To improve the fracture toughness of PMR-15 polyimide and to alleviate its high susceptibility to microcracking induced by thermal cycling, a thermoplastic polyimide, LARC-TPI, was incorporated to form a sequential semi-interpenetrating polymer network (semi-2-IPN). The imidization kinetics of LARC-TPI in the semi-IPNs were studied using a thermal gravimetric analyzer. Both the solvent and the glass transition temperature of the semi-IPN were found to have significant effects on the imidization kinetics. The kinetics could be modeled by a two-step reaction : the first step being a second-order reaction followed by a second step, which is a first-order diffusion-controlled reaction. Differential scanning calorimetry was chosen to investigate the curing of PMR-15 and PMR-15/LARC-TPI semi-IPNs. The curing process was well correlated by a first-order reaction kinetics, which suggested that the reverse Diels-Alder reaction of the Norbornene end group was the rate controlling step. The glass transition temperatures of these semi-IPNs were again found to play important an important role in dictating the curing kinetics. A higher proportion of LARC-TPI or a higher glass transition temperature of the semi-IPN prepolymer tended to result in a slower curing reaction. The optimum molding cycle of PMR-15 and PMR-15/LARC-TPI semi-IPNs were then determined from the obtained kinetics.