Solid State NMR Study of Boron Coordination Environments in

Jan 28, 2019 - Solid State NMR Study of Boron Coordination Environments in Silicone Boronate (SiBA) Polymers. Gabrielle Y. Foran , Kristopher J. Harri...
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Solid State NMR Study of Boron Coordination Environments in Silicone Boronate (SiBA) Polymers Gabrielle Y. Foran, Kristopher J. Harris, Michael A. Brook, Benjamin Macphail, and Gillian R. Goward* Department of Chemistry and Chemical Biology, McMaster University, Hamilton, Ontario, Canada L8S 4M1

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S Supporting Information *

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.

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boric acid to form “Silly Putty” or “Funny Putty”. The precise mechanism of cross-linking interaction in these materials has been the subject of some debate in the literature. Cross-linking processes have been proposed to involve hydrogen bonding through OH groups, dative cross-linking between boronic acids, dative bonding between boronic acids and oxygen atoms on the silicone backbone, and/or covalent bonds at a three- or four-coordinate boron center (Figure 2A−C). Interactions between boronic acid and silicones have previously been investigated in polysaccharides,1 such as guar, that have been mixed with borate, B−(OH)4.5 Polymerization occurs when borate condenses with diols on the polysaccharide (Figure 2D). A second condensation reaction can also occur, resulting in either intermolecular bonding (cross-linking) or intramolecular bonding (ring formation) (Figure 2E). Gel formation in this system was found to be dependent on a minimum borate concentration.5 This example illustrates that the extent of cross-linking at the boron center can be influenced by the relative proportion of boronic acid that is present in the material. The mechanical properties of SiBA elastomers, for example, Young’s moduli, show a direct correlation with boronic acid

INTRODUCTION The introduction of boronic acid groups onto a silicone backbone dramatically changes the properties of the polymer. In addition to the conversion of mobile oils into elastomeric materials, the presence of the boronic acid enhances the hydrophilic nature of the materials. The silicone boronate acid (SiBA) materials discussed here are non-Newtonian fluids. The functional boronic acid groups exist in a dynamic equilibrium between free boronic acids and boronic acid dimers, which act as cross-links;1,2 the changing interactions allow the materials to flow under their own weight at a rate that depends upon the weight fraction of boronic acids.3 SiBA are reliably synthesized via a two-step process in which vinylphenylboronic acid (VPBA) is first protected by a dimethyl-L-tartrate group and then grafted onto the silicone polymer backbone via a platinum-catalyzed hydrosilylation reaction;4 both telechelic (boronic acids on the ends of polymer chains)4 (Figure 1B-i) and pendant (boronic acids along the chains at different densities)4 (Figure 1B-ii) materials were prepared.3 The initial products of this process are oils, but upon exposure to moisture, the protecting groups undergo hydrolysis, freeing the boronic acid-functionalized end groups that then interact to form elastomeric films via a “spread and set” process.3 The most common precedent material for SiBA polymers is the unusual polymer formed when silicones are combined with © XXXX American Chemical Society

Received: November 14, 2018 Revised: January 7, 2019

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Figure 1. Two-step synthesis of SiBA: (A) protection with dimethyl-L-tartrate followed by (B) hydrosilylation of the protected VPBA to yield telechelic (B-i) and pendant (B-ii) protected Tar-SiBA. (C) The addition of moisture results in hydrolysis of the protecting group to yield SiBA elastomers. Possible cross-link bonding modes are illustrated in Figure 3.

boron NMR will by employed to elucidate boron coordination environments in SiBA elastomers.

density on the polymer, which implicates boronic acids in the cross-linking process, similar to the case with guar and borate.6 It was initially assumed that, like Silly Putty, cross-linking in the case of SiBA involves 1:1 hydrogen bonding between boronic acid groups that leads to chain extension (and an increase in viscosity), in addition to cross-linking provided by dative bonding between boronic acid sites (Figure 3).3,6 However, the exact nature of the bonding remains poorly understood. Boronic acid derivatives represent a promising alternative to traditional organic polymers,7 particularly in the field of macromolecular chemistry where these materials are known for their ability to self-assemble via reservable covalent bonding.8−10 However, further investigation into the boron coordination environments that exist in SiBA elastomers is essential as several modes of covalent bonding including boroxine formation, Lewis base coordination, spiroborate formation, and esterification are possible.11 Additionally, as stated for Silly Putty, noncovalent interactions such as hydrogen bonding can have a significant role in the structure of these supramolecular assemblies.11 To this end, solid state

