Synthesis of polymers containing donor±acceptor Schiff base in side ...

Synthesis of polymers containing donor±acceptor Schi€ base
in side chain for nonlinear optics
Abstract
M. Bagheri, A. Entezami *
Laboratory of Polymer Chemistry, Faculty of Chemistry, Tabriz University, Tabriz, Iran
Received 12 September 2000; received in revised form 19 March 2001; accepted 21 May 2001
The Schi€ bases 4-x-hydroxyalkyloxy)-N-5-nitro-2-thienylmethylene)aniline with 2, 6 and 8 methylenic units in
their alkyl group in which a nitrothienyl group as acceptor and an oxyphenyl group as donor were synthesized by
condensation of x-hydroxyalkyloxy)anilines with 5-nitro-2-thiophenecarboxaldehyde. The methacrylate monomers
4-x-methacryloyloxyalkyloxy)-N-5-nitro-2-thienylmethylene)aniline with alkylene groups of di€erent lengths were
synthesized by two di€erent routes and polymerized using a free radical initiator to produce low molecular weight
polymers useful for nonlinear optics. All the obtained compounds were characterized by conventional spectroscopic
methods. First-order hyperpolarizability b) of 4-2-hydroxyethyloxy)-N-5-nitro-2-thienylmethylene)aniline typically
was calculated using semiempirical method and the nonlinear optic properties of the same compound was studied by
second harmonic generation. Ó 2001Published by Elsevier Science Ltd.
Keywords: Nonlinear optic; Schi€ base; Donor±acceptor; Side chain polymer; Second harmonic generation; Semiempirical method
1. Introduction
The synthesis and characterization of polymers containing
nonlinear optically active structures continue to
be topic of current interest [1]. The synthesis of polymer
with side chain groups that are capable of showing
NLO response is a very active ®eld [2]. The side chain
group is usually a donor±acceptor substituted p-conjugated
compound D-p-A molecules) such as a benzene,
biphenyl, napthyl, stilbene, azobenzene and diphenyl
butadiene speci®cally designed for second-order NLO
[3]. Study on the e€ect of the p-conjugated nature to
optimize the NLO characteristics has led to development
of new molecular systems.
It is found that introducing a heterocyclic ring could
lead to enhanced molecular hyperpolarizability. For
example, replacing the benzenoid stilbene compounds
with corresponding heteroaromatic rings such as thio-
* Corresponding author. Fax: +98-411-334-0191.
E-mail address: aaentezami@yahoo.com A. Entezami).
European Polymer Journal 38 2002) 317±326
0014-3057/01/$ - see front matter Ó 2001Published by Elsevier Science Ltd.
PII: S0014 -305701)0013 5 -5
www.elsevier.com/locate/europolj
phene showed that a thiophene moiety results in more
electron delocalization in donor±acceptor compounds
than that of a benzenoid moiety [4].
The liquid-crystalline polymers containing electron
donor and acceptor groups have great potentials in
various ®elds, such as information storage and nonlinear
optics [5±9]. The side chain group is usually a donor±
acceptor substituted aromatic compound such as a
benzene, stilbene or azodye derivative [10]. Synthesis of
side chain liquid crystalline polyacrylate and copolymers
containing alkylene spaced nitro and cyano phenyl,
carbazolyl, quinoline and naphthyl mesogenic unit were
reported [8,11,12].
The goal of the present work was to synthesize a
series of donor±acceptor Schi€ base and related polymers
in the base of thiophene ring. In this paper,
we report the synthesis and characterization of the
Schi€ bases, methacrylate monomers and low molecular
weight polymers containing alkylene spaced nitrothienyl
group. Polymerization of the monomers was carried out
by a radical initiator. These compounds contain both a
nitro thienyl group acceptor) and an oxyphenyl group

318 M. Bagheri, A. Entezami / European Polymer Journal 38 2002) 317±326
donor). The ®rst order hyperpolarizability b) calculation
and second harmonic generation SHG) of Schi€
base 4-2-hydroxyethyloxy)-N-5-nitro-2-thienylmethylene)aniline
in polymethylmethacrylate PMMA) matrices
typically was studied by conventional apparatus.
