Covalent organic frameworks structure

Just as interesting as the developments in molecular boron chemistry is the recent progress in solid state, mesoporous, and microporous structures based on boron building blocks. For example, covalent organic framework structures (COFs) are now accessible in high quality and high reproducibility (Figure 8.4). ... 

Severin and coworkers reported (146) the reaction of tris(2-aminoethyl)amine and 4-formylphenylboronic acid with penta-erythritol to give, via multicomponent assembly, the boronic acid based macrobicyclic cage 35 (Fig. 25). The cage has the form of an ellipsoid with a diameter of 20.5 A and binds two Cud) ions in a fashion similar to the smaller tren-based cryptands. The reversible formation of boronic esters has also been employed to build other hollow structures such as nanotubes (147) and porous covalent organic frameworks (148,149). 

In addition to polystyrene, several other polymers have been provided with the hypercrosslinked structure, and, in addition to the postcrosslinking of preformed polymeric chains, other synthetic approaches to hypercrosslinked open networks have been developed, including the direct polymerization and polycondensation of appropriate monomers or co-monomers. The recently developed metal-organic frameworks and covalent organic frameworks constitute three-dimensional coordination and element-organic polymers with an unusually h%h free volume they fit into the new and rapidly growing class of hypercrosslinked network materials as well. 

In comparison to abundant examples of noncovalent supramolecular assemblies,relatively few structures have been built with strong covalent bonds. This is due to the fact that normal covalent bonds are not easily broken and reformed. One can manipulate the temperature and conditions so that the formation of stable imines, esters, disulfides, hydrazones, and boronate esters is reversible. Some examples of these covalent self-assembled systems include disulfide hosts and covalent organic frameworks. ... 

Figure 8.4 Covalent organic framework (COF) structure based on a condensation process followed by boroxine monomer extraction to give a 3D boroxine framework. (Reproduced from ref. 18 with the permission of Elsevier.)...

The separation factor of these organic polymer membranes is typically located in a moderate range, of around 5 and 10, but rarely higher than 20. As a rule of thumb and proven by recent publications, the membrane selectivity can be approximated as the product of the adsorption selectivity and diffusion selectivity [2]. This chapter provides a wealth of information on diffusion inside micro- and mesoporous structures using concepts and ideas that originate from Maxwell and Stefan. A molecular-level understanding of diffusion in a variety of materials such as zeolites, MOFs, covalent organic frameworks (COFs), carbon nanotubes, and cylindrical silica pores is provided with the aid of extensive data sets of molecular... 

In Chapter 6, B. M. Rombo et al. delve into the formation of boronate-linked supramolecular architectures based on boronate ester formation—for example, small molecule diesters form supramolecular self-assemblies in the solid state based on a phenyl-boron-phenyl sandwich motif in which these small oligomers link together to generate macrocycles and other polymers. The polymeric macrocyclics and linear structures demonstrate self-repair capabilities and constitute a new class of wide band-gap semiconducting materials. Through the incorporation of polyvalent boronates, covalent organic frameworks are described, which create highly crystalline, porous network materials. 

More stable nanoporous materials have been constructed by making use of recognition and self-assembly of H-bonded block copolymers. The H-bonds do not directly induce porosity, which is in contrast to the H-bonded organic frameworks. A well-defined nanoporous structure is created after breaking the H-bonds and the release of a template molecule. A great advantage is that the H-bonds can be broken under mild conditions, while for covalently bonded block copolymers harsh... 

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