Polyurea Structure Characterization by HR-MAS NMR Spectroscopy

Article pubs.acs.org/IECR

Polyurea Structure Characterization by HR-MAS NMR Spectroscopy Xubao Jiang,† Xiaoli Zhu,† Alexandre A. Arnold,*,‡ Xiang Zheng Kong,*,† and Jerome P. Claverie*,§ †

College of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China NanoQAM, Québec Center for Functional Materials, Department of Chemistry, Université du Québec at Montréal, Montréal, Québec H3C3P8, Canada § Québec Center for Functional Materials, Department of Chemistry, Université de Sherbrooke, Sherbrooke, Québec J1K2R1, Canada ‡

S Supporting Information *

ABSTRACT: Owing to the presence of abundant interchain interactions such as hydrogen bonds, polyureas (PU) are only swellable or soluble in a limited number of highly protonic solvents, and the viscosity of the solutions obtained is very high, making their chemical structure characterization hard or even impossible by standard NMR. Accurate structure analysis is also hard by solid-state NMR due to low spectral resolution. The presence of a side reaction in their synthesis generating biuret cross-links is often invoked to explain their insolubility. Here we demonstrate that High Resolution Magic Angle Spinning (HR-MAS) NMR is an efficient tool for the chemical structure analysis even for cross-linked PU. With 1H, 13C, and 1 H−15N HSQC combined, a variety of linear and cross-linked PU is analyzed by HR-MAS NMR, and conclusive information on their chemical structure is obtained, which reveals for the first time that the biuret group is absent in all PU.

1. INTRODUCTION Polyureas (PUs) are of prime importance for a great number of applications spanning from biocompatible hydrogels,1 catalyst support,2,3 microcapsules for controlled release to phase transition materials,4−8 self-healing material, and anticorrosion protective coatings.9,10 Despite more than 50 years of study, the precise chemical structure of these polymers remains controversial. It is well-known that Nuclear Magnetic Resonance (NMR) is one of the most versatile tools to study polymer chemical structure. However, the complete elucidation of PU chemical structure by NMR is hampered by their low solubility in most common solvents due to the presence of hydrogen bonds between their chains. Nevertheless, PU are soluble in a limited number of highly polar solvents,11 but the resulting solutions are highly viscous. In NMR, high viscosity translates into short T2 values and large band-widths of all resonances, leading to poor resolution and unacceptable signal over noise.12,13 The conventional solid-state NMR techniques such as cross-polarization magic-angle-spinning (MAS) usually do not offer sufficient resolution to separate adequately the resonances arising from fine chemical structure differences.14 In addition, conventional solid-state NMR probes are not optimal for 1H detection as they lack field-locking circuits and gradient coils, which prevent the use of widespread indirect detection methods such as heteronuclear single quantum correlation (HSQC) for example. Therefore, specially prepared model compounds are needed for chemical structure determination of PU or look-alikes.15 © XXXX American Chemical Society

High-Resolution (HR) MAS NMR spectroscopy has been developed to analyze materials with rheological features intermediate between those of liquids and solids, such as gels, elastomers, ionic liquids, and liquid crystals.16 The presence of MAS allows the averaging of most heteronuclear dipolar couplings and anisotropies of magnetic susceptibility, leading to spectra with liquid-like bandwidth. Using HR-MAS NMR, the mechanism of vulcanization of butadiene rubber with cyclic disulfide was elucidated.17,18 This technique was also used to shed light on the hydration mechanism of cross-linked polyamidoamines and to achieve cryogel functionalization by click reaction.1,19 Due to cross-linking and poor solubility of these materials, no other NMR technique is suitable to generate exploitable results. We report here the analysis of a series of PUs (Figure 1) by this technique and demonstrate that biuret functionality is not present in the polymer and that cross-links originate solely from the reaction of triamine with isocyanates.

