A Perspective on the History and Current Opportunities of Aqueous RAFT Polymerization

Reversible addition-fragmentation chain transfer (RAFT) polymerization has proven itself as a powerful polymerization technique affording facile control of molecular weight, molecular weight distribution, architecture, and chain end groups - while maintaining a high level of tolerance for solvent and monomer functional groups. RAFT is highly suited to water as a polymerization solvent, with aqueous RAFT now utilized for applications such as controlled synthesis of ultra-high molecular weight polymers, polymerization induced self-assembly, and biocompatible polymerizations, among others. Water as a solvent represents a non-toxic, cheap, and environmentally friendly alternative to organic solvents traditionally utilized for polymerizations. This, coupled with the benefits of RAFT polymerization, makes for a powerful combination in polymer science. This perspective provides a historical account of the initial developments of aqueous RAFT polymerization at the University of Southern Mississippi from the McCormick Research Group, details practical considerations for conducting aqueous RAFT polymerizations, and highlights some of the recent advances aqueous RAFT polymerization can provide. Finally, some of the future opportunities that this versatile polymerization technique in an aqueous environment can offer are discussed, and it is anticipated that the aqueous RAFT polymerization field will continue to realize these, and other exciting opportunities into the future.

RAFT-mediated polymerization-induced self-assembly (RAFT-PISA): current status and future directions

Polymerization-induced self-assembly (PISA) combines polymerization and self-assembly in a single step with distinct efficiency that has set it apart from the conventional solution self-assembly processes. PISA holds great promise for large-scale production, not only because of its efficient process for producing nano/micro-particles with high solid content, but also thanks to the facile control over the particle size and morphology. Since its invention, many research groups around the world have developed new and creative approaches to broaden the scope of PISA initiations, morphologies and applications, etc. The growing interest in PISA is certainly reflected in the increasing number of publications over the past few years, and in this review, we aim to summarize these recent advances in the emerging aspects of RAFT-mediated PISA. These include (1) non-thermal initiation processes, such as photo-, enzyme-, redox- and ultrasound-initiation; the achievements of (2) high-order structures, (3) hybrid materials and (4) stimuli-responsive nano-objects by design and adopting new monomers and new processes; (5) the efforts in the realization of upscale production by utilization of high throughput technologies, and finally the (6) applications of current PISA nano-objects in different fields and (7) its future directions.

End-functionalized polymers by controlled/living radical polymerizations: synthesis and applications

This review focuses on end-functionalized polymers synthesized by controlled/living radical polymerizations and the applications in fields including bioconjugate formation, surface modification, topology construction, and self-assembly.

Light-Mediated Polymerization Induced by Semiconducting Nanomaterials: State-of-the-Art and Future Perspectives

Direct capture of solar energy for chemical transformation via photocatalysis proves to be a cost-effective and energy-saving approach to construct organic compounds. With the recent growth in photosynthesis, photopolymerization has been established as a robust strategy for the production of specialty polymers with complex structures, precise molecular weight, and narrow dispersity. A key challenge in photopolymerization is the scarcity of effective photomediators (photoinitiators, photocatalysts, etc.) that can provide polymerization with high yield and well-defined polymer products. Current efforts on developing photomediators have mainly focused on organic dyes and metal complexes. On the other hand, nanomaterials (NMs), particularly semiconducting nanomaterials (SNMs), are suitable candidates for photochemical reactions due to their unique optical and electrical properties, such as high absorption coefficients, large charge diffusion lengths, and broad absorption spectra. This review provides a comprehensive insight into SNMs' photomediated polymerizations and highlights the roles SNMs play in photopolymerizations, types of polymerizations, applications in producing advanced materials, and the future directions.

Cell-Penetrating, Peptide-Based RAFT Agent for Constructing Penetration Enhancers

Peptide-polymer conjugates represent a promising class of compounds that can be used to overcome some of the limitations associated with peptides intended for therapeutic and diagnostic applications. The efficient generation of well-defined peptide/protein-polymer conjugates can promote the development of the design and synthesis of functional drugs and gene delivery platforms. In this research, a sequence defined cell penetrating peptide (i.e., Transportan 10 (TP 10))-based chain transfer agent (TP-CTA) was designed and synthesized in an automated peptide synthesizer. Thereafter, amphiphilic block copolymers poly[oligo(ethylene glycol) methyl ether acrylate]-b-poly(n-butyl acrylate) (TP-POEGA-b-PBA) were synthesized using the TP-CTA via reversible addition-fragmentation chain transfer (RAFT) polymerization. Circular dichroism (CD) spectroscopy confirmed the preservation of α-helix structure of TP 10, which is crucial for its bioactivity. Transmission electron microscopy (TEM) revealed the formation of self-assembled rod-like and vesicle nanostructures in an aqueous environment. Finally, the obtained peptide-conjugated block copolymers were demonstrated to be effective compounds for cell penetration. This method opens up a way for accessing peptide-polymer conjugates with cell-penetrating abilities.

