Polymerization of Miniemulsions Prepared from Polystyrene in Styrene Solutions. 3. Potential Differences between Miniemulsion Droplets and Polymer Particles

Abstract

In two previous papers, it was demonstrated that the addition of a small amount of polystyrene to the styrene used in preparing miniemulsions results in a substantial increase in the overall polymerization rate and the number of polymer particles formed. In this paper, this behavior is attributed to a basic difference in the abilities of miniemulsion droplets and polymer particles to capture aqueous phase free radicals. It is hypothesized that miniemulsions prepared from polystyrene in styrene solutions resemble the polymer particles formed in normal (i.e., no polymer) miniemulsion polymerizations at early conversions. This being the case, these polymer-containing droplets are able to effectlively compete with growing polymer particles for free radicals, whereas their counterparts which contain no polymer are not, and as a result a greater fraction of the initial droplets become polymer particles. Based on this mechanism, it is speculated that the presence of the polymer increases the capture efficiency of the droplets by modifying either their interior (i.e., by increasing the interior viscosity, thereby increasing the probability of a radical to propagate rather than exit) or the droplet/water interface (i.e., by disrupting a SLS/CA interfacial barrier to radical entry). Experimental results are reported which support the latter