Carbon heptad resonances, methyl

C NMR spectra are recorded for a low molecular weight atactic PP dissolved in a variety of solvents over a broad temperature range [293 - 393 K). Comparison of chemical shifts calculated via the y effect method with the observed resonances, whose relative chemical shifts are solvent independent, permits their assignment to most of the methyl heptad, methylene hexad, and methine pentad stereosequences. Agreement between observed and calculated chemical shifts requires y effects, he., upfield chemical shifts produced by a gauche arrangement of carbon atoms separated by three bonds, of ca. - 5 ppm for the methyl and methine carbons and ca. - 4 ppm for the methylene carbons. 

NMR spectrum of atactic PP were sensitive to pentad stereo-sequences. At 90.5 MHz (see Fig. 20.10), the methyl carbon resonances show sensitivity to heptad stereosequences (rmnmmmm, rrrrrr, mrmmrr, etc.) [13]. The NMR spectra of PPs are sensitive to stereosequences extending over 4 (pentads) and 6 (heptads) bonds in both directions along the PP backbone. This long-range sensitivity to microstructural detail makes NMR a valuable tool in the determination of polymer structures. 

Comparison of the methyl resonances in P-VC and PP reveals a decreased sensitivity to stereosequence for the P-VC copolymer. The methyl carbon resonances in P-VC are sensitive to pentad stereosequences, whereas in PP heptad sensitivity is observed. In Table 2.6 the C chemical shifts calculated for the methyl carbons in several heptad stereosequences of P-VC and PP are compared. As observed, the methyl carbon chemical shifts calculated for P-VC are sensitive to pentads, but PP methyl carbons show significant heptad sensitivity. This difference in stereosequence sensitivity between the methyl carbons in P-VC and PP is directly attributable to differences in their conformational behavior as embodied in their RIS models. Local bond conformations reflect pentad sensitivity in P-VC and heptad dependence in PP. In addition, note that the overall spreads in methyl carbon chemical shifts observed in P-VC and PP are 2.7 and 2.0 ppm, respectively, with the P-VC methyl carbons resonating about 1 ppm upfield from those in PP. These observations are also reproduced by the calculated chemical shifts, which employ the same y-effect (ycHj.cH = Ppm)... 

---