Noncovalent structural locking of thermoresponsive polyion complex micelles, nanowires, and vesicles via polymerization-induced electrostatic self-assembly using an arginine-like monomer

The noncovalent locking of nanostructured thermoresponsive polyion complexes can be achieved via polymerization-induced electrostatic self-assembly (PIESA) using an arginine-like cationic monomer.

Surfactant Pyrolysis-Guided in Situ Fabrication of Primary Amine-Rich Ordered Mesoporous Phenolic Resin Displaying Efficient Heavy Metal Removal

A unique strategy for the direct preparation of primary amine-functionalized ordered mesoporous phenolic resin (NH₂-MPRN) was presented. The essence of our approach avoided the side reactions in the m-nitrophenol-formaldehyde resol synthesis using m-nitrophenol and formaldehyde as the monomers. Importantly, these resols can efficiently assemble with triblock copolymer F127 to form a mesostructured composite. The final prolysis treatment removed the F127 template to produce ordered mesopores, and meanwhile, the accompanying reductive gas in situ transferred the nitro groups to primary amines. Notably, the obtained NH₂-MPRN material delivered fast toxic hexavalent chromium sorption kinetics with high uptake capacity and selectivity due to a mesoporous structure, high amine availability, and a hydrophobic surface. Interestingly, almost all of adsorbed chromium species existed as low-toxic trivalent chromium in the resin owing to the cooperative detoxification process by the neighboring primary amines and phenolic hydroxyl groups. Also, it showed the reversible detoxification for at least five times.

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

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

Ultra-fast aqueous polymerisation of acrylamides by high power visible light direct photoactivation RAFT polymerisation

The effect of visible LED power (λ_max= 402 nm, 451 nm) on kinetics and control of direct photoactivation RAFT polymerisations of acrylamide and dimethylacrylamide are investigated.

Rapid Visible Light-Mediated Controlled Aqueous Polymerization with In Situ Monitoring

We report a simple procedure for rapid, visible light-mediated, controlled radical polymerization in aqueous solutions. Based on the photoelectron transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization, fast chain propagation at room temperature in water was achieved in the presence of reductant and without prior deoxygenation. A systematic study correlating irradiation intensity and polymerization kinetics, enabled by in situ nuclear magnetic resonance spectroscopy, provided optimized reaction conditions. The versatility of this procedure was demonstrated through a rapid triblock copolymer synthesis, and incorporation of water-labile activated esters for direct conjugation of hydrophilic small molecules and proteins. In addition, this technique boasts excellent temporal control and provides a wide range of macromolecular materials with controlled molecular weights and narrow molecular weight distributions.

Photoinitiated Polymerization-Induced Self-Assembly (Photo-PISA): New Insights and Opportunities

The polymerization-induced self-assembly (PISA) process is a useful synthetic tool for the efficient synthesis of polymeric nanoparticles of different morphologies. Recently, studies on visible light initiated PISA processes have offered a number of key research opportunities that are not readily accessible using traditional thermally initiated systems. For example, visible light mediated PISA (Photo-PISA) enables a high degree of control over the dispersion polymerization process by manipulation of the wavelength and intensity of incident light. In some cases, the final nanoparticle morphology of a single formulation can be modulated by simple manipulation of these externally controlled parameters. In addition, temporal (and in principle spatial) control over the Photo-PISA process can be achieved in most cases. Exploitation of the mild room temperature polymerizations conditions can enable the encapsulation of thermally sensitive therapeutics to occur without compromising the polymerization rate and their activities. Finally, the Photo-PISA process can enable further mechanistic insights into the morphological evolution of nanoparticle formation such as the effects of temperature on the self-assembly process. The purpose of this mini-review is therefore to examine some of these recent advances that have been made in Photo-PISA processes, particularly in light of the specific advantages that may exist in comparison with conventional thermally initiated systems.

