Racemic diad

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. 

The second pair contains racemic diads and is syndiotactic by the meso, racemic argument but with the chloride atoms on the same side. 

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 influence of the chain expansion produced by excluded volume on the mean-square optical anisotropy is studied in six types of polymers (PE, PVC, PVB, PS, polylp-chlorostyrene), polylp-bromostyrenel. RIS models are used for the configuration statistics of the unperturbed chains. The mean-square optical anisotropy of PE is found to be insensitive to excluded volume. The mean-square optical anisotropy of the five other polymers, on the other hand, is sensitive to the imposition of the excluded volume if the stereochemical composition is exclusively racemic. Much smaller effects are seen in meso chains and in chains with Bernoullian statistics and an equal probability for meso and racemic diads. 

The configurational-conformational characteristics of PP are discussed by considering every polymer chain as constituted by the periodic repetition of a sequence of monomeric units in a given configuration. Calculations are presented for the special case in which mesa and racemic diads are distributed according to Bemoullian statistics. Numerical results show that the characteristic ratio of atactic PP reaches an asymptotic value of 5.34 when the size of the periodic sequence corresponds to six monomeric units. 

The proton NMR spectrum of a highly isotactic sample of PP is examined. The polymer is shown to contain 2% racemic diads occurring randomly at junctions of isotactic sequences of opposite configurations. The mean-square end-to-end distance of this polymer is measured under 9 conditions. Comparison of the value obtained with theoretical predicitons of Flory, Mark, and Abe [J. Am. Chem. Soc. 1966, 88, 639] permit an approximate measurement of the strength of the steric interactions within the PP chain. 

Conformational energies of meso and racemic diads of PS are computed as functions of skeletal bond rotations. Confinement of rotations of the phenyl groups to a small range within which they are nearly perpendicular to the plane defined by the two adjoining skeletal bonds is confirmed. Steric interactions involving the relatively large planar phenyl group virtually preclude"g" conformations. A simple, two-state RIS scheme is applicable with states at 170° and 70° for both meso and racemic dyads. 

The depolarization of light scattered at 90° by PP and PS is treated according to RIS theory. Numerical calculations are carried out as functions of the statistical weight parameter to governing interactions of second order, of the locations of the rotational states, of the chain length expressed by the number x of repeat units, and of the stereochemical composition expressed by the fraction of racemic diads. 

The dynamic RIS model, which was proposed before to investigate the dynamics of local conformational transitions in polymers, is elaborated to formulate the increase in the number of excimer-forming sites through rotational sampling. Application of the model to the meso and racemic diads in PS confirms the fact that conformational mobility of the chain plays a major role in intramolecular exclmer formation. Comparison with experiments demonstrates that the decay of the monomer fluorescence in styrene dimers is predominantly governed by the process of conformational transitions. 

Conformational energies as function of rotational angles over two consecutive skeletal bonds for both meso and racemic diads of poly(Af-vinyl-2-pyrrolidone) are computed. The results of these calculations are used to formulate a statistical model that was then employed to calculate the unperturbed dimensions of this polymer. The conformational energies are sensitive to the Coutombic interactions, which are governed by the dielectric constant of the solvent, and to the size of the solvent molecules. Consequently, the calculated values of the polymeric chain dimensions are strongly dependent on the nature of the solvent, as it was experimentally found before. 

Conformational features of meso and racemic diads of PVAc are examined using energy calculations. In contrast to other vinyl chains bearing planar substituents, the g conformation is not prohibited for this polymer. The shifts in the positions of the energy minima from perfect staggering are discussed in terms of the second order interactions. Calculated statistical weight parameters are used to reproduce the experimental data on NMR coupling constants and the characteristic ratios. 

In an effort to correlate the conformational features of polysilane derivatives with their properties, calculations are performed to determine the relative stabilities of the conformational states of the meso and racemic diads of polysilapropylene. Energy maps are constructed in terms of internal rotation angles to calculate the average properties of the chain. The calculations show that the difference In energy between the various states of the meso and racemic dlad Is small. Hence, PSP can be considered to be more flexible than the analogous carbon polymer, PP. The characteristic ratios of the unperturbed end-to-end distances for the /so- and syndiotaclic PSP are less than those for the PP of corresponding tacticity. 

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. 

Finally, the stereoselectivity of most cationic vinyl polymerizations is poor due to the sp2 hybridization of carbenium ions at carbon. In this case, attack from either side of the plane has approximately equal probability leading to similar proportions of meso and racemic diads. For the sufficiently bulky substituent(s) tacticity control improves for example, highly syndiotactic (>90%) poly(a-methylstyrene) can be prepared by cationic polymerization (70]. 

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. 

---