is the equation for determination of DPn or molecular weight of polybutylene succinate (the polymer was prepared by polycondensation of diol and diacid) from NMR spectroscopy?
TIA
The determination of the MW of polymers without a easily identifiable endgroup is difficult by NMR. You polymer can have hydroxy or carboxylic acid endgroup. The neighboring CH2 will not be sufficiently different from the other CH2 to allow quantification. It is much easier if the polymer has distinct endgroups such as methyl or vinyl. The the MW can be calculated from the integral ratio of main chain versus endgroup. It might be possible by quantitative 13C NMR where a carboxylic acid is different from a ester. The CH2OH end might also be identifiable.
Dear Fatima,
without a clear terminal (or close-to-terminal as Clemens mentioned) signal with a defined number of atoms per polymer contributing to the NMR signal it is not possible!
The integral of a NMR signal is given by
(molar concentration of a polymer)*(number of atoms in this polymer)
Thus half of the molar concentration of the polymer with twice the number of monomers unit (double MW) will lead to the same integral.
An estimate of MW may be obtained by measuring diffusion (DOSY technique) coefficient D. But you will need (several) reference polymers of same type/chemistry and known MWs for calibration
...or you find something useful in literature.
A rough estimate of the MW may be "guessed" if you only have one MW reference by a polymer-scaling law - to my remembering D(MW)~MW^(3/5) for unstructured polymers. But please check this!
If you have MW references there are more simple (SEC, gel-filtration) chromatography methods.
Good luck
Alfred
Further to the suggestions of using weights of terminal groups, it might be useful to add an "internal" reference in the spectrum: it may be a low MW component added to the sample, it should be chemically/physically neutral to components of the solvent, high purity, no MW-distribution, with known MW, and it would be preferrable is this internal standard signal sits far away from the polymer spectral lines. The idea behind the calculations is the same as that for the end groups.
On Alexanders comment,
I think that even with a internal standard you cannot discriminate a "low concentration of high MW polymer" from "high concentration of low-MW polymer" by calculation of integrals.
Kind regards
Alfred
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