ATRP in Water: Kinetic Analysis of Active and Super-Active Catalysts for Enhanced Polymerization Control

ATRP with ppb Concentrations of Photocatalysts

In atom transfer radical polymerization (ATRP), dormant alkyl halides are intermittently activated to form growing radicals in the presence of a CuI/L/X-CuII/L (activator/deactivator) catalytic system. Recently developed very active copper complexes could decrease the catalyst concentration to ppm level. However, unavoidable radical termination results in irreversible oxidation of the activator to the deactivator species, leading to limited monomer conversions. Therefore, successful ATRP at a low catalyst loading requires continuous regeneration of the activators. Such a regenerative ATRP can be performed with various reducing agents under milder reaction conditions and with catalyst concentrations diminished in comparison to conventional ATRP. Photoinduced ATRP (PhotoATRP) is one of the most efficient methods of activator regeneration. It initially employed UV irradiation to reduce the air-stable excited X-CuII/L deactivators to the activators in the presence of sacrificial electron donors. Photocatalysts (PCs) can be excited after absorbing light at longer wavelengths and, due to their favorable redox potentials, can reduce X-CuII/L to CuI/L. Herein, we present the application of three commercially available xanthene dyes as ATRP PCs: rose bengal (RB), rhodamine B (RD), and rhodamine 6G (RD-6G). Even at very low Cu catalyst concentrations (50 ppm), they successfully controlled PhotoATRP. Well-defined polymers with preserved livingness were prepared under green LED irradiation, with subppm concentrations ([PC] ≥ 10 ppb) of RB and RD-6G or 5 ppm of RD. Interestingly, these PCs efficiently controlled ATRP at wavelengths longer than their absorption maxima but required higher loadings. Polymerizations proceeded with high initiation efficiencies, yielding polymers with narrow molecular weight distributions and high chain-end fidelity. UV-vis, fluorescence, and laser flash photolysis studies helped to elucidate the mechanism of the processes involved in the dual-catalytic systems, comprising parts per million of Cu complexes and parts per billion of PCs.

Aqueous photo-RAFT polymerization under ambient conditions: synthesis of protein-polymer hybrids in open air

A photoinduced reversible addition-fragmentation chain-transfer (photo-RAFT) polymerization technique in the presence of sodium pyruvate (SP) and pyruvic acid derivatives was developed. Depending on the wavelength of light used, SP acted as a biocompatible photoinitiator or promoter for polymerization, allowing rapid open-to-air polymerization in aqueous media. Under UV irradiation (370 nm), SP decomposes to generate CO2 and radicals, initiating polymerization. Under blue (450 nm) or green (525 nm) irradiation, SP enhances the polymerization rate via interaction with the excited state RAFT agent. This method enabled the polymerization of a range of hydrophilic monomers in reaction volumes up to 250 mL, eliminating the need to remove radical inhibitors from the monomers. In addition, photo-RAFT polymerization using SP allowed for the facile synthesis of protein-polymer hybrids in short reaction times (<1 h), low organic content (≤16%), and without rigorous deoxygenation and the use of transition metal photocatalysts. Enzymatic studies of a model protein (chymotrypsin) showed that despite a significant loss of protein activity after conjugation with RAFT chain transfer agents, the grafting polymers from proteins resulted in a 3-4-fold recovery of protein activity.

Hydrophilic Poly(meth)acrylates by Controlled Radical Branching Polymerization: Hyperbranching and Fragmentation

Topology significantly impacts polymer properties and applications. Hyperbranched polymers (HBPs) synthesized via atom transfer radical polymerization (ATRP) using inimers typically exhibit broad molecular weight distributions and limited control over branching. Alternatively, copolymerization of inibramers (IB), such as α-chloro/bromo acrylates with vinyl monomers, yields HBPs with precise and uniform branching. Herein, we described the synthesis of hydrophilic HB polyacrylates in water by copolymerizing a water-soluble IB, oligo(ethylene oxide) methyl ether 2-bromoacrylate (OEOBA), with various hydrophilic acrylate comonomers. Visible-light-mediated controlled radical branching polymerization (CRBP) with dual catalysis using eosin Y (EY) and copper complexes resulted in HBPs with various molecular weights (M n = 38 000 to 170 000) and degrees of branching (2%-24%). Furthermore, the optimized conditions enabled the successful application of the OEOBA to synthesize linear-hyperbranched block copolymers and hyperbranched polymer protein hybrids (HB-PPH), demonstrating its potential to advance the synthesis of complex macromolecular architecture under environmentally benign conditions. Copolymerization of hydrophilic methacrylate monomer, oligo(ethylene oxide) methyl ether methacrylate (OEOMA500), and inibramer OEOBA was accompanied by fragmentation via β-carbon C-C bond scission and subsequent growth of polymer chains from the fragments. Furthermore, computational studies investigating the fragmentation depending on the IB and comonomer structure supported the experimental observations. This work expands the toolkit of water-soluble inibramers for CRBP and highlights the critical influence of the inibramer structure on reaction outcomes.