2. Experimental section
2.1. Materials
The purifying or drying of compounds and solvents
have been performed according to the common procedure.
4-Nitrophenol, 2-chloroethanol, 1,8-octanediol,
Scheme 1. Synthetic route of PnTh n ˆ 2, 6 and 8).
1,3-dibromopropane, stannous chloride and KHCO3
were purchased from Merck. The thiophene-2-carboxaldehyde;
1,6-hexandiol and sodium sul®de nonahydrate
were purchased from Fluka. 1-Chloro-6-hexanol and
1-chloro-8-octanol [13] were prepared according to a
literature procedure. The monomers and polymers were
synthesized according to the route outlined in Schemes 1
and 2, and further details are given below. The chemical
structures of the monomers are shown in Fig. 1.
2.1.1. 5-Nitro-2-thiophene carboxaldehyde
This compound was prepared as given in the literature
[14]. Thus to HNO3 19.2 g, d ˆ 1:51) and 140 ml
AcOH was added slowly 2-thiophenecarboxaldehyde

M. Bagheri, A. Entezami / European Polymer Journal 38 2002) 317±326 319
Fig. 1. Structure of donor±acceptor monomers.
26.9 g, 0.24 mol) in 85 ml Ac2O below 30 °C, the
mixture stirred 0.5 h at 50±60 °C, quenched in ice 1h)
and the diacetylated product washed with water. The
crude material 31g) was treated with 140 ml H2O, 140
ml EtOH, and 14 ml concentrated H2SO4, boiled for 0.5
h, treated with charcoal, ®ltered hot and cooled. Then
the ®ltrate was extracted with Et2O. The yellow needle
crystals were recrystallized from 50% EtOH to give
11.31 g 5-nitro-2-thiophene carboxaldehyde. Yield: 30%;
mp ˆ 76 °C lit [17] mp ˆ 77±77.5 °C); IR KBr): 1670
C@O), 1510 N@O), 1340 N@O) cm 1 .
Scheme 2. Synthetic route of P3Th.
2.1.2. 4-x-Hydroxyalkyloxy)aniline [12]
The synthesis of 4-8-hydroxyoctyloxy)aniline is described
as a representative case. 8-Chlorohexanol 39.5
g, 0.24 mol) was added to a solution containing 4nitrophenol
27.8 g, 0.2 mol) and KOH 16.8 g, 0.3 mol)
in 150 ml of ethanol and water mixture 1:1). Then
the mixture was re¯uxed for 48 h to give 17.5 g
4-8-hydroxyoctyloxy)nitrobenzene. The resulting 4-8hydroxyoctyloxy)nitrobenzene
17.37 g, 0.065 mol) was
reduced with sodium sul®de nonahydrate 31.2 g, 0.13
mol) in 150 ml 50% ethanol at re¯uxing temperature for
24 h. The crude product was recrystallized from toluene
to give 12.26 g 4-8-hydroxyoctyloxy)aniline. Yield:
79.5%; mp ˆ 77 °C; FTIR KBr): 3362 O±H), 3323
N±H), 3185 N±H) cm 1 ; 1 HNMR CDCl3): d 1.28±
1.59 b, 13H, HOCH2CH2)6CH2), 1.76 m, 2H, NH2),
3.66 m, 2H, CH2 ortho to OH), 3.94m, 2H, CH2OPh),
6.66 d, 2H aromatic, ortho to O), 6.75 d, 2H aromatic,
ortho to NH2); FTIR KBr): 3431 OH), 1619 C@N)
cm 1 ; UV±VIS CH2Cl2): 421nm e ˆ 13,500).
