Methyl carbon heptad

Fig. 4.17 illustrates the potential of carbon-13 NMR to detect the presence of isotactic (a), syndiotactic (b), and atatactic (c) vinyl polymers with polypropylene as sample [521], The spectrum of atactic polypropylene (Fig. 4.17(c)) displays the signals of all possible stereosequences including iso- and syndiotactic ones. Using the empirical increment systems for alkane carbon shift prediction [85, 201, 202] and including y effects of Zy = — 5 ppm specifically obtained by analysis of stereoisomeric polypropylene partial sequences between 3,5-dimethylheptane and 3,5,7,9,11,13,15-heptamethylheptadecane as a heptad model, the methyl carbon-13 shifts of all 36 possible heptads can be calculated... 

Fig.4.17. Proton broadband-decoupled l3C NMR spectra of polypropylene ((a-c) 25 MHz 200 mg/mL 1,2,4-trichlorobenzene at 140 JC (d-e) 90.52 MHz 200 mg/mL heptane at 67 X) (a) isotactic (b) syndiotactic (c) atactic sample (d) methyl carbon spectrum, simulated for calculated carbon shifts and Lorentzian signals of < 0.1 Hz width at half-height (e) experimental spectrum [521]. Numbers in (d) refer to the 36 possible heptads ...

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. 

To predict the chemical shifts observed for the methyl carbons in a-PP (see Fig. 20.10), which show sensitivity to heptad stereosequences, we simply have to calculate the trans and gauche probabdities for the backbone bonds in each of the 36 heptad stereosequences. When this is carried out with the Suter—Floiy rotational isomeric state (RIS) conformational model for PP [29] and the resultant probabilities of finding CH3 in a gauche arrangement with its y-substituents (CHs) are multiplied by ycHs—ch = 5.2ppm, we obtain the 5CH3S shown as the stick spec-... 

The polymer chain is a "fingerprint of the catalyst active centers, go that study of microstructure of the polymer chain handily provides information about the structure of active centers in the catalyst and the effect of different additives on them. The microstructure of polypropylene is most easily studied in the methyl carbon region of NMR spectra.With modem high field NMR instruments even heptad configurations of PP chain can be determined rather easily, Microstructures of polypropylene up to hexads can be examined in the methylene region of the spectra,... 

The methyl groups numbered in Structure III correspond to the central methyl carbon of the following pentads and heptads that are observed by C-13 NHR spectroscopy ... 

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)... 

Table 2.6 NMR chemical shifts calculated for the methyl carbons in several heptad stereosequences of P-VC and PP ...

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. 

Figure 2.7 (a) Conformadons of a four carbon fragment of a PP chain (b) heptad of PI observed methyl is marked by asterisk... 

---