Synthesis and properties of optically active nanostructured polymers bearing amino acid moieties by direct polycondensation of 4,4′-thiobis(2- tert-butyl-5-methylphenol) with chiral diacids

Article abstract of DOI:10.1007/s00726-011-0958-3

Four derivatives of N-trimellitylimido-l-amino acid (4a-4d) were prepared by the reaction of trimellitic anhydride (1) with the l-amino acids (2a-2d) in acetic acid as diacid monomers and were used with the aim to obtain a new family of amino acid based p

Full text of DOI:10.1007/s00726-011-0958-3

Amino Acids (2012) 42:2187–2194
DOI 10.1007/s00726-011-0958-3
ORIGINAL ARTICLE
Synthesis and properties of optically active nanostructured
polymers bearing amino acid moieties by direct polycondensation
of 4,40-thiobis(2-tert-butyl-5-methylphenol) with chiral diacids
•
Shadpour Mallakpour Samaneh Soltanian
Received: 3 June 2011 / Accepted: 7 June 2011 / Published online: 21 June 2011
Ó Springer-Verlag 2011
Abstract Four derivatives of N-trimellitylimido-L-amino
acid (4a–4d) were prepared by the reaction of trimellitic
anhydride (1) with the L-amino acids (2a–2d) in acetic acid
as diacid monomers and were used with the aim to obtain a
new family of amino acid based poly(ester-imide)s (PEI)s.
The polymerization was performed by direct polyconden-
sation of chiral diacids (4a–4d) with 4,40-thiobis(2-tert-
butyl-5-methylphenol) (5) in the presence of tosyl chloride
(TsCl), pyridine and N,N-dimethyl formamide (DMF).
Step-growth polymerization was carried out by varying the
time of heating and the molar ratio of TsCl/diacid and the
optimum conditions were achieved. The synthesized
polymers were characterized by means of specific rotation
experiments, FT-IR, 1H-NMR, X-ray diffraction tech-
niques and elemental analysis. The surface morphology of
the obtained polymers was studied by field emission
scanning electron microscopy. The result showed nano-
structure morphology of the resulting polymers. The
obtained PEIs were soluble in polar aprotic solvents such
as DMF, N,N-dimethyl acetamide, dimethyl sulfoxide,
N-methyl-2-pyrrolidone and protic solvents such as sulfuric
acid. Thermal stability and the weight-loss behavior of the
PEIs were studied by thermal gravimetric analysis (TGA)
techniques. TGA showed that the 10% weight loss
temperature in a nitrogen atmosphere was more than
402°C, therefore they had useful levels of thermal stability
associated with excellent solubility.
Keywords Poly(ester-imide) ꢀ Nanostructure ꢀ Chiral
polymers ꢀ L-Amino acid ꢀ Biodegradable
Introduction
Aromatic polyimides are well-known high-performance
polymers that show thermal stability, low flammability,
and excellent thermal, mechanical and electrical properties.
Due to these properties they have been extensively used in
aerospace, microelectronic, aviation, defense and elec-
tronic packaging industries (Wilson et al. 1990; Cassidy
1980; Mallakpour and Kowsari 2005; Guo et al. 2007a;
Faghihi and Gholizadeh 2010; Raju and Alam 2006).
However, one of the problems with most polyimides is
their poor processability, high dielectric constant, high
glass transition and limited solubility in common organic
solvents. To overcome these disadvantages many attempts
have been made by incorporating different moieties into
polyimide chain (Raju and Alam 2008; Guo et al. 2007b;
Edlund and Albertsson 2003). Thus, many copolyimides,
such as poly(amide-imide)s, poly(ester-imide)s (PEI)s and
other copolymers have been prepared (Saxena et al. 2003;
Mallakpour and Kowsari 2006a, b; Mallakpour and Shah-
mohammadi 2004). PEIs which combine many useful
properties of polyimides and polyesters are well known for
their thermal resistance coupled with easy processability.
They have received considerable attention for their excel-
lent thermal stability, mechanical properties, chemical
resistance and low melting viscosity (Hamciuc et al. 2008;
Mallakpour and Shahmohammadi 2005).
S. Mallakpour (&) ꢀ S. Soltanian
Department of Chemistry, Organic Polymer Chemistry Research
Laboratory, Isfahan University of Technology,
84156-83111 Isfahan, Islamic Republic of Iran
e-mail: mallak@cc.iut.ac.ir
S. Mallakpour
Nanotechnology and Advanced Materials Institute,
Isfahan University of Technology, 84156-83111 Isfahan,
Islamic Republic of Iran
123

