3D Covalent Organic Framework as a Metastable Intermediate in the Formation of a Double-Stranded Helical Covalent Polymer

One-Dimensional Covalent Organic Frameworks: From Design, Synthesis to Applications

As an important branch of the covalent organic frameworks (COFs) family, one-dimensional COFs (1D COFs), which are formed by the ordered arrangement of confined covalent bonds in one dimension and non-covalent interactions (van der Waals force, π-π interactions, and hydrogen bonds) in the vertical two and three dimensions has aroused much attention. Compared with two-dimensional (2D)/three-dimensional (3D) COFs, 1D COFs behaved more easily dispersing and had more opportunities for active sites exposure due to their weaker interchain/interlayer interaction, modified nonlinear edge, and pore structures. These features make them have great application potential in many fields, including catalysis, energy storage, adsorption, sensing, and others. In this minireview, we highlight the state-of-the-art advances of 1D COFs in the structure design principles of building blocks, synthesis strategies, and their related applications. Furthermore, we present an in-depth outlook on the challenges and opportunities faced by 1D COFs, aiming to offer insights for future studies in this intriguing and significant research field.

Closed-Loop Recyclable Lithium and Sodium Conducting Covalent Adaptable Networks

Within the past two decades, covalent adaptable networks (CANs) have emerged as a novel class of dynamically crosslinked polymers, combining the benefits of thermosets and thermoplastics. Although some CANs with charged side chains have been reported, CANs with negatively charged backbones remain very limited. The integration of permanent charge into the backbones upon their formation could open up important new applications. Here, we introduce a series of aliphatic spiroborate-linked ionic covalent adaptable networks (ICANs), representing a new category of dynamic ionomer thermosets. These ICANs were synthesized using a catalyst-free, scalable, and environment-friendly method. Incorporating lithium or sodium as counter cations in these networks yielded promising ion conductivity without the need of plasticizers. The dynamic nature of the spiroborate linkages in these materials allows for rapid reprocessing and recycling under moderate conditions. Furthermore, their potential as flexible solid-state electrolytes is demonstrated in a device that maintained robust conducting performance under extreme physical deformation, coupled with effective self-healing properties. This research opens new possibilities for future development of dynamic ionomer thermosets and their potential applications in flexible electronic devices.

Ionic Covalent Organic Frameworks Consisting of Tetraborate Nodes and Flexible Linkers

Covalent organic frameworks (COFs) have emerged as versatile materials with many applications, such as carbon capture, molecular separation, catalysis, and energy storage. Traditionally, flexible building blocks have been avoided due to their potential to disrupt ordered structures. Recent studies have demonstrated the intriguing properties and enhanced structural diversity achievable with flexible components by judicious selection of building blocks. This study presents a novel series of ionic COFs (ICOFs) consisting of tetraborate nodes and flexible linkers. These ICOFs use borohydrides to irreversibly deprotonate the alcohol monomers to achieve a high degree of polymerization. Structural analysis confirms the dia topologies. Reticulation is explored using various monomers and metal counterions. Also, these frameworks exhibit excellent stability in alcohols and coordinating solvents. The materials have been tested as single-ion conductive solid-state electrolytes. ICOF-203-Li displays one of the lowest activation energies reported for ion conduction. This tetraborate chemistry is anticipated to facilitate further structural diversity and functionality in crystalline polymers.

Construction of Benzoxazine-linked One-Dimensional Covalent Organic Frameworks Using the Mannich Reaction

Covalent polymerization of organic molecules into crystalline one-dimensional (1D) polymers is effective for achieving desired thermal, optical, and electrical properties, yet it remains a persistent synthetic challenge for their inherent tendency to adopt amorphous or semicrystalline phases. Here we report a strategy to synthesize crystalline 1D covalent organic frameworks (COFs) composing quasi-conjugated chains with benzoxazine linkages via the one-pot Mannich reaction. Through [4+2] and [2+2] type Mannich condensation reactions, we fabricated stoichiometric and sub-stoichiometric 1D covalent polymeric chains, respectively, using doubly and singly linked benzoxazine rings. The validity of their crystal structures has been directly visualized through state-of-the-art cryogenic low-dose electron microscopy techniques. Post-synthetic functionalizations of them with a chiral MacMillan catalyst produce crystalline organic photocatalysts that demonstrated excellent catalytic and recyclable performance in light-driven asymmetric alkylation of aldehydes, affording up to 94 % enantiomeric excess.

