Structure and Distribution of Cross-Links in Boron-Modified Phenol

Article pubs.acs.org/Macromolecules

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 Libor Kobera,† Jiri Czernek,† Magda Strečková,‡ Martina Urbanova,† Sabina Abbrent,† and Jiri Brus*,† †

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky sq. 2, 162 06 Prague 6, Czech Republic ‡ Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 040 01 Košice, Slovak Republic S Supporting Information *

ABSTRACT: 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 multiplequantum 11 B− 11 B 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 (highdensity) 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 (BIV). 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).

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INTRODUCTION The thermosetting phenol−formaldehyde (PF) resin Bakelite is one of the first fully synthetic polymers and has been known for approximately 100 years.1 Currently, alkali-catalyzed PF resins are used as composites or coating materials because of their excellent thermal stability, dielectric properties, and mechanical and flexural strength.2−5 The wide use of PF resins has promoted extensive research aimed to improve their final properties according to use. Usually, PF resins are modified by inorganic silicon or boron compounds.4−8 Such chemical modifications open new possibilities for using PF resins as for instance binders of soft magnetic composites (SMCs), fire retardants, resin transfer moldings, chainlike core−shell nanowires, green luminescent silver/phenol−formaldehyde resin core/shell spheres, and other advanced materials.5,7−10 Recently, novel SMCs composed of spherical FeSi powder covered with phenolic resin and chemically modified with silica nanoparticles (SiO2) have been prepared and investigated in detail. The designed FeSi/PF-SiO2 composites exhibited © XXXX American Chemical Society

mechanical properties similar to those of the sintered FeSi powder, whereas the resulting electric and magnetic properties of the material were quite superior with respect to those of the sintered FeSi powder.8 This material thus represents a promising SMC with a remarkable combination of mechanical, electrical, and magnetic properties. For further development and optimization of the end-use properties of the SMCs, novel modifications of PF resins with various inorganic cross-linkers have been extensively researched. An indispensable part of the rational design of SMCs is detailed knowledge of the structure of the newly developed polymeric matrixes. The ordering of untreated and cured PF resins and the structure of PF-SiO2 hybrids are wellknown.7,11,12 However, the structure of the phenol−formaldehyde resins modified by boron compounds (PFRBs) has Received: May 14, 2015 Revised: July 1, 2015

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Macromolecules Scheme 1. Schematic Representation of Polymerization of Phenol−Formaldehyde Resins under Alkaline Conditions

Solid-State NMR Spectroscopy. Solid-state NMR spectra were obtained at 11.7 T using a Bruker Avance III 500 HD WB/US NMR spectrometer with 4 mm and 3.2 mm MAS probeheads. The 13C CP/ MAS NMR spectra were recorded at a Larmor frequency of ν0(13C) = 125.783 MHz at a spinning frequency of ωr/2π = 20 kHz. The number of scans for the accumulation of the 13C CP/MAS NMR spectra was 2048. Repetition delays of 4 s and a spin lock of 1750 μs were used. The isotropic chemical shift of the 13C scale was calibrated using glycine as an external standard (176.03 ppm to the carbonyl signal). The 11B MAS NMR spectra were acquired at ν0(11B) = 160.476 MHz with a spinning frequency of ωr/2π = 11 kHz. The number of scans was 512, and the recycle delay was 2 s. All of the spectra were referenced to Na2[B4O5(OH)4]·8H2O (borax; maximum intensity of the highest-field signal was set to 1.0 ppm). Two-dimensional (2D) triple-quantum 11B 3Q/MAS NMR spectra were recorded at a spinning frequency of ωr/2π = 20 kHz using a zfilter sequence18 with excitation, reconversion, and selective pulse lengths of 4.1, 1.8, and 20 μs, respectively. The 20 μs z-filter was inserted between the reconversion and selective pulses. The obtained spectra were processed by the standard ISO-shearing transformation or by using the recently developed biaxial Q-shearing procedure.20−22 The optimal shearing parameters for the quadrupolar nuclei with a nuclear spin of I = 3/2 (11B, 23Na, etc.) were determined according to a procedure described in the literature18,20 and subsequently verified by the 11B 3Q/MAS NMR spectra of a reference compound with defined NMR parameters (borax; Supporting Information Figure S1). The 2D 11B−11B DQ/SQ MAS NMR correlation spectra19 were obtained using the BR212 symmetry-based dipolar recoupling sequence combined with the two initial loops of the fast-amplitude modulation sequence (FAM) for signal enhancement at a spinning frequency of ωr/2π = 20 kHz. The recycle delay was 2 s, the t1 evolution period consisted of 64 increments, each composed of 320 scans, and the homonuclear recoupling sequence for excitation and reconversion of 600 μs was employed. Dipolar LG-CW decoupling was used during the buildup of DQ coherence. The 2D 1H−13C and 1H−11B FSLG-HETCOR experiments17 were run with FSLG (frequency switched Lee−Goldburg) decoupling during the t1 evolution period consisting of 64 increments made of 64−128 scans with dwell time 100 μs. Rotation frequency was ωr/2π = 12.5 kHz. The intensity of spin-locking field B1 (11B) was ca. 7−15 kHz to reach efficient polarization transfer. The cross-polarization mixing time was 50−400 μs. The site-specific measurement of onebond 1H−13C dipolar couplings was achieved by the 2D PILGRIM16 and inverse-T1-filtered-PISEMAS23 experiments. The experiments were performed at spinning frequency ωr/2π = 12.5 kHz. The recycle delay was 4 s; the t1 evolution period consisted of 32−48 increments, each made of 256−400 scans. High-power dipolar decoupling (SPINAL 64) was used during detection for all of the 1D and 2D MAS NMR experiments. Frictional heating of the samples resulting from the fast rotation was compensated by active cooling. The temperature was calibrated using a Pb(NO3)2 sample at 308 K.24 Quantum Chemical Calculations. The geometries of reference compounds (e.g., H3BO3, H4BO4−) and of boron complexes representing cross-linking units were optimized using the density functional theory (DFT)-based B3LYP/6-311G** method, and subsequently the located stationary points were verified as minima

