Phosphine-based coordination cages and nanoporous coordination polymers

Recent findings in the use of multidentate phosphines to synthesise porous coordination polymers (metal-organic frameworks) and their possible precursor cages are reviewed. Additional recent investigations into using large adamantoid Age cages as polymer vertices in giant diamandoid structures are also presented. The results are discussed in terms of possible strategies for the controled synthesis of porous coordination polymers.     

Effect of surfactants on the porous structure of poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) hydrogels prepared by an emulsion templating method

Stimuli-sensitive porous hydrogels prepared with an emulsion templating method developed by the authors are potentially applicable in the medical and pharmaceutical fields; thermosensitive N-isopropylacrylamide (NIPA) hydrogels having randomly distributed sphere-like cavities have been prepared by the polymerization in an aqueous phase in an oil-in-water (O/W) emulsion, followed by the washing of oil (oleyl alcohol) microdroplets. The surfactant plays a dominant role in the preparation of porous hydrogels and the pore size. This study concerns with the surfactant effects on the stability of pre-gel O/W emulsions. The porous NIPA hydrogels were successfully prepared using the surfactants forming the stable emulsion and their internal structures and swelling properties were characterized. The O/W emulsions and the porous hydrogels prepared using various amounts of oil and surfactant were characterized. The information obtained serves for preparation of porous hydrogels having suitable porous structure for their applications.     

Self-formation phenomenon to hierarchically structured porous materials: design, synthesis, formation mechanism and applications

In this paper, we will thoroughly review a novel and versatile self-formation phenomenon that can be exploited to target porous hierarchies of materials without need of any external templates only on the basis of the chemistry of metal alkoxides and alkylmetals. These hierarchically porous materials have unique structures, which are made of either parallel funnel-like/straight macrochannels or 3D continuous interconnected macroporous foams with micro/mesoporous walls. The self-generated porogen mechanism has been proposed, leading to a series of techniques to tailor porous hierarchy, i.e. the use of different chemical precursors (single metal alkoxides, mixed metal alkoxides, single molecular precursors with two different alkoxide functionalities, alkylmetals, etc., …), the control of their hydrolysis and condensation rates (pH, chelating agents,…) and the addition of alkoxysilanes as co-reactant. Various chemical compositions from single or binary metal oxides, to aluminosilicates, aluminophosphates, silicoaluminophosphates, metallophosphates,… can be prepared, offering a panel of potential applications. Some perspectives have been proposed to transform the synthesized materials with a hierarchy of pore sizes to micro-meso-macroporous crystalline materials with zeolite architectures. The advantages of this self-formation preparation method have been discussed compared to traditional templating methods. The possibility to combine with other strategies, for example soft or hard templating, to target even more sophisticated hierarchically meso-macroporous materials with specific structure and function for various applications has been presented. The "hierarchical catalysis" concept has been re-visited.     

Macroporous Oxide Platforms Templated by Non‐Close‐Packed Spherical Copolymer Aggregates

Exclusive organic templating of macroporous oxide films is reported by using non‐close and lose packing of spherical copolymer aggregates, in combination with facile control of condensation degree/density of inorganic oxide frameworks. Unique macroporous oxide films, mainly titania showing highly porous, crystalline, and versatile properties, can be fabricated with continuous design from unusual 3‐D net‐shape to tunable spherical macrostructures, which expands the preparation of other inorganic oxide films (silica, alumina, and zirconia) and possibly adapts the use of other assembled organic polymers. The macroporous structures are helpful for effective accommodation of bulky biomoleculeshigh and diffusivity of organic molecules (useful for photocatalysts). Unusual structural variation, expansion of spherical voids, is also observed, being useful for fine tuning of optical property.     

