Indian Journal of Chemistry Vol. 44A, January 2005, pp. 58-63

Microstructure characterization of poly(2-N-carbazolylethyl acrylate) by two-dimensional NMR spectroscopy

_A S Brar*, M Markanday & S Gandhi

Department of Chemistry, Indian Institute of Technology, Delhi, New Delhi 11001 6, India Email : asbr;r@chemistryJitd.e~t.in - ~

Received 8 Septelllber 2004; revised 15 October 2004

Poly(2-N-carbazolylethyl acrylate) has been synthesized by solut ion polymerization of 2-N-carbazolyethyl acrylate with 2,2'-azobisisobutyronitri le as free radical initiator. Distortionless Enhancement by Polarization Transfer has been used to di stingui sh between the overlapping main-chain methine and side-chain methylene resonances in DC { IH } NMR spectrum. Configurational assignments of carbon and proton resonances of main-chain methylene group have been done using two-dimensional Heteronuclcar Single Quantum Correlation spectroscopy and two-dimensional Tot,ti Correlation Spectroscopy. Two and three bond order carbonlproton couplings have been investi gated using Heteronuclear Multiple Bond Correlation studies.

IPC Code: Int. C1.7: C08F 120118; GOIR 33/20

Homo- and co-polyacrylates are of academic and industrial interest because of their wide range of physical and chemical properties that can be controlled by an appropriate choice of pendant group in the polymer and design of copolymer structure. Poly(2-N-carbazolylethyl acrylate) belongs to the class of photoconductive polymers 1.3 , which finds widespread application in electrophotography, light­emitting diodes, photorefractive material s4.5 and photovoltaic devices6

.7

. The photoinduced intrachain charge transfer complexes in a photoconductive polymer ensure efficient charge transport across the polymeric materials. Most investigations have concentrated on search for a chromophore that has a high efficiency in forming such charge-transfer complexes. Higher carrier mobility can be expected in polymers where the chromophore is directly linked to the backbone despite the fact that molecular mobility of the chromophore in such cases will be depressed. This insight has g iven impetus to further research on design of photoconductive polymers in which chromophores are directly linked to main chain through chemical bonding lJ

•11 . Poly(2-N­

carbazolyle thyl acrylate)' 2 is an example of photoconductive polymer in which the carbazole moieties appear as pendant groups along the main chain of the polymer.

The determination of microstructure in polymers is useful for establishing the structure-property

relationshi p I3.14. High-resolution one-dimensional and two-dimensional NMR spectroscopy have proved to be one of the most informative and revealing techniques for the investigation of polymer

. 15·19 microstructure . Much work has been done to study the photoconductive properties of poly(2-N­carbazolylethyl acrylate) and its copolymers. However, a literature survey reveals that the microstructure of this polymer has not been reported in detail.

In this paper, we report the microstructure of poly(2-N-carbazolylethyl acry late). Unambiguous assignments have been made with the help of I3C{ IHI NMR, distortion less enhancement by polarization transfer (DEPT), 20 heteronuclear single-quantum correlation (HSQC), total correlation spectroscopy (TOCSY) and 20 het ronuclear multi bond correlation (HMBC) NMR experiments.

Materials and Methods Carbazole (96%, Aldrich) was recrystallized from

methanol. Ethylene carbonate (98%, Aldrich) and sodium hydride (50% oil dispersion, CDH Pvt. Ltd ., New Delhi) were used as supplied. Acry loyl chloride (98%, Aldrich) was distilled under reduced pressure and stored at low temperature. AIBN (Fluka) was recrystallized from methanol and stored at low temperature.

BRAR et at.: MICROSTRUCTURE CHARACTERIZATION OF POLY(2-N-CARBAZOLYLETHYL ACRYLATE) 59

Procedure Synthesis of monomer, 2-N-carbazolylethyl acrylate

A solution of carbazole (10 g, 0.06 mol) and 4.32 g (0.072 mol) sodium hydride in dry tetrahydrofuran (THF) was vigorously stirred at O°e. After 2 hours of stirring ethylene carbonate (6.38 g, 0.072 mol) was slowly added and the mixture was stirred at O°C for 30 min. and then at 40°C for three hours. To the resulting solution ice-cold water was added gradually, followed by stirring at 40°C for 3 hours. Finally, work-up of the reaction mixture was done by pouring 200 ml of water and extracting the aqueous mixture with dichloromethane (200 ml x 3). The extract was chromatographed on a silica-gel column. (75:25/ hexane: ethyl acetate) (Scheme I). [M. pt. 81-81.5°C (Lit M. pt. 83-83.5°C). 'H NMR (in CDC b) 81.35 (s, IH, OH), 3.80-3.82 (t, 2H, NCH2), 4.20-4.24 (t, 2H, OCH2), 7.00-7.90 (m, 8H, aromatic)].

