The influence of the nature of confining walls on morphology development during bulk polymerization of methyl methacrylate in a poly(acrylonitrile-ran-butadiene) network was investigated by dynamic mechanical analysis and fight and scanning electron microscopy. Some samples polymerized while confined by metallic walls showed broad damping properties between the Tg of the components and, consequently, a microheterogeneous morphology. On the other hand, the same compositions, polymerized while confined by glass walls, showed clear separation into a macroscopic three-layer structure of poly(methyl methacrylate) film. between NBR layers. This suggests that phase separation of polymeric systems under symmetric boundary conditions also depends on the nature of the boundaries.