Structural Analysis of the End Groups and Substructures of

Jul 12, 2016 - Structural Analysis of the End Groups and Substructures of Commercial Poly(ethylene terephthalate) by Multiple-WET 1H/13C NMR...
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Structural Analysis of the End Groups and Substructures of Commercial Poly(ethylene terephthalate) by Multiple-WET 1H/13C NMR Kimiko Tanaka,*,† Muneki Oouchi,*,‡ Fumiaki Hayashi,‡ Hideaki Maeda,‡ and Hiroshi Waki† †

Mitsui Chemical Analysis & Consulting Service Inc., 580-32 Nagaura, Sodegaura, Chiba, 299-0265, Japan NMR Facility, Center for Life Science Technologies, RIKEN, 1-7-22 Suehiro, Tsurumi, Yokohama, Kanagawa 230-0045, Japan

‡

S Supporting Information *

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INTRODUCTION Nuclear magnetic resonance (NMR) has undoubtedly emerged as one of the most valuable spectroscopic techniques for structural and dynamic studies of polymers. In a recent review article, de Ilarduya and Muñoz-Guerra well highlight the application of NMR to the structural study of polyesters.1 However, the structural characterization of the end groups of commercial poly(ethylene terephthalate) (PET) by solution 1H NMR has always been challenging because of poor sensitivity, severe overlaps of the main-component and solvent signals, and the insufficient dynamic range of NMR instruments. In general, low-molecular-weight model compounds2,3 and/or 13C-labeling experiments4 are used to assign the NMR signals. Microstructures bearing the vinyl end group of commercial PET were characterized using solution NMR and a model copolyester.2 In a subsequent study by Amari et al.,5 one of the three proton signals from the vinyl end group was masked by the main signal of the ethylene glycol (EG) unit, and only two of the three proton signals were identified in the 1H NMR spectrum of a PET solution. Therefore, the development of a method to suppress strong 1H signals originating from the main components (ethylene terephthalate (EG-TA)) and solvents (CDCl3, C5D5N, and HFIP-d2) is essential to observing the NMR signals from all of the end groups. Hence, the assignment of the end group signals can be accomplished without using any model compounds and/or 13C-labeling experiments. Previously, water suppression enhanced through T1 effects (WET)6−9 was combined with 2D double-quantum coherence to suppress two signals from the main component of poly(ether sulfone) and one signal from the solvent;10 for a similar purpose, double presaturation 1H NMR was used for the unsaturation characterization of polyethylene.11 In this study, we demonstrate the use of a multiple WET-NMR method implemented in multidimensional (2D and 3D) 1H and 13C chemical-shift correlation experiments12 at ultrahigh field (21.1 T) to eliminate these strong signals. As a result, we observed 1H signals from the end groups, such as vinyl, methyl ester, etc., of commercial PET with an excellent sensitivity, hence paving a path to the complete assignment of these end groups, as presented in this study (Figure 1). 13C chemical shifts were accurately measured using a 1D 13C NMR method to confirm the assignments from the 2D and 3D correlation data. In addition, multiple WET sequences were combined with the inversion recovery pulse sequence to obtain 1H spin−lattice © XXXX American Chemical Society

Figure 1. Main components and substructures observed in a commercial PET.

relaxation times, ultimately to investigate the dynamics of the end groups and substructures in PET.

