Dense melts of coarse-grained representations of poly(silylenemethylene) (PSM) and poly(dimethylsilylenemethylene) (PDMSM) have been simulated at 373 K. The PSM melt has very little structure, and the individual chains have conformations in good agreement with the prediction from the rotational isomeric state (RIS) model for the single chain. For PDMSM, however, the melt is much more strongly structured. and the individual chains have mean square dimensions 40% higher than the ones predicted by the RIS model for the single chain. Intermolecular packing interactions in the structured PDMSM melt are accompanied by expansion of individual chains. (C) 2004 Elsevier Ltd. All rights reserved.