2. EXPERIMENTAL SECTION 2.1. Materials. All the reagents were used as received. Isophorone diisocyanate (IPDI) was purchased from Degussa (German). Hexamethylene diisocyanate (HDI, AR) was obtained from Aladdin Chemicals. Ethylene diamine (EDA, Received: Revised: Accepted: Published: A

January 15, 2017 February 28, 2017 March 2, 2017 March 2, 2017 DOI: 10.1021/acs.iecr.7b00192 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Polyureas (PUs) prepared for this study.

samples were taken and centrifuged for 5 min at 12,000 rpm to separate the polymer from the solvent. The polymer obtained was washed twice with acetone prior to drying up at 80 °C for 12 h under vacuum. 2.3. PU Structure Analysis Using NMR. All spectra were recorded on a Bruker Avance III HD (Milton, ON, Canada), operated at frequencies of 599.90, 150.86, and 60.79 MHz for 1 H, 13C, and 15N, respectively. Liquid state samples (PU1) consisted of 77 mg of dry polymer dissolved in 500 μL of DMSO-d6, and their spectra were recorded using a 5 mm broadband double resonance BBFO probe. The 1H spectra consisted of 32 scans acquired with a 12.9 μs long at 90° pulse, with an acquisition time of 1 s, a recycle delay of 3 s, and a spectral width of 20 ppm. The 13C spectra consisted of 1024 scans acquired with a 4 μs long 30° pulse, an acquisition time of 1 s, a recycle delay of 2 s, and a spectral width of 260 ppm. Waltz-16 1H decoupling with a radio frequency field of 3.5 kHz was applied during recycle delays and acquisition. 1H−15N HSQC spectra were acquired using the same 1H pulse lengths, 14.2 μs long 15N 90° hard-pulses, and Garp-4 15N decoupling during acquisition at a radio frequency field of 1 kHz. Sixteen scans were accumulated for each of the 128 spectra in the indirect dimension. Spectral widths were 10 and 200 ppm in 1H and 15N dimensions, respectively. HR-MAS samples were prepared by dispersing 60 mg of dry polymer in DMSO-d6 (100−300 μL), and the dispersion was thoroughly vortexed to ensure a complete wetting of the polymers. After mild centrifugation, the resulting slurries were placed in 4 mm MAS rotors with 50 μL inserts to obtain an optimal RF and magnetic field homogeneity. The samples were spun at 10 kHz using a 4 mm HR-MAS indirect-detection double resonance probe. The temperature was raised to 60 °C in order to improve spectral resolution, and the field was locked using the DMSO-d6 signal with the samples manually shimmed. 1 H, 13C, and 1H−15N HSQC spectra were acquired under the same conditions as the liquid-state spectra, except that the pulse length in 13C and 15N 90° was reduced to 5 and 10 μs, respectively, and 4096 transients were acquired for 13C spectra. The cross-linked samples PU3 and PU5 were in addition

AR), diethylene triamine (DETA, AR), and acetone (AR) were purchased from Tianjin Damao Chemicals, China. Deuterated dimethyl sulfoxide (DMSO-d6, 99.9%) was from Sigma-Aldrich. Water was double distilled in the laboratory. 2.2. Preparation of PU Samples. PU samples were prepared via precipitation polymerization as previously reported.20,21 Five samples (PU1−PU5, Figure 1) were synthesized using either IPDI or HDI as isocyanate monomer. PU was prepared based on two different mechanisms of polymerization: copolymerization with polyamine (EDA or DETA) and copolymerization with amines in situ formed through the reaction of diisocyanate (IPDI or HDI) with water. With polymerization done straight with a polyamine, acetone was used as the solvent; whereas with the polymerization done through the reaction of diisocyanate with water, a binary solvent of H2O−acetone at 3/7 mass ratio was used. The reactants and the solvent as well as their amounts used are shown in Table 1. For a typical run, the solvent (acetone or its Table 1. Ingredients and Their Amounts (g) Used in Preparation of PU Samples PU samples

IPDI

PU1 PU2 PU3 PU4 PU5

5.0

HDI

DETA

EDA

5.0 1.5 1.5

0.47 0.42 1.4

0.58

H2O

acetone

28.5 28.5

66.5 66.5 98.0 98.0 98.0

mixture with H2O) was first charged into a glass bottle of 120 mL capacity, followed by addition of a diisocyanate (IPDI or HDI) for PU1 and PU2 or by addition of a diisocyanate with a polyamine (PU3−PU5) with the ratio of the functional groups NCO/amines (primary and secondary) fixed at 1.0. The reaction bottle was sealed off, hand shaken for about 10 s to make the mixture homogeneous, and located into a water bath at 30 °C. The polymerization was conducted under quiescent condition, i.e., without any stirring or shaking during polymerization, for 4 h.21 The initially clear solution turned turbid during the process. At end of the polymerization, B

DOI: 10.1021/acs.iecr.7b00192 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