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EXPERIMENTAL SECTION

Sample Preparation. The PET sample used in this study was purchased from Scientific Polymer Products, Inc.; its number-average molecular weight, Mn (polystyrene equivalents), was 14 400, and its weight-average molecular weight, Mw, was 59 700, as determined by gel permeation chromatography (GPC). The gel permeation chromatograph consisted of a 515 pump, a 717plus autosampler and a 2487 UV/vis detector (Waters). Both 15 and 50 mg samples of PET were separately dissolved in a CDCl3/HFIP-d2 mixture (7/3 v/v) in 5 mm OD NMR tubes to yield concentrations of 3% and 10% (w/v), respectively; 20 μL of C5D5N was subsequently added to both samples to shift any hydroxyl protons away from the δH 4−5 region.5 The 3% PET sample was used to record 1D and 2D 1H NMR spectra and T1, whereas the 10% PET was used for 13C NMR and 3D 1H NMR. NMR Measurements. All of the 1H NMR measurements were carried out on a 900 MHz Bruker Avance III HD spectrometer equipped with a 5 mm cryogenic probe (TCI), whereas 13C NMR measurements were carried out on a 600 MHz Bruker Avance III HD spectrometer equipped with a 5 mm cryogenic probe (DCH). All of the experiments were performed at 298 K. All of the NMR data were processed using the Topspin 3.2 software. Signals for CHCl3 (δH 7.24) and CDCl3 (δC 77.0) were used for chemical shift referencing. MWET(n) (multiple WET with n suppressed points) pulses were optimized by selecting 7−10 frequencies to excite resonances from among δH 8.08 (TA); 4.68 (EG); 5.62(OH); 7.24 (chloroform); 8.39, 7.88, and 7.45 (three frequencies of pyridine); 4.49 and 3.93 (two Received: May 24, 2016 Revised: July 4, 2016

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DOI: 10.1021/acs.macromol.6b01105 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. (a) 1H NMR spectrum of the 3% PET solution at 900 MHz, along with the expanded regions: δH 7.2−8.8 (a1) and 3.5−5.4 (a2). (b) MWET(7) NMR spectrum along with the same expanded regions: δH 7.2−8.8 (b1) and 3.5−5.4 (b2). Seven suppressed points (δH 8.39, 8.08, 7.88, 7.45, 7.24, 5.62, and 4.68) are marked with green arrows. The 1H peaks from the vinyl end group are marked with red arrows. Both spectra were collected using 16 scans and a 25 s repetition time (AQ + D1). Namely, “baseopt” was implemented as a digitization mode (DIGMOD) with a 55 μs sampling delay (DE).

Figure 3. Low-field spectral regions of the H/H and C/H correlation spectra for the vinyl end group of PET recorded at 900 MHz with same suppressions as described in Figure 2b. (a) MWET(7) TOCSY (black, 8 scans, repetition time = 2.3 s) and MWET(7) DQF-COSY (red, 32 scans, repetition time = 2.4 s) spectra. (b) 1D 13C NMR spectrum of the 10% PET solution at 150 MHz (30° pulse, 100 000 scans, repetition time = 2.7 s). (c) MWET(7) HSQC-TOCSY (black, 128 scans, repetition time = 5.6 s) and MWET(7) HSQC (red, 88 scans, repetition time = 2.5 s) spectra. frequencies of DEG); and 4.35 (HFIP-d2). SINC shape excitation pulses ((sin (x))/x, 0 < x < π)) with a 30 ms duration were used for the MWET(n) 1D, 2D, and 3D experiments. The MWET(n) component was added to existing pulse sequences such as doublequantum-filtered-COSY(DQF-COSY), TOCSY, HSQC, HMBC, HSQC-TOCSY, and 3D-HSQC-TOCSY.12 In the inversion recovery pulse sequence, this component was placed in front of the π/2 pulse (Supporting Information, Figure S1). As an example, in the case of n = 10, the excitation profile of an MWET(10) component is shown in the Supporting Information (Figure S2).

a. Vinyl End Group. In several reports, a small amount of the vinyl end group has been demonstrated to have been thermally generated by a β-elimination process during the polycondensation step or after the processing of the polyester at high temperatures.1,5,13 The characterization of a small amount of vinyl end group in a commercial PET using NMR is extremely difficult.1,13 Below, we present the complete assignment of the vinyl end group in a commercial PET. Figure 2 shows the 1H NMR spectrum of the 3% PET solution; this spectrum was recorded at 900 MHz with a cryogenic probe. Only one weak 1H peak from the vinyl end group is observed at δH 5.16 in the 1H NMR spectrum (Figure 2a), whereas the other two peaks from this group are masked by strong signals resulting from the main components (EGTA) and solvents. By using the MWET(7) method, we clearly observed all three 1H resonances of the vinyl end group

