This article describes the radical copolymerization of styrene (St) and n-butylmethacrylate (BMA) in a counter-rotating twin-screw extruder. In order to investigate the effect of the polymer melt viscosity on the working domain in reactive extrusion, the copolymer was crosslinked with different amounts of divinylbenzene (DVB). Crosslinking of the polymer resulted in a higher polymer melt viscosity. It was demonstrated that not only the product properties but also the process characteristics were strongly influenced by the addition of divinylbenzene to the reaction mixture. The product properties changed as expected: more crosslink agent led to a higher molecular weight or an increase in gel fraction, and a much higher melt viscosity of the polymer. Besides these effects on the product properties, the addition of DVB resulted in a more stable process, leading to an enlarged working domain.