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NMR METHODOLOGY We have applied solid state NMR to probe the character of the boron centers in SiBA polymers because the technique can be used in noncrystalline systems. 11B is a quadrupolar nucleus and therefore possesses an asymmetric distribution of its nuclear charge, causing it to interact anisotropically with the electric field gradient (EFG).12 The resultant line shape is therefore highly dependent on the symmetry of the nuclear environment. 11B tends to exist in either three-coordinate trigonal planar or four-coordinate tetrahedral environments. The differing symmetry of these environments allows them to be identified by line shape fitting.13 11B solid state NMR has previously been used in the characterization of both minerals13 and glasses.14,15 11B solid state NMR as also been used to investigate boron coordination environments in other wellunderstood systems including doped TiO216 and boroncontaining small molecules.17 Solid state 11B NMR is not typically used to study polymers due in part to the inherent B

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Figure 2. Elastomers can be formed via the condensation of boric acid with PDMS. Cross-linking via three-coordinate (A) and four-coordinate (B, C) centers is shown. Gel formation via the condensation of borate with guar polysaccharide. Single (D) and double (E) condensation reactions are possible with K2 being twice as large as K1.

Figure 3. Possible boronic acid binding motifs. Three-coordinate: (A) dative bonding between boronic acids, (B) free boronic acid, and (C) hydrogen-bonded boronic acids. Four-coordinate: (D) dative bonding between a boronic acids and (E) dative bonding between a boronic acid and oxygen on the PDMS backbone.

increased symmetry around the 11B nucleus.13,14 The asymmetry parameter, which depends on the symmetry of the ligands in the immediate coordination sphere, ranges between 0 and 1.19 Here, 11B NMR is used to elucidate the tendencies toward three- versus four-coordinate boron centers in the SiBA elastomers of interest.

challenge of employing quadrupolar NMR techniques with mobile and amorphous materials. However, Kobera et al.18 have employed the technique to study less mobile systems such as cured alkali-catalyzed PF resins. Quantum chemical calculations were performed concurrently with solid state NMR for the purpose of confirming experimentally derived quadrupolar parameters in some of these studies.17,18 However, as this technique depends on the use of optimized structures, quantum chemical calculations were not deemed to be an efficient technique for the analysis of boron coordination in SiBA elastomers as the amorphous nature of these materials makes it impossible to obtain crystal structures. These studies have shown that typical CQ values for trigonal planar boron range from 2 to 3 MHz with much lower CQ values being reported for tetrahedral environments due to

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EXPERIMENTAL SECTION

Synthesis of SiBAs. SiBA polymers were synthesized according to a previously published procedure2−4 producing pendant SiBA with boronic acid loadings (weight fraction calculated as CH2CH2C6H4B(OH)2/total MW), after hydrolysis, of 49% P-49 (x = 7, y = 7), 37% P-37 (x = 18, y = 8), and 13% P-13 (x = 75, y = 6) (Figure 1B-ii). Telechelic SiBA was produced with boronic acid loadings of 23% T23 (n = 12) and 5% T-5 (n = 79, Figure 1B-i). C

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Macromolecules Thermal Gravimetric Analysis (TGA). TGA was performed on 5−10 mg SiBA samples using a Mettler Toledo TGA-DSC 3+ system. Samples were heated from 30 to 800 °C at a rate of 10 °C/min. The experiments were performed under argon flow. Solid State NMR. Background suppressed 11B NMR spectra were collected at three different magnetic fields: 7, 11.7, and 20 T with spinning speeds of 15, 30, and 30 kHz, respectively. Background suppression was used to limit the contribution of signals from boron contained within the probe to the experimentally observed spectra. Samples were referenced to an external aqueous solution of boric acid (19.6 ppm) in all cases.20 At 7 T, spectra were collected using a 4.5 μs 90° pulse at a power level of 40 W using a 4 mm wide bore probe with 15 kHz spinning. At 11.7 T, spectra were collected using a 15 μs 90° pulse at a power level of 8.9 W using a 2.5 mm probe with 30 kHz spinning. At 20 T, spectra were collected using a 10 μs 90° pulse at 7.24 W using a 1.9 mm probe with 30 kHz spinning. Spectra collected at 11.7 and 20 T were obtained using selective pulses which were chosen to selectively excite the central transition. Excitation at 7 T was done using nonselective pulses due to significant line broadening at the lower magnetic field. All experiments were performed at room temperature. Spectra of T-5 at 7 and 20 T were collected without spinning due to difficulties associated with achieving a stable MAS rate for this low-viscosity sample. In addition to the background suppressed spectra, multiple quantum magic angle spinning (MQMAS) spectra were collected at 20 T with 30 kHz spinning. The spectra were collected using a threepulse sequence composed of an excitation pulse (1.2 μs, 275 W), a reconversion pulse (3.6 μs, 275 W), and a selective pulse (17.5 μs, 0.58 W). Because of the relatively low signal intensity obtained via triple quantum excitation and the presence of multiple unique boron chemical environments in the material, the success of this technique was highly dependent on the use of a magnet with sufficiently high field. In this case, MQMAS data were only successfully acquired at 20 T.