not yet been fully elucidated. PFRBs contain traditional organic functional groups (dimethylene ether bridges, methylol groups, and phenolic rings) and inorganic boron cross-linkers. To the best of our knowledge, the tetrahedral or trigonal configuration typical of boron atoms has not been completely clarified in these systems, and the results published in peer-reviewed journals are inconsistent. For example, Gao and Abdalla predicted the existence of trigonal coordinated (BIII) boron species in an alkali-catalyzed boron-modified PF resin,4−6 whereas the authors of other studies of the cross-linking mechanism of organic diols or polysaccharides in alkaline solutions reported the formation of tetragonal coordinated (BIV) boron species.13−15 This ambiguity raises the questions: which type of boron configuration is preferred in alkalicatalyzed PF resin modified by boronic acid, and how are these boron ligands distributed in the polymer matrix? On the basis of these facts, we have focused our attention on the preparation and subsequent detailed solid-state NMR structural characterization of PFRB systems. Therefore, a range of solid-state NMR techniques including 11B MAS and 13C CP/MAS NMR experiments, two-dimensional 1H−13C PILGRIM,16 1H−11B HETCOR,17 and 11B 3Q/MAS NMR experiments with zfilter18 as well as the recently developed advanced 11B−11B double-quantum (DQ) magic-angle-spinning (MAS) NMR technique,19 combined with the set of the density functional theory (DFT)-based geometry optimizations and subsequent 11 B NMR chemical shielding calculations, were applied. This experimental approach helped us deduce the arrangement and distribution of boron atoms in untreated and cured alkalicatalyzed PF resins.

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EXPERIMENTAL SECTION

Chemicals and Reagents. Phenol (99%, Aldrich), formaldehyde (37% aq, Aldrich), ammonia (NH3, 26% aq, Aldrich), and boric acid (H3BO3 99.5%, Lachema) were used for the synthesis of PFRBs. Ethanol (absolute, Fisher) was used as a solvent. Resin Synthesis. The PFRB resin was synthesized by a sol−gel process. H3BO3 was used as the boron source and was incorporated into the polymer matrix prepared according to the standard polycondensation reactions (Scheme 1). The initial molar reaction ratio of phenol/formaldehyde/NH3/H3BO3 was 1/1.5/0.35/0.1. Phenol, formaldehyde, and NH3 were mixed in a round-bottomed flask until the phenol was completely dissolved. The predetermined amount of H3BO3 was then added to the prepared solution. The mixture was stirred for 10 min, and subsequent separation of the water and organic phases was observed in the solution within 45 min of refluxing at 80 °C. The water phase was removed by vacuum distillation (45 min at 95 °C). The ready PFRB prepolymer was transparent, with a honey-like viscosity. Finally, a brittle and orangecolored solid PFRB polymer was obtained by heat-treating the viscous transparent prepolymer at 200 °C under ambient pressure. B

