Acid-Sensitive Polymeric Micelles Based on Thermoresponsive Block Copolymers with Pendent Cyclic Orthoester Groups

An orthoester-containing monomer, trans-N-(2-ethoxy-1,3-dioxan-5-yl)acrylamide (tNEA), was synthesized. Atom transfer radical polymerization of tNEA using a poly(ethylene glycol) (PEG) macroinitiator afforded three acid-labile thermoresponsive block copolymers: PEG-b-PtNEA(27), PEG-b-PtNEA(56), and PEG-b-PtNEA(73), These block copolymers are water-soluble at low temperatures (< 13 degrees C). Thermally induced phase transition behaviors, including the critical aggregation temperatures (CATs), of these polymers were investigated by light scattering and H-1 NMR. The results indicated that the longer the PtNEA chain length, the lower the CAT. Upon heating above the CATs, all the three polymers underwent a phase transition and formed polymeric micelles or micelle-like nanoparticles with PEG as the shell and PtNEA block as the core. Both the sizes and morphologies of the micelles were found to be affected by the heating rate and the salt concentration in the buffers. The micelles, formed through a fast heating procedure ill the buffer with a relatively high salt concentration, have a smaller size and a more compacted structure. pH-dependent destabilization of the polymeric micelles prepared from PEG-b-PtNEA(73) was studied by using light scattering and Nile Red fluorescence. The results demonstrated that hydrophobic Nile Red could be loaded in the micelles that were stable at pH 7.4, but destabilized in mildly acidic media. The dissociation of the micelles and the subsequent release of Nile Red were induced by the acid-triggered hydrolysis of the orthoester groups, which was proved by the H-1 NMR spectra.