Tetrahedral four-coordinate boron environments tend to be highly symmetrical.19,21 This configuration minimizes interaction with the EFG which results in very low CQ values.19,21 Tetrahedral boron centers with four identical ligands tend to crystallize with enough symmetry such that they interact minimally with the EFG. As a result, these samples appear to give rise to isotropic spectra under magic angle spinning (MAS) conditions.19 Quadrupolar parameters are not extracted from these line shapes. The 11B line shape for datolite, a mineral that contains BO4 tetrahedra, has a negligible CQ value (Figure 4B).22 Tetrahedral centers that are not surrounded by four identical ligands, such as the crosslinked sites studied here, tend to have small CQ values (typically around 0.5 MHz);13 these line shapes are difficult to fit directly when the high magnetic fields that are necessary to distinguish individual sites are employed. MQMAS is used here to measure and extract values for the quadrupole product (ρ) which is a combination of CQ and η parameters (eq 1). ρ = CQ 2(1 + η2 /3)

(1)

Line broadening, caused by distributions of chemical shift and EFG parameters, tends to complicate line shape fitting, as it is difficult to disentangle superimposed peaks.19,21 Unlike with dipolar coupling, anisotropy found in 11B spectra can only be partially improved by magic angle spinning (MAS) alone (section SI-1 of the Supporting Information). Typical MAS NMR, where the sample is spun at 54.7° relative to the external magnetic field, removes line broadening due to the first-order anisotropic term. Removing the second anisotropic term requires spinning the sample at an angle of 37.4° or 79.1°.19 Simultaneously spinning a sample at two angles is difficult and requires specialized probes.12 In this work and much more commonly, improved resolution of nonchemically equivalent boron sites in our elastomers is obtained by increasing the magnetic field strength in addition to performing 2D 11B MQMAS experiments. Increasing the external magnetic field strength tends to reduce the overlap of peaks from nonchemically equivalent sites because the quadrupole interaction is inversely proportional to the strength of the magnetic field.21 Although peak overlap is reduced, the resultant line narrowing removes quadrupole features from the 1D line shape, making it difficult to characterize boron sites as three- or four-coordinate.23 MQMAS is employed such that the information contained in the EFG can be obtained based on differences in chemical shift in the F2 and F1 dimensions.24 The 2D experiment produces a spectrum where the direct (F2) dimension retains the anisotropic quadrupole interaction and the indirect (F1) dimension shows an isotropic spectrum.23 The line shape in the F2 dimension is the same as that which is observed in conventional MAS NMR experiments,23,24 whereas isotropic line shapes can be observed in the F1 dimension following data processing.23−25 The absence of peak shapes from the quadrupole interaction in the F1 dimension means that previously overlapped peaks are separated from each other.23−25 Analysis of the MQMAS data and 1D line shape fitting enables us to obtain EFG parameters which can be used to draw analogies to previously studied crystalline and glassy systems (Figure 4) to interpret the 11B data and to thereby characterize the bonding environments in the SiBA elastomers.13,14,25,26 Here we can demonstrate that different boron coordination environments in these elastomeric materials can

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RESULTS AND DISCUSSION Boric acid, a fully characterized compound that in crystalline form is composed of ordered planar B(OH)3 layers, was used here as a model three-coordinate boron sample. Line shape fitting yielded CQ = 2.5 MHz and η = 0.1 (Figure 4A). This is consistent with data presented by MacKenzie and Smith for trigonal planar boron centers with three identical ligands.13

