Isotactic coupling constant

Proton NMR spectroscopy has been used to characterize the tacticity of various vinyl polymers in solution. In the case of isotactic polymers, there are two magnetically non-equivalent protons (Figure 7-34) and, as we discussed earlier in this chapter, this can result in the appearance of four bands (the chemical shift difference is of the same order of magnitude as the coupling constant, so the simple rules for mnltiplicities don t apply and we get what we called an AB pattern). On the other hand, in syndiotactic polymers the two methylene protons are equivalent and we observe only one line. Let s look at this in more detail, using poly(methyl methacrylate) (PMMA), as an example, because bands due to various tactic sequences are particularly well resolved in the spectrum of this material. 

Spectra were observed at 60 macrocycles/s and 100 macrocycles/s both at room temperature and at high temperatures, and spin-decoupling experiments were done. The difference in the chemical shifts of the two meso methylene protons at 60 macrocycles/s was found to be approximately 7 cycles/s for the isotactic three-unit model, whereas it was approximately 16 cycles/s for the isotactic two-unit model or heterotactic three-imit model. PVC spectra can be reasonably interpreted on the basis of this result. Observed values of vicinal coupling constants of model compounds were interpreted as the weighted means of those for several conformations, and the stable conformations of the models determined. 

Also in this case the inequality of the coupling constants between the two methylenic protons and the methynic ones [i] indicates that isotactic macromolecules in solution are not perfectly spiralized. Hence, it seems neither necessary nor sufBcient to suppose the presence of steric irregularities to explain the end to end distance observed by viscosimetric measurements on isotactic poly-a-olefins [ ]. 

Coupling between the methylene protons of isotactic polymers results in a typical AB pattern which is usually complicated by additional fine structure due to coupling with methine protons. The coupling constant between methylene protons is 13.5 Hz, typical... 

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