Structure of the Polymer Backbones in polyMOF Materials

MOF composites for revolutionizing blue energy harvesting and next-gen soft electronics

Metal-organic frameworks (MOFs) are porous materials with highly ordered and crystalline structures, which have earned tremendous attention in the academic community in recent years owing to their high tunability in porosity and pore structure. By integrating MOFs with soft colloids or polymers to form MOF composites, the rigidity and brittle nature of MOFs can be compensated for, thus achieving synergistic effects for a wide variety of applications. In particular, the past decade has seen the advancement of MOF composites in the budding fields of blue energy harvesting and soft electronics, which have received growing interest in the past 5 years. This review focuses on the applications of MOF composites in these two fields, and starts by examining the nanoarchitectures of MOFs, followed by the fabrication of MOF composites. Furthermore, topical advances of MOF composites in blue energy harvesting and soft electronics are reviewed and summarized, and their challenges and future opportunities are discussed as the final touch. This article provides comprehensive review and valuable insights into the development of MOF composites, which may open up new avenues for blue energy harvesting and soft electronics to solve the imminent energy crisis and to advance the wearable technology in healthcare.

Oriented PolyMOFs Enabled by Bridging Coligand for CO2 Separation

Despite enormous research efforts in recent years, polymer-metal-organic framework (polyMOF) development still faces several drawbacks, such as the substantial decrease in surface area, poor crystallinity, and monophyletic chemical structure of polyMOFs. Herein, we overcome the constraints of the coordination mode of conventional polyMOFs and report a bridging coligand strategy to prepare new types of polyMOFs, where the MOFs featuring accessible CuII sites are compelled to orientally regrow within the confined channels of semirigid PIM-1 in dimethyl sulfoxide. Coordination-substitution characteristics and solvent-modulated synthesis enable the Cu centers in MOFs to coordinate with the N atoms from PIM-1 by bridging coligand mode. The reduced particle size, enhanced ultramicroporosity, preferential orientation, and superior filler-matrix compatibility endow the polyMOF-based mixed matrix membrane with excellent CO2 separation performance, with a CO2 permeability of 4669 Barrer, and with a CO2/N2 selectivity of ∼30. This polyMOF design concept exploits a viable avenue for developing more inorganic-organic hybrid materials.

Ligand cross-links as a design element in oligo- and polyMOFs

Metal-Organic Frameworks (MOFs) constructed using cross-linked oligomeric or polymeric ligands (oligoMOFs and polyMOFs respectively) have so far relied on a handful of canonical structural blueprints, in which the cross-links have not played a significant role in determining structure. In this study, we show that cross-links between terephthalate ligands in dabco-based Zn-MOFs (DMOFs) can exert control over the overall phase landscape of resulting oligo- and polyMOFs. We find that cross-links can direct the overall topology of the resulting MOF (pcu vs. kag) based on their length or rigidity, and can influence the phase transformation behavior of the pcu network. We also show the first example of tethered ligand dimers adopting a different MOF structure to the analogous trimer and polymer. Understanding the influence of cross-links on the formation of these MOFs will help guide the design of future MOF-polymer hybrid materials.

Gelation during Ring-Opening Reactions of Cellulosics with Cyclic Anhydrides: Phenomena and Mechanisms

Cellulose esters are used in Food and Drug Administration-approved oral formulations, including in amorphous solid dispersions (ASDs). Some bear substituents with terminal carboxyl moieties (e.g., hydroxypropyl methyl cellulose acetate succinate (HPMCAS)); these ω-carboxy ester substituents enhance interactions with drug molecules in solid and solution phases and enable pH-responsive drug release. However, the synthesis of carboxyl-pendent cellulose esters is challenging, partly due to competing reactions between introduced carboxyl groups and residual hydroxyls on different chains, forming either physically or covalently cross-linked systems. As we explored ring-opening reactions of cyclic anhydrides with cellulose and its esters to prepare polymers designed for high ASD performance, we became concerned upon encountering gelation. Herein, we probe the complexity of such ring-opening reactions in detail, for the first time, utilizing rheometry and solid-state 13C NMR spectroscopy. Gelation in these ring-opening reactions was caused predominantly by physical interactions, progressing in some cases to covalent cross-links over time.

