Metal-Free Atom Transfer Radical Polymerization

Photoinduced Controlled Radical Polymerizations Performed in Flow: Methods, Products, and Opportunities

Photoinduced controlled radical polymerizations (CRPs) have provided a variety of approaches for the synthesis of polymers possessing targeted structures, compositions, and functionalities with the added capability for spatial and temporal control, presenting the potential for new materials development. However, the scalability and reliability of these systems can be limited as a consequence of dependence on uniform irradiation of the reaction to produce well-defined products. In this perspective, we highlight the utility and promise of photo-CRP approaches through an overview of the adaptation of these methodologies to photo-flow reactor systems. Special emphasis is placed on the current state-of-the-art in polymerization scalability, reactor design, and polymer scope.

Photocontrolled Growth of Cross-Linked Nanonetworks

We report the preparation of photoresponsive nanomaterials and the increase of their nanoscopic size through a "photogrowth" mechanism. The photogrowable nanonetworks (PGNNs) were synthesized by cross-linking two components, a thiolated acrylate copolymer and a symmetrical bismaleimide trithiocarbonate (TTC), utilizing thiol-maleimide click chemistry. With this strategy, nanonetwork growth was achieved through a photoinduced polymerization from the integrated trithiocarbonate by either direct photolysis or photoredox catalysis. Via direct photolysis, we generated a series of expanded particles by polymerizing methyl acrylate (MA) under irradiation with violet light (400 nm) over a period of 1, 3, and 6 h, starting from a 58 nm parent particle, resulting in particles of increased sizes of 77, 156, and 358 nm, respectively. Nanoparticle expansion reactions catalyzed by 10-phenylphenothiazine (PTH) were experienced to progress faster in 20 and 30 min to reach particle sizes of 195 and 300 nm. The addition of the photoredox catalyst to the expansion polymerizations with MA resulted in an increased control over the dispersity of the particles as well as of the promoted disassembly products. In this work, we demonstrated that nanoparticle structures designed as cross-linked networks with integrated trithiocarbonates can be expanded by photocontrolled radical polymerizations (photo-CRPs) in the presence or absence of a photoredox catalyst. These proof-of-concept experiments showcase the dynamic growth and integration of functional units into existing scaffolds and open up the possibility to prepare highly tailorable nanomaterials.

N,N-Diaryl Dihydrophenazines as Photoredox Catalysts for PET-RAFT and Sequential PET-RAFT/O-ATRP

N,N-Diaryl dihydrophenazines are employed as organic photoredox catalysts (PCs) for photoinduced electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization. The ability of these PCs to mediate PET-RAFT is heavily dependent on the ability of the PC to access a photoexcited intramolecular charge transfer state. The use of PCs displaying intramolecular charge transfer in the excited state allows for efficient PET-RAFT of a variety of monomers, including vinyl acetate, and in a wide range of solvents. The ability of these PCs to also mediate organocatalyzed atom transfer radical polymerization (O-ATRP) is exploited to perform a sequential PET-RAFT/O-ATRP block copolymerization of PMA-b-PMMA using the same PC for both polymerizations.

Organocatalytic Approach to Functional Semifluorinated Polymers Driven by Visible Light

Through the construction of an organic photocatalysis system, photoredox catalyst (PC)/additive, where PC stands for photoredox catalyst, an organocatalyzed step transfer-addition and radical-termination (O-START) polymerization irradiated by blue LED light at room temperature is realized. Different types of α,ω-diiodoperfluoroalkane A and α,ω-unconjugated diene B are copolymerized through O-START efficiently, and generate various kinds of functional semifluorinated polymers, including polyolefins and polyesters. The process is affected by several factors; solvents, additives, and feed ratio of A to B. After optimization of all these components, the polymerization efficiency is greatly improved, generating polymers with both relatively high yield and molecular weight. Considering the mild reaction condition, easy operation process, and free-of-metal-catalyst residues in the polymer product, the organocatalytic polymerization strategy provides a simple and efficient approach to functional semifluorinated polymer materials and hopefully opens up their application in high-tech fields.

