Controlled Radical Polymerization Methods Research Articles

Aqueous Photoiniferter Polymerization of Acrylonitrile.

Polyacrylonitrile (PAN) is a key industrial polymer for the production of carbon fiber for high-strength, lightweight composite material applications, with an estimated 90% of the carbon fiber market relying on PAN-based polymers. Traditionally, PAN synthesis is achieved by conventional radical polymerization, resulting in broad molecular weight distributions and the use of toxic organic solvents or surfactants during the synthesis. Additionally, attempts to improve polymer and processing properties by controlled radical polymerization methods suffer from low monomer conversions and struggle to achieve molecular weights suitable for producing high-performance carbon fiber. In this study, we present an aqueous photoiniferter (aqPI) polymerization of acrylonitrile, achieving high monomer conversion and high PAN molecular weights with significantly faster kinetics and dispersity control when compared to traditional methods. This approach allows for the unprecedented control of polymer properties that are integral for downstream processing for enhanced carbon fiber production.

Nitroxide-Mediated Controlled Radical Copolymerization of α-Trifluoromethylstyrenes with Styrenes.

Fluorinated polymers are important materials in everyday life; however, most monomers of widely used fluoropolymers are gaseous, and their polymerization is difficult in an ordinary laboratory. Therefore, partially fluorinated polymers have recently been reported. As an easy-to-handle fluorine-containing monomer, α-trifluoromethylstyrene (TFMST) can be used to produce partially fluorinated polymers with trifluoromethyl groups in the main chain; however, TFMST does not homopolymerize, and there are limited reports on its copolymerization with styrene (ST). In this study, we applied the controlled radical polymerization method, which is effective for the polymerization of ST, to the copolymerization of TFMST and ST. We also showed that nitroxide-mediated polymerization is effective. The content ratio of TFMST in the TFMST-ST copolymer can be controlled between 10% and 40% by changing its monomer ratio. Additionally, the polymerization of TFMST and ST with substituents was performed to increase structural variations. The thermal stability as well as water and oil repellency of the synthesized polymers with different composition ratios and substituents were also evaluated.

Acid-triggered radical polymerization of vinyl monomers

Reversible addition–fragmentation chain-transfer (RAFT) polymerization is one of the most versatile and robust controlled radical polymerization methods owing to its broad material scope and high tolerance to various functionalities and impurities. However, to operate RAFT polymerization, a constant supply of radicals is required, typically via exogenous thermal radical initiators that are not only challenging to transport and store, but also primarily responsible for termination and end-group heterogeneity. Here we present an acid-triggered RAFT polymerization that operates in the dark and without any conventional radical initiator. Abundant acids (for example, sulfuric acid) are shown to have a dual role initiating and accelerating the polymerization. The polymers prepared have low dispersity and high end-group fidelity. The method is compatible with a wide range of vinyl monomers and solvents, and can be applied to the synthesis of well-controlled high molecular weight block copolymers, as well as to free radical polymerization.

CuBr-mediated surface-initiated controlled radical polymerization in air.

Herein, we present a straightforward CuBr-mediated surface-initiated controlled radical polymerization (SI-CRP) method for fabricating polymer brushes using microliter volumes of reaction solution in air and at room temperature. The key advantage of this method is its ability to rapidly grow polymer brushes with oxygen tolerance, driven by the controlled disproportionation of CuI into CuII and Cu0 by CuBr and ligand. We demonstrate the successful preparation of homo-, block, patterned, and wafer-scale polymer brushes. Additionally, the catalyst in CuBr-mediated SI-CRP is reusable, long-lasting, and compatible with various monomers. This work broadens the potential of CuBr for polymer brush growth, making it accessible to both experts and non-experts.

Formation of Polyion Complex Aggregate Formed from a Cationic Block Copolymer and Anionic Polysaccharide.