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RESULTS AND DISCUSSION Resonance Assignment. We present below a complete assignment strategy for the end groups and substructures of a PET solution (Figure 1) on the basis of 1H and 13C chemical shifts and a multiple WET 1D (H), 2D (H/H, C/H), and 3D (C/H/H) correlation. B

DOI: 10.1021/acs.macromol.6b01105 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules unlike the two 1H resonances reported earlier5at δH 4.82 (dd, 1H, JHH = 2 and 6 Hz) and 5.16 (dd, 1H, JHH = 2 and 14 Hz) with similar integral values (0.98 and 1.00) and at 7.34 (dd, JHH = 6 and 14 Hz) (Figure 2b). When MWET(7) was used, the sensitivity of the signal at δH 5.16 was improved ca. 3-fold compared to that in a normal 1H NMR spectrum. The vinyl end group is assigned on the basis of the MWET(7) 2D H/H correlation spectrum (Figure 3a), wherein the peaks at δH 4.82 (b′), 5.16 (a′), and 7.34 (c′) correlate with each other. In addition, the MWET(7) 2D C/H correlation spectrum (Figure 3c) is used to obtain the C/H correlations between the 1H peaks at δH 4.82 and 5.16 with the 13C peak at δC 100.4 for a 10% PET solution (Figure 3b), i.e., δC 100.4/δH 4.82 and δC 100.4/δH 5.16 (F1/F2), whereas the 1H peak at δH 7.34 is shown to correlate with the 13C peak at δC 140.8, i.e., δC 140.8/ δH 7.34. All of the assigned 1H/13C chemical shift values associated with the vinyl end group are listed in Table 1.

Figure 4. C/H correlation spectra of the 1H high-field region of PET at 900 MHz. (a) DEPT-135 spectrum of the 10% PET solution at 150 MHz (4096 scans, repetition time = 3.9 s). (b) the overlaid C/H MWET(7) HSQC (red) and MWET(7) HSQC-TOCSY (black) correlation spectra at 900 MHz from Figure 3c. (c) 13C NMR spectrum (refer to the caption of Figure 3b). (d) MWET(7) HMBC spectrum (80 scans, repetition time = 2.9 s).

Table 1. Assigned Chemical Shifts and Spin−Lattice Relaxation Times (T1) of the PET Solutions Measured by Inversion−Recovery (IR) and the MWET(7) IR Methodsa T1(1H)/s

