Configuration isotactic

Tacticity of products. Most solid catalysts produce isotactic products. This is probably because of the highly orienting effect of the solid surface, as noted in item (1). The preferred isotactic configuration produced at these surfaces is largely governed by steric and electrostatic interactions between the monomer and the ligands of the transition metal. Syndiotacticity is mostly produced by soluble catalysts. Syndiotactic polymerizations are carried out at low temperatures, and even the catalyst must be prepared at low temperatures otherwise specificity is lost. With polar monomers syndiotacticity is also promoted by polar reaction media. Apparently the polar solvent molecules compete with monomer for coordination sites, and thus indicate more loosely coordinated reactive species. 

It is known the case of i-PP, for which the copolymerization with small amounts of ethylene tends to stabilize the y form [84] for instance, by melt crystallization of a copolymer with 6% by mol of ethylene more than 80% of the crystalline phase is in the y form [85], It is also known that the obtainment of the y form by melt crystallization, is also favored for samples of low molecular mass [86, 87] and for stereoblock fractions [88]. This seems to suggest that, whenever the preferential crystallization of the y-form is observed, there is the concomitant occurrence of a reduction in the polymer of the length of the chain stretches with polypropylene head to tail constitution and isotactic configuration. 

According to this scheme the fundamental difference in the mechanism of free-radical copolymerization of MA with TASM and of MA with alkyl acrylates is due to the fact that in the former copolymerization intermolecular coordination is involved. This coordination is similar to the effect of various complexing agents (ZnCl2, SnCft and A1C13) on free-radical homo- and copolymerization of vinyl monomers. This effect seems to favor the appearance of isotactic configurations along the main chain. 

Side groups in atactic structures and chain branching hinder crystallization of the thermoplastic polymers in contrast to unbranched and isotactic configurations which lead to increased crystallinity. With increasing crystallinity the density, strength and stiffness are increased but the transparency and processability of the plastic decrease. 

Ans. Refer to reaction 6.6. 6.22 is expected to give an isotactic polymer. On steric grounds trans-isotactic configurations of 1,5 and 3,4 bonds are... 

Small structural changes in the catalysts can produce even more remarkable results. Substitution of a methyl group in the cyclopentadienyl ring of iPr(CpHu)ZrCl2 led to the produetion of hemiisotactic polypropylene. In this structure, every second stereogeruc carbon has an isotactic configuration, and the centers in between have atactic placements. Thus, there is order and disorder along the chain in a well-defined manner. 

The isotactic configuration in which each substituent is placed on the same side of the chain. 

Fig. 4.2 Imagined planar zigzag conformations of vinyl polymers (a) the carbon backbone, (b) a regular isotactic configuration, (c) a regular syndiotactic configuration and (d) a random atactic configuration. The planar zigzag conformation might not be possible for the real chain because of steric hindrance. ( Cambridge University Press 1989.)...

As mentioned before (Section II,E,3), the determination of tac-ticity by X-ray analysis is limited by the requirement that the polymer be crystalline. For the study of poly (methyl methacrylate), which may or may not be crystalline, nuclear magnetic resonance spectroscopy has been more useful. In order to interpret the spectra, it has been found necessary to describe the stereochemistry of a unit by the configurations on both sides. Therefore, an isotactic configuration, or isotactic triad, is one where the central unit is fianked by units of the same asymmetry, that is ddd or III. Similarly, for a syndiotactic triad, the stereochemistry is did or Idl. To overcome the disadvantages of the term atactic, a new term heterotactic was introduced. The stereochemistry for heterotactic configurations is, therefore, Idd, dll, lid, and ddl. 

In Bawn and Ledwith s mechanism the asymmetry of the newly formed cation could be assured by association with the gegenion followed by rapid intramolecular solvation. Although assrmmetry could be lost in Cram and Kopecky s postulate owing to incipient free ion formation, the isotactic configuration could be recaptured as ring formation occurred. 

In addition to the polyolefins, polystyrene and polymethylmethacrylate are examples of polymers that can be made in the isotactic configuration. 

Polypropylene (PP) is also an excellent solvent resistant polymer when it is in the isotactic configuration. This isotactic configuration is highly crystalline in contrast K> the atactic form which is amorphous. Polypropylene... 

Once the axial geometry in a polymeric chain is the main factor in determining the ability of a chain to form ciystallites, the ciystalline contribution is probably due to isotactic and syndiotactic structure sequences." " In fact, saturated PHAmcl> which are able to crystallise due to their isotactic configuration, are also seen to crystallise with allq l side chains in an extended conformation to form ordered sheets, but they still show a reduced degree of crystallinity when compared to P(3HB) or P(3HB-co-3HV), owing to low crystallization rates. ... 

Because, in EPDM-60 (I), half of all propylene units exist in the isotactic configuration and the content of diene groups is lower, its segments are able to diflfiise into nonpolar SKN regions to a greater depth than segments of Elastokam 6305. 

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