For other 4-x-hydroxyalkyloxy)anilines, n ˆ 2:
yield: 30%; mp ˆ 67 °C lit [15] mp ˆ 63:8 °C); FTIR
KBr): 3450 OH), 3250, 3200 NH), n ˆ 6: yield: 65%;

320 M. Bagheri, A. Entezami / European Polymer Journal 38 2002) 317±326
mp ˆ 75 °C lit [15] mp ˆ 75:7 °C); FTIR KBr): 3354
OH), 3277, 3200 NH).
2.1.3. 4-x-Hydroxyalkyloxy)-N-5-nitro-2-thienylmethylene)aniline
HnTh)
5-Nitro-2-thiophene carboxaldehyde 4.7 g, 0.03 mol)
was dissolved in 30 ml ethanol. After an equimolar
amount of 4-x-hydroxyalkyloxy)aniline was added to
the ethanol solution, the mixture was heated at 70 °C for
1h. The product was puri®ed by recrystallization from
dichloromethane±petroleum ether 1:3, v/v) to provide
HnTh.
H2Th: Yield: 75% of brownish orange crystals;
mp ˆ 125:4 °C; 1 HNMR CDCl3): d 3.88 m, 2H, CH2,
ortho to OH), 4.2 t, 2H, CH2OPh), 6.95 d, 2H aromatic,
ortho to O), 7.29 d, 2H aromatic, ortho to N),
7.39 d, 1H aromatic, meta to NO2), 7.88 d, 1H aromatic,
ortho to NO2), 8.53 s, 1H, N@CH); FTIR
KBr): 3350 OH), 1614 C@N) cm 1 ; UV/VIS CH2
Cl2): 413 nm e ˆ 10,600).
H6Th: Yield: 90% of orange crystals; mp ˆ 99:6 °C;
1 HNMR CDCl3): d 1.28 t, 1H, OH), 1.52 m, 4H,
HOCH2)2CH2)2CH2)2), 1.64 m, 2H, HOCH2CH2),
1.84 m, 2H, CH2CH2OPh), 3.69 t, 2H, CH2 ortho to
OH), 4.01t, 2H, CH2OPh), 6.95 d, 2H aromatic, ortho
to O), 7.31m, 3H aromatic), 7.91d, 1H aromatic,
ortho to NO2), 8.57 s, 1H, N@CH); FTIR KBr): 3350
OH), 1615 C@N) cm 1 ; UV±VIS CH2Cl2): 420.8 nm
e ˆ 12,200).
H8Th: Yield: 87% of yellowish brown crystals;
mp ˆ 99:8 °C; 1 HNMR CDCl3): d 1.35±1.60 broad,
9H, HOCH2)2CH2)4CH2), 1.78±1.82 m, 4H, HOCH2-
CH2,CH2CH2OPh), 3.67 m, 2H, CH2 ortho to OH),
4.01m, 2H, CH2OPh), 6.95 d, 2H aromatic, ortho to
O), 7.31m, 3H aromatic), 7.91d, 1H aromatic, ortho
to NO2), 8.57 s, 1H, N@CH); FTIR KBr): 3431OH),
1619 C@N) cm 1 ; UV±VIS CH2Cl2): 421nm e ˆ
13,500).
2.1.4. 4-x-Methacryloyloxyalkyloxy)-N-5-nitro-2-thienylmethylene)aniline
MnTh)
The synthesis of 4-6-methacryloyloxyhexyloxy)-N-
5-nitro-2-thienylmethylene)aniline is described as a
representative case. H6Th 5 g, 0.017 mol) solution in 30
ml dichloromethane was added to a solution containing
methacrylic acid 1.47 g, 0.017 mol), dicyclohexylcarbodiimide
DCC), 3.53 g, 0.017 mol) and dimethylaminopyridine
DMAP) 1.04 g, 0.0086 mol) in 50 ml
dichloromethane at 0 °C. The reaction mixture was
stirred for 20 h at room temperature. After work-up
procedure, the product was puri®ed by recrystallization
three times from ethanol to provide MnTh.
