RAFT Dispersion Polymerization of Benzyl Methacrylate in Silicone Oil Using a Silicone-Based Methacrylic Stabilizer Provides Convenient Access to Spheres, Worms, and Vesicles

Reversible addition–fragmentation chain transfer (RAFT) solution polymerization of 3-[tris(trimethylsiloxy)silyl] propyl methacrylate (SiMA) was conducted in toluene to prepare three PSiMA precursors with mean degrees of polymerization (DP) of 12, 13, or 15. Each precursor was then chain-extended in turn via RAFT dispersion polymerization of benzyl methacrylate (BzMA) in a low-viscosity silicone oil (decamethylcyclopentasiloxane, D5). 1H NMR studies confirmed that such polymerizations were relatively fast, with more than 99% BzMA conversion being achieved within 100 min at 90 °C. Moreover, gel permeation chromatography analysis indicated that these polymerizations were well controlled, with dispersities remaining below 1.25 when targeting PBzMA DPs up to 200. A phase diagram was constructed at a constant copolymer concentration of 20% w/w. Only spherical micelles were accessible when the PSiMA15 stabilizer was utilized, as determined by transmission electron microscopy and small-angle X-ray scattering (SAXS) studies. Nevertheless, these spheres exhibited narrow size distributions and tunable z-average diameters ranging between 19 and 49 nm, as determined by dynamic light scattering. In contrast, spheres, worms, or vesicles could be prepared depending on the target PBzMA DP when utilizing the relatively short PSiMA12 precursor. Moreover, each of these nano-objects could be obtained at copolymer concentrations as low as 5% w/w. To obtain more detailed structural information, these spheres, worms and vesicles were further characterized by SAXS. PSiMA12-PBzMA55 worms formed reasonably transparent free-standing gels when prepared at copolymer concentrations as low as 5% w/w and exhibited an elastic modulus (G′) of 90 Pa at 25 °C, as judged by oscillatory rheology studies. Finally, broadening of the molecular weight distribution was observed during the long-term storage of PSiMA-PBzMA dispersions at ambient temperature. We tentatively suggest that this instability is related to hydroxyl impurities in the SiMA, which leads to cross-linking side reactions. This problem also causes incipient flocculation of the spheres and worms during the long-term storage of such dispersions at 20 °C.