Nitroxide-mediated polymerization-induced self-assembly of amphiphilic block copolymers with a pH/temperature dual sensitive stabilizer block

Abstract

Comb-like terpolymers with a polymethacrylate backbone, carrying short pendant poly(ethylene)oxide (PEO) side chains (M_n = 300 g mol⁻¹, 5 EO units), carboxylic acid groups and a few styrene (S) units [P(PEOMA₃₀₀-co-MAA-co-S)-SG1], were synthesized by nitroxide-mediated polymerization, and used to initiate the emulsion polymerization of n-butyl methacrylate and styrene at 85 °C. The macroalkoxyamine initiators were shown to display a pH/temperature double responsive behavior. Above pH 5.7, the incorporation of MAA units in the copolymers shifted the solubility transition to higher temperatures whereas below pH 5.7, the cloud points decreased with increasing MAA content due to the increased hydrophobicity of non-ionized MAA units and also possibly because of inter- or intrapolymeric H-bonding between the carboxylic acid groups of MAA and the oxygen atoms of PEO. Chain extension of the comb-like terpolymers with an immiscible poly(n-butyl methacrylate-co-styrene) block, carried out above or below the lower critical solution temperature (LCST) of the macroinitiator, resulted in electrosterically self-stabilized nano-objects with spherical, worm-like or vesicular nanostructures. The effect of the composition of the hydrophilic block, the pH and the macroinitiator concentration on the control of the polymerization, the polymerization kinetics and the particle morphology was studied in detail. Compared to their P(PEOMA₃₀₀-co-S)-SG1 or P(PEOMA₉₅₀-co-S)-SG1 homologues, the macroalkoxyamine terpolymers displayed significantly different stabilizing properties which directly influenced the phase diagram. Lastly, hybrid nanostructures consisting of silica particles decorated by self-assembled block copolymers were also reported using the macroalkoxyamine initiator containing 14 mol% of MAA. Interestingly, the polymerization performed at pH 9 resulted in partially coalesced short worms radially expanding from the silica core in a sea urchin-like morphology.