21.7 : ポリマーの分類: 立体特異性

Polymerization of a substituted vinyl monomer leads to a polymer chain with numerous chiral carbons. Based on the relative configurations of these chiral centers, a polymer can be classified as isotactic, syndiotactic, or atactic.

In the isotactic configuration, all substituents are aligned in the polymer chain backbone.

In the syndiotactic configuration, the substituents periodically alternate on either side of the polymer chain.

In an atactic polymer, the substituents are randomly oriented.

Unlike the atactic configuration, the stereoregular arrangement of substituents in isotactic and syndiotactic configurations enables the close-packing of polymer chains.

So, the crystallinity and melt transition temperature of a polymer increase with an increase in the stereospecificity of polymer chains, while an increase in random orientations makes the polymer less crystalline.

For example, isotactic polypropylene has a melting point between 160 and 170 degrees Celsius, which drops to 125 to 131 degrees in syndiotactic polymers. Conversely, atactic polypropylene is a rubbery material without a sharp melting point.