M2Th: Yield: 76% of reddish orange crystals; mp ˆ
148 °C; 1 HNMR CDCl3): d 1.89 s, 3H, CH3), 4.18 t,
J ˆ 6:3, 2H, CH2OPh), 4.44 t, J ˆ 6:6, 2H, OCOCH2),
5.53 s, 1H, HC@C), 6.08 s, 1H, HC@C), 6.9 d,
J ˆ 8:6, 2H aromatic, ortho to O), 7.2 d, J ˆ 8:6, 2H
aromatic, ortho to N), 7.26 d, J ˆ 7, 1H aromatic, meta
to NO2), 7.82 d, J ˆ 7, 1H aromatic, ortho to NO2), 8.8
s, 1H, N@CH). Anal. calcd. for C17H16N2O5S: C,
56.65%; H, 4.48%; N 7.78%. Found: C, 56.04%; H,
4.37%; N, 7.62%.
M6Th: Yield: 80% of dark orange crystals;
mp ˆ 97:8 °C; 1 HNMR CDCl3): d 1.52 m, 4H,
OCOCH2)2CH2)2CH2)2), 1.74 m, 2H, OCOCH2CH2),
1.82 m, 2H, CH2CH2OPh), 1.96 s, 3H, CH3), 4.0 t,
J ˆ 6:3, 2H, CH2OPh), 4.19 t, J ˆ 6:6, 2H, OCOCH2),
5.56 s, 1H, HC@C), 6.11 s, 1H, HC@C), 6.94 d,
J ˆ 8:7, 2H aromatic, ortho to O), 7.31d, J ˆ 8:7, 2H
aromatic, ortho to N), 7.34 d, J ˆ 7, 1H aromatic, meta
to NO2), 7.92 d, J ˆ 7, 1H aromatic, ortho to NO2),
8.57 s, 1H, N@CH). Anal. calcd. for C21H24N2O5S: C,
60.56%; H, 5.81%; N, 6.73%. Found: C, 59.99%; H,
5.76%; N, 6.76%.
M8Th: Yield: 82% for brownish red crystals;
mp ˆ 96:5 °C; 1 HNMR CDCl3): d 1.41±1.55 m, 8H,
OCOCH2)2CH2)4CH2)2OPh), 1.70 m, 2H, OC-
OCH2CH2), 1.82 m, 2H, CH2CH2OPh), 1.97 s, 3H,
CH3), 4.00 m, J ˆ 6:3, 2H, CH2OPh), 4.16 t, J ˆ 6:6,
2H, OCOCH2), 5.56 s, 1H, H2C@C), 6.12 s, 1H,
H2C@C), 6.95 d, J ˆ 8:6, 2H aromatic, ortho to O),
7.30 d, J ˆ 8:6, 2H aromatic, ortho to N), 7.34 d,
J ˆ 7, 1H aromatic, meta to NO2), 7.92 d, J ˆ 7, 1H
aromatic, ortho to NO2), 8.57 s, 1H, N@CH). Anal.
calcd. for C23H28N2O5S: C, 62.14%; H, 6.55%; N,
6.31%. Found: C, 61.57%; H, 6.71%; N, 6.38%. FTIR
KBr) for MnTh: 1719 C@O), 1618 C@N) cm 1 .
2.1.5. 4-3-Bromopropyloxy)nitrobenzene
This compound was prepared using a literature
procedure [15]. Thus, 4-nitrophenol 4.5 g, 32.4 mmol),
1,3-dibromopropane 65.4 g, 324 mmol) and potassium
carbonate 33.5 g, 242 mmol) were re¯uxed with stirring
in dry acetone 250 ml) overnight. The reaction mixture
was ®ltered hot. The residue was washed with acetone,
and the solvent removed under reduced pressure. Light
petroleum 40±60 °C) was added to the extract and the
resulting white precipitate collected. The crude product
was recrystallized twice from ethanol with hot ®lteration
to ensure the complete removal of any dimeric side
products. Yield: 3.79 g) 90%; mp ˆ 80 °C; FTIR KBr):
