Chapter 1: Introduction

Porous materials with the advantages of high surface area, unique channels, and outstanding adsorption performance are essential for industry and daily life.^1,2  It has always been the quest of chemists and material scientists to precisely control materials at the molecular level, and thus tailor-make materials for targeted properties and functions. The last 25 years have witnessed the emergence and rapid development of reticular chemistry, the study of linking molecular building blocks via strong bonds to construct extended crystalline structures, for example, metal–organic frameworks (MOFs) and covalent organic frameworks (COFs), which offers an important platform for developing advanced functional porous materials.^3,4 

Employing the principle of reticular chemistry to design and construct frameworks with targeted topology is termed “reticular synthesis”, which often starts with rigid and directional inorganic and/or organic building units that are shape persistent during the reticulation process.⁵  The formed strong bonds ensure the permanent porosity as well as the chemical and thermal stability of the crystalline networks. In this way, judicious selection of the building units together with the appropriate reaction conditions would allow for rationally structuring frameworks. In addition, the molecular building units can be substituted with ones possessing the same linking type and varied length and functionality without changing the underlying topology, and this isoreticular (IR) principle largely enriches the reticular materials.⁶  Last but not least, due to their open frameworks with high porosity, it is possible to functionalize the skeletons post-synthetically, achieving accurate adjustment on the size, shape, and chemical environment of the pores.^7–9 

Reticular chemistry is first exemplified by MOFs, in which the organic linkers and metal ions or clusters (also known as secondary building units, SBUs) can be assembled via strong bonds in a designed manner.^10–15  This is different from early coordination polymers that are generally constructed with weak bonds between single-metal nodes and neutral linkers showing limited structural stability.¹⁶  SBUs in MOFs are metal-containing units generally ranging from single metal ions to polynuclear metal clusters. Due to their rigidity and directionality, the SBU approach plays a vital role in the predesign of the structures as well as the maintenance of the pore structures in MOFs.¹⁷  The SBUs are further reticulated with polytopic organic linkers, for example, carboxylates, leading to periodic open frameworks. Due to the flexible combination of the components, more than 100 000 structures have been developed and studied. The surface areas of MOFs are mostly in the range of 1000 to 10 000 m² g⁻¹. All these unique traits make MOFs appealing for applications including gas storage and separation, catalysis, sensing, energy storage, etc.

Under the guidance of reticular chemistry, organic building units can be precisely arranged into periodically extended frameworks linked by covalent bonds, giving the second type of reticular material, COFs.^18–21  The discovery and advances of COFs have tackled the challenge of constructing 2D and 3D covalently linked extended organic solids. Since their first report in 2005, 2D and 3D COFs with diverse structures, linkages, and pore metrics have been developed.²²  To afford such periodic networks, the linkage types and reaction conditions should be carefully identified to allow for thermal dynamic control and thus the “error-checking” process.^23,24  The linkage chemistry has expanded to over 20 reactions, and typical linkage types include boroxine, boronate eater, amide, imine, etc. Organic building units ranging from rigid molecules to flexible units have been utilized for COF construction, such as arenes, coordinated strings, and large macrocycles. These highly ordered organic materials feature low density, large pore size, high surface area, and good thermal stability, showing great potential in the fields of gas storage, optoelectronics, catalysis, electronic devices, and so on.

Although MOFs and COFs exhibit great potential in a wide variety of fields, there are drawbacks limiting their full potential and further industrial applications. For example, some reticular materials show limited stability and cannot withstand the harsh conditions of practical working scenarios. In addition, these crystalline materials are mostly obtained in the form of powder that is insoluble or non-meltable, hindering further shaping into desired forms. To achieve optimal performance and satisfy the requirements from various application areas, it is of significance to introduce new features and functions into reticular materials. Controllable integration of MOFs and COFs with other functional materials offers a powerful way to endow them with special morphology, properties, and functionalities.^25–27  The rationally constructed composites or hybrids could combine the merits of the components, and even show new traits and superior performance as a result of collective and/or synergistic effects. Considerable efforts have been made to hybridize MOFs and COFs with various functional materials (e.g. polymers, metal nanoparticles, metal oxide, carbon materials, biomolecules) in sophisticated structures, and these hybrids are promising for broad applications including catalysis, separation, energy storage and conversion, biomedicine, etc.

Reticular material–polymer hybrids in particular attract extensive attention.²⁸  In contrast to the crystalline and rigid properties of reticular materials, polymers possess characteristics of flexibility, resilience, and good processability. Their integration would preserve the attributes of both parts, and the resulting sophisticated hybrids may show novel properties due to the special host–guest interactions. One of the most important benefits is that MOFs or COFs could be produced in shaped forms such as films and pellets, with the aid of polymers, which largely expands their potential application situations.^29–34  Furthermore, this hybridization strategy has brought new insights for both reticular and polymer chemistry. Efforts have been made to understand their interactions, and the resulting influences on the micro- and macro-architectures, assembly process, and properties.^35,36  With the rapid advancement in this field, reticular material–polymer hybrids have shown great potential for separation technology, sensing, biomedicine, energy storage, and conversion.

Owing to the significance of reticular material–polymer hybrids and the rapid development in this field, a book offering a comprehensive and useful source of information and progress on this topic is highly desired. With critical contributions from experts working in this field, this book contains eight chapters and starts with a brief background and overview of the theme. In Chapter 2, Seth M. Cohen and his colleague review the synthesis, properties, and characterization strategies of polyMOFs, a distinct class of MOF–polymer hybrid materials. In the following chapter, Takashi Uemura et al. discuss the progress regarding the polymer@MOF with focus on the synthesis, recognition, and hybridization methods. Jianrong Li et al., in Chapter 4, provide a comprehensive review of the synthetic approaches applied for the hybridization of MOFs and polymers based on their respective MOF/polymer interactions. The progress in MOF/polymer composite membranes, as well as their separation applications, is described by Yanshuo Li and colleagues in Chapter 5. Bo Wang et al. then summarize the applications of MOF–polymer hybrids in electrochemistry, toxic chemical protection, and biomedicine. In Chapter 7, Xiao Feng et al. present an elaborate review of COF chemistry including design concepts, synthetic strategies, and characterization methods, as well as the applications of COFs. In the last chapter, Wei Zhang and colleagues highlight the recent development of COF–linear polymer composites, with an emphasis on their synthetic approaches and emerging applications.