Supramolecular reactor in an aqueous environment: aromatic cross Suzuki coupling reaction at room temperature

[reaction: see text] We have investigated supramolecular reactors for the Suzuki coupling reactions of aryl halides with phenyl boronic acids by using self-assembly of amphiphilic rod-coil molecules in aqueous solution at room temperature. All the rod-coil molecules synthesized in this work showed to self-assemble into discrete micelles consisting of aromatic rod bundles encapsulated by hydrophilic poly(ethylene oxide) coils. We present a comparative study of rod-coil molecules' efficiency as supramolecular reactors for Suzuki coupling reaction. The closed-packed aromatic bundles play an efficient role in supramolecular reactors for the coupling reactions at room temperature. The supramolecular reactor based on hexa-p-phenylene confers unprecedented activity, allowing reactions to be performed at very low catalyst levels, without conventional heating or microwave.     

Transformation of isotropic fluid to nematic gel triggered by dynamic bridging of supramolecular nanocylinders

We have synthesized an amphiphilic triblock coil-rod-coil molecule consisting of a rigid aromatic building block and poly(ethylene oxide). The coil-rod-coil molecule was observed to assemble into a cylindrical micelle structure in aqueous solution. Notably, addition of a small amount of a rod-coil-rod molecule into the cylindrical micellar solution can induce the anisotropic gelation due to dynamic interconnection of adjacent cylindrical micelles via hydrophobic and pi-pi interactions. Depending on the concentration of the rod-coil-rod molecule, the nematic gel can be reversibly switched into an isotropic solution of cylindrical micelles.     

Electrical switching between vesicles and micelles via redox-responsive self-assembly of amphiphilic rod-coils

An aqueous vesicular system that is switchable by electric potential without addition of any chemical redox agents into the solution is demonstrated using redox-responsive self-assembly of an amphiphilic rod-coil molecule consisting of a tetraaniline and a poly(ethylene glycol) block. The vesicle membrane is split by an oxidizing voltage into smaller pucklike micelles that can reassemble to form vesicles upon exposure to a reducing voltage. The switching mechanism is explained by the packing behavior of the tetraaniline units constituting the membrane core, which depends on their oxidation states.     

基于聚环氧乙烷链和亚苯基的刚棒-线团分子自组装

含有亲水性聚环氧乙烷(PEO)链和多个亚苯基作为刚棒的刚棒-线团(rod-coil)分子,由于具有很强的π-π堆积作用和微相分离作用,可以在本体和溶液中产生有序排列形成各种组装体。材料的性能主要依靠分子聚集体来体现,因此研究分子聚集体的性质和用途成为当前超分子领域研究的重要内容。刚棒嵌段的形状影响着分子排列的方式、自组织性能和微结构,本文总结了Y型、T 型、O型、 K型、n型、螺旋桨型和哑铃型等不同形状分子的自组装行为,并展望了此类刚棒-线团分子未来研究的方向。     

Ordered nanostructures from self-assembly of rod–coil oligomers with n-shaped rod and dendritic poly(ethylene oxide) coil segment

The n-shaped rod–coil molecules consisting of an anthracene unit and two biphenyl groups connected by acetylenyl bonds as a conjugated rod segment and dendritic poly(ethylene oxide)s with different cross-sectional areas were synthesised. These new molecular structures were characterised by using ¹H NMR and MALDI-TOF-MS. The self-assembly of these molecules in the bulk state and in aqueous solution was investigated using differential scanning calorimetry, X-ray diffraction and transmission electron microscopy (TEM). In the bulk state, molecule 1a with a linear coil segment, self-organised into lamellar crystalline structures, whereas molecules 1b and 1c with di- and tetra-branched dendritic wedges did not solidify at room temperature. Dynamic light scattering and TEM experiments reveal that in aqueous solution, the molecules exhibit a strong tendency to organise into the thread-like fibres along the axial direction of cylindrical micelles or into the wide fibroid bundles via the aggregation of cylindrical micelles as the cross-sectional area of the dendritic coil segments increases.     

