Interpolymer radical coupling reaction leading to block copolymer formation has been demonstrated for the first time in the solid state and in the absence of diffusion using solid-state shear pulverization. Radical coupling is often referred to as a diffusion-controlled reaction as it lacks an activation barrier to reaction. However, pulverization can lead both to intimate mixing, creating large interfacial area between blend components, and to chain scission, yielding polymer radicals and accommodating interpolymer coupling in the solid state. Fluorescence-detection gel permeation chromatography (GPC) vas used to detect interpolymer reaction in high-molecular-weight (MW) polystyrene (PS)/pyrene-labeled PS and high-MW poly(methyl methacrylate) (PMMA)/pyrene-labeled PS blends. The latter system was chosen as PMMA/PS blend compatibilization vas recently achieved via pulverization; this compatibilization was hypothesized to originate from in situ block copolymer formation via interpolymer radical coupling. Proof of coupling was obtained in this study from pyrene fluorescence in pulverized blends at GPC elution tunes less than those of the original prone-labeled PS. With the high-MW PS/pyrene-labeled PS mixture, comparison of label on coupled chains label on chains that underwent scission indicates that similar to5% of the pyrene-labeled char undergoing scission were able to couple to radicals originating from the high-MW PS. The effect of MW, blend composition, and screw design can chain scission during pulverization was also studied.