Stereostructures of Linear and Cyclic Polyphenylsilanes Produced by Dehydrocoupling in the Presence of Group 4 Metallocene Catalysts
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Abstract
Phenylsilane was polymerized with a number of group 4 metallocene precatalysts, including (S,S)-[1,2-bis(η5-tetrahydroindenyl)ethane]titanium binaphtholate (1)/2BuLi, Cp2HfCl2 (2)/2BuLi/B(C6F5)3, and CpCp*ZrCl2 (3)/2BuLi/B(C6F5)3. With 1, polymerization proceeds extremely slowly and it is possible to observe the stepwise oligomerization to Si2, Si3, Si4, and Si5 species. One- and two-dimensional 1H and 29Si NMR experiments permit the assignment of peaks to the stereoisomers of the Si4 and Si5 compounds. For the linear oligomers PhSiH2SiHPh(SiHPh)nSiHPhSiH2Ph (n ≥ 0), the 29Si NMR resonances group cleanly into the regions δ (ppm) 58−59 (PhSiH2), 59−63 (SiHPhSiHPhSiHPh), and 64−65 (PhSiH2SiHPhSiHPh). The chiral catalyst 1 has little influence on the sterochemical outcome of the coupling reaction. Higher molecular weight polymers synthesized with 2 and 3 or obtained by fractionation of lower molecular weight polymers all have nearly identical 29Si NMR spectra. It is proposed that this spectrum is a reflection of the pentad composition of the polymers and that any deviations from randomness are a result of intrachain interactions at the growing chain end and are unrelated to the structure of the catalyst.
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