Self-assembly of Amphiphilic Linear Diblock Rod-Coil Molecules by Hydrogen Bond and π-π Stacking Interactions

Aromatic amphiphilic molecules(1) consisting of three biphenyl groups linked together with ether bonds as a rigid rod segment and poly(ethylene oxide) with the number of repeating units of 17 as a coil segment were synthesized, and their self-assembly behavior in the bulk state and aqueous solution was investigated. In bulk, molecules 1 self-assembled into 1-D lamellar structure in the solid state or smectic A phase in the liquid crystalline phase via the cooperative effects of ?-? stacking, micro-phase separation and hydrogen bond interactions. In dilute aqueous solutions, molecules 1 were observed to selfassemble into cylindrical micelles owned uniform diameter and length of hundreds of nanometers.     

Heat inhibited reactions

Acyl transfer from p-nitrophenyl trimethylacetate to hydrogen peroxide in millimolar aqueous solutions of an amphiphilic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer slows down as the temperature is raised due to partitioning of the hydrophobic ester into heat-induced micelles.     

Formation of fibril structures in polymerizable, rod-coil-oligomer-modified epoxy networks

This paper describes the in situ preparation of fibrils in epoxy networks in which the fibril-like structures are cured polymerizable rod-coil oligomers. The epoxy-terminated alpha,omega-modified PEO oligomers, which are ABA rod-coil-rod oligomers with a poly(ethylene oxide) coil unit and two aromatic azomethine liquid-crystalline rod units, were synthesized and then further blended with an epoxy precursor. Uniform nanoscale columnar structures were observed in the neat rod-coil oligomers as well as in the crosslinked liquid-crystalline state. During the curing of the blends, the supramolecular nanoscale columnar structures of the rod-coil oligomers are transformed into polymeric fibrils where the epoxy functional end groups have co-reacted with epoxy precursors to form a crosslinked network.     

Multi-morphological self-assembled structures of a rod-coil organometallic oligomer

A rod-coil amphiphilic organometallic oligomer based on o-ferrocenylcarbonyl benzoic acid (FcBA) as hydrophobic short rod and poly(ethylene glycol)₁₇ (PEG₁₇) as hydrophilic coil chain was synthesized via Friedel–Crafts acylation and esterification reaction and characterized by ¹H-NMR and FT-IR analyses. The rod-coil oligomer FcBA-PEG₁₇ with short-rod insoluble FcBA segment and long-coil PEG₁₇ segment could self-assemble into multi-morphological aggregates in water such as sphere, rod, strip and vesicle at different initial concentrations, which were characterized by transmission electron microscopy. Interestingly, two different kinds of vesicles were observed, where disfigured vesicles formed at low initial concentration and ideal vesicles at high initial concentration. And the disfigured vesicles can be transferred into ideal vesicles at lower initial concentration through the threading of rigid α-CDs onto the coil PEG₁₇ chains. A possible mechanism for the formation of multi-morphological self-assembled micelles was also proposed.     

Micellization phenomena of biodegradable amphiphilic triblock copolymers consisting of poly(beta-hydroxyalkanoic acid) and poly(ethylene oxide)

