Methylene resonances, tetrad chemical shift

The Tetrad Chemical Shift Sequence with Respect to an Increasing Field Strength for Some Representative Vinyl Polymer Backbone Methylene Carbon Resonances... 

Much literature precedent supports the assignment of tacticity in methyl acrylate polymers using NMR techniques [40,41]. In the H-NMR spectrum, the shift of the methylene protons is sensitive to dyad stereochemistry. For example, in an isotactic (meso) dyad 28, the methylene protons are chemically non-equivalent and appear as two separate sets of signals, whereas in a syndiotactic (racemic) dyad 29, the methylene protons are equivalent. The H-NMR spectrum of 27 showed multiplets at 1.89 and 1.5 ppm due to the two diastereotopic methylene protons of the isotactic dyad. The rest of the spectrum is consistent with the structure of the n=4 tetrad 27. A racemic dyad structure would have been expeeted to give resonances of intermediate shift to that of the two resonances observed for the telomer 27. This evidence strongly implies that 27 has the allisotactic configuration shown in Scheme 8-12. 

The various structural features outlined above can give rise to a large number of resonance lines in the nmr spectrum of a stereoregular copolymer. It is not a simple matter to make assignments for these resonances since there are ten possible tetrads and twenty possible pentads, and since the various tetrad or pentad resonances are usually overlapped to a considerable extent. The case can be even more complicated if all of the nuclei in a given environment do not have the same chemical shift, as for example, the methylene protons in isotactic copolymers. 

For the 6-methylene groups, the tetrad (and perhaps even the hexad) resonances appear as fine structure on the m and r dyad resonances. The assignments are superimposed on the spectra. The m peak shows additional resolved resonances resulting from mmm, mmr, and rmr placements. Substitution of an outside m with an r results in a downfield shift. When adding placements on either side, the change in chemical shift on substituting an m for an r is just the opposite. 

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