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An isotactic triad (mm) is made up of two adjacent m diads, a syndiotactic triad (also spelled syndyotactic [4]) (rr) consists of two adjacent r diads, and a heterotactic triad (rm) is composed of an r diad adjacent to an m diad. The mass fraction of isotactic (mm) triads is a common quantitative measure of tacticity.
For example, atactic polypropylene is usually amorphous and transparent while syndiotactic polypropylene, which has crystallinity ~50%, is opaque. [30] Crystallinity also affects dyeing of polymers: crystalline polymers are more difficult to stain than amorphous ones because the dye molecules penetrate through amorphous regions with greater ease.
In case of atactic polypropylene, the methyl group (-CH 3) is randomly aligned, alternating (alternating) for syndiotactic polypropylene and evenly for isotactic polypropylene. This has an impact on the crystallinity (amorphous or semi-crystalline) and the thermal properties (expressed as glass transition point T g and melting point T m ).
Different metal chlorides allowed the selective production of each form i.e., syndiotactic, isotactic and atactic polymer chains could be selectively created. However, there were further complications to be solved. If the Ziegler–Natta catalyst was poisoned or damaged then the chain stopped growing.
Coordination type polymers are also stereoregular and can be isotactic or syndiotactic instead of just atactic. This tacticity introduces crystallinity in otherwise amorphous polymers. From these differences in polymerization type the distinction originates between low-density polyethylene (LDPE), high-density polyethylene (HDPE) or even ultra ...
There are three types of tacticity: isotactic (all substituents on the same side), atactic (random placement of substituents), and syndiotactic (alternating placement of substituents). Isotactic Syndiotactic
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A polymer that lacks any regular arrangement in the position of its alkyl substituents (R) is called atactic. Both isotactic and syndiotactic polypropylene are crystalline, whereas atactic polypropylene, which can also be prepared with special Ziegler–Natta catalysts, is amorphous.