Polymer NMR Spectroscopy. XI. Polystyrene and Polystyrene Model Compounds

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Abstract
The high‐resolution NMR spectra of isotactic and ``atactic'' polystyrene in tetrachloroethylene and o‐dichlorobenzene are compared to the spectra of the model compounds, meso‐ and dl‐2,4‐diphenylpentane. Analysis shows that despite marked differences in the appearance of the polymer and model spectra—particularly in the β‐methylene region—isotactic polystyrene has the same local conformation as the meso pentane. This conformation is a segment of the 31 helical conformation which isotactic polystyrene is reported to have in the crystalline state. It is probable that this conformation is quite strongly preferred for all isotactic vinyl polymers. This does not necessarily mean, however, that such polymers exist as 31 helices in solution (although polystyrene probably does) because the chains may deviate widely from a 31 helix by undergoing mirror inversions of the local conformation with little net expenditure of energy. Thus, NMR does not at present allow us to make any larger statement concerning the over‐all polymer conformation (e.g., the end‐to‐end distance) in solution. For the dl‐pentane, two conformations are present, their proportions varying with temperature. This finding cannot, however, be readily related to the conformation of syndiotactic polystyrene (or of the syndiotactic sequences in ``atactic'' polystyrene) because the NMR spectrum of ``atactic'' polystyrene, unlike that of the isotactic, is too poorly resolved to give the necessary information. It appears that there may be other, more favorable cases for which this can be done.