Triblock terpolymer micelles: a personal outlook

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.

Isoporous Membranes from Novel Polystyrene-b-poly(4-vinylpyridine)-b-poly(solketal methacrylate) (PS-b-P4VP-b-PSMA) Triblock Terpolymers and Their Post-Modification

In this paper, the formation of nanostructured triblock terpolymer polystyrene-b-poly(4-vinylpyridine)-b-poly(solketal methacrylate) (PS-b-P4VP-b-PSMA), polystyrene-b-poly(4-vinylpyridine)-b-poly(glyceryl methacrylate) (PS-b-P4VP-b-PGMA) membranes via block copolymer self-assembly followed by non-solvent-induced phase separation (SNIPS) is demonstrated. An increase in the hydrophilicity was observed after treatment of non-charged isoporous membranes from PS-b-P4VP-b-PSMA, through acidic hydrolysis of the hydrophobic poly(solketal methacrylate) PSMA block into a hydrophilic poly(glyceryl methacrylate) PGMA block, which contains two neighbored hydroxyl (-OH) groups per repeating unit. For the first time, PS-b-P4VP-b-PSMA triblock terpolymers with varying compositions were successfully synthesized by sequential living anionic polymerization. Composite membranes of PS-b-P4VP-b-PSMA and PS-b-P4VP-b-PGMA triblock terpolymers with ordered hexagonally packed cylindrical pores were developed. The morphology of the membranes was studied with scanning electron microscopy (SEM) and atomic force microscopy (AFM). PS-b-P4VP-b-PSMA triblock terpolymer membranes were further treated with acid (1 M HCl) to get polystyrene-b-poly(4-vinylpyridine)-b-poly(glyceryl methacrylate) (PS-b-P4VP-b-PGMA). Notably, the pristine porous membrane structure could be maintained even after acidic hydrolysis. It was found that membranes containing hydroxyl groups (PS-b-P4VP-b-PGMA) show a stable and higher water permeance than membranes without hydroxyl groups (PS-b-P4VP-b-PSMA), what is due to the increase in hydrophilicity. The membrane properties were analyzed further by contact angle, protein retention, and adsorption measurements.

Self-assembly of model short triblock amphiphiles in dilute solution

In this work, a molecular theory is used to study the self-assembly of short diblock and triblock amphiphiles, with head-tail and head-linker-tail structures, respectively. The theory was used to systematically explore the effects of the molecular architecture and the affinity of the solvent for the linker and tail blocks on the relative stability of the different nanostructures formed by the amphiphiles in dilute solution, which include spherical micelles, cylindrical fibers and planar lamellas. Moreover, the theory predicts that each of these nanostructures can adopt two different types of internal organization: (i) normal nanostructures with a core composed of tail segments and a corona composed of head segments, and (ii) nanostructures with a core formed by linker segments and a corona formed by tail and head segments. The theory predicts the occurrence of a transition from micelle to fiber to lamella when increasing the length of the tail or the linker blocks, which is in qualitative agreement with the geometric packing theory and with experiments in the literature. The theory also predicts a transition from micelle to fiber to lamella as the affinity of the solvent for the tail or linker block is decreased. This result is also in qualitative agreement with experiments in the literature but cannot be explained in terms of the geometric packing theory. The molecular theory provides an explanation for this result in terms of the competition between solvophobic attractions among segments in the core and steric repulsions between segments in the corona for the different types of self-assembled nanostructures.

Compartmentalized nanoparticles in aqueous solution through hierarchical self-assembly of triblock glycopolymers

Amphiphilic block copolymers can elegantly assemble in water to form well-defined nano-objects and through smart design of the polymers it is possible to efficiently prepare functional materials for biomedical applications such as drug carriers.

Recent advances in amphiphilic polymers for simultaneous delivery of hydrophobic and hydrophilic drugs

Nanomedicine has evolved with the use of biological compounds such as proteins, peptides and DNA. These hydrophilic and often highly charged compounds require a delivery system to allow effective transport and release at the site of action. These new biological therapeutics have not replaced the more traditional smaller molecule, but instead are working synergistically to the benefit of the end user. To that end, drug delivery systems are now required to encapsulate both larger hydrophilic compounds as well as the smaller and generally more hydrophobic compound. This review highlights the emerging role in drug delivery of amphiphilic polymers that by their very nature can associate with compounds of differing physicochemical properties, in particular the role of micelles, polymersomes and nanocapsules.