1595 C@C aromatic), 1505, 1330 N@O), 1250 C±O),
1170 C±O) cm 1 .
2.1.6. 4-3-Methacryloyloxypropyloxy)nitrobenzene
This compound was prepared using a procedure described
by Imrie [16]. Methacrylic acid 1.32 g, 15.5
mmol) was stirred with potassium hydrogen carbonate
15.2 g, 15.2 mmol) for 5 min at room temperature
to form the potassium methacrylate salt. This salt was

M. Bagheri, A. Entezami / European Polymer Journal 38 2002) 317±326 321
added to 4-3-bromopropyloxy)nitrobenzene 3.69 g, 14
mmol) and hydroquinone 0.024 g, 0.22 mmol) in N,Ndimethylformamide
100 ml) and the reaction mixture
was re¯uxed with stirring at 100 °C overnight. On
cooling, the mixture was poured into the water 300
ml), the precipitate collected by ®ltration and dissolved
in dichloromethane. The organic solution was washed
with 5% aqueous NaOH and the solvent removed. The
crude product was recrystallized twice from ethanol to
produce 3 g white powder. Yield: 81%; mp ˆ 78:9 °C;
IR KBr): 1720 C@O), 1590 C@C aromatic), 1500
N@O), 1330 N@O), 1250 C±O), 1170 C±O) cm 1 ;
1 HNMRCDCl3): d 1.96 s, 3H, CH3), 2.24 m, 2H,
CH2CH2CH2), 4.20 t, 2H, CH2OPh), 4.38 t, 2H, OC-
OCH2), 5.60 s, 1H, H2C@C), 6.13 s, 1H, H2C@C), 6.96
d, 2H aromatic, ortho to O), 6.99 d, 2H aromatic,
ortho to NO2), 8.2 s, 1H, N@CH).
2.1.7. 4-3-Methacryloyloxypropyloxy)aniline
This compound was prepared using a procedure described
by Bellamy and Ou [17]. Thus a mixture of 4-3methacryloyloxypropyloxy)nitrobenzene
2.65 g, 0.01
mol), SnCl2 2H2O 11.28 g, 0.05 mol) and hydroquinone
0.015 g, 0.14 mmol) in 20 ml of absolute ethanol
was heated at 70 °C under nitrogen. After 30 min the
solution was allowed to cool down and then poured into
ice. The pH was made slightly basic pH 7±8) by addition
of 5% aqueous sodiumbicarbonate before being
extracted with ethyl acetate. The organic phase was
thoroughly washed with brine, treated with charcoal and
dried over sodium sulfate. After evaporation of the
solvent, the cured product was further puri®ed by a
silica gel chromatography column eluting with 50% ethyl
acetate in petroleum ether to give 1.53 g product as
a clear yellow oil. Yield: 65%; IR KBr): 3420, 3350
N±H), 1712 C@O), 1625, 1500 C@C aromatic), 1250
C±O), 1150 C±O) cm 1 ; 1 HNMR CDCl3): d 1.65
m, 2H, NH2), 1.97 s, 3H, CH3), 2.24 m, 2H,
CH2CH2CH2), 4.17 t, 2H, CH2OPh), 4.39 t, 2H, OC-
OCH2), 5.59 s, 1H, HC@C), 6.14 s, 1H, HC@C), 7.01
d, 2H aromatic, ortho to NH2), 7.89 d, 2H aromatic,
ortho to O).
2.1.8. 4-3-Methacryloyloxypropyloxy)-N-5-nitro-2-thienylmethylene)aniline
M3Th)
5-Nitro-2-thiophene carboxaldehyde 4.7 g, 0.03 mol)
was dissolved in 30 ml ethanol. After an equimolar
amount of 4-3-methacryloyloxypropyloxy)aniline was
added to the ethanol solution, the mixture was heated at
70 °C for 1h. The product was puri®ed by recrystallization
from dichloromethane±petroleum ether 1:3, v/v).
