Microstructure elucidation of poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate) using two-dimensional NMR spectroscopy

Article abstract of DOI:10.1016/j.molstruc.2004.08.020

The configurational assignments of poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate) prepared by solution polymerization with 2,2′- azobisisobutyronitrile (AIBN) as free radical initiator were studied by combination of one- and two-dimensional NMR spec

Full text of DOI:10.1016/j.molstruc.2004.08.020

Journal of Molecular Structure 734 (2005) 35–44
www.elsevier.com/locate/molstruc
Microstructure elucidation of poly(9-ethyl-3-hydroxymethylcarbazolyl
methacrylate) using two-dimensional NMR spectroscopy
*
Ajaib Singh Brar , Sonia Gandhi, Meghna Markanday
Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India
Received 14 July 2004; revised 3 August 2004; accepted 11 August 2004
Available online 28 October 2004
Abstract
The configurational assignments of poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate) prepared by solution polymerization with
2,20-azobisisobutyronitrile (AIBN) as free radical initiator were studied by combination of one- and two-dimensional NMR
spectroscopy. The –OCH2 and –NCH2 carbon resonances were distinguished using Distortionless Enhancement by Polarization
Transfer (DEPT) experiments. The configurational assignments for main chain methylene and a-methyl group were done by using 2D
HSQC spectroscopy. Various geminal and vicinal couplings within the configurational sequences were assigned with the help of Total
Correlation Spectroscopy (TOCSY). Two and three-bond order carbon/proton coupling were observed using Heteronuclear Multiple
Bond Correlation (HMBC).
q 2004 Elsevier B.V. All rights reserved.
Keywords: NMR; Microstructure; Poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate); Photorefractive polymer
1. Introduction
photorefractive material with the pendant group containing
carbazole moiety substituted at 3-position by electron
withdrawing group. Copolymerization of such systems
with monomers having acceptor side groups such as 3,5-
dinitrobenzoic derivatives show high degree of charge
transfer complexation as well as sensitivity to light. These
copolymers in turn show properties like electrochromism
and photoconductivity [10–12].
Recently homo and copolymer systems based on
pendant carbazole units and those with tailored chemical
structures have received considerable attention, which is
amply justified in view of the unusual electrical,
photoelectrical, thermal and other photophysical properties
[1,2]. Carbazole derivatives substituted by electron with-
drawing groups at 3-position can give rise to charge
carriers in the visible region, through an induced
intramolecular charge transfer complex. Thus, here photo-
conductivity combined with Non-Linear Optical (NLO)
activity makes it a useful photorefractive material [3–6].
These photorefractive materials have a wide range of
applications in the field of image processing techniques,
programmable optical interconnection and high-density
optical data storage [7–9]. Poly(9-ethyl-3-hydroxymethyl-
carbazolyl methacrylate) is one of such kind of
The physical and chemical properties of polymers are
influenced by their molecular level microstructure hence its
study is useful in establishing structure property relationship
[13–15]. 1D and 2D NMR spectroscopy has been found to
be most efficient technique for determining the microstruc-
ture of polymers [16–24].
In this manuscript, we report the microstructure of
poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate)
1
using H, 13C{1H} NMR, DEPT, 2D HSQC and TOCSY
NMR experiments. The a-CH3 region was found to be triad
sensitive whereas the main chainmethylene was found to be
dyad sensitive. Further studies on higher bond order
carbon/proton couplings were made possible by 2D
HMBC spectroscopy.
* Corresponding author. Tel.: C91 11 26 591 377; fax: C91 11 581 102.
E-mail address: asbrar@chemistry.iitd.ernet.in (A.S. Brar).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.08.020

