Stereochemical and compositional assignments of acrylonitrile/butylacrylate copolymers by two-dimensional NMR spectroscopy

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1 ndian Journal of Chemistry Vo l. 4 A, May 2002, pp Notes Stereochemical and compositional assignments of acrylonitrile/butylacrylate copolymers by two-dimensional NMR spectroscopy A S Brar* & 0 R Pradhan Department of Chemistry, ndi an n stitute of Tec hnology, Delhi, New Delhi 006, ndia Received 6 Seplelllber 200; revised 5 December 200 Acry lonitrile/bu tylac!'y latc (AlB) copolymcrs have becn prepared by bul k polyme ri zation using bcnzoy l pcroxidc as in itiator. 2D-heteronuclear single quantu m correlat ion (HSQC) and total correlated spectroscopy (TOCSY) spectra havc been used to resolve the complex H NMR spcctrum and to dctermine the compositi onal and confi gu rati onal seq uences of AlB copolymers. n our earlier publi cations, we have reported the microstru ctures of ac rylonitril e-alkylmethacrylate -3, acry loni tri le- meth y lacry late 4, acry loni tri le-eth y lacrylate S and acrylonitrile-butylacrylate 6 by 3C NMR spectroscopy. Brar et Ol.6 reported the reacti vity ratios using Kelen-Tudos, error in variabl e methods (EYM )7.8 and detai led in vesti gation of 3C NMR spectra of AlB copolymers. They have reported the microstructural analys is on the basis of carbonyl carbon of butyl acrylate and nitrile carbon of acy lonitril e from 3C{ H } NMR spectrum. Two-dimensional NMR spectroscopy offers a powerful tool fo r the stereochemical in vesti gation of polymers n our in vestigation we have used 3C{ H } NMR, HSQC and TOCSY to determine the compositional and confi gurational sequences of these copolymers. n continuati on of our earlier work, we report the compositional and configurational tri ad sequences of AlB copolymers. We have resolved the complex and overlapping H NMR spectrum. Experimental A series of AlB copolymers of di ffe rent compositi ons were prepared by bulk polymerizati on at 60 C using benzoy l peroxide as initiator. The conversion was kept below 0% by precipitating the copolymers in methanol. The copolymers were fu rther pu rified us ing chloroform/methano l system. 0 ( H, 3CC H} ) and 20 (HSQC, TOCS Y) NM R spectra of AlB copolymers were recorded at 25 C in CDC3, on a Bruker 300 MHz DPX spectrometer using di fferent standard pul se sequences. Other related detail s are given in our earlier papers 3-6. Results and discussion JC{' H} NMR studies The 3C{H} NMR spectrum of the copoly mer AlB (F A =0.47) in CDC3 is shown in Fig. l a along with the signal assignments. The spectral region ppm is not complex and can be assigned without ambiguity with the help of C- 3 NMR spectrum 6. n the expanded nitril e region (Fig. b) the signals around , and ppm are assigned to AAA, AA B and BAB tri ads, on the basis of vari ati on of the intensity of the signals with the copolymer compositi on. n the carbonyl reg ion (Fig. c), the intensities of the signals around , and ppm are assigned to ABA, BB A and BBB tri ads respec ti vely, on the bas is of change in the intensity of the signals with the copolymer composition. All the assignments are shown in the Fig. a-c. HSQC NMR studies The 20!3C_ H HSQC NMR spectrum of AlB copolymer (F A =0.47) is shown in Fig. 2a. The a-ch (methine) carbon signals of A unit show compositional as well as configurati onal senstivity. The cross peaks at /3.00 ( ), (2), (3) ppm are assigned to AmAmA, ArAmA and ArArA triads respecti vely, on the basis of HSQC spectrum of poly(acrylonitrile). The other cross peaks in the A-cemered methine region are assigned on the basis of change in intensity with change in copolymer composition. The cross peaks at /2.78 (4), (5) and (6) ppm are assigned to AmAB, ArAB and BAB respecti ve ly. The a -CH carbon signals of B un it show compos i ti onal triad sensiti vity. The cross pea ks at (7), /2. 53 (8) and /2.32 (9) ppm

2 NOTES 95 BBA ppm ppm (e) (-CH) B CDCll -OCH Z >C-O -CN ) ppm Fig. l- The 3qH} NMR spectrum of AlB copolymer (FA=O.47), Expanded nitrile carbon region, (c) Expanded carbonyl carbon region S <ci ppo. "'[- 35 (e ) ppo l.o ppo 3.0, a.5 r ppo ppo 2.8, Fig. 2- The 2D-HSQC spectra of AlB copolymers of composition F A =0.47, Expanded a-methine carbon region of A-monomeric unit, (c) Expanded a-methine carbon region of B-monomeric unit.

3 952 NDAN J CHEM. SEC. A, MA Y CH2-CH}-{CHZ-CH+ CN C, o OCH2-CH2-CH2-CH3 ( A) (B) OCHZ AAA BBB " """'" " ""'" " ""' " """' " """'' ""' li ll 'i'f' t",, ppm Fig. 3-The H- NMR spectrum of AlB copolymer (F A =0.47) in CDC, ( ppm ppm ppm ppm Fig. 4- The 2D-TOCSY spectrum of AlB copolymer (F A =0.47) in CDC, at low mixing time (4 ms), Expanded 2D-TOCSY spectrum of AlB copolymer (F A =0.47) in CDC, at low mixing time (4 ms). are assigned to ABA, BBA and BBB triads respectively (Fig. 2b & 2c). 'H and 2D-TOCSY NMR studies The proton spectrum along with its complete assignments is shown in Fig. 3 (F A =0.47). Once the 3C{ H} NMR spectrum of the AlB copolymers are assigned completely, the various overlapping resonance signals in ' H NMR spectrum may be assigned by one to one correlation between carbon and proton with the help of 2D-HSQC spectrum. n the H NMR spectrum, the methine protons of acrylonitrile and butyl acrylate are assigned around and ppm respectively. The B-me thylene protons of A-and B-units resonate around ppm. The methine proton signals of acrylonitrile and butyl acrylate show a multiplet, indicating that they are compositionally sensitive. These signals can be assigned with the help of TOCSY spectrum.

