title: Autonomous viscosity oscillation via metallo-supramolecular terpyridine chemistry of branched poly(ethylene glycol) driven by the Belousov-Zhabotinsky reaction.
abstract: Herein, we report an autonomous viscosity oscillation of polymer solutions coupled with the metal-ligand association/dissociation between Ru and terpyridine (tpy), driven by the Belousov-Zhabotinsky (BZ) reaction. The tpy ligand for the Ru catalyst was attached to the terminals of poly(ethylene glycol) (PEG) with different numbers of branches (linear-, tetra-, and octa-PEG). It is well known that mono-tpy coordination is stable when Ru is oxidized (Ru(tpy)(3+)), whereas bis-tpy coordination is stable when the Ru centre is reduced (Ru(tpy)2(2+)). In the oxidized state, the three different polymers existed as solutions. In contrast, when the Ru centre was reduced, gels were obtained for the tetra- and octa-PEG owing to the formation of a three-dimensional polymer network through Ru-tpy coordination. Rheological measurements confirmed that the sol-gel transition occurred much more quickly in the octa-PEG system than in the tetra-PEG system, because of the requirement of fewer crosslinking points. The polymer solutions exhibited self-oscillation of absorbance and viscosity when BZ substrates were added to the solutions of Ru(2+)-tpy-modified tetra-/octa-PEG. This indicated that the Ru(tpy)2(2+) attached to the polymer ends could work as a metal catalyst for the BZ reaction. By increasing the number of branches from 4 to 8, the amount of crosslinking changed more remarkably during the oscillation, with a maximum value closer to that necessary for gelation. Thus, viscosity oscillation with a larger amplitude in the region of higher viscosity was achieved by using octa-PEG.
authors.affiliation: Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. ueki@cross.t.u-tokyo.ac.jp ryo@cross.t.u-tokyo.ac.jp.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Ueki
authors.forename: Yumi
authors.initials: Y
authors.lastname: Takasaki
authors.forename: Kaori
authors.initials: K
authors.lastname: Bundo
authors.forename: Tomonaga
authors.initials: T
authors.lastname: Ueno
authors.forename: Takamasa
authors.initials: T
authors.lastname: Sakai
authors.forename: Yuki
authors.initials: Y
authors.lastname: Akagi
authors.forename: Ryo
authors.initials: R
authors.lastname: Yoshida