Synthesis of Flower-like Micelles That Recognize Thermal History via RAFT Aqueous Dispersion Polymerization

Flower-like micelles of block copolymer assemblies that recognize thermal history are prepared using polymerization-induced self-assembly. Such assemblies are prepared via reversible addition–fragmentation chain-transfer (RAFT) dispersion polymerization. First, a macromolecular-chain-transfer agent (macro-CTA) corresponding to the looped shell is synthesized from hydrophilic poly(ethylene glycol) methyl ether methacrylate (PEGMA) by using a difunctional RAFT agent. Subsequently, the core of the flower-like micelle is further propagated in situ from the macro-CTA by chain extension of the hydrophobic moiety without changing the aggregation number using water-soluble 2-hydroxypropyl methacrylate (HPMA) in water. Thus, the flower-like micelle of PHPMA-b-PPEGMA-b-PHPMA triblock copolymers including a PHPMA core is produced by RAFT aqueous dispersion polymerization. Although the PHPMA is hydrophobic, it changes to a lower critical solution temperature (LCST)-type thermoresponsive polymer due to the presence of the hydrophilic blocks of PPEGMA. If the assembly solution is not cooled to ∼2 °C after polymerization, it maintains the flower-like micelle morphology and shows solution (sol) behavior. However, once cooled, the flower-like micelle molecularly dissolves in water. It therefore recognizes the temperature change and adopts an intermicellar bridging as a physical gel at room temperature after reheating. Thus, the resulting assembly in water that exhibits a sol state at room temperature remains gelled at the same room temperature once it has been cooled. This is not a chemical reaction but a physical change. Once cooled, the triblock copolymer assembly in water shows an LCST-type thermoresponsiveness with reversible sol–gel changes in response to temperature changes. However, at room temperature, the polymer solution is always in a physical gel state after undergoing only one thermal change.