Editing Polypropylene (section)

==== Molecular structure – tacticity ====
[[File:Polypropylene tacticity en.svg|class=skin-invert-image]]

<small>Polypropylene can be categorized as atactic polypropylene (aPP), syndiotactic polypropylene (sPP) and isotactic polypropylene (iPP). In case of atactic polypropylene, the methyl group (-CH<sub>3</sub>) 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<sub>g</sub> and [[melting point]] T<sub>m</sub>).</small>

The term [[tacticity]] describes for polypropylene how the methyl group is oriented in the polymer chain. Commercial polypropylene is usually isotactic. This article therefore always refers to isotactic polypropylene, unless stated otherwise. The tacticity is usually indicated in percent, using the isotactic index (according to DIN 16774). The index is measured by determining the fraction of the polymer insoluble in boiling [[heptane]]. Commercially available polypropylenes usually have an isotactic index between 85 and 95%. The tacticity affects the polymer's [[Physical property|physical properties]]. As the [[methyl group]] is in isotactic propylene consistently located at the same side, it forces the macromolecule in a [[Helix|helical shape]], as also found in [[starch]]. An isotactic structure leads to a [[semi-crystalline polymer]]. The higher the isotonicity(the isotactic fraction), the greater the crystallinity, and thus also the softening point, rigidity, e-modulus and hardness.<ref name="guide" />{{rp|22}}

Atactic polypropylene, on the other hand, lacks any regularity, which prevents it from crystallization, thereby creating an [[Amorphous solid|amorphous]] material.