Facile Synthesis of Methacrylic ABC Triblock Copolymer Vesicles by RAFT Aqueous Dispersion Polymerization

Poly(glycerol monomethacrylate-block-2-hydroxypropyl methacrylate) [PGMA-PHPMA] diblock copolymer vesicles are prepared by RAFT aqueous dispersion polymerization at 70 degrees C and then used as precursors for chain extension experiments with a third comonomer. For self-blocking experiments conducted with water-soluble HPMA, good living character is obtained, as judged by the relatively low final polydispersity (M-w/M-n < 1.30). This suggests that the RAFT chain-end fidelity is high, at least over time scales of a few hours. Using water-immiscible monomers such as methyl methacrylate (MMA) or benzyl methacrylate (BzMA) also leads to efficient chain extension under "seeded" emulsion polymerization conditions. High conversions are obtained within a few hours at 70 degrees C, but polydispersities are somewhat higher for the resulting ABC triblock copolymer chains. TEM studies indicate that introduction of a second hydrophobic block within the vesicle membrane produces remarkable changes in morphology. A distinctive framboidal morphology is obtained in the case of BzMA, which is attributed to microphase separation between the PHPMA and PBzMA hydrophobic blocks. Moreover, there is a monotonic increase in the size of the observed globular features as higher degrees of polymerization for the PBzMA block are targeted. Similar but less distinctive morphological changes are also observed when using MMA as the third comonomer, which suggests that somewhat weaker microphase separation occurs in this case. If a bifunctional monomer such as ethylene glycol dimethacrylate (EGDMA) is used as the third comonomer, highly crosslinked vesicles are obtained that can withstand a surfactant challenge, unlike the precursor linear diblock copolymer vesicles. In this case it is noteworthy that much better results are obtained if the EGDMA is added last, rather than during the HPMA polymerization. The robust, reproducible nature of these aqueous formulations is expected to lead to new opportunities in the growing field of block copolymer vesicles, since they allow access to new particle morphologies in multigram quantities at relatively high solids.