Multiarm star-like unimolecular micelles with a dendritic core and a dual thermosensitive shell

Stimuli-responsive unimolecular polymeric micelles have attracted much interest because of their unique structures and properties. In this paper, the double phase transition behavior of unimolecular dendritic three-layer nanostructures with dual thermoresponsive shells is studied. By using a hydrophobic hyperbranched polyester (Bolton H40)-based macro reversible addition-fragmentation transfer (macro-RAFT) agent, the polymerizations of N-isopropylacrylamide (NIPAM) and N-isopropylmethacrylamide (NIPMAM) were conducted via the successive RAFT process. The dendritic core-shell architecture prepared, i.e. H40-poly(N-isopropylacrylamide)-block-poly(N-isopropylmethacrylamide) (H40-PNIPAM-b-PNIPMAM), exists as unimolcular polymeric micelles with hydrophobic dendritic H40 as the core, PNIPAM as the inner shell and PNIPMAM as the corona. Upon continuous heating through the lower critical solution temperatures of PNIPAM and PNIPMAM, dendritic H40-PNIPAM-b-PNIMAM exhibits two-stage thermoresponsive collapse. Different parts of the shells of H40-PNIPAM-b-PNIMAM were selectively labeled with pyrene (Py) to result in H40-PNIPAM(Py)-b-PNIMAM with inner shell labeling and H40-PNIPAM-b-PNIMAM(Py) with outer corona labeling. Laser light scattering, UV-visible transmittance, micro-differential scanning calorimetry and excimer fluorescence measurements were used in combination to probe the conformational changes of chains located at the inner layer and outer corona during the two-stage collapse upon heating. Results from all four techniques support the double phase transition behavior of this kind of three-layer dendritic polymer. (C) 2010 Society of Chemical Industry