Tacticity racemic

The presence of hydroxyl end groups in PPG can be assigned from 13C NMR data (Figure 24), with resonances (g) observed between 65-68 ppm in the spectrum shown in Figure 24. There are two different resonances here (g) due tacticity effects in the polymer (meso/racemic (m/r) isomerisation of the terminal dyad). Estimations of the amount of these ends in the polymer, along with the average molecular weight of the PPG, can be made from the 13C NMR data. 

In figure 1 we present the experimental and calculated mK values of the copolymer poly(styrene-co-p-bromostyrene). From this study (3) we were able to show unequivocally that the tacticity of this polystyrene sample is pr — 0.55, where pr is the probability of racemic dyad replication. 

NL Benoiton, YC Lee, FMF Chen. Racemization during aminolysis of activated esters of IV-alkoxycarbonylamino acids by amino acid anions in partially aqueous solvents and a tactic to minimize it. Int J Pept Prot Res 41, 512, 1993. 

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. 

When the reaction is carried out with a racemic mixture of complexes, the product is a racemic mixture of the isotactic polymers. It was of interest to see what would happen if, after formation of a chiral block with one enantiomer of the bisoxazoline ligand, an equivalent of the other enantiomer was added. It was found that an excess of ligand changes the tacticity completely and the second block was syndiotactic In these diimine palladium complexes exchange of ligand is relatively fast and it can often be observed on the NMRtime scale as a broadening in the H NMR spectra. The process may well be associative. 

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. ... 

While the situation with respect to simple vinyl polymers is straightforward, the tacticity and geometrical arguments are more complicated for more complex polymers. Here we will only briefly consider this situation. Before we move to an illustration of this let us view two related chloride-containing materials pictured below. We notice that by inserting a methylene between the two chlorine-containing carbons the description of the structure changes from racemic to meso. Thus, there exists difficulty between the historical connection of meso with isotactic and racemic with syndiotactic. 

To date, no catalytic system has been found that converts racemic PO to tactic PHBs. Since the synthesis of enantiopure PO is very expensive, only racemic PO can be used on an industrial scale. Therefore, other pathways to yield feasible materials are area of investigation. 

Although exhaustive efforts have been made in the search for biologically acceptable catalysts, there are only a few examples of low toxicity, which mainly lead to atactic polymers of little practical use. Another route to gain control over the tacticity of PHB is the transformation of cheap building blocks to enantiomericaUy pure p-BL, which can be distilled off from the catalyst and polymerized with retention of the stereochemistry by ecofriendly initiators. This route combines many advantages. At first, even toxic metal centers can be chosen since the product can easily be separated from the catalyst and secondly, any tacticity of the polymer will be available by simply mixing enantiopure p-BL with the racemic mixture in the desired ratio. In this manner a fine-tuning of the mechanical properties becomes possible and easily performable (Fig. 36). 

The most promising route towards variable isotacic PHB is a process in which PO is transferred to both enantiopure and racemic (3-BL in two parallel processes (Fig. 40). As mentioned, these monomers can be distilled off from the catalysts and polymerized directly. Thus, any degree of tacticity in the polymer can be adjusted by mixing racemic and enantiopure (3-BL, which would allow the preparation of tailor-made materials from the low-cost oil-based monomers PO and CO (cf. Sect. 8). 

Table I Tacticities of samples obtained in DME and THF, using mono- and bifunctional initiators and Na+ and Cs+ as the counterions. I,H,S isotactic (ami), heterotactic (mr/rm) and syn-diotactic (rr) triads m,r meso and racemic dyads p 2mr/H persistence ratio ( p 1 for Bernoullian statistics).

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. 

Wicke and Elgert 238) have concluded that the tight ion pair, the loose ion pair and the free carbanion of poly-a-methylstyrene generate the same tacticity. With n-butyllithium as the initiator in THF, the activation energy for meso propagation is a little greater than that for racemic propagation (15.6 and 14.9 kcal/mole respectively) the pre-exponential factors are the same 239). 

The microstructure of the discussed cycloaliphatic polymers concerns the cis-trans geometrical isomerism of the rings and the relative stereochemistry between the rings. A modified Bovey m-r nomenclature [507] provides a useful description of the microstructure of poly(methylene-l,3-cycloalkane)s, where capital letters (M for mesogenic, R for racemic) denote the stereochemistry of the rings and lower case letters ( m and r) denote the relative stereochemistry between the rings [503], Therefore, cA-isotactic, tram-isotactic, cA-syndiotactic and tram-syndiotactic cyclopolymers may be formed. As in many other cases, 13C NMR spectroscopy reveals information about both the tacticity of the polymer and the ratio of cis to treins rings. 

FIGURE 14.3 Nuclear spin symmetry relationships for main-chain radical la from PMMA, as a function of polymer tacticity. The two radicals shown have mirror plane ( meso ) or C2 axis ( racemic ) symmetry elements establishing magnetic equivalence of each set of 3-methylene protons. 

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]. 

Ungerank et al. investigated the influence of different molybdenum Schrock-type initiators 1, 2 and 3 (see Fig. 2) on the polymerization of ( )- and (-)-exo,e tdo-bis[4 -cyanobiphenyl-4-yl)oxyalkyl]norborn-5-ene-2,3-dicarboxy-lates [( )-IV-n and (-)-IV-n] [47]. The initiator had a strong influence on the Z/E ratio of the double bond in the polymer chain and therefore also on the tacticity of the polymer chain. Using racemic 2,3-disubstituted norbornene derivatives, such as ( )-IV-n, the two adjacent monomers (diads) can in principle form eight different stereoisomers, as depicted in Fig. 5 [48]. 

CHCHj-) where all the methyl (CH3) substituents are on the same side are shown schematically in Figure 2-12. (The methyl carbon atoms are shown in red to make them easier to see.) We call this arrangement isotactic, so this polymer is isotactic polypropylene. Because of steric repulsions between CH3 groups on adjacent units, the chain would not want to sit in this planar zigzag shape or conformation, but would fold into a different shape by rotating the polymer backbone bonds. More on this when we discuss conformations. Also shown in Figures 2-13 and 2-14 are two more polypropylene chains. One of these consists of units that are all racemic to one another and is called syndio-tactic. The other has a random arrangement of units and we call such chains atactic. 

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