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

One-Pot Synthesis of Cross-linked Block Copolymer Nanowires via Polymerization-Induced Hierarchical Self-Assembly and Photodimerization

Block copolymer (BCP) nanowires are crucially important in diversified fields. However, their applications typically suffer from a tedious fabrication process, heterogeneous morphology, and degradation. Herein, we propose a facile and robust approach to synthesize stilbene-containing BCP nanowires in homogeneous morphology with high stability at high solid content (5%-20% w/w) in one pot. It is realized by the polymerization-induced hierarchical self-assembly and subsequent photodimerization cross-linking under UV irradiation of stilbene mesogens in the core-forming block in the later stage of polymerization. Because of the strong liquid crystalline ordering and photodimerization of stilbene, the cross-linked nanowires can be obtained over a broad BCP composition, which show robust morphological stability when exposed to solvent dissolution tests and water dispersion stability tests. This efficient approach to stabilized nanowires with homogeneous morphology via in situ cross-linking would be useful in various fields, such as nanomedicine and Pickering emulsifiers.

CO2-Responsive Nano-Objects with Assembly-Related Aggregation-Induced Emission and Tunable Morphologies

CO₂-responsive polymeric nano-objects with assembly-related aggregation-induced emission (AIE) are obtained via polymerization-induced self-assembly (PISA) of 2-(dimethylamino)ethyl methacrylate (DMAEMA), 2-(4-formylphenoxy)ethyl methacrylate (MAEBA), and 4-(1,2,2-triphenylvinyl)phenyl methacrylate (TPEMA). These nano-objects exhibit, depending on the feed of MAEBA, a morphology evolution process from spherical micelles to vesicles. Due to the presence of DMAEMA units, CO₂ promotes morphology transformation of the nano-objects from spheres to a mixture of "jellyfish" and vesicles and vesicles to complex vesicles. Moreover, TPEMA endows the AIE feature to these nano-objects, offering a strategy to monitor the morphology evolution process in real time. Thus, this approach is significant for exploring the assembly mechanism of copolymer in polymerization-induced self-assembly and designing multistimuli-responsive polymeric nanomaterials with tunable morphologies and sizes.

A polymerization-induced self-assembly process for all-styrenic nano-objects using the living anionic polymerization mechanism

By combination of the living anionic polymerization (LAP) mechanism with the polymerization-induced self-assembly (PISA) technique, the all-styrenic diblock copolymer poly(p-tert-butylstyrene)-b-polystyrene (PtBS-b-PS) based LAP PISA was successfully developed.

Synthesis of nano-capsules via aqueous emulsion RCMP-PISA and encapsulation

Synthesis of nano-capsules using aqueous RCMP-PISA and encapsulation of rhodamine-B (Rh-B).

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.

Scalable preparation of crystalline nanorods through sequential polymerization-induced and crystallization-driven self-assembly of alternating copolymers

We report a two-step sequential polymerization-induced and crystallization-driven self-assembly (sequential PI/CDSA) of alternating copolymers to prepare micron-length crystalline nanorods with an ultrathin lamellar structure on a large scale.

Poly(ethylene glycol)-b-poly(vinyl acetate) block copolymer particles with various morphologies via RAFT/MADIX aqueous emulsion PISA

The polymerization-induced self-assembly (PISA) of amphiphilic diblock copolymers of poly(ethylene glycol)-b-poly(vinyl acetate) in water was achieved through macromolecular design via interchange of xanthate (MADIX) polymerization in emulsion.

Polymerization techniques in polymerization-induced self-assembly (PISA)

The development of controlled/“living” polymerization greatly stimulated the prosperity of the fabrication and application of block copolymer nano-objects.

Trending methods employed for polymerization induced self-assembly

Mother Nature produces a perfectly defined architecture that inspires researchers to make polymeric macromolecules for an array of functions. The present article describes recent development in the PISA to synthesize polymeric nano-objects.

Uncontrolled polymerization that occurred during photoinitiated RAFT dispersion polymerization of acrylic monomers promotes the formation of uniform raspberry-like polymer particles

Uniform raspberry-like polymer particles are prepared by a different type of photoinitiated RAFT dispersion polymerization.

Synthesis and direct assembly of linear–dendritic copolymers via CuAAC click polymerization-induced self-assembly (CPISA)

A one-pot method was developed for in situ preparation of linear–dendritic copolymer assemblies via click polymerization-induced self-assembly (CPISA).

