Kinetics and Mechanism of Microemulsion Polymerization of Hexyl Methacrylate

A kinetic analysis of the microemulsion polymerization of hexyl methacrylate in mixed dodecyltrimethylammonium bromide/didodecyldimethylammonium bromide cationic microemulsions is developed and used to test mechanistic assumptions. In the limit of no bimolecular termination and fast radical capture, analytical expressions are derived for the conversion and reaction rate as a function of time and for the dependence upon the initiator concentration of the maximum reaction rate and the time at which it occurs. The conversion at which the rate maximum occurs is predicted to be constant at 39%. The theoretical results are in excellent agreement with experimental data through to 100% conversion for reactions initiated by azobis(amidinopropane) hydrochloride (V-50) and by buffered potassium persulfate, but not for unbuffered persulfate reactions. The discrepancy is argued to be a consequence of initiator hydrolysis in the unbuffered systems, and the importance of buffering persulfate reactions in mechanistic studies is emphasized. Some of the earlier studies of microemulsion polymerization are reviewed in light of this finding.