This paper reports the studies on micelle formation of new biodegradable amphiphilic poly(ethylene oxide)-poly[(R)-3-hydroxybutyrate]-poly(ethylene oxide) (PEO-PHB-PEO) triblock copolymer with various PHB and PEO block lengths in aqueous solution. Transmission electron microscopy showed that the micelles took an approximately spherical shape with the surrounding diffuse outer shell formed by hydrophilic PEO blocks. The size distribution of the micelles formed by one triblock copolymer was demonstrated by dynamic light scattering technique. The critical micellization phenomena of the copolymers were extensively studied using the pyrene fluorescence dye absorption technique, and the (0,0) band changes of pyrene excitation spectra were used as a probe for the studies. For the copolymers studied in this report, the critical micelle concentrations ranged from 1.3 x 10(-5) to 1.1 x 10(-3) g/mL. For the same PEO block length of 5000, the critical micelle concentrations decreased with an increase in PHB block length, and the change was more significant in the short PHB range. It was found that the micelle formation of the biodegradable amphiphilic triblock copolymers consisting of poly(beta-hydroxyalkanoic acid) and PEO was relatively temperature-insensitive, which is quite different from their counterparts consisting of poly(alpha-hydroxyalkanoic acid) and PEO.     

Evolution of multicompartment micelles to mixed corona micelles using solvent mixtures

Miktoarm star triblock copolymers mu-[poly(ethylethylene)][poly(ethylene oxide)][poly(perfluoropropylene oxide)] self-assemble in dilute aqueous solution to give multicompartment micelles with the cores consisting of discrete poly(ethylethylene) and poly(perfluoropropylene oxide) domains. Tetrahydrofuran is a selective solvent for both the poly(ethylethylene) and poly(ethylene oxide) blocks, and thus in tetrahydrofuran mixed corona micelles are favored with poly(perfluoropropylene oxide) cores. The introduction of tetrahydrofuran into water induces an evolution from multicompartment micelles to mixed corona [poly(ethylethylene) + poly(ethylene oxide)] micelles, as verified by dynamic light scattering and nuclear magnetic resonance spectroscopy. A mixed solvent containing 60 wt % tetrahydrofuran corresponds to the transition point, as verified by analysis of a poly(ethylethylene)-poly(ethylene oxide) diblock copolymer in the same solvent mixtures. Furthermore, cryogenic transmission electron microscopy suggests that, as the poly(ethylethylene) block transitions from the core to the corona, the micelle morphologies evolve from disks to oblate ellipsoid micelles (with some vesicles), with worms and spheres evident at intermediate compositions.     

Nanorings from the self-assembly of amphiphilic molecular dumbbells

We have prepared amphiphilic dumbbell molecules consisting of hydrophobic alkyl chains and hydrophilic oligoether dendrons at each end of the rod segment. The molecular dumbbells, in aqueous solution, self-assemble into toroids as an intermediate nanostructure between spherical and long cylindrical micelles. The formation of toroidal structure is likely to originate from side by side connections of discrete bundles through the combination of strong hydrophobic interactions and anisotropic aggregation of rod segments.     

Rigid-flexible block molecules based on a laterally extended aromatic segment: hierarchical assembly into single fibers, flat ribbons, and twisted ribbons

Self-assembling rigid-flexible block molecules consisting of a laterally extended aromatic segment and different lengths of hydrophilic coils were synthesized and characterized. The block molecule based on a long poly(ethylene oxide) coil (1), in the melt state, shows an unidentified columnar structure, whereas the molecule with a shorter poly(ethylene oxide) coil (2) self-organizes into an oblique columnar structure. Further decrease in the poly(ethylene oxide) coil length as in the case of 3, on heating, induces a rectangular columnar structure in addition to an oblique columnar mesophase. In diethyl ether, 1 and 2 were observed to self-assemble into uniform nanofibers with bilayer packing. Remarkably, these elementary fibers were observed to further aggregate in a lateral way to form well-defined flat ribbons (1) and twisted ribbons (2) with solvent exchange of diethyl ether into methanol. Furthermore, the ribbons formed in methanol dissociated into elementary fibers in response to the addition of aromatic guest molecules. This transformation between ribbons and single fibers in response to the addition of guest molecules is attributed to the intercalation of aromatic substrates within the rigid segments and subsequent loosening of the aromatic stacking interactions. These results demonstrate that the introduction of a laterally extended aromatic segment into an amphiphilic molecular architecture can lead to the hierarchical formation from elementary fibers of nanoribbons with a tunable twist through controlled lateral interactions between aromatic segments.     

