Thermoresponsive Oligo(Ethylene Glycols)-based Dynamic Dendritic Polymers And Cyclodextrin Derivatives

Thermoresponsive polymers are among one of the most important stimuli-responsive polymers, and have received considerable interest for both fundamental researches and promising applications. It remains an important challenge to develop novel thermoresponsive polymers with easy structural modification and tunable properties. In this thesis, several series of dynamic oligo(ethylene glycols)(OEG)-based thermoresponsive dendritic polymers with tunable phase transition temperatures were efficiently fabricated by using either cyclodextrin-based host-guest interaction or dynamic covalent bond. In addition, mono-dispersed OEG-modified cyclodextrins were synthesized efficiently through one-pot reactions, which show unprecedented thermoresponsive behavior. The details are described as follows:1. Thermoresponsive supramolecular dendritic polymersCombination of the concepts of supramolecular polymers and dendronized polymers affords a novel class of side-chain supramolecular dendronized polymethacrylates. These polymers were prepared via host-guest interaction from a linear polymer carrying β-cyclodextrin (β-CD) moieties in each repeat unit (the host) and adamantyl-cored first-and second-generation oligoethylene glycol (OEG) dendrons (the guests)(Figure A). The host and guest interaction in aqueous solution led to the formation of the supramolecular polymers, which was supported by*H NMR spectroscopy,1H NMR titration and dynamic light scattering measurements. The association constants between all dendritic guests and β-CD were determined by ITC measurements. These supramolecular dendronized polymers show good solubility in water at room temperature, but exhibit thermoresponsive behavior at elevated temperatures. Their thermoresponsiveness is thus investigated with UV/Vis and1H NMR spectroscopy. The effect of dendron structures (generation and peripheral groups), hydrophilic cyclodextrin moieties, polymer concentration and molar ratio of host/guest was all examined. Based on the dynamic nature of the supramolecular polymers, supramolecular dendronized copolymers were further constructed from the cyclodextrin polymer and a mixture of methoxyl-and ethoxyl-terminated second generation dendritic guests. Their phase transition temperatures can be tuned continuously by varying the ratios of these dendritic guests of different hydrophilicities. Based on the temperature-varied proton NMR spectra, it was found that the host-guest complex starts to decompose during the aggregation process upon heating to its dehydration temperature, and this decomposition was coordinated with the hydrophilicity of the dendritic guests.Figure A. Illustration of side-chain supramolecular dendronized polymers prepared through host-guest inclusion and their collapse in water.Supramolecular dendrimers are also constructed from the cyclodextrin trimer and the OEG dendritic guests through the similar concept. Their phase transition temperatures were dependent on the generation or periphery groups of dendrons, hydrophilic CD moieties and solution concentration, which is quite different from the supramolecular polymers described above.2. Thermoresponsive cyclodextrins with switchable inclusion abilitiesA series of monodispersed α-,β-, and γ-CDs grafted with OEG units were synthesized via one-step reaction. These CD derivatives show unprecedented thermoresponsive behavior, and their phase transition temperatures (LCSTs) can be tuned by varying the OEG chain lengths and the ring size of CDs. Based on the*H NMR, UV/vis and circular dichroism spectra, these CD derivatives inherited inclusion ability from their parent counterparts to dyes below their LCSTs, and the association constants are dependent on the cavity size of CDs and the grafted OEG chains. Interestingly, inclusion of dye guests can be switched on or off reversibly by changing temperature below or above LCST. This unique inclusion behavior was exploited to fabricate CDs-based supramolecular colorimetric temperature sensor (Figure B).Figure B. Schematic drawing of thermoresponsive behavior of OEG-modified cyclodextrins and their switchable inclusion of dyes. 3. Thermoresponsive dynamic OEG-modified poly-L-lysinesA novel class of thermoresponsive poly-L-lysines was prepared via dynamic imine bond from poly-L-lysine hydrochloride and three-fold branched OEG-modified benzaldehyde. Its structure, thermoresponsive behavior and conformation are characterized by1H NMR, UV/vis and circular dichroism spectroscopy. Based on the dynamic nature of imine bond, the phase transition temperature of the OEG-modified poly-L-lysines can be easily tuned by varying pH values or the molar ratios of benzaldehyde and amine.