Photoredox Catalysis in Photocontrolled Cationic Polymerizations of Vinyl Ethers

Abstract

Metrics & MoreArticle Recommendations 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 photocontrolled 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 will 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. However, 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 will 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 will 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 selectivity in situ using light to control polymer structure. At the end of the Account, we will 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.

Photocontrolled RAFT polymerization: past, present, and future

“…Nevertheless, photomediated cationic RAFT polymerization is still at an early stage for numerous applications owing to the narrow scope of monomers, PCs, and solvents and lack of oxygen tolerance in the reaction system. The recent development of PCs 92 or use of continuous flow reactors 382 combined with further studies would expand the utility of the polymerization.…”

“…For example, imperfect control or increase in monomer conversion even without light irradiation, which has been observed in some photomediated cationic RAFT polymerizations, has been ascribed to the decomposition of the PC or slow deactivation of the CTA owing to improper balance between ground-and excited-state redox potentials of the PC and CTA. 43,[90][91][92] Slow deactivation of the CTA results in longer lifetime of active propagating species such that these species remain after the cessation of irradiation and consume monomers. This loss of control caused by the remaining radicals, however, can be utilized to induce latent RAFT polymerization in the dark using the energy stored during precedent irradiation.…”

Design and Synthesis of a New Indazole-Decorated RAFT Agent for Highly Efficient PET-RAFT Polymerization

“…Photocontrolled reversible deactivation radical polymerization (photo-RDRP) makes use of light energy to control production of polymers with precise molecular weight, composition, and architecture under ambient conditions. − Unlike cationic and anionic polymerization, − photo-RDRP is compatible with diverse solvents, including aqueous solutions. − A good example of such systems is photoinduced electron/energy transfer reversible addition–fragmentation chain transfer (PET-RAFT) polymerization catalyzed by zinc tetraphenyl porphyrin (ZnTPP), a sustainable and biocompatible photocatalyst (PC). − Notably, ZnTPP-catalyzed PET-RAFT systems are capable of controlling oxygen-present radical polymerization, because its populated triplet excited states ( 3 PC*) can transform ground-state oxygen (O 2 ) into singlet oxygen ( 1 O 2 ), allowing for subsequent elimination by vinyl monomers or the solvent . They are even compatible to open-air conditions, potentially suitable for a broad spectrum of aerobically biological and photocuring applications. − …”

Visible-Light-Induced Cationic Polymerization of Isobutylene: A Route toward the Synthesis of End-Functional Polyisobutylene

“…The use of photocatalysis in polymer synthesis has been an emerging and intensively developing area of research in the past decades. − The advantages of light as an external stimulus to promote polymerization processes consist of its availability, high energy efficiency, low cost, environmentally benign nature, and easy accessibility of cheap long-lifetime LED light sources . Moreover, recent progress in controlled/living polymerization has provided access to novel photocontrolled techniques, offering temporal and spatial control over molecular weight, functionality, and macromolecular architecture via a light “on/off” reaction setup, which is not available in conventional thermally and chemically activated processes. − …”