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In isotactic polypropylene, for example, the methyl groups are oriented on one side of the carbon backbone. This arrangement creates a greater degree of crystallinity and results in a stiffer material that is more resistant to creep than both atactic polypropylene and polyethylene.
An isotactic macromolecule consists of 100% m diads, though IUPAC also allows the term for macromolecules with at least 95% m diads if that looser usage is explained. [3] Polypropylene formed by Ziegler–Natta catalysis is an example of an isotactic polymer. [8] Isotactic polymers are usually semicrystalline and often form a helix configuration.
The structure of isotactic polypropylene. The structure of atactic polypropylene. Whether or not polymers can crystallize depends on their molecular structure – presence of straight chains with regularly spaced side groups facilitates crystallization. For example, crystallization occurs much easier in isotactic than in the atactic ...
Both isotactic and syndiotactic polypropylene are crystalline, whereas atactic polypropylene, which can also be prepared with special Ziegler–Natta catalysts, is amorphous. The stereoregularity of the polymer is determined by the catalyst used to prepare it.
Polypropylene glycol. Conventional polymerization of propylene oxide results in an atactic polymer. The isotactic polymer can be produced from optically active propylene oxide, but at a high cost. A salen cobalt catalyst was reported in 2005 to provide isotactic polymerization of the prochiral propylene oxide [3] Cobalt catalyst for isotactic ...
In the mid-to-late-1950s, atactic polypropylene (APP) was a by-product of the synthesis of isotactic polypropylene (iPP) using first and early second generation Ziegler–Natta catalysts (Z-N catalysts), which typically produced about 10-15 wt% of APP.
Segments of polypropylene, showing the slightly different structures of isotactic (above) and syndiotactic (below) polymers. The study of polymer thermodynamics helps improve the material properties of various polymer-based materials such as polystyrene (styrofoam) and polycarbonate.
In isotactic polypropylene, the molecules form a helix. Like the zigzag conformation, such helices allow a dense chain packing. Particularly strong intermolecular interactions occur when the residues of the repeating units allow the formation of hydrogen bonds , as in the case of p -aramid .