The heat-induced, solvent-free copolymn. of styrene (I) with maleic anhydride (II) was studied on a pilot scale in a loop reactor coupled to a tube reactor. Under conditions required for the manuf. of alternating copolymers (stoichiometric ratio of I to II or excess II) rapid reaction kinetics were obsd. resulting from the Trommsdorff effect, which, with the high reaction enthalpy of .apprx.80 kJ/mol. led to thermal instabilities esp. during start-up and at low recirculating rates. This was overcome by effective reactor control. The manuf. of products with low contents of II was also demonstrated possible by employing low II concns. (4-16%) in the monomer mixt. and rapid and efficient reactant mixing. [on SciFinder (R)]