Visible Light Photocatalytic Thiol–Ene Reaction: An Elegant Approach for Fast Polymer Postfunctionalization and Step-Growth Polymerization

An elegant approach for fast polymer postfunctionalization and step-growth polymerization (via addition reaction) under aerobic condition was developed from visible light photocatalytic thiol–ene “click” reaction, employing Ru­(bpy)₃Cl₂ as photoredox catalyst and <i>p</i>-toluidine as redox mediator. The nature of the photoredox catalysts, thiol substrates, and solvents were extensively investigated for this reaction with two types of alkene polymers: polybutadiene and poly­(allyl methacrylate)­s. The use of <i>N</i>-methyl-2-pyrrolidone as the solvent and <i>p</i>-toluidine as redox mediator remarkably improved the reaction rates and limited the formation of side products. Finally, this highly efficient thiol–ene reaction was employed for the synthesis of polymers by step-growth addition polymerization

Photoinduced Oxygen Reduction for Dark Polymerization

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 <i>in situ</i> 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

Diblock Copolymer Stabilized Liquid Metal Nanoparticles: Particle Settling Behavior and Application to 3D Printing

Eutectic gallium indium (EGaIn) is a liquid metal with promising applications due to its favorable thermal and electrical conductivity, low viscosity, and metallic nature. For applications, including imaging, catalysis, and nanomedicine, stable EGaIn particles with submicron diameters are required. However, the low viscosity and high density of EGaIn have typically precluded the formation of stable submicron particles due to rapid EGaIn droplet coalescence. In this work, we show that poly(acrylic acid)-block-poly(N,N′-dimethylacrylamide) copolymers are able to effectively stabilize EGaIn nanodroplets formed upon ultrasonication, where the poly(acrylic acid) block anchors the polymer to the EGaIn surface and the poly(N,N′-dimethylacrylamide) block provides colloidal stability to the particles in solution. Although the high density of EGaIn causes rapid particle settling, the behavior is predictable, which allows the average particle size to be controlled through centrifugation. We demonstrate that stable EGaIn particles with sizes on the order of 50–100 nm and narrow particle size distributions can be easily obtained using this method and further used in photopolymer resins to prepare 3D printed EGaIn–polymer hybrid materials. The predictable sizes and high stability of these EGaIn nanoparticles should allow further applications in soft-electronics, nanomedicine, catalysis, and other nanotechnology

Diblock Copolymer Stabilized Liquid Metal Nanoparticles: Particle Settling Behavior and Application to 3D Printing

Eutectic gallium indium (EGaIn) is a liquid metal with promising applications due to its favorable thermal and electrical conductivity, low viscosity, and metallic nature. For applications, including imaging, catalysis, and nanomedicine, stable EGaIn particles with submicron diameters are required. However, the low viscosity and high density of EGaIn have typically precluded the formation of stable submicron particles due to rapid EGaIn droplet coalescence. In this work, we show that poly(acrylic acid)-block-poly(N,N′-dimethylacrylamide) copolymers are able to effectively stabilize EGaIn nanodroplets formed upon ultrasonication, where the poly(acrylic acid) block anchors the polymer to the EGaIn surface and the poly(N,N′-dimethylacrylamide) block provides colloidal stability to the particles in solution. Although the high density of EGaIn causes rapid particle settling, the behavior is predictable, which allows the average particle size to be controlled through centrifugation. We demonstrate that stable EGaIn particles with sizes on the order of 50–100 nm and narrow particle size distributions can be easily obtained using this method and further used in photopolymer resins to prepare 3D printed EGaIn–polymer hybrid materials. The predictable sizes and high stability of these EGaIn nanoparticles should allow further applications in soft-electronics, nanomedicine, catalysis, and other nanotechnology

Aqueous RAFT Photopolymerization with Oxygen Tolerance

The emergence of light regulated controlled/living radical polymerization adds a new layer of control over polymerization. Light mediated polymerizations afford simple and facile route to modulate polymerization rate through manipulation of light intensity and by switching on/off the light source. Such mediation has resulted in the synthesis of 3D surfaces with both spatial and temporal control. However, these techniques present a major limitation in terms of oxygen tolerance and require the use of organic solvent. In this contribution, we report an efficient aqueous polymerization system capable of being activated under visible light in the presence of oxygen. We perform aqueous photopolymerization in the presence of water-soluble zinc porphyrin photocatalyst (Zn­(II) meso-tetra­(4-sulfonato­phenyl)­porphyrin, ZnTPPS^4–) with ascorbic acid as singlet oxygen quencher in both open and closed vessels. Polymers could be prepared without prior deoxygenation with good control over the molecular weight and polydispersity. In addition, polymerization in the presence of air could be achieved with a short inhibition period

Visible Light-Mediated Polymerization-Induced Self-Assembly in the Absence of External Catalyst or Initiator

We report the use of visible light to mediate a RAFT dispersion polymerization in the absence of external catalyst or initiator to yield nanoparticles of different morphologies according to a polymerization-induced self-assembly (PISA) mechanism. A POEGMA macro-chain transfer agent (macro-CTA) derived from a 4-cyano-4-((dodecylsulfanylthiocarbonyl)­sulfanyl)­pentanoic acid (CDTPA) RAFT agent can be activated under blue (460 nm, 0.7 mW/cm²) or green (530 nm, 0.7 mW/cm²) light and act simultaneously as a radical initiator, chain transfer agent, and particle stabilizer under ethanolic dispersion conditions. In particular, the formation of worm-like micelles was readily monitored by the increase of reaction viscosity during the polymerization; this method was shown to be particularly robust to different reaction parameters such as macro-CTAs of varying molecular weight. Interestingly, at high monomer conversion, different morphologies were formed depending on the wavelength of light employed, which may be due to differing degrees of polymerization control. Finally, the in situ encapsulation of the model hydrophobic drug, Nile Red, was demonstrated, suggesting applications of this facile process for the synthesis of nanoparticles for drug delivery applications