Block copolymers (PmMn; P20M101 and P100M98) comprising poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC, P) containing biocompatible phosphorylcholin pendants and cationic poly((3-acryloylaminopropyl) trimethylammonium chloride) (PMAPTAC, M) were synthesized via a controlled radical polymerization method. The degrees of polymerization of the PMPC and PMAPTAC segments are denoted by subscripts (PmMn). The mixture of cationic PmMn and anionic sodium chondroitin sulfate C (CS) with the pendant anionic carboxylate and sulfonate groups formed polyion complex (PIC) aggregates in phosphate-buffered saline. A charge-neutralized mixture of P20M101 with CS formed P20M101/CS PIC vesicles with a hydrodynamic radius (Rh) of 97.2 nm, zeta potential of ca. 0 mV, and aggregation number (Nagg) of 23,044. PMPC shells covered the surface of the PIC vesicles. The mixture of P100M98 and CS formed PIC spherical micelles with the PIC core and hydrophilic PMPC shells. The Rh, zeta potential, and Nagg of the PIC micelles were 26.4 nm, ca. 0 mV, and 404, respectively. At pH < 4, the carboxylate anions in CS were protonated. Thus, the charge balance in the PIC micelles shifted to decrease the core density owing to the electrostatic repulsions of the excess cations in the core. The PIC micelles dissociated at a NaCl concentration ≥0.6 M owing to the charge screening effect. The positively charged PIC micelles with excess P100M98 can encapsulate anionic dyes owing to electrostatic interaction.

Impact of Alkyl Core Substitution Kinetics in Diaryl Dihydrophenazine Photoredox Catalysts on Properties and Performance in O-ATRP.

Organocatalyzed atom transfer radical polymerization (O-ATRP) is a controlled radical polymerization method mediated by organic photoredox catalysts (PCs) for producing polymers with well-defined structures. While N,N-diaryl dihydrophenazine PCs have successfully produced polymers with low dispersity (Đ < 1.3) in O-ATRP, low initiator efficiencies (I* ~ 60-80%) indicate an inability to achieve targeted molecular weights and have been attributed to the addition of radicals to the PC core. In this work, we measure the rates of alkyl core substitution (AkCS) to gain insight into why PCs differing in N-aryl group connectivity exhibit differences in polymerization control. Additionally, we evaluate how PC properties evolve during O-ATRP when a non-core-substituted PC is used. PC 1 with 1-naphthyl groups in the N-aryl position resulted in faster AkCS (k 1 = 1.21 ± 0.16 × 10-3 s-1, k 2 = 2.04 ± 0.11 × 10-3 s-1) and better polymerization control at early reaction times as indicated by plots of molecular weight (number average molecular weight = M n) vs conversion compared to PC 2 with 2-naphthyl groups (k 1 = 6.28 ± 0.38 × 10-4 s-1, k 2 = 1.15 ± 0.07 × 10-3 s-1). The differences in rates indicate that N-aryl connectivity can influence polymerization control by changing the rate of AkCS PC formation. The rate of AkCS increased from the initial to the second substitution, suggesting that PC properties are modified by AkCS. Increased PC radical cation (PC•+) oxidation potentials (E 1/2 = 0.26-0.27 V vs SCE) or longer triplet excited-state lifetimes (τ T1 = 1.4-33 μs) for AkCS PCs 1b and 2b compared to parent PCs 1 and 2 (E 1/2 = 0.21-0.22 V vs SCE, τ T1 = 0.61-3.3 μs) were observed and may explain changes to PC performance with AkCS. Insight from evaluation of the formation, properties, and performance of AkCS PCs will facilitate their use in O-ATRP and in other PC-driven organic transformations.

Atom Transfer Radical Polymerization of Glycidyl methacrylate triblock copolymer using a Novel Fluorescence Based Bifunctional Initiator

A novel bifunctional initiator containing fluorescein, a fluorescent moiety, was prepared by reacting dihydroxy fluorescein and 2-bromoisobutyl bromide. The structure elucidation of the new initiator was carried out using various spectroscopic tools such as NMR, IR and mass spectrum. Novel, fluorescent triblock copolymers comprising of glycidyl methacrylate namely P(GMA-b-MMA-b-GMA) was successfully synthesized by the atom transfer radical polymerization (ATRP) method using CuBr as the catalyst and N, N, N1, N11, N111 – pentamethyldiethylenetriamine as a complexing agent. The polymers were characterized both by 1H NMR and GPC measurements. It was observed that as the polymerization time increased, both the conversion and the molecular weight increased linearly with time. Well controlled soluble PGMA and triblock copolymers were successfully synthesized by utilizing the controlled radical polymerization method. Fluorescent PGMA was highly fluorescent with distinguishable emission signatures from the fluorescein group and the repeating units of the polymer. The fluorescence emission spectra reveal interesting features such as red shift when compared to the non-fluorescent polymer.

