Isotacticity

Isotacticity is the degree to which a polymer chain exhibits isotactic stereochemistry, in which the substituent groups along the backbone are aligned on the same side of the chain. In practice, isotactic polymers have a regular sequence of stereocenters that facilitates efficient packing and crystallization. Isotacticity is contrasted with syndiotacticity, where substituents alternate sides, and with atactic configurations, where stereochemistry is irregular or random.

Isotactic polymers are typically derived from enantiomerically biased monomers such as propylene. Their isotacticity is largely controlled by the catalyst system and polymerization conditions; Ziegler–Natta and metallocene catalysts are known to produce high isotacticity polypropylene, whereas different catalysts or random feeds can reduce tacticity.

High isotacticity leads to increased crystallinity, higher melting temperatures, and greater stiffness and strength, with corresponding effects on barrier properties and chemical resistance. Isotactic polypropylene is a widely used example, existing in crystalline grades suitable for fibers, films, and injection-molded parts. Syndiotactic variants may also crystallize, while atactic polymers tend to be amorphous.

Quantification of isotacticity commonly uses nuclear magnetic resonance to determine tacticity distributions, such as the isotactic triad fraction (mmm) or related indices, as well as differential scanning calorimetry and X-ray diffraction to assess crystalline content.