5924
Ind. Eng. Chem. Res. 2008, 47, 5924–5933
Modeling Assessment of Microemulsion Polymerization ´ lvarez§ F. Lo´pez-Serrano,*,† J. E. Lo´pez-Aguilar,† E. Mendiza´bal,‡ J. E. Puig,‡ and J. A Facultad de Quı´mica, Departamento de Ingenierı´a Quı´mica, UniVersidad Nacional Auto´noma de Me´xico, Me´xico D.F. 04510, CUCEI, Departamento de Quı´mica e Ingenierı´a Quı´mica, UniVersidad de Guadalajara, Guadalajara, Jalisco, Me´xico 44430, and Departamento de Ingenierı´a Quı´mica e Hidra´ulica, UniVersidad Auto´noma Metropolitana-Iztapalapa, Me´xico D.F. 09340
In this work, the problem of understanding some underlying phenomena in microemulsion polymerization is addressed, with an integrodifferential (ID) approach combining notions and concepts from microemulsion kinetics and thermodynamics. The procedure is applied to previously reported experimental data on the polymerization of hexyl methacrylate (C6MA), n-butyl methacrylate (nC4MA), and styrene (STY), in microemulsions stabilized with dodecyltrimethylammonium bromide (DTAB), and a simple mechanistic threeparameter model is presented that is capable of describing all systems studied. It was found that (i) the nucleation rate does not follow a linear behavior with time as claimed previously, (ii) the propagation rate coefficient (kp) decreases at high conversions suggesting a pronounced vitreous effect, even in reactions where the glass transition temperature of the polymer is lower than the reaction temperature, (iii) radical entry to polymer particles is negligible, (iv) coagulation among particles can be disregarded, and (v) the interval corresponding to the rate decrease is also caused by a reduction of active sites. 1. Introduction Even though microemulsion and emulsion polymerizations have many common features, these two processes are kinetically different and frequently the characteristics of the final products are dissimilar.1–6 However, in contrast to emulsion polymerization, models describing the kinetics of microemulsion polymerization are scarce. Guo7 et al. presented the first microemulsion polymerization model based on thermodynamic and kinetic arguments and found for styrene (STY) that (i) particle nucleation depends exponentially on conversion, (ii) monomer content in the particles drops linearly with conversion, (iii) chain growth termination is caused by transfer to monomer, (iv) there is a low average number of radicals in the particles (