Effect of hydrophobicity of monomers on the structures and properties of 1,3:2,4-dibenzylidene-D-sorbitol organogels and polymers prepared by templating the gels

The effects of hydrophobicity of monomers on the structures and properties of 1,3:2,4-dibenzylidene-D-sorbitol (DBS) organogels and nanostructured polymers prepared by templating the self-assembled organogels were investigated in this study. Hydrophobic styrene (St), hydrophilic methyl (methacrylate) (MMA), and their mixtures were chosen as the monomers. Though the gelation time varied, the average diameters (around 10 nm) of DBS nanofibrils found in the resulting organogels did not change significantly, for monomers of different hydrophobicity, as observed by transmission electron microscopy (TEM). Nonetheless, new structures, DBS microaggregates, appeared when the MMA content in the monomers was high enough. These irregular, micrometer-sized DBS structures (microaggregates) may have formed because the aggregated DBS molecules were influenced by the MMA monomers, due to the hydrogen bonding between DBS and MMA. This was confirmed by Fourier transform infrared (FTIR) spectroscopy and could also explain the differences in the gelation time of the DBS organogels: gels form more slowly in MMA than in St because of the competing interaction, hydrogen bonding, between DBS and MMA. Subsequently, we thermally initiated the free-radical polymerization of these St/MMA co-monomers. PS/PMMA copolymers were obtained, and no macroscopic phase separation occurred after the polymerization. Finally, the porous structures of the polymers produced by the solvent extraction of the DBS templates were observed, using TEM.     

Polymers with advanced structural and supramolecular features synthesized through topochemical polymerization

Polymers are an integral part of our daily life. Hence, there are constant efforts towards synthesizing novel polymers with unique properties. As the composition and packing of polymer chains influence polymer''s properties, sophisticated control over the molecular and supramolecular structure of the polymer helps tailor its properties as desired. However, such precise control via conventional solution-state synthesis is challenging. Topochemical polymerization (TP), a solvent- and catalyst-free reaction that occurs under the confinement of a crystal lattice, offers profound control over the molecular structure and supramolecular architecture of a polymer and usually results in ordered polymers. In particular, single-crystal-to-single-crystal (SCSC) TP is advantageous as we can correlate the structure and packing of polymer chains with their properties. By designing molecules appended with suitable reactive moieties and utilizing the principles of supramolecular chemistry to align them in a reactive orientation, the synthesis of higher-dimensional polymers and divergent topologies has been achieved via TP. Though there are a few reviews on TP in the literature, an exclusive review showcasing the topochemical synthesis of polymers with advanced structural features is not available. In this perspective, we present selected examples of the topochemical synthesis of organic polymers with sophisticated structures like ladders, tubular polymers, alternating copolymers, polymer blends, and other interesting topologies. We also detail some strategies adopted for obtaining distinct polymers from the same monomer. Finally, we highlight the main challenges and prospects for developing advanced polymers via TP and inspire future directions in this area.

This perspective showcases the potential of topochemical polymerization as an effective tool for synthesizing polymers with advanced molecular and supramolecular structures.     

Selective CO2 adsorption by a triazacyclononane-bridged microporous metal-organic framework

Metal-organic frameworks constructed by self-assembly of metal ions and organic linkers have recently been of great interest in the preparation of porous hybrid materials with a wide variety of functions. Despite much research in this area and the large choice of building blocks used to fine-tune pore size and structure, it remains a challenge to synthesise frameworks composed of polyamines to tailor the porosity and adsorption properties for CO(2). Herein, we describe a rigid and microporous three-dimensional metal-organic framework with the formula [Zn(2)(L)(H(2)O)]Cl (L=1,4,7-tris(4-carboxybenzyl)-1,4,7-triazacyclononane) synthesised in a one-pot solvothermal reaction between zinc ions and a flexible cyclic polyaminocarboxylate. We have demonstrated, for the first time, that a porous rigid framework can be obtained by starting from a flexible amine building block. Sorption measurements revealed that the material exhibited a high surface area (135 m(2) g(-1)) and was the best compromise between capacity and selectivity for CO(2) over CO, CH(4), N(2) and O(2); as such it is a promising new selective adsorbent for CO(2) capture.     

