Stereochemistry of vinyl polymers

Historically vinyl polymers were the first to be classified stereochemically and the first to be studied by NMR. In some respects they are also the simplest. The basic concepts of polymer stereochemistry and its influence on NMR spectra are developed first with specific reference to vinyl polymers. The ideas introduced are then extended to other systems. 

The stereochemistry of a particular chain is specified completely by the absolute configuration of every pseudochiral carbon. However, since enantio- 

2 NMR spectra of stereoregular polymers. We consider first proton spectra. The key distinctive feature is the spectrum of the CH2 group in an 

The spectra of stereoregular polymers show a single sharp line for each chemically distinct carbon because within each type of chain, each monomer residue is identical. However, the chemical shifts for isotactic and syndiotactic chains are not the same. For example, in isotactic poly(propylene), the CH3, CH and CH2 carbons occur at 20.0, 27.1 and 44.4 ppm, respectively, whereas in syndiotactic poly (propylene), the corresponding shifts are 18.7, 27.0 and 45.4 ppm [53]. NMR does not yield an unequivocal identification of the tacticity as does the CH2 proton spectrum, but it does permit different tacticities to be distinguished. 

The tacticity sensitivity of the CXY group may extend further than triads, being influenced by the next nearest neighbours also, i.e. a sequence of five monomer units (four dyads), termed a pentad. There is a total of 16 pentad enantiomers, but only ten are distinguishable. Extending the m/r notation 

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Vinyl polymers, for which pol5rpropylene serves as a prototype, present some additional issues not encountered in chains with symmetric torsions. The physical properties of these chains depend on the stereochemical composition and stereochemical sequence of the chain, and this dependence must be reflected in Z. Two equivalent methods have been used for description of the stereochemistry of vinyl polymers. One approach uses pseudoasymmetric centers [67]. Although the fragment denoted by -CH2-CHR-CH2- does not contain a chiral center, it can be treated as though it were chiral if one CH2 group is distinguished from the other. This distinction is drawn when the bonds in the... 

Among the most useful findings was the observation that proton nmr could be employed, —even with the low frequency spectrometers of the time—, to obtain quantitative information concerning the stereochemistry of vinyl polymers. The Ziegler-Natta revolution" was then relatively new and was still generating much excitement. 

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