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 polymethylchlorosi lane prepared from based-catalyzed disproportionation of 1,1,2,2-tetra chlorodimethyldisilane has been characterized in detail by Si-29 and C -13 magic angle spinning nuclear magnetic resonance, using cross-polar ization as well as inversion recovery cross-polarization techniques, T hese techniques allow a clear insight in the protonated environment of a given nucleus, in particular to distinguish between strongly couple d 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 eff ective in distinguishing the silane and carbosilane sites, The 180-450 degrees C temperature range was investigated: the formation of carbos ilane units was clearly demonstrated by C-13 and also Si-29 NMR experi ments, The various Si-29 and C-13 sites were thus identified due to th eir polarization inversion behavior and quantified, Comparison of thes e results with a thermogravimetric analysis coupled with mass spectrom etry allowed us to propose two different mechanisms for the formation of carbosilane units in such system: at low temperature (T greater tha n or equal to 180 degrees C), it is suggested that carbosilane units a re formed via condensation reactions between Si-Cl and H-C groups, whi le at higher temperature (T greater than or equal to 389 degrees C), t he so-called ''Kumada rearrangement'' occurs.