Synthesis of poly(stearyl methacrylate)-poly(2-hydroxypropyl methacrylate) diblock copolymer nanoparticles via RAFT dispersion polymerization of 2-hydroxypropyl methacrylate in mineral oil†
Poly(stearyl methacrylate)-poly(2-hydroxypropyl methacrylate) (PSMA-PHPMA) diblock copolymer nanoparticles are synthesized via reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA) in mineral oil at 90 °C. The relatively short PSMA precursor (mean degree of polymerization = 9) remains soluble in mineral oil, whereas the growing PHPMA block quickly becomes insoluble, resulting in polymerization-induced self-assembly (PISA). Relatively high HPMA monomer conversions (≥98%) were achieved within 70 min as confirmed by in situ 1H NMR spectroscopy studies, while gel permeation chromatography (GPC) analyses indicated high blocking efficiencies and relatively narrow molecular weight distributions (Mw/Mn ≤ 1.37) for all PISA syntheses. Depending on the precise synthesis conditions, this PISA formulation can produce diblock copolymer spheres, worms or vesicles; a pseudo-phase diagram has been constructed to enable reproducible targeting of each pure phase. Thus this is a rare example of the use of a commercially available polar monomer for PISA syntheses in non-polar media that offers access to the full range of copolymer morphologies. The resulting nanoparticles were characterized using dynamic light scattering (DLS), transmission electron microscopy (TEM), oscillatory rheology and small-angle X-ray scattering (SAXS). Interestingly, PSMA9-PHPMA70 worms undergo an unusual (partial) worm-to-vesicle transition at elevated temperature. Finally, PSMA9-PHPMA50 spheres were evaluated as putative Pickering emulsifiers. Using lower water volume fractions produced water-in-oil (w/o) emulsions after high shear homogenization, as expected. However, using higher water volume fractions, shear rates or copolymer concentrations favored the formation of w/o/w Pickering double emulsions.