Heterotactic conformation

Such configurational as well as conformational effects have been also reported by MILLAN et al. in the case of nucleophilic substitution of poly(vinyl chloride) with sodium thiophenate (14) and with sodium isooctylthioglycolate or isooctylthiosalicylate (15). The authors have shown that these reactions proceed selectively on the isotactic TT diads which can only exist either in the GTTG isotactic or in the TTTG heterotactic triads, the former ones being much more reactive than the latter ones. 

Mixtures of diastereomers of 2,4,6-trlphenylheptane are epimerized. The mole fractions olisotactic, heterotactic, and syndiotactic isomers at equilibrium at 343 K are 0.217, 0.499, and 0.284, respectively. There results are interpreted according to the theory of stereochemical equilibrium. The theory of equilibria between isomers and the associated theory of the conformer populations for each isomer provide a mutually consistent interpretation of the two kinds of results, the same arbitrary parameters being used for both. Stereochemical equilibria and conformer population calculated for PS for the same parameters differ considerably from those for the oligomers. 

Stereospecificity. The adsorption of isotactic, stereotactic and atactic PMMA to silica from CDCI3 was studied by IH liquid spectra, where the methyl peaks spectrally separate into isotactic, heterotactic and syndiotactic sequences. The intensity ratios of the liquid IH signals of polymers adsorbed to silica were compared to solution spectra. In Fig. 17 the comparison of traces A and D shows that the isotactic component (mm) has significantly decreased in intensity relative to the other two components in spectrum D, and a selective adsorption of i-PMMA was concluded. The stereospecificity decreased with adsorbed amount, and was also reduced for adsorption above the conformational transition temperature. Comparison to adsorption to a hydrophobised surface, which did not show stereospecific immobilisation, proved that the silica hydroxyl groups were essential to attract the isotactic component specifically [59]. 

Spectra were observed at 60 macrocycles/s and 100 macrocycles/s both at room temperature and at high temperatures, and spin-decoupling experiments were done. The difference in the chemical shifts of the two meso methylene protons at 60 macrocycles/s was found to be approximately 7 cycles/s for the isotactic three-unit model, whereas it was approximately 16 cycles/s for the isotactic two-unit model or heterotactic three-imit model. PVC spectra can be reasonably interpreted on the basis of this result. Observed values of vicinal coupling constants of model compounds were interpreted as the weighted means of those for several conformations, and the stable conformations of the models determined. 

Figure 20 illustrates idealized classification of the stereo-controlled polymerizations of a racemic monomer that bears one center of chirality (p-BL case). For the sake of simplicity, only formation of isotactic (...-RRRRRRRR-... or. ..-SSSSSSSS-...) polymer formation is shown. In principle, this scheme conforms also to the rac-LA (containing two identical centers of chirality in one molecule) polymerization. In a more general case, formation of the syndiotactic (...-RSRSRSRS-...) and heterotactic (...-RRSSRRSS-...) polyester chains (see, e.g., structures 5) should additionally be considered vide infra). 

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