Photoredox Catalysis in Photocontrolled Cationic Polymerizations of Vinyl Ethers

CONSPECTUS: Advances in photocontrolled polymerizations have expanded the scope of polymer architectures and structures that can be synthesized for various applications. The majority of these polymerizations have been developed for radical processes, which limits the diversity of monomers that can be used in macromolecular design. More recent developments of photo-controlled cationic polymerizations have taken a step toward addressing this limitation and have expanded the palette of monomers that can be used in stimuli-regulated polymerizations, enabling the synthesis of previously inaccessible polymeric structures. This Account wi l l detail our group's studies on cationic polymerization processes where chain growth is regulated by light and highlight how these methods can be combined with other stimuli-controlled polymerizations to precisely dictate macromolecular structure. Photoinitiated cationic polymerizations are well-studied and important processes that have control over initiation. Howe v e r , we wanted to develop systems where we had spatiotemporal control over both polymer initiation and chain growth. This additional command over the reaction provides the ability to manipulate the growing polymer with an external stimulus during a polymerization, which can be used to control structure. To achieve this goal, we set out to develop a method to photoreversibly generate a cation at a growing chain end that could participate in a controlled polymerization process. We took inspiration from previous work on cationic degenerate chain transfer polymerizations of vinyl ethers that used thiocarbonylthio chain transfer agents. These polymerizations were initiated by a strong acid and gave well-defined poly(vinyl ether)s. We posited that we could remove the acid initiator in these systems and reversibly oxidize the thiocarbonylthio chain ends in these reactions with a photocatalyst to give a photocontrolled cationic polymerization of vinyl ethers. This Account wi l focus on our journey to discover cationic photocontrolled polymerizations. We will summarize our initial developments and detail our mechanistic understanding of these reactions using both organic and inorganic based photocatalysts, and we wi l l outline more recent efforts to expand cationic degenerate chain transfer polymerizations to other thioacetal initiators. Finally, we will detail how these photocontrolled cationic polymerizations can be used to switch monomer selectivit y in situ using light to control polymer structure. At the end of the Account, we wi ll discuss our vision for future potential applications of these photocontrolled cationic polymerizations in the synthesis of novel block copolymers and next generation cross-linked networks.

Automated summary

Advancements in photocontrolled polymerizations have expanded the range of polymer structures that can be synthesized, however, most research has focused on radical processes, limiting monomer diversity. Recent developments in photo-controlled cationic polymerizations have increased the palette of monomers that can be used, enabling the synthesis of previously inaccessible structures.