Polymers Bearing Covalently Linked Chiral Proline-Based Pendants: Induction of a Highly Stable Helical Sense that Mimics Double-Stranded Helical Morphology and Their Chiral Resolution Ability

Synthesis of common organic polymers bearing a helical sense in aqueous media is still an inflexible area. Here, we report proline-derived imidazolium-functionalized water-soluble, highly stable chiral polymers/chiral polyelectrolytes whose chiral pendant groups exist at the cation part of the polymer chains with different counteranions. Despite chiral pendants, chiral polymers with chloride counteranions did not respond to circular dichroism (CD) absorption spectroscopy, which suggests that they exist as random coils. Interestingly, hydroxy-containing poly-(l-)-2-OH and poly-(d-)-2-OH showed two intense CD signals (with opposite loops), suggesting that the hydroxyl anion facilitates the noncovalent interactions in the chiral polymers to realize this "random coil to helical" transformation, which failed with the chloride anion. l-Prolinate-substituted polymers (poly-(l-)-2-OH) showed a positive Cotton effect with a double-stranded, right-handed helical sense in the presence of achiral and chiral counteranions. Under identical conditions, d-prolinate-substituted chiral polymers (poly-(d-)-2-OH) showed a negative Cotton effect with a left-handed helical sense, which is exactly opposite to its counterpart. This study discusses the influence of the type of chiral center on a preferred helical sense and tries to establish the influence of chirality at the main chain vs side chain on the type and extent of helicity. The synthesized chiral polymers were successfully used for the chiral resolution of racemic d,l-alanine through enantioselective crystallization. These helical polymers showed good enantio-separation efficiency with an enantiomeric excess (ee) of up to 90%. Here, we demonstrated that by selective choice of the chiral helical polymers, a targeted enantiomer could be recrystallized.