Metal-Free Atom Transfer Radical Polymerization

Biocompatible Polymeric Analogues of DMSO Prepared by Atom Transfer Radical Polymerization

The synthesis of a sulfoxide-based water-soluble polymer, poly(2-(methylsulfinyl)ethyl acrylate) (polyMSEA), a polymeric analogue of DMSO, by atom transfer radical polymerization (ATRP) is reported. Well-defined linear polymers were synthesized using relatively low amounts of copper catalyst (1000 or 100 ppm). Two types of star polymers were synthesized by either an "arm-first" approach or a "core-first" approach using a biodegradable β-cyclodextrin core. The glass transition temperatures of both the linear polymer (16 °C) and star polymer (32 °C) were determined by differential scanning calorimetry (DSC). The lower critical solution temperature (LCST) of poly(MSEA) was estimated to be ca. 140 °C by extrapolating the LCST of a series of copolymers with NIPAM. Cytotoxicity tests revealed that both the linear and star polymers have low toxicity, even at concentrations up to 3 mg/mL.

Visible Light-Induced Metal Free Surface Initiated Atom Transfer Radical Polymerization of Methyl Methacrylate on SBA-15

Surface-initiated atom transfer radical polymerization (SI-ATRP) is one of the most versatile techniques to modify the surface properties of materials. Recent developed metal-free SI-ATRP makes such techniques more widely applicable. Herein photo-induced metal-free SI-ATRP of methacrylates, such as methyl methacrylate, N-isopropanyl acrylamide, and N,N-dimethylaminoethyl methacrylate, on the surface of SBA-15 was reported to fabricate organic-inorganic hybrid materials. A SBA-15-based polymeric composite with an adjustable graft ratio was obtained. The structure evolution during the SI-ATRP modification of SBA-15 was monitored and verified by FT-IR, XPS, TGA, BET, and TEM. The obtained polymeric composite showed enhanced adsorption ability for the model compound toluene in aqueous conditions. This procedure provides a low-cost, readily available, and easy modification method to synthesize polymeric composites without the contamination of metal.

Dual-pathway chain-end modification of RAFT polymers using visible light and metal-free conditions

We report a metal-free strategy for the chain-end modification of RAFT polymers utilizing visible light. By turning the light source on or off, the reaction pathway in one pot can be switched between either complete desulfurization (hydrogen chain-end) or simple cleavage (thiol chain-end), respectively. The versatility of this process is exemplified by application to a wide range of polymer backbones under mild, quantitative conditions using commercial reagents.

Intramolecular Charge Transfer and Ion Pairing in N,N-Diaryl Dihydrophenazine Photoredox Catalysts for Efficient Organocatalyzed Atom Transfer Radical Polymerization

Photoexcited intramolecular charge transfer (CT) states in N,N-diaryl dihydrophenazine photoredox catalysts are accessed through catalyst design and investigated through combined experimental studies and density functional theory (DFT) calculations. These CT states are reminiscent of the metal to ligand charge transfer (MLCT) states of ruthenium and iridium polypyridyl complexes. For cases where the polar CT state is the lowest energy excited state, we observe its population through significant solvatochromic shifts in emission wavelength across the visible spectrum by varying solvent polarity. We propose the importance of accessing CT states for photoredox catalysis of atom transfer radical polymerization lies in their ability to minimize fluorescence while enhancing electron transfer rates between the photoexcited photoredox catalyst and the substrate. Additionally, solvent polarity influences the deactivation pathway, greatly affecting the strength of ion pairing between the oxidized photocatalyst and the bromide anion and thus the ability to realize a controlled radical polymerization. Greater understanding of these photoredox catalysts with respect to CT and ion pairing enables their application toward the polymerization of methyl methacrylate for the synthesis of polymers with precisely tunable molecular weights and dispersities typically lower than 1.10.

Photoinduced Fe-mediated atom transfer radical polymerization in aqueous media

Photoinduced atom transfer radical polymerization with an Fe catalyst was successfully performed in aqueous media for the first time.

Controlled radical polymerization of vinyl ketones using visible light

Herein we report the photoinduced electron transfer–reversible addition–fragmentation chain transfer (PET-RAFT) polymerization of a range of vinyl ketone monomers including methyl, ethyl and phenyl derivatives, using Eosin Y as an organic photoredox catalyst and visible light.

Photocatalyzed iron-based ATRP of methyl methacrylate using 1,3-dimethyl-2-imidazolidinone as both solvent and ligand

A simple photocatalyzed Fe-based ATRP of MMA was conducted under UV irradiation using the “green” solvent DMI as both the solvent and ligand.

Polymerization of silyl ketenes using alkoxide initiators: a combined computational and experimental study

The atomic composition and chemical structure of polymers is fundamental to dictating properties and applications.

Synthesis of block copolymers by mechanistic transformation from photoinitiated cationic polymerization to a RAFT process

A novel synthetic strategy for the synthesis of block copolymers based on mechanistic transformation from photoinitiated cationic polymerization to radical addition fragmentation transfer polymerization is presented.