Progress in Photo-Responsive Polypeptide Derived Nano-Assemblies

Stimuli-responsive polymeric materials have attracted significant attention in a variety of high-value-added and industrial applications during the past decade. Among various stimuli, light is of particular interest as a stimulus because of its unique advantages, such as precisely spatiotemporal control, mild conditions, ease of use, and tunability. In recent years, a lot of effort towards the synthesis of a biocompatible and biodegradable polypeptide has resulted in many examples of photo-responsive nanoparticles. Depending on the specific photochemistry, those polypeptide derived nano-assemblies are capable of crosslinking, disassembling, or morphing into other shapes upon light irradiation. In this mini-review, we aim to assess the current state of photo-responsive polypeptide based nanomaterials. Firstly, those 'smart' nanomaterials will be categorized by their photo-triggered events (i.e., crosslinking, degradation, and isomerization), which are inherently governed by photo-sensitive functionalities, including O-nitrobenzyl, coumarin, azobenzene, cinnamyl, and spiropyran. In addition, the properties and applications of those polypeptide nanomaterials will be highlighted as well. Finally, the current challenges and future directions of this subject will be evaluated.

Advanced Materials by Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) has been successfully employed for the preparation of various advanced materials with controlled architecture. New catalysts with strongly enhanced activity permit more environmentally benign ATRP procedures using ppm levels of catalyst. Precise control over polymer composition, topology, and incorporation of site specific functionality enables synthesis of well-defined gradient, block, comb copolymers, polymers with (hyper)branched structures including stars, densely grafted molecular brushes or networks, as well as inorganic-organic hybrid materials and bioconjugates. Examples of specific applications of functional materials include thermoplastic elastomers, nanostructured carbons, surfactants, dispersants, functionalized surfaces, and biorelated materials.

Phenothiazinimides: Atom-Efficient Electrophilic Amination Reagents

Phenothiazinimides, a fairly unknown class of imines, were prepared and found to be very reactive as ultrasimple atom-efficient electrophilic amination reagents for phenols and indoles under metal-free conditions.

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

This review article highlights recent developments in the field of photochemistry and photochemical reversible deactivation radical polymerization applied to aqueous polymerizations. Photochemistry is a topic of significant interest in the fields of organic, polymer, and materials chemistry because it allows challenging reactions to be performed under mild conditions. Aqueous polymerization is of significant interest because water is an environmentally benign solvent, and the use of water enables complex polymer self-assembly and bioconjugation processes to occur. This review focuses on powerful new developments in photochemical aqueous polymerization reactions and their applications to the synthesis of well-defined polymer nano-objects and bioconjugates. It is anticipated that these aqueous photopolymerizations will enable the next generation of self-assembled structures and biohybrid materials to be developed under mild and environmentally friendly conditions.

Exploiting Charge-Transfer States for Maximizing Intersystem Crossing Yields in Organic Photoredox Catalysts

A key feature of prominent transition-metal-containing photoredox catalysts (PCs) is high quantum yield access to long-lived excited states characterized by a change in spin multiplicity. For organic PCs, challenges emerge for promoting excited-state intersystem crossing (ISC), particularly when potent excited-state reductants are desired. Herein, we report a design exploiting orthogonal π-systems and an intermediate-energy charge-transfer excited state to maximize ISC yields (Φ_ISC) in a highly reducing ( E⁰* = -1.7 V vs SCE), visible-light-absorbing phenoxazine-based PC. Simple substitution of N-phenyl for N-naphthyl is shown to dramatically increase Φ_ISC from 0.11 to 0.91 without altering catalytically important properties, such as E⁰*.

Structure-Property Relationships for Tailoring Phenoxazines as Reducing Photoredox Catalysts

Through the study of structure-property relationships using a combination of experimental and computational analyses, a number of phenoxazine derivatives have been developed as visible light absorbing, organic photoredox catalysts (PCs) with excited state reduction potentials rivaling those of highly reducing transition metal PCs. Time-dependent density functional theory (TD-DFT) computational modeling of the photoexcitation of N-aryl and core modified phenoxazines guided the design of PCs with absorption profiles in the visible regime. In accordance with our previous work with N, N-diaryl dihydrophenazines, characterization of noncore modified N-aryl phenoxazines in the excited state demonstrated that the nature of the N-aryl substituent dictates the ability of the PC to access a charge transfer excited state. However, our current analysis of core modified phenoxazines revealed that these molecules can access a different type of CT excited state which we posit involves a core substituent as the electron acceptor. Modification of the core of phenoxazine derivatives with electron-donating and electron-withdrawing substituents was used to alter triplet energies, excited state reduction potentials, and oxidation potentials of the phenoxazine derivatives. The catalytic activity of these molecules was explored using organocatalyzed atom transfer radical polymerization (O-ATRP) for the synthesis of poly(methyl methacrylate) (PMMA) using white light irradiation. All of the derivatives were determined to be suitable PCs for O-ATRP as indicated by a linear growth of polymer molecular weight as a function of monomer conversion and the ability to synthesize PMMA with moderate to low dispersity (dispersity less than or equal to 1.5) and initiator efficiencies typically greater than 70% at high conversions. However, only PCs that exhibit strong absorption of visible light and strong triplet excited state reduction potentials maintain control over the polymerization during the entire course of the reaction. The structure-property relationships established here will enable the application of these organic PCs for O-ATRP and other photoredox-catalyzed small molecule and polymer syntheses.

