One-pot synthesis of micron-sized polybetaine particles; innovative use of supercritical carbon dioxide

Polybetaines exhibit unique properties including anti-polyelectrolyte and low protein fouling behaviour, as well as biocompatibility. We recently presented the synthesis of ca. 20 nm polybetaine particles by aqueous RAFT polymerisation, but the synthesis of larger particles proved to be extremely challenging with standard emulsion and dispersion techniques being unsuccessful. Here we present the first reported synthesis of micron-sized, discrete cross-linked polybetaine particles, using polymerisation in scCO2 with methanol as a co-solvent. Discrete particles are produced only when the methanol is efficiently removed in-situ using scCO2 extraction. A relatively high crosslinking agent initial concentration (10 wt%) was found to result in the most well defined particles, and particle integrity reduced as the crosslinking agent initial concentration was decreased. A monomer loading of between 3.0x10-2 mol/L and 1.8x10-1 mol/L resulted in discrete micron sized particles, with significant agglomoration occuring as the monomer loading was increased further. A spherical morphology and extremely low size dispersity is observed by SEM analysis for the optimised particles. The particles are readily re-dispersed in aqueous solution and light scattering measurements confirm their low size dispersity.