Frontal polymerization is a promising approach for preparing polymeric materials in which a localized reaction zone propagates through an unstirred medium. Large thermal and compositional gradients occur in the front that can lead to convective instabilities that interfere with the front propagation. With monomers that produce thermoplastic polymers, the Rayleigh-Taylor instability can destroy a descending front as the more dense molten polymer "fingers" into the unreacted monomer. Ascending fronts are not possible because simple convection caused by the heat release of the reaction quenches the front. Using a saltwater dispersion, we have shown for the first time that stable descending fronts with monomers such as benzyl acrylate can propagate through a saltwater dispersion if the density of the aqueous phase is greater than the density of the polymer. However, hydrolysis followed by intermolecular anhydride formation occurs at the high front temperature, resulting in a lightly cross-linked copolymer.