Syndiotacticity

Syndiotacticity is a term used in polymer chemistry to describe the stereoregularity of tacticity along a polymer chain. In a syndiotactic polymer, the pendant substituents alternate their orientation with every other monomer along the backbone, for example the methyl groups in polypropylene alternate sides rather than all lying on the same side. This contrasts with isotactic polymers, where all substituents are on the same side, and atactic polymers, where their orientation is random.

This alternating arrangement tends to promote crystallinity in many polymers, often leading to distinct crystalline forms and higher melting points relative to the atactic form. The exact influence on properties depends on the monomer and crystal form. Common examples include syndiotactic polystyrene and syndiotactic polypropylene.

Syndiotacticity is typically achieved through stereospecific polymerization using tailored catalysts, such as certain metallocene or Ziegler–Natta systems, and is analyzed by 13C NMR to quantify diad and triad sequences, as well as by differential scanning calorimetry and X-ray diffraction to assess crystallinity.

Understanding tacticity helps explain differences in processing, mechanical performance, and optical properties among stereoregular polymers.