Meso dyads

Robyr P, Gan Z and Suter U W 1998 Conformation of raoemo and meso dyads in glassy polystyrenes from C polarization-transfer NMR Macromolecules 31 8918- 23... 

The same group of hydrogens in a meso dyad [XIII] produce a quartet of peaks two different chemical shifts, each split into two by the two hydrogens in the methylene. 

The area under the methylene peaks is proportional to the dyad concentration The singlet gives the racemic dyads and the quartet gives the meso dyads. 

Table 7.9 lists the estimated fractions of dyads of types m and r and the fractions of triads of types i, s, and h. These fractions represent the area under a specific peak (or four peaks in the case of the meso dyads) divided by the total area under all of the peaks in either the dyad or triad category. As expected for the sample labeled isotactic, 89% of the triads are of type i and 87% of the dyads are of type m. Likewise, in the sample labeled syndiotactic, 68% of the triads are s and 83% of the dyads are r. 

The number of racemic dyads in a sequence is the same as the number of syndiotactic units n. The number of meso dyads in a sequence is the same as the number of iso units nj. These can also be verified from structure [XVIII] above. 

The configuration of a center in radical polymerization is established in the transition state for addition of the next monomer unit when it is converted to a tetrahedral sp1 center. If the stereochemistry of this center is established at random (Scheme 4.1 km = k,) then a pure atactic chain is formed and the probability of finding a meso dyad, P(m), is 0.5. 

The cyclization rate kc is higher for B B than for B B dyads and a drastic influence of tacticity is observed, the meso dyads being by far more reactive than the racemic ones,as expected (8). Some characteristic Kc values measured at 20°C in NaOH 0.1 N for different systems are collected in table 5. 

These definitions are clarified by considering a portion of a polymer chain such as XVII. Chain segment XVII has a total of 9 repeating units but only 8 dyads and 7 triads. There are 6 meso dyads and 2 racemic dyads (m) —, (r) —, There are 4 isotactic, 2 heterotactic, and 1 syndiotactic triads mm) = (mr) — j, (rr) — A. 

The conformational statistics of asymmetric vinyl chains such as P2VN are well-known 126). The rotational conformers of isotactic (meso) dyads are entirely different from those of syndiotactic (dl) dyads. Frank and Harrah132) have described each of the six distinct conformers for meso and dl dyads, using the t, g+ and g nomenclature of Flory 126). Excimer-forming sites (EFS) are found in the tt and g g+ meso states, and in the degenerate tg , g t dl state. Because the rotational conformers of compounds such as l,3-bis(2-naphthyl)propane do not match those of either the iso-or syndiotactic dyads of P2VN, the propane compounds make poor models of aryl vinyl polymers. However, the rate constants of fluorescence and decay of the intramolecular excimer in polymers can usually be determined from the propane compounds (but see the exceptional case of PVK and its models133)). 

While the study of diarylpentanes is helpful in understanding the conformational behavior of aryl vinyl polymers, a simple weighting of the properties of the model compounds by the tacticity of the polymer does not yield the properties of the polymer. For example, the presence of dl dyads surrounding a meso dyad will suppress the tt conformer in the meso dyad 14fl). Thus, in order to obtain the fraction of tt meso conformers within an atactic P2VN sample, it is necessary to resort to a Monte Carlo calculation utilizing an extended product of statistical weight matrices, 26). 

Intermolecular EFS and intramolecular EFS formed by rings separated by more than three backbone atoms can be neglected in dilute miscible blends. EFS are primarily located at meso dyads in the tt conformation. 

Recently, Ewen 1291 has found that the soluble Cp2Ti(Ph)2 (Ph = phenyl)/methyl-alumoxane catalyst produces isotactic polypropylene ([m] = 0.83-0.85) at temperatures below —30 °C. The polymerization was carried out in the temperature range of —85 to 50 °C. The highest activity was achieved at —45 °C, and the isotactic (meso) dyad fraction [m] of the produced polypropylene decreased from 0.85 to 0.50 with an increase in the polymerization temperature. Figure 28 shows the time dependence of polymer yield, Mn, number of polymer chain produced per titanium atom, [N], and mjmn, obtained at —60 °C. The yield of polymer is almost proportional to time, but Rln increases to a constant value. The number of polymer chains [N] increases with time, and the value of Mw/Mn increases toward 2.0, indicating that chain transfer... 

Fig. 6. A The characteristic ratio as a function of stereoregularity, calculated for a system with R = (CH2)zCH3(z > 1). The parameters used are E /RT = 2.3, E / /RT = 5 1 and A = 20°, respectively. From Ref. 63). B The characteristic ratios for Monte Carlo chains of 100 units each as a function of fj, the fraction of meso dyads in the chain. The curve shown above represents results of calculations carried out with Et"/RT = EW/RT = 5. From Ref. 63). C The characteristic ratios for Monte Carlo chains of 200 units each as a function of fj, the fraction of meso dyads in the chain. The curve shown above represents results of calculations carried out for a temperature of 140 °C with the conformational parameters chosen as indicated, in cal. mol. The experimental values of Bovey and Heatley85) (a) are shown. From Ref.65). D The characteristic ratios for Monte Carlo chains of 200 units each as a function of fj, the fraction of meso dyads in the chain. The values of Ea and Ep in that order, in kcal mol-1, are marked on the curves. The experimental values of Cowie and Bywater81) ( ), and Noda et al.80) (a) are shown. From Ref.6 ). E The characteristic ratios for Monte Carlo chains of 200 units each as a function of fj, the fraction of meso dyads in the chain. Curves are shown for (a) Ea = 1.0, Ep = -0.6, and 6 = 58° (h) Ea= 1.2, Efl= -0.6, and 6 = 58° (c) Ea= 1.2, Ej8= -0.2, and = 56°, energies being in kcal mol-1. The experimental results of various authors are represented by points as follows Katime et al. ( )75), Katime and Roig ( ), Sakurada et ah ( )7°), Fox ( )...

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. 

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