chemical shift main TA main EG end-EG DEG end-vinyl

a d f g i j a′ b′ c′

δ Hb

δ Cc

IRb

8.08 4.68 4.45 3.99 4.49 3.93 5.16 4.82 7.34

130.0 63.5 66.9 61.2 64.6 69.0 100.4

3.26 1.25 1.24 1.09 0.96 1.01

140.8

MWET(7) IRb

unit, and the diethylene glycol end group (end-DEG), which were assigned on the basis of the NMR analysis of model copolyesters.2,3 From the MWET(7) HSQC spectrum (Figure 4b, red), two C/H correlations of DEG at δC 64.6/δH 4.49 (i) and at δC 69.0/δH 3.93 (j) are observed, whereas from the MWET(7) HSQC-TOCSY (Figure 4b black), a carbon peak at δC 64.6 is additionally correlated to the 1H peak at δH 3.93 (i.e., δC 64.6/ δH 3.93). Similarly, the carbon peak at δC 69.0 is additionally correlated to the 1H peak at δH 4.49 (i.e., δC 69.0/δH 4.49). Therefore, all of the C/H correlations for DEG are established using both spectra. The connection between the TA unit and DEG is confirmed by a long-range C/H correlation between δC 167.6 (TA carbonyl) and δH 4.49 (DEG) in the 2D MWET(7) HMBC spectrum (Figure 4d); hence, DEG is fully assigned. The end-EG group was assigned in a similar manner; the chemical shifts associated with the assigned 13C and 1H resonances are given in Table 1. Whereas the 2D NMR methods were successfully used to fully assign the C/H resonances of DEG and end-EG, the 3D C/H/H MWET(10) HSQC-TOCSY NMR method was applied for the C/H assignment of TEG and end-DEG. A 2D H/H TOCSY slice-spectrum extracted from the 3D MWET(10) HSQC-TOCSY spectrum of the 10% PET solution at δC 64.4 is shown in Figure 5a. Four H/H correlations are observed at δH 4.49/4.49 (red), δH 4.49/3.93 (red), δH 4.46/4.46 (black), and δH 4.46/3.87 (black). The 1H correlations shown in red are already assigned to DEG, whereas the 1H correlations shown in black are assigned to (q) and (r) of TEG (see Supporting Information for the 13C and 1H chemical shift values, Table S1). Similarly, the 1H correlation at δH 3.75/3.75 (s) is assigned to TEG by taking a spectral slice at δC 70.3 (Figure 5d). The carbon peak at δC 70.3 is determined to be a methylene on the basis of the DEPT-135 spectrum (Figure 4a). In a similar manner, (l), (m), (n) and (o) of endDEG in Figure 5c and 5e are assigned, whereas the H/H correlation (m) of end-DEG in Figure 5c is overlapped with (r) of TEG (refer to the Supporting Information for the chemical shifts, Table S1).

1.23 1.11 0.99 1.03 1.89 1.60 3.91

a

Refer to the Supporting Information for the chemical shifts and relaxation times of the other groups. b3% solution. c10% solution.

b. Methyl Ester End Group. The polymerization method of the polyester was determined using 1H NMR through the presence or absence of the methoxy signal of the methyl ester end group at ∼δH 3.9.2 Therein, the polymerization was achieved using a small model copolyester.2 The 1H peak of interest from the methyl ester end group of methyl terephthalate is masked by strong signals originating from the diethylene glycol (DEG) unit (Figure 2). As a consequence, the presence or absence of a methyl ester group could not be confirmed. We therefore carried out MWET(7) HSQC (Figure 4b, red) and MWET(7) HSQC-TOCSY (Figure 4b, black) experiments at 900 MHz. C/H correlation peaks from the methoxy group (δC 53.0/δH 3.93 (u)) are clearly observed in the data from both experiments. The DEPT135 spectrum of the 10% PET solution shows the methyl resonance peak at δC 53.0 (Figure 4a). Moreover, the connection between the TA unit and the methoxy group is confirmed by a long-range C/H correlation between δC 168.4 (TA carbonyl) and δH 3.93 (methyl) in the MWET(7) HMBC experiment (Figure 4d), and subsequently, the methyl ester end group is assigned. On the basis of the presence of the methyl ester end group, the PET used in this study was possibly produced via the transesterification of dimethyl terephthalate (DMT).2 c. Other End Groups and Substructures. Commercial PET also contains other substructures such as the DEG unit, ethylene glycol end group (end-EG), triethylene glycol (TEG) C

DOI: 10.1021/acs.macromol.6b01105 Macromolecules XXXX, XXX, XXX−XXX

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approximately 1/25th that of the end-EG group (Figure 2b and Supporting Information, Figure S4). Spin−Lattice Relaxation Time (T1). To study the dynamics of the end groups in PET, we carried out 1H spin− lattice relaxation time (T1) measurements using two different approaches: (a) inversion−recovery (IR) and (b) MWET(7) inversion−recovery methods. The T1 values of different groups are listed in Table 1. As evident from the results in Table 1, the two approaches resulted in similar T1 values within 5% error. The T1 values of the vinyl end group5 varied substantially in the range from 1.6 to 3.9 s. In addition, the measured T1 values of the EG, end-EG, and DEG units were observed to be in the range from 1.0 to 1.3 s, which suggests that these groups have similar dynamics in PET.

