Synthesis of latex capsules

An improved process for making an aqueous dispersion of water-insoluble polymer particles comprising:

• (a) sequentially emulsion polymerizing in an aqueous medium containing a free radical initiator, a core monomer system comprising one or more monoethylenically unsaturated monomers and at least about 5 mole percent of an ethylenically unsaturated carboxylic acid or anhydride monomer, whereby dispersed core particles are formed having an average diameter of from about 0.05 to 1 micron, and
• (b) polymerizing in the presence of the dispersed core particles resulting from (a), a shell monomer system comprising:
• at least one monoethylenically unsaturated monomer having no ionizable group to form a shell polymer on the core particles, wherein any monoethylenically unsaturated carboxylic acid in the shell monomer mixture is present in an amount of no more than about 10 mole percent of the shell monomers, wherein the equivalents of acid in said shell polymer do not exceed one-third of the equivalents of acid in said core particle, the resultant core-shell particles having an average diameter before neutralization and swelling of from about 0.07 to 4.5 microns, the relative amounts of core-forming monomer(s) and shell-forming monomer(s) being such that the ratio of the weight of the core to the weight of the total polymer in the resulting dispersed particles is from about 1:2 to 1:100, and
• (c) neutralizing said particles formed with a base so as to swell said core and form particles containing a microvoid in the core, wherein the improvement comprises swelling said core to a point where said shell explodes, forming particles containing a microvoid in the core and at least one channel connecting the microvoids to the exterior of the particle.