Quantification of spontaneous initiation in radical polymerization of styrene in aqueous miniemulsion at high temperature

The spontaneous (thermal) initiation rate (Ri,th; no added initiator) in radical polymerization of styrene in aqueous miniemulsion at 110 and 125 °C with sodium dodecylbenzenesulfonate or poly(vinyl alcohol) as surfactants (colloidal stabilizers) has been estimated using a novel approach based on the total number of chains. In qualitative agreement with previous work at lower temperatures, Ri,th was found to be 3.1-15.1 times greater than that in bulk. According to the activation energy and conversion dependence of Ri,th, the radical generation mechanism differs from that in bulk. The experimental evidence is consistent with the enhanced Ri,th in miniemulsion being related to the oil-water interface, with radical generation in the aqueous phase playing a negligible role. The implications with regards to nitroxide-mediated radical polymerization in aqueous dispersed systems are discussed.