The present invention discloses an emulsion polymerization process that comprises: (1) preparing an aqueous polymerization medium which is comprised of (a) at least one monomer, (b) a polymerization control agent, and an emulsifier, wherein the emulsifier is prepared in-situ within the aqueous polymerization medium; and (2) initiating polymerization of said monomer within the aqueous polymerization medium. The subject invention more specifically reveals an emulsion polymerization process that comprises: (1) preparing a monomer solution which is comprised of (a) at least one monomer, (b) a conjugate acid of a surfactant with a pK.sub.a of less than 14, and (c) a controlled free radical polymerization agent; (2) preparing an aqueous medium which is comprised of (a) water, and (b) a conjugate base of a weak acid wherein the pK.sub.b of the base is less than 14; and (3) mixing the monomer solution with the aqueous medium under conditions that result in the in-situ formation of an emulsifier, and (4) initiating free radical polymerization. This technique can be used to synthesize unique block polymers, such as a block polymer which is comprised of an A block and a B block, wherein the A block is a polymer block which is comprised of a vinyl aromatic monomer and wherein the B block is a polymer block which is comprised of (a) at least one monomer selected from the group consisting of acrylonitrile and methacrylonitrile and (b) a conjugated diolefin monomer, such as 1,3-butadiene or isoprene.