Tacticity by nmr

Stereochemical sequences can be characterized by high resolution nmr.

PC Fig. 6.27 shows schematically how proton nmr spectra can be analyzed in order to establish the diad and triad sequences in poly(methyl methacrylate).

• The methylene protons are in equivalent environments in a syndiotactic diad, yielding a single peak. But they are in distinguishable environments in an isotactic diad, yielding a doublet that is again split into a doublet by spin-spin interaction, for a total of four peaks of similar intensity, called an AB pattern. An atactic polymer will show the signatures of both of these diads, in the ratio established by its stereochemical composition. A physical mixture of isotactic and syndiotactic chains is not distinguishable from an atactic polymer, if one looks only at the diad content.

• The methyl protons experience different environments in isotactic, syndiotactic, and heterotactic diads. The chemical shift of the single peak is therefore different in these three cases. An atactic sample would show peaks at all three positions. A physical mixture of isotactic and syndiotactic chains would show peaks at only two of these three positions.

Even as early as the 1970's, tetrads could be resolved in the proton nmr (at 220 MHz) of poly(methyl methacrylate). Some of Bovey's work from this era is reproduced in PC Fig. 6.28 and PC Fig. 6.29. For proton nmr spectra of PMMA at higher resolution (500 MHz), see Schilling, F. C.; Bovey, F. A.; Bruch, M. D.; Kozlowski, S. A. "Observation of the Stereochemical Configurations of Poly(methyl methacrylate) by Proton Two-Dimensional J-Correlated and NOE-Correlated NMR Spectroscopy" Macromolecules 1985, 18, 1418-1427. Higher order sequences are readily observable in vinyl polymers, as shown by the assignment of hexads in atactic polypropylene, Tonelli, A. E.; Schilling, F. C. "¹³NMR Chemical Shift and the Microstructure of Polymers" Acct. Chem. Res. 1982, 14, 233-238.

Return to the index