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Chapter 13

Structure Determination of Poly(vinyl alcohol) by 2D N M R Spectroscopy

Downloaded by UNIV OF ARIZONA on January 3, 2013 | http://pubs.acs.org Publication Date: December 10, 2002 | doi: 10.1021/bk-2003-0834.ch013

A. S. Brar, R. Kumar, A. Yadav, and M. Kaur

Department of Chemistry, Indian Institute of Technology, Delhi 110016, India The spectral assignment ofpoly(vinylalcohol)(PVA) prepared by the basic hydrolysis of poly(vinyl acetate) was studied using a combination of one and two-dimensional NMR spectroscopy. The C{ H} and H NMR spectra of the homopolymer (PVA) were assigned to the configurational pentads (CH region) and tetrads (CH region). These assignments were substantiated by the use of two dimensional heteronuclear single quantum correlation (HSQC), heteronuclear single quantum correlation - total correlation spectroscopy (HSQC-TOCSY) and total correlated spectroscopy (TOCSY) experiments. 13

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Introduction The use of 2D NMR spectroscopy for characterizing the stereochemical structures of the polymers has got tremendous importance . The two dimensional techniques that couples single-bond carbon-proton and protonproton correlation spectroscopy (2D HSQC-TOCSY) can help in unambiguous stereochemical and compositional assignments of the polymers " . Levy et al. have assigned the methylene region upto tetrad and methine region upto penrad levels in the polyvinyl chloride) spectrum by the application of spin lock relay experiment. This is achieved by elucidating the conductivity of the carbons belonging to even and odd ad sequences along the polymer chain. Although polyvinyl alcohol) has been studied extensively by many coworkers Ovenall , Terao et al and Matsuzaki et al , a complete and unambiguous signal assignment of the C{ H} NMR spectrum has not been reported so far. In this M

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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.

175 paper, we assign the methylene and methine carbons upto tetrad and pentad levels respectively with the help of HSQC-TOCSY spectrum. In the 2D HSQCTOCSY experiment the assignments of the proton and the carbon signals correspond to each other to arrive at unambiguous assignments.

Experimental Poly(vinyl alcohol) was prepared by the basic hydrolysis of Poly(vinyl acetate). The homopolymer was precipitated in Hexane and purified in H 0 Hexane solvent system. All the 2D NMR spectrum was recorded in DMSO-d at 100°C on on a Bruker DPX-300 NMR spectrometer operating at 300.13 and 75.5 MHz for *H and C nuclei respectively. 2D (HSQC-TOCSY, TOCSY) NMR experiments were performed using standard pulse sequences as described in our previous papers ' .


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Results and Discussion In the HSQC-TOCSY spectrum (Figure 1) of polyvinyl alcohol) (PVA) in DMSO-dô at 100°C, the signals around 668.2-63.7 and 846.0 - 44.0 ppm are assigned to methine (-CH) and methylene (-CH ) carbons respectively. All these carbon signals show various levels of configurational sensitivity. The signals at 864.07, 865.95 and 867.73 ppm are assigned to rr, mr and mm triads respectively. These triads further shows splitting and assigned upto pentad level. In rr the signals at 863.89, 864.07 and 864.25 ppm are assigned to mrrm, mrrr and rrrr respectively. The four mr centered pentads are assigned at 865.79 (mmrm), 865.95 (mmrr + rmrm) and 866.13 ppm (rmrr), whereas the mm centered pentads at 867.55, 867.73 and 867.94 ppm are assigned as mmmm, mmmr and rmmr respectively. These assignments can be confirmed by 2D HSQC-TOCSY experiment. Since both heteronuclear (C-H) and homonuclear (H-H) correlations are established in a single 2D experiment, the resulting spectrum has the high resolution of C spectrum and high sensitivity of proton nuclei. The methine is more sensitive than methylene. In the spectrum apart from the direct (C-H) correlation crosspeaks (Figure 1 a and c), relayed coupling by magnetization transfer through the spin system (H-H) are also observed (Figure 1 b and d). The methine proton signals were assigned from the one to one correlations with the corresponding methine carbons (Figure la). The corresponding relay peaks, arisingfromthe magnetization transfer from CH protons to the neighboring C H protons confirm the methine carbon 2

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assignments (Figure 1 b). The CH proton in the rrrr pentad gives relay peak to the C H protons in rrr tetrad at 864.25/1.43 ppm whereas the mrrr pentad will give relay peaks to rrr 2



Figure 1. The expanded 2D HSQC-TOCSY (through spin-lock) spectrum showing the methine and methylene regions of polyvinyl alcohol)


