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Recently, synthesis of microporous covalent organic polymers has advanced very rapidly, partly due to the urgent need for low-density porous materials for hydrogen and methane storage. Such polymers, consisting solely of light elements (H, B, C, N, O, etc.), can be constructed by a wide variety of standard synthetic strategies in organic chemistry. In extreme cases, covalent organic frameworks (COFs) with crystalline structures and high surface area can be constructed under thermodynamic control. The COFs are obtained by carefully selecting the building blocks and the conditions of their synthesis, which is carried out in a closed system to ensure reversibility of the condensation reactions;the choice of solvent appears to be very important to slow down the reversible process. In general, however, solidification of organic polymer chains results in dense,nonporous solids because such highly flexible and translationally and conformationally mobile macromolecules tend to minimize their own void volume by efficiently packing in the solid state. In addition, such polymers are mostly formed under kinetic control and irreversible conditions;hence, the formed bonds hardly rearrange into the ideal positions to achieve highly porous and ordered structures. Consequently,molecules with high rigidity and symmetry, nonplanarity,and high connectivity have been considered as suitable monomers that can frustrate the packing of the resulting polymers, and thus afford loosely packed, porous solids. Polymers of intrinsic microporosity (PIMs) formed by means of dioxane-forming reactions exhibit apparent Brunauer–Emmet–Teller (BET) specific surface areas of 500–1000 m2 g1 depending on the structure of the monomers.Hyper-cross-linked polymers are another example of porous organic polymers that can display permanent porosity because of extensive chemical cross-links that prevent the polymer chains collapsing into dense solids.The simplest networks constructed from styrenic monomers have a wide range of BET surface areas (100–2000 m2g1) that depend mainly on the degree of cross-linking and the monomers. Amorphous conjugated microporous polymers can be synthesizedwith BET surface areas in the range 500–1200 m2 g1 by means of Pd0/CuI-catalyzed Sonogashira cross-coupling. Micropore distributions of these conjugated poly(arylene ethynylene) materials could be controlled by means of the rigid node–strut topology, in particular by the average strut length,[19a,b] and this implies that order and crystallinity are not prerequisites for precise control over porosity of materials. These previous reports strongly suggest that selection of the right monomer is a critical factorto achieve synthetic porous organic polymers with high surface area and narrow pore size distribution. |
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