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

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.