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Synthesis and characterization of porous organic polymers with different organic building units bridged by azo bonds

Porous organic polymers (POPs) are a class of crystalline or amorphous porous solid materials that can be constructed by linking different organic molecular building units with strong covalent bonds. A variety of building units and synthetic routes can be used for the synthesis of POPs, enabling the design of materials with precisely defined properties. The characteristics of POPs, such as high thermal and chemical stability, low density, large surface area, adjustable pore size and structure, make them suitable for a range of applications. One of the most important potential application of POPs is gas storage and separation, especially the capture and separation of CO2, the primary greenhouse gas. Recent studies have revealed that various synthetic approaches and careful selection of building units for the synthesis of POPs can improve interactions between POPs and CO2 molecules and increase the CO2/N2 selectivity. Here, we synthesized a series of new azo-bridged polymers based on benzene, pyridine and triazine central units. Different synthetic methods were used for the preparation of azo-bridged polymers including reductive homocoupling of aromatic nitro monomers, oxidative homocoupling of aromatic amino monomers and heterocoupling of aromatic nitro monomers and various aromatic diamines differing in length and rigidity. The influence of different building units and synthetic approaches on the structural, thermal and porosity properties of resulting materials was investigated by IR spectroscopy, 13C CP/MAS NMR spectroscopy, powder X-ray diffraction, elemental analysis, thermogravimetric analysis, N2 adsorption– desorption experiments and computational methods. The prepared polymers were characterized as amorphous solids of good thermal stability, displaying various Brunauer–Emmett–Teller (BET) surface areas. The obtained results indicated that the building units and synthetic methods have a significant effect on the porosity of the final materials. The highest BET surface areas of 606 and 351 m2 g−1 were observed for the azo-bridged polymers with pyridine and triazine central units, prepared by copper(I)-catalyzed oxidative homocoupling of amino monomer and NaBH4-mediated reductive homocoupling of nitro monomer, respectively. Introduction of linear linkers by using heterocoupling reactions of aromatic nitro monomers and aromatic diamines produced azo- bridged polymers exhibiting very low surface areas. Periodic DFT calculations and grand- canonical Monte Carlo (GCMC) simulations suggested that the introduction of linear linkers does not have a notable effect on CO2 adsorption properties of model azo-bridged polymers.

Porous organic polymers, Azo linkages ; Synthesis, Characterization

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