Tacticitet

Tacticitet, also known as tacticity, is a term in polymer chemistry that describes the stereochemical arrangement of pendant substituents along the main chain of a polymer formed from a prochiral monomer such as propene. The arrangement along the chain affects how the polymer chains pack and interact.

The three principal configurations are isotactic, syndiotactic, and atactic. In isotactic polymers all stereocenters have the same configuration along the chain. In syndiotactic polymers the configurations alternate regularly. In atactic polymers the configurations are irregular, leading to a lack of regular packing.

Tacticitet strongly influences physical properties. Isotactic and syndiotactic polymers tend to be more crystalline and have higher melting temperatures and stiffness, whereas atactic polymers are usually more amorphous with lower crystallinity and strength. These differences impact processing behavior and end-use performance, including mechanical properties, clarity, and barrier properties in plastics.

Control of tacticitet during polymerization is achieved through choice of catalyst and reaction conditions. Ziegler–Natta catalysts and certain metallocenes can favor isotactic configurations in polymers such as polypropylene, while other catalysts or ligand environments can promote syndiotactic sequences or create more atactic materials.

Characterization commonly relies on nuclear magnetic resonance (NMR) to quantify triad and higher-order tacticity fractions. Differential scanning calorimetry (DSC) and X-ray diffraction assess crystallinity and thermal properties, providing a link between stereoregularity and material performance.

Tacticitet is a foundational concept in polymer science, influencing material properties, processing, and catalyst design.