NMR Spectroscopy of Polymers in Solution and in the Solid State

Chapter 10 1

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Triple Resonance 3D NMR Study of Poly(dimethylsiloxane) MD M

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H/ C/ Si

H

Downloaded by NORTH CAROLINA STATE UNIV on January 3, 2013 | http://pubs.acs.org Publication Date: December 10, 2002 | doi: 10.1021/bk-2003-0834.ch010

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Minghui Chai , Peter L. Rinaldi , and Sanlin Hu 1

Department of Chemistry, Marshall University, Huntington, WV 25755-2520 Department of Chemistry, Knight Chemical Laboratory, The University of Akron, Akron, OH 44325-3601 DowCorning Corporation, Midland, MI 48686

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H/ C/ Si triple resonance 3D NMR combined with pulse field gradient (PFG) techniques has been utilized for characterizing poly(dimethylsiloxane) (PDMS), MD M . The signals from H- C- Si connectivities among H atoms coupled to both C and Si at natural abundance have been selectively detected, which provide information for complete resonance assignments of MD M . The study showed that the considerable spectral dispersion in Si NMR of siloxanes compared with the narrow H and C spectral ranges allows detailed examination of the structure of PDMS by the 3D H/ C/ Si NMR correlation experiment. H

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© 2003 American Chemical Society

In NMR Spectroscopy of Polymers in Solution and in the Solid State; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Introduction Poly(dimethylsiloxane) (PDMS) is the simplest of organosilicone elastomers. PDMS was commercially developed by Dow Corning in the 1960's. It possesses inertness and useful mechanical properties, and can also be readily made into various desired shapes for medical items including prostheses of different bone and soft tissue elements (in surgery), and ancillary components such as tubes, catheters, shunts and drug carriers. In addition, PDMS has shown good compatibility in clinical research. (X) So it has been a very useful synthetic polymer in medicine. In fact, silicone polymers are often employed in checking the biocompatibility of new polymers. Furthermore, organosilicones comprise an important class of compounds used in resins, and room temperature and heat-cured rubber industrial and consumer products. NMR has been a powerful technique for structural analyses of macromolecules. However, ID NMR spectra of PDMS are usually complicated due to signal overlap. Their complete characterization often requires combinations of several techniques. Multidimensional NMR techniques, especially inversely detected 3D heteronuclear shift correlation experiments, offer the opportunity to obtain the complete structural characterization by using NMR experiments alone. Biological 3D-NMR experiments are usually performed in conjunction with uniform C and N isotopic labeling. In polymer chemistry, when isotopic labeling is possible, it is often very difficult and expensive. By modifying the 3D-pulse sequence used for biopolymers, triple resonance 3D-NMR techniques have been adapted for studying the structures of polymers, which involve H - C - F , Η - ΰ - Ρ , ^ - " C - ^ S i spin systems. (2) These results have shown that 3D-NMR spectroscopy can be tremendously useful for characterizing polymer structures, even without isotopic labeling. Performance of H / C / S i triple resonance NMR (natural abundance of S i = 4.7%) is extremely challenging because it requires selective detection of the ^ - ^ C - ^ S i spin systems which are present in only 0.05% of the molecules, while suppressing the signals from the remaining 99.95% of the molecules. Nevertheless, with modern instrumentation and a stable instrument environment, such experiments are possible and can produce very useful data. (2c, 2d) The nomenclature used to define siloxane compounds combines the use of the letters M , D, Τ and Q, which represent R Si—Ο—, R Si(—O—) , RSi(—Ο—) and Si(—Ο—) units respectively, where R stands for aliphatic and/or aromatic substituents or H. In this study, substituents other than methyl groups are indicated as superscripts; for example, M = (Me HSi—Ο—). Scheme 1 shows the structure of M D ^ * . We use M D M , which is a simple but important oligomer of PDMS, to show the use of 3D H / C / S i triple resonance NMR for the characterization of PDMS structures. l 3

