Core-shell flow improver

A process for making a flow improver, comprising:

• (a) forming a plurality of core particles of a drag reducing polymer by emulsion polymerization;
• and
• (b) forming polymeric shells around at least a portion of the core particles by emulsion polymerization to thereby produce a plurality of core-shell particles,
• wherein the polymeric shell is formed around the core particle by emulsion polymerizing at least one hydrophobic monomer and at least one polymerizable surfactant in the presence of the core particle,
• wherein the core-shell particles have a mean particle size of less than 10 microns, and
• wherein the polymeric shell comprises repeat units of the at least one hydrophobic monomer in an amount in the range of from about 25 to about 98 weight percent.