Selective and Controlled Grafting from PVDF-Based Materials by Oxygen-Tolerant Green-Light-Mediated ATRP

Poly(vinylidene fluoride) (PVDF) shows excellent chemical and thermal resistance and displays high dielectric strength and unique piezoelectricity, which are enabling for applications in membranes, electric insulators, sensors, or power generators. However, its low polarity and lack of functional groups limit wider applications. While inert, PVDF has been modified by grafting polymer chains by atom transfer radical polymerization (ATRP), albeit via an unclear mechanism, given the strong C-F bonds. Herein, we applied eosin Y and green-light-mediated ATRP to modify PVDF-based materials. The method gave nearly quantitative (meth)acrylate monomer conversions within 2 h without deoxygenation and without the formation of unattached homopolymers, as confirmed by control experiments and DOSY NMR measurements. The gamma distribution model that accounts for broadly dispersed polymers in DOSY experiments was essential and serves as a powerful tool for the analysis of PVDF. The NMR analysis of poly(methyl acrylate) graft chain-ends on PVDF-CTFE (statistical copolymer with chlorotrifluoroethylene) was carried out successfully for the first time and showed up to 23 grafts per PVDF-CTFE chain. The grafting density was tunable depending on the solvent composition and light intensity during the grafting. The initiation proceeded either from the C-Cl sites of PVDF-CTFE or via unsaturations in the PVDF backbones. The dehydrofluorinated PVDF was 20 times more active than saturated PVDF during the grafting. The method was successfully applied to modify PVDF, PVDF-HFP, and Viton A401C. The obtained PVDF-CTFE-g-PnBMA materials were investigated in more detail. They featured slightly lower crystallinity than PVDF-CTFE (12-18 vs 24.3%) and had greatly improved mechanical performance: Young's moduli of up to 488 MPa, ductility of 316%, and toughness of 46 × 106 J/m3.

Visible-light-induced ATRP under high-pressure: synthesis of ultra-high-molecular-weight polymers

In this study, a high-pressure-assisted photoinduced atom transfer radical polymerization (p ≤ 250 MPa) enabled the synthesis of ultra-high-molecular-weight polymers (UHMWPs) of up to 9 350 000 and low/moderate dispersity (1.10 < Đ < 1.46) in a co-solvent system (water/DMSO), without reaction mixture deoxygenation.

Red-Light-Driven Atom Transfer Radical Polymerization for High-Throughput Polymer Synthesis in Open Air

Photoinduced reversible-deactivation radical polymerization (photo-RDRP) techniques offer exceptional control over polymerization, providing access to well-defined polymers and hybrid materials with complex architectures. However, most photo-RDRP methods rely on UV/visible light or photoredox catalysts (PCs), which require complex multistep synthesis. Herein, we present the first example of fully oxygen-tolerant red/NIR-light-mediated photoinduced atom transfer radical polymerization (photo-ATRP) in a high-throughput manner under biologically relevant conditions. The method uses commercially available methylene blue (MB+) as the PC and [X-CuII/TPMA]+ (TPMA = tris(2-pyridylmethyl)amine) complex as the deactivator. The mechanistic study revealed that MB+ undergoes a reductive quenching cycle in the presence of the TPMA ligand used in excess. The formed semireduced MB (MB•) sustains polymerization by regenerating the [CuI/TPMA]+ activator and together with [X-CuII/TPMA]+ provides control over the polymerization. This dual catalytic system exhibited excellent oxygen tolerance, enabling polymerizations with high monomer conversions (>90%) in less than 60 min at low volumes (50-250 μL) and high-throughput synthesis of a library of well-defined polymers and DNA-polymer bioconjugates with narrow molecular weight distributions (Đ < 1.30) in an open-air 96-well plate. In addition, the broad absorption spectrum of MB+ allowed ATRP to be triggered under UV to NIR irradiation (395-730 nm). This opens avenues for the integration of orthogonal photoinduced reactions. Finally, the MB+/Cu catalysis showed good biocompatibility during polymerization in the presence of cells, which expands the potential applications of this method.

