Study On The Novel Synthetic Method Of Aromatic Azo-Containing Polymers And Their Properties

Polymers containing aromatic azobenzene and its derivatives(Azo-containing polymers) have drawn considerable attention owing to their outstanding mechanical, unique physicochemical properties, and the well-known reversible trans-cis photoisomerization behavior. Azo-containing polymers have diversified potential applications in photoresponsive and optoelectronic fields, including reversible optical information storage media, optical switches, liquid crystal displays, nonlinear optical materials, light-driven molecular motores and so on. In recent years, the design and synthesis of the Azo-containing polymers with novel structures and functionalities have attracted increasing attentions. Therefore, developing a facile and effective synthetic method of preparing Azo-containing polymers is one of the important research subjects in polymer chemistry and functional materials field.In this thesis, we have developed a novel and effective method for synthesizing a series of Azo-containing polymers with different topological structures. And their photoisomerization behaviors were also investigated in detail. The detailed results were summarized as following:(1) A novel approach for preparing the main-chain Azo-containing polymers directly from bisnitroaromatic compounds photocatalyzed by Zr O2-supported gold nanoparticles(Au NPs) under mild conditions were reported for the first time. First, we employed two kinds of nitronitroaromatic compounds, 2-nitro-9,9-dioctyl-9H-fluorene(NOFL) and 1-(hexyloxy)-4-nitrobenzene(HONB), as the model compounds to confirm the catalytic activity of Au NPs under Xe lamp irradiation and investigate the optimized experimental conditions. After that, three novel bisnitroaromatic compounds, 2,7-dinitro-9,9-dioctyl-9Hfluorene(monomers A), 3,6-dinitro-9-(pentacosan-13-yl)-9H-carbazole(monomers B) and 1,12-bis(2-(dodecyloxy)-4-nitrophenoxy)dodecane(monomers C), were successfully synthesized and characterized. Then the corresponding Azo-containing polymers were prepared via the reduction condensation of monomers A, B and C, respectively. Interestingly, the reaction intermediate(azoxy units) was observed in the early statge, which then converted into azo units completely. The final Azo-containing polymers obtained do not contain azoxy units. The obtained polymers at different reaction time were characterized by 1H NMR, FT-IR spectra, UV-vis spectra, gel permeation chromatography(GPC) and MALDI-TOF MS techniques, respectively. It has not only revealed the reaction mechanism of the photo-catalytic reduction of nitronitroaromatic compound, but also achieved the recycling of the catalyst.(2) Based on the Au NPs-catalyzed reduction condensation reaction in Part I, we developed one efficient method for preparing hyperbranched Azo-containing polymers and studied the visible light-driven photoismerization of these polymers. First, two trinitrofunctionalized monomers, 2,7,12-trinitro-5,5,10,10,15,15-hexaoctyl-10,15-dihydro-5Hdiindeno[1,2-a:1’,2’-c]fluorene(TNF) and 1,3,5-tris(4-(4-(7-nitro-9,9-dioctyl-9H-fluoren-2-yl)phenoxy)butoxy)benzene(TOF), were synthesized. Then the trinitro-functionalized monomers were polymerized via a Au NPs photocatalytic process. Meanwhile the mononitro compound, 2-bromo-7-nitro-9,9-dioctyl-9H-fluorene(NOF), was introduced as end-capping reagent into the polymerization system in order to get the soluble hyperbranched Azocontaining polymers. The obtained polymers were characterized by GPC, 1H NMR, FT-IR and UV-vis spectra. Furthermore, the branching factors of the above polymers were calculated to be less than 0.25 by using the TD-GPC. The reversible visible-light-driven photoisomerization of PTOF and PTNF were also investigated in tetrahydrofuran solution. The isomerization rate constants(ke) of PTOF and PTNF were 0.165 s-1 and 0.062 s-1, respectively. These novel hyperbranched Azo-containing polymers have potential application in the visible light responsive materials field.(3) A series of monodisperse fluorene-azo oligomers(OFLn A(n = 1, 2, 3 and 4)) were precisely synthesized via the Au NPs photocatalytic process. All the obtained oligomers were well characterized by GPC, 1H NMR, FT-IR, UV-vis and MALDI-TOF MS spectra. The results demonstrates that the absorption maxima of OFLn A showed obvious red-shift from 395 nm(OFL1A) to 425 nm OFL2A), 454 nm(OFL3A) and 466 nm(OFL4A) respectively. Moreover, both the photoisomerization efficiency and rate constants(ke) decreased progressively with the increasing π-conjugated length. The ke value for tans to cis process was 0.256 s-1, 0.132 s-1, 0.116 s-1 and 0.007 s-1 for OFL1 A, OFL2 A, OFL3 A and OFL4 A respectively. Utilizing the density functional theory(DFT) computation approach, the absorption maxima of OFLn A also shows obvious red-shift from 345 nm(OFL1A) to 378 nm(OFL2A), 394 nm(OFL3A) and 403 nm(OFL4A) respectively, which agrees well with the experimental ones. DFT computation also revealed that only one end azo unit undergo photoisomerization under irradiation from OFL1 A to OFL4 A, demonstrating a Trans-[transtrans]n-cis model, revealing the inner cause for unobvious photoisomerization behavior of π-conjugated aromatic azobenzene oligomers. The current results will provide another basis guide for designing photosensitive materials from π-conjugated aromatic azo main-chain polymers.(4) Multiple response cyclic Azo-containing polymers(cyclic-PEG-Azo) was firstly designed and perepared via the efficient photocatalytic process in one-pot, which used the nitrocompound(PEG1K-NO2 and PEG2K-NO2) as the precursors. And the linear Azocontaining polymers(linear-PEG-Azo) were also successfully synthesized. The structures of linear and cyclic functional polymers were both characterized by GPC, 1H NMR, FT-IR, UV-vis and MALDI-TOF MS spectra. Furthermore the photoresponsive behaviors of cyclic-PEG-Azo were similar to that of linear-PEG-Azo. The trans to cis photoisomerization rate constants(ke) of cyclic- and linear-PEG-Azo were about 0.057 s-1. The results shows that the cyclic-PEG-Azo has higher cloud point(Tc) than the linear precursor, and the trans-cis photoisomerization also can effectively increase the Tc of the corresponding polymers.