Geometrical isomerism in polydienes

Cis/trans isomerism of backbone double bonds (more correctly Z/E isomerism in modern nomenclature) is a structural feature of polymers that may affect NMR spectra in a manner similar to tacticity in vinyl polymers. We will take the prototype chain, poly (1,4-butadiene) (-CH2CH=CHCH2 ) as an example. Denoting cis and trans isomers by c and f, the possible dyad, triad, etc. isomeric sequences can be constructed from c and t in the same way as m and r were used for vinyl polymer stereosequences. The possible dyads are cc, ct, tc and tt. In the cc and tt dyads, the two CH2 carbons are equivalent whereas in the ct and tc dyads, the two CH2 carbons are not equivalent. 

Thus if the CH2 chemical shifts depend on the structure of the two neighbouring double bonds, a total of four peaks would be expected, one assigned to each of cc and tt, and two assigned to ct/tc. Similarly, at the triad level, there are six distinct triads, four symmetrical ccc, tct, etc and ttt) and two unsymmetrical (tcc/cct and ett/tte). If the chemical shifts of the olefinic carbons of the central unit depend on triad structure, a total of eight peaks would be expected. This example shows that when relating NMR spectra to structure, due regard must be paid to the total number of distinct nuclear environments in a particular structure. 

Experimentally, the chemical shifts of both [68] and [69, 70] nuclei in poly (1,4-butadiene) are found to depend almost entirely only on the structure at the monomer level. In the CH2 region of the spectrum, cis units appear at 27.4ppm and trans units at 32.8 ppm [69]. In the olefinic region, there is a slight dependence on sequence structure [70] peaks from cis units at 128.8 ppm and trans units at 129.35 ppm are each split into a doublet of about 0.1 ppm due to a slight sensitivity to dyad structure. It should be noted that analysis of the spectra of poly(dienes) in terms of cis/trans isomerism is severely complicated by the occurrence of 1,2- as well as 1,4-addition. 

In studying polymer stereochemistry, the form of the NMR spectrum gives a rapid, unequivocal identification of the structure of stereoregular vinyl polymers. For stereo-irregular polymers, the resonance peaks are frequently split into several lines from sequences of a few monomer residues of different stereochemistry. The relative peak intensities give statistical information on 

Abraham, J. Fisher and P. Loftus, Introduction to NMR Spectroscopy, Wiley, Chichester (1988). 

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