Solvent Responsive Micelles Based on Block and Gradient Copoly(2-oxazoline)s

Nanoscale polymer discs, toroids and platelets: a survey of their syntheses and potential applications

Polymer self-assembly has become a reliable and versatile workhorse to produce polymeric nanomaterials. With appropriate polymer design and monomer selection, polymers can assemble into shapes and morphologies beyond well-studied spherical and cylindrical micellar structures. Steadfast access to anisotropic polymer nanoparticles has meant that the fabrication and application of 2D soft matter has received increasing attention in recent years. In this review, we focus on nanoscale polymer discs, toroids, and platelets: three morphologies that are often interrelated and made from similar starting materials or common intermediates. For each morphology, we illustrate design rules, and group and discuss commonly used self-assembly strategies. We further highlight polymer compositions, fundamental principles and self-assembly conditions that enable precision in bottom-up fabrication strategies. Finally, we summarise potential applications of such nanomaterials, especially in the context of biomedical research and template chemistry and elaborate on future endeavours in this space.

Multiblock Poly-ε-Caprolactones: One-Step Synthesis toward Programmable Properties

Control of monomer sequence enables predictable structure-property relationships in versatile polymeric materials. The facile synthesis of multiblock copolymers (MBCPs) with controlled chain structure is highly challenging, particularly for those prepared via one-pot copolymerization of mixed monomers. Herein, poly-ε-caprolactone MBCPs, a series of thermoplastic elastomers with tailored thermal, mechanical, rheological, and degradable properties, are synthesized by Janus polymerization. Melting temperature, tensile strength, ductility, viscosity, and enzymatic degradability are governed by block length which is in turn dictated by the monomer-to-catalyst feed ratio. The relationships between the physicochemical properties and the architectures are investigated in detail.

Real-time 31P NMR reveals different gradient strengths in polyphosphoester copolymers as potential MRI-traceable nanomaterials

Polyphosphoesters (PPEs) are used in tissue engineering and drug delivery, as polyelectrolytes, and flame-retardants. Mostly polyphosphates have been investigated but copolymers involving different PPE subclasses have been rarely explored and the reactivity ratios of different cyclic phospholanes have not been reported. We synthesized binary and ternary PPE copolymers using cyclic comonomers, including side-chain phosphonates, phosphates, thiophosphate, and in-chain phosphonates, through organocatalyzed ring-opening copolymerization. Reactivity ratios were determined for all cases, including ternary PPE copolymers, using different nonterminal models. By combining different comonomers and organocatalysts, we created gradient copolymers with adjustable amphiphilicity and microstructure. Reactivity ratios ranging from 0.02 to 44 were observed for different comonomer sets. Statistical ring-opening copolymerization enabled the synthesis of amphiphilic gradient copolymers in a one-pot procedure, exhibiting tunable interfacial and magnetic resonance imaging (MRI) properties. These copolymers self-assembled in aqueous solutions, 31 P MRI imaging confirmed their potential as MRI-traceable nanostructures. This systematic study expands the possibilities of PPE-copolymers for drug delivery and theranostics.

Gradient copolymers versus block copolymers: self-assembly in solution and surface adsorption

The structures of amphiphilic block and gradient copolymers in solution and adsorbed onto surfaces are surveyed using molecular-dynamics simulations. A bead-spring model is used to identify the general effects of the different architectures: block and gradient copolymers have equal numbers of solvophilic and solvophobic beads, and the gradient copolymer is represented by a linear concentration profile along the chain. Each type of isolated copolymer forms a structure with a globular head of solvophobic beads, and a coil-like tail of solvophilic beads. The radius of gyration of a gradient copolymer is found to be much more sensitive to temperature than that of a block copolymer due to an unravelling mechanism. At finite concentrations, both gradient and block copolymers self-assemble into micelles, with the gradient copolymers again showing a larger temperature dependence. The micelles are characterised using simulated scattering profiles, which compare favourably to existing experimental data. The adsorption of copolymers onto structureless surfaces is modelled with an attractive potential that is selective for the solvophobic beads, and the surface structures are characterised using the average height of the molecules, and the proportion of beads adsorbed. Both types of copolymer form adsorbed films with persistent micelle-like structures, but the gradient copolymers show a stronger dependence on the strength of the surface interactions and the temperature. Coarse-grained, bead-spring models allow a rapid survey and comparison of the block and gradient architectures, and the results set the scene for future work with atomistic simulations. A superficial but favourable comparison is made between the results from the bead-spring models, and atomistic simulations of a butyl prop-2-enoate/prop-2-enoic acid (butyl acrylate/acrylic acid) copolymer in n-dodecane at room temperature.

