The structure of poly(di-n-propylsilylenene thylene) (PDPrSM) has been studied by X- ray diffraction, electron diffraction and molecular modeling. The fiber X-ray data and the electron diffraction pattern of single crystals show that PDPrSM has triclinic symmetry with unit cell parameters a = 10.52 Angstrom, b = 8.66 Angstrom. c = 4.86 Angstrom,= 78.4 degrees, beta = 100.0 degrees and gamma = 98.2 degrees .The fiber repeat, 4.86 Angstrom, indicates an all-gauche conformation in the main chain, giving a 4(1) helical structure. Ab initio and semi-empirical energy calculations show that with attachment of n-alkyl side chains an all-gauche conformation for the main chain gives the lowest energy. This is in contrast to the unsubstituted polymer. poly(silaethylene), which has a minimum energy for the all-trans conformation. The all-gauche conformation of the main chain of PDPrSM relieves the steric hindrance between side chains found in the all-trans conformation. (C) 2001 Elsevier Science Ltd. All rights reserved.