This work deals with the kinetic and processing aspects of the synthesis of a polypropylene-phenol formaldehyde graft, copolymer compatibilizer. This compatibilizer is suitable for blends or alloys of polypropylene and engineering polymers having aromatic residues or functionality complementary to hydroxyl. The compatibilizer is synthesized from thermoplastic phenol formaldehyde (PF) resins and functionalized polypropylene (f-PP) by reactive extrusion. The reaction between maleic anhydride-(PP-g-MAH) and glycidyl methacrylate-functionalized (PP-g-GMA) PP and PF resins followed the predicted models normally used for solvent systems. In a twin-screw extruder, this synthesis was generally best performed in a screw profile offering very intensive mixing and, in particular, with kneading elements located early in the reaction zone. The reaction between PP-g-MAH and PF was enhanced by applying vacuum venting to the system. For the PP-g-GMA and PF reaction, no such effect of a vacuum was observed. The conversion of GMA, with or without catalyst, was generally significantly higher than for the MAH system. (C) 2000 John Wiley & Sons, Inc.