Using Dynamic Covalent Chemistry To Drive Morphological Transitions: Controlled Release of Encapsulated Nanoparticles from Block Copolymer Vesicles

Regulating Block Copolymer Assembly Structures in Emulsion Droplets through Metal Ion Coordination

In this report, we demonstrate the metal ion coordination-induced morphological transition of block copolymer assemblies under three-dimensional (3D) confinement. Polystyrene- block-poly(4-vinyl pyridine) (PS- b-P4VP) aggregates with various morphologies can be obtained by emulsion-solvent evaporation in the presence of metal ions (e.g., Pb(II) or Fe(III) ions) in the aqueous phase. Due to the coordination interaction between 4VP units and metal ions, the overall shape, internal structure, and surface composition of the particles can be tailored by varying the type and concentration of the metal ions. For example, when Pb(II) ions were employed, morphological transition of the assemblies occurred due to the formation of P4VP-Pb(II) complexes. More interestingly, when Fe(III) ions were added, hydrolysis of Fe(III) caused the reduction of the pH value of the aqueous phase, leading to the protonation of 4VP units. As a result, interfacial instability took place to trigger the splitting of emulsion droplets and then formation of nanosized micelles. Therefore, metal ion coordination is a facile strategy to tune the structure of assemblies under 3D confinement and offers an alternative approach for the design of organic-inorganic hybrid assemblies with well-tunable structures.

Bioinspired Self-Healing Hydrogel Based on Benzoxaborole-Catechol Dynamic Covalent Chemistry for 3D Cell Encapsulation

Boronic ester, one typical example of dynamic covalent bonds, has presented great potential to prepare self-healing hydrogels. However, most of currently reported hydrogels based on boronic esters are formed at pH > 8, which impeded their further use in physiological conditions. In this study, we designed two kinds of zwitterionic copolymers with benzoxaborole and catechol pendant groups, respectively. Owing to the lower pK_a value of benzoxaborole (7.2), gelation can happen easily at pH 7.4 PBS after mixing these two copolymers due to efficient formation of benzoxaborole-catechol complexations. The resulting hydrogels exhibited excellent self-healing property as well as dual pH/sugar responsiveness due to the dynamic nature of boronic ester. Moreover, benefiting from the cell membrane bioinspired 2-methacryloyloxyethyl phosphorylcholine (MPC)-based polymeric matrix, the hydrogel was further investigated for 3D cell encapsulation. The combination of biocompatible zwitterionic polymers with dynamic benzoxaborole-catechol complexation makes the hydrogels a promising platform for diverse potential bioapplications like drug delivery and tissue engineering.

Three Switchable Orthogonal Dynamic Covalent Reactions and Complex Networks Based on the Control of Dual Reactivity

Achieving complexity is central to the creation of chemical systems, inspired by natural systems. Herein we introduce a strategy of switchable orthogonal dynamic covalent chemistry (DCC) toward the regulation of complex dynamic networks. The control of dual reactivity of tautomers and resulting pathways allowed reversible covalent bonding of a large scope of primary amines, secondary amines, alcohols, and thiols with high efficiency. The selection of reaction pathways next enabled the realization of orthogonal but switchable dynamic covalent reactions (DCRs) with nucleophile pairs of amine/alcohol, alcohol/thiol, and amine/thiol by varying protonation and oxidation states. Control experiments confirmed the crucial role of dual reactivity on the stability and switchability of DCRs. The specificity toward amines, alcohols, and thiols, as well as interconversion between their corresponding assemblies, was further accomplished in one vessel, thus creating tunable communicating networks with three types of DCRs. Moreover, the switchable orthogonality combined with differential reactivity of multiple sulfonamides and nucleophiles enhanced the complexity within dynamic libraries. The generality and versatility of our approaches should facilitate their incorporation into many aspects of chemistry endeavors.

Controlling Nanomaterial Size and Shape for Biomedical Applications via Polymerization-Induced Self-Assembly

Rapid developments in the polymerization-induced self-assembly (PISA) technique have paved the way for the environmentally friendly production of nanoparticles with tunable size and shape for a diverse range of applications. In this feature article, the biomedical applications of PISA nanoparticles and the substantial progress made in controlling their size and shape are highlighted. In addition to early investigations into drug delivery, applications such as medical imaging, tissue culture, and blood cryopreservation are also described. Various parameters for controlling the morphology of PISA nanoparticles are discussed, including the degree of polymerization of the macro-CTA and core-forming polymers, the concentration of macro-CTA and core-forming monomers, the solid content of the final products, the solution pH, the thermoresponsitivity of the macro-CTA, the macro-CTA end group, and the initiator concentration. Finally, several limitations and challenges for the PISA technique that have been recently addressed, along with those that will require further efforts into the future, will be highlighted.

