Stereostructures of linear and cyclic polyphenylsilanes produced by dehydrocoupling in the presence of group 4 metallocene catalysts

Phenylsilane was polymerized with a number of group 4 metallocene precataly sts, including (S,S)-[1,2-bis(eta(5)-tetrahydroindenyl)ethane]titanium bina phtholate (1)/2BuLi, Cp2HfCl2 (2)/2BuLi/B(C6F5)(3), and CpCp*ZrCl2 (3)/2BuL i/B(C6F5)(3). With 1, polymerization proceeds extremely slowly and it is po ssible to observe the stepwise oligomerization to Si-2, Si-3, Si-4, and Si- 5, species. One- and two-dimensional H-1 and Si-29 NMR experiments permit t he assignment of peaks to the stereoisomers of the Si-4 and Si-5 compounds, For the linear oligomers PhSiH2SiHPh(SiHPh)(n)SiHPhSiH2Ph (n greater than or equal to 0), the Si-29 NMR resonances group cleanly into the regions del ta (ppm) 58-59 (PhSiH2), 59-63 (SiHPhSiHPhSiHPh), and 64-65 (PhSiH2SiHPhSiH Ph). The chiral catalyst 1 has little influence on the sterochemical outcom e of the coupling reaction, Higher molecular weight polymers synthesized wi th 2 and 3 or obtained by fractionation of lower molecular weight polymers all have nearly identical Si-29 NMR spectra. It is proposed that this spect rum is a reflection of the pentad composition of the polymers and that any deviations from randomness are a result of intrachain interactions at the g rowing chain end and are unrelated to the structure of the catalyst.