Meso-diads

When adjacent monomers in a backbone share the same stereoconfiguration, the placement is known as a meso diad. When adjacent monomers have opposing stereoconfigurations, the placement is known as a racemic diad. Thus, a purely isotactic polymer comprises all meso placements, and a syndiotactic polymer consists of all racemic placements. 

Stereodefects are the result of one or more racemic diads interrupting a sequence of meso diads. Figure 5.7 illustrates the two principal types of stereodefect. In Fig. 5.7 a), a single racemic placement results in the subsequent methyl groups being placed on the opposite side of the chain from those of the preceding sequence. In Fig 5.7 b), a pair of racemic placements interrupts the meso sequence. In this case, both the meso sequences have their methyl groups on the same side of the chains. 

Note 2 An isotactic polymer consists of meso diads. 

In principle, each enchainment of a prochiral monomer such as propene generates a chiral carbon center. The relative orientation of two such centers can therefore give rise to meso-diads or racemo-diads. Three monomers consecutively enchained in meso-fashion give rise to a mm triad, and so forth (Scheme 8.37). 

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. 

The third group consists of mixtures of racemic and meso diads. These sequences are given the name atactic or having nothing to do with tacticity or orderly arrangement. ... 

It is evident that in the racemic diade both -CH2- protons are imbedded into an identical microenvironment. Consequently, they are magnetically equivalent, absorb at the same resonance frequency v (have the same value of 6), and do not couple with each other. Therefore, the proton in a racemic diade appears as a singlet in the NMR spectrum. For the meso diade, on the other hand, it is obvious that the two -CH2- protons have a clearly different micro-environment while has two methyl groups as neighbors, there are the ester groups for proton H. Consequently, the two -CH2- protons of the meso diade are magnetically nonequivalent, absorb at different resonance frequencies Vj, and V , respectively, and couple with each other. Therefore, these protons in a meso diade appear as a set of two doublets in the NMR spectrum. 

The mean-square dipole moments of PPCS chains are calculated as a function of stereochemical composition using the RIS analysis recently published for PS. The calculations are in good agreement with the average of experimental results for atactic PPCS chains estimated to contain ca. 35% meso diads. The temperature coefficient is calculated to be negative in agreement with available experiments. 

The optical anisotropies y and molar Kerr constants mK of model compounds, and of polylp-chlorostyrene) and polyfp-bromostyrene), are determined. Averages and over all conformations of the polymer chains are calculated as functions of the fraction wm of the meso diads using the RIS model originally developed for PS. 

Fig. 9. Schematic drawing of the racemic and meso diad of polyfa-olefins)...

In sterically random propagation, the probability of meso diad generation Pm in each addition is equal to 0.5, and the ratio of triads in the chain is i h s = = 1 2 1. In an atactic polymer, the diads m and r are statistically distributed in the chain. Natta [81] called the simplest stereoregular chains isotactic —mmmm— and syndiotactic —rrrr—. Monomers with both a and / carbons unsymmetrically substituted can form diisotactic polymers... 

FIGURE 2 11 (Top) meso diad (m) (bottom) racemic diad (r). 

Two adjacent monomer units in the same chain are called meso diads when they have the same configuration, as in Figure 7-39, where it can be seen that the substituent lies on the same side of the chain when it is held in an extended conformation. In a racemic diad, snch as that shown in Figure 7-39, the substituents are opposite one another across the extended chain. 

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