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Tacticity refers to the stereochemical arrangement of atoms or groups of atoms in a polymer chain. Specifically, it describes the relative configuration of chiral centers or stereogenic units along the polymer backbone. This arrangement significantly influences the physical properties of the polymer, such as its melting point, crystallinity, solubility, and mechanical strength.

In polymers formed from monomers with a chiral center, tacticity is classified into three main types: isotactic, syndiotactic, and atactic. Isotactic polymers have all chiral centers oriented in the same direction along the polymer chain. Syndiotactic polymers have chiral centers alternating in a regular pattern. Atactic polymers have a random arrangement of chiral centers.

The tacticity of a polymer is determined during the polymerization process, particularly by the type of catalyst used. For example, Ziegler-Natta catalysts are known for their ability to control tacticity, producing isotactic or syndiotactic polymers. Atactic polymers are typically formed through radical polymerization. Understanding and controlling tacticity is crucial in polymer synthesis for tailoring material properties for specific applications. Different tacticities can lead to vastly different material behaviors, even for the same chemical repeat unit.