A solution of acryloyl chloride (5.1 g, 0.057 mol) in 20 ml of dry DCM was added dropwise under vigorous stirring to a mixture of 2-N-(hydroxyethyl) carbazole (10.0 g, 0.047 mol) and 20 ml triethylamine in 200 ml of dry DCM. The mixture was stirred overnight at O°e. After the Et3N.HCI was filtered off at room temperature, the residue was chromatographed on a silica gel column (7:3lhexane: ethyl acetate). [M.pt. 76°C (lit. M.pt. 75-76°C). 'H NMR (in CDCI3) 84.50-4.60 (m, 4H, OCH2CH2N), 5.80-6.30(m, 3H, CH2=CH-), 7.00-7.90(m, 8H, aromatic)].

Polymerization

Poly (2-N-carbazolylethyl acrylate) was prepared

by solution polymerization using AIBN (0.05 mol %) initiator in distilled toluene at 60°e. The homopolymer was precipitated in methanol. Further purification was done by reprecipitation in CHCI3/ CH30H solvent system.

NMR studies NMR experiments were performed in CDCI3 on

Bruker 300 MHz DPX spectrometer at a frequency of 300.13 MHz and 75.5 MHz for 'H and I3C {'H} NMR respectively using the standard pulse sequences as reported in our earlier publications2o

.2

' .

Results and Discussion J3C{'H} NMRstudies

13C{ 'H} NMR spectrum of poly(2-N-carbazolylethyl acrylate) with assignment of various resonance signals, is shown in Fig. I. The carbonyl carbon resonates at 8173.25-174.5 ppm showing sensitivity towards configuration . The signals around 8173.25-173.75, 8173.75-174.25 and 8174.25-174.5 ppm are assigned to mm, mr/rm and IT triads respectively. Carbons 3, 5 and la of the carbazole ring in the homopolymer were erroneously assigned by Crone and Natansohn22

. Correct assignments of the carbazole ring in poly(N-vinylcarbazole) were reported by Dias et al.23 The aromatic carbons of the pendant group in the investigated poly(2-N­carbazolylethyl acrylate) have been assigned on the basis of investigations reported by Dias et at. The aromatic carbons were found to resonate at 8108.5(C-2), 8118.0 (C-l), 8119.0 (C-4), 8122.0 (C-4a), 8124.5(C-3) and 8141.0 (C-Ia) ppm. The -OCH2

yH20H 1 )NaHITHF alOe C/3hrs yH2

b)40oC/2hrs N 2)Ethylene carbonate a}OoC/1 o:D-..-:: -------1 I

3)Water(O°C} a)40oC/3hrs ~ ~ carbazole ethylene carbonate 2-N-hydroxyethyl carbazole

CH20H I f H2,OCC£H-CH2

cc:o~H2 1 )Hydroquinone, tnethyl amine/dry CH2CI2 yH2

I ~ ~ + H C=CHCOCI (OoC/overmght) CCDN -..-:: 1 2 - 1 1

~ h- 2)Acryloyl chlonde h- ~ 2·N·hydroxyethyl carbazole acryloyl chlonde

2-N·carbazolylethyl acrylate

Scheme I-Synthesis of 2-N-carbazolylethyl acrylate.

60 INDIAN J CHEM, SEC A, JANUARY 2005

mr/rm

---,-~~.------.,-.......4._-.----,--- .. _ 175 174 173

>c=o {'\

ppm 160

1a {'\

140 120

2 "

-, 100 BO 60 40 20

Fig. I-The DC{ 'H} NMR spectrum of poly(2-N-carbazolylethyl acrylate).

carbon of the side chain appears as a singlet around 862.5 ppm indicating that it is not sensitive to configurational sequence of the homopolymer.

The spectral region around 838.8-42.1 ppm is overlapped and can be assigned to backbone methine (-CH) carbon and -NCH2 carbon of the side chain. The overlapped carbon regions can be resolved with the assistance of a DEPT -135 spectrum. The backbone methy lene (-CH2) carbon appears as multiplets around 832.2-38.8 ppm, indicating its sensitivity towards various configurational sequences. This region appears as a broad signal due to the bulky pendant group.

