Amphiphilic Diblock Copolymers with a Moderately Hydrophobic Block: Toward Dynamic Micelles

Modern Trends in Polymerization-Induced Self-Assembly

Polymerization-induced self-assembly (PISA) is a powerful and versatile technique for producing colloidal dispersions of block copolymer particles with desired morphologies. Currently, PISA can be carried out in various media, over a wide range of temperatures, and using different mechanisms. This method enables the production of biodegradable objects and particles with various functionalities and stimuli sensitivity. Consequently, PISA offers a broad spectrum of potential commercial applications. The aim of this review is to provide an overview of the current state of rational synthesis of block copolymer particles with diverse morphologies using various PISA techniques and mechanisms. The discussion begins with an examination of the main thermodynamic, kinetic, and structural aspects of block copolymer micellization, followed by an exploration of the key principles of PISA in the formation of gradient and block copolymers. The review also delves into the main mechanisms of PISA implementation and the principles governing particle morphology. Finally, the potential future developments in PISA are considered.

pH-Dependent Solution Micellar Structure of Amphoteric Polypeptoid Block Copolymers with Positionally Controlled Ionizable Sites

While solution micellization of ionic block copolymers (BCP) with randomly distributed ionization sites along the hydrophilic segments has been extensively studied, the roles of positionally controlled ionization sites along the BCP chains in their micellization and resulting micellar structure remain comparatively less understood. Herein, three amphoteric polypeptoid block copolymers carrying two oppositely charged ionizable sites, with one fixed at the hydrophobic terminus and the other varyingly positioned along the hydrophilic segment, have been synthesized by sequential ring-opening polymerization method. The presence of the ionizable site at the hydrophobic segment terminus is expected to promote polymer association toward equilibrium micellar structures in an aqueous solution. The concurrent presence of oppositely charged ionizable sites on the polymer chains allows the polymer association to be electrostatically modulated in a broad pH range (ca. 2-12). Micellization of the amphoteric polypeptoid BCP in dilute aqueous solution and the resulting micellar structure at different solution pHs was investigated by a combination of scattering and microscopic methods. Negative-stain transmission-electron microscopy (TEM), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS) analyses revealed the dominant presence of core-shell-type spherical micelles and occasional rod-like micelles with liquid crystalline (LC) domains in the micellar core. The micellar structures (e.g., aggregation number, radius of gyration, chain packing in the micelle) were found to be dependent on the solution pH and the position of the ionizable site along the chain. This study has highlighted the potential of controlling the position of ionizable sites along the BCP polymer to modulate the electrostatic and LC interactions, thus tailoring the micellar structure at different solution pH values in water.

Protected Poly(3-sulfopropyl methacrylate) Copolymers: Synthesis, Stability, and Orthogonal Deprotection

Because of their permanent charge, strong polyelectrolytes remain challenging to characterize, in particular, when they are combined with hydrophobic features. For this reason, they are typically prepared through a postmodification of a fully hydrophobic precursor. Unfortunately, these routes often result in an incomplete functionalization or otherwise require harsh reaction conditions, thus limiting their applicability. To overcome these problems, in this work a strategy is presented that facilitates the preparation of well-defined strong polyanions by starting from protected 3-sulfopropyl methacrylate monomers. Depending on the chemistry of the protecting group, the hydrophobic precursor could be quantitatively converted into a strong polyanion under nucleophilic, acidic, or basic conditions. As a proof of concept, orthogonally protected diblock copolymers were synthesized, selectively deprotected, and allowed to self-assemble in aqueous solution. Further conversion into a fully water-soluble polyanion was achieved by deprotecting the second block as well.

Application of molecular docking in elaborating molecular mechanisms and interactions of supramolecular cyclodextrin

The cyclodextrin (CD)-based supramolecular nanomedicines have attracted growing interest because of their superior characteristics, including desirable biocompatibility, low toxicity, unique molecular structure and easy functionalization. The smart structures of CD impart host-guest interaction for meeting the multifunctional needs of disease therapy. However, it faces challenges in formulation design and inclusion mechanism clarification of the functional supramolecular assemblies owing to the complicated structures and mechanisms. Fortunately, molecular docking helps the researchers to comprehend the interaction between the drug and the target molecule for achieving high-through screening from the database. In this review, we summarized the category and characteristics of molecular docking along with the properties and applications of CD. Significantly, we highlighted the application of molecular docking in elaborating molecular mechanisms and simulating complex structures at molecular levels. The issues and development of CD and molecular docking were also presented to provide beneficial reference and new insights for supramolecular nano-systems.