Yield: 96%; mp ˆ 119 °C; FTIR KBr): 1720 C@O),
1618 C@N) cm 1 ; 1 HNMR CDCl3): d 1.96 s, 3H,
CH3), 2.21m, 2H, CH2CH2CH2), 4.11 t, J ˆ 6:3,
2H, CH2OPh), 4.38 J ˆ 6:6, t, 2H, OCOCH2), 5.59
s, 1H, HC@C), 6.13 s, 1H, HC@C), 6.96 d, J ˆ 8:6,
2H aromatic, ortho to O), 7.31d, J ˆ 8:6, 2H aromatic,
ortho to N), 7.34 d, J ˆ 7, 1H aromatic, meta to NO2),
7.92 d, J ˆ 7, 1H aromatic, ortho to NO2), 8.57 s, 1H,
N@CH). Anal. calcd.: C, 57.74%; H, 4.85%; N, 7.49%.
Found: C, 57.49%; H, 4.88%; N, 7.31%.
2.1.9. Polymerization
Polymerization of MnTh was carried out in dry
DMF solution with azobisisobutyronitrile) AIBN; 3
mol%) as an initiator at 70 °C for 20 h under vacuum.
The resulting polymer was puri®ed by reprecipitation
using a methanol. This was repeated until the polymer
contained no trace of monomer.
2.2. Methods
Spectroscopic characterization utilized the following
instrumentation: Melting points were recorded with an
electrothermal apparatus. Number-average molecular
weights of the resulting polymers were determined with a
maxima 820 gel permeation chromatography GPC)
analysis instrument run time: 50 min, column temperature:
50 °C). IR and FT-IR spectra were recorded on a
Mattson sirius 100 and Shimadzu Model FT-IR-8101 M
spectrometer, respectively. 1 HNMR spectra were taken
on a FT-NMR 300 and 400 MHz) Brucker versus,
TMS in CDCl3. Elemental analyses were performed by
Heareus CHN-O-RAPID analyzer. UV±VIS spectra
were recorded on a Shimadzu UV-265FW spectrometer
in dichloromethane solvent.
2.2.1. Film preparation
Thin ®lm of host polymer PMMA) doped with 3±5%
Schi€ base H2Th) or dopant by weight was made by dip
coating onto indium±tin oxide ITO) coated glass slides
from a mixture in dichloromethane solution. This ®lm
was employed for SHG measurements.
2.2.2. Corona poling
The ®lm was poled at 82 °C for 1h using a coronadischarge
setup to orient the NLO chromophore. The
poling voltage was around 6 kV with a tip-to-plane
distance of about 1.2 cm. The ®lm thickness of 3±4 lm
was measured by a microscope.
2.2.3. Second harmonic generation measurements
The Maker Fringe method [18] was employed
to measure the SHG intensity of poled ®lms. The apparatus
employed for SHG measurements, schematically
shown in Scheme 3, was a Q-switched Nd:YAG
laser L ˆ 1:064 lm) with a pulse with of 10 ns was used
as the fundamental beam. The generated second harmonic
SH) wave ®ltered from the fundamental and
detection was accomplished by FEM-100 photomultiplier.

322 M. Bagheri, A. Entezami / European Polymer Journal 38 2002) 317±326
3. Results and discussion
3.1. Monomer synthesis
The Schi€ bases 4-x-hydroxyalkyloxy)-N-5-nitro-2thienylmethylene)aniline
HnThs, n ˆ 2, 6, and 8) were
prepared from 4-N-hydroxyalkyloxy)anilines and 5nitro-2-thiophene
carboxaldehyde in high yield.