Porous biomaterials obtained using supercritical CO 2- water emulsions

Highly porous, hydrophilic porous matrices were fabricated by using a high internal phase supercritical-CO2 (scCO2) emulsion templating technique. The novel aspect of the work resides in the combination of a natural biopolymer (dextran) as the building component of the matrices and of an environmentally benign solvent (supercritical-CO2) as the pore-generating phase. The synthetic route to the porous biomaterials involved the preliminary functionalization of the dextran chains with methacrylic moieties, formation of a scCO2-in-water concentrated emulsion, and curing of the external phase of the emulsion by radical polymerization. As the emulsion stabilizer a perfluoropolyether surfactant was chosen. The matrices obtained exhibit highly interconnected, trabecular morphologies. The porous biomaterial morphologies were qualitatively characterized by scanning electron microscopy (SEM) and the evaluation of void and interconnect sizes was carried out on the micrographs taken with the light microscope. To tailor the morphologies of the porous structures, the influence of the volume fraction of the internal phase and of the surfactant/internal phase ratio was investigated. It was established that the variation of the volume fraction of the internal phase exerted only a limited influence on void and interconnect sizes. On the contrary the increase of surfactant concentration alters dramatically the distribution of void size, a large proportion of the void space enclosed within the matrix being attributable to voids with a diameter exceeding 100 microm. The free toxic solvent process of fabrication of the porous structures, the high water content, the expected biocompatibility, and the mechanical properties that resemble natural tissues make these porous hydrogels potentially useful for tissue engineering applications.     

Microgel Capsules Tailored by Droplet‐Based Microfluidics

Microgel capsules are micrometer‐sized particles that consist of a cross‐linked, solvent‐swollen polymer network complexed with additives. These particles have various applications, such as drug delivery, catalysis, and analytics. To optimize the performance of microgel capsules, it is crucial to control their size, shape, and content of encapsulated additives with high precision. There are two classes of microgel‐capsule structures. One class comprises bulk microcapsules that consist of a polymer network spanning the entire particle and entrapping the additive within its meshes. The other class comprises core–shell structures; in this case, the microgel polymer network just forms the shell of the particles, whereas their interior is hollow and hosts the encapsulated payload. Both types of structures can be produced with exquisite control by droplet‐based microfluidic templating followed by subsequent droplet gelation. This article highlights some early and recent achievements in the use of this technique to tailor soft microgel capsules; it also discusses applications of these particles. A special focus is on the encapsulation of living cells, which are very sensitive and complex but also very useful additives for immobilization within microgel particles.     

Helical structures of conjugate polymers created by oxidative polymerization using synthetic lipid assemblies as templates

The morphology of conjugate polymers (such as poly(ethylenedioxythiophene), poly(pyrrole), and poly(aniline)) can be controlled in their polymerization processes, by applying the concept of the templating method to oxidative polymerization. As oxidative polymerization of these monomers produces cationic intermediates, the anionic assemblies can act as potential templates due to the mutual electrostatic attractive force. Oxidative polymerization of ethylenedioxythiophene (EDOT), pyrrole, and aniline was carried out using helical superstructures of synthetic lipid assemblies as templates. Interestingly, we have found that oxidative polymerization of these monomers results in novel polymeric aggregates, such as a helical-tape structure and an intertwined helical structure, and that both the right-handed and left-handed helical structures can be created by a change in the hydrophilic head groups. This is the first example of helical superstructures composed of conjugate polymers that have been designed utilizing a convenient templating method.     

Colloids of pure proteins by hard templating

Colloidal particles from pure proteins are favorable over composite colloids (usually polymer-based) for applications in drug delivery and biocatalysis. This is due to degradation issue and protein unfolding. Hard templating based on porous CaCO₃ cores has been recently adopted for fabrication of pure protein colloids. In comparison to conventional techniques, the templating offers (i) a control over particles size and (ii) mild preparation conditions without any additives, shear forces, and exposure to high temperature or gas-water interface. In this review, the current achievements in CaCO₃-based templating of protein colloids are given. The focus is on physicochemical and material properties of the colloids such as stability, mechanical properties, and internal structure. These properties are considered as a function of pH, ionic strength, and protein denaturation degree. Understanding of these basic aspects gives an option to formulate the protein colloids by hard templating achieving desired particle properties that is crucially important for future applications.     