Compartmentalization of an ABC triblock copolymer single-chain nanoparticle via coordination-driven orthogonal self-assembly

We present coordination-driven intramolecular orthogonal self-assembly of ABC triblock copolymer into protein-like compartmentalized SCNP, whose sub-10 nm ultrafine subdomains are discrete and can respond to aqueous surroundings individually.

Observed Photoenhancement of RAFT Polymerizations under Fume Hood Lighting

Given the recent findings of exogenous radical initiator/catalyst-free reversible addition-fragmentation chain transfer (RAFT) radical polymerization under both UV and visible light irradiation, the effect of standard laboratory lighting conditions (fluorescent tube lights) on traditional RAFT reactions, that is, those conducted in the presence of a thermally activated radical initiator, remains unknown. This is investigated in the current study, where a significant "photoenhancement" is observed for most cases under typical RAFT reaction conditions, indicating that fume hood lights can contribute to the generation of radicals in RAFT reactions. Given the observed emission spectrum of a typical fluorescent light source, the photoenhancement is proposed to occur through a visible light activation pathway. These findings are crucial for ensuring maximum reproducibility of controlled polymerizations conducted in the presence of typical sources of irradiation encountered in a standard chemical laboratory.

Visible Light-Mediated Polymerization-Induced Self-Assembly in the Absence of External Catalyst or Initiator

We report the use of visible light to mediate a RAFT dispersion polymerization in the absence of external catalyst or initiator to yield nanoparticles of different morphologies according to a polymerization-induced self-assembly (PISA) mechanism. A POEGMA macro-chain transfer agent (macro-CTA) derived from a 4-cyano-4-((dodecylsulfanylthiocarbonyl)sulfanyl)pentanoic acid (CDTPA) RAFT agent can be activated under blue (460 nm, 0.7 mW/cm²) or green (530 nm, 0.7 mW/cm²) light and act simultaneously as a radical initiator, chain transfer agent, and particle stabilizer under ethanolic dispersion conditions. In particular, the formation of worm-like micelles was readily monitored by the increase of reaction viscosity during the polymerization; this method was shown to be particularly robust to different reaction parameters such as macro-CTAs of varying molecular weight. Interestingly, at high monomer conversion, different morphologies were formed depending on the wavelength of light employed, which may be due to differing degrees of polymerization control. Finally, the in situ encapsulation of the model hydrophobic drug, Nile Red, was demonstrated, suggesting applications of this facile process for the synthesis of nanoparticles for drug delivery applications.

Light-Controlled Radical Polymerization: Mechanisms, Methods, and Applications

The use of light to mediate controlled radical polymerization has emerged as a powerful strategy for rational polymer synthesis and advanced materials fabrication. This review provides a comprehensive survey of photocontrolled, living radical polymerizations (photo-CRPs). From the perspective of mechanism, all known photo-CRPs are divided into either (1) intramolecular photochemical processes or (2) photoredox processes. Within these mechanistic regimes, a large number of methods are summarized and further classified into subcategories based on the specific reagents, catalysts, etc., involved. To provide a clear understanding of each subcategory, reaction mechanisms are discussed. In addition, applications of photo-CRP reported so far, which include surface fabrication, particle preparation, photoresponsive gel design, and continuous flow technology, are summarized. We hope this review will not only provide informative knowledge to researchers in this field but also stimulate new ideas and applications to further advance photocontrolled reactions.

Synthetic polycations with controlled charge density and molecular weight as building blocks for biomaterials

A series of polycations prepared by RAFT copolymerization of N-(3-aminopropyl)methacrylamide hydrochloride (APM) and N-(2-hydroxypropyl)methacrylamide, with molecular weights of 15 and 40 kDa, and APM content of 10-75 mol%, were tested as building blocks for electrostatically assembled hydrogels such as those used for cell encapsulation. Complexation and distribution of these copolymers within anionic calcium alginate gels, as well as cytotoxicity, cell attachment, and cell proliferation on surfaces grafted with the copolymers were found to depend on composition and molecular weight. Copolymers with lower cationic charge density and lower molecular weight showed less cytotoxicity and cell adhesion, and were more mobile within alginate gels. These findings aid in designing improved polyelectrolyte complexes for use as biomaterials.

Visible-light induced controlled radical polymerization of methacrylates with Cu(dap)2Cl as a photoredox catalyst

Under visible light irradiation, block copolymers of PPEGMA-b-PMMA with high molecular weights and narrow molecular weight distributions are obtained starting from a PPEGMA macroinitiator in the presence of the Cu(dap)₂Cl/Me₆TREN catalytic system.

