Configurational sensitivity

The C-13 spectrum of polystyrene, shown In Figures 2 and 3, contains two regions where stereochemical Information can be extracted. There are nine methylene resonances and at least 20-22 aromatic quaternary carbon resonances. No other carbons In polystyrene e lblt a configurational sensitivity. Tentative assignments have been made for the methylene carbons based on an assumed Bernoulllan behavior (IJ). 

There are only three unique triad combinations, mm, mr and rr thus a methyl configurational sensitivity to just nearest neighbor configurations would produce only three resonance in the methyl region of the C-13 spectrum. From an earlier spectrum of the amorphous polymer, we noted at least ten methyl resonances. We must therefore consider the situation where the next-nearest as well as nearest neighbor configurations are affecting the chemical shift, that is. 

In a similar manner. It can be shown that the backbone methylene carbons are sensitive to an even number of structural units with the simplest configurational sensitivity being dyad. 

These relationships are also valuable for testing assignments In different regions of the same C-13 NMR polymer spectrum where different configurational sensitivities are shown. 

Figure 5 shows the Raman spectra of several isomers of /J-carotene (Koyama et al., 1983 Hu et al., 1997), each of which exhibits a unique spectral pattern. The Raman lines can be correlated to the cis-trans configurations as follows (Koyama et al. 1988c Hu et al., 1997) (1) The frequency of the strongest Raman line due to the in-phase C=C stretchings is configuration-sensitive the C=C stretching frequency increases in the order, aA-trans < 1-cis < 9-cis <13-... 

Quinting and Cai [62] carried out high-resolution C-NMR and proton NMR measurements to determine the tacticity of poly(n-butyl methacrylate) (PBMA) with particular focus on the peak assignments for the n-butyl side chain. Free-radical and anionic PBMA were examined, with the former being predominantly syndiotactic and the latter isotactic. Proton NMR resonances for the n-alkyl chain of these polyacrylics show a combination of effects from configurational sensitivity and homonuclear scalar interactions. A combination of J-resolved proton NMR and proton- C-heteronuclear correlated 2D-NMR spectra was used to characterise the long-range chemical shift effects due to tacticity. 

The ester methyl resonance could in principle give the same information but in most solvents, as in chloroform, it is not configurationally sensitive). In Fig. 2, we see three a-methyl proton resonances having the same chemical shifts but very different intensities in each spectrum. From their measured areas, the triad probabilities are readily determined. 

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... 

---