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Macromolecules by calculating the harmonic vibrational frequencies (all real for each structure). These geometries, available as Supporting Information, were subjected to calculations of the NMR chemical shielding by combining the GIAO approach25,26 with the B3LYP/6-311G** method. The resulting GIAO-B3LYP/6-311G** strategy was successfully applied to describe the NMR chemical shielding tensors in peptides.27 Here its reliability was investigated by exploiting the D3h and S4 symmetry of H3BO3 and H4BO4−, respectively, and applying a number of higher-level techniques (not shown) to obtain the difference between boron chemical shielding in the two systems. Thus, the GIAO-B3LYP/6-311G** values were found to be sufficiently close to those obtained by computationally much more demanding methods. The Gaussian 09 suite of programs was used with default settings.28

(assigned as C3) were converted during the curing process to short methylene bridges (marked as C1 in Figure 1; left side). Two unusual signals at 45 and 55 ppm in the 13C CP/MAS NMR spectra (marked as C2) were detected for both systems. These resonances were attributed to 2-hydroxybenzylamine and 2,2-dihydroxydibenzylamine, respectively,29 and indicated the incorporation of ammonia into the polymer matrix.32 To experimentally support the supposed immobilization of the phenyl rings, we performed measurements of motionally averaged 1H−13C dipolar couplings. As demonstrated by the 1 H−13C dipolar spectrum (Figure 2a), two distinct fractions of

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RESULTS AND DISCUSSION Solid-state NMR spectroscopy was used for the structural study of the as-synthesized untreated (u-PFRB) and the thermally converted (c-PFRB) boron-modified PF resins. First, basic characterization of both types of PFRB resins by 13C CP/MAS NMR and 11B MAS NMR techniques was performed; the resulting spectra are depicted in Figure 1. All of the peaks in the 13 C CP/MAS NMR and 11B MAS NMR spectra were assigned according to literature data11−15,29,30 and subsequently verified by DFT calculations (Table 1).

Figure 1. 13C CP/MAS NMR (left) and 11B MAS NMR (right) spectra of u-PFRB system (a) and c-PFRB resin (b).

Figure 2. 2D 1H−13C PILGRIM NMR spectra u-PFRB system (a) and c-PFRB resin (b).

A comparison of the 13C CP/MAS NMR data of the u-PFRB and the c-PFRB resins suggests extensive immobilization of the phenol rings combined with the formation of morphology heterogeneities31 induced by the thermal treatment indicated by the huge broadening of the corresponding signals C5−C7 in the range of 110−160 ppm in the latter system. Furthermore, the polyoxymethylene oligomers (signal C4) and ether bridges

the aromatic rings were found in the uncured u-PFRB system. The main fraction was characterized by a large 1H−13C dipolar coupling constant (ca. 22 kHz), reflecting immobilized aromatic rings, whereas the secondary fraction was indicated by inner dipolar doublets (Figure 2a) showing a distribution of considerably smaller 1H−13C dipolar couplings (5−17 kHz). These inner doublets thus represent aromatic rings executing

Table 1. 13C and 11B NMR Chemical Shift Assignment of Boron-Modified Phenol−Formaldehyde Resins C NMR δ (ppm)

13

35−45 45−60 65−75 80−90 110−123 123−137 145−162

assignment

symbol

methylene bridges benzylamines dimethylene ether bridges polyoxymethylene oligomers unsubstituted o- and p-carbons unsubstituted m- and substituted o- and p-carbons phenoxy carbons

1 2 3 4 5 6 7

11

B NMRa δ (ppm)

coordination and assignment

symbol

18.4

H3BO3

a

3.0 0.9 −0.3 −0.5 −2.4

B(OH)4− monoester complex diester complex monoester complex diester complex

b c d e f

B NMR chemical shifts δ (ppm) were obtained from the 11B MQ/MAS NMR spectra (see Supporting Information).

a11

C

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Figure 3. Biaxially Q-sheared 11B MQ/MAS NMR spectra of the u-PFRB (a) and c-PFRB resins (b) with skyline projections made at resonance frequencies estimated from the 1D 11B MAS NMR spectrum of u-PFRB.

