Rubber modified prepolymers were synthetized through a nucleophilic addition of diamines to aromatic bismaleimides, using N,N＇bismaleimido-4-4＇diphenylmethane (BMI), 1,4-piperazine (PyP), and an amino-terminated polydimethylsiloxane elastomer (PDMS). Thermal stability was investigated by thermogravimetric analysis (TGA) in order to elucidate the influence of diamines. Physical and rheological changes during the curing process were monitored by differential scanning calorimetry (DSC) and dynamic mechanical tests (DMA). The degree of cross-linking of the sample with PDMS was higher than that of the sample without PDMS. This effect might be due to a larger chain mobility provided by the rubber during the cure.