In this study, a novel bi-order model combined with zero- and first-order kinetics was proposed for the decomposition of PMMA (MW=120,000 g/mol) in concentrated HNO₃ by microwave irradiation. To develop and validate this model, Fourier Transform Infrared spectroscopy, scanning electron microscopy, fractional-life method, the gravimetric analysis and Newton's method were utilized. Rate constants, activation energies, the pre-exponential factors and the weight fractions (φ) via main-chain scission for the decomposition at 423-453 K were derived from this model. The zero-order reaction was observed dominant at 423-443 K, while the first-order reaction dominated at 453 K and 473 K. The digestion efficiency increased as HNO₃ was increased to >3 mL at 423 K-443 K. At 473 K, the digestion was almost 100% when HNO₃ volume was >3 mL. The estimated φ values increased with HNO₃ volume at 423 and 443 K, but varied insignificantly at 453 and 473 K.
