Controlled Radical Polymerization Techniques Research Articles

Hybrid Bonding Bottlebrush Polymers Grafted from a Supramolecular Polymer Backbone.

Bottlebrush polymers, macromolecules consisting of dense polymer side chains grafted from a central polymer backbone, have unique properties resulting from this well-defined molecular architecture. With the advent of controlled radical polymerization techniques, access to these architectures has become more readily available. However, synthetic challenges remain, including the need for intermediate purification, the use of toxic solvents, and challenges with achieving long bottlebrush architectures due to backbone entanglements. Herein, we report hybrid bonding bottlebrush polymers (systems integrating covalent and noncovalent bonding of structural units) consisting of poly(sodium 4-styrenesulfonate) (p(NaSS)) brushes grafted from a peptide amphiphile (PA) supramolecular polymer backbone. This was achieved using photoinitiated electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization in water. The structure of the hybrid bonding bottlebrush architecture was characterized using cryogenic transmission electron microscopy, and its properties were probed using rheological measurements. We observed that hybrid bonding bottlebrush polymers were able to organize into block architectures containing domains with high brush grafting density and others with no observable brushes. This finding is possibly a result of dynamic behavior unique to supramolecular polymer backbones, enabling molecular exchange or translational diffusion of monomers along the length of the assemblies. The hybrid bottlebrush polymers exhibited higher solution viscosity at moderate shear, protected supramolecular polymer backbones from disassembly at high shear, and supported self-healing capabilities, depending on grafting densities. Our results demonstrate an opportunity for novel properties in easily synthesized bottlebrush polymer architectures built with supramolecular polymers that might be useful in biomedical applications or for aqueous lubrication.

Regulating copolymer architecture using photoiniferter polymerization to direct composite morphology

Due to mild conditions and straightforward synthetic routes, photoiniferter polymerization has received increasing use as a facile method for preparation of polymers with highly controlled structure. However, low quantum yields inherent to thiocarbonylthio species result in polymerization speeds that are considerably slower than traditional controlled radical polymerization techniques. To improve polymerization speed while maintaining high level control, it is critical to understand relationships between reaction parameters and ultimate polymer structure. In this work, initiating light intensity, reactant concentration, and monomer pendant groups were manipulated in the photoiniferter polymerization of acrylate monomers to explore the effect of these conditions on reaction kinetics and molecular weight control. Polymerization rate of n-butyl acrylate was significantly enhanced by increasing light intensity rivaling speeds of conventional thermally initiated RAFT polymerization while enabling more precise control over molecular weight. Interestingly, photoiniferter rate asymptotically approached saturation at relatively high light intensities. Moreover, degradation of trithiocarbonate species occurred at greater intensities, ultimately resulting in moderate increases in molecular weight. Increased reactant concentration enabled faster rates with bulk polymerization, possibly due to reduced macro radical mobility and termination. Additionally, comparing photoiniferter polymerization of methyl acrylate and hydroxyethyl acrylate showed that greater polarity considerably enhanced polymerization speeds. Finally, the utility of precisely controlling polymer structure was demonstrated by synthesizing and incorporating a series of amphiphilic block copolymers with increasing molecular weight into a photocurable epoxy resin. The well-defined block copolymer structures enabled controllable phase separation and thermomechanical properties in epoxy composites. This work showed that photoiniferter polymerization speeds could be significantly enhanced using elementary reaction conditions, and that these conditions enable facile production of well-defined copolymers allowing tailored composite morphology and properties.

Star polyethylenes by coordinative polymerization

AbstractWith their distinct star architecture, star polymers show some desirable materials properties and have received extensive research interest. To date, the advancements in living anionic and controlled radical polymerization techniques have empowered the versatile synthesis of a variety of star polymers from several groups of monomers that are amenable to these polymerization techniques. Despite the fact that polyethylene is the largest produced commodity polymer worldwide, the synthesis of star polyethylenes as a unique polyethylene grade has somehow lagged due to the limited availability of powerful transition metal catalysts that can effectively facilitate ‘living’ ethylene polymerization for the construction of star architectures. Nevertheless, some progress had been made over the past two decades. In this review, we attempt to summarize the developments in the area, with an emphasis on synthesis strategies.

