Direct Synthesis and Stimulus-Responsive Micellization of Y-Shaped Hydrophilic Block Copolymers

Abstract

Two bifunctional atom transfer radical polymerization (ATRP) macroinitiators were synthesized via Michael addition of 2 equivalents of 2-hydroxyethyl acrylate to two commercial monoamine-capped poly(alkylene oxides), Jeffamine XTJ-506 and Jeffamine XTJ-507, followed by esterification using excess 2-bromoisobutyryl bromide. 1H NMR and MALDI−TOF mass spectra (MALDI−TOF MS) were consistent with the expected structures for these two ATRP macroinitiators. A range of well-defined, Y-shaped, pH-responsive or thermoresponsive block copolymers was synthesized using these macroinitiators by polymerizing various hydrophilic methacrylic monomers via ATRP in methanol at 20 °C. The stimulus-responsive micellization of selected Y-shaped block copolymers in aqueous solution was investigated by 1H NMR, dynamic light scattering (DLS), and surface tensiometry. For comparative purposes, their linear diblock copolymer counterparts were also synthesized, and their micellization properties in aqueous solution were investigated. DLS studies indicated that the nonlinear architectures of selected Y-shaped block copolymers usually resulted in micelles with differing dimensions to those formed by the corresponding linear diblock copolymers. DLS studies also confirmed that one of these new Y-shaped block copolymers exhibited “schizophrenic” thermoresponsive behavior in aqueous solution. Near-monodisperse spherical micelles were obtained at higher temperatures (above 45 °C), whereas polydisperse nonspherical micelles appear to be formed at lower temperatures (below 12 °C).