Preparation And Photoresponsive Behavior Of Novel Cyclic Azobenzene Polymers

Azobenzene polymers combine the unique optical trans-cis-trans isomerization behavior of azobenzene and the good processability and mechanical properties of polymer materials. These azobenzene polymers can be potentially applied in fascinating photo-responsive variations, such as liquid crystal displays, optical data storage, nonlinear optical materials and so on. Cyclic polymers have attracted more and more attentions in recent years due to their unique topological structures and characteristic properties in both solution and bulk state.Therefore, develop and design of new cyclic azobenzene polymers with the novelty structure and excellent optical activity has been an important assignment for polymer chemistry.In this thesis, we designed and synthesized two kinds of azobenzene polymers with the novel structures. Their trans-cis isomerization and so caused properties change were investigated. The detailed researches were summarized as the following:(1) Anα-azide,ω-alkyne A-B type azobenzene monomer, 3'-ethynylphenyl- [4-hexyl-(2-azido-2-methyl-propionate)phenyl]azobenzene (EHPA), was synthesized and used to generate PEHPA precursor(linear-PEHPA) via step-growth polymerization by thermal 1,3-dipolar cycloaddition in bulk. The subsequent end-to-end intramolecular coupling reaction under high dilution and"click"conditions leads to efficient preparation of cyclic-PEHPA. Gel permeation chromatography (GPC), 1H-NMR and Fourier transform infrared (FT-IR) spectra all confirmed the complete transformation of linear-PEHPA to cyclic-PEHPA.Because of the absent chain ends, the cyclic-PEHPAs showed higher glass transition temperatures than the linear-PEHPAs with the same molecular weight. But this difference increased with decreasing molecular weight of the polymers. The decomposition temperatures (Tds) of cyclic-PEHPAs and the corresponding linear-PEHPAs were almost the same, and thermally stable up to 320oC under nitrogen atmosphere. However, at higher temperatures, the decomposition of the cyclic ones proceeds more slowly than the linear ones. Furthermore, the photoisomerization of trans–cis and cis–trans recovery of the linear-PEHPAs and cyclic-PEHPAs in DMF were investigated by irradiation with UV light and visible light, respectively. Comparing with the precursor linear-PEHPAs, the cyclic-PEHPAs have shown a little faster trans-cis-trans photoisomerization ability. These intriguing results obtained from photoisomerization of cyclic main-chain azobenzene polymers are worthy of further theoretical considerations.(2) We report on the preparation of well-defined cyclic azobenzene poly(N-isopropylacrylamide) (cyclic-azo-PNIPAM) via ATRF and click chemistry. A novel ATRP initiator bearing with azobenzene and alkyne groups, 2-Bromo-2-methyl-propionic acid 4-(4-prop-2-ynyloxy-phenylazo)-phenyl ester (BMPAPE), was synthesized and successfully used as the ATRP initiator to initiate the polymerization of N-isopropylacrylamide (NIPAM). Then linear-azo-PNIPAM-Br reacted with NaN3 to transform the terminal chloride into azide group to get linear-azo-PNIMA-N3. The subsequent end-to-end intramolecular coupling reaction under high dilution and"click"conditions leaded to efficient preparation of cyclic-azo-PNIPAM. GPC, 1H-NMR and FT-IR spectra all confirmed the complete transformation of linear-azo-PNIPAM-N3 to cyclic-azo-PNIPAM. Comparing with the linear-azo-PNIMA-N3 with the same molecular weight, the cyclic-azo-PNIPAM have shown a little faster trans-cis photoisomerization ability.