Localised polymerisation of acrylamide using single-barrel scanning electrochemical cell microscopy

Single-barrel scanning electrochemical cell microscopy has been adapted for polymerisation of acrylamide in droplet cells formed at gold electrode surfaces. Localised electrochemical atom transfer radical polymerisation enables controlled synthesis and deposition of polyacrylamide or synthesis of polyacrylamide brushes from initiator-functionalised electrode surfaces.

Fully Oxygen-Tolerant Visible-Light-Induced ATRP of Acrylates in Water: Toward Synthesis of Protein-Polymer Hybrids

Over the last decade, photoinduced ATRP techniques have been developed to harness the energy of light to generate radicals. Most of these methods require the use of UV light to initiate polymerization. However, UV light has several disadvantages: it can degrade proteins, damage DNA, cause undesirable side reactions, and has low penetration depth in reaction media. Recently, we demonstrated green-light-induced ATRP with dual catalysis, where eosin Y (EYH2) was used as an organic photoredox catalyst in conjunction with a copper complex. This dual catalysis proved to be highly efficient, allowing rapid and well-controlled aqueous polymerization of oligo(ethylene oxide) methyl ether methacrylate without the need for deoxygenation. Herein, we expanded this system to synthesize polyacrylates under biologically relevant conditions using CuII/Me6TREN (Me6TREN = tris[2-(dimethylamino)ethyl]amine) and EYH2 at ppm levels. Water-soluble oligo(ethylene oxide) methyl ether acrylate (average M n = 480, OEOA480) was polymerized in open reaction vessels under green light irradiation (520 nm). Despite continuous oxygen diffusion, high monomer conversions were achieved within 40 min, yielding polymers with narrow molecular weight distributions (1.17 ≤ D̵ ≤ 1.23) for a wide targeted DP range (50-800). In situ chain extension and block copolymerization confirmed the preserved chain end functionality. In addition, polymerization was triggered/halted by turning on/off a green light, showing temporal control. The optimized conditions also enabled controlled polymerization of various hydrophilic acrylate monomers, such as 2-hydroxyethyl acrylate, 2-(methylsulfinyl)ethyl acrylate), and zwitterionic carboxy betaine acrylate. Notably, the method allowed the synthesis of well-defined acrylate-based protein-polymer hybrids using a straightforward reaction setup without rigorous deoxygenation.

Scaling-Up an Aqueous Self-Degassing Electrochemically Mediated ATRP in Dispersion for the Preparation of Cellulose-Polymer Composites and Films

Electrochemically mediated atom transfer radical polymerization (eATRP) is developed in dispersion conditions to assist the preparation of cellulose-based films. Self-degassing conditions are achieved by the addition of sodium pyruvate (SP) as a ROS scavenger, while an aluminum counter electrode provides a simplified and more cost-effective electrochemical setup. Different polyacrylamides were grown on a model cellulose substrate which was previously esterified with 2-bromoisobutyrate (-BriB), serving as initiator groups. Small-scale polymerizations (15 mL) provided optimized conditions to pursue the scale-up up to 1000 mL (scale-up factor ~67). Cellulose-poly(N-isopropylacrylamide) was then chosen to prepare the tunable, thermoresponsive, solvent-free, and flexible films through a dissolution/regeneration method. The produced films were characterized by Fourier-transform infrared (FTIR), scanning electron microscopy (SEM), dynamic scanning calorimetry (DSC), and thermogravimetric analysis (TGA).