Controlling the Degree of Interpenetration in Chiral Three-Dimensional Covalent Organic Frameworks via Steric Tuning

Network interpenetration plays a crucial role in functionalizing porous framework materials. However, controlling the degree of interpenetration in covalent organic frameworks (COFs) to influence their pore sizes, shapes, and functionalities still remains a significant challenge. Here, we demonstrate a steric tuning strategy to control the degree of COF interpenetration and modulate their physicochemical properties. By imine condensations of 1,1'-bi-2-naphthol-derived tetraaldehydes bearing different alkyl substituents with the monomer tetra(p-aminophenyl)-methane, we synthesized and characterized a family of two-component and three-component chiral COFs with different interpenetrated dia networks. The alkyl groups are periodically appended on the pore walls, and their types/contents that can be synthetically tuned control the interpenetration degree of COFs by minimizing repulsive interactions between the alkyl groups. Specifically, the COF with -OH groups adopts an interpenetrated dia-c5 topology, those with -OMe/-OEt groups take an interpenetrated dia-c4 topology, whereas those with the bulky -OnPr/-OnBu groups exhibit a noninterpenetrated dia-c1 topology. The multivariate COFs with both -OH and -OnBu groups display either a noninterpenetrated or dia-c5 topology, depending on the proportion of -OnBu groups. The extent of interpenetration in COFs significantly affects their porosity, thermal stability, and chemical stability, resulting in varying selective performances in the adsorption and separation of dyes and asymmetric catalysis. This work highlights the potential of using steric hindrance to tune and control interpenetration, porosity, stability, and functionalities of COFs materials, broadening the range of their applications.

New Advances in Covalent Network Polymers via Dynamic Covalent Chemistry

Covalent network polymers, as materials composed of atoms interconnected by covalent bonds in a continuous network, are known for their thermal and chemical stability. Over the past two decades, these materials have undergone significant transformations, gaining properties such as malleability, environmental responsiveness, recyclability, crystallinity, and customizable porosity, enabled by the development and integration of dynamic covalent chemistry (DCvC). In this review, we explore the innovative realm of covalent network polymers by focusing on the recent advances achieved through the application of DCvC. We start by examining the history and fundamental principles of DCvC, detailing its inception and core concepts and noting its key role in reversible covalent bond formation. Then the reprocessability of covalent network polymers enabled by DCvC is thoroughly discussed, starting from the significant milestones that marked the evolution of these polymers and progressing to their current trends and applications. The influence of DCvC on the crystallinity of covalent network polymers is then reviewed, covering their bond diversity, synthesis techniques, and functionalities. In the concluding section, we address the current challenges faced in the field of covalent network polymers and speculates on potential future directions.

Dynamic Entwined Topology in Helical Covalent Polymers Dictated by Competing Supramolecular Interactions

Naturally occurring polymeric structures often consist of 1D polymer chains intricately folded and entwined through non-covalent bonds, adopting precise topologies crucial for their functionality. The exploration of crystalline 1D polymers through dynamic covalent chemistry (DCvC) and supramolecular interactions represents a novel approach for developing crystalline polymers. This study shows that sub-angstrom differences in the counter-ion size can lead to various helical covalent polymer (HCP) topologies, including a novel metal-coordination HCP (m-HCP) motif. Single-crystal X-ray diffraction (SCXRD) analysis of HCP-Na revealed that double helical pairs are formed by sodium ions coordinating to spiroborate linkages to form rectangular pores. The double helices are interpenetrated by the unreacted diols coordinating sodium ions. The reticulation of the m-HCP structure was demonstrated by the successful synthesis of HCP-K. Finally, ion-exchange studies were conducted to show the interconversion between HCP structures. This research illustrates how seemingly simple modifications, such as changes in counter-ion size, can significantly influence the polymer topology and determine which supramolecular interactions dominate the crystal lattice.

"Homoleptic" Tetracoordinate Boron Compounds

"Homoleptic" tetracoordinate boron compounds, in which the central boron atom links to four identical atoms, are a special and important family of boron compounds. During the past decades, they have been extensively employed in inorganic, organic, macromolecular, and materials chemistry. Many of them exhibit a diverse range of outstanding properties, and therefore, the synthesis and application of those compounds have emerged as a hot research topic in modern boron chemistry. This review summarizes and discusses the "homoleptic" tetracoordinate boron compounds, which are organized according to the kinds of atoms coordinated to the central boron.