Figure 4. 11B spectra of boric acid (A) and datolite (B). (A) was acquired experimentally at 7 T with 15 kHz spinning. (B) is a simulated spectrum that was created based on data obtained by Hansen et al.22 Three- and four-coordinate boron centers result in very different 11B line shapes. D

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e.g., Tar-T-23 and Tar-P-49 (Figure 1), are oils. Differences in cross-link density can potentially be attributed to two factors: the weight percent boronic acid and the relative positioning of the boronic acid groups. Previous work, based on only two SiBA samples, proposed that the spacing between boronic acids was ultimately more important.6 Here, the site-specific resolution of cross-linked sites was examined by solid state NMR to facilitate a more quantitative analysis of the relative proportion of four-coordinate boron and therefore the nature of cross-linking. That is, the influence of boronic acid spacing, telechelic or pendant, and boronic acid loading on crosslinking can be determined. An assignment of the structural nature of various boron coordination environments is also of interest. Several possibilities for boronic acid dimer interactions include B−OH···O−B hydrogen bonds (Figure 3C), B−O−B covalent bonds (Figure 3D), a mixture of the two, and Lewis acid/base interactions between boron and oxygen atoms on silicone chains (Figure 3E). It was anticipated that the quantification of three- and four-coordinate boron environments may be used to determine the most likely structural motifs in the cross-linked elastomers. Background suppressed 11B spectra are collected at three different magnetic fields: 7, 11.7, and 20 T (Figure 6A−C). At lower magnetic fields, nonequivalent boron sites are superimposed upon one another, making it difficult to extract specific conclusions regarding the number of boron sites present in each material and their respective coordination numbers (Figure 6A,B). It can, however, be concluded that multiple boron coordination environments are present as appropriate line shape fitting cannot be achieved using a single quadrupolar line shape. Spectra that were collected at 20 T are sufficiently well resolved to reveal individual boron sites (Figure 6C). Three distinct groups of sites can be observed in P-49 and P-37 (Figure 6C). At lower boron loading, two distinct groups of sites are observed (Figure 6C). It can be reasonably assumed that the narrow peak, located around 9 ppm, corresponds to a four-coordinate boron environment and that the sites with the highest chemical shifts (20−30 ppm range) correspond to three-coordinate boron environments. These assumptions are based on typical chemical shift ranges for three- and fourcoordinate boron centers.13 However, because of overlap in typical chemical shift ranges and the general lack of quadrupolar line shape features at high field, the sites with chemical shifts between 13 and 20 ppm in P-49 and P-37 cannot readily be assigned to a particular type of coordination environment. Additional NMR techniques must be used to more accurately characterize these sites using quadrupole parameters such that boron coordinate environments can be proposed. To this end, MQMAS NMR was performed at 20 T with 30 kHz spinning (Figure 7). The purpose of this experiment was to separate chemically nonequivalent sites and to obtain more easily verifiable values for the quadrupolar parameters of each site. The quadrupolar product (ρ, eq 1) is calculated for each site based on the differences between the chemical shifts in the direct and indirect dimensions. As the value of η is fixed between 0 and 1, a range of quadrupole parameters with which a peak can be fit is obtained. Five distinct boron sites are fit in P-49 (section SI-3). Values of CQ and η were obtained by fitting the F2 projection line shapes from the MQMAS data. Sites with isotropic chemical shifts between 24 and 27 ppm have CQ values ranging from 2.5 to 2.9 MHz. Sites with

be distinguished based on their quadrupolar line shapes and quadrupolar parameters.19,21 Boronic acid loadings in the SiBA elastomers were verified using thermogravimetric analysis. SiBA materials were prepared in two different structural motifs, pendent and telechelic (Figure 1), with different boronic acid loadings that were calculated based on the mass of grafted boronic acid relative to the total mass of the sample (section SI-2). Pendant samples were prepared with 49%, 37%, and 13% boronic acid, respectively (P-49, P-37, and P-13), while telechelic SiBA samples were produced with boronic acid loadings of 23% and 5% boronic acid loadings (T-23 and T-5) (section SI-2). Boronic acid loading was readily verified via thermogravimetric analysis (Figure 5). When subjected to thermal degradation in

Figure 5. Thermogravimetric decomposition profiles of VPBA and SiBA materials acquired between 30 and 800 °C with a 10 °C/min heating rate under an argon atmosphere. T-5 was not analyzed via TGA due to low viscosity.