chemical shifts of different borate anions in tetrahedral coordination occur within a narrow range. Therefore, to achieve better resolution of the signals, well-established 2D 11B 3Q/MAS NMR experiments were applied. However, the obtained standard (ISO-sheared) 11B 3Q/MAS NMR spectra of the prepared resins still showed basically unresolved distribution of the 11B NMR resonances of BIV sites (Figure 3a). In an attempt to better separate the individual spectral contributions, we applied an alternative approach to processing the obtained data. This biaxial Q-shearing transformation generally allows for a nearly complete separation of the quadrupolar broadening contributions (second-order anisotropy splitting and quadrupole-induced chemical shift) from the isotropic chemical shifts. In this way, weak variations in the resonance frequencies originally hidden behind the quadrupolar effects can be resolved.20−22 As expected, the skyline projections of the biaxially Qsheared 11B 3Q/MAS NMR spectra more clearly demonstrated the multiple-site character of the 11B NMR signals of tetracoordinated BIV species (Figure 3b). The as-synthesized u-PFRB resin was characterized by three relatively well-evolved signals (B2, B3, and B4), whereas the thermally treated c-PFRB resin was represented by the two slightly high-field shifted resonances B5 and B6. Moreover, the existence of multiple BIV sites in the c-PFRB resin was further confirmed by the 11B−11B DQ/SQ MAS NMR spectroscopy discussed later. The spectral characteristics derived from 2D 11B 3Q/MAS NMR spectra are summarized in Table 2. On the basis of the analogy found for the cross-linking mechanism of organic diols and polysaccharides by boric

local motions with relatively high-amplitude reorientations. In contrast, in the 1H−13C dipolar spectrum of cured c-PFRB system the inner dipolar frequencies had completely disappeared, and the aromatic rings were exclusively characterized by the large 1H−13C dipolar coupling constant of ca. 23−22 kHz (Figure 2b). These findings thus confirm progressive immobilization of polymer segments induced by the heat treatment. The 11B MAS NMR spectra (Figure 1, right side) clearly indicated that a significant part of the originally trigonalcoordinated boron atoms (BIII) of boric acid (H3BO3, with an isotropic chemical shift at δiso = 18.3 ppm and a quadrupolar coupling constant of CQ = 2.5 MHz (Supporting Information Figure S2), was transformed into species resonating at δiso from 1 to −3 ppm and CQ = 0.3 MHz. The reduced chemical shift and the low quadrupolar coupling constant indicated a spherically symmetric distribution of the electron density around the boron nucleus, thus confirming the transformation of tricoordinated boron atoms to tetrahedral-coordinated units (BIV). Traces of residual boric acid (BIII, signal B1) were observed to completely disappear in c-PFRB during the thermal treatment. Moreover, as a result of quenched segmental motions, the narrow spectral components of the BIV resonances (Figure 1a) considerably broadened, leading to a single relatively unresolved signal (Figure 1b). Because of the quadrupolar character of the 11B nuclei combined with the small range of chemical shifts of BIV units, the recorded 11B resonances were partially overlapped, which could lead to misinterpretation of the obtained spectra. Moreover, the literature data33 indicated that the 11B NMR D

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Macromolecules Table 2. 11B NMR Chemical Shifts δiso and δBQ of Borax and Both PFRB Resins Extracted from the ISO-Sheared 11B MQ/ MAS NMR and Biaxially Q-Sheared 11B MQ/MAS NMR Spectra, Respectivelya compd borax u-PFRB

c-PFRB

species

δ1 (ppm)

δ2 (ppm)

δiso (ppm)

δBQ (ppm)

BIV BIII H3BO3 B(OH)4− monoester diester monoester diester

1.9 22.9 21.8 3.5 1.1 0.2 −0.5 −2.2

1.5 12.6 12.6 2.4 0.6 −0.7 −0.6 −2.7

1.75 19.1 18.4 3.1 0.9 −0.3 −0.5 −2.4

1.73 18.1 18.4 3.0 0.9 −0.1 −0.3 −2.3

a

The chemical shift values from the ISO-sheared 11B MQ/MAS NMR spectra were obtained using equation ∂iso = (17F1 + 10F2)/27, where F1 and F2 values were extracted from central of gravity of detected signals. The chemical shift values from biaxially Q-sheared 11B MQ/ MAS NMR spectra, defined as δBQ, were obtained from slices directly.