Photoassisted Radical Depolymerization.

Controlled radical polymerization techniques enable the synthesis of polymers with predetermined molecular weights, narrow molecular weight distributions, and controlled architectures. Moreover, these polymerization approaches have been routinely shown to result in retained end-group functionality that can be reactivated to continue polymerization. However, reactivation of these end groups under conditions that instead promote depropagation is a viable route to initiate depolymerization and potentially enable closed-loop recycling from polymer to monomer. In this report, we investigate light as a trigger for thermal depolymerization of polymers prepared by reversible-addition-fragmentation chain-transfer (RAFT) polymerization. We study the role of irradiation wavelength by targeting the n → π* and π → π* electronic transitions of the thiocarbonylthio end-groups of RAFT-generated polymers to enhance depolymerization via terminal bond homolysis. Specifically, we explore depolymerization of polymers with trithiocarbonate, dithiocarbamate, and p-substituted dithiobenzoate end groups with the purpose of increasing depolymerization efficiency with light. As the wavelength decreases from the visible range to the UV range, the rate of depolymerization is dramatically increased. This method of photoassisted depolymerization allows up to 87% depolymerization efficiency within 1 h, results that may further the advancement of recyclable materials and life-cycle circularity.

Enhanced Electrochemical Performance of NCM811 Cathodes with Functionalized PVDF Graft Copolymer Binders

Nickel-rich Li(Ni x Co y Mn1–x–y O2) (x ≥ 0.6) (NCM) cathodes are regarded as the predominant cathode materials for next-generation Li-ion batteries due to their high specific energy density. However, undesired structural disruption and thermal instability were observed with the increase of Ni content e.g. Ni0.8Co0.1Mn0.1(NCM811), which are often attributed to the transition metal (TM) ion dissolution, phase change, oxygen release and microcracks formed on the secondary particles during cycling. Therefore, the rapid capacity loss and poor capacity retention have hindered the successful practical applications for NCM811 cathodes. To solve these abovementioned problems, many strategies have been proposed. One of most effective way is to design and explore functional and novel polymer binder materials. Although poly(vinylidene fluoride) (PVDF), as the benchmark binder in battery community, has been widely used for decades, lacking of functional groups and poor chemical interactions with the active particles makes it no longer the optimal choice for more challenging cathodes.Herein, to explore the possibilities and broaden the range of functional polymers for boosting the performance of NCM811 cathodes, we propose grafting from poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) to obtain PVDF-based polymers with a comb-like architecture, where the PVDF-CTFE was chosen as a substrate backbone to provide the initiating sites for further modification. Atom transfer radical polymerization (ATRP), the controlled radical polymerization technique was used to obtain well-controlled grafted chains with low dispersity and desired length and architecture. A random copolymer of poly(ethylene glycol) methyl ether acrylate and poly(tert-butyl acrylate) (PEGMEA-co-PtBA) was first grafted from the PVDF-CTFE backbone, then after hydrolysis of PtBA to poly(acrylic acid) (PAA), the PVDF-CTFE-g-PEGMEA-co-PAA graft copolymers were obtained. Benefiting from the ethylene oxide conductive segments in PEGMEA, the Li ion diffusion and transport was significantly improved. The chelation of TM ions to -COOH groups from PAA would effectively mitigate the TM dissolution and prevent phase transition of NCM811. To test the electrochemical performances of NCM811 cathodes prepared with the designed polymer binder, PVDF and PVDF-CTFE were compared as the control groups in this study. The rate performance of the Li|NCM811 cells showed that PVDF-CTFE-g-PEGMEA-co-PAA binder gave comparable discharge capacities to PVDF and PVDF-CTFE at 0.1C-2C (Figure 1a), however, under 4C with faster charging/discharging, the discharge capacities of PVDF-CTFE-g-PEGMEA-co-PAA were much higher than the two control groups. When increasing the NCM811 weight ratio from 85 wt% to 93 wt%, where the contribution from the conductive carbon is less, the differences were even more significant (Figure 1b). PVDF-CTFE-g-PEGMEA-co-PAA delivered higher capacity at all rates, especially at 2C and 4C, suggesting faster Li ion transport and enhanced electronic properties in the cathodes. Figure 1. Rate performance of Li|NCM811 cells with binder PVDF (grey), PVDF-CTFE (orange) and PVDF-CTFE-g-PEGMEA-co-PAA (green) in the voltage range of 2.9–4.3 V using NCM811/carbon black/binder weight ratio of a) 85/10/5 and b) 93/4/3. Figure 1