an inert atmosphere between 400 and 650 °C, long chain polydimethylsiloxane polymers (PDMSthe parent polymers of SiBA) undergo a single-step degradation which can be attributed to the breaking/re-forming of Si−O bonds.27 The products resulting from the degradation reaction, mostly cyclic tetramers ((Me2SiO)4), are volatile, causing significant mass loss.27 VPBA, the parent boron-containing material, underwent two decomposition events which resulted in a total mass loss of 72% (Figure 5). The decomposition profile of the SiBA materials more closely matched the decomposition profile of PDMS, as a single decomposition event is observed (Figure 5). The percent mass loss increases with decreasing boronic acid loading (Figure 5), which suggests that the majority of the observed mass loss is a result of the degradation of the PDMS backbone. The resultant mass is composed of decomposed boronic acid and can be used to rank materials based on boronic acid content. It was previously demonstrated that, after exposure to water, pendant SiBA materials have larger Young’s moduli (P-53 2285 kPa, P-37 2192 kPa, and P-13 2149 kPa) than the telechelic SiBA materials (T-23 171 kPa versus T-5 154 kPa),6 due to higher cross-link densities in the pendant samples. Dimethylsilicones are not capable of self-cross-linking. Therefore, the cross-linking in SiBA is associated with the boronic acid group; note that the tartrate-protected SiBA compounds, E

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sites correspond to three-coordinate boron environments which tend to have CQ values ranging from 1.5 to 3.5 MHz depending on the symmetry of the boron center.13 The site with an isotropic chemical shift of 9.3 ppm has a CQ of 0.64 MHz. This site is therefore attributed to an asymmetric fourcoordinate boron center. Similar results are obtained by performing the MQMAS experiment on P-37 (section SI-4). Errors for the line shape fitting of the MQMAS data tended to be small (∼10%) as the line shapes from the F2 projections were narrow and individually resolved. The experiment was not attempted on the materials with lower boron loading over concerns for achieving appreciable signal, as MQMAS experiments tend to yield significantly less signal relative to what can be achieved in a typical 1D NMR experiment. However, on the basis of data from the elastomers with higher boron loading, it can be concluded that the materials with lower boron loading contain two boron sites with chemical shifts between 20 and 30 ppm and the four-coordinate boron site with a chemical shift around 9 ppm (sections SI-5−7). Therefore, similar quadrupole parameters were used to fit these line shapes. Errors in line shape fitting were greater in samples with lower boron loading as it was not efficient to collect MQMAS spectra. Line shape fitting for all SiBA materials at each magnetic field can be found in the Supporting Information (sections SI-3−7). The same quadrupole parameters and number of sites were used to fit each line shape at each magnetic field to confirm the suitability of the MQMAS NMR-derived fits. Errors in the determination of line broadening and chemical shift tended to be greater at lower magnetic field due to the superposition of nonequivalent sites. The three- and four-coordinate environments that are confirmed by MQMAS are used to determine the structure of each boron site (Figure 3). These structural motifs are now linked to specific line shapes of the 11B spectra, illustrated in Figure 8, where the motif labels A−E are retained between Figures 3 and 8. It is well understood that the boronic acid groups are responsible for cross-linking in these materials.3,6 The transformation from oil to elastomer upon the removal of the tartrate protecting group (Figure 1) has been documented via IR spectroscopy (section SI-8). Elastomer formation is marked by the presence of a peak at 3300 cm−1 which corresponds to a hydrogen-bonded O−H stretching vibration.28 It can be concluded that hydrogen bond formation is one of the methods of boron cross-linking that occurs in SiBA elastomers (Figure 8C). As hydrogen bonding was identified via IR in T-23 (section SI-8), one of the three-coordinate boron sites with chemical shifts between 20 and 30 ppm must correspond to the hydrogen-bonded dimer (Figure 8C). NonNewtonian fluids are characterized by a tendency to exist in an equilibrium between the dimerized and free states.29 As the elastomers with lower boron loading tend to be the most fluid, it can be assumed that the second three-coordinate site with a chemical shift between 20 and 30 ppm corresponds to free boronic acid (Figure 8B). As boronic acid dimerization is typically associated with lower CQ values,30 the site with an isotropic chemical shift of 26.6 ppm is assigned to the dimerized structure (Figure 8C). The site with a chemical shift of 24.0 ppm is assigned to the free boronic acid (Figure 8B). An additional set of three-coordinate boron peaks exists between 13 and 20 ppm in P-49 and P-37 (Figure 6C, sections SI-3,4). These sites were assigned to the three-coordinate boron environment that exists when a B−O−B dative bond is formed (Figures 3A and 8A). These sites are not observed in

