Cyclic Amphiphilic Polymer Containing Main-Chain Azobenzene: Click Synthesis And Investigation On Its Properties

Due to the lack of chain ends and less flexible as well as smaller conformation entropy of polymer chains, cyclic polymers show many particularly excellent physical performances such as higher glass transition temperature, larger refractive index, smaller hydrodynamic volume, lower viscosity and friction coefficient in comparison with their linear counterparts. Azobenzene is a kind of reversible photoisomerization chromophore, many outstanding properties of cyclic amphiphilic polymers containing azobenzene in solution by self-assembly are observed.In this article, cyclic amphiphilic polymers containing azobenzene in the main-chain were designed and synthesized for the first time. Then, the thermal properties, photoisomerization, self-assembling behaviors and photoresponse performances of the cyclic polymers as well as their relevant mechanisms were investigated compared with the corresponded linear precursors. The major researches were as follows:1. The cyclic amphiphilic polymers(C1, C2) consisting of multiple hydrophobic azobenzene chromophores in the cyclic main-chain tethered with hydrophilic tetraethylene glycol monomethyl ether(TEG) units had been successfully synthesized by Cu(I)-catalyzed azide–alkyne cycloaddition(Cu AAC) intramolecular cyclization in DMF dilute solution from the α-alkyne-ω-azide hetero-difunctional linear precursors(L1, L2) with different molecular weights, which were synthesized by a thermal catalyzed “click” step-growth polymerization in bulk of the α-alkyne-ω-azide A-B type azobenzene monomer. The macromolecular structures of linear and cyclic polymers were fully characterized by gel permeation chromatography(GPC), 1H nuclear magnetic resonance(1H NMR), fourier transform infrared(FT-IR) spectra and matrix-assisted laser desorption ionization time-of-flight(MALDI-TOF) mass spectrometry.2. The thermal properties, photoisomerization, fluorescence emission, self-assembly behaviors as well as photoresponse performances of cyclic polymers were investigated compared with the linear counterparts. The detailed results were as follows:(1) The glass transition temperatures(Tgs) of cyclic and linear polymers were evaluated by differential scanning calorimetry(DSC). The Tg of the cyclic polymers with lower molecular weight was higher than that of the linear ones. Furthermore, these differences of Tgs between cyclic and linear polymers decreased with an increase in molecular weights of the polymers.(2) The trans-cis-trans photoisomerization behaviors of cyclic polymers and linear polymers in THF were investigated by irradiation with UV light and visible light. The trans-cis and cis-trans isomerization of cyclic polymers were slightly faster than those of their linear counterparts.(3) The fluorescence emission and photoresponse performances of cyclic polymers in THF were investigated in contrast with linear analogues. Cyclic polymers showed significantly a fluorescence enhancement than the corresponded linear ones. The fluorescence emission of both cyclic polymers and linear analogues increased by irradiation with 365 nm UV light until a trans-cis photostationary states of azobenzene were reached. Soon after, the fluorescence emission of both cyclic polymers and linear analogues decreased with 435 nm visible light irradiation until a cis-trans photostationary states of azobenzene were reached. Furthermore, the reversible variation of fluorescence intensity of polymers could be observed by alternative irradiation with UV light and visible light, and the photo-switchable fluorescence change could be periodically repeated five times, indicating a good optical switch performance of polymers.(4) The self-assembly behavior of cyclic polymers in THF/H2 O was investigated compared with the linear counterparts, together with the photoresponsive behaviors of self-assembled aggregates. The topography and sizes of the formed aggregates were confirmed by transmission electron microscopy(TEM) and dynamic light scattering(DLS). Both of linear and cyclic polymers in mixed solution of THF/H2 O could self-assemble into spherical aggregates. It was observed that the aggregates sizes of cyclic polymers were significantly smaller than those of corresponded linear ones. The aggregates of both linear and cyclic polymers became smaller or even dissociated by irradiation with 365 nm UV light. However, the topography and sizes of the aggregates could revert to their original states by irradiation with 435 nm visible light. The aggregates of cyclic polymers had slower photoresponsive behaviors relative to the linear ones.