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139 Scheme 1 Me

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Me


Me Ο

it

Me Ο

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Ο

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Me

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Experimental H

M D M was obtained from Dow Corning Corporation. 200 mg of this colorless oil was dissolved in 0.7 ml CDC1 solvent and put into a 5-mm NMR tube for the following NMR measurements. All NMR spectra were collected at 25 °C on a Varian Omtyplus 750 MHz NMR spectrometer equipped with four RF channels, a Performa II ζ axial pulse field gradient (PFG) accessory, a 5 mm Varian H / C / X (where X is tunable over the range of the resonance frequencies from N to C d ) triple resonance four channel probe with a PFG coil (for ID H , 2D and 3D NMR experiments), and a 5 mm Varian H / X ( X = P- N) switchable probe with a PFG coil (for ID C and S i NMR experiments). The solvent was also used as the internal reference for *H and C chemical shifts. TMS was used as an external reference for Si chemical shifts. All data were processed with Varian's VNMR software on a SUN Ultra10 workstation. 3

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ID NMR The *H spectrum was acquired at 750 MHz using a 3.5 s acquisition time, 4.2 ps (30°) pulse width and 16 transients. The C spectrum was acquired at 188.6 MHz with WALTZ-16 modulated Ή decoupling using a 1.2 s acquisition time, 4.2 ps (45°) pulse width and 256 transients. The Si spectrum was acquired at 149 MHz with WALTZ-16 modulated H decoupling using a 1.4 s acquisition time, 13.4 ps (90°) pulse width, 5 s delay and 256 transients. 1 3

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2D NMR The ^ S i long range PFG-HMQC(3J 2D NMR spectrum was collected with *H and Si 90° pulses of 12.7 and 22.0 μ 8 , respectively, a relaxation delay 29


140 n

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of 1 s, Δ = (2 χ J )~ = 73.5 ms (optimized for 2 bond 'H-^Si correlations), 5000 Hz and 6000 Hz spectral windows in the Wf ) and Si(/}) dimensions and a 0.045 s acquisition time with Si GARPf^) decoupling; 8 transients were averaged for each of 512 real ti increments. The gradient strengths of three 2.0 ms PFGs were 0.325, 0.325, and -0.129 Tm" , respectively. The 'H^C PFG-HMQC3 2D NMR spectrum was collected with H and C 90° pulses of 12.9 and 25.0 μβ, respectively, a relaxation delay of 3 s, Δ = (2 χ % c ) " = 3.57 ms (optimized for 1 bond H - C correlations), 5000 Hz and 600 Hz spectral windows in the H(f ) and C(/}) dimensions and a 0.05 s acquisition time with C GARP decoupling. 8 transients were averaged for each of 256 real ti increments. The gradient strengths of three 2.0 ms PFGs were 0.216, 0.216, and -0.109 Tm' , respectively. The 2D-NMR data were processed with sinebell weighting; spectra were displayed in the magnitude-mode in both dimensions; 2D FT was performed on a 1024 χ 1024 matrix. HSt

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3D NMR ]

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The 3D-NMR spectrum was obtained with 90° pulses for H , C and Si of 13.8 ps, 28.0 ps and 23.0 ps, respectively, relaxation delay 1 s, Δ = 2.08 ms (l/(4x 7 //), J c = 120 Hz), τ = 10 ms (1/(4x0^,), = 59 Hz), acquisition time = 0.05 s (with simultaneous C and Si GARP decoupling); 4 transients were averaged for each of 2 χ 18 increments during ti and 2 χ 18 increments during t . Evolution times were incremented to provide a 5000 Hz spectral window in f (*H), a 2800 Hz spectral window in// ( Si), and a 600 Hz spectral window in f ( C) dimensions in the 3D-NMR experiment. The durations and amplitudes of the gradient pulses were 3, 1 and 1 ms; and 0.541, 0.325, and 0.0645 T/m, respectively. Thefirstgradient pulse serves as a homospoil pulse, so its value relative to the other two is not critical. The total experiment time was 5.3 hours. The data were zero filled to 512x512^512 and weighted with a shifted sinebell function before Fourier transformation. ;

