The pyrolysis of polypropylene was modeled at the mechanistic level to predict the formation of low molecular weight products. Differential equations were developed that describe the evolution of the moments of structurally distinct polymer species. Unique polymer groups were devised that allowed the necessary polymeric features for capturing the pyrolysis chemistry to be tracked, while maintaining a manageable model size. The conversion among the species was described using typical free radical reaction types, including intermolecular hydrogen abstraction, midchain beta-scission, end-chain beta-scission, intramolecular hydrogen transfer, radical addition, bond fission, radical recombination, and disproportionation. The model included over 24 000 reactions and tracked 213 species (27 products tracked with molecular weights below 215 amu). The intrinsic kinetic parameters (a frequency factor and activation energy for each reaction) were obtained from data in the literature and previous modeling work in our laboratory. 1,2 The model predictions for the evolution of the yields of five major alkenes and five major alkanes compare well with experimental data obtained in our laboratory for the pyrolysis of polypropylene over a temperature range of 350-420 degreesC. In addition, literature data(3) for the evolution of the polypropylene molecular weight was captured by incorporating weak backbone links modeled as peroxide bonds.