The structure of poly(di-n-propylsilylenemethylene)

The structure of poly(di-n-propylsilylenemethylene) (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 Å, b = 8.66 Å, c = 4.86 Å, α = 78.4°, β = 100.0° γ = 98.2°. The fiber repeat, 4.86 Å, indicates an all-gauche conformation in the main chain, giving a 4₁ 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.