Fine Synthesis And Performance Of Cyclic Amphiphilic Random Copolymer Bearing Azobenzene Side Chains

In this work, novel cyclic amphiphilic random copolymers bearing azobenzene at the side chains by combination of atom transfer radical polymerization (ATRP) and CuAAC "Click" chemistry are prepared. Comparing with the corresponding linear copolymers, the properties of the cyclic polymers were thoroughly investigated. The detailed researches are as follows:(1)[(6-(4-benzyloxy)hexyloxy)-(4-methoxyphenylazo)]ethylene (BHME) and t-butyl acrylate (t-BA) were polymerized by ATRP using an alkynyl-ended initiator (PBB) to generate random copolymers of α-alkynyl-ω-azide-linear-P(BHMEm-co-t-BAn) bearing azobenzene at the side chains. The ended bromine at the polymers were converted into azide groups. The subsequent end-to-end intramolecular coupling reaction under high dilution and "Click" conditions led to efficient preparation of cyclic-P(BHMEm-co-t-BAn) of narrow polydistribution-index. The structures of cyclic polymers were verified by various techniques such as gel permeation chromatography (GPC),1HNMR and fourier transform infrared (FT-IR).(2) The linear-P(BHMEm-co-t-BAn) and cyclic-P(BHMEm-co-t-BAn) were selectively hydrolyzed by CF3COOH, through which the linear-and cyclic-P(BHMEm-co-AAn) were obtained. The structures of the copolymers after hydrolysis were ascertained by FT-IR,1H NMR and13C NMR.(3) The Tgs of the linear-and cyclic-P(BHMEm-co-AAn) were measured by differential scanning calorimetry (DSC) and the values of cyclic-polymers were higher than that of coresponsding linear counterparts because of the absence of dangling chain ends. In addition the difference increased more obvious with decreasing size of the cyclic polymers.(4) The photoisomerization of linear-and cyclic-P(BHMEm-co-AAn) in THF were investigated. The results showed that the cyclic-P(BHMEm-co-AAn) displayed higher rate of trans-to-cis photoisomerization and cis-to-trans recovery than the respective linear precursors. The linear-and cyclic-P(BHMEm-co-AAn) could both form spherical aggregates by self-assembling in the mixed solvent of ethanol/THF. The morphologic sizes of the cyclic-polymer were significantly smaller than the linear ones under the same conditions. After irradiation with UV light for different time, the spherical aggregates from both linear-and cyclic-P(BHME10-co-AA9) were gradually disrupted until the trans-to-cis photoisomerization reached the steady state. The irradiated solutions were then placed in the dark at room temperature. Most of the spherical aggregates reformed and reached an initial state. These results showed that amphiphilic cyclic polymers have more promising potential applications than the linear precursors. The size and morphology of aggregates were examined using transmission electron microscopy (TEM) and dynamic light scattering (DLS). The reasons of different performances between linear-and cyclic-P(BHMEm-co-AAn) were further explored.