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Results and Discussion 1D-NMR Spectra ]

The 750 MHz H spectrum (Figure la) exhibits six groups of resonances: the silicon hydride proton at 4.74 ppm, and five different methyl protons between 0.06 - 0.23 ppm. From the relative intensities of the methyl resonances (see integration values in the spectrum), it is possible to assign the M - C H proton resonances to the signals at 0.116 ppm. Also the doublet at 0.21 ppm 3


141 H

can be assigned to the protons of M - C H because of the adjoining silicon hydride proton coupling. At this very high magnetic field (750 MHz), five methyl protons can be resolved, the three peaks at 0.07 - 0.1 ppm can't be directly assigned from the ID spectrum. However, the 188.6 MHz C NMR spectrum (Figure lb) shows five peaks within 0.8 - 2.2 ppm region corresponding to five different methyl groups of M D M Also, based on the relative intensities of these signals, the peak at 2.02 ppm can be assigned to the carbons of M-CH . The other resonances can't be determined just from the ID spectrum. In the Si NMR spectra of siloxanes, the resonances from, M-Si, DSi, T-Si and Q-Si are distinctive because S i resonance frequencies strongly depend on the number of directly bonded oxygen atoms. For instance, the resonance from M type silicon (R SiO-,) is normally found around +10 ppm; and the resonance from D type silicon (R Si(0-) , R = alkyl, not H) is normally found around -20 ppm. So, according to these empirical rules/5,) in S i NMR spectrum (Figure lc), the peak at 7.06 ppm can be attributed to the M silicon of MD;3M ; three resonances at -20.13, -21.59 and -22.17 ppm are from D-type silicons of M D M . The silicon of M is a silicon hydride, which normally appears around -10 ppm in the Si NMR spectrum. Thus the resonance at 7.14 ppm is from the silicon of M . For D-type silicons, the detailed assignment can be made based on the comparison with the Si NMR spectrum of M D M . Based on the reported Si chemical shift assignments of D-type silicon atoms in MD M,(5c) it is possible to assign the resonance at -21.59 ppm in Figure lc to the Di silicon and the resonance at -22.17 ppm to the D silicon. The resonance at -20.13 ppm belongs to the silicon of D . Now based on the assigned silicon resonances, multidimensional NMR techniques can be employed to disperse H and C signals, that fall in narrowfrequencyranges of the ID spectra, into more dimensions, and to correlate these signals with the reliably assigned silicon resonances in the multidimensional NMR spectra. Thus additional structural information can be obtained for siloxane compounds via nD NMR spectroscopy. 3

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2D NMR Spectra ]

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The 2D H - C PFG-HMQC NMR spectrum (Figure 2a) of M D M exhibits crosspeaks from directly bonded 'H-^C atoms. Some of the assignments for H and C resonances made in ID NMR spectra can be confirmed from the 2D spectrum. For example the H - C correlations of the M - C H and M - C H groups can be identified. The *H- C correlations for three D-type methyl groups can also be observed clearly in the spectrum. However, the detailed assignments for individual D-CH groups cannot be obtained. The 2D ^ - ^ S i long-range PFG-HMQC NMR spectrum (Figure 2b) of M D M provides information about 2-bond H- Si correlations. Because 3

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Figure 1. One-dimensional NMR spectra ofMD3^: (a) 750 MHz H spectrum; φ) 188.6MHz Cspectrum; (c) 149MHz Si spectrum. 13

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Figure 2. The 2D PFG-HMQC NMR spectra ofMDM*: (a) H- C PFGHMQC spectrum acquired with delays based on J c 140 Hz; and (b) *H- Si long-range PFG-HMQC spectrum acquired with delays based on J si = 6.8 Hz. 1

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resonances in the ID S i spectrum can be reliably assigned, the *H resonance assignments can be clearly made from this 2D H- Si long-range correlation spectrum, as labeled in Figure 2b. !