The donor±acceptor monomers MnThs, n ˆ 2, 6
and 8) were successfully synthesized contain both an
oxyphenyl group electron donor) and a nitro thienyl
group electron acceptor) within the same molecules, by
condensation of a x-hydroxyalkyloxy)anilines with a
nitrothiophene carboxaldehyde derivative, followed by
esteri®cation of the terminal hydroxyl group with
methacrylic acid Scheme 1). For the ®nal puri®cation of
the monomers, the recrystallization from the ethanol
solution were used because the Schi€'s base decomposed
where it was puri®ed by silica gel column chromatography
[12]. DCC was used to keep preparation of
monomer milder than those with acid chloride reported
in the literature for a similar monomer containing Nmethyl
carbazole [12] or nitrobenzene [7]. A higher yield
76±82%) was obtained, as compared to 37±56% with
the acid chloride route. It is important to maintain
neutral condition when monomer is very sensitive to
both acid and base and hydrolyzes easily.
The other homologue of MnThs, the 4-3-methacryloyloxypropyloxy)-N-5-nitro-2-thienylmethylene)ani
Scheme 3. Schematic representation of experimental apparatus.
line M3Th) was synthesized from di€erent route depending
on starting material, 1,3-dibromopropane, according
with Scheme 2. Stannous chloride reduction of
nitro group to amine was carried out as a mild, selective,
inexpensive and general method [17]. Under the conditions
used, other reducible or acid sensitive groups are
recovered unchanged. Condensation of 4-3-methacryloyloxy
propyloxy)aniline with nitro thiophene derivative
was given monomer M3Th) in high yield 96%).
3.2. Polymer synthesis and characterization
All methacrylate polymers containing a 5-nitro-2thienyl)aniline
group were prepared by solution polymerization
with azobisisobutyronitrile) as a radical
initiator. The molecular weights, polydispersities and
average degree of polymerization for the PnTh series are
listed in Table 1. All the monomers were given only low
molecular weights polymers resulting from propagation
through the bond. In the di€erent conditions of polymerization
such as time and temperature resulting
crosslinked and insoluble products. Formation of oligomers
and crosslinked products by radical polymerization
of methacrylate derivatives of ®ve-membered heterocyclics
were already reported [19±21].
1 HNMR spectra of M2Th and P2Th are shown in
Fig. 2. The peaks due to the methacryloyl unit in the
monomer at 5.53±6.08 ppm were replaced by the backbone
methylene absorption in the polymethacrylate at
Table 1
Molecular weights, polydispersities and average degree of polymerization for the PnTh n ˆ 2, 3, 6 and 8)
PnTh Mng mol 1 ) Mwg mol 1 ) PD DP UV/VIS CH2Cl2) Yield %)
n ˆ 2 3087 3308 1.07 9 kmax ˆ 414 38
e ˆ 4:3 104 n ˆ 3 29514551 1.76 7 kmax ˆ 416 40
e ˆ 5:6 104 n ˆ 6 2254 3262 1.44 6 kmax ˆ 419 42
e ˆ 1:9 104 n ˆ 7 2359 2949 1.25 7 kmax ˆ 420 42
e ˆ 1:7 104

M. Bagheri, A. Entezami / European Polymer Journal 38 2002) 317±326 323
Fig. 2. 1 HNMR spectra of a) monomer M2Th), b) polymer P6Th) in chloroform solution.
0.78±1.52 ppm. Since the intensity of the C±H proton
peak of the Schi€ base linkage in 1 HNMR spectrum
of the polymer was corresponded to that expected, the
relatively labile C@N bond in the polymer was not decomposed
during polymerization. Moreover, the aromatic
proton peaks of the phenylene group and thienyl group
resonances were seen at 6.8±7.1and 7.1±7.78 ppm, respectively.
The observed chemical shift showed that the
polymer contains the designed chromophoric unit.
