This study was concerned with the chemical modification process of an atactic polypropylene from polymerization reactor byproducts with p-phenylen-bismaleamic acid to obtain a new grafted polymer supposed to play an important role as an interfacial agent in heterogeneous systems based on polymers. The grafting process was undertaken with the aid of the Box-Wilson experimental worksheet, with two independent variables (or controlled factors), the level of polar monomer to be grafted in the polymer and the concentration of the initiator, and with the agitation rate kept constant. Each item of the experimental design was performed at four different reaction times. This latter let us ascertain the transient nature of the process and the existence of a critical point in the concentration of both reactants conducting the system to the maximum grafting yield. (C) 2003 Wiley Periodicals, Inc.