- Stimuli-responsive polymers based on calixarenes and crown ethers
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Stimuli-responsive polymers based on calixarenes and crown ethers
Poly(azocalixarenes) with varying length of their lower rim substituents (C4 (n-butyl), C12(n-dodecyl), and TEG (tetraethylene glycol)) have been synthesized and reported, see Fig. 1 and Refs [1-3]. Reversible photo-isomerisation of these polymers from their predominantly trans to cis conformation is possible (Figs. 2 and 3) and referenced to low molar mass azobenzene analogues. Formation of a dynamic intermolecular complex between the polymers and a low molar mass pyridinium-based guest has been studied before and after the irradiation, see Fig. 3. The polymer chain conformation has been shown to affect the binding and release of pyridinium derivatives. Irradiation induces slight changes in the cone conformation of the calixarene units. This in turn affects the degree of host-guest interaction, which is decreased when the polymers are in the cis-rich state. The complexation can thus be controlled by adjusting the photostationary state of the poly(azocalixarenes). Experiments with low molar mass model compounds show that the complex formation between the pyridinium guest and azocalixarene host is possible when the latter ones are polymeric. Model di-azocalixarenes do not form similar complexes. Hence, our experiments show that the combination of the known properties of calixarenes and those of photo-switchable azo-bridges with a polymeric structure leads to the construction of templates for hosting of low molar mass guests, with a tunable release mechanism.
Polymers consisting of azocalixarenes in the main chain and tetraethyleneglycol monomethyl ether chains in the lower rim of the calixarene units undergo reversible photo-isomerization between the trans and the cis forms, the extent of which depends on the solvent. A lower critical solution temperature (LCST) type behavior is observed for aqueous solutions of the polymers, which is strongly affected by the molar mass and concentration. More importantly, the same polymers exhibit an upper critical solution temperature (UCST) type transition in alcohols. It is shown that the temperature of the phase transition in alcohols decreases proportionally to the decrease in the trans content of the samples thus offering a unique possibility to reversibly tune the UCST behavior by adjusting the irradiation exposure time. An exciting photo-assisted writing on solutions of the polymer in alcohols is demonstrated, see video.
Video: Photo-assisted writing application on ethanol solution of tegylated poly(azocalixarene)s.
We thank the Academy of Finland (project numbers 127329 and 260486) and Oskar Öflunds Foundation for financial support.
 Novel Photo-Switchable Polymers Based on Calixarenes, Wiktorowicz, S.; Aseyev, V.; Tenhu, H. Polym. Chem. 2012, 3, 1126-1129; DOI: 10.1039/c2py20020a.
 Influence of Photo-Isomerisation on Host-Guest Interaction in Poly(azocalixarenes), Wiktorowicz, S.; Aseyev, V.; Tenhu, H. Polym. Chem. 2013, DOI:10.1039/C3PY00194F.
 Using Light To Tune Thermo-Responsive Behavior and Host–Guest Interactions in Tegylated Poly(azocalixarene)s, Wiktorowicz, S.; Tenhu, H.; Aseyev, V. Macromolecules 2013, 46, 6209–6216; DOI: 10.1021/ma4011457.
Writing With Light In A Polymer Mixture, Sanderson, K. Chemical & Engineering News, ACS; ISSN 0009-2347. Web Date: August 2, 2013: http://cen.acs.org/articles/91/web/2013/08/Writing-Light-Polymer-Mixture.html