Star amphiphilic block copolymers: synthesis via polymerization-induced self-assembly and crosslinking within nanoparticles, and solution and interfacial properties

The star amphiphilic block copolymer of star s-PNIPAM-b-PS is synthesized and it shows characteristics significantly different from those of the linear block copolymer counterpart.

100th Anniversary of Macromolecular Science Viewpoint: Heterogenous Reversible Deactivation Radical Polymerization at Room Temperature. Recent Advances and Future Opportunities

Heterogenous reversible deactivation radical polymerization (RDRP) has become an important method for the preparation of a diverse set of well-defined polymer materials in dispersed systems. Conducting heterogeneous RDRP at room temperature seems to be a minor adjustment in polymerization technique but this will lead to a great opportunity for functional polymer synthesis, developing of interesting heterogeneous RDRP systems, and better mechanistic insights into heterogeneous RDRP. In this Viewpoint, we highlight some recent advances of room-temperature heterogeneous RDRP that are challenging to achieve via traditional thermally initiated heterogeneous RDRP. We hope that this Viewpoint can provide some inspiration for both experts in this field and new comers, as well as nonexperts who are interested in preparing their own polymer materials by conducting room-temperature heterogeneous RDRP.

Diblock Copolymer Core-Shell Nanoparticles as Template for Mesoporous Carbons: Independent Tuning of Pore Size and Pore Wall Thickness

Latex templating using core-shell particles represents a unique opportunity to design mesoporous carbons with a high level of control on textural properties. This new class of organic colloid templates is synthesized by polymerization-induced self-assembly (PISA) in which a solvophilic poly(hydroxyethyl acrylate) (PHEA) homopolymer is chain extended with a solvophobic polystyrene (PS) via a photomediated reversible-addition-fragmentation-transfer (RAFT) polymerization. The resultant PHEA-b-PS diblock copolymer nanoparticles exhibit a PS core stabilized by a PHEA shell, with two blocks characterized by a low molecular weight dispersity (1.1-1.3) and an adjustable degree of polymerization (DP). The core-shell structured nanoparticles are used as soft template for the formation of mesostructured carbons from phloroglucinol and glyoxylic acid in methanol solution. A micro- and mesostructured cellular foam is obtained having uniform, interconnected, and narrowly distributed mesopores ranging between 15 and 30 nm in diameter, a specific surface area up to 719 m² g^-1, and a total pore volume of (0.4-1.3) cm³ g^-1. The mesopore size can be controlled by adjusting the diameter of the PS core (16-29 nm), while the wall thickness can be tailored independently by varying the size of the solvated PHEA shell (5-25 nm). An increase of PHEA block's DP from 25 to 85 gradually extends the stabilizing shell dimension, thus increasing the wall thickness up to 10 nm, and causing the shift from interconnected to isolated mesopores. By comparison, much thinner walls (2-3 nm) are obtained with conventional latex templates such as polystyrene nanoparticles or colloidal silica. Decreasing PHEA DP to 17 induces the formation of copolymer vesicles that can be used as template to create mesoporous carbons with nonspherical mesopores.

Probing the mechanism for hydrogel-based stasis induction in human pluripotent stem cells: is the chemical functionality of the hydrogel important?

It is well-known that pluripotent human embryonic stem cells (hPSC) can differentiate into any cell type. Recently, we reported that hPSC colonies enter stasis when immersed in an extremely soft hydrogel comprising hydroxyl-functional block copolymer worms (I. Canton, N. J. Warren, A. Chahal, K. Amps, A. Wood, R. Weightman, E. Wang, H. Moore and S. P. Armes, ACS Centr. Sci., 2016, 2, 65-74). The gel modulus and chemical structure of this synthetic hydrogel are similar to that of natural mucins, which are implicated in the mechanism of diapause for mammalian embryos. Does stasis induction occur merely because of the very soft nature of such hydrogels or does chemical functionality also play a role? Herein, we address this key question by designing a new hydrogel of comparable softness in which the PGMA stabilizer chains are replaced with non-hydroxylated poly(ethylene glycol) [PEG]. Immunolabeling studies confirm that hPSC colonies immersed in such PEG-based hydrogels do not enter stasis but instead proliferate (and differentiate if no adhesion substrate is present). However, pluripotency is retained if an appropriate adhesion substrate is provided. Thus, the chemical functionality of the hydrogel clearly plays a decisive role in the stasis induction mechanism.