ω-Unsaturated methacrylate macromonomers as reactive polymeric stabilizers in mini-emulsion polymerization

Polymer latexes of poly(benzyl methacrylate) P(BzMA) were synthesized by mini-emulsion polymerization, using hexadecane as the hydrophobe and ω-unsaturated methacrylate-based macromonomers as a reactive stabilizer. The amphiphilic macromonomers were synthesized by...

Amphiphilic Asymmetric Diblock Copolymer with pH-Responsive Fluorescent Properties

Stimuli-responsive polymers with changeable fluorescent properties have numerous applications in sensing, bioimaging, and detection. Here we describe the facile synthesis of a pH-responsive amphiphilic asymmetric diblock copolymer of acrylic acid and butyl acrylate that incorporates a polarity-sensitive fluorophore. The asymmetric structure enhances the stimuli-responsive behavior: as the environmental pH decreases, the fluorescent intensity of the asymmetric diblock copolymer gradually increases, whereas its symmetric block counterpart shows limited and stepwise change. Besides, this remarkable difference was demonstrated to be concentration-independent, as similar emission behavior was found for both polymers at lower concentrations. These results indicate that the fluorescence properties of the copolymer can be adjusted by rationally designing the copolymer structure. This work provides a novel and general strategy for the design and synthesis of polymeric materials with encapsulated structures showing stimuli-responsive fluorescent properties to be applied as fluorescent probes with a smoothly varying response curve rather than the simple on-off switch that is typical of block copolymer systems.

Cholesterol-Functionalized Linear/Brush Block Copolymers for Metal-Incorporated Nanostructures with Modulated Core Density and Enhanced Self-Assembly Efficiency

Metal-mediated self-assembly of chelating double-hydrophilic block copolymer has become a facile preparation strategy of great importance for the metal-chelated hybrid nanostructures. Herein, we present a delicate control over the morphology regulation of metal-chelated nanostructures by a terminal modification of polymer building blocks with mesogenic cholesterol. Such a molecular design motif at an end of chelating linear/brush-type block copolymer imparts not only additional hydrophobicity for enhanced cohesive force to facilitate the metal-mediated self-assembly, but also significant morphological alteration of a metal-chelated core that otherwise generally forms a spherical interior with cholesterol-free block copolymers. The presence of cholesterol entities localized at the central core further allows for the density modulation of the final Pt^II-chelated nanostructures while maintaining the colloidal stability, comparable to that of the cholesterol-free nanoparticles in physiological conditions. This metal-mediated assembly strategy with modified polymer building blocks can provide a potential platform for the delivery of inorganic agents.

Effect of Hydrophilic Monomer Distribution on Self-Assembly of a pH-Responsive Copolymer: Spheres, Worms and Vesicles from a Single Copolymer Composition

A series of copolymers containing 50 mol % acrylic acid (AA) and 50 mol % butyl acrylate (BA) but with differing composition profiles ranging from an AA-BA diblock copolymer to a linear gradient poly(AA-grad-BA) copolymer were synthesized and their pH-responsive self-assembly behavior was investigated. While assemblies of the AA-BA diblock copolymer were kinetically frozen, the gradient-like compositions underwent reversible changes in size and morphology in response to changes in pH. In particular, a diblock copolymer consisting of two random copolymer segments of equal length (16 mol % and 84 mol % AA content, respectively) formed spherical micelles at pH >5, a mix of spherical and wormlike micelles at pH 5 and vesicles at pH 4. These assemblies were characterized by dynamic light scattering, cryo-transmission electron microscopy and small angle neutron scattering.

Direct synthesis via RAFT of amphiphilic diblock polyelectrolytes facilitated by the use of a polymerizable ionic liquid as a monomer

A novel method to prepare amphiphilic block polyelectrolytes with a strongly hydrophobic block under homogeneous conditions is presented here.

Tuning Size and Morphology of mPEG-b-p(HPMA-Bz) Copolymer Self-Assemblies Using Microfluidics

The careful design of nanoparticles, in terms of size and morphology, is of great importance to developing effective drug delivery systems. The ability to precisely tailor nanoparticles in size and morphology during polymer self-assembly was therefore investigated. Four poly(ethylene glycol)-b-poly(N-2-benzoyloxypropyl methacrylamide) mPEG-b-p(HPMA-Bz) block copolymers with a fixed hydrophilic block of mPEG 5 kDa and a varying molecular weight of the hydrophobic p(HPMA-Bz) block (A: 17.1, B: 10.0, C: 5.2 and D: 2.7 kDa) were self-assembled into nanoparticles by nanoprecipitation under well-defined flow conditions, using microfluidics, at different concentrations. The nanoparticles from polymer A, increased in size from 55 to 90 nm using lower polymer concentrations and slower flow rates and even polymer vesicles were formed along with micelles. Similarly, nanoparticles from polymer D increased in size from 35 to 70 nm at slower flow rates and also formed vesicles along with micelles, regardless of the used concentration. Differently, polymers B and C mainly self-assembled into micelles at the different applied flow rates with negligible size difference. In conclusion, this study demonstrates that the self-assembly of mPEG-b-p(HPMA-Bz) block copolymers can be easily tailored in size and morphology using microfluidics and is therefore an attractive option for further scaled-up production activities.

