Kumada rearrangement

Figure 6 provides a series of 29 Si MAS NMRs taken of PMS samples heated to selected temperatures. The Kumada rearrangement occurs at 400 °C and essentially phase pure SiC (2-4 nm crystallites by TEM) crystallizes at 1000 °C. Holding the polymer for periods of 5-10 h at 800 or 900 °C also results in crystallization. Additional MAS29Si NMR studies on samples pyrolyzed to 1400 °C showed no changes in peak position (from —15.7 ppm), suggesting the formation of well-defined /1-SiC crystalline phases at 1000 °C. 

FIGURE 6. - Si MAS NMR spectra recording the pyrolytic transformation of polymethylsilane, —[MeSiH] —, heated to selected temperatures for 1 h. Note the Kumada rearrangement to polycarbosilane, —[H2S1CH2L—. that occurs at 400°C. Crystalline /8-SiC forms at 1000°C... 

This work is consistent with the previously observed low temperature (200°Q onset of the Kumada rearrangement in polymethylsilane.20 The lower... 

However, interest in this technology was ignited by the extensive and pioneering work of Yajima and co-workers (6, 7). In an early work, Yajima and Hayashi (6) applied the known Kumada rearrangement to Burkhard s poly(dimethylsilane) polymer and obtained melt-spinnable polycarbosilane polymers (equation 2). 

In any case, various chemical reactions with different activation energies proceed competitively at such high temperatures. Perhaps the direct trapping of -Si-CH2 radicals by Si-H groups is favored to explain Si-CH2-Si formation in the case of PVS, while the Kumada rearrangement process, requiring many Si-Si bonds for methyl group insertion, is a minor reaction. 

Polysilanes have been the first class of precursors used to prepare silicon carbide ceramics. In all cases, on pyrolysis, polysilanes are converted into polycarbosilanes by a Kumada rearrangement prior to the formation of an amorphous silicon carbide network. Several synthetic routes including dehydro-polymerization, ring-opening polymerization of strained cyclosilanes, polymerization of masked disilenes, or base catalyzed disproportionation reactions of disilanes have been described to prepare linear or branched polysilanes but despite its drawbacks the Wurtz-coupling route remains the method applied most, especially to prepare linear polysilanes. 

The conversion of the polysilane into a polycarbosilane proceeds via a Kumada rearrangement [42] ... 

The decomposition chemistry of most polysilanes above 300 °C is quite complex and involves restructuring of the polymer backbone as well as substantial cleavage of the side groups. Generdly a poly(alkyl)silane must go through a carbosilane intermediate prior to formation of the SiC network (Kumada rearrangement) [125] ... 

The difference between the formula for PSP, namely SiCzHs, and that of the Yajima type PCSs from the Kumada rearrangement of PDMS, e.g., SiCi rrOoraHazo, suggests that the latter... 

PCSZ precursor melts and precursor fibers with a -Si-CH2-NH-Si- backbone (via the Kumada rearrangement) having tailored Si/C/N ratios [8-11] [18] [24],... 

To date, two precursor types have been identified that transform to nearly phase pure SiC. These are polymethylsilane (PMS, —CHsHSi—) and polysilaethylene (equation 33), which are related by the fact that heating PMS >300 °C transforms it via the Kumada rearrangement to polysilaethylene. 

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