Electron Microscopic Study on Aerosol-Assisted Synthesis of Aluminum Organophosphonates Using Flexible Colloidal PS-b-PEO Templates

A wide variety of synthetic approaches from homogeneous precursor solutions have so far been developed for precise structural design of materials in multiscale. In organic templating approaches for porous materials design, we have recently developed a new approach to fabricate colloidal polystyrene-block-poly(oxyethylene) (PS-b-PEO) templated large pores that can be controlled in thick films of aluminum organophosphonate (AOP). In this study, we extended this approach using colloidal PS-b-PEO aggregates to aerosol-assisted synthesis for the fabrication of spherical particles. Structural variations (morphology and porous structure) depended on the synthetic conditions, which were mainly investigated by using electron microscopies (SEM and TEM). In addition to the insight on the colloidal PS-b-PEO templating of spherical pores in AOP spheres, it was found that colloidal PS-b-PEO aggregates were flexible for further design of pore shape that was strongly affected by external morphology. In this context, we proposed this method as flexible colloidal PS-b-PEO templating to fabricate unusual macroporous structures during morphological control from precursor solutions containing colloidal PS-b-PEO aggregates. The insights will be promising for precise construction of unique devices using porous materials templated by colloidal organic aggregates. In addition, we found a useful water adsorption-desorption behavior over the macroporous AOP bulky powders when the macropores were connected through large pores, which is also significant for future development of AOP-based porous materials.     

Second‐Order Nonlinear Optical Dendrimers and Dendronized Hyperbranched Polymers

Abstract : Second‐order nonlinear optical (NLO) dendrimers with a special topological structure were regarded as the most promising candidates for practical applications in the field of optoelectronic materials. Dendronized hyperbranched polymers (DHPs), a new type of polymers with dendritic structures, proposed and named by us recently, demonstrated interesting properties and some advantages over other polymers. Some of our work concerning these two types of polymers are presented herein, especially focusing on the design idea and structure–property relationship. To enhance their comprehensive NLO performance, dendrimers were designed and synthesized by adjusting their isolation mode, increasing the number of the dendritic generation, modifying their topological structure, introducing isolation chromophores, and utilizing the Ar‐Ar^F self‐assembly effect. To make full use of the advantages of both the structural integrity of dendrimers and the convenient one‐pot synthesis of hyperbranched polymers, DHPs were explored by utilizing low‐generation dendrons as big monomers to construct hyperbranched polymers. These selected works could provide valuable information to deeply understand the relationship between the structure and properties of functional polymers with dendritic structures, but not only limited to the NLO ones, and might contribute much to the further development of functional polymers with rational design.     

Multiple‐Level Porous Polymer Monoliths with Interconnected Cellular Topology Prepared by Combining Hard Sphere and Emulsion Templating for Use in Bone Tissue Engineering

A combination of hard sphere and high internal phase emulsion templating gives a platform for synthesizing hierarchically porous polymers with a unique topology exhibiting interconnected spherical features on multiple levels. Polymeric spheres are fused by thermal sintering to create a 3D monolithic structure while an emulsion with a high proportion of internal phase and monomers in the continuous phase is added to the voids of the previously constructed monolith. Following polymerization of the emulsion and dissolution of the templating structure, a down‐replicating topology is created with a primary level of pores as a result of fused spheres of the 3D monolithic structure, a secondary level of pores resulting from the emulsion's internal phase, and a tertiary level of interconnecting channels. Thiol‐ene chemistry with divinyladipate and pentaerythritol tetrakis(3‐mercaptopropionate) is used to demonstrate the preparation of a crosslinked polyester with overall porosity close to 90%. Due to multilevel porosity, such materials are interesting for applications in bone tissue engineering, possibly simulating the native sponge like bone structure. Their potential to promote ossteointegration is tested using human bone derived osteoblasts. Material–cell interactions are evaluated and they reveal growth and proliferation of osteoblasts both on surface and in the bulk of the scaffold.     

