Syntheses and self-assembly of poly(benzyl ether)-b-poly(N-isopropylacrylamide) dendritic-linear diblock copolymers

This article describes the syntheses and solution behavior of model amphiphilic dendritic-linear diblock copolymers that self-assemble in aqueous solutions into micelles with thermoresponsive shells. The investigated materials are constructed of poly(benzyl ether) monodendrons of the second generation ([G-2]) or third generation ([G-3]) and linear poly(N-isopropylacrylamide) (PNIPAM). [G-2]-PNIPAM and [G-3]-PNIPAM dendritic-linear diblock copolymers have been prepared by reversible addition-fragmentation transfer (RAFT) polymerizations of N-isopropylacrylamide with a [G-2]- or [G-3]-based RAFT agent, respectively. The critical micelle concentration (cmc) of [G-3]-PNIPAM(220), determined by surface tensiometry, is 6.3 x 10(-6) g/mL, whereas [G-2]-PNIPAM(235) has a cmc of 1.0 x 10(-5) g/mL. Transmission electron microscopy results indicate the presence of spherical micelles in aqueous solutions. The thermoresponsive conformational changes of PNIPAM chains located at the shell of the dendritic-linear diblock copolymer micelles have been thoroughly investigated with a combination of dynamic and static laser light scattering and excimer fluorescence. The thermoresponsive collapse of the PNIPAM shell is a two-stage process; the first one occurs gradually in the temperature range of 20-29 degrees C, which is much lower than the lower critical solution temperature of linear PNIPAM homopolymer, followed by the second process, in which the main collapse of PNIPAM chains takes place in the narrow temperature range of 29-31 degrees C. (c) 2006 Wiley Periodicals, lnc.