2188
S. Mallakpour, S. Soltanian
Amino acids as a basic component of proteins are no
direct polycondensation of natural amino acid based
diacids (4a–4d) with aromatic diol (5). Presence of bulky
t-butyl groups in this diol increases the solubility of the
resulting polymers. Due to the use of amino acid contain-
ing monomers to synthesize PEIs, it is anticipated that this
macromolecules have potential biodegradability and bio-
compatibility behavior.
doubt very biocompatible with human tissue. They are
expected to be not only the biocompatible materials but
also the sources of chemically functional materials. Of
particular interest is the utilization of amino acids as nat-
ural or biological building blocks, in the preparation of
synthetic, functional polymers and nanostructured systems.
They are widely used as key components of biopolymers
including polymer drugs (Wu et al. 2003; Sutthasupa et al.
2007; Nagai et al. 2004; O’Reilly 2010). With the recent
considerable advances in nanoscience and nanotechnology,
a variety of nanostructured polymers have been devised to
possess useful electronic, optical and physicochemical
properties for a broad range of advanced applications.
Examples include the use of polymer nanoparticles as drug
delivery devices, and polymer nanofibers as conducting
wires (Dai 2004).
Experimental
Materials
The following analytical grade chemicals were purchased
from Fluka Chemical Co. (Buchs, Switzerland) and Merck
Chemical Co. L-Phenylalanine, L-leucine, L-isoleucine,
L-methionine, and TTMP were used as received. Trimel-
litic anhydride (1) was purified by recrystallization in an
acetic acid/acetic anhydride mixture (3/1) and dried in
vacuum at 60°C for 6 h. DMF and N-methyl-2-pyrrolidone
(NMP) were dried over BaO and then distilled under
reduced pressure.
Chiral polymers including those bearing main or side
chain amino acid units are used extensively in the
pharmaceutical industry for enantio-selective separation
of drugs. The main potential medical applications for
synthetic a-amino acid based polymers are bioresolvable
sutures, screws or plates, and drug delivery systems
(controlled release and targeting) (Feng et al. 2007; Deng
et al. 2002). They are also used as chiral media for
asymmetric synthesis, polymeric chiral catalysts and
chiral liquid crystals in ferroelectrics and non-linear
optical devices (Mallakpour and Rafiemanzelat 2005;
Mallakpour and Moghaddam 2006; Mallakpour and
Mirkarimi 2010).
Instruments
The inherent viscosities (ginh) of the polymers were
determined with a Cannon–Fenske Routine Viscometer
(Germany) at the concentration of 0.5 g/dL at 25°C. Spe-
cific rotations were measured by a Jasco Polarimeter
(Japan). Proton nuclear magnetic resonance (1H-NMR,
500 MHz) spectra were recorded on Bruker (Germany)
Avance 500 instrument at room temperature (RT), using
solution in deuterated dimethylsulfoxide (DMSO-d6). FT-
IR spectra were recorded on a spectrophotometer (Jasco-
680, Japan) using KBr pellets. Band intensities are assigned
as weak (w), medium (m), shoulder (sh), strong (s), and
broad (br). The X-ray diffraction (XRD) patterns were
recorded using a Philips X’PERT MPD diffractometer (Cu
Ka radiation: k = 0.154056 nm at 40 kV and 30 mA) over
the 2h range of 10°–100° at a scan rate of 0.05°/min.
Thermal gravimetric analysis (TGA) data for polymers
were taken on a STA503 WinTA instrument in a nitrogen
atmosphere at a rate of 20°C/min. Elemental analyses were
performed by Leco, CHNS-932. Field emission scanning
electron microscopy (FE-SEM) was done using HITACHI
(S-4160).
A Vilsmeier adduct derived from an arylsulfonyl
chloride such as tosyl chloride (TsCl) and N,N-dimeth-
ylformamide (DMF) in pyridine (Py) was successfully
used by Higashi and co-workers for the synthesis of
aromatic polyesters by the direct polycondensation
of aromatic dicarboxylic acids and bisphenols and also of
hydroxybenzoic acids. The reaction of the Vilsmeier
adduct can activate carboxyl groups of dicarboxylic acids,
thus, could be used for a wide variety of dicarboxylic
acids as a suitable condensing agent (Higashi et al. 1984,
1999).
4,40-thiobis (2-tert-butyl-5-methylphenol) (TTMP) is a
hindered thiophenol antioxidant which is known as an
antioxidant for polyolefins. Under certain conditions, it has
been found that such a compound does not offer suffi-
ciently long-term antioxidant characteristics to polyolefins.
It has been also recognized that the polymeric form of
TTMP gives long term antioxidant protection to polyole-
fins (Hirsch 2006).
Monomer synthesis
The aim of this study is utilization of amino acid
building blocks for efficient synthesis of new thermally
stable and optically active nanostructured PEIs. The reac-
tion promoted by TsCl in Py and DMF was applied to the
N-Trimellitylimido-L-amino acids (4a–4d) as diacid
monomers were prepared according to our published arti-
cles (Mallakpour et al. 2000, 2003, 2008); Mallakpour and
Kolahdoozan 2007) and are shown in Scheme 1.
123