Toward Green Atom Transfer Radical Polymerization: Current Status and Future Challenges

Reversible-deactivation radical polymerizations (RDRPs) have revolutionized synthetic polymer chemistry. Nowadays, RDRPs facilitate design and preparation of materials with controlled architecture, composition, and functionality. Atom transfer radical polymerization (ATRP) has evolved beyond traditional polymer field, enabling synthesis of organic-inorganic hybrids, bioconjugates, advanced polymers for electronics, energy, and environmentally relevant polymeric materials for broad applications in various fields. This review focuses on the relation between ATRP technology and the 12 principles of green chemistry, which are paramount guidelines in sustainable research and implementation. The green features of ATRP are presented, discussing the environmental and/or health issues and the challenges that remain to be overcome. Key discoveries and recent developments in green ATRP are highlighted, while providing a perspective for future opportunities in this area.

Ultrafast Visible-Light-Induced ATRP in Aqueous Media with Carbon Quantum Dots as the Catalyst and Its Application for 3D Printing

Photoinduced atom transfer radical polymerization (ATRP) has been proved to be a versatile technique for polymer network formation. However, the slow polymerization rates of typical ATRP limited its application in the field of additive manufacturing (3D printing). In this work, we introduced carbon quantum dots (CQDs) for the first time to the ATRP in aqueous media and developed an ultrafast visible-light-induced polymerization system. After optimization, the polymerization could achieve a high monomer conversion (>90%) within 1 min, and the polydispersity index (PDI) of the polymer was lower than 1.25. This system was then applied as the first example of ATRP for the 3D printing of hydrogel through digital light processing (DLP), and the printed object exhibited good dimensional accuracy. Additionally, the excellent and stable optical properties of CQDs also provided interesting photoluminescence capabilities to the printed objects. We deduce this ATRP mediated 3D printing process would provide a new platform for the preparation of functional and stimuli-responsive hydrogel materials.

Aqueous electrochemically-triggered atom transfer radical polymerization

Simplified electrochemical atom transfer radical polymerization (seATRP) using Cu^II–N-propyl pyridineimine complexes (Cu^II(NPPI)₂) is reported for the first time. In aqueous solution, using oligo(ethylene glycol) methyl ether methacrylate (OEGMA), standard electrolysis conditions yield POEGMA with good control over molecular weight distribution (Đ_m < 1.35). Interestingly, the polymerizations are not under complete electrochemical control, as monomer conversion continues when electrolysis is halted. Alternatively, it is shown that the extent and rate of polymerization depends upon an initial period of electrolysis. Thus, it is proposed that seATRP using Cu^II(NPPI)₂ follows an electrochemically-triggered, rather than electrochemically mediated, ATRP mechanism, which distinguishes them from other Cu^IIL complexes that have been previously reported in the literature.

Simplified electrochemical atom transfer radical polymerization (seATRP) using Cu^II-pyridineimine complexes is reported and follows a previously unreported electrochemically triggered mechanism.

Dispersity within Brushes Plays a Major Role in Determining Their Interfacial Properties: The Case of Oligoxazoline-Based Graft Polymers

Many synthetic polymers used to form polymer-brush films feature a main backbone with functional, oligomeric side chains. While the structure of such graft polymers mimics biomacromolecules to an extent, it lacks the monodispersity and structural purity present in nature. Here we demonstrate that side-chain heterogeneity within graft polymers significantly influences hydration and the occurrence of hydrophobic interactions in the subsequently formed brushes and consequently impacts fundamental interfacial properties. This is demonstrated for the case of poly(methacrylate)s (PMAs) presenting oligomeric side chains of different length (n) and dispersity. A precise tuning of brush structure was achieved by first synthesizing oligo(2-ethyl-2-oxazoline) methacrylates (OEOXMAs) by cationic ring-opening polymerization (CROP), subsequently purifying them into discrete macromonomers with distinct values of n by column chromatography, and finally obtaining poly[oligo(2-ethyl-2-oxazoline) methacrylate]s (POEOXMAs) by reversible addition-fragmentation chain-transfer (RAFT) polymerization. Assembly of POEOXMA on Au surfaces yielded graft polymer brushes with different side-chain dispersities and lengths, whose properties were thoroughly investigated by a combination of variable angle spectroscopic ellipsometry (VASE), quartz crystal microbalance with dissipation (QCMD), and atomic force microscopy (AFM) methods. Side-chain dispersity, or dispersity within brushes, leads to assemblies that are more hydrated, less adhesive, and more lubricious and biopassive compared to analogous films obtained from graft polymers characterized by a homogeneous structure.