Unexpected toroidal micelles formed from St/MMA gradient copolymers

Toroidal micelles are of great interest and rarely observed in gradient copolymer systems. Herein, we report massive toroidal micelles formed from styrene (St)/methyl methacrylate (MMA) gradient copolymers using a common solvent mixing method followed by a cooling-heating procedure. Furthermore, we demonstrate that the obtained toroidal morphology is sensitively dependent on a heat treatment procedure. Solely spherical micelles are obtained by a common solvent mixing method. These spherical micelles could be transformed into toroidal micelles via vesicles during a cooling-heating process. When a reverse heating-cooling process is adopted, no toroidal micelles formed. Thus, these results add new members to the family of toroidal micelles and reveal pathway dominating morphologies in gradient copolymer micelles.

A unified kinetic Monte Carlo approach to evaluate (a)symmetric block and gradient copolymers with linear and branched chains illustrated for poly(2-oxazoline)s

The synthesis of well-defined gradient, block-gradient and di-block copolymers with both asymmetric and symmetric compositions considering hydrophilic and hydrophobic monomer units is relevant for application fields, such as drug/gene delivery...

Micellization of Sequence-Controlled Polyurethane Ionomers in Mixed Aqueous Solvents

While the impact of compositional parameters such as block length and ionic content on the micellization of (polymeric) amphiphiles is widely investigated, the influence of monomer sequence has received far less attention until recently. Here, we report the synthesis of two sequence-controlled polyurethane ionomers (PUIs) prepared via a stepwise coupling-deprotection strategy, and compare their solution association in aqueous-organic mixtures. The two PUIs are highly similar in mass and overall composition, yet differ markedly in the sequence of building blocks. PUI-A2 comprises a polytetrahydrofuran (pTHF) block connected to an alternation of isophorone diamine (IPDA) and dimethylolpropionic acid (DMPA) units that together are also arranged in a blockwise manner. The result is a macromolecular structure with a comparatively hydrophobic tail (pTHF) and a hydrophilic headgroup, which structure is reminiscent of those of traditional surfactants, albeit much larger in size. PUI-S2 instead resembles a bolaamphiphilic architecture with a pTHF midblock connected on either end to a singly charged segment comprising DMPA and IPDA. We detect micellization below a threshold cosolvent volume fraction (φsolv) of 0.4 in aqueous-organic mixtures with tetrahydrofuran (THF), ethanol, and isopropyl alcohol. We use scattering tools to compare the aggregation number (N agg) and hydrodynamic radius (R h) of PUI-S2 and PUI-A2 micelles. Irrespective of the solvent composition, we observe in the micellar window of φsolv < 0.4, lower N agg for PUI-S2 micelles compared to PUI-A2, which we attribute to packing restraints associated with its bolaamphiphilic architecture. The increase in micellar size with increasing φsolv is much more pronounced for PUI-S2 than for PUI-A2. The micellar mass decreases with increasing φsolv for both PUIs; the effect is modest for PUI-S2 compared to PUI-A2 and is not observed in the most apolar cosolvent studied (THF). Upon the approach of the micellization boundary φsolv ≈ 0.4, both types of PUI micelles become less compact in structure, as (in most cases) PUIs are released and as micellar dimensions increase.