177 (564.07/1.45 ppm) and mrr (564.07/1.43 ppm). The third rr centered pentad mrrm shows one relay crosspeak at 563.89/1.45 ppm. The CH proton in mmrm pentad show relay peaks to the C H protons inrarar(565.79/1.51 ppm) and mrm (565.79/1.46 ppm) tetrads, whereas the mmrr and rmrm pentads show relay peaks to the C H protons in mmr (565.95/1.51 ppm), mrr (565.95/1.45 ppm), and rmr (565.95/1.49 ppm), mrm (565.95/1.45 ppm) tetrads respectively. The other mr centered pentad (rmrr) show relay peak to rmr (566.13/1.49 ppm) and mrr (566.13/1.45 ppm) tetrads. Similarly the CH protons in the mm centered (mmmm, mmmr and rmmr) pentads will show CH/CH three bond coupling with mmm (567.55/1.48 ppm), mmm (567.73/1.48 ppm), mmr (567.73/1.51 ppm), rmm (567.94/1.51 ppm) tetrads. All these assignments are summarized in the Table 1. 2

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Table 1. The methine Carbon Crosspeaks and Their Corresponding Pentads Peak No.

Pentads

1 2 3 4 5

mrrm mrrr rrrr mmrm mmrr + rmrm

6 7 8 9

rmrr mmmm mmmr rmmr

Corresponding Tetrads crosspeaks (ppm) mrr (63.89/1.45) + rrm (63.89/1.45) mrr (64.07/1.45) + rrr (64.07/1.43) rrr (64.25/1.43) + rrr (64.25/1.43) mmr (65.79/1.51) + mrm (65.79/1.46) mmr (65.95/1.51) + mrr (65.95/1.45) rmr (65.95/1.49) + mrm (65.95/1.46) rmr (66.13/1.49) + mrr (66.13/1.45) mmm (67.55/1.48) + mmm (67.55/1.48) mmm (67.73/1.48) + mmr (67.73/1.51) rmm (67.94/1.51) + mmr (67.94/1.51)

Thus in the C H protons region the r centered tetrads are assigned around 51.15-1.47 ppm whereas the m centered tetrads are assigned around 51.43-1.60 ppm. After assigning the C H protons, one can assign the methylene carbon of PVA. The methylene carbon assignments of PVA given by us are similar to those given by Matsuzaki et al . The methylene carbon in rrr (545.39 ppm) tetrad shows one cross peak at 51.43 ppm. The rmr (545.03 ppm) tetrad shows two crosspeaks to the two non-equivalent protons at 51.49 and 51.45 ppm (Figure lc). The crosspeaks at 544.53/1.51 and 544.53/1.46 ppm are assigned to mmr and mrm tetrads respectively. Similarly the C H protons in the mmm tetrad is assigned at the crosspeak 544.16/1.48 ppm. These assignments of the methylene carbons can also be confirmed by analyzing the corresponding relay peaks to the methine protons. 2

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Conclusions Poly(vinyl alcohol) prepared by the basic hydrolysis of poly(vinyl acetate) was characterized by two-dimensional NMR spectroscopy. The methylene and methine carbon resonances were assigned to tetrad and pentad configurational sequences. These assignments were justified with the help of 2D HSQC-TOCSY NMR experiment.


Acknowledgements The authors wish to thank the Department of chemistry, Indian Institute of Technology, Delhi and Council of Scientific and Industrial Research (CSIR), India, for providing the funding and NMR facility for this work.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

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Dong, L.; Hill, D.J.T.; O'Donnell, J.H.; Whittakar, A.K. Macromolecules 1994, 27, 1830. Bulai, Α.; Jimeno, M.L.; Roman, J.S. Macromolecules 1995, 28, 7363. Suchoparek, M., Spevacek, J. Macromolecules 1993, 26, 102. Asakura, T.; Nakayama, N.; Demura, M.; Asano, A. Macromolecules 1992, 25, 4876. Dutta, K.; Mukherjee, M.; Brar, A.S. J Polym Sci, Part A, Polym Chem 1999, 37, 551. McCord, E.F.; Shaw, Jr. W.H.; Hutchinson, R.A. Macromolecules 1997, 30, 246. Crowther, M. W.; Szeverenyi, Ν. M.; Levy, G. C. Macromolecules 1986, 19, 1333. Ovenall, D.W. Macromolecules 1984, 17, 1458. Terao, T.; Maeda, S.; Saika, A. Macromolecules 1983, 16, 1535. Matsuzaki, K.; Uryu, T.; Asakura, T. "NMR Spectroscopy and Stereoregularity of Polymers", Japan Scientific Societies Press, Tokyo 1996. Dutta, K.; Brar, A.S. J Polym Sci, Part A, Polym Chem 1999, 37, 3922.


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