Effects of Other Important Factors

semi-batch and continuous emulsion polymerizations are usually carried out in stirred tank reactors, where agitation by a stirrer is necessary. The type of stirrer chosen and its stirring speed can often affect the rate of polymerization, the number of polymer particles and their size distribution (PSD), and the molecular weight of the polymer produced. However, the effect of stirring on emulsion polymerization has never been the main research parameter in research programs [241]. This is probably due to the conflicting results obtained so far by various researchers. 

Shunmukham et al. [242] studied the effect of stirring on the emulsion polymerization of St, and concluded that violent agitation decreased the rate of polymerization, as shown in Fig. 8. 

Schoot et al. [243], on the other hand, criticized Shunmukham s conclusion, stating that this strange effect of agitation observed by Shunmukham might have been due to the absorption of contaminant oxygen into the reaction mix- 

At lower stirring speeds, on the other hand, stirring controls the rate of monomer transport from the monomer droplets to the polymer particles, thereby controlling the rate of polymerization. The rate-determining step is usually the monomer-transport step from the monomer droplets to the aqueous phase, because the monomer-transport step from the aqueous phase to the polymer particles is much faster than the former step due to the much greater total surface area of the polymer particles compared to that of the monomer droplets. 

At low emulsifier concentrations near the CMC, an increase in the degree of agitation results in a reduction of the emulsifier used for the formation of polymer particles (like micelles). This is because the monomer droplets become smaller as the degree of agitation is increased, and so the amount of emulsifier adsorbed onto the surfaces of the monomer droplets increases in proportion to the increased surface area of the monomer droplets. This brings about a decrease in the number of polymer particles produced, and so a decrease in the rate of polymerization. 

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