Efficient occlusion of oil droplets within calcite crystals

It is well known that oil and water do not mix. Similarly, the incorporation of oil droplets within inorganic crystals is highly counter-intuitive because there is a large difference in surface energy for these two components. Nevertheless, herein we demonstrate the efficient occlusion of ∼250-500 nm oil droplets within 20-40 μm calcite crystals. These droplets are stabilized using various amphiphilic poly(methacrylic acid)-poly(n-alkyl methacrylate) diblock copolymer emulsifiers. Both copolymer concentration and diblock compositions affect the extent of occlusion, with optimized conditions producing calcite crystals containing up to 11% oil by mass. Moreover, compressive forces exerted by the growing crystals cause significant deformation of the oil droplets during occlusion. In principle, this protocol enables the incorporation of water-insoluble dyes or hydrophobic nanoparticles within calcite, which is a cheap, naturally-occurring and environmentally-benign mineral. The single crystal nature of this host lattice ensures efficient retention of such guests, while lowering the solution pH leads to triggered release via acid dissolution.

Ring-Opening Polymerization of N-Carboxyanhydride-Induced Self-Assembly for Fabricating Biodegradable Polymer Vesicles

Polymerization-induced self-assembly (PISA) is regarded as one of the most important strategies in macromolecular nanotechnology, as it can create a wide range of nanoparticles at high concentrations and on a large scale. However, open-to-air PISA with biodegradable product is still a complicated challenge, as traditional PISA is usually carried out under oxygen-free conditions to afford nonbiodegradable polymers. To meet the above challenges, we propose a convenient one-pot open-to-air ring-opening polymerization (ROP) of N-carboxyanhydride (NCA)-induced self-assembly (NCA-PISA) at 10 °C, without the need for degassing, heating, catalysts, or chain transfer agents. The morphologies of nanoparticles depend on the ratio of the initiator to the monomer and the solid content. Polymer vesicles can be fabricated when the ratio and the solid content are 1:20 and 20%, respectively. Overall, this versatile one-pot NCA-PISA provides an insight into facilely fabricating biodegradable nanoparticles in air.

Structural Difference in Macro-RAFT Agents Redirects Polymerization-Induced Self-Assembly

Polymerization-induced self-assembly (PISA) has served as a versatile platform for the large-scale preparation of well-defined block copolymer nano-objects. It appears to be "common sense" that block copolymers with narrow molecular weight distributions are inevitable. In this study, we have conducted the direct comparison of reversible addition-fragmentation transfer (RAFT)-mediated PISA of 2-hydroxypropyl methacrylate (HPMA) using polymethacrylate- and polyacrylate-based macro-RAFT agents. Taking advantage of the poor RAFT controllability of polyacrylate-based macro-RAFT agents with respect to HPMA, uniform submicron-sized polymeric microspheres were prepared by photoinitiated RAFT-mediated PISA of HPMA. The diameter of polymeric microspheres can be precisely controlled by further chain-extension of PHPMA. Finally, uniform epoxy-functionalized multicompartment block copolymer particles (MBCPs) were prepared by a two-step photoinitiated RAFT-mediated PISA with poly(glycidyl methacrylate) (PGlyMA) as the third block. The performance of MBCPs as Pickering emulsifiers was evaluated in detail. This study not only expands the scope of RAFT-mediated PISA for preparing well-defined polymer particles but also provides important insights into the mechanism of RAFT-mediated PISA.

Tuning the membrane permeability of polymersome nanoreactors developed by aqueous emulsion polymerization-induced self-assembly

Polymeric vesicles (or polymersomes) are hollow bilayer structures consisting of an inner aqueous compartment enclosed by a hydrophobic membrane. Vesicular constructs are ubiquitous in nature and perform a variety of functions by compartmentalizing molecules into disparate environments. For polymer chemists, the synthesis of vesicles can be readily accomplished using polymerization-induced self-assembly (PISA), whereby pure vesicle morphologies can be easily accessed by tuning initial reaction parameters. Research into polymersomes is motivated primarily by the fact that hydrophilic cargo such as drug molecules, DNA, or enzymes can be encapsulated and protected from the often harsh conditions of the surrounding environment. A key factor governing the capability of vesicles to retain and protect their cargo is the permeability of their hydrophobic membrane. Herein, we demonstrate that membrane permeability of enzyme-loaded epoxy-functionalized polymersomes synthesized by aqueous emulsion PISA can be modulated via epoxide ring-opening with various diamine cross-linkers and hydrophobic primary amines. In general, membrane cross-linking or amine conjugation resulted in increased polymersome membrane thickness. Membrane modification was also found to decrease permeability in all cases, as measured by enzymatically-catalysed oxidation of an externally administered substrate. Functionalization with hydrophobic amines resulted in the largest reduction in enzyme activity, suggesting significant blocking of substrate diffusion into the central aqueous compartment. This procedurally facile strategy yields meaningful insight into how the chemical structure of the membrane influences permeability and thus could be generally applied to the formulation of polymeric vesicles for therapeutic applications.

