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1. Photocontrolled cationic degenerate chain transfer polymerizations via thioacetal initiators

   https://doi.org/10.1039/D0PY01100B  

   Sifri, Renee J.; Kennedy, Audrey J.; Fors, Brett P. (October 2020, Polymer Chemistry)

   null (Ed.)

   Recent developments in photocontrolled polymerizations have facilitated the development of previously inaccessible materials. While photocontrolled radical polymerizations have been extensively studied, related processes involving cationic polymerizations are underexplored and limited to RAFT processes. In this study, we disclose a visible light, temporally controlled cationic polymerization of vinyl ethers utilizing thioacetals and a photoredox catalyst. We demonstrate a broad scope of thioacetal initiators that achieve a well-controlled polymerization by recapping propagating chains via photocatalyst turnover in combination with a degenerate chain transfer process through sulfonium intermediates. Furthermore, we show that a photocatalyst with a more reducing ground state reduction potential allows for enhanced control and excellent temporal regulation of polymerization. 

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2. A Perspective on the History and Current Opportunities of Aqueous RAFT Polymerization

   https://doi.org/10.1002/marc.202200414  

   Fortenberry, Alexander W.; Jankoski, Penelope E.; Stacy, Evan K.; McCormick, Charles L.; Smith, Adam E.; Clemons, Tristan D. (July 2022, Macromolecular Rapid Communications)

   Abstract Reversible addition‐fragmentation chain transfer (RAFT) polymerization has proven itself as a powerful polymerization technique affording facile control of molecular weight, molecular weight distribution, architecture, and chain end groups ‐ while maintaining a high level of tolerance for solvent and monomer functional groups. RAFT is highly suited to water as a polymerization solvent, with aqueous RAFT now utilized for applications such as controlled synthesis of ultra‐high molecular weight polymers, polymerization induced self‐assembly, and biocompatible polymerizations, among others. Water as a solvent represents a non‐toxic, cheap, and environmentally friendly alternative to organic solvents traditionally utilized for polymerizations. This, coupled with the benefits of RAFT polymerization, makes for a powerful combination in polymer science. This perspective provides a historical account of the initial developments of aqueous RAFT polymerization at the University of Southern Mississippi from the McCormick Research Group, details practical considerations for conducting aqueous RAFT polymerizations, and highlights some of the recent advances aqueous RAFT polymerization can provide. Finally, some of the future opportunities that this versatile polymerization technique in an aqueous environment can offer are discussed, and it is anticipated that the aqueous RAFT polymerization field will continue to realize these, and other exciting opportunities into the future. 

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3. Enhancing Temporal Control and Enabling Chain‐End Modification in Photoregulated Cationic Polymerizations by Using Iridium‐Based Catalysts

   https://doi.org/10.1002/anie.201804111  

   Kottisch, Veronika; Supej, Michael J.; Fors, Brett P. (June 2018, Angewandte Chemie International Edition)

   Abstract Gaining temporal control over chain growth is a key challenge in the enhancement of controlled living polymerizations. Though research on photocontrolled polymerizations is still in its infancy, it has already proven useful in the development of previously inaccessible materials. Photocontrol has now been extended to cationic polymerizations using 2,4,6‐triarylpyrylium salts as photocatalysts. Despite the ability to stop polymerization for a short time, monomer conversion was observed over long dark periods. Improved catalyst systems based on Ir complexes give optimal temporal control over chain growth. The excellent stability of these complexes and the ability to tune the excited and ground state redox potentials to regulate the number of monomer additions per cation formed allows polymerization to be halted for more than 20 hours. The excellent stability of these iridium catalysts in the presence of more nucleophilic species enables chain‐end functionalization of these polymers. 

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4. Achieving molecular weight distribution shape control and broad dispersities using RAFT polymerizations

   https://doi.org/10.1039/D1PY00399B  

   Rosenbloom, Stephanie I.; Sifri, Renee J.; Fors, Brett P. (January 2021, Polymer Chemistry)

   null (Ed.)

   Reversible addition–fragmentation chain-transfer (RAFT) polymerizations are one of the most versatile and powerful polymerization techniques for the synthesis of complex macromolecular architectures. While RAFT polymerizations often give polymers with narrow molecular weight distributions (MWDs), commodity plastics often have broad MWDs to give targeted properties and processability. Thus, new methods to precisely control both MWD breadth and shape are essential for fine-tuning polymer properties for next generation materials. Herein, we report a simple method for controlling polymer MWD features in thermally activated radical RAFT and redox activated cationic RAFT polymerizations by means of metered additions of chain transfer agents. 

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5. Reusable polymer brush-based photocatalysts for PET-RAFT polymerization

   https://doi.org/10.1039/D2PY00966H  

   Bell, Kirsten; Freeburne, Sarah; Wolford, Adam; Pester, Christian W. (November 2022, Polymer Chemistry)

   This contribution discusses the control over polymerizations using a heterogeneous photocatalyst based on fluorescein polymer brushes tethered to micron-scale glass supports (FPB@SiO 2 ). FPB@SiO 2 -catalyzed photoinduced electron/energy transfer-reversible addition–fragmentation chain transfer (PET-RAFT) polymerization is shown to provide high conversions, controlled molecular weights and narrow molecular weight distributions for a variety of monomers. Moreover, the beads can catalyze PET-RAFT on gram scales, in the presence of oxygen, while allowing full catalyst recovery through simple filtration. Finally, their high shelf-life allows for multiple polymerizations and user-friendly access to precision macromolecules under mild reaction conditions even after prolonged (months) storage time. 

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