Figure 2. Liquid NMR spectra of PU1. a). 1H NMR spectrum (inset: H-bonded urea proton region); b). 13C NMR spectrum (carbonyl region); c). 1 H−15N HSQC spectrum. All spectra were taken in DMSO-d6 at 60 °C.

the presence of a urea unit. Second, Okuto’s report indicated that biuret reacts with primary amines at room temperature. As the synthesis of PUs is generally performed with an equal molar ratio of amine to isocyanate, the biuret group should not be present in all PU in this work. However, the presence of the biuret group has been claimed in a variety of PU.25−27 In view of these conflicting reports, five PU samples were prepared (Figure 1), and their chemical structures were analyzed by NMR in order to elucidate the presence of the biuret group. Theoretically, samples PU1, PU2, and PU4 are all linear PU. However, only PU1 was soluble in DMSO at 60 °C, whereas PU2 and PU4 were swollen to form viscous gels. It is unclear whether this behavior was owing to the formation of Hbonding network or to biuret cross-linking.28 Samples PU3 and PU5 were cross-linked as they were prepared with DETA, a trifunctional amine. PU5 formed a viscous gel in DMSO, while PU3 appeared as a wet powder. Figure 2 shows the 1H liquid NMR spectrum of PU1. The NH urea protons resonate between 5.0 and 6.0 ppm and are shown in insets of Figure 2a. Based on the PU1 structure, two NH resonances should be observed in a 1:1 ratio, corresponding to the NH group attached to methylene CH2 and methyne CH. The spectrum of 1H−15N HSQC (Figure 2c) demonstrates that one set of NH resonates between 5.3 and 5.6 ppm (correlated to 15N at −282 ppm), whereas the other set of NH resonates between 5.6 and 5.9 ppm (correlated to 15N at −305 ppm). The presence of several poorly resolved resonances of the two protons of the urea group indicated different environments for these two protons, which could either be due to the lack of regioregularity of the polymer chain

analyzed using a double resonance 4 mm MAS solid-state NMR probe. The NMR parameters were identical to the HR-MAS 13 C analysis except that the acquisition time was reduced to 50 ms, and high-power TPPM 1H decoupling was used during data acquisition.

3. RESULTS AND DISCUSSION The reaction of a polyamine with a diisocyanate leads to PU formation (Figure 1). Alternatively, diamine is formed by reacting diisocyanate with water. The amines thus formed in situ react also with isocyanate to form PU. Biuret groups can also be formed when an isocyanate reacts with a urea group.21 While the formation of allophanate is well-established in polyurethanes,22 the presence of biuret groups in PU remains ambiguous. The presence of biuret moiety in PU was demonstrated by Okuto,23 based on chemical shifts assignments obtained with model compounds.24 In these studies, the biuret group was formed by heating a linear PU at 140 °C in the presence of excessive isocyanate. The author also degraded the biuret linkages with a probe amine at room temperature and analyzed the resulting polymer by 1H NMR. This method is only efficient for the polymers with biuret or allophanate as the only cross-linking units and is not applicable for the majority of the PU cross-linked by multifunctional amines (PU3 and PU5 for examples, Figure 1). Furthermore, it is of low sensitivity, as the lowest amount of the biuret group detectable was 3%.23 Despite the limitations of this method, it outlined several key factors for biuret reactivity. First, biuret is only formed at high temperature with excessive isocyanate in C


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Figure 3. Analysis of PU samples by HR-MAS NMR. a). 13C NMR of PU4 (carbonyl region); b). 1H−15N HSQC of PU4; c). 13C NMR of PU3 (carbonyl region); d). 13C NMR of PU5 (carbonyl region); e). 1H−15N HSQC of PU5. All spectra were taken in DMSO-d6 at 60 °C.