Light-regulated growth of polymer chains from the surface of RAFT agent primed mesoporous silica nanoparticles

Reversible addition-fragmentation chain transfer (RAFT) polymerization is one of the extensively explored and studied controlled radical polymerization methods. Recently, in comparison to the conventional RAFT polymerization process, visible light-regulated photo-induced electron/energy transfer (PET)-RAFT polymerization has picked up interest. With an ever-increasing desire for greener and more sustainable methods for synthesis, techniques such as PET-RAFT polymerization can be well adapted as an alternative approach for performing a variety of syntheses. Keeping this in mind we step forward with the synthesis of mesoporous silica nanoparticles containing an in-built RAFT agent using a stepwise co-condensation method. Herein, we report a facile synthesis of an organic-inorganic hybrid system in which surface-initiated RAFT polymerization was performed using several monomers (hydrophobic and hydrophilic) from RAFT agent primed mesoporous silica nanoparticles (RAFT-MSNs). The surface-initiated RAFT polymerization was carried through thermally initiated polymerization using AIBN and visible light-regulated PET-RAFT polymerization using Eosin Y as a photocatalyst. The light-regulated PET-RAFT polymerization was carried out smoothly in a shorter time frame as compared to the thermally initiated RAFT polymerization. A variety of characterization methods were used to confirm the successful grafting of polymeric chains from the surface of RAFT-MSNs. Hence, by appropriate polymer functionalization of mesoporous silica nanoparticles with stimuli-responsive polymers or polymers bearing functional groups, these organic-inorganic hybrids can be studied in a wide variety of applications such as drug delivery, catalysis, nanoconfinement, etc.

Preparation of microbeads grafted with poly(2-(acryloyloxy)ethyl]trimethylammonium chloride) cationic polyelectrolyte as recyclable and effective adsorbents for organic dyes

Brush polymers have been widely used as a suitable replacement for many adsorbents due to their economical cost, reusability, and mechanical strength. Here, cationic polyelectrolyte microbeads poly(2-hydroxyethyl methacrylate/ethylene dimethacrylate)-g-poly(2-(acryloyloxy)ethyl]trimethylammonium chloride) marked as p(HEMA/EDMA)-g-p(AEtMAC) were prepared by a combination of suspension and controlled radical polymerization methods. Microbeads displayed nonporous, spherical, and uniform morphology with a size of about 250 μm. Thermal stability decreased after grafting of p(AEtMAC) onto the surface of p(HEMA/EDMA)-Br and the final microbeads p(HEMA/EDMA)-g-p(AEtMAC) exhibited the onset degradation temperature of 200 °C. The adsorption of Coomassie Brilliant Blue G250 (CBB) and methyl orange (MO) anionic dyes at different adsorbent dosages (0.5–3 g L−1) and constant concentration of dye (50 mg L−1) was investigated. The adsorbent at the optimum dosage of 2 g L−1 was selected. More than 90% of the mentioned dyes were removed in the optimum pH range of 5–6. The maximum adsorption capacities 145 and 135 mg dye g−1 adsorbent were obtained for MO and CBB dyes, respectively, at ambient temperature and 300 mg L−1 dye concentration. Adsorptions of the MO and CBB on the adsorbent were well fitted to Temkin and Langmuir isotherm models, respectively, which obeyed the quasi-second-order kinetic model. Adsorption enthalpy change values for MO and CBB were found to be 32.17 and 17.59 KJ mol−1, correspondingly, which indicated the exothermic property and the physical nature of the adsorption. Furthermore, the entropy change values of 130 and 70 J mol−1 were determined for MO and CBB, respectively. Positive values of entropy changes are related to the increase in irregularity during dye adsorption on the cationic adsorbent.

A comparison of RAFT and ATRP methods for controlled radical polymerization.

Reversible addition-fragmentation chain-transfer (RAFT) polymerization and atom transfer radical polymerization (ATRP) are the two most common controlled radical polymerization methods. Both methods afford functional polymers with a predefined length, composition, dispersity and end group. Further, RAFT and ATRP tame radicals by reversibly converting active polymeric radicals into dormant chains. However, the mechanisms by which the ATRP and RAFT methods control chain growth are distinct, so each method presents unique opportunities and challenges, depending on the desired application. This Perspective compares RAFT and ATRP by identifying their mechanistic strengths and weaknesses, and their latest synthetic applications.