Enzyme-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization: Precision polymer synthesis via enzymatic catalysis

Enzyme-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization provides a sustainable strategy for efficient production of well-defined polymers under mild conditions. Horseradish peroxidase (HRP), a heme-containing metalloenzyme, catalyzes oxidation of acetylacetone (ACAC) by hydrogen peroxide (H₂O₂) to generate ACAC radicals, initiating polymerization of vinyl monomers. This HRP/H₂O₂/ACAC ternary initiating system is applied to RAFT polymerization of different types of vinyl monomers. Furthermore, to overcome the inherent limitation of necessity for oxygen-free conditions, another enzyme, glucose oxidase (GOx) or pyranose 2-oxidase (P2Ox), with excellent deoxygenation capability, is introduced to consume oxygen by catalyzing oxidation of glucose to generate H₂O₂. The generated H₂O₂ is directly supplied to HRP catalysis for radical generation. Both GOx-HRP and P2Ox-HRP cascade catalysis afford RAFT polymerization with oxygen tolerance. In this chapter, we mainly focus on detailed synthetic protocols of RAFT polymerizations initiated by HRP/H₂O₂/ACAC ternary initiating system and P2Ox-HRP cascade catalysis. The general characterization and analytical methods used in these enzyme-initiated RAFT polymerizations are also included.

Light-Triggered Reversible Slimming of Azobenzene-Containing Wormlike Nanoparticles Synthesized by Polymerization-Induced Self-Assembly for Nanofiltration Switches

Photoresponsive wormlike block copolymer nanoparticles (NPs) have potential applications in versatile fields, but their preparation suffers from narrow worm phase region and tedious approaches. In this work, azobenzene-containing wormlike NPs based on poly(methylacrylic acid)-b-poly(4-((4-butylphenyl)diazenyl)phenyl methacrylate) are prepared via polymerization-induced self-assembly at high solids concentration in ethanol. The pure wormlike NPs occupy a remarkably broad region in the morphological phase diagram because of the rigid nature of the core-forming block. These wormlike NPs expand resulting from trans-cis transformation upon UV irradiation, and slim near to the original state via visible light irradiation. The diameter and its variation amplitude of worms increase with the chain length of core-forming block. Moreover, a nanofiltration switch for rhodamine B is assembled to illustrate one of its potential applications by remote trigger using light.

Predicting Monomers for Use in Aqueous Ring-Opening Metathesis Polymerization-Induced Self-Assembly

Aqueous polymerization-induced self-assembly (PISA) is a well-established methodology enabling in situ synthesis of polymeric nanoparticles of controllable morphology. Notably, PISA via ring-opening metathesis polymerization (ROMPISA) is an emerging technology for block copolymer self-assembly, mainly due to its high versatility and robustness. However, a limited number of monomers suitable for core-forming blocks in aqueous ROMPISA have been reported to date. In this work, we identified seven monomers for use as either corona- or core-forming blocks during aqueous ROMPISA by in silico calculation of relative hydrophobicity for corresponding oligomeric models. The predicted monomers were validated experimentally by conducting ROMPISA using our previously reported two-step approach. In addition to predictive data, our computational model was exploited to identify trends between polymer hydrophobicity and the morphology of the self-assembled nano-objects they formed. We expect that this methodology will greatly expand the scope of aqueous ROMPISA, as monomers can be easily identified based on the structure-property relationships observed herein.

