Synthesis of diblock copolymer spheres, worms and vesicles via RAFT aqueous emulsion polymerization of hydroxybutyl methacrylate

Abstract

There are many literature examples of reversible addition–fragmentation chain transfer (RAFT) aqueous emulsion polymerization that produce only kinetically-trapped spheres, even when targeting highly asymmetric diblock copolymer compositions. Recently, we postulated that the aqueous solubility of the vinyl monomer was likely to be a key parameter for overcoming this morphological limitation. In the present study, the RAFT aqueous emulsion polymerization of hydroxybutyl methacrylate (HBMA) has been revisited using a relatively short non-ionic poly(glycerol monomethacrylate) (PGMA) precursor as a steric stabilizer block. HBMA was selected for its relatively high aqueous solubility (∼25 g dm⁻³ at 50 °C). Conversions of more than 99% were achieved within 2 h at 50 °C using a low-temperature azo initiator (VA-044), as indicated by ¹H NMR studies. Gel permeation chromatography analysis confirmed that high blocking efficiencies and relatively low dispersities (M_w/M_n < 1.37) could be achieved under these conditions. A pseudo-phase diagram was constructed by systematically increasing the PHBMA target DP from 10 to 120 and varying the copolymer concentration between 5 and 20% w/w. Only spheres, vesicles or mixed phases were accessible at 5% w/w copolymer concentration, with higher concentrations being required to access a pure worm phase. Transmission electron microscopy (TEM) and small-angle X-ray scattering studies indicated the formation of well-defined diblock copolymer worms and vesicles when targeting longer PHBMA blocks. The evolution in copolymer morphology when targeting PGMA₄₁-PHBMA₁₂₀ vesicles was monitored using TEM. This technique revealed intermediate morphologies that are strikingly similar to those reported during the preparation of PGMA₄₇-PHPMA₂₀₀ vesicles via RAFT aqueous dispersion polymerization (A. Blanazs, J. Madsen, G. Battaglia, A. J. Ryan and S. P. Armes, J. Am. Chem. Soc., 2011, 133, 16581). This suggests that the formation of vesicles via RAFT aqueous emulsion polymerization occurs via essentially the same mechanism. Finally, linear PGMA₄₁-PHBMA₁₁₀ vesicles were evaluated as putative Pickering emulsifiers for the stabilization of n-dodecane droplets in water. Such nano-objects survive high-shear homogenization and stabilize genuine Pickering emulsions, unlike linear PGMA₄₅-PHPMA₂₀₀ vesicles (K. L. Thompson, P. Chambon, R. Verber and S. P. Armes, J. Am. Chem. Soc., 2012, 134, 12450).