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

Using high-resolution magic angle spinning (MAS) solid-state NMR spectroscopy and density-functional theory (DFT) calculations, we determine the microstructure of the silicon-based functional polymer poly(1,4Si(6)) arising from the dehydrocoupling polymerization of cyclosilane 1,4Si(6). H-1-Si-29 refocused-INEPT solid-state NMR experiments allow the unambiguous determination of the number of attached protons to a silicon atom for each Si-29 NMR signal in 1,4Si(6) and poly(1,4Si(6)). One-dimensional H-1 -> Si-29 cross-polarization MAS (CPMAS) NMR spectra of poly(1,4Si(6)) show the development of SiH resonances upon polymerization, and peak integration indicates an average degree of polymerization of 20. The H-1 -> Si-29 CPMAS spectrum of poly(1,4Si(6)) also shows two sets of isotropic signals, suggesting the presence of at least two distinct species. Two-dimensional Si-29 dipolar double-quantum-single-quantum and single-quantum- single-quantum homonuclear correlation NMR spectra reveal similar connectivity in the two species, pointing to a stereochemical and/or conformational heterogeneity. DFT calculations on trimer models predict that chair or twist-boat conformations and with trans or cis diastereomers are all energetic minima. Si-29 chemical shift calculations of the lowest-energy structures show that conformers and stereoisomers are expected to give rise to distinct Si-29 NMR peaks and likely explain the appearance of multiple sets of Si-29 NMR signals. The strategy outlined here is expected to be widely useful for the microstructural elucidation of silicon-based functional polymers.