Phase transition behavior of unimolecular dendritic three-layer nanostructures with dual thermoresponsive coronas is studied. Successive reversible addition-fragmentation transfer (RAFT) polymerizations of N-isopropylacrylamide (NIPAM) and 2-(dimethylamino)ethyl methacrylate (DMA) were conducted using fractionated fourth-generation hyperbranched polyester (Bolton H40) based macroRAFT agent. At lower temperatures (<20 degrees C), dendritic macromolecules H40-poly(N-isopropylacrylamide)-poly(2-(dimethylamino)ethyl methacrylate) (H40-PNIPAM-PDMA) exist as unimolcular core-shell-corona nanostructures with hydrophobic H40 as the core, swollen PNIPAM as the inner shell, and swollen PDMA as the corona. PNIPAM and PDMA homopolymers undergo phase transitions at their lower critical solution temperatures (LCST), which are found to be 32 degrees C for PNIPAM and 40-50 degrees C for PDMA, respectively. Upon continuously heating through the LCSTs of PNIPAM and PDMA, such dendritic unimolecular micelles exhibit two-stage thermally induced collapse. This process is reversible with a two-stage reswelling upon cooling. Laser light scattering, micro-differential scanning calorimetry, and excimer fluorescence measurements are used to investigate the double phase transitions.

This paper describes the double phase transition behavior of a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brush at the surface of a hydrophobic core. Reversible addition-fragmentation transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) was conducted by using a hyperbranched polyester (Boltorn H40) based macroRAFT agent. The resultant multiarm star block copolymer (H40-PNIPAM) exists as unimolecular micelles with hydrophobic H40 as the core, densely grafted PNIPAM brush as the shell. A combination of laser light scattering (LLS) and microdifferential scanning calorimetry (micro-DSC) studies of H40-PNIPAM in aqueous solution reveals double phase transitions of the PNIPAM corona, which is in contrast to the fact that free PNIPAM homopolymer in aqueous solution exhibits a lower critical solution temperature (LCST) at approximately 32 degrees C. The first phase transition takes place in the broad temperature range 20-30 degrees C, which can be tentatively ascribed to the n-cluster-induced collapse of the inner region of the PNIPAM brush close to the H40 core; the second phase transition occurs above 30 degrees C, which can be ascribed to the outer region of PNIPAM brush. Employing the RAFT chain extension technique, the inner and outer part of PNIPAM brush were then selectively labeled with pyrene derivatives, respectively; temperature-dependent excimer fluorescence measurements further support the conclusion that the inner part of PNIPAM brush collapses first at lower temperatures, followed by the collapse of the outer part at higher temperatures.