Relation of conformation to NMR properties

Carbon chemical shifts. The chemical shift patterns in the NMR spectrum of a polymer with chiral centres usually contain much more detailed confor- 

The halogenated polymers are slightly less tractable, but they are are in any case known to give shifts that depend somewhat upon solvent, especially in the methylene region. However, the spectrum of poly(methylmethacrylate) (PMMA) cannot be predicted by any reasonable conformational analysis [34]. In this case the shift of the methyl carbon in the M-trans chain should be insensitive to the stereochemistry at its neighbouring centres, for the PMMA chain is known from other measurements to have this predominantly a -trans stereochemistry. Yet the shifts of its carbons show a very large dependence upon tacticity, in both the solution and crystalline states. 

Methyl resonances Methylene resonances Methine resonances 

In principle, it should be possible to use the conformational information that one has obtained from the above analyses in order to also understand the proton NMR shifts. But unfortunately no simple shift theory is available in the general case. It is also difficult to assess the precise orientation, and hence shift effects, of substituent groups such as phenyl. 

It was shown in section 4.3 that often lengthens significantly for all carbons in isotactic chain segments in dissolved polymers [37]. This may be explained by the RIS analysis in the previous section, given the reasonable assumption that the transitions between conformations is rapid. Furthermore, the analysis shows that the relaxation parameters are insensitive to the rate of the local motions, within this limit. For example, an i-PMMA dyad must have two equally weighted conformations, loosely tg and gt, on symmetry grounds. The calculation described above also finds that tt has a very similar weighting. In contrast, an s-PMMA dyad has only one main conformation, tt, with a smaller contribution from gg. Thus more extensive local motion is to be anticipated in short i-PMMA sequences than in short s-PMMA sequences, in the same overall chain. Similar observations and calculations have been made with poly(acrylonitrile). 

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