Multifunctional thermoplastic main-chain benzoxazine prepolymers were synthesized and systematically varied in their structure in order to function as high-performance toughener additives. Their unique chemical composition allows multiple covalent crosslinking with many thermoset network systems including benzoxazines and epoxides in conjunction with a defined chemical induced phase separation (CIPS) upon curing. This was successfully shown using a benzoxazine-based thermoset resin matrix as an example. The corresponding morphologies were addressable in a predictable manner and brought into context with the obtained macroscopic mechanical and thermal properties. In this relationship the CIPS process was classified and compared with the literature in more general means for advanced morphology control by differentiating between covalently attached and so-called gradient domain structures. The prepolymers were characterized by (1)H NMR, FT-IR, DSC and TGA. The thermoset morphologies were investigated by TEM and AFM. The fracture toughness (K(Ic)) and the elastic modulus (E) were measured by fracture and three point bending experiments. Thermal properties of the resulting films have been tested by DMA. (C) 2011 Elsevier Ltd. All rights reserved.