Optimization of the RAFT polymerization conditions for the in situ formation of nano-objects via dispersion polymerization in alcoholic medium
Hydrophilic polymer brushes based on poly(ethylene glycol) methyl ether acrylate (P(PEGA454)) or poly(ethylene glycol) methyl ether methacrylate (P(PEGMA475)), both having a trithiocarbonate end group, were prepared in water–dioxane (9 : 1) at 44 °C via RAFT polymerization, and subsequently used in RAFT dispersion polymerization of styrene in isopropanol at 90 °C. RAFT reaction conditions were first optimized to prepare P(PEGA454) and P(PEGMA475) macro-RAFT agents at high monomer conversions (>90%) and very low fraction of dead chains (<1%). The macro-RAFT agents were then shown to have similar efficiency in terms of reinitiating and controlling the polymerization of styrene in dispersion polymerization. Both polymer brushes allowed the preparation of well-defined amphiphilic diblock copolymers (P(PEGA454)-b-PS and P(PEGMA475)-b-PS) which self-assemble in situ into nano-objects with various morphologies. Using relatively long chain P(PEGA454) or P(PEGMA475) macro-RAFT agents (DP ≈ 75) leads to the formation of near uniform spherical nano-particles with diameters ranging from 30 to 140 nm, depending on the targeted DP of the PS block. In contrast, TEM and DLS studies demonstrated that using a shorter P(PEGA454) or P(PEGMA475) macro-RAFT agent (DP ≈ 20) enables the formation of worm-like micelles, vesicles and large compound vesicle morphologies, in addition to spheres. Cryo-TEM was used to confirm polymerization induced morphology transition, rather than morphologies obtained via self-assembly driven by selective solvent or solvent evaporation during the preparation of samples for characterization.