Stereoselective Helix-Sense-Selective Cationic Polymerization of N-Vinylcarbazole Using Chiral Lewis Acid Catalysis

Abstract

Helical polymers with a defined main-chain atropoisomeric conformation are important materials in high value applications such as nonlinear optics and chiral separations. Currently, no methods exist for the cationic helix-sense-selective polymerization of prochiral vinyl monomers, which limits access to a number of potentially valuable optically active helical polymers. Here, we demonstrate the first stereoselective cationic helix-sense-selective polymerization of a prochiral vinyl monomer, which provides access to optically active helices of poly­(N-vinylcarbazole). Chiral bis­(oxazoline)-scandium Lewis acids serve as chiral counterions to polymerize N-vinylcarbazole into highly isotactic (up to 94% meso triads) polymers. Mechanistic investigations uncovered the distinct phenomenon that are responsible for independent control of conformational (i.e., helicity) and configurational (i.e., tacticity) stereochemistry. Polymer helicity was strongly influenced by the stereoselectivity of the first monomer propagation, whereas polymer tacticity was dictated by the thermodynamically controlled conformation of the growing polymer chain end. Overall, this method expands the suite of accessible helical polymers through helix-sense-selective polymerization and provides mechanistic insight into how polymer tacticity and helicity can be controlled independently.

Asymmetric Ion-Pairing in Stereoselective Vinyl Polymerization

“…The Leibfarth group recently reported the first example of a stereoselective cationic HSSP of a vinyl monomer, Nvinylcarbazole (NVC), using chiral scandium bis(oxazoline) (BOX) Lewis acids. 56 The proposed mechanism was that ionization of a hemiaminal of NVC and methanol initiated polymerization and generated a chiral counteranion concomitantly, which provided a highly isotactic and helically enriched polymer (Figure 10A). Use of a chiral substituted BOX ligand led to a marked increase in tacticity across the board (70% to >85% mm, at room temperature) compared to an achiral variant, highlighting the importance of the chiral character of the counterion (Figure 10A).…”

“…In all cases, chiral ligands outperformed the achiral 1,1-diphenylethanol control ligand (Figure A). A similar phenomenon has been seen in various systems, highlighting the importance of chirality in the catalysts. , …”

Anion Migrated Ring Opening and Rearrangement in Anionic Polymerization Induced C7 and C8 Polymerizations

“…The establishment of novel polymerization mechanisms has always been the focus of polymer chemists because it is one of the important pillars in polymer chemistry. In particular, numerous polymers with unprecedented structures, such as backbones, − sequences, , configurations, , helices, , crystallizations, etc., can thus be explored with the progress achieved in polymerization methods. As polymerization mechanisms continue to become more advanced, regulation of the carbon skeleton structure of backbones has recently attracted considerable attention because slight changes in the backbones will cause substantial discrepancies and produce unexpected breakthroughs in studies of polymer properties.…”

From a single helix to a helical porous metalloenzyme catalyst based on temperature sensitive polyionic liquids

“…2 The construction of helical polymers is mainly realized by introducing functional groups into small molecule monomers through intermolecular hydrogen bonds or supramolecular interactions. [2][3][4][5][6][7][8][9][10][11][12][13] There are obvious shortcomings in the synthesis of helical polymers, which restrict their further development and application. On one hand, the realization of a helical structure usually requires the introduction of chiral molecules.…”