Palladium-catalyzed Mizoroki-Heck reactions in water using thermoresponsive polymer micelles

Palladium-catalyzed Mizoroki-Heck reactions were carried out in water using thermoresponsive polymer micelles. The micelles were generated from thermoresponsive block copolymers consisting of a poly(N-isopropylacrylamide) (PNIPAAm) segment and a hydrophilic segment such as nonionic poly(ethylene glycol) (PEG) (2) and anionic poly(sodium p-styrenesulfonate) (PSSNa) (9). These copolymers exhibited lower critical solution temperature (LCST) behavior at ca. 40–50?°C and showed thermal stimuli-induced formation and dissociation of micelles. The copolymers formed micelles in aqueous solution at higher temperature, where catalytic reactions proceeded. At lower temperature, the micelles dissociated to form a clear solution, enabling efficient extraction of the products from aqueous reaction mixture. In the presence of these copolymers, palladium complexes catalyzed the coupling reactions between aryl iodides and alkene compounds inside the hydrophobic micelle cores in water under relatively milder conditions. Extraction of the products from the aqueous solution of 2 or 9 was found to be efficient enough in comparison with conventional surfactants.     

Reversible Metallo‐Supramolecular Block Copolymer Micelles Containing a Soft Core

An amphiphilic metallo‐supramolecular poly(ethylene‐co‐butylene)‐block‐poly(ethylene oxide) diblock copolymer containing a bis(2,2′:6′,2″‐terpyridine)ruthenium(II) complex as a supramolecular connection between the two constituting blocks was used to prepare stable aqueous micelles. The micelles were characterized by dynamic light scattering and atomic force microscopy. Individual micelles were observed together with aggregates of micelles. Only the addition of a large excess of competitive ligand caused the cleavage of the very stable ruthenium complex.     

Functional and site-specific macromolecular micelles as high potential drug carriers

Since several years, macromolecular micelles based on amphiphilic block copolymers have attracted much interest as drug carriers. These micelles show a long term blood circulation time resulting from their small diameter and the steric repulsion created by the poly(ethylene oxide) chains which constitute micelle corona, as well as from their high thermodynamic stability. Besides this long term blood circulation time generating a passive targeting, an active targeting, chemical or physical affinity targeting, might allow the preparation of more efficient drug carriers. In order to obtain such double targeting properties, we have prepared two kinds of macromolecular micelles. The first one is based on amphiphilic poly(ethylene oxide)/poly(β-benzyl -aspartate) ---PEO/PBLA--- block copolymers having hydroxy groups at the free end of PEO chains. As a result of their structure, such micelles have hydroxy groups on their outer-shell which can be further modified in order to introduce a targeting moiety (sugar, etc.). The characteristics (diameter, critical micellar concentration (cmc), drug loading capacity) have been determined. Moreover, doxorubicin loaded -hydroxy PEO/PBLA micelles have been shown to be slightly more cytotoxic than the corresponding -methoxy PEO/PBLA micelles. The second type of micelles is based on thermosensitive amphiphilic poly(N-isopropyl acrylamide)/polystyrene ---PIPAAm/PSt--- block copolymers. Such micelles have a small diameter and a low cmc in addition to thermosensitivity properties which are similar to those of PIPAAm.     