Effect of the microstructure of n-butyl acrylate/N-isopropylacrylamide copolymers on their thermo-responsiveness, self-organization and gel properties in water

HYPOTHESIS

Polymer composition, microstructure, molar mass, architecture… critically affect the properties of thermoresponsive polymers in aqueous media.

EXPERIMENTS

The behaviour of n-isopropylacrylamide and n-butyl acrylate-based copolymers of variable composition and structure (statistical, diblock or triblock) was studied in solution at different temperatures and concentrations with turbidimetry measurements, differential scanning calorimetry, electronic microscopy, rheology and scattering experiments.

FINDINGS

This study illustrates how it is possible through chemical engineering of the microstructure of amphiphilic thermoresponsive polymers to modulate significantly the self-assembly, morphological and mechanical properties of these materials in aqueous media. Statistical structures induced a strong decrease of cloud point temperature compared to block structures with similar composition. Moreover, block structures lead below the transition temperature to the formation of colloidal structures. Above the transition temperature, the formation of colloidal aggregates is observed at low concentrations, and at higher concentrations the formation of gels. Neutron scattering and light scattering measurements show that for a given composition diblock structures lead to smaller colloids and mesoglobules than their triblock counterparts. Moreover, diblock structures, compared to triblock analogs, allow the formation of gels that do not demix with time (no synaeresis) but that are softer than triblock gels.

Gradient and asymmetric copolymers: the role of the copolymer composition profile in the ionization of weak polyelectrolytes

The ionization of weak polyelectrolytes can be altered by controlling the composition profile of the comonomers along the chain.

CO2 -Activated Reversible Transition between Polymersomes and Micelles with AIE Fluorescence

Fluorescent polymersomes with both aggregation-induced emission (AIE) and CO₂ -responsive properties were developed from amphiphilic block copolymer PEG-b-P(DEAEMA-co-TPEMA) in which the hydrophobic block was a copolymer made of tetraphenylethene functionalized methacrylate (TPEMA) and 2-(diethylamino)ethyl methacrylate (DEAEMA) with unspecified sequence arrangement. Four block copolymers with different DEAEMA/TPEMA and hydrophilic/hydrophobic ratios were synthesized, and bright AIE polymersomes were prepared by nanoprecipitation in THF/water and dioxane/water systems. Polymersomes of PEG₄₅ -b-P(DEAEMA₃₆ -co-TPEMA₆ ) were chosen to study the CO₂ -responsive property. Upon CO₂ bubbling vesicles transformed to small spherical micelles, and upon Ar bubbling micelles returned to vesicles with the presence of a few intermediate morphologies. These polymersomes might have promising applications as sensors, nanoreactors, or controlled release systems.

A mild and quantitative route towards well-defined strong anionic/hydrophobic diblock copolymers: synthesis and aqueous self-assembly

Well-defined hydrophobic/strong anionic diblock copolymers were synthesized through a protected hydrophobic intermediate. Their self-assembly in aqueous solution was subsequently studied.

Self-Assembly Behavior and pH-Stimuli-Responsive Property of POSS-Based Amphiphilic Block Copolymers in Solution

Stimuli-responsive polymeric systems containing special responsive moieties can undergo alteration of chemical structures and physical properties in response to external stimulus. We synthesized a hybrid amphiphilic block copolymer containing methoxy polyethylene glycol (MePEG), methacrylate isobutyl polyhedral oligomeric silsesquioxane (MAPOSS) and 2-(diisopropylamino)ethyl methacrylate (DPA) named MePEG-b-P(MAPOSS-co-DPA) via atom transfer radical polymerization (ATRP). Spherical micelles with a core-shell structure were obtained by a self-assembly process based on MePEG-b-P(MAPOSS-co-DPA), which showed a pH-responsive property. The influence of hydrophobic chain length on the self-assembly behavior was also studied. The pyrene release properties of micelles and their ability of antifouling were further studied.

Aqueous solution behaviour of novel water-soluble amphiphilic copolymers with elevated hydrophobic unit content

The synthesis and aqueous solution behaviour of water-soluble copolymers poly(OEGMA-stat-styrene)-b-PDMAPMA with a relatively high content of styrene units were explored.