Poly(2-oxazoline)- and Poly(2-oxazine)-Based Self-Assemblies, Polyplexes, and Drug Nanoformulations-An Update

For many decades, poly(2-oxazoline)s and poly(2-oxazine)s, two closely related families of polymers, have led the life of a rather obscure research topic with only a few research groups world-wide working with them. This has changed in the last five to ten years, presumably triggered significantly by very promising clinical trials of the first poly(2-oxazoline)-based drug conjugate. The huge chemical and structural toolbox poly(2-oxazoline)s and poly(2-oxazine)s has been extended very significantly in the last few years, but their potential still remains largely untapped. Here, specifically, the developments in macromolecular self-assemblies and non-covalent drug delivery systems such as polyplexes and drug nanoformulations based on poly(2-oxazoline)s and poly(2-oxazine)s are reviewed. This highly dynamic field benefits particularly from the extensive synthetic toolbox poly(2-oxazoline)s and poly(2-oxazine)s offer and also may have the largest potential for a further development. It is expected that the research dynamics will remain high in the next few years, particularly as more about the safety and therapeutic potential of poly(2-oxazoline)s and poly(2-oxazine)s is learned.

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.

Gradient copolymer micelles: an introduction to structures as well as structural transitions

Exhibiting variation of the composition along a chain, gradient copolymers bring new blood to the old story of polymeric micelles. The gradient chain structure results in some special features in micellar structures and leads to unique structural transitions, potentially leading to new properties and applications. Henceforth, gradient copolymer micellar structures and their transitions from the viewpoint of soft matter physics will be reviewed. Concepts such as a diffuse interface, shrinkage-stretching of micelles, and intrinsic temperature responsiveness are summarized from current research, which highlight new characteristic structures, relaxation modes and novel properties of micelles, respectively.

Impact of charge switching stimuli on supramolecular perylene monoimide assemblies

The development of stimuli-responsive amphiphilic supramolecular nanostructures is an attractive target for systems based on light-absorbing chromophores that can function as photosensitizers in water. We report here on a water soluble supramolecular carboxylated perylene monoimide system in which charge can be switched significantly by a change in pH. This was accomplished by substituting the perylene core with an ionizable hydroxyl group. In acidic environments, crystalline supramolecular nanoribbons with dimensions on the order of 500 × 50 × 2 nm form readily, while in basic solution the additional electrostatic repulsion of the ionized hydroxyl reduces assemblies to very small dimensions on the order of only several nanometers. The HOMO/LUMO levels were also found to be sensitive to pH; in acidic media the HOMO/LUMO levels are -5.65 and -3.70 eV respectively versus vacuum, whereas is in basic conditions they are -4.90 and -3.33 eV, respectively. Utilizing the assemblies as photosensitizers in photocatalytic production of hydrogen with [Mo3S13]2- as a catalyst at a pH of 4, H2 was generated with a turnover number of 125 after 18 hours. Charge switching the assemblies at a pH of 9-10 and using an iron porphyrin catalyst, protons could again be reduced to hydrogen and CO2 was reduced to CO with a turnover number of 30. The system investigated offers an example of dynamic photosensitizing assemblies that can drive reactions in both acidic and basic media.

Solvent-control over monomer distribution in the copolymerization of 2-oxazolines and the effect of a gradient structure on self-assembly

The effect of a polymerization solvent on the monomer distribution in gradient copolymers is demonstrated and the effect of the monomer gradient on the copolymer self-assembly behavior is shown.

