Metal-Free Atom Transfer Radical Polymerization

One-step synthesis of poly(methacrylate)-b-polyester via “one organocatalyst, two polymerizations”

In a “one organocatalyst, two polymerizations” system, triarylsulfonium hexafluorophosphate salt could spontaneously catalyze photo-ATRP and ROP. A well-defined PTMC-b-PMMA block copolymer was successfully synthesized in one-step.

Aluminium(iii) and zinc(ii) complexes of azobenzene-containing ligands for ring-opening polymerisation of ε-caprolactone and rac-lactide

The ability to control the outcome of polymerisations using an external stimulus remains a formidable challenge.

Thiol ligand capped quantum dot as an efficient and oxygen tolerance photoinitiator for aqueous phase radical polymerization and 3D printing under visible light

We report here a rapid visible-light-induced radical polymerization in aqueous media photoinitiated by only ppm level thiol ligand capped cadmium selenide quantum dots. The photoinitiation system could be readily employed for photo 3D printing.

A covalent organic framework as a photocatalyst for atom transfer radical polymerization under white light irradiation

TFPPy-Td-COFs have been synthesized to serve as heterogeneous photocatalysts for mediating photo-induced ATRP with copper as a co-catalyst under white light irradiation.

Recent advancements in the development of molecular organic photocatalysts

Research in the development of molecular organic photocatalysts for applications in chemical syntheses has burgeoned in recent years. While organic photosensitizers have been known for over a century, tuning the properties of these molecules to increase photocatalytic efficiencies is now of growing importance. The properties that help improve the performance of organic photocatalysts include: a wider range of redox potentials, increased molar absorptivity (ε) in the visible spectrum, increased quantum yields (Φ), long-lived excited-state lifetimes (ns to μs), and increased chemical stability. This review examines some of the recent advancements in the development of molecular organic photocatalysts, specifically cyanoarenes, acridinium dyes, phenazines, thiazines, oxazines, and xanthenes, with respect to these properties and examines the chemical synthesis routes now achieved by organic photocatalysts.

Achieving Ultrahigh Molecular Weights with Diverse Architectures for Unconjugated Monomers through Oxygen-Tolerant Photoenzymatic RAFT Polymerization

Achieving well-defined polymers with ultrahigh molecular weight (UHMW) is an enduring pursuit in the field of reversible deactivation radical polymerization. Synthetic protocols have been successfully developed to achieve UHMWs with low dispersities exclusively from conjugated monomers while no polymerization of unconjugated monomers has provided the same level of control. Herein, an oxygen-tolerant photoenzymatic RAFT (reversible addition-fragmentation chain transfer) polymerization was exploited to tackle this challenge for unconjugated monomers at 10 °C, enabling facile synthesis of well-defined, linear and star polymers with near-quantitative conversions, unprecedented UHMWs and low dispersities. The exquisite level of control over composition, MW and architecture, coupled with operational ease, mild conditions and environmental friendliness, broadens the monomer scope to include unconjugated monomers, and to achieve previously inaccessible low-dispersity UHMWs.

Organo-photoredox-Catalyzed Atom-Transfer Radical Substitution of Alkenes with α-Carbonyl Alkyl Halides

A light-driven atom-transfer radical substitution (ATRS) and carboesterification reaction of alkenes with alkyl halides has been developed using PTH as the organo-photoredox catalyst. Two types of products were obtained, depending on the additive and solvent used during the reaction. Primary, secondary, and tertiary alkyl halides reacted to give the ATRS products. This protocol has several advantages: it requires mild reaction conditions and a low catalyst loading and exhibits a broad substrate scope and good functional group tolerance. Mechanistic studies indicate that alkyl radicals might be generated as the key intermediates via photocatalysis, providing a new direction for ATRS reactions.

Solvent Effects and Side Reactions in Organocatalyzed Atom Transfer Radical Polymerization for Enabling the Controlled Polymerization of Acrylates Catalyzed by Diaryl Dihydrophenazines

