Structure and Distribution of Cross-Links in Boron-Modified Phenol–Formaldehyde Resins Designed for Soft Magnetic Composites: A Multiple-Quantum ¹¹B–¹¹B MAS NMR Correlation Spectroscopy Study

Despite the extensive use of boron-modified phenol–formaldehyde polymers as insulating materials in soft magnetic composites (SMCs), the structure and arrangement of the inorganic cross-linking units in these systems have not been fully elucidated. To clarify the structure, configuration, and distribution of the boron cross-links in these materials, phenol–formaldehyde resins modified by boric acid were synthesized and characterized using advanced multiple-quantum ¹¹B–¹¹B MAS NMR correlation techniques combined with the quantum chemical geometry optimizations and the subsequent ¹¹B NMR chemical shielding calculations. The analyses of the resulting spectra revealed a well-evolved (high-density) phenol–formaldehyde polymer network additionally strengthened by nitrogen and boron cross-links. The boron-based cross-links were attributed to monoester (ca. 10%) and diester (ca. 90%) complexes (six-membered spirocyclic borate anions) with strictly tetrahedral coordination (B^IV). During the thermal treatment, the monoester and diester borate complexes underwent additional transformation in which the spirocyclic borate anions were more tightly incorporated into the polymer matrix via additional N-type (amino) cross-links. A ¹¹B–¹¹B double-quantum correlation MAS NMR experiment revealed that the majority of the monoester and diester borate complexes (ca. 80%) were uniformly distributed within and effectively isolated by the polymer matrix, with an average ¹¹B···¹¹B interatomic distance greater than 6 Å. A non-negligible part of the spirocyclic borate anion complexes (ca. 20%), however, existed in pairs or small clusters in which the average ¹¹B···¹¹B interatomic distance was less than 5.5 Å. In addition, the formation of homodimers (diester–diester) was demonstrated to be preferred over the formation of heteroclusters (monoester–diester).