Molecular dynamics simulations of merging process of two isolated cyclic chains and that of linear chains have been performed, in order to find the difference between the two kinds of chain in bulk and in vacuum, where surface of the system restrains thousands of configurations showing up. Analysis indicates that in such confinement the chain ends at most moments float on the surface layer, and their mobility is much larger than the mean value of all atoms in linear chains, or that in cyclic chains. Comparison of the merging processes of cyclic chains and their linear counterparts was intensively carried out by several means of analyzing the trajectory files in statics and in dynamics. It was found that the segments of cyclic chains showed almost the same behavior of motion with that of linear chains. This is different diffusion behavior from that in bulk systems, where the cyclic chain has much higher diffusion rate. This modeling therefore indicates that in these confined systems because of the surface energy, the end groups are involuntarily kept on the surface, and lost the capacity of leading chain reptation through the other polymer chains, which is possibly the origin that the difference of diffusion behavior between the linear chain and the cyclic chain in the bulk almost disappears in the case of the confinement. (C) 2004 Elsevier Ltd. All rights reserved.