Investigation of the effects of a solvent on the photophysical and redox properties of the photoredox catalyst (PC), N,N-di(2-naphthyl)-5,10-dihydrophenazine (PC 1), revealed the opportunity to use tetrahydrofuran (THF) to modulate the reactivity of PC 1 toward achieving a controlled organocatalyzed atom transfer radial polymerization (O-ATRP) of acrylates. Compared with dimethylacetamide (DMAc), in tetrahydrofuran (THF), PC 1 exhibits a higher quantum yield of intersystem crossing (ΦISC = 0.02 in DMAc, 0.30 in THF), a longer singlet excited-state lifetime (τ Singlet = 3.81 ns in DMAc, 21.5 ns in THF), and a longer triplet excited-state lifetime (τ Triplet = 4.3 μs in DMAc, 15.2 μs in THF). Destabilization of 1 •+, the proposed polymerization deactivator, in THF leads to an increase in the oxidation potential of this species by 120 mV (E 1/2 0 = 0.22 V vs SCE in DMAc, 0.34 V vs SCE in THF). The O-ATRP of n-butyl acrylate (n-BA) catalyzed by PC 1 proceeds in a more controlled fashion in THF than in DMAc, producing P(n-BA) with low dispersity, Đ (Đ < 1.2). Model reactions and spectroscopic experiments revealed that two initiator-derived alkyl radicals add to the core of PC 1 to form an alkyl-substituted photocatalyst (2) during the polymerization. PC 2 accesses a polar CT excited state that is ~40 meV higher in energy than PC 1 and forms a slightly more oxidizing radical cation (E 1/2 0 = 0.22 V for 1 •+ and 0.25 V for 2 •+ in DMAc). A new O-ATRP procedure was developed wherein PC 1 is converted to 2 in situ. The application of this method enabled the O-ATRP of a number of acrylates to proceed with moderate to good control (Đ = 1.15-1.45 and I* = 83-127%).

Organocatalyzed Photoredox Radical Ring-Opening Polymerization of Functionalized Vinylcyclopropanes

Organocatalyzed photoredox radical ring-opening polymerization (rROP) of vinylcyclopropanes (VCPs) is employed for the synthesis of polymers with controlled molecular weight (MW), dispersity, and composition. Herein, we report the study on the rROP of a variety of VCP monomers bearing diverse functional groups (such as amide, alkene, ketal, urea, hemiaminal ether, and so on) under organocatalyzed conditions with varying light sources and temperature. Notably, VCP monomers bearing natural product functionality or their derivatives can be polymerized in a controlled manner to produce poly(VCPs) with predictable MW, low dispersity, tunable composition, high thermal stability, and tailored glass transition temperature (T g), ranging 39 to 107 °C. Lastly, successful "grafting through" synthesis of molecular brush copolymers containing 1.0 or 5.0 kDa polydimethylsiloxane (PDMS) side chains from readily accessible EtVCP-PDMS macromonomers further demonstrates the robustness of this organocatalyzed photoredox rROP.

Organocatalytic Approach to Photochemical Lignin Fragmentation

Herein, an organocatalytic method for photochemical C-O bond cleavage of lignin systems is reported. The use of photochemistry enabled fragmentation of the β-O-4 linkage, the primary linkage in lignin, provides the fragmentation products in good to high yields. The approach was merged with reported oxidation conditions in a one-pot, two-step platform without any intermediary purification, suggesting its high fidelity. The future utility of the organocatalytic method was illustrated by applying the visible light-mediated protocol to continuous flow processing.

The photocatalytic mechanism of organic dithienophosphole derivatives as highly efficient photo-redox catalysts

The use of organic photo-redox catalysts to initiate well-controlled photochemical reactions has aroused great interest. The development of visible light-driven photocatalytic reactions, which enable rapid and efficient synthesis of fine products, is highly desired from the perspective of being able to achieve low cost, good reversibility, and environmental friendliness. Herein, the organic photocatalytic cycle, with organic dithienophosphole (DTP) derivatives Ph-DTP and TPA-DTP as the photo-redox catalysts, and iodonium salt (Ar2I+) and ethyl 4-(dimethylamino)benzoate (EDB) as the respective acceptor and donor substrates, is fully analyzed by using density functional theory and dissociative electron transfer theory. We show that the strong redox potentials in the excited state as well as the sufficiently long-lived excited state of both DTP derivatives are a robust driving force for activating the electron acceptor Ar2I+ in the activation process. Moreover, the activation barriers of electron transfer are only 0.43-11.9 kcal mol-1 for the different activation pathways. During the deactivation process, the reaction energy profiles indicate that EDB plays a vital role in reducing DTPs˙+ to their initial states. Importantly, the activation barriers and rate constants in both activation and deactivation processes obtained in this study are better than those of classic Cu-based and metal-free Ph-PTZ-based photo-redox catalysts. The excellent performance of both DTP derivatives thus enables them to be highly efficient organic photo-redox catalysts.

