A process for producing high impact polystyrene including feeding at least one vinyl aromatic monomer, an elastomer, and a free radical initiator to a first linear flow reactor to form a reaction mixture. Polymerizing the reaction mixture in the first linear flow reactor to a point below the point at which phase inversion occurs to produce a first polymerization mixture and feeding the first polymerization mixture from the first linear flow reactor to a second linear flow reactor. Polymerizing the reaction mixture in the second linear flow reactor to at least a phase inversion point of the mixture to produce a second polymerization mixture and feeding the second polymerization mixture from the second linear flow reactor to at least a third linear flow reactor for post-inversion polymerization of the second polymerization mixture. The product stream can have an ESCR value of at least 10% toughness retained with less than 10 wt % rubber content.