In situ solvent free methods for making anhydride based graft copolymers which involve mixing an ethylenically unsaturated monomer, an anhydride monomer, either a monofunctional polyglycol having a hydroxyl or amine terminal group or a polyfunctional polyglycol having a weight average molecular weight of greater than 5,000 and at least one terminal hydroxyl group, and a free radical initiator to form a mixture. The mixture is heated to form a copolymer having repeat units derived from the ethylenically unsaturated monomer and anhydride monomer, and a copolymer grafted polyglycol comprising a main polyglycol backbone having copolymer side chains which are heated to form an anhydride based graft copolymer. The anhydride based graft copolymers are especially useful as soil release agents in detergent formulations.