Miniemulsion copolymerizations of methyl methacrylate and butyl acrylate in the presence of reactive costabilizer

The storage stability and free radical polymerizations of miniemulsions comprising methyl methacrylate (MMA), butyl acrylate (BA), and a reactive costabilizer stearyl methacrylate (SMA) were investigated. The Ostwald ripening rate increases with increasing MMA content in the monomer mixture. Both the pseudo-two-component model and empirical equation with one adjustable parameter k adequately predicted the Ostwald ripening rate data. For the empirical model, the least-squares best fit technique gave a value of k equal to 677.5 and values of Ostwald ripening rate and water solubility equal to (8.8 ± 0.2) x 10⁻²¹ cm³/s and 1.8 x 10⁻⁹ cm³/cm³ for SMA, respectively. These two models were combined to impart some physical insight to the parameter k. The kinetic studies showed that the polymerization rate increased with increasing MMA content. This is closely related to the nature of the constituent monomers MMA and BA and the particle nucleation mechanisms. The reactive costabilizer SMA is not hydrophobic enough to completely eliminate the Ostwald ripening effect, thereby increasing the probability of polymer reactions in the continuous aqueous phase. Thus, in addition to monomer droplet nucleation, particle nuclei can be generated in the aqueous phase via homogeneous nucleation. The extent of homogeneous nucleation increased with increasing MMA content and, as a result, the number of reaction loci available for the major polymerization to take place followed the same trend.