Reactive copolymers of alkyl methacrylates containing glutaric anhydride units are useful in compatibilizing blends of two or more polymers to improve physical properties of the blend. The reactive copolymers are especially useful when the polymers to be blended are incompatible with each other, but separately compatible with the reactive copolymer. Compatibilization by the reactive copolymer may be enhanced in some cases by catalysis of the grafting reaction between the reactive copolymer and one or more of the polymers to be blended.