Investigating the Microstructure of Poly(cyclosilane)\nby 29Si Solid-State NMR Spectroscopy and DFT Calculations

Using high-resolution magic\nangle spinning (MAS) solid-state NMR spectroscopy and density-functional\ntheory (DFT) calculations, we determine the microstructure of the\nsilicon-based functional polymer poly­(1,4Si6) arising from the dehydrocoupling polymerization of\ncyclosilane 1,4Si6. 1H–29Si refocused-INEPT solid-state NMR experiments\nallow the unambiguous determination of the number of attached protons\nto a silicon atom for each 29Si NMR signal in 1,4Si6 and poly­(1,4Si6). One-dimensional 1H → 29Si cross-polarization\nMAS (CPMAS) NMR spectra of poly­(1,4Si6) show the development of SiH resonances upon polymerization,\nand peak integration indicates an average degree of polymerization\nof 20. The 1H → 29Si CPMAS spectrum of\npoly­(1,4Si6) also shows two sets\nof isotropic signals, suggesting the presence of at least two distinct\nspecies. Two-dimensional 29Si dipolar double-quantum–single-quantum\nand single-quantum–single-quantum homonuclear correlation NMR\nspectra reveal similar connectivity in the two species, pointing to\na stereochemical and/or conformational heterogeneity. DFT calculations\non trimer models predict that chair or twist-boat conformations and\nwith trans or cis diastereomers\nare all energetic minima. 29Si chemical shift calculations\nof the lowest-energy structures show that conformers and stereoisomers\nare expected to give rise to distinct 29Si NMR peaks and\nlikely explain the appearance of multiple sets of 29Si\nNMR signals. The strategy outlined here is expected to be widely useful\nfor the microstructural elucidation of silicon-based functional polymers.