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A.S. Brar et al. / Journal of Molecular Structure 734 (2005) 35–44
2. Experimental
was separated and dried. Column chromatography was carried
out on silica gel column (85:15/hexane:ethyl acetate).
Yield: 87%; m.p.: 82 8C (lit 81 8C).
2.1. Materials
1H NMR (CDCl3): d10.08 (s, 1H, KCHO), d1.45 (t, 3H,
KCH3), d4.37 (q, 2H, KCH2), d7.25–8.59 (m, aromatic).
(b) Synthesis of 9-ethyl-3-methylolcarbazole (Scheme 1(b)).
A solution of 0.56 g of sodium borohydride in 10 mL of 0.4 N
sodium hydroxide was added dropwise to a stirred mixture of
5 g (0.023 moles) of 9-ethyl-3-formylcarbazole and 50 mL
ethanol. The mixture was stirred below its boiling point for 2 h
after it became a clear, colourless solution. White crystals were
obtained on cooling. The solvent was then removed at room
temperature and the product was filtered, dried and recrystal-
lized from cyclohexane/benzene (1:1 mixture)
9-Ethyl carbazole (97%, Aldrich), phosphoryl chloride
(98%, Thomas Baker), sodium borohydride (95%, CDH),
methacryloyl chloride (97%, Lancaster) were used as
purchased. N,N-dimethylformamide (99%, s.d.Fine-Chem,
Ltd, India) was distilled under reduced pressure and 2,20-
azobisisobutyronitrile (Fluka) was recrystallized using
methanol and stored at low temperature.
2.2. Synthesis (Scheme 1)
Yield: 97%; m.p.: 86.5–87 8C.
(a) Synthesis of 9-ethyl-3-formylcarbazole (Scheme 1(a)).
To 2.5 g (0.034 moles) of N,N-dimethylformamide cooled to
0 8C, 5.16 g (0.033 moles) of phosphoryl chloride was added
dropwise. The above mixture was stirred and heated to 40 8C.
To this 6 g (0.03 moles) of 9-ethyl carbazole (I) was added.
After 10 h the mixture was poured into 100 mL of water at
45 8C and stirred until the oil phase got crystallized. The
reaction mixture was worked up by extracting the aqueous
mixture with dichloromethane (100 mL!3). Organic layer
1H NMR (CDCl3): d1.77 (s, 1H, KOH), d4.83 (s, 2H,
KCH2O), d1.41 (t, 3H, KCH3), d4.36 (q, 2H, KCH2),
d7.20–8.09 (m, aromatic).
(c) Synthesis of 9-ethyl-3-hydroxymethylcarbazolyl
methacrylate (Scheme 1(c)). A mixture of 1.045 g (0.01
moles) of methacryloyl chloride and 10 mL dichloro-
methane (DCM) was added dropwise (for 15 min) under
vigorous stirring to a solution of 2.3 g (0.01 moles) of
Scheme 1. Synthesis of poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate).

A.S. Brar et al. / Journal of Molecular Structure 734 (2005) 35–44
37
9-ethyl-3-methylolcarbazole, 50 mL of dry DCM, and
1.01 g (0.01 moles) of triethylamine (cooled at 0 8C). The
mixture was stirred overnight and Et3NH$HCl was filtered
off. After removing DCM in vacuo, the product was
chromatographed on silica gel column (8:2/hexane:ethyl
acetate).
were accumulated with 2 s delay time. DEPT measurements
were carried using standard pulse sequence with a J
modulation time of 3.7 ms (JCHZ135 Hz) with 2 s delay
time. The pulse sequence invigpt and inv4gplplrnd of the
Bruker software were used to record gradient HSQC and
HMBC experiments, respectively. The spectra were
acquired with 512 increments in the F1 dimension and
2048 data points in the F2 dimension. Total correlation
spectroscopy (TOCSY) experiment was performed using
standard pulse sequence. 32 scans were accumulated for 512
experiments with 2 s delay time [25].
Yield: 81%
1H NMR: d5.56 (s, 1H, CH2(i)), d6.15 (s, 1H, CH2(ii)),
d1.97 (s, 3H, CH3(iii)), d5.37 (s, 2H, KCH2O), d4.35
(q, 2H, KNCH2), d1.42 (t, 3H, CH3(iv)), d7.23–8.13
(m, aromatic).
(d) Polymerization. Poly(9-ethyl-3-hydroxymethylcarba-
zolyl methacrylate) was prepared by solution polymeriz-
ation using AIBN (0.05 mole%) in distilled toluene as
solvent at 60 8C. The polymer obtained was filtered and
dissolved in minimum amount of chloroform. Reprecipita-
tion was further carried out in methanol.
3. Results and discussion
3.1. 13C{1H} NMR studies
The various resonance signal assignments in 13C{1H}
NMR spectrum for poly(9-ethyl-3-hydroxymethylcarbazo-
lyl methacrylate) are shown in Fig. 1. The aliphatic unit of
poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate) has
structural similarities to the poly(methyl methacrylate). The
carbonyl carbon resonances constituted three envelopes of
resonances (as shown by poly(methyl methacrylate)) from
2.3. NMR measurements
NMR spectra were recorded on Bruker DPX-300
spectrometer in CDCl3. The 1H and 13C{1H} NMR
measurements were made at frequencies of 300.13 and
75.5 MHz, respectively. In 13C{1H} NMR spectra 2K scans
Fig. 1. 13C{1H} NMR spectrum of poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate).