Polymerization-Induced Self-Assembly Using Visible Light Mediated Photoinduced Electron Transfer–Reversible Addition–Fragmentation Chain Transfer Polymerization

The ruthenium-based photoredox catalyst, Ru­(bpy)₃Cl₂, was employed to activate reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization via a photoinduced electron transfer (PET) process under visible light (λ = 460 nm, 0.7 mW/cm²). Poly­(oligo­(ethylene glycol) methyl ether methacrylate) was chain extended with benzyl methacrylate to afford in situ self-assembled polymeric nanoparticles with various morphologies. The effect of different intrinsic reaction parameters, such as catalyst concentration, total solids content, and cosolvent addition was investigated with respect to the formation of different nanoparticle morphologies, including spherical micelles, worm-like micelles, and vesicles. Importantly, highly pure worm-like micelles were readily isolated due to the in situ formation of highly viscous gels. Finally, “ON/OFF” control over the dispersion polymerization was demonstrated by online Fourier transform near-infrared (FTNIR) spectroscopy, allowing for temporal control over the nanoparticle morphology

Organic Electron Donor–Acceptor Photoredox Catalysts: Enhanced Catalytic Efficiency toward Controlled Radical Polymerization

In this study, we designed and synthesized novel organic single electron donor–acceptor molecules containing a free base porphyrin and a thiocarbonylthio group. The porphyrin acts as a light-harvesting antenna and donates an excited electron upon light irradiation to the electron-accepting thiocarbonylthio group. The excited electronic state of the donor–acceptor generates a radical from the thiocarbonylthio compound to activate a living radical polymerization in the presence of monomers. Thus, these donor–acceptor systems play the roles of highly efficient photoredox catalysts and radical initiators. The presence of both donor and acceptor in a single molecule enhanced the electron transfer efficiency in comparison to the donor/acceptor mixture and consequently greatly increased polymerization rates of vinyl monomers under visible light irradiation. The polymerizations mediated by these electron donor–acceptor photoredox catalysts were investigated under green (λ_max = 530 nm, 0.7 mW/cm²) and red (λ_max = 635 nm, 0.7 mW/cm²) lights, which exhibited great control over molecular weights, molecular weight distributions, and end-group functionalities

Exploiting Metalloporphyrins for Selective Living Radical Polymerization Tunable over Visible Wavelengths

The use of metalloporphyrins has been gaining popularity particularly in the area of medicine concerning sensitizers for the treatment of cancer and dermatological diseases through photodynamic therapy (PDT), and advanced materials for engineering molecular antenna for harvesting solar energy. In line with the myriad functions of metalloporphyrins, we investigated their capability for photoinduced living polymerization under visible light irradiation over a broad range of wavelengths. We discovered that zinc porphyrins (i.e., zinc tetraphenylporphine (ZnTPP)) were able to selectively activate photoinduced electron transfer–reversible addition–fragmentation chain transfer (PET-RAFT) polymerization of trithiocarbonate compounds for the polymerization of styrene, (meth)­acrylates and (meth)­acrylamides under a broad range of wavelengths (from 435 to 655 nm). Interestingly, other thiocarbonylthio compounds (dithiobenzoate, dithiocarbamate and xanthate) were not effectively activated in the presence of ZnTPP. This selectivity was likely attributed to a specific interaction between ZnTPP and trithiocarbonates, suggesting novel recognition at the molecular level. This interaction between the photoredox catalyst and trithiocarbonate group confers specific properties to this polymerization, such as oxygen tolerance, enabling living radical polymerization in the presence of air and also ability to manipulate the polymerization rates (<i>k</i>_p^app from 1.2–2.6 × 10^–2 min^–1) by varying the visible wavelengths

Oxygen Tolerance Study of Photoinduced Electron Transfer–Reversible Addition–Fragmentation Chain Transfer (PET-RAFT) Polymerization Mediated by Ru(bpy)₃Cl₂

This study reports a highly efficient photoredox catalyst, Ru­(bpy)₃Cl₂, capable of controlling the polymerization of methacrylates, acrylates, and acrylamides in the presence of thiocarbonylthio compounds via a photoinduced electron transfer–reversible addition–fragmentation chain (PET-RAFT) process. This polymerization technique was performed in a closed vessel in the presence or absence of air. Online Fourier transform near-infrared spectroscopy (FTNIR) was employed to monitor the monomer conversions of methyl methacrylate, methyl acrylate, and <i><i><i>N,N</i></i></i>′-dimethylacrylamide in the presence or absence of air. Interestingly, after an induction period, the polymerization proceeded in the presence of air to yield well-defined polymers (PDI < 1.20). The polymers were characterized by ¹H NMR, UV–vis spectroscopy, and gel permeation chromatography. Excellent end-group retention was also demonstrated by NMR, UV–vis, and successive chain extensions of the resulting homopolymers to yield diblock and multiblock copolymers (decablock copolymers)