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The rate-accelerating effects of camphorsulfonic acid (CSA) on nitroxidemediated styrene miniemulsion polymerization were studied. Polymerizations were initiated with benzoyl peroxide (BPO) as an initiator and mediated with either 2,2,6,6tetramethylpiperidinyloxy (TEMPO) or 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy (OH-TEMPO). Although CSA has been used to accelerate the rate in bulk nitroxidemediated polymerizations, it has not been well studied in emulsion/miniemulsion. With dispersed systems, the effectiveness of CSA is likely to be affected by partitioning between the aqueous and organic phases. In styrene miniemulsion experiments performed over a range of conditions, the effect of adding CSA varied from negligible to significantly increasing the final conversion and molecular weight, depending on the nitroxide:BPO ratio. At a ratio of nitroxide:BPO ϭ 1.7, the effect of CSA addition is small, whereas the final conversion and molecular weight are dramatically enhanced by CSA addition when the nitroxide:BPO ratio is 3.6. CSA is most effective in enhancing the rate and molecular weight when the initial free-nitroxide concentration is higher. The magnitude of the rate and molecular weight enhancement was similar for TEMPO and OH-TEMPO despite their differences in water solubility.

The introduction of the aqueous phase into a living radical polymerization increases the complexity of the kinetics by creating the possibility of species partitioning between the aqueous and organic phases, and introducing aqueous phase reactions which could play a significant role particularly in chain initiation and/or particle nucleation. We have conducted a series of styrene miniemulsion polymerizations in which the solubility of initiator and nitroxide have been systematically varied. Experiments were run using either water‐soluble (potassium persulphate) or oil‐soluble (benzoyl peroxide) initiator, and either TEMPO or 4‐hydroxy‐TEMPO. These two nitroxides vary considerably in their water solubility. The effects of initiator and nitroxide solubility in water on conversion‐time behaviour, molecular weight and initiator efficiency are presented.

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