Meso-isotactic/syndiotactic configurations

In polymers made of dis-symmetric monomers, such as, for example, poly(propylene), the stmcture may be irregular and constitutional isomerism can occur as shown in figure C2.1.1(a ). The succession of the relative configurations of the asymmetric centres can also vary between stretches of the chain. Configuration isomerism is characterized by the succession of dyads which are named either meso, if the two asymmetric centres have the same relative configurations, or racemo if the configurations differ (figure C2.1.1(b )). A polymer is called isotactic if it contains only one type of dyad and syndiotactic if the dyad sequence strictly alternates between the meso and racemo fonns. 

The meaning of the notation uu should be explained. As there is never a (local) mirror plane or twofold axis expressing the relationship between two or three units in alkene/CO polymers, meso and rac cannot be applied. Instead one uses u and /, for unlike and like. Unlike means that two subsequent centres have different absolute configurations, thus they form a syndiotactic polymer, while like means that they have the same configuration and thus they form an isotactic polymer (see Figure 12.23). 

When it is necessary to specify the internal stereochemistry of the group, a prefix is required. In vinyl polymers there are meso (m) and racemic (r) diads and mm, mr, rr triads. The latter may be called isotactic, heterotactic and syndiotactic triads, respectively. Stereoregular vinyl polymers can be defined in terms of the regular sequences of diads thus an isotactic vinyl polymer consists entirely of m diads, i.e., it corresponds to the following succession of relative configuration -mmmmmm-, whereas a syndiotactic vinyl polymer consists entirely of r diads, corresponding to the sequence -rrrrrrr-. Similarly, a vinyl polymer consisting entirely of mr (= rm) triads is called a heterotactic polymer. 

The Fischer projections show that isotactic placement corresponds to meso or m-place-ment for a pair of consecutive stereocenters. Syndiotactic placement corresponds to racemo (for racemic) or r-placement for a pair of consecutive stereocenters. The configurational... 

In the simplest case, when the structure of the propagating chain does not affect the configuration of the generated diad, the formation probabilities of meso and racemic diads, Pm and Pr, are related as Pr = (1 — Pm). Chain structure obeys Bernoulli statistics as if the added units were selected at random from a reservoir in which the fraction Pm of the total amount is m, and the fraction (1-Pm) is r. An isotactic polymer will be formed for Pm - 1, and a syndiotactic polymer for Pm -> 0. Within these limits the chains will consists of randomly ordered m and r structures. 

Figure 1.6 Configurational sequences, (a) Spatial representations of meso (m) and racemic (r) configurations, (b) Planar projections of m and r configurations, (c) Isotactic (mm), heterotactic (mr), and syndiotactic (rr) triads.

PVA samples with different tacticities, such as isotactic (iso-), atactic (at-) and syndiotactic (syn-) ones were used. The degree of polymerization, and the fractions of mm, mr and rr triads, are shown in Table 20.2, where m and r indicate the meso and racemic dyads, respectively. The CP/MAS NMR spectra for the three kinds of PVA gels with different tacticity (9% of polymer concentration) are shown in Fig. 20.7. As described in Section 20.4.1, the CH peaks are composed of both the three sharp peaks corresponding to the triad configurations (mm, mr and rr) and the three broad peaks (I, II and III at about 77, 71 and 65 ppm, respectively). 

Much literature precedent supports the assignment of tacticity in methyl acrylate polymers using NMR techniques [40,41]. In the H-NMR spectrum, the shift of the methylene protons is sensitive to dyad stereochemistry. For example, in an isotactic (meso) dyad 28, the methylene protons are chemically non-equivalent and appear as two separate sets of signals, whereas in a syndiotactic (racemic) dyad 29, the methylene protons are equivalent. The H-NMR spectrum of 27 showed multiplets at 1.89 and 1.5 ppm due to the two diastereotopic methylene protons of the isotactic dyad. The rest of the spectrum is consistent with the structure of the n=4 tetrad 27. A racemic dyad structure would have been expeeted to give resonances of intermediate shift to that of the two resonances observed for the telomer 27. This evidence strongly implies that 27 has the allisotactic configuration shown in Scheme 8-12. 

Discussion of the effect of the polymer backbone in Sec. 3.4 of this chapter already provided examples of highly isotactic po-ly(f )-endo,exo-5,6-di [n-[4 -(4"-methox-yphenyl)phenoxy]alkyl]carbonyl bicy-clo[2.2.1]hept-2-ene s [190] and syndiotactic poly n-[4 -(4"-methoxyphenyl)phen-oxyjalkoxy methacrylate s [42, 44] which crystallize and form more ordered meso-phases than those of the corresponding atactic polymers (Fig. 15). Although more flexible backbones are more able to achieve the conformation necessary to order, the side chains are evidently already attached to the polymer backbone of these tactic polymers with the proper configuration to order, which obviates the need to distort their conformation for such purposes. 

There are three types of monomer insertions with respect to the pendent methyl groups the meso, racemic. [14]. Meso insertion produces a polymer with the methyl groups in the same spatial position, which is referred to as isotactic polymer racemic insertion produces a polymer with the methyl groups in alternating locations, referred to as the syndiotactic polymer. When the monomer insertion is random and nonstereospecific, a noncrystalline atactic polymer is produced. These three forms of polypropylene are schematically represented in the following chain configurations ... 

Figure 3-2. The various configurations of propylene heptamers. The arrows point to the pseudo-asymmetric carbon atoms. (I, IV) Isotactic ( meso"). (II, III) Syndiotactic ( racemic ).

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