Atactic

Atactic refers to a polymer in which the substituent groups along the main chain are placed at random with respect to the chain’s stereochemistry. In polymer chemistry, tacticity describes the relative orientation of pendant groups, and atactic means there is no uniform stereochemical order. This randomness typically arises when polymerization proceeds without stereochemical control, such as in non-selective radical polymerizations.

The lack of regular stereoregularity gives atactic polymers a predominantly amorphous character. They generally do not crystallize well, leading to low densities, broad or absent melting transitions, and often lower glass-transition temperatures. As a result, atactic polymers tend to be softer, more flexible, and more soluble in common solvents than their isotactic or syndiotactic counterparts, and they are usually transparent rather than crystalline.

Common examples include atactic polystyrene and atactic poly(methyl methacrylate) (PMMA); many commercially produced PMMAs and related vinyl polymers are atactic. The physical properties of atactic polymers can be advantageous for applications requiring clarity, impact resistance, or easier processing, though performance can be inferior to more ordered stereoregular forms in terms of strength and solvent resistance.

Analytical methods such as nuclear magnetic resonance (NMR) spectroscopy and differential scanning calorimetry (DSC) are used to assess tacticity and thermal behavior. Stereoregular polymers (isotactic or syndiotactic) can be synthesized with stereoselective catalysts, but atactic polymers arise when such control is absent or intentionally relaxed.