Structural Arrangements in Polysilanes

For a triad of silicon atoms, there are four possibilities M,M (isotactic), R,R (syndiotactic), and M,R and R,M (heterotactic) the last two are equivalent since the polymer chain may be read in either direction. If there is no stereochemical preference, as in an atactic polymer, the amounts of isotactic, syndiotactic, and heterotactic triads would be 1 1 2, the heterotactic arrangement being twice as probable. 

If the relative configuration of two nearest neighbors on each side is considered, there are ten independent arrangements  

Various methods may be used to examine configurations of polysilanes, but 29Si NMR spectroscopy has been the most useful. Silicon-29, like carbon-13, has spin 1/2 and a relatively low abundance, 4.7%. Nuclear magnetic resonance (NMR) spectroscopy using 29Si has been important for the characterization of siloxane polymers, and is proving to be equally useful for polysilanes. 

In arylmethyl polymers the results are quite different. For the best studied example, (PhSiMe) , the 29Si NMR pattern consists of three broad lines, centered at -39.2, -39.9 and -41.2 ppm, evidently corresponding to different tacticities of atom 

RRRM = MRRR RRMR = RMRR RRMM = MMRR RMRM = MRMR 

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