A facile method for preparation of uniform polymeric vesicles with tunable size

Vesicle size and size uniformity determine whether the vesicle can realize its full potential in a wide range of biological and biomedical applications. Herein, we reported a simple yet general cosolvent method to directly prepare uniform polymeric vesicles with a tunable size ranging from 100 nm to 150 nm formed from the amphiphilic diblock copolymer poly(4-vinylpyridine)-block-polystyrene (P4VP77-b-PS318). The uniform polymeric vesicles can be easily obtained by adding plenty of selective solvent only once into the polymer solution and then resting overnight. It is found that uniform vesicles are formed with a fixed size. There is no further fusion between vesicles. The PDI of the vesicles measured by DLS can be as low as 0.03. Moreover, the size of these uniform vesicles can be controlled by the content of the selective solvent. The higher the content of the selective solvent, the smaller the vesicles that form. Monte Carlo simulation was also performed in this study. The simulation results indicated that the vesicle size indeed decreases with an increase in the content of the selective solvent. Two reasons leading to the decrease in vesicle size are elucidated. In addition, the simulations reveal that the size uniformity of the vesicle is essentially determined by the interactive enthalpy of the system.

Morphology Evolution of Stimuli-Responsive Triblock Copolymer Modulated by Polyoxometalates

Polyoxometalate (POM) H₃PMo₁₂O₄₀ was coassembled with stimuli-responsive triblock copolymer poly(ethylene oxide)- block-polystyrene- block-poly(2-(dimethylamino)ethyl methacrylate) (PEO- b-PS- b-PDMAEMA) by electrostatic interactions. Depending on the POM contents, the hybrid complexes can self-assemble into a series of morphologies: micelles, rods, toroids, and vesicles. Unlike traditional morphology transition of amphiphilic block copolymer derived from a broad range of hydrophobic volume fractions, POM-induced morphology transitions just occurred in a narrow range of volume fractions. The length of rod micelles exponentially decreased with solvent compositions (tetrahydrofuran/H₂O). The hybrid assemblies showed acid-base responsibility due to the PDMAEMA block. Rod micelles could further assemble and disassemble reversibly upon adding acid/base. Fluorescent polyoxometalate Na₉EuW₁₀O₃₆ was also complexed with PEO- b-PS- b-PDMAEMA to prepare fluorescent vesicles. The vesicles showed off-on switchable fluorescence behavior accompanied with reversible vesicle-to-micelle transformation in response to pH stimuli.

New Insights into RAFT Dispersion Polymerization-Induced Self-Assembly: From Monomer Library, Morphological Control, and Stability to Driving Forces

Polymerization-induced self-assembly (PISA) has been established as an efficient, robust, and versatile approach to synthesize various block copolymer nano-objects with controlled morphologies, tunable dimensions, and diverse functions. The relatively high concentration and potential scalability makes it a promising technique for industrial production and practical applications of functional polymeric nanoparticles. This feature article outlines recent advances in PISA via reversible addition-fragmentation chain transfer dispersion polymerization. Considerable efforts to understand morphological control, broaden the monomer library, enhance morphological stability, and incorporate multiple driving forces in PISA syntheses are summarized herein. Finally, perspectives on the future of PISA research are discussed.

Covalent layer-by-layer films: chemistry, design, and multidisciplinary applications

Covalent layer-by-layer (LbL) assembly is a powerful method used to construct functional ultrathin films that enables nanoscopic structural precision, componential diversity, and flexible design. Compared with conventional LbL films built using multiple noncovalent interactions, LbL films prepared using covalent crosslinking offer the following distinctive characteristics: (i) enhanced film endurance or rigidity; (ii) improved componential diversity when uncharged species or small molecules are stably built into the films by forming covalent bonds; and (iii) increased structural diversity when covalent crosslinking is employed in componential, spacial, or temporal (labile bonds) selective manners. In this review, we document the chemical methods used to build covalent LbL films as well as the film properties and applications achievable using various film design strategies. We expect to translate the achievement in the discipline of chemistry (film-building methods) into readily available techniques for materials engineers and thus provide diverse functional material design protocols to address the energy, biomedical, and environmental challenges faced by the entire scientific community.

