Copolymeric resin binder powders

The method of making a copolymer powder having a residual monomer content therein of less than 0.05 percent by weight and having a primary particle size less than one micron in diameter, said powder being adaptable to rapid solution in an organic solvent to form a solution for coating a core comprising a therapeutically active ingredient, which method comprises spray drying an aqueous dispersion of copolymer particles at a temperature such that the temperature of the copolymer particles does not exceed the minimum film-forming temperature of the copolymer, said dispersion being prepared by the aqueous emulsion polymerization of monomers to give particles of a size less than one micron in diameter of a physiologically tolerable copolymer comprising

• (a) 5 to 80 percent by weight of a water-soluble monomer selected from the group consisting of .alpha.,.beta.-unsaturated mono- and dicarboxylic acids, acrylamide, methacrylamide, hydroxy lower alkyl esters of acrylic acid and methacrylic acid, monoalkyl- and dialkyl-amino lower alkyl esters of acrylic acid and methacrylic acid, quaternary ammonium salts of such amino lower alkyl acrylates and methacrylates, vinyl pyrrolidone, and vinyl imidazole;
• and
• (b) 95 to 20 percent by weight of a monomer selected from the group consisting of styrene, vinyl acetate, and alkyl esters of acrylic acid and methacrylic acid having 1 to 10 carbon atoms in the alkyl portion thereof;
• the relative amounts of monomers (a) and (b) in said copolymer being such that the minimum film-forming temperature thereof is greater than 80.degree. C., said copolymer further being water-insoluble in one portion of the region between pH 1.5 and pH 8 and water-soluble or water-swellable in another portion of said region.