Figure 6. Background suppressed 11B spectra of each elastomer collected at a magnetic field of 7 T with 15 kHz spinning (A), 11.7 T with 30 kHz spinning (B), and 20 T with 30 kHz spinning (C). The spectrum of the T-5 sample was collected without spinning at 7 and 20 T (A, C).

isotropic chemical shifts between 14.5 and 17 ppm have CQ values ranging from 1.5 to 2.1 MHz. Both of these groups of F

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Figure 7. Sheared MQMAS 11B spectrum of P-49 collected at 20 T with 30 kHz spinning. The differences in chemical shift between the direct and indirect dimensions were used to calculate CQ and η for each site. Projections of the isotropic dimension for each site are shown on the right.

Figure 8. 1D spectrum of P-49 acquired at 20 T with 30 kHz spinning. The line shape is fit using the quadrupole parameters that were obtained from MQMAS NMR with the dashed line showing the sum of the fits. Each site is labeled with the corresponding boron coordination environment from Figure 3 with the symbol R being used to denote the VPBA group and the PDMS chain.

An alternative explanation for the cross-linking involves the formation of boroxines. Boroxines are B−O trimers resulting from the dehydration of boronic acids.31 Normally, the equilibrium lies far to the side of boronic acids in the presence of water, but of course, many of the boronic acid groups will reside within a silicone environment, which could have a very low water content. Boroxines derived from phenylboronic acids exhibit a notable shift of the ortho to boron aryl protons in the 1H NMR spectrum, from about 7.7 to 8.1 ppm.32 Proton NMR spectra of T-23 and P-13 (section SI-9) contain signals at 7.7 ppm and do not contain signals above 8 ppm, suggesting

P-13, T-23, and T-5 (Figure 6C) due to fewer four-coordinate boron sites being present in these samples compared to those with higher boronic acid loadings. These sites are believed to correspond to dimers, as opposed to free boronic acids, as dimerized boronic acid sites tend to have lower CQ values.30 CQ values for the dimerized structure described in Figure 8A were ∼2 MHz as opposed to the 3 MHz that was observed for the free boronic acid (Figure 8B, section SI-3). It is anticipated that these sites (18.2 ppm and 1.54 MHz, 16.3 ppm and 2.03 MHz) correspond to strong and weakly bound dimers, respectively, based on differences in CQ between these sites. G

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more favorable when more boronic acid is present whereas B− O−Si bonding is not dependent on having available neighboring boronic acid groups. Unlike for spin 1/2 nuclei, the integrated area of NMR signals cannot be used to directly quantify populations of quadrupolar nuclei. This is because rf pulse power varies with CQ, meaning that individual sites are not uniformly excited by each pulse.34 Therefore, excitation is dependent on the spin of the nucleus of interest as well as the symmetry of the coordination environment at each individual site.34 However, because of the significant decrease in the strength of the quadrupolar interaction at high field,21 it is believed that normalized integrated areas of the fourcoordinate boron peak acquired at 20 T may be used to determine the relative proportion of four-coordinate boron in the SiBA elastomers (Figure 9A). In general, the relative proportion of four-coordinate boron tends to increase with increasing boronic acid loading. The highest proportion of four-coordinate boron is found in P-49 and P-37, which have significantly higher boronic acid loadings than the other samples (Figure 9). The percent fourcoordinate boron decreases substantially when boronic acid loading decreases to 13% in the pendant SiBA samples and again when pendent SiBA is compared with telechelic SiBA (Figure 9). Although the relationship between boronic acid loading and the relative proportion of four-coordinate boron is not linear, the observed trend suggests that dative bonding is more favorable when more boronic acid is present and that dative bonding is more likely to occur when density of boronic acid is higher (pendent versus telechelic). These findings show that conditions which place boronic acid groups closer together result in a greater incidence of dative bonding (Figure 3D). The influence of the proximity of boronic acid groups on dative bonding suggests that dative bonding occurs via the formation of B−O−B rather than B−O−Si bonds (Figure 3D vs 3E). The quantitative data presented in Figure 9 suggests that boronic acid density (pendent versus telechelic) may be the more influential factor in determining the incidence of dative bonding (Figure 3D). The increased likelihood of dative bonding in elastomers containing pendant boronic acids is demonstrated when P-13 and T-23 are compared. T-23 contains a higher percentage of boronic acid but has a lower relative proportion of four-coordinate boron centers (Figure 9A). The difference between elastomers with pendant and telechelic boronic acids is most intriguing. Pendant boronic acids have fewer degrees of freedom of motion as a consequence of the flanking polymer chains, on both sides, when compared to telechelic compounds, which are tethered on one end. The difference between elastomers with pendant and telechelic boronic acids is most clearly illustrated when Young’s moduli of these samples are compared (Figure 9B).6 The Young’s moduli of the pendant samples are an order of magnitude larger than those of the telechelic samples (Figure 9B), indicating that the pendant samples are significantly more rigid than the telechelic samples. The increased tendency for four-coordinate boron centers to form in the pendant elastomers suggests that four-coordinate cross-links may be “more efficient” with pendant boronic acids than with telechelic moieties, possibly because the equilibrium between free and dimerized boronic acids favors the four-coordinate boron in the former case.