Polymer-Metal-Organic Frameworks (polyMOFs) Based on Tailor-Made Poly(alkenamer)s

Polymeric linkers used to construct porous, crystalline polymer-metal-organic frameworks (polyMOFs) are predominantly based on macromolecules with metal-coordinating ligand units (e.g., 1,4-benzenedicarboxylic acid, H2bdc) included in the primary polymer backbone. Polymers with ligand units as pendants or dangling side chain substituents have been far less explored for the synthesis of polyMOFs, despite the fact that such systems may have distinct properties and could take advantage of a variety of chain polymerization methods. Prevailing reports are based on nonliving polymerized linkers with H2bdc pendants that generated polyMOFs with key shortcomings in controlling the polymerization, tailoring polymeric linker composition and polyMOF properties, accessing porosity, etc. Herein, polymers containing H2bdc units as pendants were designed and synthesized via controlled olefin-metathesis polymerization. These poly(alkenamer)s were subsequently assembled into porous, crystalline networks with an isoreticular MOF (IRMOF) lattice topology. These polymer architectures and polymerization methodologies provide access to the formation of polyMOFs with tailored characteristics, including controlled composition, narrow dispersity, and side chain functionalization.

Unexpected Molecular Sieving of Xylene Isomer Using Tethered Ligand in Polymer-Metal-Organic Frameworks (polyMOFs)

Promising advances in adsorption technology can lead to energy-efficient solutions in industrial sectors. This work presents precise molecular sieving of xylene isomers in the polymer-metal-oragnic framework (polyMOF), a hybrid porous material derived from the parent isoreticular MOF-1 (IRMOF-1). PolyMOFs are synthesized by polymeric ligands bridged by evenly spaced alkyl chains, showing reduced pore sizes and enhanced stabilities compared to its parent material due to tethered polymer bridge within the pores while maintaining the original rigid crystal lattice. However, the exact configuration of the ligands within the crystals remain unclear, posing hurdles to predicting the adsorption performances of the polyMOFs. This work reveals that the unique pore structure of polyIRMOF-1-7a can discriminate xylene isomers with sub-angstrom size differences, leading to highly selective adsorption of p-xylene over other isomers and alkylbenzenes in complex liquid mixtures (αpX/OM = 15 and αpX/OME = 9). The structural details of the polyIRMOF-1-7a are elucidated through computational studies, suggesting a plausible configuration of alkyl chains within the polyMOF crystal, which enable a record-high p-xylene selectivity and stability in liquid hydrocarbon. With this unprecedented molecular selectivity in MOFs, "polymer-MOF" hybridization is expected to meet rigorous requirements for high-standard molecular sieving through precisely tunable and highly stable pores.

PolyMOF nanoparticles constructed from intrinsically microporous polymer ligand towards scalable composite membranes for CO2 separation

Integrating different modification strategies into a single step to achieve the desired properties of metal-organic frameworks (MOFs) has been very synthetically challenging, especially in developing advanced MOF/polymer mixed matrix membranes (MMMs). Herein, we report a polymer-MOF (polyMOF) system constructed from a carboxylated polymer with intrinsic microporosity (cPIM-1) ligand. This intrinsically microporous ligand could coordinate with metals, leading to ~100 nm-sized polyMOF nanoparticles. Compared to control MOFs, these polyMOFs exhibit enhanced ultramicroporosity for efficient molecular sieving, and they have better dispersion properties in casting solutions to prepare MMMs. Ultimately, integrating coordination chemistries through the cPIM-1 and polymer-based functionality into porous materials results in polyMOF/PIM-1 MMMs that display excellent CO2 separation performance (surpassing the CO2/N2 and CO2/CH4 upper bounds). In addition to exploring the physicochemical and transport properties of this polyMOF system, scalability has been demonstrated by converting the developed MMM material into large-area (400 cm2) thin-film nanocomposite (TFN) membranes.

Metal-Organic Frameworks Meet Polymers: From Synthesis Strategies to Healthcare Applications

Metal-organic frameworks (MOFs) have been at the forefront of nanotechnological research for the past decade owing to their high porosity, high surface area, diverse configurations, and controllable chemical structures. They are a rapidly developing class of nanomaterials that are predominantly applied in batteries, supercapacitors, electrocatalysis, photocatalysis, sensors, drug delivery, gas separation, adsorption, and storage. However, the limited functions and unsatisfactory performance of MOFs resulting from their low chemical and mechanical stability hamper further development. Hybridizing MOFs with polymers is an excellent solution to these problems, because polymers-which are soft, flexible, malleable, and processable-can induce unique properties in the hybrids based on those of the two disparate components while retaining their individuality. This review highlights recent advances in the preparation of MOF-polymer nanomaterials. Furthermore, several applications wherein the incorporation of polymers enhances the MOF performance are discussed, such as anticancer therapy, bacterial elimination, imaging, therapeutics, protection from oxidative stress and inflammation, and environmental remediation. Finally, insights from the focus of existing research and design principles for mitigating future challenges are presented.

