syndiotactical

Syndiotactical describes a type of stereoregularity found in certain polymers in which the substituent groups on successive stereocenters alternate sides along the polymer backbone. This alternating arrangement is known as syndiotacticity. The term is used interchangeably with syndiotactic when referring to the polymer chain as a whole, and syndiotactically as an adverb.

In polymer chemistry, tacticity is a key factor in crystallinity and physical properties. Syndiotactic polymers are contrasted with isotactic polymers, where all substituents lie on the same side, and atactic polymers, where substituents are arranged randomly. The syndiotactic arrangement often promotes crystalline order and distinct melting behavior, though the exact properties depend on monomer identity and other structural features.

Common examples of syndiotactic polymers include syndiotactic polystyrene and syndiotactic polypropylene, both of which can exhibit higher crystallinity and different thermal and mechanical properties compared with their atactic or certain isotactic analogues. The ability to produce syndiotactic polymers depends on stereoselective polymerization processes, typically employing specialized catalysts such as metallocenes or other stereocontrolled Ziegler–Natta systems and carefully chosen reaction conditions.

Characterization of syndiotacticity relies on analytical techniques such as nuclear magnetic resonance (NMR) spectroscopy, which can reveal the alternating arrangement of substituents, and differential scanning calorimetry or X-ray diffraction to assess crystallinity. While the standard term in literature is syndiotactic, the adjective syndiotactical is occasionally used to describe materials or processes exhibiting this pattern.

See also: tacticity, isotactic, atactic, polymer crystallinity, metallocene catalysis.