Erythro-tacticity

Double bonds present along a polymer chain are stereoisomeric centers, which may have a cis or trans configuration. Polymers of 1,3-dienes with 1,4 additions of the monomeric units contain double bonds along the chains and may contain up to two stereoisomeric tetrahedral centers. Stereoregular polymers can be cis or trans tactic, isotactic or syndiotactic, and diisotactic or disyndio-tactic if two stereoisomeric tetrahedral centers are present. In the latter case erythro and threo structures are defined depending on the relative configurations of two chiral carbon atoms.1... 

The combination of cis-trans isomerism with iso-syndio and erythro-threo dispositions gives complex stractures as exemplified by the 1,4 polymers of 1-or 4-monosubstituted butadienes, such as 1,3-pentadiene (72, 73), and 2,4-pentadienoic acid (74, 75) and of 1,4-disubstituted butadienes, for example, sorbic acid (76). This last example is described in 32-35 (Scheme 6, rotated Fischer projection). Due to the presence of three elements of stereoisomerism for each monomer unit (two tertiary carbons and the double bond) these polymers have been classed as tritactic. Ignoring optical antipodes, eight stereoregular 1,4 structures are possible, four cis-tactic and four trans-tactic. In each series (cis, trans) we have two diisotactic and two disyndiotactic polymers characterized by the terms erythro and threo in accordance with the preceding explanation. It should be noted that here the erythro-threo relationship refers to adjacent substituents that belong to two successive monomer units. 

The optical activity of the erythro-di-iso-trans-tactic polysorbates taking into account the order of magnitude of their molecular weight, which was drawn from viscosity measurements and from the number of... 

To rigorously confirm the microstructure of the oligomers, and by analogy the polymer, crystals of dimer 5 were grown from cold methylene chloride and subjected to single crystal X-ray diffraction analysis. An ORTEP plot of 5 is presented in Fig. 4.21 along with appropriate labehng. The tacticity of this diad was determined to be erythro-diisotactic. 

Table 9. Erythro selectivity [e] in the initiation process and main chain tacticity of propylene polymerization. 

FIGURE 6.1 Newman projections of various stereoregular forms (a) isotactic, (b) syndio-tactic, (c) atactic, (d) erythro-di-isotactic, (e) threo-di-isotactic, and (f) di-syndiotactic. 

Tacticity — Chlorinated-frans-PBD Diisotactic poly(erythro- (6,10)... 

Several possibilities exist for ditacticity in macromolecules formed by polymerizing 1,2-disubstituted ethylenes of the type RHC=CHR, structures (XVIII)-(XX). It can be seen that each unit contains two different asymmetric carbon atoms in the chain. The original definitions of tacticity have been extended to include these modifications and Newman s (1956) definitions of erythro and threo structures. Structures (XVIII)-(XX) illustrate ifereo-diisotactic, erythro-diisotactic, and disyndiotactic poisoners, respectively. 

Obviously as the complexity of the monomer increases, greater orders of tacticity are possible. To date only one form of a tritactic polymer has been isolated, and that by polsonerizing an ester of trans-trans sorbic acid (CH3CH=CHCH=CHCOOR) (Natta et al., 1960b). The polymer, which contained three elements of stereoisomerism, two optical and one geometrical, was identified as erythro-diiso-trans tactic. 

Tactic polymers exhibiting tacticity, i.e., regularity in the stereochemical configuration of its constitutional repeating units isotactic, syndiotactic, erythro, and threo. 

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