This chapter discusses the effect of the structure of the reactive diblock on the morphology. The rubber particles are responsible for stress concentration that results in multiple crazing or multiple shear deformation, depending on the matrix. The mechanism commonly accepted for the toughening of polyamide by finely dispersed rubber particles relies on the localized yielding of the matrix material induced by cavitation or debonding of the rubber. In the former case, the small particles might consist essentially of triblock copolymers, whereas the larger particles might result from the phase separation of the unreacted diblock copolymers restricted in size by the triblock copolymer that forms a stabilizing envelope. The typical cucumber-like core-shell was observed with the same characteristic features whatever the matrix, which confirms that the molecular architecture and composition of the ABC triblock have a decisive effect on the nanostructures that are formed in a variety of C matrices ranging from semicrystalline polyamide12 to glassy styrene-acrylonitrile.