Copolymers formed by reactions between functional groups on two different polymers are known to stabilize the morphology formed during melt blending. Large effects on morphology have been reported, but it has been difficult to determine the amount or rate of block copolymer formation. We have anionically synthesized carboxylic acid terminal polystyrene (PS-COOH) and epoxy terminal poly(methyl methacrylate) (PMMA-E) of a narrow molecular weight distribution. To compare homogeneous coupling PS-E was also synthesized. The amount of block copolymer formed by coupling of PS-COOH with PMMA-E to form the ester was measured by size exclusion chromatography. PMMA-E (30%, Mn = 43 000) was melt mixed with PS-COOH (Mn = 42 600) in a small (0.3-g) parallel plate mixer with a maximum shear rate of 20 s-1. PMMA-E particle diameter was 0.6 μm and stable after 5 min of mixing at 180 °C. The block copolymer content increased to 2% at 20 min. The reaction is slow enough that the chain ends can sample the interface many times before reacting. Assuming that reaction can occur only at the interface (about 5 nm thick) gives reaction rates about twice as high as for homogeneous coupling. This indicates that the chain end concentration is higher in the interface or that interfacial reactions are more efficient. A random MMA-glycidal methacrylate copolymer coupled with PS-COOH at a similar reaction rate. Addition of a catalyst (N,N-dimethyldodecylamine) which is insoluble in both polymers increased the heterogeneous but not the homogeneous coupling.