atactische

Atactische, or atactic, describes a polymer whose stereochemical arrangement of pendant groups along the main chain is random rather than ordered. The term is used in polymer chemistry and appears in several languages, including Dutch and English. It contrasts with isotactische (isotactic) and syndiotactische (syndiotactic) polymers, which have regular, repeating patterns of substituents. In an atactic polymer, the side groups lack a long-range order, producing a stochastic sequence along the chain.

The concept arises from the study of tacticity, a key aspect of polymer microstructure. The tacticity of a polymer is largely determined by the polymerization mechanism and reagents. Free-radical polymerization and other uncontrolled routes often yield predominantly atactic products, especially for vinyl monomers such as styrene. More controlled methods can favor isotactic or syndiotactic configurations, depending on catalysts and reaction conditions. Because atactic sequences are random, the resulting polymers can differ substantially from their ordered counterparts.

Properties of atactic polymers are largely shaped by the absence of regular packing. They are typically amorphous, with little or no crystallinity, which can lead to transparency and different mechanical behavior compared with crystalline analogues. They often exhibit broader processing windows but lower stiffness and strength than their crystalline equivalents. The exact properties depend on the monomer and overall chain architecture.

Characterization of tacticity commonly uses 13C NMR to quantify the distribution of stereochemical configurations, alongside differential scanning calorimetry and X-ray diffraction. Atactic polymers, such as atactic polystyrene, serve as important references in polymer physics and materials science. See also tacticity, isotactic, and syndiotactic.