Solvent Effects on the Activation Rate Constant in Atom Transfer Radical Polymerization

Oxidative Decarbonylation of an Azacalixpyridine-Supported Mo(0)-Tricarbonyl to a Mo(VI)-Trioxo Complex with Dioxygen in Solution and on Au(111): Determination of Molecular Mechanism

The conversion of an azacalixpyridine-supported Mo(0) tricarbonyl into a Mo(VI) trioxo complex with dioxygen (O2) is investigated in homogeneous solution and in a molecular film adsorbed on Au(111) using a variety of spectroscopic and analytical methods. These studies in particular show that the dome-shaped carbonyl complex adsorbed on the metal surface has the ability to bind and activate gaseous oxygen, overcoming the so-called surface trans-effect. Furthermore, the rate of the conversion dramatically increases by irradiation with light. This observation is explained with the help of complementary DFT calculations and attributed to two different pathways, a thermal and a photochemical one. Based on the experimental and theoretical findings, a molecular mechanism for the conversion of the carbonyl to the oxo complex is derived.

Block Copolymers of Polyolefins with Polyacrylates: Analyzing and Improving the Blocking Efficiencies Using MILRad/ATRP Approach

Despite their industrial ubiquity, polyolefin-polyacrylate block copolymers are challenging to synthesize due to the distinct polymerization pathways necessary for respective blocks. This study utilizes MILRad, metal-organic insertion light-initiated radical polymerization, to synthesize polyolefin-b-poly(methyl acrylate) copolymer by combining palladium-catalyzed insertion-coordination polymerization and atom transfer radical polymerization (ATRP). Brookhart-type Pd complexes used for the living polymerization of olefins are homolytically cleaved by blue-light irradiation, generating polyolefin-based macroradicals, which are trapped with functional nitroxide derivatives forming ATRP macroinitiators. ATRP in the presence of Cu(0), that is, supplemental activators and reducing agents , is used to polymerize methyl acrylate. An increase in the functionalization efficiency of up to 71% is demonstrated in this study by modifying the light source and optimizing the radical trapping condition. Regardless of the radical trapping efficiency, essentially quantitative chain extension of polyolefin-Br macroinitiator with acrylates is consistently demonstrated, indicating successful second block formation.

Reactivity Prediction of Cu-Catalyzed Halogen Atom Transfer Reactions Using Data-Driven Techniques

In catalysis, linear free energy relationships (LFERs) are commonly used to identify reaction descriptors that enable the prediction of outcomes and the design of more effective catalysts. Herein, LFERs are established for the reductive cleavage of the C(sp3)-X bond in alkyl halides (RX) by Cu complexes. This reaction represents the activation step in atom transfer radical polymerization and atom transfer radical addition/cyclization. The values of the activation rate constant, kact, for 107 Cu complex/RX couples in 5 different solvents spanning over 13 orders of magnitude were effectively interpolated by the equation: log kact = sC(I + C + S), where I, C, and S are, respectively, the initiator, catalyst, and solvent parameters, and sC is the catalyst-specific sensitivity parameter. Furthermore, each of these parameters was correlated to relevant descriptors, which included the bond dissociation free energy of RX and its Tolman cone angle θ, the electron affinity of X, the radical stabilization energy, the standard reduction potential of the Cu complex, the polarizability parameter π* of the solvent, and the distortion energy of the complex in its transition state. This set of descriptors establishes the fundamental properties of Cu complexes and RX that determine their reactivity and that need to be considered when designing novel systems for atom transfer radical reactions. Finally, a multivariate linear regression (MLR) approach was adopted to develop an objective model that surpassed the predictive capability of the LFER equation. Thus, the MLR model was employed to predict kact values for >2000 Cu complex/RX pairs.

Mechanically Driven Atom Transfer Radical Polymerization by Piezoelectricity

Targeting sustainable and eco-friendly polymer synthesis, we demonstrate here a synergistically catalyzed atom transfer radical polymerization (ATRP) induced and controlled by interplay between ball milling (BM) and piezoelectric nanoparticles (piezoNPs). BM-induced electron transfer can be achieved through piezoNPs deformation under impact force, serving as an external stimulus to mediate polymerization. The ppm level of copper loading is sufficient in fabrication of a polymer with well-defined molecular weight and low polydispersity. High-molecular-weight polymers ranging from 33 to 74 kDa were prepared successfully through DMSO-assisted grinding. Besides, its good performance on availability of water as liquid-assisted grinding additive, the recyclability of piezoNPs, and the formation of cross-linker-free composite resin make our ATRP approach a green and practical option alongside the existent heat-, electro-, and photo-induced methods.