The use of electrostatic association for rapid RAFT synthesis of histamine polyelectrolyte in aqueous solutions at and below 25 °C

Electrostatic association for rapid and quantitative synthesis of well-defined polyelectrolytes in dilute aqueous solutions at and below 25 °C.

Temperature and pH dual-responsive poly(vinyl lactam) copolymers functionalized with amine side groups via RAFT polymerization

A series of statistical copolymers based on cyclic N-vinyl lactams and N-vinylformamide were synthesized via RAFT polymerization. Tempertaure/pH dual responsive polymers were obtained via hydrolysis the copolymers in alkaline conditions.

Preparation and study of multi-responsive polyampholyte copolymers of N-(3-aminopropyl)methacrylamide hydrochloride and acrylic acid

Multi-responsive polyampholytes show LCST and UCST behaviour at different pH values, based on electrostatic and hydrogen bonding interactions.

A visible-light responsive zirconium metal–organic framework for living photopolymerization of methacrylates

Visible-light-induced living radical polymerization of methacrylates by using robust zirconium metal–organic frameworks.

Use of Polyion Complexation for Polymerization-Induced Self-Assembly in Water under Visible Light Irradiation at 25 °C

Polyion complexation (PIC) as the driving force of polymerization-induced self-assembly (PISA), that is, PIC-PISA, is explored. Reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization of NH₃⁺-monomer 2-aminoethylacrylamide hydrochloride (AEAM) can be achieved in water under visible light irradiation at 25 °C, using nonionic poly2-hydroxypropylmethacrylamide (PHPMA) macromolecular chain transfer agent in the presence of anionic poly(sodium 2-acrylamido-2-methylpropanesulfonate) (PAMPS) PIC-template. Sphere-to-network transition occurs, owing to the PIC of PAMPS with growing chains upon reaction close to isoelectric point (IEP); thereafter, the increase of electrostatic repulsion promotes the split of networks and the rupture of spheres into fragments. Therefore, the free-flowing solution becomes viscous liquid and free-standing physical gel, and then back into viscous and free-flowing liquid. Such a PIC-PISA is appealing for gene delivery because the size and surface charge are variable on demand and at high solids.

Polymerization-Induced Self-Assembly Using Visible Light Mediated Photoinduced Electron Transfer-Reversible Addition-Fragmentation Chain Transfer Polymerization

The ruthenium-based photoredox catalyst, Ru(bpy)₃Cl₂, was employed to activate reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization via a photoinduced electron transfer (PET) process under visible light (λ = 460 nm, 0.7 mW/cm²). Poly(oligo(ethylene glycol) methyl ether methacrylate) was chain extended with benzyl methacrylate to afford in situ self-assembled polymeric nanoparticles with various morphologies. The effect of different intrinsic reaction parameters, such as catalyst concentration, total solids content, and cosolvent addition was investigated with respect to the formation of different nanoparticle morphologies, including spherical micelles, worm-like micelles, and vesicles. Importantly, highly pure worm-like micelles were readily isolated due to the in situ formation of highly viscous gels. Finally, "ON/OFF" control over the dispersion polymerization was demonstrated by online Fourier transform near-infrared (FTNIR) spectroscopy, allowing for temporal control over the nanoparticle morphology.

Directional supracolloidal self-assembly via dynamic covalent bonds and metal coordination

An emerging strategy towards the sophistication of supramolecular nanomaterials is the use of supracolloidal self-assembly, in which micelles or colloids are used as building blocks. Binding directionality can produce nanostructures with attractive properties. Herein, we present a new directional supracolloidal self-assembly by virtue of dynamic covalent bonds and metal coordination in water. Conjugation of a ligand precursor to a water-soluble block copolymer through dynamic covalent bonds leads to the dehydration and micellization of the functionalized polymer. Reversible reaction facilitates the permeation of metal ions into core-shell interfaces. Conversely, metal-coordination promotes reaction over the interfaces. Cu(ii)-coordination occurs overwhelmingly inside each isolated micelle. However, Zn(ii)-coordination induced a directional self-assembly whose nanostructures evolve stepwise from nanorods, nanowires, necklaces, and finally to supracolloidal networks scaling-up to several tens of micrometres. Post-reactions of simultaneous dynamic covalent bond conversion and Zn(ii)-coordination over the core-shell interfaces endow these supracolloidal networks with a huge specific surface area for hydrophobic dative metal centres accessible to substrates in water. Water-soluble shells play important roles in directional supracolloidal assembly and in the stabilization of nanostructures. Thus the directional self-assembly provides a versatile platform to produce metallo-hybridized nanomaterials that are promising as enzyme-inspired aqueous catalysts.