Unravelling the Excellent Chemical Stability and Bioavailability of Solvent Responsive Curcumin-Loaded 2-Ethyl-2-oxazoline-grad-2-(4-dodecyloxyphenyl)-2-oxazoline Copolymer Nanoparticles for Drug Delivery

A new gradient copolymer has been synthesized by the living cationic ring-opening polymerization of hydrophilic 2-ethyl-2-oxazoline with lipophilic 2-(4-dodecyloxyphenyl)-2-oxazoline (EtOx-grad-DPOx). The prepared copolymer is capable of assembling in water to yield polymeric nanoparticles that are successfully loaded with an anticancer agent, curcumin. Self-assembly of the copolymer was found to be tuned by the polarity as well as the hydrogen bonding ability of solvents. Solvent took distinctive role in the preparation of unloaded and curcumin-loaded nanoparticles. The stability of the nanoparticles was increased by curcumin loading promoted by curcumin-polymer interactions. Further, the chemical stability of curcumin in water is largely enhanced inside the polymeric nanoparticles. Curcumin-loaded (EtOx-grad-DPOx) copolymer nanoparticles showed excellent stability in the biological medium, low cytotoxicity, and concentration dependent uptake by U87 MG and HeLa cells, which indicate the possibility of their efficient application in drug delivery.

Gradient helical copolymers: synthesis, chiroptical properties, thermotropic liquid crystallinity, and self-assembly in selective organic solvents

A series of novel gradient copolymers R-(−)-poly(StN-grad-C8) were synthesized through atom transfer radical copolymerization of an achiral styrenic monomer, N,N-dimethyl-4-ethenylbenzamide (M-StN), and a chiral bulky vinylterphenyl monomer, (−)-2,5-bis{4′-[(R)-sec-octyloxycarbonyl]phenyl}styrene (R-(−)-M-C8).

Block and Gradient Copoly(2-oxazoline) Micelles: Strikingly Different on the Inside

Herein, we provide a direct proof for differences in the micellar structure of amphiphilic diblock and gradient copolymers, thereby unambiguously demonstrating the influence of monomer distribution along the polymer chains on the micellization behavior. The internal structure of amphiphilic block and gradient co poly(2-oxazolines) based on the hydrophilic poly(2-methyl-2-oxazoline) (PMeOx) and the hydrophobic poly(2-phenyl-2-oxazoline) (PPhOx) was studied in water and water-ethanol mixtures by small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), static and dynamic light scattering (SLS/DLS), and ¹H NMR spectroscopy. Contrast matching SANS experiments revealed that block copolymers form micelles with a uniform density profile of the core. In contrast to popular assumption, the outer part of the core of the gradient copolymer micelles has a distinctly higher density than the middle of the core. We attribute the latter finding to back-folding of chains resulting from hydrophilic-hydrophobic interactions, leading to a new type of micelles that we refer to as micelles with a "bitterball-core" structure.

Thermodynamics of the multi-stage self-assembly of pH-sensitive gradient copolymers in aqueous solutions

The self-assembly thermodynamics of pH-sensitive di-block and tri-block gradient copolymers of acrylic acid and styrene was studied for the first time using isothermal titration calorimetry (ITC) and dynamic light scattering (DLS) performed at varying pH. We were able to monitor each step of micellization as a function of decreasing pH. The growth of micelles is a multi-stage process that is pH dependent with several exothermic and endothermic components. The first step of protonation of the acrylic acid monomer units was accompanied mainly by conformational changes and the beginning of self-assembly. In the second stage of self-assembly, the micelles become larger and the number of micelles becomes smaller. While solution acidity increases, the isothermal calorimetry data show a broad deep minimum corresponding to an exothermic process attributed to an increase in the size of hydrophobic domains and an increase in the structure's hydrophobicity. The minor change in heat capacity (ΔCp) confirms the structural changes during this exothermic process. The exothermic process terminates deionization of acrylic acid. The pH-dependence of the ζ-potential of the block gradient copolymer micelles exhibits a plateau in the regime corresponding to the pH-controlled variation of the micellar dimensions. The onset of micelle formation and the solubility of the gradient copolymers were found to be dependent on the length of the gradient block.

Block copolymers based on 2-methyl- and 2-phenyl-oxazoline by metallocene-mediated cationic ring-opening polymerization: synthesis and characterization

The cationic polymerization of oxazolines, lactones and vinyl ethers can be efficiently promoted by metallocene complexes activated by floroaryl borates.

