Meso placements

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. 

Figure 2.19 Sketches of some stereochemically variable chains, of polypropylene (two meso placements), poly(methyethylphosphazene) (two racemic placements), and poly(methylphenyl-siloxane) (meso placement followed by racemic placement).

In a triad, the focus is on the central methine between two neighboring monomer residues. An isotactic triad (mm) is produced by two successive meso placements ... 

These equations are similar to those assumed for the reactivity ratio determination. In contrast to what has been observed for conventional styrene-MMA copolymers, however, these equations indicate that a substantial proportion of the (SMM+MMS)-type resonance appears to occur in the C-area. The proportion of methoxy resonance observed in the C-area, in fact, exceeds P(SMS) by a substantial amount for many of the copolymers. This can be due to the assumption of an inadequate model for the copolymerization reaction, to the use of incorrect reactivity ratios and cyclization constants for the calculations or to an inadequate understanding of the methoxy proton resonance patterns of S/MMA copolymers. It is possible that intramolecular reactions between propagating radicals and uncyclized methacrylic anhydride units present on propagating chains result in the formation of macrocycles. Failure to account for the formation of macrocycles would result in overestimation of rc and rc and in underestimation of the proportions of MMA units in SMS triads in the derived S./MMA copolymers. This might account for the results obtained. An alternate possibility is that a high proportion (>50%) of the M-M placements in the copolymers studied in this work can be expected to have meso placements (], J2), whereas only a small proportion of such placements ( 20%) are meso in conventional S/MMA copolymers. Studies with molecular models (20) have indicated that the methoxy protons on MMA units centered in structures such as the following can experience appreciable shielding by next nearest styrene units. 

Assuming Bernoullian statistics, V=(l-a)2/a2, where a is the probability of a meso placement in the completely equilibrated polymer (or in polystyrene). V values of 0.62-0.65 thus imply that polystyrene can be characterized by a a value of 0.44. 

A typical isotactic PP has 96-99% of mmmm pentads. Modelling of the crystallinity of isotactic PP assumed that at least 14 successive meso placements are necessary for a crystal to form this corresponds to the smallest observed crystallite of 3nm length. Figure 2.6a shows such sequences, in 100 chains having a = 0.9, as straight lines. Sequences were slid by one another to maximise the number of like sequences, indicating the likely crystalline areas (Fig. 2.6b). The model predicts that a needs to be 0.95 for the PP to have a crystallinity of 50%. 

Spin-spin interactions between nuclei in copolymer chains also cause signal broadening and interfere with resolution. Such interactions make it difficult to obtain information about copolymer structure from studies of methine or methylene resonance in copolymers. Methylene protons in racemic placements in vinyl copolymers should be detected as triplets whereas those in meso placements should yield twelve peak patterns. The high multiplicity of such signals causes them to be broad and this can hinder resolution efforts if the chemical shifts of the uncoupled methylene protons in the various sequences do not differ appreciably. 

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. 

We have Instead derived four simpler cases that are consistent with the pentad intensity distributions. In the general case and the two site model we treat the polymers as having an. ..rrrrrrmmrrrrrrmrrrrrrr... microstructure and as being a mixture of. ..rrrrrmmrrrrr.. and. ..rrrrrmrrrrr... polymers. In the final analysis we simplify the problem by treating the statistics of the m and mm triads individually assuming the events leading to each of them are Independent of each other. It is shown that the variation in the meso placements in the slurry polymerizations with low monomer concentrations is consistent with isomerization reactions of coordinatlvely unsaturated complexes. 

When the relative amounts of various stereosequences can be measured by nmr, it is possible to discern what statistical relationships exist among the various structural features and thus learn something about the stereo-regulating processes occurring during polymer formation. In such considerations, stationary probabilities (e.g., P(m), P(mr), etc.) may be used to represent the relative amounts of dyads, triads and n-add placements and conditional probabilities (e.g., P(m/m), P(r/mm), etc.) may be used to represent the probabilities that m or r placements follow particular sequences in the polymer. The expression P(m/r), for example, refers to the probability that a meso placement follows a racemic placement in a polymer chain. 

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