Kinetics of polymer bond scission, the initial step in thermal decomposition of polyethylene and other vinyl polymers, was investigated as a function of the number of repeat units using an improved Reactive Molecular Dynamics (RMD) approach, which is introduced here. The rate of scission per bond is shown to depend on the degree of polymerization, an effect not captured by the conventional practice of modeling polymer decomposition with small-molecule Arrhenius parameters. In the new approach, implemented in an open-source C++ code RxnMD, well-behaved reactive force fields are generated using switching functions that activate and attenuate terms obtained from a conventional, non-reactive force field. In this way, predefined reaction types are modeled accurately by interpolating smoothly between non-reactive potential energy terms describing reactants, transition states, and products. (C) 2011 Elsevier Ltd. All rights reserved.