Threodiisotactic polymers

Fig. 8-14 Syn addition to a trans 1,2-disubstituted ethylene to yield the threodiisotactic polymer. After...

The polymerization cir-l-rf-propene by traditional Ziegler-Natta initiators in hydrocarbon solvents yields the erythrodiisotactic structure, while under similar solvent conditions anionic polymerization of cis- -d-methyl acrylate yields the threodiisotactic polymer. Explain the factor(s) responsible for this difference. 

Fig. 1.2 Steric arrangement in macromolecules (R = phenyl), (a) Isotactic polymer, (b) syndiotactic polymer (c) erythrodiisotactic polymer, and (d) threodiisotactic polymer...

While most polymers contain only one chiral or asymmetrical center in each repeat unit, it is possible to have diisotacticity where two different substituents are present at chiral centers. These isomers are labeled erythro- and threodiisotactic and erythro- and threosyndiotactic isomers (Figure 2.6). This topic is further described in Appendix J. 

II. 2.1.4. Cyclic Olefin Polymers Benzofuran (16) gives an optically active polymer by cationic polymerization with AlEtCl2 or A1C13 in the presence of an optically active cocatalyst such as P-phenylalanine and 10-camphorsulfonic acid [12,48-50], The optically active polymer is considered to have an erythro- or threodiisotactic structure with no plane of symmetry. Initiator systems of AlCl3/(-)-menthoxytriethyltin, -germanium, and -silicon also give an optically active polymer [51,52],... 

Explain what is meant by erythrodiisotactic, threodiisotactic and polymers. Illustrate. Do the same for erythrodisyndiotactic and threodisyndiotactic. 

When 1,2-disubstituted olefins are polymerized with Ziegler-Natta catalysts, the ditacticity of the products depends on the mode of addition. It also depends on the structure of the monomer, whether it is cis or trans. A threodiisotactic structure results from a syn addition of a trans monomer. A syn addition of a cis monomer results in the formation of an erythrodiisotactic polymer. For instance, cis and /ra/i -l- Z-propylenes give erythro and threo diisotactic polymers, respectively. To avoid 1,2-interactions in the fully eclipsed conformation, the carbon bond in the monomer units rotate after the addition of the monomer to the polymeric chain. 

Helix-sense-selective polymerization has also been attempted for several bulky monomers related to acrylates including an acrylonitrile derivativeand a-substituted acry-lates. " " Triphenylmethyl crotonate (TrC, 53) affords optically active helical polymers by the polymerization using DDB-FlLi and PMP-FlLi complexes. The polymers possess a nearly complete threodiisotactic structure. Although the polymers indicate relatively small specific rotation ([a]o 5.6° and 7.4 ° for the samples with DP =15 and 36, respectively), the optical activity is considered to be based on an excess helicity because the rotation was lost when the polymers were converted to methyl esters. 

Cyclic olefins afford optically active polymers by asymmetric synthesis polymerization. The first example was the polymerization of benzofuran (265) with AlEtCl2 or AICI3 in the presence of optically active cocatalysts such as p-phenylalanine and 10-CSA the polymer is considered to possess the chiral erythro- or threodiisotactic structure (266a or 266b)... 

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