Synthesis and properties of optically active nanostructured polymers
2189
Scheme 1 Synthesis of diacids
(4a–4d)
O
O
*
O
O
O
H2N
COOH
CH3COOH
HO
H
N
O
*
COOH
H
HO
R
O
OH
R.T
H
R
1
2a-2d
3a-3d
Reflux
O
O
N
HO
*
COOH
H
R
O
4a-4d
CH3
R:
CH2Ph
CH2CH
CHCH3
CH2CH2SCH3
CH2CH3
CH3
a
c
b
d
Polymer 6c. FT-IR (KBr, cm-1): m = 3483 (w, br), 2964
(s), 2917 (m), 2875 (m), 1781 (m), 1745 (s), 1725 (s), 1679
(m), 1606 (w), 1482 (s), 1451 (m), 1381 (s), 1280 (s), 1242
(s), 1217 (s), 1189 (s), 1167 (s), 1092 (s), 1059 (m), 1032
(w), 1009 (w), 727 (m) cm-1 (Mallakpour et al. 2011).
Polymer synthesis
ꢀ
The synthesis of the PEI 6a was used as an example to
illustrate the general synthetic route used to produce the
PEIs. To a solution of TsCl (0.28 g; 1.47 9 10-3 mol) with
Py (0.20 ml; 2.5 9 10-3 mol) after 30 min stirring at room
temperature, 0.09 ml DMF (1.22 9 10-3 mol) was added.
After 30 min, the mixture was added dropwise to a solution
of diacid 4a (0.10 g; 2.94 9 10-4 mol) in Py (0.20 ml).
The mixture was kept at room temperature for 30 min and
then TTMP (5) (0.1 g; 2.94 9 10-4 mol) was added and
the whole solution was stirred at 120°C for 6 h. Then the
viscous liquid was precipitated in 30 ml of methanol to
yield 0.187 g (96%) of the polymer 6a.
Polymer 6d. FT-IR (KBr, cm-1): m = 3484 (w, br), 2962
ꢀ
(s), 2915 (m), 2870 (m), 1779 (w), 1741 (s), 1725 (s), 1677
(w), 1603 (w), 1482 (s), 1439 (m), 1382 (s), 1364 (s), 1281
(s), 1242 (s), 1217 (s), 1189 (s), 1167 (s), 1092 (s), 1059
(m), 1033 (w), 1009 (w), 925 (w), 894 (w), 841 (w), 727
(m)cm-1.1H-NMR (500 MHz, DMSO-d6, d, ppm):
d = 0.91–1.26 (m, 18H), 2.00 (s, 3H), 2.20–2.24 (m, 6H),
2.46 (m, 2H), 2.51 (m, 2H), 5.44 (m, 1H), 6.75–7.23 (s, 4H,
Ar–H), 8.15 (s, 1H), 8.41 (s, 1H), 8.55 (s, 1H), 9.66–9.69
(m, OH end group) ppm.
FT-IR (KBr, cm-1): m = 3448 (w, br), 2960 (m), 2914
ꢀ
(w), 2870 (w), 1781 (w), 1748 (s),1725 (s), 1604 (w), 1482
(m), 1453 (w), 1383 (s), 1366 (m), 1281 (m), 1242 (m),
1217 (s), 1167 (s),1092 (s), 1059 (w), 1031 (w), 726 (m),
699 (w) cm-1.1H-NMR (500 MHz, DMSO-d6, d, ppm):
d = 0.92–1.25 (m, 18H), 2.15 (s, 6H), 3.46 (m, 1H), 3.63
(m, 1H), 5.64 (m, 1H), 6.75–7.19 (m, 9H, Ar–H), 8.07 (s,
1H), 8.34 (s, 1H), 8.49 (s, 1H), 9.66–9.68 (m, OH end
group) ppm (Mallakpour et al. 2011).
Results and discussion
Synthesis of PEIs
The PEIs 6a–6d were prepared by the direct polyconden-
sation reaction of an equimolar mixture of diol 5 with
chiral diacids (4a–4d) in a system of TsCl/Py/DMF
(Scheme 2). For the polymerization of optically active
diacids with aromatic diol, a Vilsmeier adduct was pre-
pared which a detailed mechanism is illustrated in
Scheme 3. Thus, TsCl was dissolved in Py to yield sulfo-
nium salt (I) and stirred for 30 min (aging time) followed
by addition of DMF and stirring for 30 min until Vilsmeier
adduct (II) was formed, as suggested before (Higashi et al.
1984, 1999). The reaction mixture was added to a solution
of diacid in Py to produce activated diacid (III). Later, diol
Elemental analysis: calcd. for (C40H39NO6S)n: C,
72.59%; H, 5.94%; N, 2.12%; S, 4.84%. Found: C,
72.30%; H, 5.97%; N, 2.31%; S, 5.00%.
The other PEIs 6b-6d were prepared following a similar
procedure.
Polymer 6b. FT-IR (KBr, cm-1): m = 3481 (w, br),
ꢀ
2960 (s), 2917 (m), 2871 (m), 1779 (m), 1745 (s), 1725 (s),
1604 (w), 1482 (s), 1449 (m), 1382 (s), 1369 (s), 1279 (s),
1241 (s), 1218 (s), 1189 (s), 1167 (s), 1123 (w), 1092 (s),
1058 (m), 1008 (w), 727 (m) cm-1
.
123

