Synthesis and self-assembly behaviors of four-armed amphiphilic polystyrene-b-poly(N-isopropylacrylamide) copolymers

Abstract

Four-armed amphiphilic block copolymers polystyrene-b-poly(N-isopropyl acrylamide) (PSt-b-PNIPAAM)4 were synthesized by atom transfer radical polymerization (ATRP) in two steps. Star narrow dispersed polystyrene, (PSt-Br)4, with controlled number-average molecular weight was firstly synthesized by ATRP of styrene (St) using pentaerythritol tetrakis (2-bromoisobutyrate) (4Bri-Bu) as four-armed initiator. Then, (PSt-b-PNIPAAM)4 was prepared using (PSt-Br)4 as macroinitiator by ATRP. The structures of (PSt-Br)4 and (PSt-b-PNIPAAM)4 were confirmed by characterization by nuclear magnetic resonance (1H NMR). The apparent viscosity of the four-armed (PSt-b-PNIPAAM)4 was significantly lower than that of the linear PSt-b-PNIPAAM with the same amount of repeat units of PSt and PNIPAAM. The self-assembly behavior of the four-armed amphiphilic block copolymers (PSt-b-PNIPAAM)4 in mixed solution (DMF/H2O) and the lower critical solution temperature (LCST) of the resulting micelles were investigated by scanning electron microscopy (SEM), dynamic light scattering (DLS) and UV-VIS spectroscopy. The results show that the size of the mono-dispersed spherical micelles decreased with the increment of the chain length of PNIPAAM in the block copolymers, while LCST increased.