Aqueous RAFT Synthesis of Micelle-Forming Amphiphilic Block Copolymers Containing N-Acryloylvaline. Dual Mode, Temperature/pH Responsiveness, and “Locking” of Micelle Structure through Interpolyelectrolyte Complexation

Temperature- and pH-responsive, micelle-forming, amphiphilic block copolymers were prepared from N,N -dimethylacrylamide (DMA), N -isopropylacrylamide (NIPAM), and N -acryloylvaline (AVAL) utilizing aqueous reversible addition−fragmentation chain transfer (RAFT) polymerization. A series of block copolymers were synthesized by employing DMA as a macro-chain transfer agent to mediate the statistical copolymerization of NIPAM with AVAL. Structural organization and solution behavior were investigated utilizing dynamic light scattering, two-dimensional NMR spectroscopy, and transmission electron microscopy. It has been demonstrated that the critical micellization temperature for the block polymers can be tuned to range from ≈10 to 36 °C by adjusting the solution pH. Micelles with apparent hydrodynamic diameters from 45 to 86 nm are formed between pH 2 and 5. Above pH 5, a sufficient number of the AVAL units are deprotonated which prevents micellization. The extent of pH and temperature changes on the apparent hydrodynamic diameters have been illustrated via 3-D plots. Significantly, micelles assembled within a specified range of pH and temperature can be “locked” by interpolyelectrolyte complexation of anionic AVAL segments with those of a cationic polymer, in this case a RAFT-generated poly([ar-vinylbenzyl]trimethylammonium chloride) (PVBTAC). When the temperature is lowered to room temperature, the polymeric micelles remain “locked” in their multimeric structures which remain dispersed in water. Addition of 0.3 M NaCl to the aqueous solution results in dissociation of the complexes into the respective water-soluble components.