Development of novel aptasensor for ultra-sensitive detection of myoglobin via electrochemical signal amplification of methylene blue using poly (styrene)-block-poly (acrylic acid) amphiphilic copolymer

Amplification of electrochemical signal in order to betterment of limit of detection in determination of biomarkers has an important role in early detection of some dangerous diseases such as cancers. For this purpose, in this research, two types of poly (styrene)-block-poly (acrylic acid) amphiphilic copolymer (PS61-b-PAA596 and PS596-b-PAA61) were synthesized by controlled radical polymerization method via reversible addition-fragmentation chain transfer polymerization (RAFT) technique. Chemical structure of block copolymers was confirmed by FT-IR spectroscopy and their surface morphology was assessed by scanning electron microscopy (SEM). Self-assembly of these block copolymers into polymeric vesicles (polymersomes), loading and release efficiency of methylene blue as an electroactive indicator were investigated in DMF and THF solvents. On the basis of our findings PS61-b-PAA596 has better capability for loading and release of MB than PS596-b-PAA61. Then the obtained methylene blue-loaded polymersome successfully used for development of an aptasensor toward determination of trace amounts of myoglobin. The proposed aptasensor showed a wide linear range from 1.0 aM to 1.0 μM with an ultra-low detection limit of 0.73 aM. Applying this amplification strategy, determination of myoglobin in real samples was successfully performed.

Polystyrene-based Amphoteric Diblock Copolymers with Upper Critical Solution Temperature (UCST) in Aqueous Solutions

Amphoteric diblock copolymers (S10Vn, n = 5, 10, and 20) composed of anionic poly(sodium p-styrenesulfonate) and cationic poly(vinylbenzyl trimethylammonium chloride) blocks were prepared via a controlled radical polymerization method. S10V10 was insoluble in pure water owing to its electrostatic interactions, but S10V5 and S10V20 were soluble in pure water. S10V5 and S10V10 exhibited upper critical solution temperature behaviors, and the transition temperature decreased with increasing sodium chloride concentration.

Structure-property relationships of tailored imidazolium- and pyrrolidinium-based poly(ionic liquid)s. Solid-like vs. gel-like systems

In this paper, we present facile synthetic strategies enabling production of well-defined poly(ionic liquids) PILs based on 1-vinyl-4-benzyl-3-methylimidazolium- (VBIm) and 1-vinyl-4-benzyl-3-methylpyrrolidium (VBPy) ionic liquids (IL) possessing different counterions (chloride Cl−vs. bis(trifluoromethanesulfonyl)imide NTf2-) (VBIm/NTf2,VBIm/Cl, VBPy/NTf2,VBPy/Cl). The reaction routes have been selected to design two types of polyelectrolytes i) linear homopolymers showing solid-like behavior and ii) grafted copolymers composing of poly(siloxane) segments extended with analogous ionic domains that shows gel-like behavior. The former one has been produced via controlled radical polymerization methods (CRP) such as atom transfer radical polymerization (ATRP) or reversible addition fragmentation chain-transfer polymerization (RAFT), whereas the second one using thiol-ene coupling photo-polymerization. It was found that the glass transition temperature, Tg, of the synthesized herein linear homopolymer-based systems (both VBIm/NTf2 and VBPy/NTf2- -based) increases with their molecular weight, Mn, similarly as reported for its N-conjugated analogs described in our previous study [Maksym et al.,Polym. Chem., 2017, 8, 5433]. However, they are characterized by markedly lower conductivity, σdc (σdc~10−15 S/cm vs.σdc~10−16S/cm for PVBIm/NTf2vs. PVBPy/NTf2- at Tg determined from calorimetric experiments) when compared to N-conjugated one (σdc~10−9 S/cm for poly(N-vinyl-imidazole)/NTf2). Interestingly, the ionic conductivity value at Tg of grafted is similar to its corresponding linear analogs. Nevertheless, at T = 293K both anhydrous linear and grafted imidazolium-based PILs are characterized by high σdc, reaching values σdc~10−4S/cm and σdc~10−7S/cm, respectively.

Method grows polymers without damaging cells

Modifying the surface of cells with synthetic polymers offers a strategy for manipulating cellular behavior. Usually researchers make such modifications by attaching premade polymers to functional groups on the cell surface, but that “grafting-to” approach isn’t very efficient. An alternative approach involves “grafting from,” in which a polymer grows from an initiator on the cell surface. But this has been hard to do in ways that keep the cells—especially mammalian cells—alive. H. Tom Soh of Stanford University; Jia Niu and Craig J. Hawker of the University of California, Santa Barbara; and coworkers report an approach using a light-mediated controlled radical polymerization method that is gentle enough to use with mammalian cells (Nat. Chem. 2017, DOI: 10.1038/nchem.2713). The researchers used cell-surface-anchoring derivatives of 2-(butylthiocarbonothioyl)propionic acid as the chain-transfer agents that are attached to the cells and initiate the reaction. The polymer chain (shown) grows through the add...

