Sterically stabilized emulsion polymerization of 2-ethylhexyl acrylate

Abstract

Batch emulsion polymerizations of 2-ethylhexyl acrylate (EHA) initiated by ammonium peroxodisulfate (APS), stabilized by the hydrophilic non-ionic emulsifier Tween-20, were investigated. Two non-stationary rate intervals appear at very high emulsifier concentration and four rate intervals at very low emulsifier concentration. The slight increase in the maximal rate with increasing emulsifier concentration is discussed in terms of the polymer gel formation. The average number of radicals per particle (¯n) mostly observed much below 0.5 is discussed in terms of the polymerization at the particle surface. The desorption rate constant k′des,N (Nomura modul, N-model) for the emulsion polymerization of EHA is ca. one order in magnitude larger than that reported for the emulsion polymerization of styrene. The stronger desorption of radicals suggests the O'Toole/Ugelstad model (k′des,T, T-model). The k′des,T values are one order of magnitude larger than the k′des,N ones. The formation of less stable monomeric (tertiary) radicals and the polymerization in the cross-linked particles (at the particle surface) induce desorption of radicals. The absorption rate of radicals (ρ a) by the particles decreases with increasing weight fraction of emulsifier. The close-packed interfacial layer forms a barrier for entering radicals. This includes the increased chain transfer to the emulsifier followed by the exit of emulsifier transferred emulsifier radicals to the continuous phase.