Mechanisms and kinetics of isothermal polymerization of N,N'-bismaleimide-4,4'-diphenylmethane (BMI) with 5,5-dimethylbarbituric acid (55BTA) in the presence of triphenylphosphine (Ph3P, as a base catalyst for aza-Michael addition reaction) were investigated. Both model-free and model-fitting methods were used to determine triplet kinetic parameters, and comparable kinetic parameters obtained. Based on the model-free method, average activation energy (E-alpha) and pre-exponential factor (A(alpha) are 33 +/- 1 kJ mol(-1) and 8267 +/- 2 min(-1) in the a range 0.1-0.9, respectively, with g(alpha) = [(1 - alpha)(-0.1) - 1)/0.1 [i.e. f(alpha) = (1 - alpha)(1.1)]. As to the model-free method, overall activation energy (E) and pre-exponential (A) are 32 +/- 1 kJ mol(-1) and 4583 +/- 1 min(-1) with the g(alpha) = [(1 - alpha)-(11 - 1]/0.1. Aza-Michael addition reaction mechanism played an important role in the polymerization of BMI/55BTA in the presence of Ph3P at low temperature. By contrast, free radical polymerization predominated in the BMI/55BTA system at higher temperature.