2-D HSQC NMR studies

Investigation of the HSQC spectrum facilitates complete assignment of the 'H NMR spectrum. The expanded aliphatic rer; ion of the HSQC spectrum is shown in Fig. 2. The methine group appears as a cross peak centered at 841.13/1.87 ppm. It does not show any kind of configurational sensitivity. The spectral region from 832.3-38.8/0.7-1.5 ppm represents the p­methylene group. This group shows sensitivity to various configurational sequences. The meso configuration of dyad gives two crosspeaks due to two methylene protons having different environment and the racemic configuration gives one crosspeak in between these two crosspeaks. The methylene protons of r dyad and m dyad are depicted in Scheme 2. The cross peaks around 8 34.2411.28 and 8 34.28/0.74

1 'I ------------.~ ,

o~ [ m m

<.CH 40

ppm

.~, ~,~, ~, -,,~~, -I~--o--r-·_r___r-~.,..---,--,-

ppm 2.0 ; .5 1.0

Fig. 2-The expanded HSQC spectrum showing the methine and p-methylene region of the polymer.

ppm are assigned to the meso (m) dyad (a and b) and the cross peak around 8 34.27/1.06 ppm is assigned to the racemic (r) dyad of the methylene region, respectively.

The aromatic region is overlapped in the 'H NMR spectrum. The protonated aromatic carbons of the carbazole ring can be resolved with the help of expanded aromatic region of the HSQC spectrum. Hence, peaks centered at 8108.5/7.1, 8119/1.7, 811817.8 and 8124.517 .2 ppm can be assigned to (C-2)-(H-2) [II] , (C-4)-(H-4) [IV] , (C-l)-(H-I) [I] and (C-3)-(H-3) [III] respectively as shown in Fig. 3.

BRAR el at.: MICROSTRUCTURE CHARACTERIZATION OF POLY(2-N-CARBAZOLYLETHYL ACRYLATE) 61

~ ~~

CH2 I

~ ~~

I

CH, I -

CH, CH2 I - I o 0

I I I I I I CH3EHe L~ HA Lo H

--c C--C C---C--C-I I I I I I C=O He CH3 He CH3 H

I

P m

Scheme 2--m eso( m) and racemic(r) confi guration of the polymer.

11--------··-------·--------------------] 1

....J i

JI =1 ~ ! ~ i

. I -ll L_

2 --- COD

1-120 i

1\

I ~130 _ __ .. _ .. _ ... _____ _ .. __________ ! ;ppm

..,.-,-,~---, ----. --·I-·-'--~ --'··-I-··· .----.--.-.----,-----.---r'- .----.-----.--"l-ppm 8.5 ·S.O 7.5 7.0 6.5

Fig_ }-- The ex panded HSQC spectrum showing the aromat ic region.

The overlapped peaks of - OCH2 and - NCH 2

protons in 'H NMR spectrum can be resolved with the help of aliphatic region of HSQC spectrum. The -OCH2 and -NCH2 are found to resonate at 862.5/4.0 and 042.11 4.15 ppm respectively as shown in Fig. 4.

cCtf(54;c.=:-~d)§lW? ) .NCH 2

./

I ~ 40 I . I I I i

I i 45

I r II ! ~ 50 I I i I

I ~ 55 i

Fig. 4-The expanded HSQC spectrum showing the -OCH2 and­NCH2 region.

------~ 0 5 ,

1.0

1.5

2.0

2.5

ppm ~'~~~'~~~~I~~~'~~~

ppm 2.5 2.0 1.5 1_0 0.5

Fig. 5--The ex panded TOCSY spectrum show ing the methine and ~-methy lene region.

2-D TOCSY NMR studies

2-D TOCSY NMR enables us to understand the connectivity between the different protons and confirm the various couplings in the polymer chain. The expanded TOCSY spectrum showing the methine and ~-methylene region of the homopolymer is shown in Fig. 5.

62 INDIAN J CHEM, SEC A, JANUARY 2005

ppm 8· 7 6 5 4 3 2 a

Fig. 6--The completely assigned IH NMR spectrum of poly(2-N­carbazolylethyl acrylate).

The vicinal couplings between the methine protons and the methylene protons in various configurational sequences can be clearly seen in the TOCSY spectrum. The two protons HA and HB of the m dyad due to coupling with the methine proton shows two cross-correlation peaks in the TOCSY spectrum centered at 01.87/1.3 (2) and 01.85/0.74 (3) ppm respectively. The methylene protons of the r dyad show a single cross-correlation peak in the TOCSY spectrum in between the two crosspeaks (2) and (3) centered at 01.86/1.04 (1) ppm.

The geminal coupling between ~-methylene protons (HA and HB) of the m dyad results in a cross­correlation peak centered at 01.03/0.7 (4) ppm. Thus using HSQC in conjugation with TOCSY enabled to strengthen assignments. Using these assignments 'H NMR spectrum was assigned as shown in Fig. 6.