Design and evaluation of pH-sensitive liposomes constructed by poly(2-ethyl-2-oxazoline)-cholesterol hemisuccinate for doxorubicin delivery

In this study, a novel material, poly(2-ethyl-2-oxazoline)-cholesterol hemisuccinate (PEtOz-CHEMS), was synthesized to construct pH-sensitive liposomes. The structure of PEtOz-CHEMS was confirmed by thin-layer chromatography, Fourier transform infrared spectroscopy, and (1)H NMR. Anticancer fluorescent drug doxorubicin (DOX) was encapsulated into the liposomes. Compared with conventional liposomes (CL), CHEMS modified liposomes (CH-L) and PEGylated liposomes (PEG-L), the PEtOzylated liposomes (PEtOz-L) showed an acidic pH-induced increase in particle size. At pH 6.4, the heme release of PEtOz-L group was close to that of the positive control group, whereas that of CL, CH-L and PEG-L was close to that of the negative control group. In vitro drug release studies demonstrated that DOX was released from PEtOz-L in a pH-dependent manner, and the release of DOX from conventional DOX liposomes (CL-DOX), DOX loaded CH-L (CH-DOX-L) and PEGylated DOX liposomes (PEG-DOX-L) had no pronounced differences under each pH medium. In vitro cellular uptake assays showed that PEtOz-DOX-L indicated a significant fluorescence intensity at pH 6.4 compared with at pH 7.4. CL-DOX, CH-DOX-L and PEG-DOX-L did not achieve any obvious diversity at different pH conditions. Confocal laser scanning microscopy images showed that PEtOz-DOX-L can fuse with the endosomal membrane under acidic conditions of endosome, release DOX into the cytoplasm, then gather into the nucleus. Therefore, PEtOz can help liposomes achieve "endosomal escape". The in vitro cytotoxicity experiment results on A375 cells showed that PEtOz-DOX-L resulted in lower cell viability than CL-DOX, CH-DOX-L and PEG-DOX-L under low pH conditions. These results confirm that the pH-responsive PEtOz was a promising material for intracellular targeted delivery system and might be used for overcoming the "PEG dilemma".

Droplet synthesis of well-defined block copolymers using solvent-resistant microfluidic device

Well-defined diblock copolymers were synthesized via an exothermic RAFT route by a droplet microfluidic process using a solvent-resistant and thermally stable fluoropolymer microreactor fabricated by a non-lithographic embedded template method. The resulting polymers were compared to products obtained from continuous flow capillary reactor and conventional bulk synthesis. The droplet based microreactor demonstrated superior molecular weight distribution control by synthesizing a higher molecular weight product with higher conversion and narrow polydispersity in a much shorter reaction time. The high quality of the as-synthesized block copolymer PMMA-b-PS led to a generation of micelles with a narrow size distribution that could be used as a template for well-ordered mesoporous silica with regular frameworks and high surface areas.

Solubility and self-assembly of amphiphilic gradient and block copolymers in supercritical CO2

This work aims at demonstrating the interest of gradient copolymers in supercritical CO(2) in comparison with block copolymers. Gradient copolymers exhibit a better solubility in supercritical CO(2) than block copolymers, as attested by cloud point data. The self-assembly of gradient and block copolymers in dense CO(2) has been characterized by Small-Angle Neutron Scattering (SANS), and it is shown that it is not fundamentally modified when changing from block copolymers to gradient copolymers. Therefore, gradient copolymers are advantageous thanks to their easier synthesis and their solubility at lower pressure while maintaining a good ability for self-organization in dense CO(2).

New micellar morphologies from amphiphilic block copolymers: disks, toroids and bicontinuous micelles

This review discusses recent advances of the self-assembly of amphiphilic block copolymers into novel micellar architectures in dilute solutions. The formation of multi-compartment, disk-like, toroidal and bicontinuous micelles and the macromolecular architectures that give rise to these morphologies are reviewed and discussed.