The diffusion effects on chain extension reactions, using carboxyl-terminated polyamide 12 (PA 12, say CA in this paper) as a model reactant with bisoxazolines (OO), were simulated by the stochastic Monte Carlo (MC) method. The chain length dependence and detailed diffusive behavior of each chain were considered based on the reptation theory. It is found that the diffusion effects retard the progress of chain extension reactions and, lead to a lower coupling efficiency and a lower molecular weight. The simulated results indicate that the diffusion effects can make the final molecular weight distribution (MWD) wider. In the presence of diffusion and with the progress of p, peaks in the evolution curves of weight-average molecule weight ((M) over bar (w)) and valleys in the evolution curves of polydispersity index, d, are observed, respectively, when p is small enough, which is different from those without diffusion effects, and has been analyzed in detail in this paper. The results also show that the effects of reactant degradation make the MWD of the reaction system wider, and weaken the diffusion effects on the coupling reaction. (c) 2006 Elsevier Ltd. All rights reserved.