Isotactic triad stereosequence

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. 

Sodium montmorillonite can also be used to polymerize polar monomers between the lamellae. Here too, the organization of monomer molecules within the monolayers influences the structure of the resultant polymers [212, 213]. Poly(methyl methacrylate) formed in sodium montmorillonite is composed of short, predominantly isotactic stereosequences [211]. The percentage of isotactic component increases with an increase in the ion exchanging population on the surface of the mineral and is independent of the temperature between 20 and 160°C. In this way, it is possible to vary the population of isotactic triads at will up to 50% composition [205]. 

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). 

The 220-MHz H NMR spectra of three polypropylene (PP) samples are presented in Fig. 20.8 [5-7]. Note the apparent greater resolution of the spectra in (a) and (c) recorded for the stereoregular samples (isotactic and syndiotactic) than for atactic PP in (b). The impression of degraded resolution in the spectrum for atactic PP is a consequence of the overlapping of many slightly different resonance frequencies or chemical shifts corresponding to the various triad and tetrad stereosequences present in the atactic sample. Only the rr (rrr) and mm (mmm) triads (tetrads) are present in the stereoregular syndiotactic and isotactic samples, respectively. 

A number of recent papers describe the use of high resolution NMR spectroscopy in identifying stereosequence distribution in polypropylene oxide (9-11,17-20). Using o -deuterated polypropylene oxide and 220 HMz NMR, Ogunl et al (19) have shown that the shift in methylene absorbtion can be used to identify the structures assigned to tail-to-tail linkages and to isotactic and syndiotactic triads. They showed that the crystalline fraction of polymer prepared with a dlethyl-zinc-water catalyst system did not contain a measurable amount of tail-to-tail linkages. 

By comparing the NMR resonance intensities with the predicted values given by the Bernoullian and first-order Markov (or higher), one can investigate the stereospecific polymerization mechanism. This approach has been used to define the mechanism for the free radical and anionically polymerized PMMA. The peak intensities for the measured stereosequences in PMMA from a free-radical polymerization are shown in Table 7.8. Using a probability of isotactic addition (P ,) equal to 0.24, the fraction of the triad, tetrad and pentads was compared with the measured values. The agreement suggests that the Bernoullian mechanism applies. 

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