Modulating Benzothiadiazole‐Based Covalent Organic Frameworks via Halogenation for Enhanced Photocatalytic Water Splitting

Two‐dimensional covalent organic frameworks (2D COFs), an emerging class of crystalline porous polymers, have been recognized as a new platform for efficient solar‐to‐hydrogen energy conversion owing to their pre‐designable structures and tailor‐made functions. Herein, we demonstrate that slight modulation of the chemical structure of a typical photoactive 2D COF (Py‐HTP‐BT‐COF) via chlorination (Py‐ClTP‐BT‐COF) and fluorination (Py‐FTP‐BT‐COF) can lead to dramatically enhanced photocatalytic H₂ evolution rates (HER=177.50 μmol h^?1 with a high apparent quantum efficiency (AQE) of 8.45 % for Py‐ClTP‐BT‐COF). Halogen modulation at the photoactive benzothiadiazole moiety can efficiently suppress charge recombination and significantly reduce the energy barrier associated with the formation of H intermediate species (H*) on polymer surface. Our findings provide new prospects toward design and synthesis of highly active organic photocatalysts toward solar‐to‐chemical energy conversion.     

复杂中空结构材料的构筑及能源应用

随着能源问题的日益突显,开发新型多功能材料以满足能源存储与转换应用的需求变得尤为重要.在众多功能材料中,复杂中空结构材料由于其独特的结构和物理化学特性而备受关注.本文综合评述了复杂中空结构材料的普适性构筑方法(硬模板法、软模板法、自模板法、次序模板法和选择性刻蚀法)及在能源方面的应用(锂/钠/钾离子电池、锂硫电池、超级电容器、电催化、光催化及染料敏化电池等).最后,对复杂空心结构研究领域存在的问题及未来的发展方向进行了展望.     

硅基催化结构合成的策略

Two approaches to synthesize silicon-based catalytic structures that aim at capturing the properties and functionalities of natural enzymes are described in this brief review:unit-by-unit synthesis of macromolecular units and templating/imprinting synthesis of nanocages. The unit-by-unit approach mimics the peptide synthesis method,offers atomic control of the structure,but is inefficient in synthesizing large structures such as nanocages. The templating/imprinting method is more suitable for nanocages at the sacrifice of atomic control,and the nanocages obtained are shown to possess properties exhibited by enzyme cavities.     

Tough interconnected polymerized medium and high internal phase emulsions reinforced by silica particles

Emulsion templating using high internal phase emulsions is an effective route to prepare low density and high porosity macroporous polymers known as polymerized high internal phase emulsions (polyHIPEs). Conventional polyHIPEs, synthesized from surfactant stabilized w/o emulsions have low permeabilities and poor mechanical properties. We present interconnected open macroporous low density nanocomposites produced by polymerizing the continuous phase of emulsion templates, which contained styrene, polyethyleneglycoldimethacrylate, and silylated silica particles. Polyethyleneglycoldimethacrylate and the silylated silica particles acted as crosslinker. The functionalized silica particles were incorporated into the polymer, which resulted in a significant improvement of the mechanical properties of the polyHIPEs without affecting the interconnected and permeable pore structures. The polyHIPEs contained up to 60 wt % silylated silica particles. Young's modulus of the reinforced macroporous polymers increased up to 600% compared with nonreinforced macroporous polymers. The mechanical performance was further increased by increasing the foam density of the macroporous nanocomposites from around 200 to 370 g/cm³ by raising the organic phase volume of the emulsion templates from 20 to 40 vol %. The macroporous polymers synthesized from less concentrated emulsions also possessed interconnected open porous although less permeable structures. The polyHIPE nanocomposites have a permeability of about 200 mD, whereas the polyMIPE nanocomposites still have permeabilities of around 50 mD. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1979–1989, 2010     

Meso- to Macro-Porous Oxides as Semiconductor Gas Sensors

The present article reviews our latest studies directed to improving gas sensing properties of semiconductor metal oxides, SnO₂ and TiO₂, by controlling their porous structure in the range of nano- to micro-meter size. Thermally stable mesoporous oxide powders and submicron-size porous thick films were fabricated by a templating method employing supramolecular surfactants or polymethylmethacrylate microspheres and also by a sol–gel process in a polyethylene glycol matrix. Advantages of these methods in improving the sensing properties are overviewed and then remaining problems are discussed.