Solvent Coordination Effect on Copper-Based Molecular Catalysts for Controlled Radical Polymerization

The equilibrium of copper-catalyzed atom transfer radical polymerization was investigated in silico with the aim of finding an explanation for the experimentally observed solvent effect. Various combinations of alkyl halide initiators and copper complexes in acetonitrile (MeCN) and dimethyl sulfoxide (DMSO) were taken into consideration. A continuum model for solvation, which does not account for the explicit interactions between the solvent and metal complex, is not adequate and does not allow the reproduction of the experimental trend. However, when the solvent molecules are included in the coordination sphere of the copper(I,II) species and the continuum description of the medium is still used, a solvent dependence of process thermodynamics emerges, in fair agreement with experimental trends.

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.

Experimental determination of activation rate constant and equilibrium constant for bromo substituted succinimide initiators for an atom transfer radical polymerization process

Alkyl bromides are used as initiators in most of the atom transfer radical polymerization (ATRP) process and play an important role for controlling the ATRP equilibrium. In this work, the effect of solvent on equilibrium constant of ATRP (K ATRP) and rate constant of activation (k act) of three isomeric alkyl bromides [namely, N-phenyl(3-bromo-3-methyl)succinimide, N-phenyl(3-bromo-4-methyl)succinimide, and N-phenyl(3-bromomethyl)succinimide] is reported. The k act and K ATRP values of alkyl bromide are determined experimentally using UV–Vis-NIR spectrometry. The termination rate constant for model compound is calculated using DOSY NMR spectroscopy. The k act and K ATRP values for the mentioned alkyl bromides are determined in five different polar solvent and the effect of polarity is observed. The obtained values of k act and K ATRP of N-phenyl(3-bromo-3-methyl)succinimide in acetonitrile at 25 °C is 6.60 × 10−2 L mol−1 s−1 and 1.42 × 10−9, respectively. These values are quite comparable with the experimentally determined reported k act and K ATRP of values of acrylates and benzyls initiators. Alternatively, the investigation of possible chain initiation activity for the ATRP process for the mentioned alkyl bromides is carried out theoretically using density functional theory (DFT) method [B3LYP/6-31+G(d) level]. A good correlation is obtained between the experimentally determined and theoretically calculated K ATRP values of studied alkyl bromides in chosen solvents. Significantly, it is found that the values of k act and K ATRP of alkyl bromides is solvent dependent and the magnitude values of the k act and K ATRP increases with increasing the solvent polarity. The proposed bromo substituted succinimides can be used as the initiator for the polymerization of acrylates, benzyls, maleimides, and itaconimides monomer under the selected solvent system.

Enhancing the efficiency of semiconducting quantum dot photocatalyzed atom transfer radical polymerization by ligand shell engineering

Manipulating the ligand shell of semiconducting quantum dots (QDs) has proven to be a promising strategy to enhance their photocatalytic performance for small molecule transformations, such as H₂ evolution and CO₂ reduction. However, ligand-controlled catalysis for macromolecules, which differ from small molecules in penetrability and charge transfer behavior due to their bulky sizes, still remains undiscovered. Here, we systematically investigate the role of surface ligands in the photocatalytic performance of cadmium selenide (CdSe) QDs in light-induced atom transfer radical polymerization (ATRP) by using thiol-based ligands with various polarities and chain lengths. A highly enhanced polymerization efficiency was observed when 3-mercapto propionic acid (MPA), a short-chain and polar ligand, was used to modify the CdSe QDs' surface, achieving high chain-end fidelity, good temporal control, and a dispersity of 1.18, while also tolerating a wide-range of functional monomers ranging from acrylates to methacrylates and fluorinated monomers. Transient absorption spectroscopy and time-resolved photoluminescence studies reveal interesting mechanistic details of electron and hole transfers from the excited QDs to the initiators and 3-MPA capping ligands, respectively, providing key mechanistic insight of these ligand controlled and QD photocatalyzed ATRP processes. The thiolate ligands were found to serve as an efficient hole acceptor for QDs, which facilitates the formation of a charge-separated state, followed by electron transfer from the conduction band edge to initiators and ultimately suppressing charge recombination within the QD.

