Chemical shift heptad

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. 

The properties of a copolymer depend on its composition, monomer sequence and stereochemical structure. Although compositional analysis can be achieved by several methods other than NMR spectroscopy, quantitative data on monomer sequence distribution can only be obtained from NMR spectroscopy. I3C NMR chemical shifts of C=0 carbons of PMMA are sensitive to pentad to heptad stereochemical sequences. The C=0 carbon signals for the copolymers of methacrylates are also sensitive to triad comonomer sequence. Thus it should be difficult to assign both tactic and comonomer sequence signals, especially in the case of copolymers with low stereoregularity. 

Although the aromatic C-l carbon resonance patterns observed for partially epimerized polystyrenes are readily interpreted using the above considerations, this is not the case for the pattern observed for polystyrene (or for the completely equilibrated polymer). The six-area pattern that is so clearly evident in the spectra of the partially epimerized polymers is not evident in the spectrum of polystyrene. It seems that resonances of heptads or nonads with high r-contents have chemical shifts that correspond to valleys observed in the spectra of the partially epimerized polymers. This causes the demarcation between pentad resonance patterns to become obscure when the r-content is about 0.5. This complication should disappear as the r-content increases from 0.5 and resonances due to heptads or nonads with high m-contents diminish in concentration. Unfortunately the unavailability of polystyrenes with high r-contents at the present time prevents this possibility from being pursued. 

Smmmmmm can be measured from the chemical shift of isotactic polystyrene or from that of the largest signal in polymers epimerized to low extents and is 146.40 ppm for the conditions employed in the present study. Accordingly, the chemical shift of any heptad can be calculated by the following general formulas and specific examples. 

Minor adjustments were made in chemical shifts calculated using Axyz values to improve the quality of fit between simulated and observed spectra. The heptad chemical shifts used to simulate... 

Figures 1-6 compare observed aromatic C-l carbon resonance spectra with simulated spectra based on the heptad chemical shifts given in Table VI and on heptad stereosequence concentrations calculated by Monte Carlo simulation of the epimerization process, using V=0.65. The simulation spectra reproduce the general features of the observed spectra very well and can be considered to be in at least semi-quantitative agreement with the observed spectra. The agreement between observed and simulated spectra might be improved if spectra with higher S/N ratios were employed and if additional parameter adjustments were made. It seems, however, that the heptad assignments developed in this work are reasonably correct.

Chemical shift ppm 6 Oxymethylene pentades Oxymethylene heptades... 

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-... 

Having assigned all heptad stereosequence dependent C NMR resonances in a-PP [11,13], integration of the resonances provides us with a detailed accounting of how much of each stereosequence is present. Such information is needed to test various statistical models of PP polymerization [18]. Furthermore, the close agreement between observed and calculated chemical shifts provides strong confirmation of the Suter-Flory rotational isomeric state (RIS) conformational model for PP [29]. 

In some cases, notably the spectrum of poly(propylene) [56] and poly-(vinyl alcohol) [57], the CXY chemical shifts may show resolvable stereochemical sensitivity over even longer ranges. At the next highest level, a sequence of seven monomer units or heptad, there is a total of 64 enantiomers of which 36 are distinguishable. 

The simulated spectrum was obtained [13] by assuming Lorentzian peaks of <0.1 ppm width at half height for each of the 36 heptad chemical shifts c culated by the y-gauche effect method. The relative intensities or heights of these heptad peaks were then adjusted to obtain the best simulation of the observed spectrum. 

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 ...

The chemical shift calculation (y-effect method) based on the y-effect of the chemical shift and the rotational isomeric state model (RIS model) has been developed as a reliable method for predicting chemical shift differences among pentad, hexad, and heptad sequences in various polyolefins [47-49, 14, 50, 51]. chemical shift assignments of tactic pentad and heptad sequences in polypropylene have been provided by this method [47-49]. Hayashi and co-workers [45,46] confirmed that the chemical shift due to the y-effect is also sensitive to different comonomer sequences in ethylene-propylene copolymers. Asakura and co-workers [52] have demonstrated that... 

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