on which high-resolution spectra are hard to acquire (Figure S4). The HR-MAS approach was therefore carried out. The 1H−15N HSQC spectrum of PU5 (Figure 3e) shows one major NH resonance at 5.9 ppm in 1H NMR coupled to two types of N atoms. By comparing the 1H−15N HSQC spectrum of PU4 (Figure 3b) with PU2 (Figure S3), the 15N resonating at −302.3 ppm and −297.4 ppm is respectively attributed to the N atoms of DETA and HDI (−NHCONH−). Interestingly, a smaller NH resonance at 5.6 ppm from the HDI molecule (shown by 1H resonance) is present in the spectrum of PU5. This is attributed to the N (−297.6 ppm) atom of HDI (−NHCONR1R2) after reaction with the secondary amine of DETA. Since branching urea groups (i.e., the urea formed by reaction of an isocyanate with a secondary amine) are clearly detected in 1H and 1H−15N HSQC spectra of PU5, the presence of two overlapping peaks in 13C NMR at 158.6 and 157.9 ppm (Figure 3d) can be simply attributed to ureas of type RNHCONHR and RNHCONR1R2, respectively. With these attributions done, the interpretation of the 13C spectrum of PU3 becomes facile (Figure 3c). The resonances of PU3 at 158.5 and 157.6 ppm correspond respectively to the ureas CH2NHCONHCH2 and CHRNHCONHCH2. A resonance around 157.8 ppm seems also present but severely hidden by overlapping with the peak at 157.6 ppm, yielding the nonsymmetrical aspect of the two resonances. This resonance at 157.8 ppm is attributed to the branched urea (RNHCONR1R2).

(head to tail or head to head connection of the IPDI monomer) or to the presence of various H-bond patterns. The 13C NMR spectrum of PU1 (Figure 2b) shows three CO resonances in a 1:2:1 ratio. Based on the work of Desilets et al.29 and on chemical shift predictions by ChemDraw Software, the peaks at 158.5, 157.6, and 156.6 ppm are respectively attributed to the urea attached by methylene on both sides, the urea by one methylene and one methine at each side, and finally the urea by methine at both sides. The 1:2:1 ratio indicates a lack of regioregularity in the polymer, indicating that the reaction of the in situ formed amine has no selectivity toward the two different isocyanate functions of IPDI. This 13C spectrum seems to reveal the absence of the biuret group, which is reportedly expected in the range of 155− 156 ppm.29,30 The sample PU1 was the only sample soluble in DMSO at 60 °C and analyzed by liquid-state NMR. The analysis for the rest of the samples was pursued with HR-MAS NMR. For sample PU2 (Figures S1, S2, S3), only one type of the urea group was detected, as shown by a single resonance for the NH−CO−NH fragment in 1H, 13C, and 1H−15N HSQC NMR. For the sample PU4, two 13C resonances (Figure 3a) were observed in a 1:1 ratio at 158.8 and 157.8 ppm, corresponding respectively to the urea with methylene on both sides and that with methylene and methine on each side. The 1H−15N HSQC spectrum (Figure 3b) was also consistent with the structure of the polymer, with 15N resonances at −302 ppm for the CH2NHCO urea group and at −283 ppm for the CHRNHCO group. More complex 13C spectra were obtained for cross-linked PU3 and PU5 (Figure 3c and 3d). Owing to the reduced mobility in these samples, these cross-linked polymers can be analyzed by HR-MAS, and high-power proton decoupling is also likely beneficial for 13C detection. Unfortunately, this requires the use of a dedicated solid-state NMR spectrometer,

4. CONCLUSIONS The above results provide a clear demonstration that the HRMAS NMR technique is an efficient technique to analyze crosslinked PU. The analysis is remarkably rapid (2 min for 1H spectrum, 40 min for 1H−15N HSQC, and 50 min for 13C spectrum) and performs well in terms of signal over noise and resolution with conventional liquid-state NMR. The HR-MAS D


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C NMR analysis clearly indicates that the biuret group, expected at 156 ppm, is absent in these samples, within the limit of the sensitivity of the 13C analysis, estimated to be around 1%. Therefore, intermolecular H-bonds are responsible for the insolubility of linear PU (PU2 and PU4). For the crosslinked samples PU3 and PU5, in addition to the intermolecular H-bonds, their lack of solubility is also due to the presence of branched urea groups from the reaction of the isocyanate with the polyamine. We envision that HR-MAS NMR spectroscopy is the method of choice for the chemical structure analysis of a variety of H-bonded polymers such as polyamides, polyurethanes, or ionomers.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00192. HR-MAS NMR spectra of PU2 and 13C MAS solid state NMR of PU3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.A.). *E-mail: [email protected] (X.Z.K.). *E-mail: [email protected] (J.C.). ORCID

Xiang Zheng Kong: 0000-0002-1725-4619 Jerome P. Claverie: 0000-0001-7363-1186 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by National Nature Science Foundation of China (NSFC, Grant Nos. 21274054, 21304038, 51473066) and the Fonds de Recherche du Québec Nature et Technologies. J.C. acknowledges a travel grant from China Scholarship Council.

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Polyurea Structure Characterization by HR-MAS NMR Spectroscopy

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