Assemblies of Polyacrylonitrile-Derived Photoactive Polymers as Blue and Green Light Photo-Cocatalysts for Cu-Catalyzed ATRP in Water and Organic Solvents

Photoluminescent nanosized quasi-spherical polymeric assemblies prepared by the hydrothermal reaction of polyacrylonitrile (PAN), ht-PLPPAN, were demonstrated to have the ability to photo-induce atom transfer radical polymerization (ATRP) catalyzed by low, parts per million concentrations of CuII complex with tris(2-pyridylmethyl)amine (TPMA). Such photo induced ATRP reactions of acrylate and methacrylate monomers were performed in water or organic solvents, using ht-PLPPAN as the photo-cocatalyst under blue or green light irradiation. Mechanistic studies indicate that ht-PLPPAN helps to sustain the polymerization by facilitating the activation of alkyl bromide species by two modes: 1) green or blue light-driven photoreduction of the CuII catalyst to the activating CuI form, and 2) direct activation of dormant alkyl bromide species which occurs only under blue light. The photoreduction of the CuII complex by ht-PLPPAN was confirmed by linear sweep voltammetry performed under illumination. Analysis of the polymerization kinetics in aqueous media indicated even though CuI complexes comprised only 1-1.4% of all Cu species at equilibrium, they exhibited high activation rate constant and activated the alkyl bromide initiators five to six orders of magnitude faster than ht-PLPPAN.

ATRP-ARGET of a Styrene Monomer onto Modified Natural Rubber Latex as an Initiator

Atom transfer radical polymerization with an activator regenerated by electron transfer (ATRP-ARGET) was performed for graft copolymerization of styrene onto natural rubber in the latex stage as a heterogeneous system. Deproteinized and subsequently brominated natural rubber particles in the latex stage were subjected to graft copolymerization of styrene on their surfaces in the presence of an activated ATRP catalyst. ¹H NMR spectroscopy and size exclusion chromatography (SEC) characterized the particles. Ozonolysis was performed to deduct the polyisoprene contribution to SEC. Graft copolymerization in heterogeneous media by extraction with an acetone/2-butanone solution. Both the linear evolution of the molecular weight versus monomer conversion and the high grafting efficiency associated with a narrow molecular weight distribution of the resulting grafted polystyrene confirm a living radical behavior.

Electrochemistry for Atom Transfer Radical Polymerization

Atom Transfer Radical Polymerization (ATRP) is the most powerful and most employed technology of Controlled Radical Polymerization (CRP) to produce polymers with well-defined architecture, that is, composition, topology, and functionality. Several hundreds of papers are published every year on ATRP processes, mainly based on empiric experimental procedures. Electrochemistry powerfully entered in the field of ATRP about 10 years ago, providing important contributions both to the further development of the process and to a better understanding of its mechanism. Five main issues took advantage of electrochemistry and/or its synergism with ATRP: i) understanding the mechanism of ATRP activation; ii) determination of thermodynamic parameters; iii) determination of activation and deactivation rate constants; iv) the SARA ATRP vs SET-LRP dispute: the role of Cu⁰ ; v) electrochemically-mediated ATRP.

Catalytic Halogen Exchange in Supplementary Activator and Reducing Agent Atom Transfer Radical Polymerization for the Synthesis of Block Copolymers

Synthesis of block copolymers (BCPs) by catalytic halogen exchange (cHE) is reported, using supplemental activator and reducing agent Atom Transfer Radical Polymerization (SARA ATRP). The cHE mechanism is based on the use of a small amount of a copper catalyst in the presence of a suitable excess of halide ions, for the synthesis of block copolymers from macroinitiators with monomers of mismatching reactivity. cHE overcomes the problem of inefficient initiation in block copolymerizations in which the second monomer provides dormant species that are more reactive than the initiator. Model macroinitiators with low dispersity are prepared and extended to afford well-defined block copolymers of various compositions. Combined cHE/SARA ATRP is therefore a simple and potent polymerization tool for the copolymerization of a wide range of monomers allowing the production of tailored block copolymers.

