Isothermal Reversible Switch Of Transmittance Of A Temperature-responsive Homopolymer Solution On The Basis Of Photocontrolled Selective Complexation Of Cyclodextrin

Host-guest inclusion complexation is a simple method of tuning cloud point ofthermosensitive polymers. For example, cyclodextrins (CD) with suitable size caninclude the side group of thermosensitive polymer and therefore change thehydrophilicity of the side group, which causes the increase of the cloud point ofpolymer solution. The inclusion complexation of side group of polymer withcyclodextrin is a fast exchange equilibrium. In the thermosensitive polymer solution,the higher the concentration of CD is, the more inclusion complex is formed, whichcauses a larger increase of the cloud point.Photocontrolled supramolecular system based on CD and azo compounds is a hotresearch topic in supramolecular chemistry field.4-(Phenylazo)benzylpyridiniumbromide (Azo-Py~+) is a water-soluble derivative of azobenzene, trans and cis formsof which can be reversibly interconverted upon light irradiation. Trans-Azo-Py~+canbe well-recognized by α-CD. However, when trans-Azo-Py~+is transformed tocis-Azo-Py~+, α-CD cannot include the cis isomer any more because of the mismatchbetween the host and guest.In this work, poly(ethoxypropylacrylamide)(PEPAm) was studied. According toour experience, α-CD could form supramolecular complex with side chain of PEPAm,which probably causes the increase of the cloud point of PEPAm solution. The twosupramolecular systems of PEPAm/α-CD and Azo-Py~+/α-CD were combined. InPEPAm/α-CD/Azo-Py~+solution, photocontrolled inclusion and exclusion of Azo-Py~+with α-CD can be used to change the concentration of free α-CD, which can causereversible changes of the cloud point of PEPAm solution.In the second chapter, we synthesized PEPAm by radical polymerization, andinvestigated effects of different sizes of CD on cloud point of PEPAm solution. α-CDshowed a significant effect on cloud point of PEPAm whereas β-CD and γ-CD hadalmost no effect. The inclusion complexation between α-CD and the side group of PEPAm was confirmed by the NOESY NMR spectrum. The inclusion stoichiometryof α-CD and PEPAm equals1, which was determined with Job’s method. Anassociation constant of6.5±1.1L/mol was obtained by NMR titration. Finally, thecloud point of PEPAm increased with an increase in the molar concentration of α-CD.The relationship between cloud point and the concentration of α-CD can be describedwith an equation based on inclusion complexation equilibrium.In the third chapter, Azo-Py~+is added to PEPAm/α-CD solution. Most of α-CDformed inclusion complexes with Azo-Py~+, because the association constant ofAzo-Py~+and α-CD is much larger than that of PEPAm and α-CD. The cloud point ofthe solution decreased owing to the decrease in the concentration of free α-CD. Astrans-Azo-Py~+was isomerized to cis-Azo-Py~+upon irradiation with UV light, theconcentration of free α-CD was increased because the association constant ofcis-Azo-Py~+and α-CD is less than that of trans-Azo-Py~+and α-CD. Consequently, thecloud point of PEPAm solution was increased. Succeeding irradiation with visiblelight led to reverse conversion of Azo-Py~+configuration. The cloud point wasdecreased owing to the decrease in the concentration of free α-CD. Furthermore, thetransmittance of the solution can be changed reversibly for at least5cycles withalternate irradiations of UV and visible light at38°C.