the present invention discloses a process for manufacturing monovinylaromatic compounds containing an elastomer to provide increased toughness and strength . one particular embodiment of the invention discloses a process for manufacturing high impact polystyrene wherein styrene monomer containing a dissolved polybutadiene rubber is flowed into an elongated upflow stirred reactor containing three reaction zones , wherein the styrene and rubber are polymerized past the inversion point , then discharged to at least one additional reactor to continue their conversion to polymer .