Covalent connections between metal-organic frameworks and polymers including covalent organic frameworks

Hybrid composite materials combining metal-organic frameworks (MOFs) and polymers have emerged as a versatile platform for a broad range of applications. The crystalline, porous nature of MOFs and the flexibility and processability of polymers are synergistically integrated in MOF-polymer composite materials. Covalent bonds, which form between two distinct materials, have been extensively studied as a means of creating strong molecular connections to facilitate the dispersion of "hard" MOF particles in "soft" polymers. Numerous organic transformations have been applied to post-synthetically connect MOFs with polymeric species, resulting in a variety of covalently connected MOF-polymer systems with unique properties that are dependent on the characteristics of the MOFs, polymers, and connection modes. In this review, we provide a comprehensive overview of the development and strategies involved in preparing covalently connected MOFs and polymers, including recently developed MOF-covalent organic framework composites. The covalent bonds, grafting strategies, types of MOFs, and polymer backbones are summarized and categorized, along with their respective applications. We highlight how this knowledge can serve as a basis for preparing macromolecular composites with advanced functionality.

MOF/polymer hybrids through in situ free radical polymerization in metal-organic frameworks

We use the free radical polymerization initiator 4,4'-azobis(cyanovaleric acid) coordinated to the open metal sites of metal-organic frameworks (MOFs) to give rise to highly uniform MOF/polymer hybrids. We demonstrate this strategy on two robust zirconium MOFs (NU-1000 and MOF-808), which are the most effective catalysts for degradation of chemical warfare nerve agents. The resulting hybrid materials maintain their hydrolytic catalytic activity and have substantially improved adhesion to polypropylene and activated carbon textile fibers, yielding highly robust MOF/polymer/textile hybrid systems. These composites are suitable for the green production of active protective clothing and filters capable of detoxifying organophosphorus warfare agents.

Tethering Effects in Oligomer-Based Metal-Organic Frameworks

Metal-organic frameworks (MOFs) can be constructed using conventional molecular linkers or polymeric linkers (polyMOFs), but the relationship and relative properties of these related materials remain understudied. As an intermediate between these two extremes, a library of oligomeric ligand precursors (dimers, trimers) was used to prepare a series of oligomeric-linker MOFs (oligoMOFs) based on the prototypical IRMOF-1 system. IRMOF-1 was found to be remarkably tolerant to a wide variety of oligomeric linkers, the use of which greatly enhanced the MOF yield and prevented framework interpenetration. Tether length-dependent ordering of ligand and metal cluster orientations was also observed in these oligoMOFs. Improved low-humidity stability was found in oligoIRMOF-1 samples, with surface area preservation varying as a function of tether length and a complete suppression of crystalline hydrolysis products for all oligoIRMOF-1 materials. These findings pave the way toward a better understanding of the structure-function relationships between monomeric, oligomeric, and polymeric MOFs and highlight an underutilized strategy for tuning MOF properties.

Amino acids and doxorubicin as building blocks for metal ions-driven self-assembly of biodegradable polyprodrugs for tumor theranostics

On-demand designed theranostics nanoagents show promising applications for next-generation precision-and-personalized oncotherapy. Researchers have since aimed to develop nanoplatforms that can efficiently deliver drugs and contrast medium to tumor and release active ingredients in response to tumor microenvironment (TME) conditions. Herein, we propose a modular strategy, and develop a series of nanoplatforms based on metal-coordinated-polyprodrugs for cancer theranostics. The polyprodrugs were synthesized through a click-reaction between amino acid and doxorubicin (DOX) with dipropiolate. The backbones of the polyprodrugs had intrinsic sensitivities to pH and/or GSH, and provided abundant -COOH, -NH2, or -S-S- to chelate with functional metal ions and further self-assembled to form different morphologies. Dicysteine, which contains disulfide bond (-S-S-), was chosen to copolymerize with DOX and triethylene glycol dipropiolate (TEP) to prepare the pH/GSH dual-responsive polyprodrug poly(dicysteine-co-TEP-co-DOX) (pDTD), then separately coordinated with Gd3+, Fe3+, and Mn2+ to construct nanoplatforms pDTD@M (M representing the metal ions). In vitro and in vivo investigations suggest the metal-coordinated-polyprodrug nanoplatforms have good magnetic resonance imaging (MRI) ability and efficient tumor-growth inhibition with high safety. The design strategy of nanoplatforms based on metal-coordinated-polyprodrugs provides a new idea for on-demand construction of promising theranostics agents. STATEMENT OF SIGNIFICANCE: Compared to small molecule antitumor drugs, polymeric drugs have high drug loading ratio and are easily enriched at the tumor site to achieve improved therapy efficacy. This work utilizes click reactions to link amino acids with anticancer drugs to produce polymeric drugs that are degraded in response to tumor microenvironment and released small molecule antitumor drugs mainly in tumor sites, and subtly utilizes the coordination of amino acid to chelate MRI functional metal ion to realize enhanced MRI imaging mediated tumor therapy. This strategy provides a new idea for the convenient construction of polymeric drugs for tumor theranostics.

