Synthesis of Highly Transparent Diblock Copolymer Vesicles via RAFT Dispersion Polymerization of 2,2,2-Trifluoroethyl Methacrylate in n-Alkanes

Abstract

RAFT dispersion polymerization of 2,2,2-trifluoroethyl methacrylate (TFEMA) is performed in n-dodecane at 90 °C using a relatively short poly(stearyl methacrylate) (PSMA) precursor and 2-cyano-2-propyl dithiobenzoate (CPDB). The growing insoluble poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) block results in the formation of PSMA-PTFEMA diblock copolymer nano-objects via polymerization-induced self-assembly (PISA). GPC analysis indicated narrow molecular weight distributions (Mw/Mn ≤ 1.34) for all copolymers, with 19F NMR studies indicating high TFEMA conversions (≥95%) for all syntheses. A pseudo-phase diagram was constructed to enable reproducible targeting of pure spheres, worms, or vesicles by varying the target degree of polymerization of the PTFEMA block at 15-25% w/w solids. Nano-objects were characterized using dynamic light scattering, transmission electron microscopy, and small-angle X-ray scattering. Importantly, the near-identical refractive indices for PTFEMA (1.418) and n-dodecane (1.421) enable the first example of highly transparent vesicles to be prepared. The turbidity of such dispersions was examined between 20 and 90 °C. The highest transmittance (97% at 600 nm) was observed for PSMA9-PTFEMA294 vesicles (237 ± 24 nm diameter; prepared at 25% w/w solids) in n-dodecane at 20 °C. Interestingly, targeting the same diblock composition in n-hexadecane produced a vesicle dispersion with minimal turbidity at a synthesis temperature of 90 °C. This solvent enabled in situ visible absorption spectra to be recorded during the synthesis of PSMA16-PTFEMA86 spheres at 15% w/w solids, which allowed the relatively weak n→I band at 515 nm assigned to the dithiobenzoate chain-ends to be monitored. Unfortunately, the premature loss of this RAFT chain-end occurred during the RAFT dispersion polymerization of TFEMA at 90 °C, so meaningful kinetic data could not be obtained. Furthermore, the dithiobenzoate chain-ends exhibited a λmax shift of 8 nm relative to that of the dithiobenzoate-capped PSMA9 precursor. This solvatochromatic effect suggests that the problem of thermally labile dithiobenzoate chain-ends cannot be addressed by performing the TFEMA polymerization at lower temperatures.