acid,13−15 we propose formation of mono- and diester borate complexes in PF systems described in Scheme 2. Consequently, the three detected resonances B2, B3, and B4 were attributed to B(OH) 4 − ions and to mono- and diester complexes, respectively. As indicated by the isotropic chemical shifts at 0.9 and −0.3 ppm, the monoester and diester complexes are probably represented by six-membered spirocyclic borate anions (Figures 4c and 4d). Subsequent deconvolution of the 11 B MAS NMR spectra further revealed that the diester complexes represented more than 90% of the cross-linking units in the thermally treated c-PFRB resins, whereas the monoester units formed only 10% of the cross-links. As listed in Table 2, the signals of the tetrahedrally coordinated boron atoms were significantly shifted to a lower position after the thermal treatment. The observed shift of the signal to a lower frequency indicated additional structural transformation of the spirocyclic borate anions. Most likely, these species were more tightly incorporated into the polymer matrix through additional N-type (amino) cross-links,34 as schematically indicated in Figures 4e,f. Narrowing of the BIV signal in both dimensions of the biaxially Q-sheared 11B MQ/ MAS NMR spectrum of the thermally treated c-PFRB resin indicated that the promoted conversion of B(OH)4− and the spirocyclic borate anions was accompanied by an increase in uniformity of the local structures of the boron species. For preliminary experimental verification of the proposed sixmembered spirocyclic borate anions we applied 1H−11B correlation spectroscopy. In this respect, we focused on probing the 1H polarization transfer from oxymethylene hydrogen atoms (ca. 3−4 ppm) to boron centers and

Figure 4. Schematic representation of the boron species present in the prepared boron-modified phenol−formaldehyde polymers.

comparing this process with the efficiency of competitive 1H polarization transfer from aromatic protons (ca. 6−7 ppm, Figure 5). As shown in Figure 6 in the proposed six-membered spirocyclic borate anions the interatomic distance between oxymethylene protons and boron centers should be significantly shorter (ca. 2.6−3.3 Å) than the distance between boron atoms and aromatic protons (ca. 4.0 Å). As demonstrated in Figure 5, for short cross-polarization mixing time (50 μs) the 1H−11B correlation signal involving oxymethylene hydrogen atoms was considerably more intensive than the signal reflecting the competitive polarization transfer from aromatic protons. For longer mixing times the relative intensities of both correlation signals were inverted as a result of a higher number of aromatic protons involved in the polarization transfer. These findings, confirming that the average 1H−11B interatomic distance between −OCH (O− CH2) protons and boron atoms is shorter than the average

Scheme 2. Mechanism Behind the Formation of Mono- and Diester Borate Complexes in PF Resins

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However, the limited spectral resolution in 1H dimension does not allow for explicit reconstruction of the local borate anions structures. Therefore, to describe the boron crosslinking units in the prepared PF systems in more detail, and to confirm the proposed assignment of the detected 11B MAS NMR signals, we performed a series of quantum chemical geometry optimizations followed by predictions of the 11B and NMR chemical shielding values. The simplest structural models representing typical mono- and diester boron complexes are shown in Figure 6a. The six-membered ring consisting of boron atom substituted by two oxygens coupled by a chain of three carbon units is the central motif of all the optimized structures. In the molecular models representing thermally converted PF resins the methylene (−O−CH2−) carbons are further substituted by amino groups. As reference compounds boric acid, borate anion (B(OH)3 and B(OH)4−) and the recently reported cyclic five-member guanosine−borate monoesters and guanosine−borate diesters35,36 were employed in the DFT calculations (for optimized geometries of the reference compounds see Supporting Information). As graphically presented in Figure 6b, the 11B NMR chemical shielding parameters calculated for all the reference compounds as well as for the proposed structural models nicely fit their experimental counterparts (i.e., the assigned 11B NMR chemical shifts). Specifically, the computational procedure correctly predicted not only low 11B shielding for the five-membered borate esters but also a striking difference between the guanosine−borate monoester and diester.35,36 The obtained data thus provide a clear proof for reliability of the applied computational approach. Consequently, this analysis corroborates the suggested low-frequency shift of 11B NMR signals induced by thermal treatment. Thus, not only the formation of exclusively six-membered spirocyclic mono- and diester borate complexes as the secondary cross-linking units of PFRB resins but also an extensive formation of amino bridges (cross-links) in the vicinity of boron atoms occurring during the thermal conversion are both confirmed. To complete and further support our findings, the values of the 13C NMR shielding parameters were also calculated at the B3LYP/6-311G** level, and the corresponding chemical shifts were approximated by an internal reference using phenoxy carbons resonating at ca. 153 ppm (see Supporting Information). The combination of a high gyromagnetic ratio and high natural isotopic abundance (80%) predetermined the 11B nucleus to be an efficient probe into the distribution and clustering of boron species. Because of the strong dipolar interactions, the 11B···11B interatomic distances could be traced to ca. 5.5 Å. In this respect, sophisticated hardware and methodological development of the past 10 years resulted in a design of a range of double-quantum dipolar recoupling techniques highly efficient for half-integer quadrupolar nuclei.17,37,38 Moreover, in the case of organic compounds also the proton-driven spin-diffusion (PDSD)39 experiment can be successfully applied. As we recently demonstrated,40 however, the 11B−11B PDSD experiment required the application of relatively long mixing times to achieve efficient long-range polarization transfer (50−100 ms). As a consequence, substantial coherence was lost during the relaxation processes. In contrast, using the 11B−11B DQ technique, which involved much shorter recoupling periods (0.5−2 ms), resulted in considerably better sensitivity.41 Therefore, for probing the structure of boron-modified phenol−formaldehyde polymers, we applied the recently developed 11B−11B DQ experiment