Reconstruction of Block Copolymer Micelles to Long-Range Ordered Dense Nanopatterns Via Light-Tunable Hydrogen-Bonding Association

Controlling the orientation and long-range order of nanostructures is a key issue in the self-assembly of block copolymer micelles. Herein, a versatile strategy is presented to transform one-component oxime-based block copolymer micelles into long-range ordered dense nanopatterns. Photoisomerization provides a straightforward and versatile approach to convert the hydrogen-bonding association from inward dimerization (E-type oxime motifs, slightly desolvated in ethyl acetate) into outward interchain association (Z-type ones, highly desolvated in ethyl acetate). This increases the glass transition temperature in bulk and converts swollen micelles into compact spherical micelles in solution. The reconstruction of these micelles on various substrates demonstrates that the phase transformation enables reconstruction of spherical micelles into mesoscopic sheets, nanorods, nanoworms, nanowires, networks, and eventually into long-range ordered and densely packed textile-like and lamellar nanopatterns on a macroscopic scale by adjusting E/Z-oxime ratio and solvent-evaporation rate.

Visible-Light-Controlled Living Radical Polymerization from a Trithiocarbonate Iniferter Mediated by an Organic Photoredox Catalyst

Living radical polymerization of acrylates and acrylamides from trithiocarbonate iniferters using a compact fluorescent lamp (CFL) bulb and 10-phenylphenothiazine as an organic photoredox catalyst is reported. With this system, chain growth can be efficiently switched between "on" and "off" in response to visible light. Polymer molar masses increase linearly with conversion, and narrow molar mass distributions are obtained. The excellent fidelity of the trithiocarbonate-iniferter enables the preparation of triblock copolymers from macro-iniferters under the same visible-light mediated protocol, using UV light without a photoredox catalyst or under traditional thermally induced RAFT conditions. We expect that the simplicity and efficiency of this metal-free, visible-light-mediated polymerization will enable the synthesis and modification of a range of materials under mild conditions.

A well-defined, versatile photoinitiator (salen)Co-CO2CH3 for visible light-initiated living/controlled radical polymerization

The control of the polymerization of a wide range of monomers under mild conditions by a single catalyst remains a major challenge in polymer science. We report a versatile, well-defined organocobalt salen complex to control living radical polymerization of different categories of monomers, including acrylates, acrylamides and vinyl acetate, under visible light irradiation at ambient temperature. Both household light and sunlight were effectively applied in the synthesis of polymers with controlled molecular weights and narrow polydispersities. Narrowly dispersed block copolymers (Mw/Mn < 1.2) were obtained under various conditions. The structures of the polymers were analyzed by 1H NMR, 2D NMR, 13C NMR, GPC, MALDI-TOF-MS and isotopic labeling experiments, which showed that the ω and α ends of the polymer chains were capped with (salen)Co and -CO2CH3 segments, respectively, from the photoinitiator (salen)Co-CO2CH3. The ω end was easily functionalized through oxygen insertion followed by hydrolysis from 18O2 to -18OH. This robust system can proceed without any additives, and offers a versatile and green way to produce well-defined homo and block copolymers.

Programmable self-assembly of a cystamine-block copolymer in response to pH and progressive reduction–ionization–oxidation

A water-soluble cystamine-block copolymer undergoes air/pH-mediated programmable self-assembly/reconstructions simply stemming from the unique environment-mediated reaction complexity of the cystamine-functionalized unit.

UV light-initiated RAFT polymerization induced self-assembly

Reversible addition–fragmentation chain transfer (RAFT) polymerization induced self-assembly (PISA) initiated by UV light is exploited as a new strategy to prepare polymeric nanomicelles at room temperature.

Improving photo-controlled living radical polymerization from trithiocarbonates through the use of continuous-flow techniques

Herein, we report simple flow reactor designs that enable photo-controlled living radical polymerization (photo-CRP) from trithiocarbonates (TTCs) with significant enhancements in scalability and reaction rates compared to the analogous batch reactions.