Difference Between Atactic Isotactic and Syndiotactic Polymer

The key difference between atactic isotactic and syndiotactic polymer is that the atactic polymers have their substituents in a random manner and isotactic polymers have their substituents in the same side, whereas the syndiotactic polymers have their substituents in an alternating pattern.

Tacticity is a chemical concept that describes the relative stoichiometry of adjacent chiral centres in a macromolecule. A macromolecule is a large molecule such as a polymer. Tacticity is important in determining the properties of a polymer. It is because the structure of a polymer regulates other properties like rigidity, crystallinity, etc.

CONTENTS

1. Overview and Key Difference
2. What is Atactic Polymer
3. What is Isotactic Polymer
4. What is Syndiotactic Polymer
5. Side by Side Comparison – Atactic Isotactic vs Syndiotactic Polymer in Tabular Form
6. Summary

What is Atactic Polymer?

An atactic polymer is a polymer material where the substituents in a carbon chain are arranged in a random manner. Usually, polymers that form via free radical polymerization has this structure; for example, polyvinyl chloride. Atactic polymers have an amorphous structure due to the random arrangement of substituent groups.

However, these atactic polymers are important in polymer technology, e.g. polystyrene. Although it is atactic, we can obtain a syndiotactic polystyrene material if we use a specialized catalyst. However, industrially produced polystyrene is atactic because manufacturers do not use specific catalysts. Since there are randomly arranged substituent groups, the polymer chains cannot be arranged in an ordered manner. Therefore, polystyrene is a semi-crystalline material.

When considering the structure of a polymer material, we call two adjacent polymer units as a “diad”. If the diad has two identical units, oriented in the same way, we call it a meso diad. In atactic polymers, the percentage of these meso diads ranges from 1-99%.

What is Isotactic Polymer?

An isotactic polymer is a polymer which has the substituents on the same side of the carbon chain. That means; all the substituents of the polymer material are located on the same side of the backbone of the polymer.

For example, industrially prepared polypropylene is isotactic. Its production method is Ziegler-Natta catalysis. Usually, these polymers are semi-crystalline. They show a helix configuration. Also, this material contains 100% meso diads.

What is Syndiotactic Polymer?

Syndiotactic polymers are polymer materials which have the substituents in an alternating pattern. Therefore, substituent groups have alternate positions along the backbone of the polymer.

If we produce polystyrene via metallocene catalysis polymerization, it gives a syndiotactic polystyrene material and it is a crystalline material. The polymer contains 100% racemo diads (the diad contains two units oriented in opposition).

What is the Difference Between Atactic Isotactic and Syndiotactic Polymer?

There are three types of polymers according to the tacticity: atactic polymers, isotactic polymers, and syndiotactic polymers. The key difference between atactic isotactic and syndiotactic polymer is that atactic polymers have their substituents in a random manner and isotactic polymers have their substituents in the same side, whereas the syndiotactic polymers have their substituents in an alternating pattern.

Moreover, atactic polymers are mostly amorphous and isotactic polymers are semi-crystalline, whereas syndiotactic polymers are mostly crystalline. Besides, another significant difference between atactic isotactic and syndiotactic polymer is that atactic polymers have 1-99% meso diads while isotactic polymers have 100% meso diads and syndiotactic polymers have 100% racemo diads.

The below infographic summarizes the difference between atactic isotactic and syndiotactic polymer, side by side.

Summary – Atactic Isotactic vs Syndiotactic Polymer

There are three types of polymers according to their tacticity: atactic polymer, isotactic polymer, and syndiotactic polymer. The key difference between atactic isotactic and syndiotactic polymer is that the atactic polymers have their substituents in a random manner and the isotactic polymers have their substituents in the same side, whereas the syndiotactic polymers have their substituents in an alternating pattern.

Reference:

1. “Tacticity.” Wikipedia, Wikimedia Foundation, 3 Mar. 2019, Available here.

Image Courtesy:

1. “Atactic-2D-skeletal” Public Domain) via Commons Wikimedia
2. “Isotactic-polypropylene-3D-balls” By Benjah-bmm27 (talk · contribs) – Own work (Public Domain) via Commons Wikimedia
3. “Syndiotactic-polypropylene-3D-balls” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia