Solid State NMR Study of Boron Coordination Environments in Silicone Boronate (SiBA) Polymers

Abstract

Silicone polymers possess very unusual properties when compared to organic polymers. The addition of grafted boronic acid groups allows for elastomeric film formation through self-association and enhances compatibility with biological systems by increasing elastomer miscibility with aqueous systems, pH tunability, and the ability to bind to saccharides. Boronic acid dimerization was reported to be the origin of cross-linking in silicone boronic acids, but the boron environments involved in this process remain poorly understood. Solid state 11B NMR was used to investigate the boron coordination environments in these materials. 11B quadrupolar line shape fitting, a method previously used to characterize minerals and amorphous glasses, revealed structural information, including coordination number and coordination sphere symmetry. Chain extension in these materials was attributed to hydrogen bonding between boronic acids and could be identified by the presence of three-coordinate boron sites. Cross-linking between boronic acid sites through four-coordinate, dative bonded boron centers was found to be the origin of elastomer formation; the oxygen Lewis bases on the silicone backbone do not appear to play a role. The proportion of boronic acid in the material and location of the boronic acid sitestelechelic versus pendantboth impacted cross-linking in these materials.

Self-Healing Amorphous Polymers with Room-Temperature Phosphorescence Enabled by Boron-Based Dative Bonds

“…The absorption of amide I and II bands occurs at lower frequencies in p-BAA-F than that in p-BAA-P (Figure 1a and Figure S2b), showing that p-BAA-F has the higher degree of hydrogen bonding and thus a stronger hydrogen-bonded environment than p-BAA-P. 46 The symmetrical narrow peak in solid-state 11 B NMR spectra (Figure 1b) demonstrates the tetrahedral four-coordinated structure of boron atoms. 47 The four-coordinated boron centers of p-BAA-P and p-BAA-F can be further proved by XPS. The B 1s signal (Figure 1c) can be fitted with two peaks with binding energies at 191.3 and 190.8 eV, corresponding to B−O and B−N dative bonds, respectively.…”

Self-Restructuring of Polyhydromethylsiloxanes by the Hydride Transfer Process: A New Approach to the Cross-Linking of Polysiloxanes and to the Fabrication of Thin Polysiloxane Coatings

“…The silicone networks obtained as a result of these reactions are contaminated with metals, which lowers their thermal, dielectric, and optical properties. New condensation and addition methods for cross-linking polysiloxanes have recently been introduced, but they require laborious syntheses [ 13 , 22 , 23 ]. Silicone polymers can also be cross-linked by free radicals formed at high temperatures from various azo or peroxide initiators [ 1 , 2 , 24 ].…”

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

“…High-resolution magic angle spinning (MAS) solid-state NMR (SSNMR) spectroscopy is a powerful technique to probe microstructure and self-assembly in both organic and inorganic polymers. − Conventional one-dimensional (1D) 13 C SSNMR spectroscopy provides information about the identity of the constitutional repeat units present in the polymer and can also be used to quantify and identify crystalline and amorphous domains. − NMR-active nuclei other than 13 C may also be used to probe polymers by SSNMR spectroscopy. For example, 11 B SSNMR spectroscopy provided insight into the variable boron coordination environments in silicone boronate polymers . One-dimensional 29 Si SSNMR experiments have been previously used to characterize linear polysilanes, such as poly­(dimethylsilylene), , poly­(di- n -hexylsilane), and poly­(di- n -R x silylene- co -di- n -R x +1 silylene) copolymers .…”

“…For example, 11 B SSNMR spectroscopy provided insight into the variable boron coordination environments in silicone boronate polymers. 47 One-dimensional 29 Si SSNMR experiments have been previously used to characterize linear polysilanes, such as poly(dimethylsilylene), 48,49 poly(di-nhexylsilane), 50 and poly(di-n-R x silylene-co-di-n-R x+1 silylene) copolymers. 51 More complex heteronuclear correlation (HET-COR), homonuclear correlation, spin diffusion, and dipolar coupling measurement experiments are often required to fully understand the solid-state structure and dynamics of polymers.…”