Chiral centre poly

Fig. 11 (a) Schematic polymer structure of poly-7 OEt. Phenylene rings are omitted in order to simplify, (b) Molecular model of repeating structure of poly-7 OEt. Four chiral centres on each of two cyclobutane rings in both sides are enantiomeric to each other. 

If the Me2C(CN)- group derived from [j ,)5- C]AIBN is attached to a monomeric unit without a chiral centre, as for poly(vinylidene chloride) or the 1,4-unit in poly butadiene, there is only one set of signals from the methyl carbons. The same result is found for poly(ethyl acrylate) [11] even though the monomeric unit possesses a chiral site in this case, the two sets must be almost coincident presumably because the terminal meso and racemic placements of the monomeric unit produce very similar shielding of the methyl groups in the initiator fragments. 

Figure 2. Possibilities for the placement of backbone chiral centres (tacticity) in poly(4-methyl cyclopentene)...

We can define the environment of each nucleus in terms of the cis or trans disposition of nearby double bonds, using as many double bonds as necessary to account for the fine structure observed in the spectrum (often the two nearest double bonds are sufficient, rarely more than three). Figure 1 shows an example using the two nearest double bonds in poly(4-methyl cyclopentene).Many ROMP polymers contain chiral or pseudo-chiral centres in the polymer backbone, and the relative placement of these centres will also affect the NMR spectrum observed. These effects are usually defined in terms of the relative stereochemistry of adjacent chiral centres in the backbone as meso or racemic. Figure 2 shows an example using a sample of poly(4-methyl cyclopentene) in which all the double bonds are trans. 

The first microstnictural issue of linear homopolymer chains that we examine is tacddty, which we illustrate with spectra from two systems from our own work the poly(alkyl <7anoacrylates) [—CHj—QCN)COOR—], which constitute a vinylidene systmn the spectra of which are shown in Figure 1.4, and the polyaJkene sulphides and sulphones [—CHj—CHR—S—] and [—CH2— CHR—SO2—], spectra of which are shown in Figure 1.5. We show tneso or m dyad structures oi two of these polymers in Scheme 4. Note how the two chiral centres of the first polymer appear to be equivalent, but for the second polymer the equivaloice is less immediatdy evident, for the residues contain three bonds... 

Once prepared [14], the two enantiomeric MPA polymers, poly-(f )-2 and poly-(5)-2, showed null CD spectra in a number of solvents, suggesting the presence of analogous populations of both helical senses. Thus, despite the presence of stereogenic centres at the pendants, the resulting polymer was racemic in its axial chirality. NMR, Raman and differential scaiming calorimetry (DSC) [15-18] studies pointed to cis-cisoid configurations at their polyene backbones. 

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