Para fluoro-thiol clicked diblock-copolymer self-assembly: Towards a new paradigm for highly proton-conductive membranes

Sulfonated sPPFS-b-PBuA diblock and statistical copolymers based on 2,3,4,5,6-pentafluorostyrene PFS and butyl acrylate BuA were elaborated for Proton Exchange Membrane Water Electrolyser (PEMWE) purposes. The block copolymers (BCP) were synthetized by Nitroxide Mediated Polymerization NMP, a controlled radical polymerization technique that yields a well-defined molar mass and a low dispersity material. These diblock-copolymers have the ability to self-assemble due to the immiscibility of the two macromolecular segments PPFS and PBuA. Statistical copolymers of the similar chemical composition were synthetized by both controlled radical polymerization NMP in solution and by free radical polymerization FRP in emulsion as waterborne dispersed polymer with highest molar mass. The copolymers were sulfonated by a mild click-reaction, namely an organo-catalyzed nucleophilic substitution reaction with sodium 3-mercapto-1-propanesulfonate (SMPS) at low temperature. The morphology of the sulfonated diblock-copolymer BCP was studied by SAXS and AFM, revealing a nano-phase-segregated sulfonated membrane. The mechanical properties of the membranes were improved by ionic crosslinking with polybenzimidazole (PBI-OO). Finally, the conductive properties of the sulfonated BCPs and statistical copolymers were investigated as a function of parameters such as the morphology of the BCP, the molar mass, and the sulfonation degree of the materials.

Lemonade as a rich source of antioxidants: Polymerization of 2-(dimethylamino)ethyl methacrylate in lemon extract

Controlled radical polymerization techniques have become increasingly widespread in the interdisciplinary space of polymer chemistry for receiving predetermined structures designed for specific applications. In this contribution, the freshly squeezed lemon extract was used for the polymerization of 2-(dimethylamino)ethyl methacrylate (DMAEMA) by activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) concept at ambient conditions in an air atmosphere. Lemon extract is a rich source of many bioactive compounds that can play a role of reducing agents in ATRP, e.g. ascorbic acid (vitamin C) or citric acid. Therefore lemon extract-accelerated ARGET ATRP provided well-defined PDMAEMA structures, accelerating the polymerization about 3–4 times compared to syntheses in water. The chain extension experiment confirmed the preserved chain-end functionality of prepared PDMAEMA. The proposed concept can be scaled up for the industry due to (a) a facile reaction setup that does not need degassing or heating – the polymerization at room temperature in open to air conditions, hence the use of a readily available reactor is sufficient, (b) no need to use laboratory-grade chemical reducing agents because of their high content in lemon extract, (c) the synthesis in the environmentally-friendly aqueous reaction medium, (d) very low catalyst concentration in the reaction mixture. • Lemon extract is a reach source of reducing agents for ARGET ATRP concept. • Polymerization of DMAEMA in freshly squeezed lemon extract. • ARGET ATRP in lemon extract accelerates the polymerization of DMAEMA compared to synthesis in water. • A reaction setup for polymerization of DMAEMA that can be easily scalable for industrial application.

Design of biointerfaces composed of soft materials using controlled radical polymerizations.