Interrogation of O-ATRP Activation Conducted by Singlet and Triplet Excited States of Phenoxazine Photocatalysts

Organocatalyzed ATRP (O-ATRP) is a growing field exploiting organic chromophores as photoredox catalysts (PCs) that engage in dissociative electron-transfer (DET) activation of alkyl-halide initiators following absorption of light. Characterizing DET rate coefficients (k_act) and photochemical yields across various reaction conditions and PC photophysical properties will inform catalyst design and efficient use during polymerization. The studies described herein consider a class of phenoxazine PCs, where synthetic handles of core substitution and N-aryl substitution enable tunability of the electronic and spin characters of the catalyst excited state as well as DET reaction driving force (ΔG_ET⁰). Using Stern-Volmer quenching experiments through variation of the diethyl 2-bromo-2-methylmalonate (DBMM) initiator concentration, collisional quenching is observed. Eight independent measurements of k_act are reported as a function of ΔG_ET⁰ for four PCs: four triplet reactants and four singlets with k_act values ranging from 1.1 × 10⁸ M^-1 s^-1, where DET itself controls the rate, to 4.8 × 10⁹ M^-1 s^-1, where diffusion is rate-limiting. This overall data set, as well as a second one inclusive of five literature values from related systems, is readily modeled with only a single parameter of reorganization energy under the frameworks of the adiabatic Marcus electron-transfer theory and Marcus-Savéant theory of DET. The results provide a predictive map where k_act can be estimated if ΔG_ET⁰ is known and highlight that DET in these systems appears insensitive to PC reactant electronic and spin properties outside of their impact on the driving force. Next, on the basis of measured k_act values in selected PC systems and knowledge of their photophysics, we also consider activation yields specific to the reactant spin states as the DBMM initiator concentration is varied. In N-naphthyl-containing PCs characterized by near-unity intersystem crossing, the T₁ is certainly an important driver for efficient DET. However, at DBMM concentrations common to polymer synthesis, the S₁ is also active and drives 33% of DET reaction events. Even in systems with low yields of ISC, such as in N-phenyl-containing PCs, reaction yields can be driven to useful values by exploiting the S₁ under high DBMM concentration conditions. Finally, we have quantified photochemical reaction quantum yields, which take into account potential product loss processes after electron-transfer quenching events. Both S₁ and T₁ reactant states produce the PC^•+ radical cation with a common yield of 71%, thus offering no evidence for spin selectivity in deleterious back electron transfer. The subunity PC^•+ yields suggest that some combination of solvent (DMAc) oxidation and energy-wasting back electron transfer is likely at play and these pathways should be factored in subsequent mechanistic considerations.

Tuning the Thermoresponsive Behavior of Surface-Attached PNIPAM Networks: Varying the Crosslinker Content in SI-ATRP

The synthesis and thermoresponsive properties of surface-attached poly(N-isopropylacrylamide)-co-N,N'-methylene bisacrylamide (PNIPAM-co-MBAM) networks are investigated. The networks are formed via SI-ARGET-ATRP ("grafting-from") on thiol-based initiator-functionalized gold films. This method is reliable, well controlled, fast, and applicable to patterned surfaces (e.g., nanopores) for networks with dry thicknesses >20 nm. Surface-attached PNIPAM-co-MBAM gels are swollen below their volume phase transition temperature but above collapse without complete expulsion of water (retain ∼50 vol %). The swelling/collapse transition is studied using complementary SPR and QCMD techniques. The ratio between swollen and collapsed heights characterizes the thermoresponsive behavior and is shown to not depend on network height but to vary with MBAM content. The higher the proportion of the crosslinker, the lower the magnitude of the phase transition, until all responsiveness is lost at 5 mol % MBAM. The temperature range of the transition is broadened for more crosslinked PNIPAM-co-MBAM gels but remains centered around 32 °C. Upon reswelling, less crosslinked networks display sharp transitions, while for those containing ≥3 mol % MBAM, transitions remain broad. This tunable behavior persists for gels on nanostructured gold surfaces. Investigating PNIPAM-co-MBAM networks on gold plasmonic nanowell arrays is a starting point for expanding their scope as thermo-controlled nanoactuators.

Antifouling and Flux Enhancement of Reverse Osmosis Membrane by Grafting Poly (3-Sulfopropyl Methacrylate) Brushes

A commercial thin film composite (TFC) polyamide (PA) reverse osmosis membrane was grafted with 3-sulfopropyl methacrylate potassium (SPMK) to produce PA-g-SPMK by atom transfer radical polymerization (ATRP). The grafting of PA was done at varied concentrations of SPMK, and its effect on the surface composition and morphology was studied by Fourier-Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), optical profilometry, and contact angle analysis. The grafting of hydrophilic ionically charged PSPMK polymer brushes having acrylate and sulfonate groups resulted in enhanced hydrophilicity rendering a reduction of contact angle from 58° of pristine membrane sample labeled as MH0 to 10° for a modified membrane sample labeled as MH3. Due to the increased hydrophilicity, the flux rate rises from 57.1 L m-2 h-1 to 71.2 L m-2 h-1, and 99% resistance against microbial adhesion (Escherichia coli and Staphylococcus aureus) was obtained for MH3 after modification.

Photoactive rose bengal-based latex via RAFT emulsion polymerization-induced self-assembly

Rose bengal shell- or core-functionalized acrylic latex synthesized by RAFT emulsion PISA: interfacial photosensitized ¹O₂ production under visible light.

