Poly isotactic diads

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. 

Now let us examine simple vinyl polymers, with only one site of substitution per repeat unit. When we look at a polymer chain, we focus only on combinations of diads or couples. For our discussion, we will use segments of poly (vinyl chloride). The geometries can be divided into three general groups. The first group, in which the substitutes, or here the chloride atoms, are all identical with adjoining neighbors, comprise meso diads. Polymers or sections of polymers that contain meso diads are referred to as isotactic. 

If there is no regular arrangement of vertical lines in the adapted Fischer projection or alkyl substituents in the planar zigzag projection (Figure 3.2), then an atactic structure occurs. Atactic poly(a-olefin)s are characterized by the appearance in their chains of m diads and r diads in equal amounts. These diads constitute heterotactic mr triads and rm triads as well as isotactic mm triads and syndiotactic rr triads which also appear in equal amounts. 

Figure 3.45 Typical steric defects (pentad distribution of stereoerrors) in a (mainly) isotactic poly(a-olefin) chain (a) isolated r diad, characteristic of chain end stereocontrol (b) pair of r diads, characteristic of enantiomorphic site stereocontrol during the propagation...

Figure 4-24. Dependence of the reciprocal molar optical rotaton,, with tacticity content of isotactic, x, and syndiotactic, Xs, diads for poly (S)-4-methyl-l-hexene. Optical purity of initial monomer was 93% (according to measurements by P. Pino et al).

On the other hand, stereocomplexes and solvatophobic bonds are specific to macromolecular multimerization. Certain polymers with mutually complementary stereostructures form what are called stereocomplexes whose stoichiometry is dependent on the stereosequence length. Examples of such stereocomplexing are provided by the pairs poly(7-benzyl-D-glutamate) with poly(7-benzyl-L-glutamate) and isotactic with syndiotactic poly(methyl methacrylates) (PMMA). The specific enthalpy of stereocomplex formation with it- and st-PMMA is, for example, a linear function of the syndiotactic diad mass fraction, with a maximum at Wst = 0.58 (Figure 6-11). If, however. 

Figure 8. NMR spectrum in CDCI3 of poly(1,2-epoxypropane) prepared by the polymerization with chloroaluminum tetraphenyl-porphyrin (1c). m, r, I, S and H represent meso, racemo (diad) and isotactic, syndiotactic, and heterotactic (triad) sequences, respectively.

Stereocontrol in the polymerization of these acrylates was more difficult compared with that in the polymerization of methacrylates. The specific rotations ([ajses ) of poly(TrA) and poly(PDBSA) obtained by the asymmetric polymerization were much smaller than those of the corresponding polymethacrylates prepared under similar conditions and were up to+102° (ligand PMP, diad isotacticity 70%) and -94° (ligand DDB, diad isotacticity 61%), respectively. The isotactic part of the polymers is considered to have a helical conformation with excess helicity. 

The syndiotactic-rich polymer, poly[3-(2,2,2-triphenylacetylamino)prop-ionic acid] (syndfo-poly[( -Ala-OH)-MA]), and the isotactic-rich polymer, (/so-poly[(j0-Ala-OH)-MA]), were examined to investigate the effect of polymer tacticity involving the side chain of the ligand upon CaCOs crystallization (Fig. 20). The diad contents for sy/7dm-poly[(y0-Ala-OH)-MA and zso-poly[(j0-Ala-OH)-MA] were determined as 77% and 70%, respectively, determined by the NMR spectra in solution of the ester-formed polymers. An extremely broad amide NH H-NMR signal for deproto-... 

Several complications are encountered in attempts to interpret the o-aromatic proton resonance patterns of poly(3,4,5-trideuteriostyrene) in terms of its microstructure. Firstly, the resonance patterns cannot be reconciled with a Bernoullian distribution of meso and racemic diads. It seems that the resonance of units centered in rrmr tetrads may occur in the lowest field area rather than in the central (heterotactic) area, as expected. When this assumption is made, the probability of an isotactic (meso) placement in polystyrene, as prepared by free radical methods at 50°, is 0.3, a rather reasonable value. Of course, it is also possible that the microstructure of polystyrene may not be describable by Bernoullian statistics. 

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