Construction of dual-functional polymer nanomaterials with near-infrared fluorescence imaging and polymer prodrug by RAFT-mediated aqueous dispersion polymerization

The performance of functional polymer nanomaterials is a vigorously discussed topic in polymer science. We devoted ourselves to investigating polymer nanomaterials based on near-infrared (NIR) fluorescence imaging and polymer prodrug in this study. Aza-boron dipyrromethene (BODIPY) is an important organic dye, having characteristics such as environmental resistance, light resistance, high molar extinction coefficient, and fluorescence quantum yield. We incorporated it into our target monomer, which can be polymerized without changing its parent structure in a polar solvent and copolymerized with water-soluble monomer to improve the solubility of the dye in an aqueous solution. At the same time, the hydrophobic drug camptothecin (CPT) was designed as a prodrug monomer, and the polymeric nanoparticles (NPs) with NIR fluorescence imaging and prodrug were synthesized in situ in reversible addition-fragmentation chain transfer (RAFT)-mediated aqueous dispersion polymerization. The dynamic light scattering (DLS) and transmission electron microscopy (TEM) revealed the final uniform size of the dual-functional polymeric NPs morphology. The dual-functional polymeric NPs had a strong absorption and emission signal in the NIR region (>650 nm) based on the fluorescence tests. In consideration of the long-term biological toxicity, confocal laser scanning microscopy (CLSM) results indicated that the dual-functional NPs with controlled drug content exhibited effective capability of killing HeLa cells. In addition, in vivo imaging of the dual-functional NPs was observed in real time, and the fluorescent signals clearly demonstrated the dynamic process of prodrug transfer.

Glassy worm-like micelles in solvent and shear mediated shape transitions

The glassiness of polymer melts is generally considered to be suppressed by small dimensions, added solvent, and heat. Here, we suggest that glassiness persists at the nanoscale in worm-like micelles composed of amphiphilic diblock copolymers of poly(ethylene oxide)-polystyrene (PS). The glassiness of these worms is indicated by a lack of fluorescence recovery after photobleaching as well as micron-length rigid segments separated by hinges. The coarse-grained molecular dynamics studies probe the dynamics of the PS in these glassy worms. Addition of an organic solvent promotes a transition from hinged to fully flexible worms and to spheres or vesicles. Simulation demonstrates two populations of organic solvent in the core of the micelle-a solvent 'pool' in the micelle core and a second population that accumulates at the interface between the core and the corona. The stable heterogeneity of the residual solvent could explain the unusual hinged rigidity, but solvent removal during shear-extension could be more effective and yield - as observed - nearly straight worms without hinges.

Synthesis of High Molecular Weight Poly(glycerol monomethacrylate) via RAFT Emulsion Polymerization of Isopropylideneglycerol Methacrylate

High molecular weight water-soluble polymers are widely used as flocculants or thickeners. However, synthesis of such polymers via solution polymerization invariably results in highly viscous fluids, which makes subsequent processing somewhat problematic. Alternatively, such polymers can be prepared as colloidal dispersions; in principle, this is advantageous because the particulate nature of the polymer chains ensures a much lower fluid viscosity. Herein we exemplify the latter approach by reporting the convenient one-pot synthesis of high molecular weight poly(glycerol monomethacrylate) (PGMA) via the reversible addition-fragmentation chain transfer (RAFT) aqueous emulsion polymerization of a water-immiscible protected monomer precursor, isopropylideneglycerol methacrylate (IPGMA) at 70 °C, using a water-soluble poly(glycerol monomethacrylate) (PGMA) chain transfer agent as a steric stabilizer. This formulation produces a low-viscosity aqueous dispersion of PGMA-PIPGMA diblock copolymer nanoparticles at 20% solids. Subsequent acid deprotection of the hydrophobic core-forming PIPGMA block leads to particle dissolution and affords a viscous aqueous solution comprising high molecular weight PGMA homopolymer chains with a relatively narrow molecular weight distribution. Moreover, it is shown that this latex precursor route offers an important advantage compared to the RAFT aqueous solution polymerization of glycerol monomethacrylate since it provides a significantly faster rate of polymerization (and hence higher monomer conversion) under comparable conditions.