Electrochemical Oxidative C-H Amination of Phenols: Access to Triarylamine Derivatives

Dehydrogenative C-H/N-H cross-coupling serves as one of the most straightforward and atom-economical approaches for C-N bond formation. In this work, an electrochemical reaction protocol has been developed for the oxidative C-H amination of unprotected phenols under undivided electrolytic conditions. Neither metal catalysts nor chemical oxidants are needed to facilitate the dehydrogenation process. A series of triarylamine derivatives could be obtained with good functional-group tolerance. The electrolysis is scalable and can be performed at ambient conditions.

Ultrasonication-Induced Aqueous Atom Transfer Radical Polymerization

A new procedure for ultrasonication-induced atom transfer radical polymerization (sono-ATRP) in aqueous media was developed. Polymerizations of oligo(ethylene oxide) methyl ether methacrylate (OEOMA) and 2-hydroxyethyl acrylate (HEA) in water were successfully carried out in the presence of ppm amounts of CuBr₂ catalyst and tris(2-pyridylmethyl)amine ligand when exposed to ultrasonication (40 kHz, 110 W) at room temperature. Aqueous sono-ATRP enabled polymerization of water-soluble monomers with excellent control over the molecular weight, dispersity, and high retention of chain-end functionality. Temporal control over the polymer chain growth was demonstrated by switching the ultrasound on/off due to the regeneration of activators by hydroxyl radicals formed by ultrasonication. The synthesis of a well-defined block copolymer and DNA-polymer biohybrid was also successful using this process.

Benzimidazolium Naphthoxide Betaine Is a Visible Light Promoted Organic Photoredox Catalyst

Benzimidazolium naphthoxide (^-ONap-BI⁺) was first synthesized and utilized as an unprecedented betaine photoredox catalyst. Photoexcited state of ^-ONap-BI⁺ generated by visible light irradiation catalyzes the reductive deiodination as well as desulfonylation reactions in which 1,3-dimethyl-2-phenylbenzimidazoline (Ph-BIH) cooperates with as an electron and hydrogen atom donor. Significant solvent effects on the reaction progress were discovered, and specific solvation toward imidazolium and naphthoxide moieties of ^-ONap-BI⁺ was proposed.

Semiconductor Quantum Dots as Photocatalysts for Controlled Light-Mediated Radical Polymerization

Light-mediated radical polymerization has benefited from the rapid development of photoredox catalysts and offers many exceptional advantages over traditional thermal polymerizations. Nevertheless, the majority of the work relies on molecular photoredox catalysts or expensive transition metals. We exploited the capability of semiconductor quantum dots (QD) as a new type of catalyst for the radical polymerization that can harness natural sunlight. Polymerizations of (meth)acrylates, styrene, and construction of block copolymers were demonstrated, together with temporal control of the polymerization by the light source. Photoluminescence experiments revealed that the reduction of alkyl bromide initiator by photoexcited QD is the key to this light-mediated radical polymerization.

Organocatalyzed, Visible-Light Photoredox-Mediated, One-Pot Minisci Reaction Using Carboxylic Acids via N-(Acyloxy)phthalimides

An improved, one-pot Minisci reaction has been developed using visible light, an organic photocatalyst, and carboxylic acids as radical precursors via the intermediacy of in situ-generated N-(acyloxy)phthalimides. The conditions employed are mild, demonstrate a high degree of functional group tolerance, and do not require a large excess of the carboxylic acid reactant. As a result, this reaction can be applied to drug-like scaffolds and molecules with sensitive functional groups, enabling late-stage functionalization, which is of high interest to medicinal chemistry.