Construction of Supramolecular Assemblies from Self‐Organization of Amphiphilic Molecular Isomers

Amphiphilic coil‐rod‐coil molecules, incorporating flexible and rigid blocks, have a strong affinity to self‐organize into various supramolecular aggregates in bulk and in aqueous solutions. In this paper, we report the self‐assembling behavior of amphiphilic coil‐rod‐coil molecular isomers. These molecules consist of biphenyl and phenyl units connected by ether bonds as the rod segment, and poly(ethylene oxide) (PEO) with a degree of polymerization of 7 and 12 as the flexible chains. Their aggregation behavior was investigated by differential scanning calorimetry, thermal optical polarized microscopy, small‐angle X‐ray scattering spectroscopy, and transmission electron microscopy. The results imply that the molecular structure of the rod building block and the length of the PEO chains dramatically influence the creation of supramolecular aggregates in bulk and in aqueous solutions. In the bulk state, these molecules self‐organize into a hexagonal perforated lamellar and an oblique columnar structure, respectively, depending on the sequence of the rod building block. In aqueous solution, the molecule with a linear rod segment self‐assembles into sheet‐like nanoribbons. In contrast, its isomer, with a rod building block substituted at the meta‐position of the aryl group, self‐organizes into nanofibers. This is achieved through the control of the non‐covalent interactions of the rod building blocks.     

Self-assembly and micellization of amphiphilic rod-coil block oligomer at the mica-water interface

In this paper, in situ atomic force microscopy has been used to investigate the micellization and self-assembling structure of an amphiphilic rod-coil block oligomer (EO16OPV) containing a conjugated oligo(phenylene vinylene) dimer and poly(ethylene oxide) at the mica-water interface. It is found that EO16OPV molecules have strong adsorption and aggregation properties on mica. In the wide concentration range from above the critical micelle concentration (cmc) to far below the cmc, a closely packed layer of stripe-like micelles with two preferred orientations can be formed at the mica-water interface. A cylindrical micelle structure for the stripes is proposed. We demonstrate that the stripe-like micelles formed on mica originate from different micellization processes at solution concentrations above and below the cmc. The origins of the strong micellization properties and oriented arrangement of the stripes are also discussed.     

Two-compartment micellar assemblies obtained via aqueous self-organization of synthetic polymer building blocks

We synthesized a symmetric linear ABCBA pentablock copolymer consisting of poly(ethylene oxide), poly(gamma-benzyl l-glutamate), and a poly(perfluoro ether) (fluorolink). The different blocks are highly immiscible with each other and form two-compartment micelles of mainly cylindrical shape in aqueous solution with lengths in the range of 100 to 200 nm and diameters of about 24 nm. The poly(perfluoro ether) (C blocks) forms the liquidlike center of the micelles (d = 6 nm). This is surrounded by a first shell of ca. 2 nm thickness consisting of beta-sheets of poly(gamma-benzyl l-glutamate) (B blocks) and a second 7 nm shell of poly(ethylene oxide) (A blocks). The A blocks provide water solubility, and the B and C blocks form separated hydrophobic compartments. This work is a contribution to the development of multicompartment micelles devoted to mimic transport proteins such as serum albumins in long-term development.     

Micellar ordered structure effects on high‐resolution CE‐SSCP using Pluronic triblock copolymer blends

Pluronic F108 block copolymers have shown a great promise to achieve the desirable high resolution in the conformation‐sensitive separation of ssDNA using CE‐SSCP. However, fundamental understanding of the structures and properties of Pluronic matrix affecting the resolution is still limited. Unlike conventional gel‐forming homopolymers, Pluronic F108 block copolymers are amphiphilic macromolecules consisting of poly(ethylene oxide)‐b‐poly(propylene oxide)‐b‐poly(ethylene oxide) triblock copolymers, which are capable of forming a highly ordered micellar structure in aqueous solution. In this study, we have performed a series of experiments by blending different types of Pluronic polymers to control the formation of micelles and to study the correlation between separation and rheological characteristics of Pluronic gels affecting the resolution of CE‐SSCP. Our experiments have been specifically designed to elucidate how the micellar structure affects the resolution of CE‐SSCP upon altering the size uniformity and constituent homogeneity of the micelles. Our results suggest that uniformly sized micelle packing is the primary structural feature of Pluronic gel matrix for the high‐resolution separation, while the size and constituent of the micelle themselves need to be considered as secondary factors.