Triad stereosequences

A monosubstituted vinyl monomer yields polymers having three regioregular arrangements of configuration (Fig. 5), described by triad stereosequences. The isotactic structure has all R groups on the same side of the backbone (mm) the syndiotactic structure has R groups on alternate sides of the polymer s backbone (rr) and the heterotactic or atactic structure has R groups randomly oriented on either side of the polymer s backbone. 

Microstructures of Poly(chlorofluoroethylene)s The carbon-13 NMR spectrum of PVCF consists of a —CH2— resonance at 54.1 ppm and a —CFC1— resonance at 108.8 ppm. There is no splitting of these lines due to tacticity, nor are there any other resonances to indicate the presence of regioirregular monomer sequences. However the polymer is stereoirregular, as shown by the fluorine-19 NMR spectrum in Figure 1. There are three principal resonances spread by 3 ppm owing to triad stereosequences, with some pentad fine structure which is barely resolved. 

The major splitting of the isoregic resonance into the three peaks is due to triad stereosequences, as noted above. Assuming that polymerization at low temperature in urea favors syndiotactic propagation, we assign these peaks to isotactic, heterotactic, and syndiotactic sequences in order of increasing field as... 

The bar graph in Figure 7 compares the triad stereosequence probabilities for the PVF samples examined here with values calculated for an equilibrium stereochemical distribution which would result from epimerization at 50 °C (18). All PVF samples, except for the urea polymer, rather fortuitously have nearly an equilibrium stereosequence distribution. Furthermore, both the precursor PVCF and final PVF derived therefrom have nearly identical p(m) values and stereosequence distributions, so that the effect of racemization during reductive dechlorination (19) is not apparent. 

The isotactic (mm), heterotactic (mr), and syndiotactic (rr) triad stereosequence probabilities for (1) commercial PVF, (2) PVF derived from PVCF, (3) PVF derived from PCFE, (4) urea PVF and (5) PVF epimerized at 50°C 08). Note that the observed heterotactic probability is mr + rm. 

Li et al. (10) used HCF triple resonance 3D NMR to assign the H, and " F resonances of poly(l-chloro-l-fluoroethylene) (PCFE). Figure 8 shows the ID NMR spectra of this polymer. The H spectrum contains very little useful information and the C spectrum shows essentially two clusters of resonances firom C-F and CHa groups. The F spectrum contains three groups of resonances in ca. 1 2 1 ratio, fi om mm, nor/rm and rr triad stereosequences. Application of HCF triple resonance 3D-NMR is particularly usefiil in this case, since has a natural abundance of 100% and an enormous chemical shift dispersion. The sensitivity of this erqreriment is comparable to that of a simple double resonance H C -HMQC experiment. 

Ordinarily, it is possible to distinguish between mm/rr and mr/rm triad stereosequences using standard NMR experiments, but distinction between the resonances of mm and rr triads can be made only if a spectrum from a stereoregular polymer of known relative configuration is available. Triple resonance 3D-NMR techniques combined with isotopic labelling have provided powerful tools for biomolecular structure determination which have tremendous potential applications in polymer chemistry [63-66]. 

Once the triad stereosequences are determined from examination of single slices, the relative stereochemistry of adjacent diads in the chain can be determined by looking for identical C-H crosspeaks in different F slices. For example, in Figure 6.12(a), the A pair of crosspeaks do not occur in the other two slices therefore, type A methylenes show crosspeaks only to F atoms in mm. 

Table 9. Dyad and Triad Stereosequences Initiated on S Configuration Sites and their Probabilities for the General Case Model.

Ordinarily, it is possible to distinguish between mm/rr and mr/rm triad stereosequences using standard NMR experiments, however, distinction between the resonances of... 

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