Monodisperse oxidative-sensitive polymer nanoparticles from dispersion polymerization of N-acryloyl thiomorpholine

Abstract

Aqueous emulsion or suspension polymerizations are well established industrial processes with widespread applications. Dispersion polymerizations, on the contrary, are rarely found for the preparation of stable particle dispersions, which is certainly related to the limited number of suitable monomers. Here, we report that N-acryloyl thiomorpholine (NAT) is an interesting candidate for aqueous dispersion polymerizations, as it is fully miscible with water but forms insoluble polymers at low degrees of polymerization. Polymerization in pure water results in an uncontrolled precipitation but with the addition of a surfactant well-defined dispersions with monomodal size distributions are obtained if low monomer concentrations (≤4 wt%) are used. Higher concentrations (10 wt%) broaden the dispersity of the resulting particles, but still result in stable dispersions, while 30 wt% cause a solidification of the solution as particles start to fuse. Crosslinked particles can be obtained by addition of a bisacrylamide comonomer. The thioether moiety of NAT further enables postpolymerization modifications including its oxidation to a more polar sulfoxide. Kinetic studies revealed that oxidation in H2O2 follows a bulk process causing a swelling of the particles after reaching a critical degree of oxidation (35%). In case of the non-crosslinked particles, disintegration occurs only at ≥ 60% conversion of the thioethers, while the particles remain intact for days at lower degrees of oxidation. The oxidation of the crosslinked particles results in water-swollen microgels. Overall, the straightforward process allows access to monodisperse reactive polymer nanoparticles of variable size (∼ 60 – 100 nm) that could be used as well-defined templates or carrier materials.