Transition-Metal-Free Suzuki-Type Cross-Coupling Reaction of Benzyl Halides and Boronic Acids via 1,2-Metalate Shift

Cross-coupling of organoboron compounds with electrophiles (Suzuki-Miyaura reaction) has greatly advanced C-C bond formation and has been well received in medicinal chemistry. During the past 50 years, transition metals have played a central role throughout the catalytic cycle of this important transformation. In this process, chemoselectivity among multiple carbon-halogen bonds is a common challenge. In particular, selective oxidative addition of transition metals to alkyl halides rather than aryl halides is difficult due to unfavorable transition states and bond strengths. We describe a new approach that uses a single organic sulfide catalyst to activate both C(sp³) halides and arylboronic acids via a zwitterionic boron "ate" intermediate. This "ate" species undergoes a 1,2-metalate shift to afford Suzuki coupling products using benzyl chlorides and arylboronic acids. Various diaryl methane analogues can be prepared, including those with complex and biologically active motifs. The reactions proceed under transition-metal-free conditions, and C(sp²) halides, including aryl bromides and iodides, are unaffected. The orthogonal chemoselectivity is demonstrated in the streamlined synthesis of highly functionalized diaryl methane scaffolds using multi-halogenated substrates. Preliminary mechanistic experiments suggest both the sulfonium salt and the sulfur ylide are involved in the reaction, with the formation of sulfonium salt being the slowest step in the overall catalytic cycle.

Photocatalysis in organic and polymer synthesis

This review, with over 600 references, summarizes the recent applications of photoredox catalysis for organic transformation and polymer synthesis. Photoredox catalysts are metallo- or organo-compounds capable of absorbing visible light, resulting in an excited state species. This excited state species can donate or accept an electron from other substrates to mediate redox reactions at ambient temperature with high atom efficiency. These catalysts have been successfully implemented for the discovery of novel organic reactions and synthesis of added-value chemicals with an excellent control of selectivity and stereo-regularity. More recently, such catalysts have been implemented by polymer chemists to post-modify polymers in high yields, as well as to effectively catalyze reversible deactivation radical polymerizations and living polymerizations. These catalysts create new approaches for advanced organic transformation and polymer synthesis. The objective of this review is to give an overview of this emerging field to organic and polymer chemists as well as materials scientists.

Photoinduced Cu(II)-Mediated RDRP to P(VDF-co-CTFE)-g-PAN

Photoinduced Cu(II)-mediated reversible deactivation radical polymerization (RDRP) was employed to synthesize poly(vinylidene fluoride-co-chlorotrifluoroethylene)-graft-polyacrylonitrile (P(VDF-co-CTFE)-g-PAN). The concentration of copper catalyst (CuCl₂) loading was as low as 1/64 equivalent to chlorine atom in the presence of Me₆-Tren under UV irradiation. The light-responsive nature of graft polymerization was confirmed by "off-on" impulsive irradiation experiments. Temporal control of the polymerization process and varied graft contents were achieved via this photoinduced Cu(II)-mediated RDRP.

A light-mediated metal-free atom transfer radical chain transfer reaction for the controlled hydrogenation of poly(vinylidene fluoride-chlorotrifluoroethylene)

A light-mediated atom transfer radical chain transfer process is proposed for the controlled hydrogenation of P(VDF-CTFE) using metal-free photocatalyst.

PET-RAFT polymerisation: towards green and precision polymer manufacturing

Photoinduced electron/energy transfer-reversible addition–fragmentation chain transfer (PET-RAFT) process has opened up a new way of precision polymer manufacturing to satisfy the concept of green chemistry.

Preparation of semifluorinated poly(meth)acrylates by improved photo-controlled radical polymerization without the use of a fluorinated RAFT agent: facilitating surface fabrication with fluorinated materials

Semifluorinated poly(meth)acrylates are prepared under both organocatalyzed and catalyst-free photo-controlled radical polymerization conditions from simple RAFT agents.

Photoinduced metal-free atom transfer radical polymerizations: state-of-the-art, mechanistic aspects and applications

Photoinduced atom transfer radical polymerization has recently been the center of intensive research in synthetic polymer chemistry because of the unique possibility of topological and temporal control in addition to precise control of macromolecular structure offered by conventional ATRP.

Hyperbranched polypeptides synthesized from phototriggered ROP of a photocaged Nε-[1-(2-nitrophenyl)ethoxycarbonyl]-l-lysine-N-carboxyanhydride: microstructures and effects of irradiation intensity and nitrogen flow rate

A new photocaged amino acid monomer N_ε-(1-(2-nitrophenyl)ethoxycarbonyl)-l-lysine-N-carboxyanhydride (NPE-Lys NCA) was designed to directly synthesize hyperbranched polypeptides by phototriggered ROP.