Role of External Field in Polymerization: Mechanism and Kinetics

The past decades have witnessed an increasing interest in developing advanced polymerization techniques subjected to external fields. Various physical modulations, such as temperature, light, electricity, magnetic field, ultrasound, and microwave irradiation, are noninvasive means, having superb but distinct abilities to regulate polymerizations in terms of process intensification and spatial and temporal controls. Gas as an emerging regulator plays a distinctive role in controlling polymerization and resembles a physical regulator in some cases. This review provides a systematic overview of seven types of external-field-regulated polymerizations, ranging from chain-growth to step-growth polymerization. A detailed account of the relevant mechanism and kinetics is provided to better understand the role of each external field in polymerization. In addition, given the crucial role of modeling and simulation in mechanisms and kinetics investigation, an overview of model construction and typical numerical methods used in this field as well as highlights of the interaction between experiment and simulation toward kinetics in the existing systems are given. At the end, limitations and future perspectives for this field are critically discussed. This state-of-the-art research progress not only provides the fundamental principles underlying external-field-regulated polymerizations but also stimulates new development of advanced polymerization methods.

Molecular recognition with soft biomaterials

Biomacromolecules and engineered materials can achieve molecular recognition if they engage their ligand with properly oriented and chemically complementary moieties. Recently, there has been significant interest in fabricating recognitive soft materials, which possess specific affinity for biological analytes. We present a summary and evaluation of current recognitive materials for biosensing, drug delivery, and regenerative medicine applications. We highlight the impact of material composition on the extent and specificity of ligand adsorption, citing new theoretical and empirical evidence. We conclude with a guide for synthesizing and characterizing novel recognitive materials, as well as recommendations for ligand selection and experimental design.

Atom Transfer Radical Polymerization for Biorelated Hybrid Materials

Proteins, nucleic acids, lipid vesicles, and carbohydrates are the major classes of biomacromolecules that function to sustain life. Biology also uses post-translation modification to increase the diversity and functionality of these materials, which has inspired attaching various other types of polymers to biomacromolecules. These polymers can be naturally (carbohydrates and biomimetic polymers) or synthetically derived and have unique properties with tunable architectures. Polymers are either grafted-to or grown-from the biomacromolecule's surface, and characteristics including polymer molar mass, grafting density, and degree of branching can be controlled by changing reaction stoichiometries. The resultant conjugated products display a chimerism of properties such as polymer-induced enhancement in stability with maintained bioactivity, and while polymers are most often conjugated to proteins, they are starting to be attached to nucleic acids and lipid membranes (cells) as well. The fundamental studies with protein-polymer conjugates have improved our synthetic approaches, characterization techniques, and understanding of structure-function relationships that will lay the groundwork for creating new conjugated biomacromolecular products which could lead to breakthroughs in genetic and tissue engineering.

Axially Ligated Mesohemins as Bio-Mimicking Catalysts for Atom Transfer Radical Polymerization

Copper is the most common metal catalyst used in atom transfer radical polymerization (ATRP), but iron is an excellent alternative due to its natural abundance and low toxicity compared to copper. In this work, two new iron-porphyrin-based catalysts inspired by naturally occurring proteins, such as horseradish peroxidase, hemoglobin, and cytochrome P450, were synthesized and tested for ATRP. Natural protein structures were mimicked by attaching imidazole or thioether groups to the porphyrin, leading to increased rates of polymerization, as well as providing polymers with low dispersity, even in the presence of ppm amounts of catalysts.

Biocatalytic "Oxygen-Fueled" Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) can be carried out in a flask completely open to air using a biocatalytic system composed of glucose oxidase (GOx) and horseradish peroxidase (HRP) with an active copper catalyst complex. Nanomolar concentrations of the enzymes and ppm amounts of Cu provided excellent control over the polymerization of oligo(ethylene oxide) methyl ether methacrylate (OEOMA₅₀₀ ), generating polymers with high molecular weight (M_n >70 000) and low dispersities (1.13≤Đ≤1.27) in less than an hour. The continuous oxygen supply was necessary for the generation of radicals and polymer chain growth as demonstrated by temporal control and by inducing hypoxic conditions. In addition, the enzymatic cascade polymerization triggered by oxygen was used for a protein and DNA functionalized with initiators to form protein-b-POEOMA and DNA-b-POEOMA bioconjugates, respectively.