Rate enhanced nitroxide-mediated miniemulsion polymerization: effect of nitroxide water solubility

Nitroxide-mediated radical polymerization of styrene in miniemulsion based on in situ surfactant generation of potassium oleate has been conducted at 130 °C for three nitroxides of vastly different water solubilities; 4-stearoyl-TEMPO, TEMPO and 4-hydroxy-TEMPO. These miniemulsions contain high levels of surfactant (120 wt% relative to styrene) and have particle diameters of <20 nm, and as such compartmentalization effects are important. The TEMPO-based system proceeded with extraordinarily high polymerization rate (initial rate ∼30 times higher than in the corresponding bulk system), much faster than ever before achieved, while control/livingness remained satisfactory. The results for the other two nitroxides were consistent with the propensity (or lack thereof) of the free nitroxide to exit to the aqueous phase – the 4-stearoyl-TEMPO system (negligible nitroxide exit) exhibited a polymerization rate similar to the corresponding bulk system with good control/livingness, whereas the 4-hydroxy-TEMPO system polymerized much faster than the corresponding bulk system, accompanied by loss of control. Consumption of free nitroxide caused by the presence of oleic acid may also influence these polymerizations.