Probing the Microstructure of Poly(Cyclosilane)

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(Left) Simplified polymeric microstructures of poly(cyclosilane) consisting of a trans (blue) and cis (red) diastereomer with chair and twisted-boat cyclosilanering conformations. (Right, Top) Natural abundance 2D 29Si double-quantum-single-quantum homonuclear correlation NMR spectrum. (Right, Bottom) Comparison of the experimental and DFT calculated 29Si NMR spectra for both diastereomersThe synthesis of silicon-based soft matter is a growing field, with representative achievements in colloidal nanocrystals, integrated nanocrystal-polymer hybrids, donor-acceptor chromophores, molecular electrons and conjugated polymers. A comprehensive understanding of a structure-function relationship facilitates the rational design and development of next-generation materials. However, polymers are often challenging to characterize because a lack of long-range periodic order and poor solubility hinder single-crystal X-ray diffraction and solution NMR spectroscopy studies, respectively. This paper provides a detailed protocol to more precisely elucidate the microstructure of silicon-based polymers in the context of the dehydrocoupling polymerization of cyclosilane 1,4Si6 to poly(1,4Si6) using high-resolution solid-state NMR spectroscopy and density-functional theory (DFT) calculations. 1H-29Si refocused-INEPT NMR experiments allowed for the unambiguous determination of the number of attached hydrogens to each 29Si NMR signal in 1,4Si6 and poly(1,4Si6). 1D 1H→29Si cross-polarization magic-angle spinning NMR spectra showed the development of SiH resonance upon polymerization and peak integration suggested and average degree of polymerization of 20. 2D 29Si double-quantum-single-quantum and single-quantum-single-quantum homonuclear correlation NMR experiments recorded at natural isotopic abundance revealed the connectivity within the distinct polymeric microstructures. DFT calculations of simplified trimer models suggested that the two lowest energy structures correspond to trans and cis diastereomers with chair and twist-boat cyclosilane ring conformations, respectively. This study supports the future optimization of structure-property relationships by revealing key details of the microstructure of polycyclosilanes. Furthermore, the approaches outlined here should be useful to determine the molecular structure, conformation and connectivity of other silicon-based polymers and materials.

R.W. Dorn, E.A. Marro, M.P. Hanrahan, R.S. Klausen,*  A.J. Rossini.* Investigating the Microstructure of Poly(Cyclosilane) by 29Si Solid-State NMR Spectroscopy and DFT Calculations , Chemistry of Materials 31, 9168−9178 (2019), DOI: 10.1021/acs.chemmater.9b03606.