Complex Formation Between Lysozyme and Stabilized Micelles with a Mixed Poly(ethylene oxide)/Poly(acrylic acid) Shell

The electrostatic complexation between lysozyme and stabilized polymeric micelles (SPMs) with a poly(acrylic acid) (PAA) or a mixed poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) shell (SPMs with a mixed shell, SPMMS) and a temperature-responsive poly(propylene oxide) (PPO) core was investigated by means of dynamic, static, and electrophoretic light scattering. The SPMs and different types of SPMMS used resulted from the self-assembly of PAA-PPO-PAA triblock copolymer chains, or PAA-PPO-PAA and PEO-PPO-PEO triblock copolymer chain mixtures (with varying chain lengths and molar ratios) in aqueous solutions at pH 10 and the subsequent cross-linking of their PPO cores via loading and photo-cross-linking of pentaerythritol tetraacrylate (PETA). The solution behavior, structure and properties of the formed complexes at pH 7 and 0.01 M ionic strength, were studied as a function of the protein concentration in the solution (the concentration of the stabilized micelles was kept constant) or equivalently the ratio of the two components. The complexation process and properties of the complexes proved to be dependent on the protein concentration, while of particular interest was the effect of the structure of the shell of the SPMs on the stability/solubility of the complexes. Finally, the fluorescence and mid infrared spectroscopic investigation of the structure of the complexed protein showed that, although a small stretching of the protein molecules occurred in some cases, no protein denaturation takes place upon complexation.

Synthesis and Self-Assembly of Amphiphilic Triblock Terpolymers with Complex Macromolecular Architecture

Two star triblock terpolymers (PS-b-P2VP-b-PEO)₃ and one dendritic-like terpolymer [PS-b-P2VP-b-(PEO)₂]₃ of PS (polystyrene), P2VP (poly(2-vinylpyridine)), and PEO (poly(ethylene oxide)), never reported before, were synthesized by combining atom transfer radical and anionic polymerizations. The synthesis involves the transformation of the -Br groups of the previously reported Br-terminated 3-arm star diblock copolymers to one or two -OH groups, followed by anionic polymerization of ethylene oxide to afford the star or dendritic structure, respectively. The well-defined structure of the terpolymers was confirmed by static light scattering, size exclusion chromatography, and NMR spectroscopy. The self-assembly in solution and the morphology in bulk of the terpolymers, studied by dynamic light scattering and transmission electron microscopy, respectively, reveal new insights in the phase separation of these materials with complex macromolecular architecture.

Asymmetric block copolymers for supramolecular templating of inorganic nanospace materials

This review focuses on polymeric micelles consisting of asymmetric block copolymers as designed templates for several inorganic nanospace materials with a wide variety of compositions. The presence of chemically distinct domains of asymmetric triblock and diblock copolymers provide self-assemblies with more diverse morphological and functional features than those constructed by EOn POm EOn type symmetric triblock copolymers, thereby affording well-designed nanospace materials. This strategy can produce unprecedented nanospace materials, which are very difficult to prepare through other conventional organic templating approaches. Here, the recent development on the synthesis of inorganic nanospace materials are mainly focused on, such as hollow spheres, tubes, and porous oxides, using asymmetric triblock copolymers.

Preparation of cylindrical multi-compartment micelles by the hierarchical self-assembly of ABC triblock polymer in solution

Amphiphilic linear ABC triblock copolymer PnBA₂₈-b-PS₃₇-b-P2VP₇₃ was prepared by the RAFT method. Spherical patchy micelles and cylindrical MCMs were formed in different steps of its two-step hierarchical self-assembly in selected solvents.

Complex Micelles with Glucose-Responsive Shells for Self-Regulated Release of Glibenclamide

Complex micelles with a hydrophobic poly(ϵ-caprolactone) (PCL) core and a mixed P(Asp-co-AspPBA)/PEG shell were prepared through co-assembly of two block copolymers PCL-b- P(Asp-co-AspPBA) and PEG-b-PCL in basic aqueous solutions. The P(Asp-co-AspPBA) chains (Asp = aspartic acid; AspPBA = aspartamidophenylboronic acid) collapsed and formed a shell layer around the PCL core at neutral pH while the soluble PEG chains stabilised the micelles. The collapsed P(Asp-co-AspPBA) polymer becomes soluble under higher glucose concentration and collapses onto the PCL core reversibly at lower glucose concentration. Self-regulated release of glibenclamide from the complex micelles was achieved based on the reversible change of P(Asp-co-AspPBA) chain mobility in response to the change of glucose concentration. As a result, polymeric micelles with glucose-responsive on-off switches were successfully developed.

