Polypropylene methyl carbons, chemical shift

In Section lU-B we showed that the chemical shift of methyl carbons in polypropylene can be calculated considering the relative position of the adjacent substituents, expressed in configurational terms (m or r, Oor 1). An alternative interpretation, with a better physical basis, takes into account only conformational aspects. Experimental evidence from hydrocarbon spectra has established that the presence of a gauche conformation between carbon atoms separated by three bonds causes an upfield shift(7 effect) (206). For flexible molecules the magnitude of this shift depends on the fraction of conformers having gauche interactions with the observed carbons (207). 

The chemical shift literature represents an interesting blend of philosophical approaches. Since frequencies are measured with great accuracy and linewidths are narrow in solution, chemical shifts are exquisitely sensitive reporters of local molecular environment. For example, the relative stereochemistry of the methyl groups, separated by up to five monomer units (13 bonds) along the chain, in polypropylene can be distinguished by carbon NMR (see Figure 6). 

FIGURE 2.3 A pentad consists of five repeat units. The NMR chemical shift of a given methyl carbon is sensitive to the relative stereochemistry of the pentad (m = meso and r = racemo). A statistical distribution of pentads is observed in the H NMR spectrum of the methyl region of atactic polypropylene. 

Since all of the methods discussed above have been applied to analysis of the methyl carbon resonances in polypropylene, the assignment of these resonances will be covered in detail to illustrate the methods. Nine methyl carbon resonances are observed (Figure 7), the lowest field one of which has the same chemical shift as the methyl carbons in isotactic polypropylene and... 

The configurational sensitivity falls within a range from triad to pentad for most vinyl polymers. In non-crystalline polypropylenes, three distinct regions corresponding to methylene (46 ppm), methine (28 ppm) and methyl (20 ppm) carbons are observed in the C-NMR spectrum. (The chemical shifts are reported with respect to an internal tetramethylsilane (TMS) standard.) The spectrum of a 1,2,4-trichlorobenzene solution at 125 "C of a typical amorphous PP is shown in... 

Hgure 6 The 90 MHz methyl spectrum of atactic polypropylene in 1,2,4-trichlorobenzene at 100°. Empirical shift predictions for different stereoisomers reflecting the meso (m) or racemic (r) relative orientation of neighboring methyl groups. (Reprinted with permissbn from Schilling FC and Tonelli AE (1980) Carbon-13 nuclear magnetic resonance of atactic polypropylene. Macromolecules 13 270 American Chemical Society.)... 

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