HMBC NMR studies

The heteronuc1ear multiple bond correlation (HMBC) experiment proves to be very useful in correlating proton and carbon nuclei via long range couplings. The assignrnent of the signals of the non­protonated aromatic carbons for the carbazole ring has been achieved by this technique as correlation of these quaternary carbons with protons via 3 JCH

couplings can be easily seen in the HMBC spectrum. The HMBC spectrum of the aromatic region is presented in Fig. 7. It can be clearly seen that each quaternary carbon correlates with protons via 3 JCH

couplings. The non-protonated carbons were found to be

positioned at 0122.0 and 0141.0 ppm using l3C{ 'H} NMR and DEPT studies. HMBC studies further confirm the positions of the respective quaternary

t, 0\

~ j.li --~/\,-\.J '"

I 100

I .-2 -~.~ 00-1

! 110

I 3

J ..s~4 ~ 7~~B

120

::J "I 1 130 I

-1 I

~~12 <000-13 [ 140

11 L I , , ,

ppm 8 6 5 4

Fig. 7-The expanded HMBC NMR spectrum of the aromatic region.

carbons C-4a and C-la. C-4a centered at 0122.0 ppm is found to couple with H-4, H-3 and H-I, while C- l a shows coupling with H-4 (11), H-2 (12) and side­chain -NCH2 (13) protons. The coupling of C-l a with side chain -NCH2 protons confirms that this non­protonated carbon is the one adjacent to 'N' atom of carbazole ring. The protonated carbon C-2 shows connectivity with H-4 (1) and H-l (2), C-l with H-2 (4) and H-3 (3), C-4 with H-2 (6) and H-3 (5), C-5 with H-4 (7), H-3 (9) and H-l (8) and C-3 with H-4 (10) protons. Henceforth, this further confirms the position of the various protonated carbons of the carbazole ring.

Acknowledgement The authors wish to thank the CSIR, New Delhi and

DST, New Delhi, India for providing the financial support to carry out this work .

References I Grazulevicius J V, Strohriegl P, Pielichowski J &

Pielichowski K, Prog Polym Sci, 28 (2003) 1297. 2 Biswas M & Das S K, Polymer, 23 (1982) 1713. 3 Chang D M, Gromelski S, Rupp R & Mulvaney J E, J Polym

Sci Chem Ed, 15 (1977) 571. 4 Burroughes J H, Bradley D, Brown A R, Marks R N,

Mackay K, Friend R H, Burn P L & Holmes A B, Natllre, 342 (1990) 539.

5 Sanda F, Nakai T, Kobayashi N & Masuda T, Macromolecules, 37 (2004) 2703.

6 Meerholz K, Volodin L B, Sandalphon, Kippelen B & Peyghambarian N, Nature, 71 (1994) 497.

BRAR et al. : MICROSTRUCTURE CHARACTERIZATION OF POLY(2-N-CARBAZOLYLETHYL ACRYLATE) 63

7 Wang G, Qian S, Xu J, Wang W, Liu X & Li F, Physica, 16 Brar A S & Puneeta, Indian J Chelll, 43A (2004) 2281. Part S, 279 (2000) 116. 17 Spevacek J, Suchoparek M & A1awi S A, Polymer, 36 (1995)

8 Oshima R, Biswas M, Wad a T & Uryu T, J Polym Sci, 4125. Polym Lett Ed, 23 (1985) 151.

18 Kim Y & Harwood H J, Polymer, 43 (2002) 3229. 9 Chen Y, He Y, Wang F, Chen H & Gong Q, Polymer, 42

(2001) 1101. 19 Matsuzki K, Uryu T & Asakwa T, NMR Spectroscopy and

10 Chen Y, Chen Z, Gong Q & Schroers M, Mat Lett, 57 (2003) Stereoregularity of Polymers, (Scientific Society, Tokyo)

2271. 1996, pp 173.

11 Kim D W, Moon H, Park S Y & Hong S I I, React FlIIlct 20 Brar A S, Dutta K & Kapur G S, Macromolecules, 28 (1995) Polym, 42 (1999) 73. 8735.

12 Uryu T, Ohkawa H & Oshima R, Macromolecules, 20 (1987) 21 Dutta K & Brar A S, J Polym Sci. Part A: Polym Chern, 37 712. (1999) 3922.

I3 Bruch M D, Macromolecules, 21 (1988) 2707. 14 Koenig J L, Chemical Microstructure of Polymer Chains, 22 Crone G & Natansohn A, J Polym Sci. Part A: Polym Chem.

(Wiley Interscience, New York) 1980, pp 217. 30 (1992) 1665.

15 Brar A S, D R Pradhan & Sunita Hooda, Indian J Chem, 43A 23 Karali A, Foudakis G E & Dais P, Macromolecules. 33 (2004) 2066. (2000) 3180.