Blending block copolymer micelles in solution; Obstacles of blending

Amphiphilic block copolymers can assemble into a variety of structures on the nanoscale in selective solvent. The micelle blending protocol offers a simple unique route to reproducibly produce polymer nanostructures. Here we expand this blending protocol to a range of polymer micelle systems and self-assembly routes. We found by exploring a range of variables that the systems must be able to reach global equilibrium at some point for the blending protocol to be successful. Our results demonstrate the kinetics requirements, specifically core block glass transition temperature, Tg, and length of the block limiting the exchange rates, for the blending protocol which can then be applied to a wide range of polymer systems to access this simple protocol for polymer self-assembly.

The self-assembly of copolymers with one hydrophobic and one polyelectrolyte block in aqueous media: a dissipative particle dynamics study

The reversible self-assembly of copolymers with one hydrophobic and one polyelectrolyte block.

Effect of Micellization on the Thermoresponsive Behavior of Polymeric Assemblies

The chain density of polymer micelles, dictated by their aggregation number (N_agg), is an often overlooked parameter that governs the macroscopic behavior of responsive assemblies. Using a combination of variable-temperature light scattering, turbidimetry, and microcalorimetry experiments, the cloud point and thermal collapse of micellar poly(N-isopropylacrylamide) (pNIPAM) corona chains at lower temperatures than the cloud point were found to be largely independent of the micelle's N_agg. By controlling the core composition, the degree of hysteresis associated with the thermal transition was found to increase as a function of core hydrophobicity. We performed this study on well-characterized micelles with tunable N_agg values, composed of a thermoresponsive corona (pNIPAM) and a nonresponsive core block poly(n-butyl acrylate-co-N,N-dimethylacrylamide) (p(nBA-co-DMA)), which were synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization. This allowed for a distinction to be made between thermoresponsive behavior at both the molecular and macroscopic level. The study of the subtle differences between these behaviors was made possible using a combination of complementary techniques. These results highlight the critical need for consideration of the effect that self-assembly plays on the responsive behavior of polymer chains when compared with free unimers in solution.

Tuning the aggregation behavior of pH-responsive micelles by copolymerization

A series of pH sensitive amphiphilic diblock copolymers where the associating block is a copolymer have been synthesized. Their pH sensitive behavior has been explored by a range of analytical techniques and the nanostructures formed are highly sensitive to both pH and the associating block composition.

Polymeric Janus nanoparticles from triblock terpolymer micellar dimers

Well-defined polymeric Janus nanoparticles have been synthesized by a novel method of combining self-assembly of simple ABC linear triblock terpolymers into nanostructured dimers and crosslinking of the conjunction between the opposite hemispheres.

Slow dynamics in transient polyelectrolyte hydrogels formed by self-assembly of block copolymers

Transient polyelectrolyte hydrogels were formed by self-assembly of triblock copolyelectrolytes with a central hydrophilic block, poly(acrylic acid) (PAA), and two hydrophobic end blocks, poly(n-butyl acrylate(50%)-stat-acrylic acid(50%)) [P(nBA(50%)-stat-AA(50%))]. The relaxation of the concentration fluctuations was investigated by dynamic light scattering as a function of the concentration, the pH, the temperature, and the ionic strength. A relatively fast mode was observed at all polymer concentrations caused by cooperative diffusion of the polymers. Above the critical percolation concentration a second slow relaxation mode was observed caused by a linear displacement of small heterogeneities in the network with constant velocity. The relative amplitude of the slow mode was determined by the strength of the electrostatic repulsion. The velocity of the displacement in the transient network is shown to be directly correlated to the terminal relaxation time of the shear modulus and has the same Arrhenius temperature dependence. Both the velocity of the displacement and the mechanical relaxation strongly slow down with decreasing degree of ionization below 0.7 and increasing ionic strength above 0.5 M. A ballistic relaxation process has been reported earlier for colloidal gels, and the present study shows that it can also occur in polymeric networks.

Kinetics of the sphere-to-rod like micelle transition in a pluronic triblock copolymer

The kinetics of the sphere-to-rod transition was studied in aqueous micelle solutions of triblock copolymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) pluronic P103 (PEO(17)PPO(60)PEO(17)). This transition was triggered by a temperature jump from the sphere phase to the rod phase and monitored with dynamic light scattering. The combination of the scattering intensity and the hydrodynamic radius were used to show that the micelles grow steadily as rods throughout the growth process. The transition was found to exhibit a single exponential behavior even in the case of large deviations from equilibrium. The linear increase in the decay rate with increasing copolymer concentration shows that the transition is dominated by a mechanism involving fusion and fragmentation of proper micelles. The decays of the sphere-to-rod transition were simulated for two pathways: random fusion fragmentation and successive addition of spherical micelles to rods. We show that micelle growth most likely occurs via random fusion-fragmentation. The second order rate constant for fusion and the fragmentation rate are calculated for the case of random fusion-fragmentation.