A study on the preparation of organogel based functional polymers

Abstract

An organogel generally has a three dimensional network structures caused by physical or chemical cross-linking and contains a large amount of organic solvent molecules. A physically cross-linked network structure is produced by the self-assembly of small molecules promotes whereas a chemically cross-linked network structure forms by the polymerization of bi-, tri- or multifunctional monomers or the cross-linking reaction between polymer chains. In this study, functional polymers were prepared using reactive organogels and their properties and application possibilities were investigated. Firstly, thermochromic polymer nanocomposite films were prepared from polymerizable organogels. Organogelator 1 had a structure within which 3,4,5-tris(ω-decenyl)benzamide groups were attached to a quarterthiophene core through amide bonds. Organogelator 2 had the same structure except that tris(ω-decenyl) groups were replaced by tris(n-decanyl) groups. The PMMA nanocomposite films were prepared by the photopolymerization of the organogels formed in MMA. The film containing 1 (0.5 wt%) showed reversible thermochromism. The emission under 365 nm irradiation was changed from orange to bright green by heating up to 120 oC and returned to its initial orange by cooling. To the contrary, the PMMA composite film prepared from the organogel of 2 (0.5 wt%) didnt show a reversible thermochromic property. Organogelator 1 with polymerizable terminal vinyl groups was covalently embedded in the PMMA matrix, but 2 didnt. The reversible thermochromism was likely caused by the thermally reversible conformational change of quarterthiophene units in the polymer fibers. Secondly, a chemical gel prepared from the Sonogashira-Hagihara reaction between 1,4-diiodobenzene and 1,3,5-triethynylbenzene was used to preparing a compressible and monolithic hierarchical porous polymer (HM). The polymers with an acid (HM-A) and base functionality (HM-B) were prepared by sulfonation of HM and by an additional Sonogashira-Hagihara reaction with a monomer bearing an amine group in the presence of HM, respectively. HM-A having sulfonic acid group and HM-B with amine group also showed compressibility, monolithic properties, and hierarchically porous structures. They were cut and fitted into a syringe sequentially and the syringe was used as a semi-continuous flow reactor. The acid catalyzed deacetalization reaction of benzaldehyde dimethyl acetal and base catalyzed Knoevenagel condensation reaction between benzaldehyde and maloronitrile were carried out in the semi-continuous flow reactor. The tandem reaction in the semi-continuous flow reactor required less solvent and time than in a conventional batch type reactor. The reactor could be recycled several times without a significant decrease in the reaction efficiency. Thirdly, a chemical gel was formed with a commercial polyurethane sponge. A compressible and hierarchically porous polymer composite (PUS-MOP-A) was prepared by Sonogashira-Hagihara coupling reaction of 1,3,5-triethynylbenzene, 1.4-diiodobenzene and 2,5-diiodobenzoic acid in a polyurethane sponge (PUS). 2,5-Diiodobenzoic acid was used as a co-monomer to provide acidic functionality to the pore surface. The microporous organic polymer (MOP-A) formed inside the PUS network showed fibrous morphology when 1,4-diiodobenzene was used as a major aryl halide. For the synthesis of PUS-MOP-A, the molar ratio between 1,4-diiodobenzene and 2,5-diiodobenzoic acid was chosen as 4:1. The Brunauer-Emmett-Teller (BET) surface area of PUS-MOP-A was 306 m2g-1. PUS-MOP-A was treated with KOH, which converted the carboxyl groups on the MOP-A backbone to the carboxylate anions. The resulting polymer composite (PUS-MOP-Aa) absorbed water quickly, showing a water contact angle of 0o. PUS-MOP-Aa to remove chemical pollutants in an aqueous solution was studied using a cationic dye, Methylene Blue (MB) and an anionic dye, Methylene Orange (MO) as a model chemical. PUS-MOP-Aa could be manually compressed and released in an aqueous solution of MB, resulting in the fast dye removal. When an aqueous solution contained both the anionic and the cationic dye, PUS-MOP-Aa preferentially removed the cationic dye. PUS-MOP-Aa was recyclable after removing the absorbed dyes by treating with an acid and washing. Lastly, a compressible heterogeneous catalyst containing a Pd nanoparticles encapsulated microporous polymer (S-M-Pd) that sharing the same hierarchical porous polymer (PUS-MOP-A) was studied. Pd catalysts, used for the Sonogashira-Hagihara reaction, were exploited as the precursors for the generation of Pd NPs. 2,5-Diiodobenzoic acid was introduced to adjust wettability to aqueous environment. S-M-Pd has a hierarchical pore structure and Brunauer-Emmett-Teller (BET) surface area of 270 m2g-1. It showed good mechanical stability against compressive stress. S-M-Pd was successfully used for the 4-nitrophenol reduction reaction and the Suzuki-Miyaura coupling reaction. A compression and release of the S-M-Pd in the reaction mixture allowed the reactants to access the catalyst more easily. The 4-nitrophenol reduction reaction with repeated compression and release was 6 times faster than in static conditions. The cylindrical S-M-Pd was fitted into the syringe, and was used as a semi-continuous flow reactor for the methylene blue reduction. The reactor showed no undesirable leakage and was used for several successive reactions without further purification.