Si-29 and C-13 NMR investigation of the polysilane-to-poly(carbosilane) conversion of poly(methylchlorosilanes) using cross/polarization and inversion recovery cross-polarization techniques

The polysilane-to-polycarbosilane transformation of polymethylchlorosilane prepared from based-catalyzed disproportionation of 1,1,2,2-tetrachlorodimethyldisilane has been characterized in detail by Si-29 and C-13 magic angle spinning nuclear magnetic resonance, using cross-polarization as well as inversion recovery cross-polarization techniques, These techniques allow a clear insight in the protonated environment of a given nucleus, in particular to distinguish between strongly coupled nuclei such as (CH2)-C-13 and moderately coupled ones such as (CH3)-C-13. For the first time, the IRCP sequence was also used to probe the environment of Si-29 nuclei in such systems and proved to be very effective in distinguishing the silane and carbosilane sites, The 180-450 degrees C temperature range was investigated: the formation of carbosilane units was clearly demonstrated by C-13 and also Si-29 NMR experiments, The various Si-29 and C-13 sites were thus identified due to their polarization inversion behavior and quantified, Comparison of these results with a thermogravimetric analysis coupled with mass spectrometry allowed us to propose two different mechanisms for the formation of carbosilane units in such system: at low temperature (T greater than or equal to 180 degrees C), it is suggested that carbosilane units are formed via condensation reactions between Si-Cl and H-C groups, while at higher temperature (T greater than or equal to 389 degrees C), the so-called ''Kumada rearrangement'' occurs.