Abstract: A process for the impact modification of a plastic with a rubber polymer to form a thermoplastic resin in a reactor extruder is provided. The process includes the steps of dissolving a rubber polymer in a solvent selected from the group consisting of ethylenically unsaturated monomers and non-polymerizable organic compounds to form a feed solution, introducing the feed solution into the feed section of a twin screw reactor extruder, introducing a separate feed stream containing a material in sufficient amount to precipitate the rubber polymer, heating the feed solution and feed stream under pressure to a temperature sufficient for polymerization to begin, polymerizing the monomers with the rubber polymer in the reactor extruder to form a thermoplastic resin and extruding the thermoplastic resin through a die. A novel ABS thermoplastic resin is also provided having a uniform distribution of rubber particles dispersed within the plastic phase of the resin, the rubber particles ranging in size of from about 0.