Soft interface materials have an immense potential for the improvement of biointerfaces, which are the interface of biological and artificially designed materials. Controlling the chemical and physical structures of the interfaces at the nanometer level plays an important role in understanding the mechanism of the functioning and its applications. Controlled radical polymerization (CRP) techniques, including atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain-transfer (RAFT) polymerization, have been developed in the field of precision polymer chemistry. It allows the formation of well-defined surfaces such as densely packed polymer brushes and self-assembled nanostructures of block copolymers. More recently, a novel technique to prepare polymers containing biomolecules, called biohybrids, has also been developed, which is a consequence of the advancement of CRP so as to proceed in an aqueous media with oxygen. This review article summarizes recent advances in CRP for the design of biointerfaces.

Living polymer networks prepared by controlled radical polymerization techniques

Much has been done towards the traditional usage of controlled radical polymerization, but with a far lesser degree of work in polymer networks. Here, we cover both parent and daughter networks, and finish by looking at their future potential.

Facile Access to Polar-Functionalized Ultrahigh Molecular Weight Polyethylene at Ambient Conditions

Facile Access to Polar-Functionalized Ultrahigh Molecular Weight Polyethylene at Ambient Conditions

Recent Advances in the Synthesis of Polymer-Grafted Low-K and High-K Nanoparticles for Dielectric and Electronic Applications.

The synthesis of polymer-grafted nanoparticles (PGNPs) or hairy nanoparticles (HNPs) by tethering of polymer chains to the surface of nanoparticles is an important technique to obtain nanostructured hybrid materials that have been widely used in the formulation of advanced polymer nanocomposites. Ceramic-based polymer nanocomposites integrate key attributes of polymer and ceramic nanomaterial to improve the dielectric properties such as breakdown strength, energy density and dielectric loss. This review describes the “grafting from” and “grafting to” approaches commonly adopted to graft polymer chains on NPs pertaining to nano-dielectrics. The article also covers various surface initiated controlled radical polymerization techniques, along with templated approaches for grafting of polymer chains onto SiO2, TiO2, BaTiO3, and Al2O3 nanomaterials. As a look towards applications, an outlook on high-performance polymer nanocomposite capacitors for the design of high energy density pulsed power thin-film capacitors is also presented.

Enhanced Humoral Immune Response by High Density TLR Agonist Presentation on Hyperbranched Polymers

AbstractProphylactic vaccines often exploit adjuvants to drive potent and directed immune responses. Small‐molecule toll‐like receptor (TLR) agonists are promising adjuvant candidates; however, the poor pharmacokinetics and rapid distribution of these relatively hydrophilic small molecules results in systemic side effects that have limited their usage to topical application. In this study, TLR7/8 agonists are covalently attached to the highly dense end‐groups of hyperbranched polymers synthesized by controlled radical polymerization techniques. Subcutaneous administration of these adjuvants results in a potent and prolonged type I interferon response. When co‐administered in a model subunit vaccine, with ovalbumin or the HIV envelope glycoprotein gp120, these hyperbranched polymeric adjuvants potentiate an enhanced immunoglobulin response and near‐immediate type‐switching to IgG2c, suggesting they promote the type of strong T helper cell 1 immune response that is desirable for the treatment of intracellular pathogens such as mycobacterial and viral infections. Controlling the pharmacokinetics of potent TLR agonists through conjugation to hyperbranched polymers, therefore, enables the development of potent adjuvants in vaccines to drive durable and high‐quality humoral immunity.

Structure–property relationships of core-substituted diaryl dihydrophenazine organic photoredox catalysts and their application in O-ATRP

Photoinduced organocatalyzed atom-transfer radical polymerization (O-ATRP) is a controlled radical polymerization technique that can be driven using low-energy, visible light and makes use of organic photocatalysts.

Iron-Based Catalytically Active Complexes in Preparation of Functional Materials

Iron complexes are particularly interesting as catalyst systems over the other transition metals (including noble metals) due to iron’s high natural abundance and mediation in important biological processes, therefore making them non-toxic, cost-effective, and biocompatible. Both homogeneous and heterogeneous catalysis mediated by iron as a transition metal have found applications in many industries, including oxidation, C-C bond formation, hydrocarboxylation and dehydration, hydrogenation and reduction reactions of low molecular weight molecules. These processes provided substrates for industrial-scale use, e.g., switchable materials, sustainable and scalable energy storage technologies, drugs for the treatment of cancer, and high molecular weight polymer materials with a predetermined structure through controlled radical polymerization techniques. This review provides a detailed statement of the utilization of homogeneous and heterogeneous iron-based catalysts for the synthesis of both low and high molecular weight molecules with versatile use, focusing on receiving functional materials with high potential for industrial application.

