NMR Spectroscopy of Polymers in Solution and in the Solid State

Chapter 12

Downloaded by UNIV OF MISSOURI COLUMBIA on August 11, 2013 | http://pubs.acs.org Publication Date: December 10, 2002 | doi: 10.1021/bk-2003-0834.ch012

Compositional and Configurational Assignments of Acrylonitrile Copolymers by 2D NMR Spectroscopy A. S. Brar Department of Chemistry, Indian Institute of Technology, Delhi 110016, India

The copolymers of acrylonitrile with methyl methacrylate (A/M), Glycidyl methacrylate (A/G) and acrylic acid (A/B) of different monomer concentrations were prepared. The carbon -13 and proton spectra of these copolymers are overlapping and complex. The complete spectral assignment of C- and H - N M R spectra were done with the help of Distortionless Enhacement by Polarisation transfer (DEPT) and two dimensional C- Η Heteronuclear Single Quantum Correlation (HSQC) and Total Correlated spectroscopy (TOCSY) experiments. The methylene, methine and methyl ( A / M and A/G) carbon resonances show both stereochemical (triad level) and compositional (dyad, triad, tetrad, pentad and hexad level) sensitivity. 2D Double Quantum Filtered Correlated Spectroscopy (DQFCOSY) experiment was used in A / B copolymers to assign the complex H NMR spectrum to different compositional sequences. 13

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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 It is well known that N M R spectroscopy is a powerful technique for structure determination of macromolecules. In case of macromolecules, C and *H N M R spectra are quite complex and overlapped, hence microstructure determination of such molecules is carried using usual ID N M R techniques coupled with 2D-NMR techniques such as H E T C O R and T O C S Y ' . D E P T is used extensively for analysis of the overlapping carbon resonances in C - N M R spectra ' . H E T C O R gives information not only on direct carbon to proton linkages, but also on compositional and configurational sequences in polymers. T O C S Y , both low and high mixing time is further used to resolve broad and overlapping proton spectra of polymers and also to determine intramolecular chain structure of polymers. Acrylontrile copolymers are used in textile industry and as precursor of carbon fibers. In this chapter, we will discuss the applications of various I D and 2D- N M R techniques on the microstructure characterization in terms of compositional and configurational sequences of different acrylonitrile copolymers. 13

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Acrylonitrile/Methyl methacrylate (A/M) copolymers 13

The complex and overlapped ΰ{*Η} N M R spectrum of A / M copolymer is resolved using DEPT-135 and H E T C O R spectra. The overlapping methine and methyl carbon region are further resolved and assigned to pentad compositional and configurational level, using DEPT-90 and DEPT-135 subspectra. Using C - * H inverse HETCOR, the methylene carbon region is first assigned to A A (δ30.0-38.0 ppm), A M (038.0-45.0 ppm) and M M (645.0-55.0 ppm) centered dyads as shown in fig.l. Further splitting within these dyads is assigned to tetrad level on the basis of change in intensity of signals with copolymer composition. The three signals in A A dyad are assigned to A A A A (634.0-35.5 ppm), A A A M (634.0-35.5 ppm) and M A A M (635.5-38.0 ppm). The splitting within these tetrad sequences can be assigned to hexad compositional sequences, on the basis of change in intensity with change in copolymer composition using *H- C inverse H E T C O R spectra. In fig.l, the cross peaks in H E T C O R spectrum at 633.19/2.16, 633.40/2.04, 634.88/2.14, 635.37/1.97, 636.65/2.10, 637.14/1.89, and 638.25/1.72 ppm can be assigned to A A A A A M (MAAAAA), M A A A A M , AAAAMA, M A A A M M , AMAAMA, A M A A M M and M M A A M M , respectively. 13

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Figure 1. The methylene region of the inverse-HETCOR spectrum of A/M copolymer.



169 The A M ( M A ) dyad can also be divided into three tetrads ,ΑΑΜΑ (538.039.9 ppm), A A M M + A M A M ( 539.9-42.0 ppm) and M A M M ( 642.0-45.0 ppm). These tetrads further show hexad compositional sensitivity. The three M M centered tetrads A M M A , A M M M ( M M M A ) and M M M M are assigned around 545.0-47.5, 547.5-51.5 and 550.5-55.5 ppm, respectively. The further splitting in the M M M M and M M M A tetrads can be assigned to hexad sequences , on the basis of variation in the intensities of the peaks with the composition . The methylene carbon signals at 554.34, 552.87, 551.40, 549.78 and 548.03 ppm can be assigned to M M M M M M , M M M M M A ( A M M M M M ) , A M M M M A + M M M M A M , M M M M A A and A M M M A A , respectively. Once ^ C ^ H } N M R spectrum is assigned completely, it is easy to assign complex and overlapping *H- N M R spectrum with the help of inverse H E T C O R experiment. A l l the methine proton signals are assigned to various pentad sequences.

