Synthesis and characterization of core-shell-type polymeric micelles from diblock copolymers via reversible addition-fragmentation chain transfer

A method was developed to enable the formation of nanoparticles by reversible addition-fragmentation chain transfer polymerization. The thermoresponsive behavior of polymeric micelles was modified by means of micellar inner cores and an outer shell. Polymeric micelles comprising AB block copolymers of poly(N-isopropylacrylamide) (PIPAAm) and poly(2-hydroxyethylacrylate) (PHEA) or polystyrene (PSt) were prepared. PIPAAm-b-PHEA and PIPAAm-b-PSt block copolymers formed a core-shell micellar structure after the dialysis of the block copolymer solutions in organic solvents against water at 20 degrees C. Upon heating above the lower critical solution temperature (LCST), PIPAAm-b-PHEA micelles exhibited an abrupt increase in polarity and an abrupt decrease in rigidity sensed by pyrene. In contrast, PIPAArn-b-PSt micelles maintained constant values with lower polarity and higher rigidity than those of PIPAAm-b-PHEA micelles over the temperature range of 20-40 degrees C. Structural deformations produced by the change in the outer polymer shell with temperature cycles through the LCST were proposed for the PHEA core, which possessed a lower glass-transition temperature (ca. 20 degrees C) than the LCST of the PIPAAm outer shell (ca. 32.5 degrees C), whereas the PSt core with a much higher glass-transition temperature (ca. 100 degrees C) retained its structure. The nature of the hydrophobic segments composing the micelle inner core offered an important control point for thermoresponsive drug release and the drug activity of the thermoresponsive polymeric micelles.