Thermally induced changes in amphiphilicity drive reversible restructuring of assemblies of ABC triblock copolymers with statistical polyether blocks

ABC triblock copolymers in which a block with stimulus-dependent solvophilicity resides between solvophilic and solvophobic end blocks can undergo reversible transitions between different thermodynamically stable assemblies in the presence or absence of stimulus. As a new example of such a copolymer system, thermoresponsive poly (ethylene oxide)-b-poly (ethylene oxide-stat-butylene oxide)-b-poly (isoprene) (E-BE-I) triblock copolymers with narrow molecular weight distributions (M^w/M_n: 1.05-1.18) were prepared by sequential living anionic and nitroxide-mediated radical polymerizations. The specific copolymers examined (9.0 ≤ M_n ≤ 14.4 kg/mol, 14% ≤ wt % isoprene ≤35%) form near-spherical aggregates with narrow size distributions at 25 °C. The thermoresponsive behavior of these polymers was studied by applying cloud point, DLS, and TEM measurements to a representative polymer, E_2.3BE_5.3I_2.3. The transformation of polymer aggregates from spherical micelles to vesicles (polymersomes) at elevated temperatures was detected by DLS and TEM studies, both with and without cross-linking of polymer assemblies. The rate of transformation with E-BE-I systems is more rapid than that observed for poly (ethylene oxide)-b-poly (N-isopropylacrylamide)-b-poly (isoprene) assemblies, suggesting that interchain hydrogen bonding of responsive blocks after dehydration plays an important role in the kinetics of aggregate rearrangement.