A mechanistic study of the low pressure pyrolysis of linear siloxanes

Abstract

Matrix isolation IR spectroscopy has been used to study the vacuum pyrolysis of 1,1,3,3-tetramethyldisiloxane (L1), 1,1,3,3,5,5-hexamethyltrisiloxane (L2) and 3H,5H-octamethyltetrasiloxane (L3) at ca. 1000 K in a flow reactor at low pressures. The hydrocarbons CH₃, CH₄, C₂H₂, C₂H₄, and C₂H₆ were observed as prominent pyrolysis products in all three systems, and amongst the weaker features are bands arising from the methylsilanes Me₂SiH₂ (for L1 and L2) and Me₃SiH (for L3). The fundamental of SiO was also observed very weakly. By use of quantum chemical calculations combined with earlier kinetic models, mechanisms have been proposed involving the intermediacy of silanones Me₂SiO and MeSiHO. Model calculations on the decomposition pathways of H₃SiOSiH₃ and H₃SiOSiH₂OSiH₃ show that silanone elimination is favoured over silylene extrusion.