Metal-Free ATRP Catalyzed by Visible Light in Continuous Flow

ATRP of methyl methacrylate catalyzed by Eosin Y, an inexpensive and an environmental benign dye, was performed in a continuous flow reactor made of FEP tubing and irradiated by visible light green LEDs. The reaction under flow conditions was significantly more rapid and controlled compared to that in batch giving 90% of polymerization after only 3 h of irradiation. The formed polymers in flow have M n measured by GPC and DOSY NMR in accordance with the theoretical values and show low dispersities (Ð < 1.5). The livingness of the polymers has been confirmed by LED on and LED off experiments and by the synthesis of block copolymers. The protocol described herein serves as a "proof of concept" of using Eosin Y as a photocatalyst for controlled polymerization and of using 1D and 2D NMR for polymer characterization. The protocol could be replicated in the future for other reversible-deactivation radical polymerizations.

Dynamic, multimodal hydrogel actuators using porphyrin-based visible light photoredox catalysis in a thermoresponsive polymer network

Hydrogels that can respond to multiple external stimuli represent the next generation of advanced functional biomaterials. Here, a series of multimodal hydrogels were synthesized that can contract and expand reversibly over several cycles while changing their mechanical properties in response to blue and red light, as well as heat (∼50 °C). The light-responsive behavior was achieved through a photoredox-based mechanism consisting of photoinduced electron transfer from a zinc porphyrin photocatalyst in its excited state to oligoviologen-based macrocrosslinkers, both of which were integrated into the hydrogel polymer network during gel formation. Orthogonal thermoresponsive properties were also realized by introducing N-isopropyl acrylamide (NIPAM) monomer simultaneously with hydroxyethyl acrylate (HEA) in the pre-gel mixture to produce a statistical 60 : 40 HEA : NIPAM polymer network. The resultant hydrogel actuators - crosslinked with either a styrenated viologen dimer (2V4+-St) or hexamer (6V12+-St) - were exposed to red or blue light, or heat, for up to 5 h, and their rate of contraction, as well as the corresponding changes in their physical properties (i.e., stiffness, tensile strength, Young's modulus, etc.), were measured. The combined application of blue light and heat to the 6V12+-St-based hydrogels was also demonstrated, resulting in hydrogels with more than two-fold faster contraction kinetics and dramatically enhanced mechanical robustness when fully contracted. We envision that the reported materials and the corresponding methods of remotely manipulating the dynamic hydrogels may serve as a useful blueprint for future adaptive materials used in biomedical applications.

Development of Environmentally Friendly Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) is one of the most successful techniques for the preparation of well-defined polymers with controllable molecular weights, narrow molecular weight distributions, specific macromolecular architectures, and precisely designed functionalities. ATRP usually involves transition-metal complex as catalyst. As the most commonly used copper complex catalyst is usually biologically toxic and environmentally unsafe, considerable interest has been focused on iron complex, enzyme, and metal-free catalysts owing to their low toxicity, inexpensive cost, commercial availability and environmental friendliness. This review aims to provide a comprehensive understanding of iron catalyst used in normal, reverse, AGET, ICAR, GAMA, and SARA ATRP, enzyme as well as metal-free catalyst mediated ATRP in the point of view of catalytic activity, initiation efficiency, and polymerization controllability. The principle of ATRP and the development of iron ligand are briefly discussed. The recent development of enzyme-mediated ATRP, the latest research progress on metal-free ATRP, and the application of metal-free ATRP in interdisciplinary areas are highlighted in sections. The prospects and challenges of these three ATRP techniques are also described in the review.

Stimuli-Responsive Rifampicin-Based Macromolecules

This paper presents the modification of the antibiotic rifampicin by an anionic polyelectrolyte using a simplified electrochemically mediated atom transfer radical polymerization (seATRP) technique to receive stimuli-responsive polymer materials. Initially, a supramolecular ATRP initiator was prepared by an esterification reaction of rifampicin hydroxyl groups with α-bromoisobutyryl bromide (BriBBr). The structure of the initiator was successfully proved by nuclear magnetic resonance (1H and 13C NMR), Fourier-transform infrared (FT-IR) and ultraviolet-visible (UV-vis) spectroscopy. The prepared rifampicin-based macroinitiator was electrochemically investigated among various ATRP catalytic complexes, by a series of cyclic voltammetry (CV) measurements, determining the rate constants of electrochemical catalytic (EC') process. Macromolecules with rifampicin core and hydrophobic poly (n-butyl acrylate) (PnBA) and poly(tert-butyl acrylate) (PtBA) side chains were synthesized in a controlled manner, receiving polymers with narrow molecular weight distribution (Mw/Mn = 1.29 and 1.58, respectively). "Smart" polymer materials sensitive to pH changes were provided by transformation of tBA into acrylic acid (AA) moieties in a facile route by acidic hydrolysis. The pH-dependent behavior of prepared macromolecules was investigated by dynamic light scattering (DLS) determining a hydrodynamic radius of polymers upon pH changes, followed by a control release of quercetin as a model active substance upon pH changes.

