A large number of improved high-temperature stable semi-interpenetrating polymer networks (semi-IPNs) were synthesized by condensing copolymerized resins with castor oil-based polyurethane using ethylene glycol dimethacrylate (EGDM) as a crosslinker. The structures of these semi-IPNs were studied using various characterization techniques such as LR and nuclear magnetic resonance (NMR) spectra, and the thermal behavior of the semi-IPNs was also studied by thermogravimetric analysis (TGA). A plausible degradation mechanism was suggested in our previous communication.