Stereoselective Polymerization of 3,6-Disubstituted N-Vinylcarbazoles.

Poly(N-vinylcarbazole) (PNVC-H) is a valuable nonconjugated photoconductive polymer, but the free radical polymerization conditions typically used for its synthesis do not control polymer stereochemistry and are not tolerant to many substituted N-vinylcarbazoles. Here, we report the stereoselective cationic polymerization of a series of 3,6-disubtituted N-vinylcarbazole derivatives using a chiral scandium-bis(oxazoline) Lewis acid catalyst. The combination of asymmetric ion-pairing catalysis and inherent monomer stereoelectronics facilitated stereoselective polymerization at room temperature, which enabled the polymerization of less soluble 3,6-disubstituted-N-vinylcarbazole derivatives. Isotactic halogen-substituted PNVCs demonstrated self-assembly in solution through halogen-halogen bonding, which was not observed in their atactic counterparts. Initial spectral characterization displayed a wide range of excitation-emission profiles for substituted PNVCs, which demonstrate the promise of these materials as a new class of nonconjugated photoconductive polymers for optoelectronic applications. Overall, these results showcase a diverse class of isotactic poly(N-vinylcarbazoles), highlight the benefits of identifying alternative stereocontrol mechanisms for polymerization, and expand the suite of accessible nonconjugated hole-transport materials.

Mechanistic Insights into the Stereoselective Cationic Polymerization of N-Vinylcarbazole.

The synthesis of stereoregular polymers through ionic mechanisms using asymmetric ion-pairing (AIP) catalysis is emerging as an effective strategy to achieve differentiated material properties from readily available building blocks. Stereoselective cationic polymerization in particular is primed for advancement using AIP by leveraging the breadth of Brønsted and Lewis acid small-molecule catalysis literature; however, mechanistic studies that address polymer-specific phenomena are scarce and, as a result, the lack of mechanistic understanding has limited catalyst design. In a recent study, we demonstrated the only example of a stereoselective and helix-sense-selective cationic vinyl polymerization of N-vinylcarbazole using chiral scandium-bis(oxazoline) Lewis acids. To better understand the mechanism of this highly stereoselective polymerization and elicit design principles for future advances, we present a combined experimental and computational study into the relevant factors that determine tacticity and helicity control. Key mechanistic experiments suggest two competing elementary steps-chain-end conformation equilibration and propagation-whose relative rates can be influenced by monomer concentration, isotope effects, and catalyst design to tune tacticity. In contrast, helicity is influenced by complex relationships between the stereoselectivity of the first monomer propagation and a time-dependent initiator-catalyst mixing time. The more complete understanding of stereoselective cationic polymerization through AIP developed herein provides insights into polymer-specific mechanisms for stereocontrol, which we believe will motivate continued catalyst discovery and development for stereoselective vinyl polymerization.

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

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.

100th Anniversary of Macromolecular Science Viewpoint: The Past, Present, and Future of Stereocontrolled Vinyl Polymerization.

The thermomechanical properties exhibited by synthetic macromolecules can be directly linked to their tacticity, or the relative stereochemistry of repeat units. The development of stereoselective coordination-insertion polymerization, for example, led to the discovery of isotactic polypropylene, now one of the most widely produced commodity plastics in the world. Widespread interest in controlling polymer tacticity has led to a variety of stereoselective polymerization methodologies; however, this area of polymer science has lagged behind when compared to the ability to control molecular weight, dispersity, and composition. Despite decades of advancements, many stereoregular vinyl polymers remain unknown, particularly those comprised of polar functionality or derived from renewable resources. This Viewpoint provides an overview of recent developments in stereocontrolled polymerization, with an emphasis on propagation mechanism, and highlights successes, limitations, and future challenges for continued innovation.