Photoinduced Oxygen Reduction for Dark Polymerization

Abstract

Photopolymerization systems for controlled/living radical polymerization (CLRP) have often been dependent on continuous irradiation to sustain radical production. Although this approach offers an opportunity to impose spatial and temporal control, it remains an energy inefficient process. As energy storage for CLRP remains an unexplored area in polymer chemistry, it may provide an opportunity for designing energy efficient polymerization. In this contribution, we propose a novel energy storage system where in situ production of hydrogen peroxide from molecular oxygen was achieved after a brief period of visible light irradiation in the presence of photo-organocatalyst and ascorbic acid. Upon ceasing irradiation, the slow generation of hydroxyl radicals from hydrogen peroxide in the presence of ascorbic acid allows for continuous radical generation in the dark. The highlight of this system stems from the fact that irradiation as brief as 5 min allows storage of enough energy as hydrogen peroxide to perform continuous polymerization to reach high monomer conversions in the dark. In addition, these aqueous polymerizations do not require nitrogen purging as oxygen is required for the production of hydrogen peroxide which becomes the radical source that initiates the polymerization. Interestingly, the amount of oxygen present in the reaction mixture affects the rate of polymerization. The system was found to be robust and versatile as it is able to accommodate different monomer families (acrylate, acrylamide, and methacrylate) and RAFT agents (dithiobenzoates and trithiocarbonates). Finally, this approach can help to solve one of the major limitations of photopolymerization pertaining to light penetration.

An Oxygen‐Tolerant PET‐RAFT Polymerization for Screening Structure–Activity Relationships

“…[21][22][23] When we applied this system to flow polymerisations,the ZnTPP was able to perform two functions in the polymerisation when activated by visible light (at 560 nm); 1) converting triplet oxygen to singlet oxygen which is irreversibly trapped by the DMSO solvent, and 2) activating the RAFT agent to provide asource of initiation for the CRP. We recently reported an ew photoinduced RAFT (PET-RAFT) polymerization technique using zinc tetraphenylporphyrin (ZnTPP) as acatalyst, which can enable oxygen tolerant polymerisations in organic solvents such as DMSO.…”

Photochemistry for Well‐Defined Polymers in Aqueous Media: From Fundamentals to Polymer Nanoparticles to Bioconjugates

“…The peroxide degrades into hydroxyl radicals, which initiated the polymerizations and resulted in conversions up to 80% with no further irradiation. Further investigation found that conversion was indeed proportional to the amount of air present in the system at reaction start, and reinitiation by addition of air and several minutes of irradiation was also possible . This process is shown in Scheme .…”

“…Proposed mechanism of dark‐driven photopolymerization by storing of photochemical energy. Adapted with permission . Copyright 2017, American Chemical Society.…”

“…In 2017, Boyer and co‐workers also reported an indirect aqueous photoinitiated RAFT system wherein a photocatalyst (Eosin Y or Rose Bengal) generated singlet oxygen under a short irradiation period . The oxygen is then reduced either by triethylamine or ascorbic acid to hydrogen peroxide.…”

Hydroperoxide Traces in Common Cyclic Ethers as Initiators for Controlled RAFT Polymerizations

“…They utilized the so‐called Fenton reaction, i.e., the cleavage of hydrogen peroxide into a hydroxo anion and a hydroxyl radical in the presence of Fe 2+ ions, for initiating ultrafast and well‐controlled RAFT polymerizations of acrylamides and acrylates. Moreover, Boyer and co‐workers investigated the photo‐organocatalyst eosin Y in combination with ascorbic acid to reduce oxygen to hydrogen peroxide and subsequently generate initiating hydroxyl radicals . In addition, enzymes can be utilized to in situ produce hydrogen peroxide from oxygen .…”