Bis-Hydrophilic Block Terpolymers via RAFT Polymerization: Toward Dynamic Micelles with Tunable Corona Properties

We present the synthesis of well-defined bis-hydrophilic block terpolymers with two outer hydrophilic blocks and an inner hydrophobic block together with studies concerning their colloidal aggregates formed in water. The investigations aim at preparation of dynamic micelles with tunable corona properties. Highly functionalized poly(ethylene oxide) macro-chain-transfer agents (PEO-CTAs) of two molecular weights (2 and 5 kDa) are used as mediating agents in reversible addition fragmentation chain transfer (RAFT) polymerization. The synthesis is accomplished by first polymerizing n-butyl acrylate as a hydrophobic block and then chain extending the diblock copolymers further with various (meth)acrylamide derivatives, acrylamide (AAm), N-isopropylacrylamide (NIPAAm), N,N-diethylacrylamide (DEAAm,) and N-(2-hydroxypropyl)methacrylamide (HPMA). Due to the high degree of functionalization of the PEO-CTA, the blocking efficiency is near quantitative and the diblock copolymers can be obtained easily in a wide range of compositions and with an excellent control of the molecular weights and polydispersities (< 1.15). Similarly, chain extension with the different (meth)acrylamide proceeds with very high blocking efficiencies to obtain well-defined block terpolymers. The hydrophilic-to-hydrophobic balance as well as the chain lengths of the hydrophilic blocks can be adjusted as desired. The second part of this study describes the aqueous solution characteristics of the micellar aggregates of the block terpolymers. A significant effect of the preparation pathway (direct dissolution or dialysis from a common solvent) on the type of formed aggregates is found, indicating a strong influence of the dissolution kinetics. The self-assembled aggregates are of dynamic character as they can undergo fusion and fission processes, induced by both temperature and time. Large-scale rearrangement of the architectures are possible as ensured by the low glass-transition temperature of the hydrophobic block, poly(n-butyl acrylate). Depending on the hydrophilic-to-hydrophobic balance and the pair of hydrophilic end blocks employed, spherical micelles, worm-like micelles, and vesicles can be found. The corona structure of the micelles can be tuned by changing the length and type of hydrophilic polymers used.