Scalable preparation of alternating block copolymer particles with inverse bicontinuous mesophases

Block copolymer particles with controlled morphologies are of great significance in nanomaterials and nanotechnology. However, ordered inverse morphologies are difficult to achieve due to complex mechanism and formation conditions. Here we report scalable preparation of amphiphilic alternating block copolymer particles with inverse bicontinuous mesophases via polymerization-induced self-assembly (PISA). Concentrated dispersion copolymerizations (up to 40% solid content) of styrene (St) and pentafluorostyrene (PFS) employing a short poly(N,N-dimethylacrylamide) (PDMA29) stabilizer block lead to the formation of well-defined, highly asymmetric PDMA29-b-P(St-alt-PFS)x block copolymers with precise compositions and various morphologies, from simple spheres to ordered inverse cubosome mesophases. The particle morphology is affected by the molecular weight, solid content, and nature of the cosolvents. The cubosome structure is confirmed by electron microscopies and small angle X-ray scattering spectroscopy. This scalable PISA approach offers facile access to ordered inverse mesophases, significantly expanding the PISA morphology scope and enabling its applicability to the materials science fields.

Photoinitiated Polymerization-Induced Self-Assembly via Visible Light-Induced RAFT-Mediated Emulsion Polymerization

Aqueous emulsion polymerization is one of the most commonly used techniques in industry for the production of polymer latexes. In this contribution, we present photoinitiated polymerization-induced self-assembly (photo-PISA) based on aqueous visible light-induced reversible addition-fragmentation chain transfer (RAFT)-mediated emulsion polymerization at room temperature. A wide range of morphologies including spheres, worms, and vesicles have been achieved at room temperature by modulating reaction parameters. Additionally, this method enables access to inorganic nanoparticles-loaded vesicles by adding inorganic nanoparticles at the beginning of the polymerization. Finally, an oxygen-tolerant RAFT-mediated emulsion polymerization has been developed, allowing the synthesis of polymer nano-objects at low volumes (e.g., in a 96-well plate). This study is expected to expand the scope of photo-PISA for the preparation of various block copolymer nano-objects in water at room temperature.

Alkyl α-Hydroxymethyl Acrylate Monomers for Aqueous Dispersion Polymerization-Induced Self-Assembly

Baylis-Hillman reaction was used to afford a series of four alkyl α-hydroxymethyl acrylates, methyl (MHMA), ethyl (EHMA), isopropyl (iPrHMA), and n-butyl (nBHMA) α-hydroxymethyl acrylate, with tunable water solubility. MHMA and EHMA with high water solubility were identified as suitable candidates for aqueous dispersion polymerization-induced self-assembly (PISA). PISA of EHMA and MHMA using poly(ethylene glycol) macromolecular chain transfer agents (PEG₄₅-CTA and PEG₁₁₃-CTA) was investigated under either thermal or photoinitiation at 40-70 °C. Photo-PISA at low temperatures provided both morphological transition and PEG₄₅-PEHMA_x block copolymers with narrow molecular weight distributions. iPrHMA with moderate water solubility was used for dispersion-emulsion polymerization with the formation of vesicles being observed.

Exploiting Wavelength Orthogonality for Successive Photoinduced Polymerization-Induced Self-Assembly and Photo-Crosslinking

We report a facile benchtop process for the synthesis of cross-linked polymeric nanoparticles by exploiting wavelength-selective photochemistry to perform orthogonal photoinduced polymerization-induced self-assembly (Photo-PISA) and photo-crosslinking processes. We first established that the water-soluble photocatalyst, zinc meso-tetra(N-methyl-4-pyridyl) porphine tetrachloride (ZnTMPyP) could activate the aqueous PET-RAFT dispersion polymerization of hydroxypropyl methacrylate (HPMA). This photo-PISA process could be conducted under low energy red light (λ_max = 595 nm, 10.2 mW/cm²) and without deoxygenation due to the action of the singlet oxygen quencher, biotin (vitamin B₇), which allowed for the synthesis of a range of nanoparticle morphologies (spheres, worms, and vesicles) directly in 96-well plates. To perform wavelength selective nanoparticle cross-linking, we added the photoresponsive monomer, 7-[4-(trifluoromethyl)coumarin] methacrylamide (TCMAm) as a comonomer without inhibiting the evolution of the nanoparticle morphology. Importantly, under red light, exclusive activation of the photo-PISA process occurs, with no evidence of TCMAm dimerization under these conditions. Subsequent switching to a UV source (λ_max = 365 nm, 10.2 mW/cm²) resulted in rapid cross-linking of the polymer chains, allowing for retention of the nanoparticle morphology in organic solvents. This facile synthesis of cross-linked spheres, worms, and vesicles demonstrates the utility of orthogonal light-mediated chemistry for performing decoupled wavelength selective chemical processes.