The structure of poly(diethylsilylenemethylene) (PDESM), poly(di-n-butylsilylenemethylene) (PDESM), poly(di-n-pentylsilylenemethyl- ene) (PDPeSM) and poly(di-n-hexylsilylenemethylene) (PDHSM) have been studied using X-ray and electron diffraction methods. The structure of poly(di-n-propylsilylenemethylene) (PDPrSM) has been studied and reported in a separate paper. PDBSM, PDPrSM, PDPeSM and PDHSM all have similar packing with a two dimensional monoclinic ab-projection of the unit cell. PDESM with the shortest alkyl side chain, has a nearly orthorhombic unit cell (gamma* = similar to 88 degrees) containing two chains (four repeat units), one at the corner and the second nearly in the center of the ab-projection of the unit cell. The backbone conformation of each of these polysilylenemethylenes (PSMs) is all-gauche, giving them a 4(1) helical structure. The length of the alkyl side chains affects the packing without changing the backbone conformation. (C) 2001 Published by Elsevier Science Ltd.