Racemic dyad configuration

When measuring vinyl polymer tactlclty, one prefers the longest complete n-ad distribution available as well as the translated simplest comonomer distribution, possibly m versus r. An alternative exists to the m versus r distribution In the form of number average or mean sequence lengths. If any vinyl homopolymer Is viewed conceptually as a copolymer of meso and racemic dyads, mean sequence lengths can be determined for continuous runs of both meso and racemic configurations (32), that Is,... 

In addition to viewing a vinyl homopolymer conceptually as a copolymer of meso and racemic dyads, one may also consider the mean sequence length of "like" configurations (M). In this Instance, the polymer chain Is seen as a succession of different lengths of co-orlented configurations from one to "n", the longest sequence of like configurations, that Is,... 

As was true in the case of mean sequence lengths for meso and racemic dyads, the necessary relationships can be used to develop corresponding equations for any particular n-ad distribution. A favorable point concerning the concept of "like" configurations is that it attaches a physical significance to the racemic distribution. 

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. 

Carbon-13 nuclear magnetic resonance (NMR) is the most useful method of assessing tacticity. By C-13 NMR it is possible to assess the different sequential distributions of adjacent configurational units that are called dyads, triads, tetrads and pentads. The two possible dyads are shown in Fig. 1.5. A chain with 100% meso dyads is perfectly isotactic whereas a chain with 100% racemic dyads is perfectly syndiotactic. A chain with a 50/50 distribution of meso and racemic dyads is atactic. 

Configurational aspects are apparent in publications concerning the fluorescence characteristics of polymers of differing tacticity and of model compounds. It has been shown that the intensity ratio of excimer to monomer fluorescence is greater in fluid solutions of isotactic polystyrene [62,63] and poly(p-methyl-styrene) [64,65] relative to that of the atactic polymers. This phenomenon has been attributed in the case of poly(p-methy1styrene) to the existence of a lesser energy barrier to excimer formation in meso dyads compared to the racemic dyad [65]. Similar conclusions of direct relevance to excimer formation in polystyrene were made by Bokobza et al [55] in studies of intramolecular excimer formation in model compounds. 

It should be stressed that this treatment of polymer stereochemistry only deals with relative configurations whether a substituent is "up or down" with respect to that on a neighboring unit. Therefore, the smallest structural unit which contains stereochemical information is the dyad. There are two types of dyad meso (m), where the two chiral centers have like configuration, and racemic /-), where the centers have opposite configuration (Figure 4.1). 

Although the definitions of isotactic, syndiotactic, and atactic polymers according to International Union of Pure and Applied Chemistry (IUPAC) rules are well established in terms of succession of mesa (m) or racemic (r) dyads,12 the symbolism of (+) and (—) bonds allows the easy treatments of possible configurations in cases of any complexity.1 Moreover, the (+) or (—) character of the bonds in a polymer chain is strictly related to the accessibility of gauche+ or gauche conformations of the bonds and, therefore, to the formation of right-handed or left-handed helical conformations.1... 

Another important application of NMR to polymer systems is the elucidation of the stereochemical configurations of Polymer chains. Poly (methyl methacrylate) was first studied by Bovey in 1960. It is now possible to analyse for the statistical frequency of occurrence of all possible combinations of up to four successive pairs of units (dyads) capable of occurring with either the same (meso) or opposite (racemic) configurations. 

The dyads in commercial poly(vinyI chloride) (PVC) are about 0.55% racemic, indicating short runs of syndiotactic monomer placements. The absence of a completely atactic configuration is reflected in the low levels of crystallinity in this polymer, which have a particular influence on the processes used to shape it into useful articles. 

Broadening this comparison to include copolymers prepared by both early and late transition metal catalysts, the results discussed immediately above show that Ci-symmetric zirconocenes such as 9/MAO produce only copolymers with isolated norbornene units or alternating structures (at 30 C), mainly with isotactic (meso) configurations. C2-symmetric zirconocenes such as 2/MAO readily produce norbornene dyads that are exclusively meso-linkcd (isotactic). In accordance with their catalyst structures, Q-symmetric zirconocenes such as 8/MAO produce norbornene dyads with a rac-linkage (syndiotactic), although with a generally lower stereoselectivity. Palladium a-diimine catalysts, despite the homotopic nature of their coordination sides (that would be expected to give a mixture of meso and racemic blocks), produce norbornene dyads that are solely rac-connected. This behavior can be attributed to a chain-end control type polymerization mechanism. 

Tacticity arises in these polymers from the relative orientation of the five membered rings in the polymer backbone as shown in structure (32). Thus, depending on the configuration of the adjacent double bond c/ -syndiotactic cis racemic, c ), cis isotactic cis meso, c ), trans syndiotactic trans racemic, t ) and /raw -isotactic trans meso, /m) dyads result. 

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