4 NOTES dtt J pp. 3.0 pp. " "j '" " 'i i ' ', pp. 3 2 ',, pp. 3.0 f (0) Fig. 5- The 2D-TOCSY spectrum of AlB copolymer (F A =0.47) in CDO] at high mi xing time (80 ms), Expanded 2D-TOCSY spectrum of AlB copolymer (F A =0.47 ) in CDC) at hi gh mixing time (80 ms). At shorter mixing time, there is direct coupling (AM spin type) between the bonded protons whereas at hi gher mixing time there is relay coupling (AMX spin type) through magneti zation transfer. Figures 4a and Sa show th e TOCSY spectra of th e A/B copolymer (F A =0.47 ) in CDCl 3 at two mixing times, 4 and 80 ms. The cross peaks of methyl/methylene protons at /2.2 (), /.88 (2),83.08/.92 (3) ppm are assigned to AmAmA, AmArA and ArArA tri ad sequences. n the AAB triads, the central methine proton shows three-bond coupling with two types of meth ylene protons (A and B type) at 82.82/2.06 (4), /.79 (5) and /. 85 (6) ppm, assigned to ArAB+AmAB, AAmB and AArB triads respectively. The cross peak at /.69 (7) is assigned to BAB tri ad. Simi larl y, in th e ABA triad region, th e methine proton shows three-bond coupling between the methylene protons at /.92 (8) ppm. The cross peaks at (9) (BmBA) and /2.09 ( 0) (BrB A) ppm are due to th e three-bond coupling between the methine proton and meth ylene protons of the B types. The other three-bond coupling between the methine proton and th e meth ylene protons of A type is at /.85 ( ) (BBA) ppm. The threebond coupling between methine and meth ylene protons at /.70 ( 3) ppm is assigned to BBB triad. The cross peak at /.99 (2) ppm is due to the coupling of the methine proton with th e - methylene protons in the poly(butylacrylate). All these assignments have been made on the basis of th e assignments done in the homopolymer, po ly(butylacrylate) (Fig. 4b). The cross peak at ( 9) ppm is a relayed coupling between the methine protons of AA A and AAB triads, which accounts for AAAB tetrad. The methine proton of BAB triad fraction ( ppm) shows a relayed coupling with methine proton of ABA tri ad fraction ( ppm). This cross peak at /2.68 ppm is assigned to BABA (A BAB) (20) tetrad. Similarly, the cross peak at /2.5 (2 ) ppm is assigned to AABB tetrad. The methine proton of BBA (ABB) ( ppm) shows a relayed coupling with methine proton of BBB ( ppm), which accounts fo r BBBA (22) tetrad. The other cro s peaks at 8.93/.59 ( 5), 8.60/.39 ( 6), 8.33/ 0.95 (L 7), (8), 8.59/0.93 (23), 84.09/ 0.92 (24) and 84.0/.38 (25) are due to th e coupli ng in the branched chain of the copolymer. The cross peak at (4) is assigned as due to the coupling of - m e th y l e n e protons (head-to-tail lin kage) of both th e monomeri c units in the copolymer [Figs 4 (a & b), 5 (a & b) ]. The overlapping and broad signals in th e carbon and proton NMR spectra of acrylonitrile/butylacrylate copolymers are assigned completely to vari ous compositional and configurational sequences with the

5 954 NDAN J CHEM. SEC. A. MA Y 2002 help of HSQC. TOCSY (4 ms and 80 ms) experiments. The methine carbon resonances are assigned to triad compositional and configurational sequences. Acknowledgement The authors wish to thank to the Council for Scientific and ndustrial Research, New Delhi, ndia, for providing financial support. References Kapur G S & Brar A S, Polymer, 32 (99) 2. 2 Kapur G S & Brar A S, J polym Sci, Polym Chern Edn, 29 ( 99) Kapur G S & Brar A S, Makrollol Chem, 92 (99) Brar AS & Sunila, j polym Sci, PolYl Chell Edn, 30 (992) Brar A S & Sunila, Eur Polym J, 28 (992) Brar A S & Sunila, Polymer, 34 (993) Kelen T & Tudos F, J macrollol Chem, A9 ( 975). 8 Dube M, Sanyel R A, Penlidis A, 0' Driscoll K F & Reilly P M, J po/y Sci, Chem Edn, 29 (99) Dong L, Hill D J T, 0' Donnell J H & Whittaker A K, Macromolecules, 27 (994) Bulai A, Jimeno M L & Roman J S, Macromolecules, 28 (995) Suchoparek M & Spevacek J, Macromolecules, 26 ( 993) Asakura T, Nakayama N, Demura M & Asano A, Macromolecules, 25 (992) Brar A S & Dutta K, Macrolllol Chelll Phys, 99 ( 998) Brar AS, Dutta K & Kapur G S, Macromolecules, 28 (995) Dutta K & Brar A S, J poly Sci, Par A, Polym Ch ell, 37 (999) Brar A S & Dutta K, J poly Sci, Pari A, PolYl Chelll, 37 (999) 533.