Figure 5. 1H−11B FSLG HETCOR NMR spectra of the thermally converted PFRB resin (c-PFRB) measured with cross-polarization mixing times τm = 50 and 400 μs (left and right, respectively).

Figure 6. (a) B3LYP/6-311G** optimized geometries of models for the six-membered cyclic borate monoesters and diesters and thermally converted six-membered cyclic borate diesters. (b) Correlation graph of experimentally determined 11B NMR isotropic chemical shifts and their corresponding isotropic chemical shieldings as calculated by the GIAO-B3LYP/6-311G** approach. Local structures of five-membered cyclic borate monoesters and diesters as well as the corresponding 11B MAS NMR chemical shifts were adopted from the literature.35,36 Geometries and the corresponding isotropic chemical shielding values were newly calculated using the same approach (for the optimized models of five-membered cyclic borate esters see Supporting Information).

distance between boron atoms and aromatic protons, are in a good agreement with the proposed structure of the sixmembered spirocyclic borate fragments. F

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Macromolecules based on the BR212 recoupling sequence,19 the efficiency of which was optimized and tested on a system with known geometry (borax, Supporting Information Figure S4). On the basis of the obtained optimized excitation/reconversion time (τmix = 0.6 ms), the 11B−11B distances could be detected to ca. 5.5 Å.40 When analyzing the recorded 11B−11B DQ MAS NMR spectrum of the c-PFRB resin (Figure 7), we first estimated the

Figure 8. B3LYP/6-311G** optimized geometries of local structures in “diester−diester” pairs of thermally altered N-cross-linked spirocyclic borate anions.

distance is ranging from ca. 4.6 up to 5.6 Å in these clusters, thus allowing an efficient evolution of double-quantum correlation signals. Moreover, formation of these clusters, particularly the more compact ones, is accompanied by the additional increase in the 11B NMR isotropic chemical shielding. This finding is in accordance with the experimentally observed low-frequency shift of the double-quantum 11B−11B MAS NMR correlation signal of these paired boron moieties (Figure 7), confirming thus partial clustering (pairing) of the N-cross-linked spirocyclic borate anions.

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Figure 7. Scaled 1D 11B MAS NMR (a) and 2D 11B−11B DQ MAS NMR spectra of the c-PFRB resin (b). The 1D 11B DQF MAS NMR spectrum was used as the upper skyline projection. Both the 1D singlepulse 11B MAS NMR and 11B DQF MAS NMR spectra were measured using the same number of scans (NS = 320).