pH-responsive polymeric micelles with core-shell-corona architectures as intracellular anti-cancer drug carriers

Polymeric micelles with core-shell-corona nanoarchitecture were designed for intracellular therapeutic anti-cancer drug carriers. Poly(styrene-b-acrylic acid-b-ethylene glycol) (PS-b-PAA-b-PEG) asymmetric triblock copolymer underwent self-assembly in aqueous solution to form spherical micelles with hydrophobic PS core, anionic PAA shell and hydrophilic PEG corona. The anti-cancer drug (doxorubicin, DOX) was successfully incorporated into the polymeric micelles. The in vitro release experiment confirmed that the release of DOX from the micelles was inhibited at pH 7.4. In contrast, an accelerated release of DOX was observed at mildly acidic conditions such as pH 4.5. The excellent biocompatibility of our PS-b-PAA-b-PEG-based micelles made the synthesized nano-carrier best suited for the delivery of anti-cancer drugs.

Polymeric Surfactants: Novel Agents with Exceptional Properties

This article presents recent progress in the field of polymeric surfactants made of permanently amphiphilic block copolymers or of stimulus-sensitive ones. We highlight key points in the design of amphiphilic macromolecules, to yield polymer surfactants with tailor-made properties, as well as recently developed and still challenging application fields for this new class of surfactants. The efficiency boosting of amphiphilic block copolymers as co-surfactants in microemulsions is discussed, as are surface modification by polymer surfactants, and stabilization of dispersions. Moreover, the use of block copolymers in nanosciences is presented, for instance as a tool for nanomaterial fabrication, or for biomedical and cosmetic applications in bio-nanotechnology. Finally, self-assembly and applications of some newly developed “exotic” amphiphilic block copolymer structures as new surface-active materials will be highlighted.

Polymer micelles with hydrophobic core and ionic amphiphilic corona. 1. Statistical distribution of charged and nonpolar units in corona

Polymer micelles with hydrophobic polystyrene (PS) core and ionic amphiphilic corona from charged N-ethyl-4-vinylpyridinium bromide (EVP) and uncharged 4-vinylpyridine (4VP) units spontaneously self-assembled from PS-block-poly(4VP-stat-EVP) macromolecules in mixed dimethylformamide/methanol/water solvent. The fraction of statistically distributed EVP units in corona-forming block is β = [EVP]/([EVP]+[4VP]) = 0.3-1. Micelles were transferred into water via dialysis technique, and pH was adjusted to 9, where 4VP is insoluble. Structural characteristics of micelles were investigated both experimentally and theoretically as a function of corona composition β. Methods of dynamic and static light scattering, electrophoretic mobility measurements, sedimentation velocity, transmission electron microscopy, and UV spectrophotometry were applied. All micelles possessed spherical morphology. The aggregation number, structure, and electrophoretic mobility of micelles changed in a jumplike manner near β ~ 0.6-0.75. Below and above this region, micelle characteristics were constant or insignificantly changed upon β. Theoretical dependencies for micelle aggregation number, corona dimensions, and fraction of small counterions outside corona versus β were derived via minimization the micelle free energy, taking into account surface, volume, electrostatic, and elastic contributions of chain units and translational entropy of mobile counterions. Theoretical estimations also point onto a sharp structural transition at a certain corona composition. The abrupt reorganization of micelle structure at β ~ 0.6-0.75 entails dramatic changes in micelle dispersion stability in the presence of NaCl or in the presence of oppositely charged polymeric (sodium polymethacrylate) or amphiphilic (sodium dodecyl sulfate) complexing agents.