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3D NMR Spectrum Figure 3 shows the selected 2D slices, which were taken at each of the ^Si chemical shifts (/}) from the 3D W ^ ^ S i spectrum of MDsM . In order to narrow the relatively wide spectral window of Si for better resolution in the 3D spectrum, a narrow Si spectral window was carefully selected so that the S i resonance at 7.06 ppm would fold into a non-interfering region near -11.6 ppm. Later, the chemical shift was corrected according to the value obtained from the ID S i spectrum. This triple resonance U/ C/ Si heteronuclear correlation 3D experiment was set to selectively detect the 1-bond ^ - ^ C and 1-bond C - S i correlations in the structure of M D M . Therefore, the connectivity between each type of methyl group and its attached silicon atom can be proven from the 3D spectrum. Assignments are labeled in each ^ slice related to the chemical shifts of the directly bound S i atoms in the / ; dimension. The chemical shift assignments from this 3D experiment are summarized in Table 1. 11

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Table 1. Chemical Shift Assignments for MD M

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Chemical Shifts (ppm)

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0.116

2.02

7.06

0.075

1.37

-21.59

0.100

1.26

-22.17

0.095

1.09

-20.13

0.210

0.94

M D2

D M

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-7.14

H

4.743 (Si-H)

Conclusion Triple resonance 3D-NMR experiments can be useful for studying polymeric structures without resorting to isotopic labeling, even when the nuclei involved are present in low natural abundance. This study of MD3M shows that the considerable spectral dispersion obtained in the Si NMR spectra of siloxanes, compared with the narrow *H and C chemical shift ranges, permits detailed examination of the structure of PDMS by the 3D H / C / S i NMR correlation experiment. These techniques can also be useful for characterizing star-branched polymers which contain NMR active nuclei, polymers with low concentrations of heteroatoms (e.g. at the chain end or at low occurrence branch points) and many organometallic compounds. H

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Acknowledgement We would like to acknowledge the NSF (DMR-9617477 and DMR0073346) for support of this research and the Kresge Foundation and donors to the Kresge Challenge program at the University of Akron for funds used to purchase the 750 MHz NMR instrument.

References 1.

Gumargalieva, K. Z.; Zaikov, G. E.; Moiseev, Yu. V. In Polymer in Medicine; Zaikov, G. E., Ed; Nova Science Publisher, Inc.: Commack, NY, 1998; Ch. 1.


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4. 5.

(a). Li,L.;Rinaldi, P. L. Macromolecules, 1997, 30, 520. (b). Saito, T.; Medsker, R. E.; Harwood, H. J.; Rinaldi, P. L. J. Magn. Resort. Ser. A 1996, 120, 125. (c). Chai, M.; Saito, T.; Pi, Z.; Tessier, C.; Rinaldi, P. L. Macromolecules 1997, 30, 1240. (d). Chai, M.; Pi, Z.; Tessier,C.;Rinaldi,P.L.J.Am. Chem. Soc. 1999, 121, 273. Vuister,G.W.;Boelens,R.;Kaptein,R.;Hurd,R.E.;John,B.;van Zijl,P.C.M.J.Am. Chem. Soc. 1991, 113, 9688. Shaka,A.J.;Barker,P.B.;and Freeman, R. J. Magn. Reson. 1986, 64, 547-552. (a) Takayama, Y. In The Chemistry of Organic Silicon compounds; Zappoport, Z.; Apeloig, Y., Eds., Vol. 2, Part 1; John Wiley & Sons, Inc.: New York, NY, 1998, Ch. 6. (b) Smith, A. L. The Analytical Chemistry of Silicones; John Wiley & Sons, Inc.: New York, NY, 1992; Vol. 112. (c) Williams, E. A. Annual Report on NMR Spectroscopy, 1983, 15, pp 235-289. (d) Marsmann, H, In NMR Basic Principles and Progress; Diehl, P.; Fluck, E.; Kosfeld, R., Eds.; Springer-Verlag: New York, NY, 1981, Vol. 17, pp 97.


NMR Spectroscopy of Polymers in Solution and in the Solid State

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