Effect of length and functionality of the linker between the main chain and the chiral pendant on the helical nature of chiral poly(ionic liquids)

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Abstract
Induction of “helical sense” through external stimuli over the conformational liability of a linker between the main chain of a polymer to its respective pendants is rare. The objective of this work was to understand the influence of functionality as well as the chain length of the linker in a chiral polymer over its helical sense. We synthesized two sets of imidazolium-functionalized chiral poly(ionic liquids) bearing chirality at the cation part. In the first set of chiral PILs (poly-(L- and D-)-2, 3 and 4-Cl), chiral pendants were attached to the main chain through amide bonding. In the second set (poly-(L- and D-)-5, 6 and 7-Cl), it was through methylene bonding, with a linker length of 2, 3 and 4, respectively. All the polymers contained chiral pendant groups but initially they did not respond to circular dichroism (CD) spectroscopy, suggesting that their chains existed as random coils. Interestingly, chiral PILs bearing an amide linkage showed induced CD with a conformational arrangement from random coil to helical via secondary interactions through the help of external stimuli (i.e., achiral amine or temperature). Later, the polymers showed a memorized effect even after removal of the external stimuli completely. A decrease in helical sense was observed as the linker length increased from two to four, indicating the critical role of the chain length of the linker. However, neither of the external stimuli could elicit any induced CD in the second set of chiral polymers, in which the amide linkage was absent. Therefore, our study highlighted the sensitivity of the functionality that allows noncovalent interaction to bring chiral pendants into a helical array.

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