Thermoreversible crystallization-driven aggregation of diblock copolymer nanoparticles in mineral oil

A poly(behenyl methacrylate)37 (PBeMA37) macromolecular chain transfer agent is utilized for the reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization of benzyl methacrylate (BzMA) directly in mineral oil at 90 °C. Polymerization-induced self-assembly (PISA) occurs under these conditions, yielding a series of sterically-stabilized PBeMA37-PBzMA x diblock copolymer spheres of tunable diameter as confirmed by dynamic light scattering (DLS) and transmission electron microscopy (TEM) studies. Rheological studies indicate that a relatively transparent, free-flowing, concentrated dispersion of non-interacting 32 nm PBeMA37-PBzMA100 spheres at 50 °C forms a turbid, paste-like dispersion on cooling to 20 °C. Turbidimetry and differential scanning calorimetry (DSC) studies conducted on solutions of PBeMA37 homopolymer in mineral oil suggest that this switchable colloidal stability is linked to crystallization-induced phase separation exhibited by this stabilizer block. Indeed, variable-temperature small-angle X-ray scattering (SAXS) indicates that a loose mass fractal network of strongly interacting spheres is formed on cooling to 20 °C, which accounts for this thermoreversible sol-gel transition. Moreover, SAXS, DSC and wide-angle X-ray scattering (WAXS) analyses indicate that the behenyl (C22H45) side-chains first form crystalline domains comprising adjacent stabilizer chains within individual spherical nanoparticles, with subsequent crystallization between neighboring nanoparticles leading to the formation of the mass fractal aggregates.

Hydrogels Based on Dynamic Covalent and Non Covalent Bonds: A Chemistry Perspective

Hydrogels based on reversible covalent bonds represent an attractive topic for research at both academic and industrial level. While the concept of reversible covalent bonds dates back a few decades, novel developments continue to appear in the general research area of gels and especially hydrogels. The reversible character of the bonds, when translated at the general level of the polymeric network, allows reversible interaction with substrates as well as responsiveness to variety of external stimuli (e.g., self-healing). These represent crucial characteristics in applications such as drug delivery and, more generally, in the biomedical world. Furthermore, the several possible choices that can be made in terms of reversible interactions generate an almost endless number of possibilities in terms of final product structure and properties. In the present work, we aim at reviewing the latest developments in this field (i.e., the last five years) by focusing on the chemistry of the systems at hand. As such, this should allow molecular designers to develop a toolbox for the synthesis of new systems with tailored properties for a given application.

Ultra-tough injectable cytocompatible hydrogel for 3D cell culture and cartilage repair

In this work, we developed a very facile strategy, i.e. dual dynamic crosslinking, to prepare a high performance injectable hydrogel. Poly(vinyl alcohol) (PVA) was crosslinked by 4-carboxyphenylboronic acid (CPBA) through the generation of borate bonding and ionic interaction to bridge the polymer chains in the presence of calcium ions. The dynamic gathering of CPBA could induce a self-reinforcing effect inside the hydrogel matrix, leading to high tensile and compressive moduli of the hydrogel over 1.0 MPa including the highest compressive modulus up to 5.6 MPa. Meanwhile, the mechanical properties of the hydrogel can be broadly and accurately tuned. And owing to the flexible PVA network, the hydrogel is ultra-tough, showing maximum tensile strain, tensile and compressive fracture energies up to 1600%, 600 kJ m^-2 and 25 kJ m^-2, respectively. Besides, the dynamic bonding overcomes the barriers to forming an injected strong hydrogel, e.g. to obtain a modulus and a fracture energy exceeding 1.0 MPa and 40 kJ m^-2, by using a commercial dual-syringe kit under physiological conditions. Such a mild gelation procedure benefits the administration, 3D encapsulation and proliferation of cells of the hydrogels. The application of the PVA hydrogel was demonstrated by effective cartilage repair.

pH-Responsive diblock copolymers with two different fluorescent labels for simultaneous monitoring of micellar self-assembly and degree of protonation

Facile labelling of both blocks of a pH-responsive diblock copolymer with different fluorophores allows monitoring of polymer aggregation and deprotonation.

