Stimuli-responsive Dynamic Covalent Dendritic Polymers

Dendritic polymers including dendrimers, hyperbranhced polymers and dendronized polymers are highly branched macromolecules with three-dimensional architecture and complex topological features. They exhibit very different properties compared to their linear analogues. Dynamic covalent chemistry(DCC) is an efficient tool for constructing polymers with dynamic and tunable properties. Herein, a series of dynamic covalent dendritic polymers carrying three-fold branched oligoethylene glycol(OEG) dendrons in the pendent groups were prepared through the dynamic covalent reactions. They show characteristic thermoresponsive behavior and their LCSTs(Lower critical solution temperature) could be tuned by either structure characteristics or by DCC. Moreover, the application of these dendritic polymers in to the fields of ion recognition, drug delivery, as well as self-healing hydrogel were also investigated. The details are described as followings: 1. Dynamic acylhydrazone dendrimersThermoresponsive dynamic covalent dendrimers containing acylhydrazone bond were efficiently synthesized. First generation(G1) dendronized acylhydrazones with methoxy or ethoxy terminal substituents and second generation(G2) acylhydrazone dendrimers with ethoxy terminal substituents were formed through the dynamic covalent reactions between aldehyde and acylhydrazine. The formation of acylhydrazone in aqueous solutions with different p H were investigated by 1H NMR. Their phase transitions induced by temperature were followed by turbidity measurements using UV/vis spectroscopy and their phase transiton temperatures were determined. All dendrimers show thermoresponsive properties, and their phase transiton temperatures can be varied by changing the structures and adding competitive hydrazine with different hydrophilicities. The selectivity of acylhydrazone dendrimers toward a series of metal ions were further investigated with the UV/vis spectroscopy and fluorescence spectroscopy. G1 acylhydrazone dendrimers displayed high selectivity to Cu2+ in organic solvents with fluorescence enhancement. 2. Thermoresponsive dynamic covalent dendronized polymersA series of novel thermoresponsive dynamic covalent dendronized polymers are prepared. First generation(G1) dendronized polyacylhydrozones with a methoxy or an ethoxy terminal substituent and second generation(G2) dendronized polyacylhydrozones with ethoxy terminal substituent were synthesized through the dynamic covalent reactions between di(aldehyde) and di(acylhydrazine) in the presence of acid catalyst. The molecular weights were determined by Gel Permeation Chromatography/Multiangle Laser Light Scattering(GPC/MALLS). The weightaverage molecular weights of these polymers are influenced by the monomer concentrations and complementarity of monomer hydrophilicity. These dynamic polymers show remarkable macromolecular effect in enhancing high constitutional stability even at strong acidic solutions. The morphologies of these polymers were also investigated by atomic force microscopy. We directly visualize the individual dynamic covalent polymer chains on substrates. All polymers show thermoresponsive properties, and their phase transition temperatures can be varied between 37.3 to 75.6 oC by changing peripheral groups of the dendrons. G1 polymer shows switchable specific interaction with Cu2+ mediated with the phase transitions. 3. Thermoresponsive hyperbranched poly(acylhydrazones)A series of hyperbranched poly(acylhydrazones) with different branched architectures were prepared by traditional A2 + B3 polycondensation of di(aldehyde) carrying three-fold branched oligoethylene glycol(OEG) units in the pendent groups and hydrophilic tri(hydrazine) with a feeding molar ratio. Owing to their peripheral densely packed OEG side chains, these polymers show characteristic thermoresponsive behaviors and low cytotoxicity. The LCSTs of these polymers could be changed by the branched architecture and p H. The surplus acylhydrazine units in the polymer periphery provide chance for further modification to form functional materials. Stimuliresponsive hydrogels were formed through crosslinking of dendronized hyperbranched poly(acylhydrazones) with dialdehyde-terminated polyethylene glycol(PEG). The mechanical properties of these hydrogels were evaluated by oscillatory rheology. They also show interesting self-healing behavior. In addition, the anticancer drug doxorubicin(DOX) was conjugated to the acylhydrazine terminal groups of hyperbranched polymers. Drug release experiments showed that DOX released fast from the polymer conjugates both at acidic condition and above LCSTs. 4. Thermoresponsive dendronized polyazomethinesThanks to the thermoresponsive property of dendritic monomers carrying dialdehyde groups, dynamic covalent dendronized conjugated polymers were synthesized via reaction between dialdehyde and aromatic diamine in water above phase transition temperature of the dendritic monomer. These polymers show characteristic thermoresponsive behaviors, which take great effect on the conjugated backbones. As a result of the reversible nature of the dynamic covalent bond, the obtained polymers were found to respond to changes in p H by reversible depolymerization.