CONCLUSIONS Ammonium-catalyzed PF resins represent low-molecularweight precursors that can be converted into high-molecularweight polymers with a dense three-dimensional network through the coadding specific cross-linkers and performing a subsequent thermal treatment. In this way, a wide range of polymeric materials with tunable physicochemical and mechanical properties can be synthesized. Recently, novel soft magnetic composites of spherical FeSi powder covered with chemically modified phenolic resins exhibited a remarkable combination of mechanical, electrical, and magnetic properties. For this reason, a boron-modified PF matrix of soft magnetic composites was synthesized via a sol−gel process in a basic aqueous medium. The formation of a PF network containing incorporated benzylamines was monitored with 13C MAS NMR, whereas the structure of inorganic boron complexes (cross-links) was probed by 1D 11B MAS NMR and 2D 11B MQ/MAS NMR spectroscopy combined with the DFT structural optimizations and subsequent 11B NMR chemical shielding calculations. In combination, these ss-NMR spectroscopic analyses showed that the PF resin was cross-linked via tetragonally coordinated boron species (BIV). These units formed three types of boronbased complexes: (i) free borate anions, (ii) monoesters, and (iii) diesters, with the latter two represented by six-membered spirocyclic borate anions. The subsequent thermal curing process was demonstrated to result in the complete consumption of the free borate anions and considerable transformation of the spirocyclic borate anions that became more tightly incorporated into the polymer matrix via additional N-type cross-links. The continuing conversion of B(OH)4− and spirocyclic borate anions during the thermal treatment was accompanied by an increase in the uniformity of the local structures of the boron species. By applying 11B−11B DQ MAS NMR correlation spectroscopy, we showed that approximately 80% of the boron cross-links were homogeneously and uniformly dispersed within the polymer network

total efficiency of the excitation of the DQ coherence. Although the reported maximum efficiency of the BR212 recoupling sequence17 was approximately 10−15%, in our case, the DQF signal reached only 2−4% of the single-quantum signal (Figure 7a), indicating that the vast majority of boron species (75− 80%) in the c-PFRB resin were not involved in dipolar interactions. Because the boron atoms were separated by the organic phase, the boron−boron interatomic distances were considerably larger than ca. 5.5−6.0 Å, and the boron crosslinks, whose typical local structure is demonstrated in Figure 6, were rather homogeneously and uniformly dispersed in the prepared systems. However, approximately 20% of the cross-linking units formed pairs or small clusters in which the boron−boron distances were shorter than 5.0−5.5 Å. In the recorded 11B−11B DQ MAS NMR spectrum of the c-PFRB (Figure 7b), these pairs (clusters) were characterized by three autocorrelation signals. Weak autocorrelation signals resonating at 1.6 and −0.9 ppm in the DQ dimension reflected the existence of “monoester−monoester” pairs formed by residual nonaltered six-membered spirocyclic borate anions (B3−B3) and N-crosslinked six-membered spirocyclic borate anions (B4−B4), respectively. In contrast, the strong autocorrelation signal at −4.0 ppm was attributed to “diester−diester” pairs of thermally altered N-cross-linked spirocyclic borate anions (B5−B5). The formation of pairs of spirocyclic borate anions could, in principle, result from random concentration fluctuations and/or from the cyclization reactions leading to the formation of the clusters in which two borate spirocyclic species are mutually coupled by amino- or methylamino linkers (Figure 8). As revealed by the DFT geometry optimization, the boron−boron G

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Article

Macromolecules

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and separated by an organic matrix, where the average boron− boron distance was greater than 6 Å. The remaining fraction of the cross-linking units formed pairs or small clusters in which the boron−boron distances were shorter than 5.5 Å. The determined, basically homogeneous distribution of the crosslinks (benzylamines and boron units) was thus demonstrated to enhance the final properties of these PF polymers by increasing their cross-linking density. This finding opens new possibilities for the application of these PF polymers in the design of new types of SMCs.

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ASSOCIATED CONTENT

S Supporting Information *

Details of solid-state NMR experimental parameters, processing of 11B MQ/MAS NMR spectra, and DFT optimized local structures of boron clusters and reference compounds used for 11 B NMR shielding calculations. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01037.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank COST Action MP1202 HINT (Ministry of Education, Youth and Sports of the Czech Republic: LD14010) for financial support. Computational resources were partially provided under the program “Projects of Large Infrastructure for Research, Development, and Innovations” LM2010005 and in the Center CERIT Scientific Cloud, part of the Operational Program Research and Development for Innovations, reg. no. CZ. 1.05/3.2.00/08.0144.

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DOI: 10.1021/acs.macromol.5b01037 Macromolecules XXXX, XXX, XXX−XXX