Heterotactic stereosequences

Although several techniques have been used to characterize stereosequence distribution, we suggest that the percent crystallinity and temperature of melting measurements are more generally applicable than any other technique presently available. Bovey and co-workers (7) showed how NMR measurements can be used to determine the triad distribution in polymers such as polymethyl methacrylate in which there is sufficient difference between the NMR spectra corresponding to syndiotactic, isotactic, and heterotactic triads to allow quantitative measurements to be made. This type of measurement unfortunately is restricted to few systems and would lead to a unique description of the stereostructure of the chain only when a model involving one or two probability parameters is applicable (See Appendix I). 

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. 

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 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. 

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