Synthesis and Solution Behavior of Doubly Responsive Hydrophilic Block Copolymers

This dissertation presents the synthesis of stimuli-sensitive hydrophilic polymers, particularly doubly responsive hydrophilic block copolymers, by controlled radical polymerizations and the study of their solution behavior in water. By incorporating a small amount of stimuli-responsive groups into the thermosensitive block of a hydrophilic block copolymer, the lower critical solution temperature (LCST) of the thermosensitive block can be tuned by a stimulus and multiple micellization/dissociation transitions can be achieved by combining two external triggers. Chapter 1 describes the synthesis and thermosensitive properties of two new watersoluble polystyrenics with a short oligo(ethyl glycol) pendant from each repeat unit and the study of hydrophobic end group effects on cloud points of thermosensitive polystyrenics. Well-defined polymers were prepared from monomer-based initiators via nitroxide-mediated polymerization and the alkoxyamine end groups were removed by tri(n-butyl)tin hydride, yielding thermoresponsive polystyrenics with essentially no end groups. The results showed that hydrophobic end groups could significantly change the cloud points and the molecular weight dependences of cloud points of polystyrenics. Chapter 2 presents the synthesis of thermo- and light-sensitive hydrophilic block copolymers, poly(ethylene oxide)-b-poly(ethoxytri(ethylene glycol) acrylate-co-onitrobenzyl acrylate), and their responsive behavior in dilute aqueous solutions. Dynamic light scattering and fluorescence spectroscopy studies showed that these copolymers were molecularly dissolved in water at lower temperatures and self-assembled into micelles at temperatures above the LCST of the thermosensitive block. Upon UV irradiation, the oiv nitrobenzyl group was cleaved and the LCST of the thermosensitive block was increased, causing the dissociation of micelles into unimers. The resultant copolymers underwent thermo-induced reversible micellization at higher temperatures. Chapter 3 describes multiple micellization/dissociation transitions of thermo- and pH-sensitive hydrophilic block copolymers, poly(ethylene oxide)-b-poly(methoxydi(ethylene glycol) methacrylate-co-methacrylic acid), in response to temperature and pH changes. The LCST of the thermosensitive block can be reversibly tuned and precisely controlled by solution pH. Chapter 4 presents the study on multiple sol-gel-sol transitions of a 20.0 wt % aqueous solution of poly(ethylene oxide)-b-poly(ethoxytri(ethylene glycol) acrylate-co-o-nitrobenzyl acrylate) induced by temperature changes and UV irradiation. The solution underwent thermo-induced sol-gel-sol transitions. Upon UV irradiation to dissociate micelles, the gel was transformed into a free-flowing liquid, which upon heating underwent sol-gel-sol transitions again.