21.8: Polymer Classification: Stereospecificity
Polymerization generates chiral centers along the entire backbone of a polymer chain. Accordingly, the stereochemistry of the substituent group has a significant effect on polymer properties. Polymers formed from monosubstituted alkene monomers feature chiral carbons at every alternate position in the polymer backbone. Relative to the predominant orientation of substituents at the adjacent chiral carbons, the polymer can exist in three different configurations: isotactic, syndiotactic, and atactic.
In the isotactic configuration, the substituents are generally positioned on the same side of the polymer backbone. In the syndiotactic configuration, the substituents periodically alternate on both sides of the polymer chain. In the atactic configuration, the substituents orient randomly. Figure 1 depicts the comparison of substituents’ arrangement in the isotactic, syndiotactic, and atactic polypropylene polymer chains.
Figure 1: Structural configurations of isotactic polypropylene (top), syndiotactic polypropylene (middle), and atactic polypropylene (bottom) chains.
The more regular arrangement of substituents in isotactic and syndiotactic configurations facilitates the close-packing of polymer chains and increases the polymer's density, crystallinity, and melt transition temperature. On the other hand, an increase in the fraction of the atactic configuration makes loosely bound polymer chains, which reduces the density and crystallinity of the polymer.
For example, the melting temperature for commercial isotactic polypropylene is 160 to 170 °C, depending on the quantity of atactic traces present, while for syndiotactic polypropylene, it is 125 to 131 °C. In contrast, atactic polypropylene is an amorphous rubbery material without a sharp melting point. So, control over the stereospecificity of polymer chains is important while synthesizing polypropylene for commercial applications, such as temperature-resistant tubes and bottles.