Systematic Review on the Effects, Roles and Methods of Magnetic Particle Coatings in Magnetorheological Materials

Magnetorheological (MR) material is a type of magneto-sensitive smart materials which consists of magnetizable particles dispersed in a carrier medium. Throughout the years, coating on the surface of the magnetic particles has been developed by researchers to enhance the performance of MR materials, which include the improvement of sedimentation stability, enhancement of the interaction between the particles and matrix mediums, and improving rheological properties as well as providing extra protection against oxidative environments. There are a few coating methods that have been employed to graft the coating layer on the surface of the magnetic particles, such as atomic transfer radical polymerization (ATRP), chemical oxidative polymerization, and dispersion polymerization. This paper investigates the role of particle coating in MR materials with the effects gained from grafting the magnetic particles. This paper also discusses the coating methods employed in some of the works that have been established by researchers in the particle coating of MR materials.

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.

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.

Solubility behaviour of random and gradient copolymers of di- and oligo(ethylene oxide) methacrylate in water: effect of various additives

Poly[oligo(ethylene oxide)] based gradient and random copolymers with different compositions are synthesized via Cu-based atom transfer radical polymerization. The solubility behavior of these copolymers in pure water and in the presence of different salts, surfactants and ethanol is investigated. According to dynamic light scattering results, the lower critical solution temperature (LCST) depends on the structure of the copolymer and changes slightly in the presence of additives. Good cosolvents like ethanol can increase the LCST through dissolving the collapsed copolymer chains to some extent. The same effect is observed for surfactants that make the copolymer solution more stable by preventing aggregation. Above a certain concentration of surfactant, depending on the copolymer structure, the solution is stable at all temperatures (no LCST). The effect of salts on the solubility of the copolymers follows the Hofmeister series and it is related linearly to the salt concentration. Based on their affinity to the copolymer, the salts can increase or decrease the LCST. There is a considerable difference in phase transition changes for gradient or random copolymers after salt addition. While both copolymers show a two-step phase transition in the presence of different salts, the changes in the hydrodynamic radius and normalized scattering intensity are rather broad for random compared to gradient copolymers. Contrary to what was expected, varying the cations has no distinguishable effect on the LCST for both copolymers. All chlorides decrease the LCST. This decrease is almost the same for gradient copolymers and fluctuates for random copolymers.

Under pressure: electrochemically-mediated atom transfer radical polymerization of vinyl chloride

Electrochemically mediated ATRP (eATRP) of vinyl chloride (VC), a less activated monomer, was successfully achieved. It is the first report on eATRP of a gaseous monomer under pressure.

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.

Translating Surface-Initiated Atom Transfer Radical Polymerization into Technology: The Mechanism of Cu0-Mediated SI-ATRP under Environmental Conditions

The exceptional features of Cu⁰-mediated surface-initiated atom transfer radical polymerization (Cu⁰ SI-ATRP), and its potential for implementation in technologically relevant surface functionalizations are demonstrated thanks to a comprehensive understanding of its mechanism. Cu⁰ SI-ATRP enables the synthesis of multifunctional polymer brushes with a remarkable degree of control, over extremely large areas and without the need for inert atmosphere or deoxygenation of monomer solutions. When a polymerization mixture is placed between a flat copper plate and an ATRP-initiator-functionalized substrate, the vertical distance between these two overlaying surfaces determines the tolerance of the grafting process toward the oxygen, while the composition of the polymerization solution emerges as the critical parameter regulating polymer-grafting kinetics. At very small distances between the copper plate and the initiating surfaces, the oxygen dissolved in the solution is rapidly consumed via oxidation of the metallic substrate. In the presence of ligand, copper species diffuse to the surface-immobilized initiators and trigger a rapid growth of polymer brushes. Concurrently, the presence and concentration of added Cu^II regulates the generation of Cu^I-based activators through comproportionation with Cu⁰. Hence, under oxygen-tolerant conditions, the extent of comproportionation, together with the solvent-dependent rate constant of activation (k_act) of ATRP are the main determinants of the growth rate of polymer brushes.

Expanding the monomer scope of linear and branched vinyl polymerisations via copper-catalysed reversible-deactivation radical polymerisation of hydrophobic methacrylates using anhydrous alcohol solvents

The use of anhydrous alcohols for Cu-catalysed reversible-deactivation radical polymerisation of a wide range of hydrophobic methacrylates has been explored in detail.

Liquid salts as eco-friendly solvents for atom transfer radical polymerization: a review

Liquid salts, comprising ionic liquids and eutectic mixtures, are organic compounds/mixtures characterized by a low melting point that have been emerging as a very promising eco-friendly solvent for atom transfer radical polymerization.