that boroxines are not present in these materials. Additionally, boroxines can be identified by strong characteristic signals in the IR spectrum at 1340, 1300, and 700 cm−1 resulting from E′ and A″2 vibrational modes.33 These are not present in the IR spectrum of T-23 (section SI-8). Quantum chemical calculations were not used to support the experimentally determined quadrupolar parameters and, by extension, the proposed boron coordination environments due in part to the complexity of the MQMAS spectra (Figure 7) and the impossibility of obtaining crystal structures for the SiBA elastomers. Quantum chemical calculations were used by Perras and Bryce to support quadrupole parameters that were extracted from the MQMAS analysis of several well-understood boron-containing small molecules.17 Although the quadrupolar parameters predicted by quantum mechanical calculation closely matched the experimentally obtained values, the calculated isotropic chemical shifts differed significantly.17 Because of the presence of multiple superimposed boron environments in the SiBA elastomers (Figure 6C), even at a magnetic field of 20 T, significant variances in the isotropic chemical shift may make it difficult to reproduce the experimentally obtained spectrum. Additionally, it is not possible to obtain crystal structures for the SiBA elastomers presented in this work due to the dynamic and amorphous nature of these materials. The lack of crystal structures makes solid state NMR a more efficient method of predicting the structure of boron coordination environments. The structure of the four-coordinate boron site is also of interest. Dative bonding may occur via either a B−O−B bond between two boronic acids (Figure 3D) or via a B−O−Si bond between a boronic acid and an oxygen on the PDMS backbone (Figure 3E). It is of interest to determine the relative proportion of four-coordinate boron as a function of boronic acid loading (Figure 9A) as B−O−B bonding would become

Figure 9. (A) Relative proportion of four-coordinate boron. (B) Young’s modulus as a function of boronic acid loading. H

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CONCLUSION Boron coordination environments were analyzed in SiBA elastomers using solid state 11B NMR which afforded good resolution of distinct boron coordination environments when MQMAS experiments were performed. An analysis of the quadrupole parameters of each site confirmed that all SiBA elastomers contained three- and four-coordinate boron environments. These experiments were, however, limited to SiBA elastomers containing relatively high amounts of boronic acid. Experimentally determined quadrupole parameters where extrapolated to systems with lower boron loading as significant increases in experimental time would be required to adequately characterize these materials directly. Three-coordinate boron sites were identified as being boronic acids dimerized by hydrogen bonding and free boronic acids. Four-coordinate sites were assigned to dative bonded boronic acids. The observed dependence of dative bonding on the amount of boronic acids present as well as boronic acid density suggests that dative bonding occurs via the formation of B−O−B bonds in these materials. Boronic acid density was deemed to be the more important factor in boronic acid cross-linking with fourcoordinate centers being more likely to form in pendant materials. Understanding how boronic acid loading and boronic acid density influence the formation of four-coordinate boron and the resultant elasticity of SiBA elastomers has important consequences when these materials are considered for use in biomedical applications such as contact lenses.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02442.

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Sections SI-1 to SI-9 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. ORCID

Michael A. Brook: 0000-0003-0705-9657 Gillian R. Goward: 0000-0002-7489-3329 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Natural Sciences and Engineering Council of Canada for financial support and Dr. Laura Zepeda-Velazquez for helpful discussions.

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REFERENCES

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DOI: 10.1021/acs.macromol.8b02442 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b02442 Macromolecules XXXX, XXX, XXX−XXX