Acrylonitrile/Glycidylmethacrylate (A/G) copolymers 13

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The C { H } N M R spectrum of A / G copolymer is quiet complex and overlapping. The overlapping methine and methylene carbon signals are resolved with the help of DEPT-135. The methine carbon signals are first assigned to three broad triad compositional sequences A A A , A A G and G A G . Further splitting in these are assigned to pentad compositional and configurational sequences using DEPT-90 spectra of A / G copolymer of different compositions. The methylene carbon resonances are assigned to tetrad compositional sequence, using DEPT- 135 N M R spectra. Using 2D- inverse H E T C O R N M R spectra the methylene group carbon region is first assigned to AA(532.0-38.0 ppm), AG(538.5-44.6 ppm) and GG(544.6-55.0 ppm) centered dyads. Further splitting within these dyads is assigned first to tetrad level and then to hexad compositional sequences, on the basis of change in intensity with change in copolymer composition. The methyl carbon signals overlap with the methine carbon signals, which can be resolved by DEPT experiments. However, the non overlapping methyl proton signals could be clearly assigned around 50.60-1.5 ppm and show both compositional and configurational sensitivity. The methyl proton region can be split into three broad envelops that vary with copolymer composition and are assigned to A G A (51.19-1.5 ppm), G G A ( A G G ) (50.98-1.19 ppm) and G G G (50.80-0.98 ppm). These triad fractions further show signals that can be assigned to configurational or compositional sequences. In the G G G triad region, the signals at 51.06, 0.88 and 0.83 ppm are assigned to A G G G A , G G G G A (AGGGG) and G G G G G pentad sequences on the basis of the change in intensity with the copolymer composition. These assignments can also be confirmed by the inverse-HETCOR spectrum as shown in fig.2, where



Figure 2. Methyl region of the inverse-HETCOR spectrum of A/G copolymer (F = 0.48). A


171 the cross peaks at 817.00/1.06, 16.51/0.88 and 16.06/0.83 ppm are assigned to these three GGG-centered pentads, respectively. Similarly, in the A G G (GGA) triad region, the three GGA-centered pentads are assigned at 518.61/1.14 ( A G G A A ) , 17.79/1.06 ( A G G A G + G G G A A ) and 17.36/1.02 (GGGAG) ppm (fig.2). The signals in the A G A triad region, which do not change with the copolymer composition, are assigned to A G A (δ 1.24 ppm), A G A ( A G A ) (δΐ.31 ppm) and A G A (δΐ.35 ppm). Further compositional sequences with in these triads are assigned with the help of the inverse-HETCOR spectrum. Thus with the help of the inverse-HETCOR experiment, overlapping methyl carbon signals can be assigned without ambiguity. m


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Acrylonitrile/ Acrylic Acid (A/B) copolymers The methine carbon signals of A - unit shows configurational and compositional sensitivity. The multiplets can be assigned to rr, rm(mr) and mm triad respectively, on the basis of signal assigned in homopolymer spectra. Further splittings within these can be assigned to compositional sequences. Similarly, the methine carbon signals of B - unit shows triad compositional sensitivity. The D Q F C O S Y spectrum of A / B copolymer recorded in D M S O - d is shown in fig. 3. In the D Q F C O S Y spectrum, the multiplets in the methine proton region of A - unit can be assigned to triad compositional sequences. The cross peaks at δ3.14/2.07, 3.02/2.05, 3.02/1.90 and 2.92/1.78 ppm can be assigned to A A A , A A B ( B A A ) and B A B . In the A A B triad fraction, two cross peaks are seen as the methine proton is coupled to two different types of methylene protons. The A A A triad further shows configurational sensitivity. The cross peaks at δ3.22/2.17, 3.14/2.07 and 3.12/2.03 ppm are assigned to AmAmA, A m A r A and A r A r A respectively. The methine protons at δ3.22, 3.14 and 3.12 ppm show three bond coupling with the methylene proton at δ2.17, 2.07 and 2.03 ppm which further shows three bond coupling with the methylene proton at δ 1.94, 1.86 and 1.80 ppm. Thus accounting for the head to head linkages. The absence of any methine cross peaks indicates that there are no C H / C H coupling in the polymers. Similarly, in A A B and B A B triad fractions the methine protons at δ2.94 and 2.98 ppm shows three bond coupling with the methylene proton at δ ΐ. 8 8 and 1.81 ppm which further shows three bond coupling with the methylene proton at δ 1.39 and 1.52 ppm, respectively, accounting for the head to head linkages in these triad fractions (AAB and B A B ) . In the methine proton region of B - unit, the multiplets can be assigned to triad compositional sequences. The cross peaks at δ2.32/1.59, 2.55/1.78 and 2.70/1.94 ppm can be assigned to B B B , 6



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Figure 3. The 2D-DQFC0SY copolymer in DMSO-d^

spectrum of the acrylonitrile/acrylic

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173 B B A and A B A triads sequences respectively. The B B B triads further shows configurational sensitivity The cross peaks at 52.26/1.55, 2.32/1.59 and 2.38/1.66 ppm are assigned to BmBmB, BmBrB and BrBrB triad fractions respectively on the basis of assignment done in the homopolymer. The methine protons at δ2.34, 2.53 and 2.64 ppm show three bond coupling with the methylene protons at δ 1.84, 1.98 and 2.06 ppm respectively, which further shows three bond coupling with the methylene proton at δ1.46, 1.56 and 1.67 ppm. Thus accounting for the head to head linkages in triad fractions B B B , B B A and A B A , respectively.

Conclusions The overlapping and broad signals in the carbon and proton spectra was assigned completely to various compositional and configurational sequences with the help of inverse H E T C O R and T O C S Y / D Q F C O S Y experiments.

References 1. Brar, A . S.; Dutta, K . ; Hekmatyar, S. K . J. PolymerSci.1998, 36,1081 2. Brar, A . S.; Dutta, K . Macromolecules 1998, 31,4695 3. Brar, A . S.; Dutta, K . Eur. Polym. J. 1998, 34, 1585 4. Grobelny, J.; Kotas, A. Polymer 1995, 36, 1363. 5. Brar, A . S.; Shiv charan Polymer 1996, 37,2451. 6. Brar, A . S.; Malhotra, M. Macromolecules 1996, 29. 7470 7. Suchoparek, M.; Spevacek, J. Macromolecules 1993, 26, 102 8. Mijangoes, C.; Lopez, D . Macromolecules 1995, 28, 1364.


NMR Spectroscopy of Polymers in Solution and in the Solid State

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