Fig. 3a shows the UV/VIS spectrum of the H6Th,
M6Th, and P6Th. The absorption maximum in the UV/
VIS spectra of the monomer was similar to that of the

324 M. Bagheri, A. Entezami / European Polymer Journal 38 2002) 317±326
Fig. 3. UV/VIS spectra of a) H6Th i), M6Th ii) and P6Th iii); b) P2Th i), P3Th ii), P6Th iii) and P8Th iv) measured in dichloromethane.
Schi€ base. The small interaction caused by attachment
of the chromophores to the polymer backbone could be
detected by UV. The absorption band near 400 nm hypsochromically
shifted and has diminished in intensity
for polymer. As shown in the Fig. 3b increasing the
length of the alkyl chain in the spacer group from ethyl
to the 1,8-octyl group leads to a shift of the maximum
absorption to the longer wavelengths by a maximum of
8 nm).
3.3. Second-order molecular nonlinearity
Replacing the homocyclic ring of a p-conjugated
system with heteoaromatic rings also leads to enhanced
second-order NLO response properties [22]. It has been
speculated that the heteroaromatic rings such as thiophene
acts as an additional donor in the bridge. It
could also be possible that the reduced aromaticity of
the thiophene compound to the benzene ring might result
in enhanced molecular hyperpolarizability. The
hyperpolarizability of synthesized donor±acceptor Schi€
base in the base of thiophene ring was calculated using
the semiempirical method. This chromophore as a candidate
was proposed to the synthesized compounds because
a chromophore in a polymer it has b several times
that of the same chromophore as a monomer [23].
Therefore the polymers are capable of showing the NLO
response superior than H2Th.

At First, for calculation of b, the molecular geometry
was optimized using the AM1Hamiltonian in the Hyperchem
4.5 package [24]. Then this optimized geometry
was used for calculation of b in the MOPAC 6 quantum
chemical package [25]. The obtained amount of b for
H2Th was 7:196 10 30 esu.
3.4. Second harmonic generation measurements
M. Bagheri, A. Entezami / European Polymer Journal 38 2002) 317±326 325
For second harmonic generation SHG) or frequency
doubling, the NLO chromophores in the polymer matrix
must be aligned in to a noncentrosymmetric orientation.
Electric-®eld-induced chromophore orientation occurs
in regions of su cient local free volume and segmental
mobility. Corona poling techniques involving large
magnitude electric ®eld, applied across the polymers
®lm, was used to orient the chromophores into the
noncentrosymmetric structure required to obtain a second
harmonic signal. A scheme of an apparatus used to
measure SHG is shown in Scheme 3. Light is generated
by a Q-switched Nd:YAG laser with a k ˆ 1:064 lm
fundamental yielding the SHG signal at a wavelength of
532 nm).
As shown in Fig. 4, with changing the incident angle
the amount of I2x is also changed and reach to a maximum.
This maximum is the interference point of between
harmonic boundary waves and harmonic free
waves. Because, according to the Maker et al. equation
[26], I2xaA…/†d 2 sin 2 pl/2lc), the second harmonic intensity
have oscillation behavior that related to the
phase mismatch between boundary wave and free wave.
In Maker et al. equation, I2x is intensity of second
harmonic, A/) is the transition factor, d is the NLO
coe cient, / is the incident angle, l is the ®lm thickness
and lc is the coherent length and lc ˆ k/4nx cos hx
n2x cos hx)n is refractive index and h is refractive angle).
Fig. 4. SHG in PMMA doped with H2Th.
The obtained curve in the base of Maker method was
exhibited qualitatively the second harmonic generation
of the molecule. Therefore these new NLO materials
have been used for SHG application.
4. Conclusion
The synthesis of four members of a polymethacrylate
containing donor±acceptor Schi€ bases in side chain
have been reported. GPC studies showed that the radical
polymerization of monomers containing a nitro thienyl
group at the end of molecules gave low molecular weight
polymers which was soluble in THF, CH2Cl2, DMF and
DMSO. These compounds with electron donor and acceptor
groups can be considered as NLO materials with
potential application in nonlinear optic devices.
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