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 Me2Si=O and MeSiH=O. Model calculations on the decomposition pathways of H3SiOSiH3 and H3SiOSiH2OSiH3 show that silanone elimination is favoured over silylene extrusion.

A Study of Pyrolysis of Polymethylsiloxanes by Fourier Transform Infrared

“…Apparently, such a spectrum reflects a complex composition of the gas phase formed by a large number of molecules with various chemical bonds: Si-O-Si (1000-1100 cm −1 , Si-C-Si and Si-O-C (900-1100 cm −1 3 9 and Si-C in the range of 800 cm −1 . 23 The HMDSO pyrolysis study demonstrates [24][25][26] that the transformation begins with the siloxane bonds break with formation of a silyl radical and a silanone molecule:…”

Dimethylsilanone Generation from Pyrolysis of Polysiloxanes Filled with Nanosized Silica and Ceria/Silica

“…On the one hand, many works, in particular those dealing with the polymer's industrial applications, suggest that polydimethylsiloxane‐based materials are well suited for foam control in food processing owing to their thermal stability and chemical inertness . On the other hand, various linear and cyclic PDMS have been reported to lead to thermal generation of the silicon analogue of acetone, that is, dimethylsilanone [(CH 3 ) 2 Si=O], which is believed to be an extremely reactive compound . Indeed, polydimethylsiloxane‐based silicon greases, which are believed to be chemically “inactive entities”, have been the subject of two review articles .…”

“…Several experimental studies have suggested that silanones are formed as intermediates in the thermal reactions of low‐molecular organosilicon compounds such as silylperoxides and hydrosilylperoxides, siloxetanes, silenes, hydridosilylketenes, allyloxysilanes, alkoxyvinylsilanes, and polysilylated diazomethanes . Linear and cyclic PDMS were reported to generate dimethylsilanone . These claims of dimethylsilanone formation were usually based on kinetic data and chemical trapping experiments .…”

“…[9,10] On the other hand, various linear and cyclic PDMS have been reported to lead to thermal generation of the silicon analogue of acetone, that is, dimethylsilanone[ (CH 3 ) 2 Si= O],w hich is believed to be an extremely reactive compound. [15][16][17][18][19][20][21] Indeed, polydimethylsiloxane-based silicon greases, which are believed to be chemically "inactive entities", have been the subject of two review articles. [22,23] These reviews indicated that PDMS can actually function as as ource of (CH 3 ) 2 Si=O, leading to undesirable side reactions that yield various exotic molecular and supramolecular compounds.…”

“…[16,[34][35][36][37][38][39] Linear and cyclic PDMS were reported to generate dimethylsilanone. [15][16][17][18][19][20][21] These claims of dimethylsilanone formationw ere usually based on kinetic dataa nd chemical trapping experiments. [16][17][18] Amongt he direct physical methods, matrixi solation IR spectroscopy was used to identify the involvement of this compound as an intermediate in the thermal redistributiono fs omel ow-molecular cyclic and linear siloxanes.…”