SuFEx – a selectively triggered chemistry for fast, efficient and equimolar polymer–polymer coupling reactions

The synergy between controlled radical polymerization methods and click chemistry enables the design of complex and well-defined materials.

Poly(alkyl methacrylate)-grafted silica nanoparticles in polyethylene nanocomposites

Surface-initiated reversible addition-fragmentation chain transfer (SI-RAFT) polymerization has been widely used to synthesize various polymers grafted from nanoparticles (NPs) for incorporation into polymer nanocomposites. It is believed that these grafted polymer brushes, with a similar chemistry as the matrix polymer, can be employed to improve NP dispersion by reducing unfavorable interactions between the inorganic NPs and organic matrices. While controlled radical polymerization methods do not allow the polymerization of polyolefins, a substitute strategy is controllably attaching polyolefin-like polymers onto the NP surface. In the present work, the SI-RAFT polymerization was used to anchor poly(hexyl, lauryl, and stearyl methacrylate) on silica NPs, showing good control of the polymerizations. The long alkyl side chains can create an “olefin-like” interface and improve the compatibility of modified particles with polyolefins. Subsequently, we investigated the dispersion of these poly(alkyl methacrylate)-modified silica NPs in linear low density polyethylene (LLDPE). Poly(stearyl methacrylate)-grafted silica NPs (PSMA-g-SiO2) demonstrated improved dispersion of particles when compared to shorter alkyl side chain methacrylates. TEM images showed that the dispersion of these particles was highly dependent upon the molecular weight and density of the grafted PSMA chains. Differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and dynamic mechanical analysis (DMA) were used to characterize these nanocomposites. SAXS showed that the inter-particle distance (distribution of particle spacings) in the semicrystalline state was broader than in the melt, suggesting that particles spacing was affected by the polyethylene crystallization particularly at lower loadings. Nanocomposites at low loadings, 0.5 wt% core content, showed significant improvement in storage modulus due to the compatible particle-matrix interface. Further increases in particle loadings, however reversed this trend likely due to the increase in soft PSMA content.

Macro- and near-mesoporous monoliths by medium internal phase emulsion polymerization: A systematic study

Abstract The synthesis of a series of poly(ethylene oxide)- b -polystyrene copolymers with different block lengths was performed by radical addition fragmentation chain transfer. These amphiphilic copolymers were tested as stabilizers for water-in-oil medium internal phase emulsion (MIPE) templating polymerization and the formation of polyMIPEs with controlled morphology. Aside from the structure of the emulsion stabilizer, several parameters susceptible to influence the size of the cavities and the interconnectivity of the porous monoliths were probed including the choice of the comonomers, treatment of the emulsion by ultrasound, the use of controlled radical polymerization method for the network formation as well as interfacial initiation. Interconnected cellular monoliths were produced. The polymerization of the ultrasonicated water-in-ethylhexylacrylate/divinylbenzene MIPE notably led to near-mesoporous open-cell material. Mechanical properties and specific surface areas of the polyMIPEs were also investigated and discussed.

Efficient click-addition sequence for polymer–polymer couplings

Controlled radical polymerization methods and click chemistry form a versatile toolbox for creating complex polymer architectures.

Synthesis of polyacrylonitrile by reversible-deactivation radical polymerization and its application as electrode materials for electrochemical double layer capacitors

Polyacrylonitrile (PAN) was firstly synthesized using Fe(acac)2 as the mediator via “living”/controlled radical polymerization method.

Poly(N-vinylcaprolactam): a thermoresponsive macromolecule with promising future in biomedical field.

Poly(N-vinylcaprolactam) (PNVCL) is a thermoresponsive and biocompatible polymer that raises an increasing interest in the biomedical area, especially in drug delivery systems (DDS) that include micelles, hydrogels, and hybrid particles. The thermoresponsiveness of PNVCL, used alone or in combination with other stimuli- responsive polymers or particles (pH, magnetic field, or chemicals), is often key in the loading and/or release process in these DDS. The renewed focus on this polymer, which is known for decades, is to a large extent due to recent progress in synthetic strategies. Especially, the advent of efficient controlled radical polymerization (CRP) methods for NVCL monomer gives now access to unprecedented well-defined NVCL-based copolymers with unique properties. This Review article addresses up-to-date synthetic aspects, biological features, and biomedical applications of the latest NVCL-containing systems.