Photochemical regulation of a redox-active olefin polymerization catalyst: controlling polyethylene microstructure with visible light

The utility of photoredox chemistry is expanded to include microstructural control of polyolefins.

Externally controlled atom transfer radical polymerization

ATRP can be externally controlled by electrical current, light, mechanical forces and various chemical reducing agents. The mechanistic aspects and preparation of polymers with complex functional architectures and their applications are critically reviewed.

A photocatalyst immobilized on fibrous and porous monolithic cellulose for heterogeneous catalysis of controlled radical polymerization

Fibrous and porous monolithic cellulose was employed to immobilize a photocatalyst for heterogeneously catalysing controlled radical polymerization, which provides superior catalyst recyclability.

Synthesis of poly(3-hexylthiophene) based rod–coil conjugated block copolymers via photoinduced metal-free atom transfer radical polymerization

Photo-mediated metal-free ATRP for the synthesis of P3HT-based rod–coil copolymers.

A sustainable photocontrolled ATRP strategy: facile separation and recycling of a visible-light-mediated catalyst fac-[Ir(ppy)3]

A visible-light-mediated catalyst (fac-[Ir(ppy)₃]) in situ separation and recycling ATRP system for PEG-based water-soluble monomers was constructed.

Industrial development of reversible-deactivation radical polymerization: is the induction period over?

The commercial applications of polymers produced by reversible-deactivation radical polymerization are reviewed here.

Metal Free Reversible-Deactivation Radical Polymerizations: Advances, Challenges, and Opportunities

A considerable amount of the worldwide industrial production of synthetic polymers is currently based on radical polymerization methods. The steadily increasing demand on high performance plastics and tailored polymers which serve specialized applications is driven by the development of new techniques to enable control of polymerization reactions on a molecular level. Contrary to conventional radical polymerization, reversible-deactivation radical polymerization (RDRP) techniques provide the possibility to prepare polymers with well-defined structures and functionalities. The review provides a comprehensive summary over the development of the three most important RDRP methods, which are nitroxide mediated radical polymerization, atom transfer radical polymerization and reversible addition fragmentation chain transfer polymerization. The focus thereby is set on the newest developments in transition metal free systems, which allow using these techniques for biological or biomedical applications. After each section selected examples from materials synthesis and application to biomedical materials are summarized.

Facile modification of electrospun fibrous structures with antifouling zwitterionic hydrogels

Electrospinning technology can easily produce different shaped fibrous structures, making them highly valuable to various biomedical applications. However, surface contamination of biomolecules, cells, or blood has emerged as a significant challenge to the success of electrospun devices, especially artificial blood vessels, catheters and wound dressings etc. Many efforts have been made to resist the surface non-specific biomolecules or cells adsorption, but most of them require complex pre-treatment processes, hard-to-remove metal catalysts or rigorous reaction conditions. In addition, the stability of antifouling coatings, especially in complex conditions, is still a major concern. In this work, inspired by the interpenetrating polymer network and reinforced concrete structure, an efficient and facile strategy for modifying hydrophobic electrospun meshes and tubes with antifouling zwitterionic hydrogels has been introduced. The resulting products could efficiently resist the adhesion of proteins, cells, or even fresh whole blood. Meanwhile, they could maintain the shapes and mechanical strength of the original electrospun structures. Furthermore, the hydrogel structures could retain stable in a physiological condition for at least 3 months. This paper provided a general antifouling and hydrophilicity surface modification strategy for various fibrous structures, and could be of great value for many biomedical applications where antifouling properties are critical.

A Facial Strategy for Catalyst and Reducing Agent Synchronous Separation for AGET ATRP Using Thiol-Grafted Cellulose Paper as Reducing Agent

Atom Transfer Radical Polymerization (ATRP) has been a powerful tool to synthesize well-defined functional polymers, which are widely used in biology, drug/gene delivery and antibacterial materials, etc. However, the potential toxic residues in polymer reduced its service life and limited its applications. In order to overcome the problem, in this work, a novel polymerization system of activators generated by electron transfer for atom transfer radical polymerization (AGET ATRP) for synchronous separation of the metal catalyst and byproduct of reducing agent was developed, using thiol-grafted cellulose paper (Cell-SH) as a solid reducing agent. The polymerization kinetics were investigated in detail, and the “living” features of the novel polymerization system were confirmed by chain-end analysis and chain extension experiment for the resultant polymethyl methacrylate (PMMA). It is noted that the copper residual in obtained PMMA was less than 20 ppm, just by filtering the sheet-like byproduct of the reducing agent.