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A.S. Brar et al. / Journal of Molecular Structure 734 (2005) 35–44
Fig. 2. Expanded 13C{1H} NMR spectrum showing carbonyl carbon resonances of poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate).
175.9–176.6, 176.7–177.4 and 177.5–178.5 ppm (Fig. 2)
were assigned to mm, mr/rm and rr, respectively [25,26].
The a-methyl region showed splitting into two regions
corresponding to rr and mr/rm triads from low to high
chemical shift (Fig. 3). Triads rr and mr/rm constituted
single peaks at 16.78 and 18.88 ppm, respectively. The
methyl (KNCH2CH3) group showed an intense resonance
at 13.8 ppm (Fig. 1).
present in 13C{1H} spectrum and absent in DEPT spectrum
were assigned to quaternary carbons by the comparison of
these two spectra. Quaternary carbon resonances showed
configurational sensitivity up to triad level. The peaks
centered at 45.03, 45.39 and 45.9 ppm were assigned to rr,
rm/mr and mm (as shown by poly(methyl methacrylate))
triads, respectively (Fig. 5). The methylene groups of –
OCH2 and –NCH2 gave resonances at 67.69 and 37.41 ppm,
respectively and the position of the peaks were further
confirmed by 2D experiments. The backbone methylene
showed configurational sensitivity as it appeared as a
multiplet around 52.11–55.77 ppm (Fig. 1).
The triad concentrations were calculated from the area
under these resonance signals. From these triad fractions,
using the Bernoullian statistics, the probability of the meso
addition to the growing polymer chain (Pm) was calculated.
The average value of Pm from the triad fractions is 0.19.
The experimentally and statistically determined triad
fractions of various carbon resonances are shown in
Table 1. The theoretical values were calculated from
Bernoullian statistics using PmZ0.19 while the experi-
mental values were determined from 13C{1H} NMR
spectrum.
The aromatic carbons were found to resonate at around
105.53–141.15 ppm where peaks at 122.85, 139.75 and
140.21 ppm were assigned to quaternary carbons C-b, C-3
and C-a, respectively as shown in Fig. 1. The various other
assignments for the aromatic carbons C-4 and C-5 were
based on the assignments made by Simionescu et al. [27]
for the carbazole ring. The aromatic carbons were found to
resonate at 108.39 (C-1, C-8), 119.06 (C-2), 120.79 (C-5),
121.46 (C-4), 125.82 (C-6) and 127.02 ppm (C-7), as
shown in Fig. 1.
The –CH2 resonances were assigned with the help of
DEPT-135 experiments as they appear as negative peaks in
DEPT-135 spectrum (Fig. 4). The resonances which were
Fig. 3. Expanded 13C{1H} NMR spectrum showing a-methyl carbon resonances of poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate).

A.S. Brar et al. / Journal of Molecular Structure 734 (2005) 35–44
39
Table 1
54.41/1.34 and 54.32/1.99 ppm were assigned to meso (m)
dyad arising due to non-equivalent protons HA and HB,
respectively (Fig. 7).
The experimentally and statistically determined triad fractions of various
carbon resonances
The various assignments of aromatic region in 1H NMR
and 13C{1H} NMR were done with the help of expanded
aromatic region of HSQC spectrum. Hence the crosspeaks
10, 11, 12, 13, 14, 15 and 16 centered at 107.99/6.89,
126.91/7.02, 118.82.7.03, 108.16/7.05, 125.59/7.23,
121.29/7.69 and 120.49/7.86 ppm, respectively can be
assigned to C-8, C-7, C-2, C-1, C-6, C-4 and C-5,
respectively as shown in Fig. 8.
Triads
Triad fractions
Theoretical
Quaternary
carbon
a-Methyl
Carbonyl
rr
0.65
0.31
0.04
0.67
0.30
0.03
0.63
0.33
0.04
0.66
0.31
0.03
rm(mr)
mm
3.2. 2D HSQC NMR studies
2D HSQC enables unambiguous assignments of various
1
resonance signals in 13C{1H} and H NMR spectra. The
3.3. 2D TOCSY NMR spectra studies
expanded HSQC for aliphatic is shown in Fig. 6. The
crosspeaks 1, 2 and 3 centered at 13.37/1.06, 37.19/3.86 and
67.39/4.66 ppm were assigned to methyl (–NCH2CH3),
–NCH2 and –OCH2, respectively. The a-methyl region was
found to be configurational sensitive and crosspeak 4 and 5
centered at 16.37/0.71 and 18.76/0.93 ppm were assigned to
rr and rm/mr triads, respectively (Fig. 6).
2D TOCSY NMR is useful in understanding various
proton couplings in different configurations of the polymer.
The assignments done for –NCH2 and –OCH2 in HSQC
spectrum were further confirmed by 2D TOCSY exper-
iments as protons of NCH2 showed vicinal coupling with
methyl (KNCH2CH3) group resulting in cross-correlation
peak ‘i’ centered at 1.09/3.92 ppm, as shown in Fig. 9.
The geminal coupling between backbone b-methylene
protons in meso configuration (HA and HB) results in cross-
correlation peak ‘ii’ centered at 1.99/1.33 ppm (Fig. 9).
Hence the assignments done in HSQC spectra were further
confirmed by TOCSY experiments.
The backbone b-methylene group showed sensitivity to
various configurational sequences resulting in crosspeak 6
(Fig. 6). The two methylene protons of r dyad will be under
similar environment giving a single crosspeak in 2D HSQC
spectrum, while the two methylene protons of m dyad will
give two crosspeaks in 2D HSQC as shown in Fig. 7.
The crosspeak 7 centered at 54.71/1.79 ppm is assigned to
racemic (r) dyad whereas the crosspeaks 8 and 9 centered at
With the help of all these studies various assignments in
1H NMR spectrum were done, as shown in Fig. 10.
Fig. 4. DEPT-135 NMR spectrum of poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate).