Coordination-induced assemblies of quantum dots in amphiphilic thermo-responsive block copolymer micelles: morphologies, optical properties and applications

Thermo-responsive dual-emitting QD/BCP assemblies with QDs located in the core (CDMs), shell (SDMs) and the interface (IDMs) between the core and the shell of micelles were constructed via coordination-driven assemblies for the selective detection of TNP and Hg²⁺ ions.

Using Host-Guest Chemistry to Tune the Kinetics of Morphological Transitions Undertaken by Block Copolymer Vesicles

Host-guest chemistry is exploited to tune the rate at which block copolymer vesicles undergo morphological transitions. More specifically, a concentrated aqueous dispersion of poly(glycerol monomethacrylate-co-glycidyl methacrylate)-poly(2-hydroxypropyl methacrylate) [P(GMA-co-GlyMA)-PHPMA] diblock copolymer vesicles was prepared via polymerization-induced self-assembly (PISA). The epoxy groups in the GlyMA residues were ring-opened using a primary amine-functionalized β-cyclodextrin (NH₂-β-CD) in order to prepare β-CD-decorated vesicles. Addition of azobenzene-methoxypoly(ethylene glycol) (azo-mPEG) to such vesicles results in specific binding of this water-soluble macromolecular reagent to the β-CD groups on the hydrophilic P(GMA-co-GlyMA) stabilizer chains. Such host-guest chemistry induces a morphological transition from vesicles to worms and/or spheres. Furthermore, the rate of this morphological transition can be tuned by UV/visible-light irradiation and/or guest molecule competition. This novel molecular recognition strategy offers considerable scope for the design of new stimulus-responsive diblock copolymer vesicles for targeted delivery and controlled release of cargoes.

Emulsion Solvent Evaporation-Induced Self-Assembly of Block Copolymers Containing pH-Sensitive Block

A simple yet efficient method is developed to manipulate the self-assembly of pH-sensitive block copolymers (BCPs) confined in emulsion droplets. Addition of acid induces significant variation in morphological transition (e.g., structure and surface composition changes) of the polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) assemblies, due to the hydrophobic-hydrophilic transition of the pH-sensitive P4VP block via protonation. In the case of pH > pKa_(P4VP) (pKa _(P4VP) = 4.8), the BCPs can self-assemble into pupa-like particles because of the nearly neutral wetting of PS and P4VP blocks at the oil/water interface. As expected, onion-like particles obtained when pH is slightly lower than pKa_(P4VP) (e.g., pH = 3.00), due to the interfacial affinity to the weakly hydrophilic P4VP block. Interestingly, when pH was further decreased to ∼2.5, interfacial instability of the emulsion droplets was observed, and each emulsion droplet generated nanoscale assemblies including vesicles, worm-like and/or spherical micelles rather than a nanostructured microparticle. Furthermore, homopolymer with different molecular weights and addition ratio are employed to adjust the interactions among copolymer blocks. By this means, particles with hierarchical structures can be obtained. Moreover, owing to the kinetically controlled processing, we found that temperature and stirring speed, which can significantly affect the kinetics of the evaporation of organic solvent and the formation of particles, played a key role in the morphology of the assemblies. We believe that manipulation of the property for the aqueous phase is a promising strategy to rationally design and fabricate polymeric assemblies with desirable shapes and internal structures.

Stimulus-responsive block copolymer nano-objects and hydrogels via dynamic covalent chemistry

Herein we demonstrate that dynamic covalent chemistry can be used to induce reversible morphological transitions in block copolymer nano-objects and hydrogels. Poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) (PGMA-PHPMA) diblock copolymer nano-objects (vesicles or worms) were prepared via polymerization-induced self-assembly. Addition of 4-carboxyphenylboronic acid (CPBA) leads to the formation of phenylboronate ester bonds with the 1,2-diol pendent groups on the hydrophilic PGMA stabilizer chains; such binding causes a subtle reduction in the packing parameter, which in turn induces either vesicle-to-worm or worm-to-sphere transitions. Moreover, CPBA binding is pH-dependent, so reversible transitions can be achieved by switching the solution pH, with relatively high copolymer concentrations leading to associated (de)gelation. This distinguishes these new physical hydrogels from the covalently cross-linked gels prepared using dynamic covalent chemistry reported in the literature.