Polymer synthesis by mimicking nature's strategy: the combination of ultra-fast RAFT and the Biginelli reaction

A two-stage method has been developed by mimicking nature's protein synthesis strategy to prepare plenty of polymers using limited monomers.

Visible-light-induced living radical polymerization using in situ bromine-iodine transformation as an internal boost

A new visible-light-induced methodology, termed as “bromine-iodine transformation activated living radical polymerization”, was successfully established to build a “bridge” between ATRP and iodine-mediated LRP techniques.

Photoinduced controlled radical polymerization of methacrylates with benzaldehyde derivatives as organic catalysts

Under 23 W CFL irradiation, block copolymers are obtained starting from a P_n-I macroinitiator in the presence of a benzaldehydic molecule-based catalytic system.

Oxygen tolerant photopolymerization for ultralow volumes

A benchtop approach is developed for the synthesis of various polymeric architectures using an aqueous Reversible Addition–Fragmentation chain Transfer (RAFT) photopolymerization technique.

Investigation into the mechanism of photo-mediated RAFT polymerization involving the reversible photolysis of the chain-transfer agent

A new dithiocarbamate with a N-carbazole Z group is synthesized and investigated as a chain-transfer agent (CTA) in a photo-mediated RAFT polymerization mechanism involving its partial and reversible photolysis.

Facile synthesis of size-tunable superparamagnetic/polymeric core/shell nanoparticles by metal-free atom transfer radical polymerization at ambient temperature

Size-tunable superparamagnetic/polymeric core/shell nanoparticles with uniform distribution was fabricated based on metal-free atom transfer radical polymerization at ambient temperature.

Conventional Type II photoinitiators as activators for photoinduced metal-free atom transfer radical polymerization

A novel methodology for photoinduced metal-free Atom Transfer Radical Polymerization (ATRP) by using conventional Type II photoinitiators such as benzophenone, thioxanthone, isopropyl thioxanthone and camphorquinone as sensitizers is presented.

Chlorophyll a crude extract: efficient photo-degradable photocatalyst for PET-RAFT polymerization

This work demonstrates use of spinach extracts for living radical polymerization bypassing catalyst synthesis/purification, degassing and catalyst removal procedures.

Block copolymer synthesis in one shot: concurrent metal-free ATRP and ROP processes under sunlight

A completely metal-free strategy was developed by combining Atom Transfer Radical Polymerization (ATRP) and Ring Opening Polymerization (ROP) for the syntheses of block copolymers.

RAFT iniferter polymerization in miniemulsion using visible light

Methodology for the successful implementation of RAFT (4-cyano-4[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid (CDTPA)) iniferter polymerization of butyl methacrylate in miniemulsion using visible light (green light; λ_max = 530 nm) has been developed.

Natural RAFT Polymerization: Recyclable-Catalyst-Aided, Opened-to-Air, and Sunlight-Photolyzed RAFT Polymerizations

The successful sunlight-photolyzed reversible addition-fragmentation chain transfer (RAFT) photopolymerization can be reversibly activated and deactivated by irradiation with sunlight in the absence of photocatalyst and photoinitiator. In the present work, the thiocarbonylthio compounds (dithiobenzoate, trithiocarbonate, and xanthate) can all be employed to carry out the polymerization under sunlight irradiation acting as an initiator, chain transfer agent, and termination agent. Moreover, it was demonstrated that the recyclable-catalyst-aided, opened-to-air, and sunlight-photolyzed RAFT (ROS-RAFT) polymerizations can be successfully carried out to fabricate precise and predictable polymers in the presence of the recyclable magnetic semiconductor nanoparticles (NPs). The oxygen tolerance is likely attributed to a specific interaction between NPs and oxygen.

Metallocene-Containing Homopolymers and Heterobimetallic Block Copolymers via Photoinduced RAFT Polymerization

We report the synthesis of cationic cobaltocenium and neutral ferrocene containing homopolymers mediated by photoinduced reversible addition-fragmentation chain transfer (RAFT) polymerization with a photocatalyst fac-[Ir(ppy)3]. The homopolymers were further used as macromolecular chain transfer agents to synthesize diblock copolymers via chain extension. Controlled/"living" feature of photoinduced RAFT polymerization was confirmed by kinetic studies even without prior deoxygenation. A light switch between ON and OFF provided a spatiotemporal control of polymerization.

Metal-Free Atom Transfer Radical Polymerization of Methyl Methacrylate with ppm Level of Organic Photocatalyst

It is well known that the recently developed photoinduced metal-free atom transfer radical polymerization (ATRP) has been considered as a promising methodology to completely eliminate transition metal residue in polymers. However, a serious problem needs to be improved, namely, large amount of organic photocatalysts should be used to keep the controllability over molecular weights and molecular weight distributions. In this work, a novel photocatalyst 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) with strong excited state reduction potential is successfully used to mediate a metal-free ATRP of methyl methacrylate just with parts per million (ppm) level usage under irradiation of blue light emitting diode at room temperature, using ethyl α-bromophenyl-acetate as a typical initiator with high initiator efficiency. The polymerization kinetic study, multiple controlled "on-off" light switching cycle regulation, and chain extension experiment confirm the "living"/controlled features of this promising photoinduced metal-free ATRP system with good molecular weight control in the presence of ppm level photocatalyst 4CzIPN.