Progress in the Free and Controlled Radical Homo- and Co-Polymerization of Itaconic Acid Derivatives: Toward Functional Polymers with Controlled Molar Mass Distribution and Architecture.

Polymeric derivatives of itaconic acid are becoming increasingly more interesting for research and industry because itaconic acid is accessible from renewable resources. In spite of the structural similarity of poly(itaconic acid derivatives) to poly(methacrylates), they are much less reactive, homopolymerize only sluggishly by free radical polymerization (FRP), and are often obtained with low molar masses and conversions. This has so far limited their use. The reasons for the low reactivity of itaconic acid derivatives (including itaconimides, diitaconates, and diitaconamides) are combined steric and electronic effects, as demonstrated by the body of literature on the FRP homopolymerization kinetics of these monomers which is summarized herein. These problems can be solved to a large extent by using controlled radical polymerization (CRP) techniques, notably atom transfer radical polymerization (ATRP) and reversible addition and fragmentation chain transfer radical polymerization (RAFT). By optimizing the reaction conditions for the ATRP and RAFT of itaconic acid derivatives, in particular the reaction temperature, linear relations between molar mass and conversion are obtained in many cases, and homopolymers with high molar masses and reasonably narrow polydispersity indices become accessible. This review presents the state-of-the-art FRP and CRP of itaconic acid derivatives, and highlights functional polymers obtained by these methods.

Direct Polymerization Approach to Synthesize Acid-Degradable Block Copolymers Bearing Imine Pendants for Tunable pH-Sensitivity and Enhanced Release.

The development of effective approaches to synthesize smart amphiphilic block copolymers (ABPs) exhibiting acid-responsive degradation through the cleavage of acid-labile imine bonds is extensively explored for controlled release of encapsulated biomolecules, particularly in drug delivery. Here, a new approach based on direct polymerization utilizing a controlled radical polymerization technique to synthesize acid-degradable ABPs bearing pendant imine linkages in hydrophobic block is reported. The approach centers on the synthesis of a novel methacrylate bearing benzoic imine group that can be polymerized to form the hydrophobic imine pendant block. The formed ABPs respond to mild acidic pHs equivalent to tumoral and endosomal/lysosomal acidic environments. This causes the dissociation of self-assembled nanoassemblies through change in their hydrophilic/hydrophobic balance upon the cleavage of pendant imine linkages to the corresponding aldehyde and primary amine, thus leading to the enhanced release of encapsulated drugs. The proof-of-concept results suggest that this robust approach is versatile to further design advanced nanoassemblies responding to dual/multiple stimuli, thus being more effective to intracellular drug delivery.

Amphiphilic zwitterionic-PDMS-based surface-modifying additives to tune fouling-release of siloxane-polyurethane marine coatings

Amphiphilic coatings have shown promising performance for marine applications to deter and limit biofouling. Hydrophobic marine coatings are unable to deter marine organisms that prefer hydrophobic surfaces for settlements, thus a series of amphiphilic additives were prepared and introduced to a hydrophobic system to attain surface amphiphilicity and improved performance without changing the base matrix. In this work, we report the successful synthesis of amphiphilic additives where highly incompatible blocks of hydrophobic polydimethylsiloxane (PDMS) and hydrophilic poly(sulfobetaine methacrylate) (poly(SBMA)) were connected, using ARGET ATRP (activators regenerated by electron transfer atom transfer radical polymerization) controlled radical polymerization technique. The surface characterization confirmed the presence of self-migrated amphiphilic additives to the surface of hydrophobic coating systems, and biological assessments indicated that the additives desirably improved fouling-release performance of the base hydrophobic system against macroalgal spores (Ulva linza) and barnacles (Amphibalanus amphitrite) while these additives had no detrimental effect on ability of the base matrix to release bacteria (Cellulophaga lytica), diatoms (Navicula incerta), and mussels (Geukensia demissa). Furthermore, this study not only reported the outstanding contribution of poly(SBMA)-PDMS additives towards contending with marine biofouling as well as the facile preparation of amphiphilic additives, but also concluded that several factors should be considered in the design of these additives for tailoring hydrophobic coatings: 1) PDMS block with a molecular weight of 1000 M¯n is preferred over higher molecular weights of PDMS; 2) hydrophilic portion between a range of 50% to 80% provides the desired amphiphilicity on a surface; and 3) poly(SBMA) block size does not necessarily impact the effectiveness of an additive.