Impacts of Performing Electrolysis During Organocatalyzed Atom Transfer Radical Polymerization

An electrochemical variant of organocatalyzed atom transfer radical polymerization (O-ATRP) is developed and investigated. Inspired by electrochemically mediated atom transfer radical polymerization (eATRP), potentiostatic electrolysis is used to manipulate the catalyst's redox states in O-ATRP to understand whether deactivation in O-ATRP can be enhanced to improve polymerization control. During the course of this work, several possible side reactions are investigated, and the electrochemical apparatus is optimized to reduce side reactions at the counter electrode. This electrochemically modified O-ATRP method (eO-ATRP) is then studied at different applied potentials, under different irradiation conditions, and with two photoredox catalysts to understand the impact of electrolysis on polymerization control. Ultimately, although electrolysis was successfully used to improve polymerization control in O-ATRP, some additional challenges have been identified. Several key questions are postulated to guide future work in this area.

In-situ syntheses of graft copolymers by metal-free strategies: combination of photoATRP and ROP

A completely metal-free and environmentally friendly strategy is demonstrated for the preparation of graft copolymers by combining photoinduced Atom Transfer Radical Polymerization (ATRP) and Ring Opening Polymerization (ROP). Polymerizations are simultaneously realized in a one-pot manner. For this purpose, bare vinyl monomers, vinyl monomers with hydroxyl functional groups, and lactone monomers were simultaneously polymerized under visible light using specific catalysts. While vinyl monomers construct the main chain, the lactone monomers were polymerized from the hydroxyl functions present at the side chain. Spectral and chromatographic analyses prove that the utilized strategy is successful in the preparation of graft copolymers controlled molecular weights and narrow distributions.

NIR Light-Induced ATRP for Synthesis of Block Copolymers Comprising UV-Absorbing Moieties

NIR exposure at 790 nm activated photopolymerization of monomers comprising UV-absorbing moieties by using [CuII /(TPMA)]Br2 (TPMA=tris(2-pyridylmethyl)amine) in the ppm range and an alkyl bromide as initiator. Some of them comprised structural elements selected either from those showing proton transfer or photocycloaddition upon UV excitation. Polymers obtained comprise living end groups serving as macroinitiator for controlled synthesis of block copolymers with relatively narrow molecular weight distributions. Chromatographic results indicated formation of block copolymers produced by this synthetic approach. Free-radical polymerization of monomers pursued for comparison exhibited the expected broader dispersity of molecular weight compared to photo-ATRP. Polymerization of these monomers by UV photo-ATRP failed on the contrary to NIR photo-ATRP demonstrating the UV-filter function of the monomers. This work conclusively provides a new approach for the polymerization of monomers comprising UV-absorbing moieties through photo-ATRP in the NIR region. This occurred in a simple and efficient pathway. However, studies also showed that not all monomers chosen successfully proceeded in the NIR photo-ATRP protocol.

Amphiphilic Polymeric Nanoparticles for Photoredox Catalysis in Water

Photoredox catalysis has recently emerged as a powerful synthesis tool in organic and polymer chemistry. In contrast to the great achievements realized in organic solvents, performing photocatalytic processes efficiently in aqueous media encounters several challenges. Here, it is presented how amphiphilic single-chain polymeric nanoparticles (SCPNs) can be utilized as small reactors to conduct light-driven chemical reactions in water. By incorporating a phenothiazine (PTH) catalyst into the polymeric scaffold, metal-free reduction and C-C cross-coupling reactions can be carried out upon exposure to UV light under ambient conditions. The versatility of this approach is underlined by a large substrate scope, tolerance towards oxygen, and excellent recyclability. This approach thereby contributes to a sustainable and green way of implementing photoredox catalysis.