Coordination Polymer Glasses with Lava and Healing Ability for High-Performance Gas Sieving

Coordination polymer (CP) glasses offer a way to tackle the fabrication challenges encountered by inorganic porous membranes and show great potentials for size-exclusive gas separation. However, their processability and performance still cannot simultaneously meet the requirements for high-performance membrane separation. Herein, we have developed a series of CP glasses (M-P-dmbIm, M=Zn, Cd, Cu, and Mn), which possess low vitrification temperature as well as low viscosity (η) and lave capability above the transition temperatures. The derived glass (a_g M-P-dmbIm) membranes show outstanding performances for H₂ /CO₂ , H₂ /N₂ , and H₂ /CH₄ separation, which all far surpass the Robeson upper bound and even rival against the best of the state-of-the-art gas separation membranes. The low viscosities not only allow us to hot-cast or hot-press the CP glasses into thin membranes within 5 min without sacrificing their selectivity and permeability, but also endow the resulted glass membranes with healing ability.

Signal Deconvolution and Generative Topographic Mapping Regression for Solid-State NMR of Multi-Component Materials

Solid-state nuclear magnetic resonance (ssNMR) spectroscopy provides information on native structures and the dynamics for predicting and designing the physical properties of multi-component solid materials. However, such an analysis is difficult because of the broad and overlapping spectra of these materials. Therefore, signal deconvolution and prediction are great challenges for their ssNMR analysis. We examined signal deconvolution methods using a short-time Fourier transform (STFT) and a non-negative tensor/matrix factorization (NTF, NMF), and methods for predicting NMR signals and physical properties using generative topographic mapping regression (GTMR). We demonstrated the applications for macromolecular samples involved in cellulose degradation, plastics, and microalgae such as Euglena gracilis. During cellulose degradation, ¹³C cross-polarization (CP)-magic angle spinning spectra were separated into signals of cellulose, proteins, and lipids by STFT and NTF. GTMR accurately predicted cellulose degradation for catabolic products such as acetate and CO₂. Using these methods, the ¹H anisotropic spectrum of poly-ε-caprolactone was separated into the signals of crystalline and amorphous solids. Forward prediction and inverse prediction of GTMR were used to compute STFT-processed NMR signals from the physical properties of polylactic acid. These signal deconvolution and prediction methods for ssNMR spectra of macromolecules can resolve the problem of overlapping spectra and support macromolecular characterization and material design.

Scalable crystalline porous membranes: current state and perspectives

Crystalline porous materials (CPMs) with uniform and regular pore systems show great potential for separation applications using membrane technology. Along with the research on the synthesis of precisely engineered porous structures, significant attention has been paid to the practical application of these materials for preparation of crystalline porous membranes (CPMBs). In this review, the progress made in the preparation of thin, large area and defect-free CPMBs using classical and novel porous materials and processing is presented. The current state-of-the-art of scalable CPMBs with different nodes (inorganic, organic and hybrid) and various linking bonds (covalent, coordination, and hydrogen bonds) is revealed. The advances made in the scalable production of high-performance crystalline porous membranes are categorized according to the strategies adapted from polymer membranes (interfacial assembly, solution-casting, melt extrusion and polymerization of CPMs) and tailored based on CPM properties (seeding-secondary growth, conversion of precursors, electrodeposition and chemical vapor deposition). The strategies are compared and ranked based on their scalability and cost. The potential applications of CPMBs have been concisely summarized. Finally, the performance and challenges in the preparation of scalable CPMBs with emphasis on their sustainability are presented.

Inside polyMOFs: layered structures in polymer-based metal-organic frameworks

In this report, we explore the internal structural features of polyMOFs consisting of equal mass ratios of metal-coordinating poly(benzenedicarboxylic acid) blocks and non-coordinating poly(ethylene glycol) (PEG) blocks. The studies reveal alternating lamellae of metal-rich, crystalline regions and metal-deficient non-crystalline polymer, which span the length of hundreds of nanometers. Polymers consisting of random PEG blocks, PEG end-blocks, or non-coordinating poly(cyclooctadiene) (COD) show similar alternation of metal-rich and metal-deficient regions, indicating a universal self-assembly mechanism. A variety of techniques were employed to interrogate the internal structure of the polyMOFs, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and small-angle synchrotron X-ray scattering (SAXS). Independent of the copolymer architecture or composition, the internal structure of the polyMOF crystals showed similar lamellar self-assembly at single-nanometer length scales.

In this report, we explore the internal structural features of polyMOFs consisting of equal mass ratios of metal-coordinating poly(benzenedicarboxylic acid) blocks and non-coordinating poly(ethylene glycol) (PEG) blocks.