An improved version of the reactive molecular dynamics method is presented. The method, which extends conventional (force-field-based) molecular dynamics to modeling chemical reactions, is used to simulate the thermal decomposition of polyisobutylene. The results of the simulations are generally consistent with experimental observations. A quantitative analysis of the results shows that the rate constant of the key initiation reaction, backbone scission, depends on the size of the molecular model of the polymer. This implies that the kinetics of some elementary reactions that take place in a polymer melt are affected by the macromolecular nature of the environment. (C) 2004 Elsevier Ltd. All rights reserved.