Kari Vijayakrishna

The reversible protonation behaviour of an ethyl piperazine-bearing dithiocarbamate-based RAFT agent enables the synthesis of block copolymers of the type poly(MAM)- b -poly(LAM) using both blocks as macro-chain transfer agents.

1. Chirality drives life on Earth. It is the L-amino acids and D-sugars embedded in peptides, proteins, and carbohydrates, respectively, that create the physiological environment’s homochirality.[1...

The synthesized poly(cationic- co -anionic) based hydrogels are very effective in simultaneous removal of dyes (both cationic and anionic dyes) and heavy metals from an aqueous medium.

Cinchona alkaloids are naturally occurring chiral molecules that have emerged as catalysts in asymmetric organocatalysis especially in enantioselective transformations because of their inherent chirality and unique structural features. Immobilizing these alkaloids on polymeric supports has significantly influenced their use in catalysis. Polymer-anchored cinchona alkaloids combine the catalytic efficacy of cinchona derivatives with additional benefits of polymer chain, such as ease of recovery, recyclability, reusability, and reduced environmental impact. These polymer-supported cinchona alkaloids have found wide applications in enantioselective reactions such as Michael addition, aldol condensations, Henry reaction, dimerization reaction, dihydroxylation, and benzylation. Various strategies have been employed for anchoring cinchona alkaloids onto polymers, including covalent attachment of alkaloids in the polymer side chain or main chain, and ionic attachment of alkaloids via quaternization in the main chain or side chain of polymer. This review focuses on the various synthetic methodologies for the preparation of polymer-anchored cinchona alkaloids and their application in numerous asymmetric transformations. Graphical Abstract

Synthetic helical polymers have gained tremendous attention due to their superior efficiency in mimicking the complex self‐assembly “helix” of biological systems. Prolinate‐based chiral polyelectrolytes bearing chloride counter anion (poly‐(L‐)‐Pro‐Cl) is circular dichroism (CD) silent and exist as random coils. For instance, the same chiral polymer with methoxide counter anion (poly‐(L‐)‐Pro‐OMe) shows CD response with preferred‐handed helical sense. Here we have used partial anion exchange of chloride anion with methoxide as a simple and effective technique to bring the induced CD, eventually helical sense to poly‐(L‐)‐Pro‐Cl. Even with 1 mol% of methoxide exchange, the obtained chiral random copolymer poly‐(L‐)‐Pro‐Cl_0.99‐co‐poly‐(L‐)‐Pro‐OMe_0.01 showed induced CD. The random copolymer poly‐(L‐)‐Pro‐Cl_0.50‐co‐poly‐(L‐)‐Pro‐OMe_0.50 with 50 mol% of the methoxide anion exchanged, showed induced CD identical to that of poly‐(L‐)‐Pro–OMe. In partial anion exchange‐induced random copolymers, methoxide‐containing polymer units act as sergeants and chloride‐containing units act as soldiers. Through the “sergeants‐ and soldiers‐like effect,” partial anion exchange reactions trigger the helical sense into the polymer chains. CD analysis provided strong support for the study. Here, the study provides a novel approach to the helical amplification phenomenon, which corresponds to the “sergeants‐ and soldiers‐like effect.”

Vinyl acetate (VAc) is one of the least active unconjugated monomers among the less activated monomers (LAMs), resulting in highly reactive and unstable propagating radicals. A significant obstacle in achieving well-defined high molar mass polyvinyl acetate (PVAc) is the occurrence of chain transfer reactions during radical polymerization. Towards the controlled polymerization of VAc, we have employed two different dithiocarbamate-based chain transfer agents CTA-1 and CTA-2 bearing ethyl piperazine and pyrrole as Z stabilizing groups, respectively. In CTA-1, the lone pair of electrons on the nitrogen atom is not delocalized, reducing the double-bond character of the CS bond and enhancing its zwitterionic nature. Conversely, in the pyrrole-based CTA-2, the nitrogen's lone pair of electrons will be delocalized within the aromatic system of the Z group, resulting in a less zwitterionic nature. CTA-1 exhibited excellent control over VAc polymerization, yielding a high molar mass of 132 kDa with a low dispersity of 1.31. In contrast, CTA-2 showed poor control when attempting high molar mass VAc polymerization. This observable change in the polymerization behaviour with CTA-1 and CTA-2 can be attributed to the availability of a lone pair of electrons on the nitrogen of the stabilizing Z group, which deactivates the thiocarbonyl group by reducing its double bond character. To understand the underlying reasons, ab initio and DFT calculations were conducted to investigate the neutral or zwitterionic forms of CTA-1 and CTA-2. The Mulliken charges on each atom and the β-scission of the S–R bond in these RAFT-adduct radicals of both CTAs were measured. The results revealed that CTA-1 exists in a zwitterionic form, while CTA-2 remains in a neutral form, explaining the superior performance of CTA-1 in VAc polymerization. The other thermodynamic aspects of the S–R β-scission of RAFT-adduct radicals of CTA-1 and CTA-2 were also calculated to support the above hypothesis.

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.