Progress and Perspectives Beyond Traditional RAFT Polymerization.

The development of advanced materials based on well‐defined polymeric architectures is proving to be a highly prosperous research direction across both industry and academia. Controlled radical polymerization techniques are receiving unprecedented attention, with reversible‐deactivation chain growth procedures now routinely leveraged to prepare exquisitely precise polymer products. Reversible addition‐fragmentation chain transfer (RAFT) polymerization is a powerful protocol within this domain, where the unique chemistry of thiocarbonylthio (TCT) compounds can be harnessed to control radical chain growth of vinyl polymers. With the intense recent focus on RAFT, new strategies for initiation and external control have emerged that are paving the way for preparing well‐defined polymers for demanding applications. In this work, the cutting‐edge innovations in RAFT that are opening up this technique to a broader suite of materials researchers are explored. Emerging strategies for activating TCTs are surveyed, which are providing access into traditionally challenging environments for reversible‐deactivation radical polymerization. The latest advances and future perspectives in applying RAFT‐derived polymers are also shared, with the goal to convey the rich potential of RAFT for an ever‐expanding range of high‐performance applications.

Synthesis of Hyperbranched Polymers via Metal‐Free ATRP in Solution and Microemulsion

AbstractAtom transfer radical polymerization (ATRP), as one of the most successful controlled radical polymerization techniques, has been broadly used by polymer chemists and nonspecialists for synthesis of various functional materials, although the use of copper as traditional catalyst often results in undesired color or properties. The first homopolymerization of an initiable monomer, that is, inimer, is reported via metal‐free ATRP using 10‐phenylphenothiazine (Ph‐PTH) as photocatalyst in both solution and microemulsion media. Although polymerizations of inimers in both media can be carried out, only the microemulsion polymerization of methacrylate‐based inimer 1 effectively confines the random bimolecular reaction within each segregated latex and produces hyperbranched polymers with high molecular weight and low polydispersity. Several experimental parameters in the microemulsion polymerization of inimer 1 are subsequently studied, including the Ph‐PTH amount, the solids content of microemulsion, and the light source of irradiation. The results not only provide an effective method to tune the structure and molecular weights of hyperbranched polymers in confined‐space polymerization, but also expand the toolbox of using metal‐free ATRP method for synthesizing highly branched polymers in controlled manner.

General Way To Construct Micro- and Mesoporous Metal-Organic Framework-Based Porous Liquids.

We report a new class of porous liquids (PLs) using internally functionalized metal-organic framework (MOF) particles as pore carriers and poly(dimethylsiloxane) as bulky solvents. Using a generalizable noncovalent surface-initiated controlled radical polymerization technique, a series of isoreticular UiO-66 particles were dispersed in a liquid PDMS matrix with excellent homogeneity and colloidal stability. Benefiting from the inherent properties of PDMS, the PLs exhibit low vapor pressure, high thermal stability, and fluidity down to -35 °C. Attributed to the bulkiness of PDMS and its inherent high permeability, the sorption properties of the MOF fillers can be largely retained in their respective PLs as confirmed by low-pressure CO2, N2, Xe, and H2O sorption isotherms. The permanent porosity of the PLs can also be largely preserved even after 15 months of storage. Finally, we demonstrate that by tuning the molecular weight and polymer chain architecture of PDMS, it is possible to preserve the permanent porosity of a mesoporous MOF, MIL-101(Cr), within a PL.