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CONCLUSION In summary, we have demonstrated the utility of multiple WET 2D and 3D solution NMR methods for the structural and dynamic studies of a commercial PET. In the present study, the difficulty associated with the severe overlap of signals from the main components (TA and EG) and solvents with those of the substructures was effectively overcome through the use of an MWET(n) component. We applied 1D 13C NMR with a highly sensitive cryogenic probe and multiple WET 1D (H), 2D (H/ H, C/H), and 3D (C/H/H) correlation experiments at ultrahigh field (21.1 T) to achieve complete assignment of all of the end groups without using any model compounds. In addition, the 1H spin−lattice relaxation times (T1) of the end groups and substructures were obtained by adding the MWET(n) component to the inversion recovery sequence. Our result from T1 measurements shows that the EG, end-EG, and DEG units are associated with similar dynamics. We believe that 1D and multidimensional NMR methods combined with the MWET(n) suppression presented in this work should serve as a robust tool for the characterization of substructures of not only polyesters but also other synthetic polymers, such as polyamides and polyurethanes.

Figure 5. 2D H/H TOCSY slices extracted from the 3D MWET(10)HSQC-TOCSY spectrum of the 10% PET solution measured with 10 points of multiple suppression, where 3 additional points at δH 4.35 (HFIP-d2), 4.49 and 3.93 (two DEG signals) were suppressed in addition to the MWET(7) in the caption of Figure 2b. The 2D slices in the carbon dimension were extracted at (a) δC 64.4, (b) δC 66.9, (c) δC 69.0, (d) δC 70.3, and (e) δC 71.8 for 13C NMR. Positive peaks are shown in black, whereas partially suppressed negative peaks are shown in red. The total experimental time for the 3D measurement was ca. 2 d (16 scans, repetition time = 1.95 s, TOCSY(DIPSI2) mixing time = 80 ms). Details of the 3D experiment include, spectral width (SW): 9,058 (13C) × 10,799 (1H) × 14,423 (1H) Hz, acquired data points: 48 (t1) × 96 (t2) × 2,048 (t3) and processed data points: 256 (F1) × 512 (F2) × 2,048 (F3).

The presence of a small amount of the isophthalate (IA) unit3 is confirmed from the 1H peaks at δH 8.66 (s, 1H), 8.22 (d, 2H, JHH = 8 Hz), and 7.55 (t, 1H, JHH = 8 Hz) and the 13C peaks at δC 128.8, 131.1, and 134.8 (Figure 6). On the basis of small chemical composition of IA as calculated from its 1H peak integral values (approximately 0.1 mol % from Figure S3b), this unit may originate from impurities and/or additives. The concentrations of each group are determined by the integral intensity of the 1H resonance and the peak height of the 13C resonance in the 1D NMR spectra (Supporting Information, Figures S3, S4, and S7). The calculated concentrations of these groups decrease in the order EG ≫ DEG > end-EG > methyl ester end group > TEG > end-DEG, vinyl end group. The integrated area of the vinyl end group was

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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.macromol.6b01105.

Figure 6. Aromatic regions of the H/H and C/H correlation spectra of PET recorded at 900 MHz in Figure 3. (a) MWET(7) TOCSY (black) and MWET(7) DQF-COSY (red) spectra. (b) 1D 13C NMR spectrum of the 10% PET solution at 150 MHz. (c) MWET(7) HSQC-TOCSY (black) and MWET(7) HSQC (red) spectra. D

DOI: 10.1021/acs.macromol.6b01105 Macromolecules XXXX, XXX, XXX−XXX

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Chemical shift table, pulse sequences, and additional NMR data (PDF) Bruker programs for pulse (ZIP)

AUTHOR INFORMATION

Corresponding Authors

*(K.T.) E-mail: [email protected]. *(M.O.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS All NMR experiments were performed at the RIKEN NMR Facility as an activity of the “NMR Open Sharing and Platform Program” supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors would like to thank Prof. Toshifumi Hiraoki of Hokkaido University for helpful discussions.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b01105 Macromolecules XXXX, XXX, XXX−XXX