Hydrocarbon based stabilisers for the synthesis of cross-linked poly(2-hydroxyethyl methacrylate) particles in supercritical carbon dioxide†
An environmentally-friendly process to prepare well-defined cross-linked particles of poly(2-hydroxyethyl methacrylate) (PHEMA) was investigated. The dispersion polymerisation of 2-hydroxyethyl methacrylate (HEMA) with a cross-linker ethylene glycol dimethacrylate (EGDMA) was performed in supercritical carbon dioxide (scCO2). Stabilisers based only on vinyl acetate (VAc) and vinyl pivalate (VPi) random copolymers P(VAc-stat-VPi) were employed thus avoiding the use of silicone or fluorine based stabilisers which are normally required in scCO2. The effect of molecular weight and composition of the P(VAc-stat-VPi) stabilisers on the size and shape of the produced microparticles was investigated. A copolymer stabiliser with 56 : 44 (VAc : VPi) molar composition (Mn = 12.8 kg mol−1) was found to be the most efficient, resulting in well-defined spherical cross-linked PHEMA particles with a diameter of 1.2 μm. This particular stabiliser was found to provide good steric stabilisation for a range of cross-linker ratios and the addition of a co-monomer such as N-vinylpyrrolidone (NVP) allowed tuning of the properties of the microparticulate products. Finally, the stabiliser plays a second very important role. Partial hydrolysis of the ester groups of the stabiliser located on the surface of the microparticles leads to a hydrophilic surface with alcohol moieties and hence facilitates dispersion of the microparticles into water leading to valuable microgels.