Pentads, structural feature

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. 

Since there are six observable tetrads and ten observable pentads, it is a formidable task to make resonance assignments for these structxiral features. However, there are a number of general approaches for doing this. These include (a) the use of stationary or "necessary n-add relationships" to correlate the relative intensities of resonances (b) the development of empirical relationships to correlate differences in chemical shift due to structural differences ... 

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