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A.S. Brar et al. / Journal of Molecular Structure 734 (2005) 35–44
Fig. 5. Expanded 13C{1H} NMR spectrum of quaternary carbon of poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate).
3.4. HMBC NMR studies
aromatic carbons via 13C{1H} NMR and DEPT experiments
were further confirmed by HMBC experiments.
HMBC NMR spectroscopy can be used to study long-
range couplings between proton and carbon nuclei. The
tentative assignments done for quaternary and non-protonated
The crosspeak ‘a’ centered at 37.45/1.12 ppm was
assigned to coupling between NCH2 proton with methyl
group. Carbon of –OCH2 group showed coupling with H-4
Fig. 6. Expanded 2D HSQC spectrum showing couplings in the aliphatic region of poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate).

A.S. Brar et al. / Journal of Molecular Structure 734 (2005) 35–44
41
Fig. 7. Methylene region in 2D HSQC spectrum of poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate).
of aromatic ring resulting in crosspeak ‘b’ centered at
67.75/7.78 ppm (Fig. 11). This confirms that the substitution
is at third position of carbazole ring.
strengthened by the expanded HMBC spectrum of aromatic
region.
The crosspeaks ‘c’ and ‘d’ were assigned to couplings
of quaternary carbon C-a with H-5 and H-6. The
quaternary carbon C-3 couples with H-4 and H-2 giving
The assignments done for aromatic carbons and
protons in 13C{1H} and 1H NMR spectra were further
Fig. 8. Expanded 2D HSQC spectrum showing couplings in aromatic region.

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A.S. Brar et al. / Journal of Molecular Structure 734 (2005) 35–44
Fig. 9. 2D TOCSY spectrum showing couplings of –NEt and HA–HB protons of m centered methylene dyad of poly(9-ethyl-3-hydroxymethylcarbazolyl
methacrylate).
rise to crosspeaks ‘e’ and ‘f’ whereas C-b shows
connectivity with H-1, H-4, H-5 and H-8 resulting in
crosspeaks ‘g’, ‘h’, ‘i’ and ‘j’, respectively. The various
other carbon–proton couplings in carbazole ring are C1–
H2, C2–H1, C5–H6, C7–H5, C6–H8 and C6–H5 resulting
in crosspeaks ‘k’, ‘l’, ‘m’, ‘n’, ‘o’ and ‘p’, respectively as
shown in Fig. 12.
4. Conclusions
The microstructure of poly(9-ethyl-3-hydroxymethylcar-
bazolyl methacrylate) prepared by solution polymerization
was analysed by one- and two-dimensional spectroscopy.
1D (1H, 13C{1H}and DEPT) and 2D (HSQC, TOCSY and
HMBC) NMR techniques were studied to assign
1
Fig. 10. H NMR spectrum of poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate).

A.S. Brar et al. / Journal of Molecular Structure 734 (2005) 35–44
43
Fig. 11. 2D HMBC spectrum showing higher bond order couplings for –NEt and –OCH2 groups.
methylenes, a-methyl and aromatic region of the homo-
polymer. The backbone methylene and a-methyl were
assigned to dyad and triad configurational sequence,
respectively. These assignments were further justified with
the help of 2D HSQC and TOCSY experiments. The higher
carbon/proton bond order couplings for –OCH2, –NCH2 and
aromatic region were studied by HMBC. Explicit assign-
ments of carbon/proton resonances and the analysis of
Fig. 12. Expanded 2D HMBC spectrum for the aromatic region of poly(9-ethyl-3-hydroxymethylcarbazolyl methacrylate).

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A.S. Brar et al. / Journal of Molecular Structure 734 (2005) 35–44
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troscopy thus establishing the microstructure of the
homopolymer.
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Acknowledgements
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New York, 1996. pp. 525.
The authors wish to thank Department of Science and
Technology (DST), India and Council of Scientific and
Industrial Research (CSIR), India for providing the financial
support to carry out this work.
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