Polymer micelles with hydrophobic core and ionic amphiphilic corona. 2. Starlike distribution of charged and nonpolar blocks in corona

Mixed polymer micelles with hydrophobic polystyrene (PS) core and ionic amphiphilic poly(4-vinylpyridine)/poly(N-ethyl-4-vinylpyridinium bromide) corona (P4VP/PEVP) spontaneously self-assembled from mixtures of PS-b-PEVP and PS-b-P4VP macromolecules in dimethylformamide/methanol/water selective solvent. The fraction of PEVP units in corona was β = [PEVP]/([PEVP] + [P4VP]) = 0.05-1.0. Micelles were transferred into pure water via dialysis technique and pH was adjusted to 9, where P4VP blocks are insoluble. Structural characteristics of micelles as a function of corona composition β were investigated. Methods of dynamic and static light scattering, electrophoretic mobility measurements, sedimentation velocity, transmission electron microscopy, and UV spectrophotometry were applied. Spherical morphology with core (PS)-shell (P4VP)-corona (PEVP) organization was postulated. Micelles demonstrated a remarkable inflection in structural characteristics near β ~ 0.5-0.7. Above this region, aggregation number (m), core and corona radii of mixed micelles coincided with those of individual PS-b-PEVP micelles. When β decreased below 0.5, dramatic growth of aggregation number was observed, accompanied by growth in micelle size and stretching PEVP chains. At β below 0.2, dispersions of mixed micelles were unstable and easily precipitated upon addition of NaCl. Scaling relationships between micelle characteristics and β were obtained via minimization the micelle free energy, taking into account electrostatic, osmotic, volume, and surface contributions. Theoretical estimations predicted dramatic influence of β on aggregation number, m ~ β(-3). This result is in general agreement with experimental data and confirms the correctness of the core-shell-corona model. The inflection in micelle characteristics entails drastic changes in micelle dispersion stability in the presence of oppositely charged polymeric (sodium polymethacrylate) or amphiphilic (sodium dodecyl sulfate) complexing agents.

Highly symmetric patchy multicompartment nanoparticles from the self-assembly of ABC linear terpolymers in C-selective solvents

Multicompartment micelles, especially those with highly symmetric surfaces such as patchy-like, patchy, and Janus micelles, have tremendous potential as building blocks of hierarchical multifunctional nanomaterials. One of the most versatile and powerful methods to obtain patchy multicompartment micelles is by the solution-state self-assembly of linear triblock copolymers. In this article, we applied the simulated annealing method to study the self-assembly of ABC linear terpolymers in C-selective solvents. Simulations predict a variety of patchy and patchy-like multicompartment micelles with high symmetry and also yield a detailed phase diagram to reveal how to control the patchy multicompartment micelle morphologies precisely. The phase diagram demonstrates that the internal segregated micellar structure depends on the ratio between the volume fractions of the two solvophobic blocks and their incompatibility, whereas the overall micellar shape depends on the copolymer concentration. The relationship between the interfacial energy, stretching energy of chains and the micellar morphology, micellar morphological transition are elucidated by computing the average contact number among the species, the mean square end-to-end distances of the whole terpolymers, the AB blocks in the terpolymers, the AB diblock copolymers, and angle distribution of terpolymers. The anchoring effect of the solvophilic C block on micellar structures is also examined by comparing the morphologies formed from ABC terpolymers and AB diblock copolymers.

Triggered disassembly of hierarchically assembled onion-like micelles into the pristine core-shell micelles via a small change in pH

The size and surface property of nanomaterial-based delivery systems administered intravenously play important roles in their cell uptake and in vivo distribution. Both of them should be capable of self-evolution in order to achieve efficient targeting performance. A facile strategy was proposed to manipulate both the size and surface property of polymeric micelles. It was found that the hierarchical assembly between trimethylated chitosan-g-poly(ε-caprolactone) (TMC-PCL) micelles and carboxyethyl chitosan-g-poly(ethylene glycol) (CEC-PEG) could produce onion-like micelles with enlarged size and PEGylated surface. The onion-like micelles could withstand the ionic strength of plasma and competitive exchange with BSA, and abruptly disassemble into the pristine TMC-PCL micelles via a small change in pH. By varying the degree of carboxyethylation, the disassembly pH could be modulated to the range of the tumoral microclimate pH. In contrast with TMC-PCL micelles, which displayed high cytotoxicity and endocytic ability towards C6 glioma cells, the onion-like micelles were cell-friendly and internalized by the cells at a very low level. Doxorubicin was used as a model chemotherapeutic agent and incorporated within TMC-PCL micelles. Dox release from both TMC-PCL micelles and the onion-like micelles was very slow under normal physiological conditions and displayed excellent pH sensitivity. Cell viability of Dox-loaded micelles was also investigated.