Organic Electronics: An El Dorado in the Quest of New Photocatalysts for Polymerization Reactions

Photoinitiated polymerization has been the subject of continued research efforts due to the numerous applications in which this polymerization technique is involved (coatings, inks, adhesives, optoelectronic, laser imaging, stereolithography, nanotechnology, etc.). More recently, photopolymerization has received renewed interest due to the emergence of 3D-printing technologies. However, despite current academic and industrial interest in photopolymerization methodologies, a major limitation lies in the slow rates of photopolymerization. The development of new photoinitiating systems aimed at addressing this limitation is an active area of research. Photopolymerization occurs through the exposure of a curable formulation to light, generating radical and/or cationic species to initiate polymerization. At present, photopolymerization is facing numerous challenges related to safety, economic and ecological concerns. Furthermore, practical considerations such as the curing depth and the competition for light absorption between the chromophores and other species in the formulation are key parameters drastically affecting the photopolymerization process. To address these issues, photoinitiating systems operating under low intensity visible light irradiation, in the absence of solvents are highly sought after. In this context, the use of photoredox catalysis can be highly advantageous; that is, photoredox catalysts can provide high reactivities with low catalyst loading, permitting access to high performance photoinitiating systems. However, to act as efficient photoredox catalysts, specific criteria have to be fulfilled. A strong absorption over the visible range, an ability to easily oxidize or reduce as well as sufficient photochemical stability are basic prerequisites to make these molecules desirable candidates for photoredox catalysis. Considering the similarity of requirements between organic electronics and photopolymerization, numerous materials initially designed for applications in organic electronics have been revisited in the context of photopolymerization. Organic electronics is a branch of electronics and materials science focusing on the development of semiconductors devoted to three main research fields; organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic solar cells (OSCs). The contribution of organic electronics to the field of electronics is important as it paves the way toward cheaper, lighter, and more energy efficient devices. In the present context of photopolymerization, materials that were investigated as photocatalysts were indifferently organic semiconductors used for transistors, charge-transport materials, and light-emitting materials used in electroluminescent devices or conjugated polymers and small molecule dyes for solar cells. In this Account, we summarize our latest developments in elaborating on photocatalytic systems based on these new classes of compounds. Through an in-depth understanding of the parameters governing their reactivities and our efforts to incorporate these materials into photoinitiating systems, we provide new knowledge and a valuable insight for future prospects.

Organocatalyzed Atom Transfer Radical Polymerization Using N-Aryl Phenoxazines as Photoredox Catalysts

N-Aryl phenoxazines have been synthesized and introduced as strongly reducing metal-free photoredox catalysts in organocatalyzed atom transfer radical polymerization for the synthesis of well-defined polymers. Experiments confirmed quantum chemical predictions that, like their dihydrophenazine analogs, the photoexcited states of phenoxazine photoredox catalysts are strongly reducing and achieve superior performance when they possess charge transfer character. We compare phenoxazines to previously reported dihydrophenazines and phenothiazines as photoredox catalysts to gain insight into the performance of these catalysts and establish principles for catalyst design. A key finding reveals that maintenance of a planar conformation of the phenoxazine catalyst during the catalytic cycle encourages the synthesis of well-defined macromolecules. Using these principles, we realized a core substituted phenoxazine as a visible light photoredox catalyst that performed superior to UV-absorbing phenoxazines as well as previously reported organic photocatalysts in organocatalyzed atom transfer radical polymerization. Using this catalyst and irradiating with white LEDs resulted in the production of polymers with targeted molecular weights through achieving quantitative initiator efficiencies, which possess dispersities ranging from 1.13 to 1.31.

Metal-free reductive coupling of C[double bond, length as m-dash]O and C[double bond, length as m-dash]N bonds driven by visible light: use of perylene as a simple photoredox catalyst

Perylene, a simple polycyclic aromatic hydrocarbon, was used as a photoredox catalyst to enable the reductive coupling reaction of aromatic aldehydes, ketones, and an imine under visible-light irradiation using a white LED.

Chemoselective Radical Dehalogenation and C-C Bond Formation on Aryl Halide Substrates Using Organic Photoredox Catalysts

Despite the number of methods available for dehalogenation and carbon-carbon bond formation using aryl halides, strategies that provide chemoselectivity for systems bearing multiple carbon-halogen bonds are still needed. Herein, we report the ability to tune the reduction potential of metal-free phenothiazine-based photoredox catalysts and demonstrate the application of these catalysts for chemoselective carbon-halogen bond activation to achieve C-C cross-coupling reactions as well as reductive dehalogenations. This procedure works both for conjugated polyhalides as well as unconjugated substrates. We further illustrate the usefulness of this protocol by intramolecular cyclization of a pyrrole substrate, an advanced building block for a family of natural products known to exhibit biological activity.