Aqueous Micellar Gels of Multiresponsive Hydrophilic ABA Linear Triblock Copolymers

This dissertation presents the synthesis of a series of well-defined multiresponsive hydrophilic ABA linear triblock copolymers and the study of their aqueous micellar gels. By incorporating a small amount of stimuli-responsive groups into thermosensitive outer blocks of ABA triblock copolymers, the lower critical solution temperatures (LCST) of thermosensitive blocks can be modified by external stimuli. Consequently, the sol-gel transition temperatures (T_sol-gel) of their aqueous solutions can be altered.

Chapter 1 describes the synthesis and solution behavior of a series of thermo- and light-sensitive triblock copolymers, poly(ethoxytri(ethylene glycol) acrylate-co-o-nitrobenzyl acrylate)-b-poly(ethylene oxide)-b-poly(ethoxytri(ethylene glycol) acrylate-co-o-nitrobenzyl acrylate), with different contents of light-responsive o-nitrobenzyl groups. Aqueous solutions of these block copolymers with a 10.0 wt% concentration exhibited thermo-induced sol-gel transitions. Upon UV irradiation, the hydrophobic o-nitrobenzyl groups were cleaved, resulting in an increase in the LCST and consequently gel-to-sol transitions. The UV-irradiated solutions again underwent temperature-induced sol-gel transitions but at higher temperatures. The change of T_sol-gel was, in general, larger for the copolymer with a higher o-nitrobenzyl content after UV irradiation. Chapter 2 presents the synthesis of thermo- and enzyme-responsive ABA triblock copolymers, poly(ethoxydi(ethylene glycol) acrylate-co-4-((dihydroxyphosphoryl)oxy)butyl acrylate)-b-poly(ethylene oxide)-b-poly(ethoxydi(ethylene glycol) acrylate-co-4-((dihydroxyphosphoryl)oxy)butyl acrylate), and the enzyme-induced formation of thermoreversible micellar gels from their moderately concentrated aqueous solutions at 37 °C. The dephosphorylation by acid phosphatase decreased the LCST of thermosensitive outer blocks from above to below 37 °C. The enzyme-induced gelation of 7.9 wt % aqueous polymer solutions at pH 4.4 was monitored by rheological measurements. The T_sol-gel decreased and the gel strength increased with the increase of reaction time. The gels formed were thermoreversible.

Chapter 3 presents the synthesis of two thermo- and pH-sensitive tertiary amine-containing ABA triblock copolymers and the sol-gel transitions of their aqueous solutions with a 10 wt% concentration at different pH values. Chapter 4 describes the use of reversible addition-fragmentation chain transfer (RAFT) polymerization for the synthesis of well-defined thermosensitive polymethacrylates and polyacrylates. Eight chain transfer agents were synthesized. The RAFT polymerizations of alkoxyoligo(ethylene glycol) (meth)acrylates using these chain transfer agents were well controlled, producing well-defined polymers. A summary of this dissertation research and future work are presented in Chapter 5.