Solvent-dependent photochemical dynamics of a phenoxazine-based photoredox catalyst

Organic substitutes for ruthenium and iridium complexes are increasingly finding applications in chemical syntheses involving photoredox catalysis. However, the performance of these organic compounds as electron-transfer photocatalysts depends on their accessible photochemical pathways and excited state lifetimes. Here, the UV-induced dynamics of N-phenyl phenoxazine, chosen as a prototypical N-aryl phenoxazine organic photoredox catalyst, are explored in three solvents, N,N-dimethyl formamide, dichloromethane and toluene, using ultrafast transient absorption spectroscopy. Quantum chemistry calculations reveal the locally excited or charge-transfer electronic character of the excited states, and are used to assign the transient electronic and vibrational bands observed. In toluene-d8, complete ground-state recovery is (31 ± 3) % by internal conversion (IC) from the photo-excited state (or from S1 after IC but before complete vibrational relaxation), (13 ± 2) % via direct decay from vibrationally relaxed S1 (most likely radiative decay, with an estimated radiative lifetime of 13 ns) and (56 ± 3) % via the T1 state (with intersystem crossing (ISC) rate coefficient k ISC = (3.3 ± 0.2) × 108 s−1). In dichloromethane, we find evidence for excited state N-phenyl phenoxazine reaction with the solvent. Excited state lifetimes, ISC rates, and ground-state recovery show only modest variation with changes to the solvent environment because of the locally excited character of the S1 and T1 states.

Pressure-assisted solvent- and catalyst-free production of well-defined poly(1-vinyl-2-pyrrolidone) for biomedical applications

In this work, we developed a fast, highly efficient, and environmentally friendly catalytic system for classical free-radical polymerization (FRP) utilizing a high-pressure (HP) approach. The application of HP for thermally-induced, bulk FRP of 1-vinyl-2-pyrrolidone (VP) allowed to eliminate the current limitation of ambient-pressure polymerization of ‘less-activated’ monomer (LAM), characterized by the lack of temporal control yielding polymers of unacceptably large disperisites and poor result reproducibility. By a simple manipulation of thermodynamic conditions (p = 125–500 MPa, T = 323–333 K) and reaction composition (two-component system: monomer and low content of thermoinitiator) well-defined poly(1-vinyl-2-pyrrolidone)s (PVP) in a wide range of molecular weights and low/moderate dispersities (M_n = 16.2–280.5 kg mol⁻¹, Đ = 1.27–1.45) have been produced. We have found that HP can act as an ‘external’ controlling factor that warrants the first-order polymerization kinetics for classical FRP, something that was possible so far only for reversible deactivation radical polymerization (RDRP) systems. Importantly, our synthetic strategy adopted for VP FRP enabled us to obtain polymers of very high M_n in a very short time-frame (0.5 h). It has also been confirmed that VP bulk polymerization yields polymers with significantly lower glass transition temperatures (T_g) and different solubility properties in comparison to macromolecules obtained during the solvent-assisted reaction.

High-pressure classical free-radical polymerization allowed to eliminate the current limitation of the ambient-pressure synthesis of 1-vinyl-2-pyrrolidone and production of well-defined polymers.

Lead Halide Perovskite Nanocrystals as Photocatalysts for PET-RAFT Polymerization under Visible and Near-Infrared Irradiation

A key challenge of harvesting solar energy for chemical transformations is the scarcity of photocatalysts with broad activation wavelength and easily tunable band structures. Here, we introduce lead halide perovskite (CsPbBr₃) nanocrystals as band-edge-tunable photocatalysts for efficient photoinduced electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization. PET-RAFT polymerization of various functional monomers is successfully conducted using a broad range of irradiation sources ranging from blue to red light (460 to 635 nm), resulting in polymer products with narrow dispersity (Đ = 1.02-1.13) and high degree of chain-end fidelity. Furthermore, the giant two-photon absorption cross-section of CsPbBr₃ enables activation with a light source in the near-infrared region (laser pulses centered at 800 nm) for the PET-RAFT process.

Designing with Light: Advanced 2D, 3D, and 4D Materials

Recent achievements and future opportunities for the design of 2D, 3D, and 4D materials using photochemical reactions are summarized. Light is an attractive stimulus for material design due to its outstanding spatiotemporal control, and its ability to mediate rapid polymerization under moderate reaction temperatures. These features have been significantly enhanced by major advances in light generation/manipulation with light-emitting diodes and optical fiber technologies which now allows for a broad range of cost-effective fabrication protocols. This combination is driving the preparation of sophisticated 2D, 3D, and 4D materials at the nano-, micro-, and macrosize scales. Looking ahead, future challenges and opportunities that will significantly impact the field and help shape the future of light as a versatile and tunable design tool are highlighted.