MECHANISM OF PYROLYSIS OF POLYCARBOSILANES - POLY(SILYLETHYLENE) AND POLY(DIMETHYLSILYLETHYLENE)

The thermal degradation of linear polycarbosilanes, poly(silylethylene), (Si(H2)C2H4)n, and poly(dimethylsilylethylene), (Si(Me2)C2H4)n, having the same backbone an differing by the nature of the substituents of the silicon atoms, was investigated by means of thermogravimetric analysis coupled to mass spectrometry and solid-state NMR spectroscopy. Both fragmentation and cross-linking mechanisms appear strongly dependent on the substituents on the silicon atoms, as shown by the nature of the gases and volatile organosilicon compounds evolved and the pyrolysis yields. In the case of poly(dimethylsilylethylene), random chain scission via a simple free-radical mechanism explains the formation of the volatile products detected during the degradation. On the other hand, in the case of poly(silylethylene), the fast elimination of hydrogen at temperatures as low as 380-degrees-C and the formation of SiH2Et chain ends cannot be accounted for by a free-radical mechanism and suggests the formation of silylene species. Thus, the formation of silylenes by 1,1-elimination of H-2, followed by insertion and rearrangement reactions, would provide an efficient way of cross-linking, whereas the formation of silylenes by 1,1-cleavage of Si-C and Si-H bonds would account for the formation of SiH2Et chain ends and the fragmentation of the polymer.