2190
S. Mallakpour, S. Soltanian
Scheme 2 Polycondensation
reaction of aromatic diol 5 with
diacids (4a–4d)
O
O
TsCL/Py/DMF
S
S
O
+
OH 4a-4d
HO
O
O
*
N
C
H
O
R
5
n
CH3
CH2CH
6a-6d
R:
CH2Ph
CHCH3
CH2CH2SCH3
CH2CH3
CH3
a
c
b
d
CH3
..
N
O
H
Table 1 Effect of the reaction time and the molar ratio of TsCl/
diacid on the inherent viscosity (ginh) and yield of PEI 6a at 120°C
C
CH3
:
N
N
gainh
(dL/g)
Cl
O
Polymer
TsCl/diacid
(mol/mol)
Reaction
time (h)
Yield
(%)
Ar
S
Cl
O
S
-Py
O
Ar
TsCl
PEI 6a
PEI 6a
PEI 6a
PEI 6a
PEI 6a
a
3
3
5
5
8
2
4
4
6
6
72
75
89
96
95
0.16
0.16
0.18
0.19
0.19
(I)
H3C
Ar'
HOOC
COOH
Cl
N
CH
O
OSO2Ar
-ArSO3H
H3C
(II)
Measured at a concentration of 0.5 g/dL in DMF at 25° C
O
H2C
Cl
H3C
CH3
HO
Ar'' OH
Table 2 Some physical properties of PEIs 6a-6d
N
C
O
Ar'
O
C
N
Cl
H
H
-DMF
-HCl
Polymer
Yield (%)
gainh
CH3
25
a
½aꢁD
(III)
PEI 6a
PEI 6b
PEI 6c
PEI 6d
a
96
74
76
87
0.19
0.15
0.14
0.16
?16.41
?14.40
?15.32
-14. 42
O
O
O
Ar'
O
Ar''
n
PEI 6a-6d
O
Measured at a concentration of 0.5 g/dL in DMF at 25°C
*
N
Ar' =
R:
Ar =
H3C
H
R
O
measuring the specific rotations of polymers (Table 2). As
shown in Table 2 all of the obtained polymers show optical
rotation and therefore are optically active.
4a-4d
CH3
CH2CH
CH3
S
5
CH2Ph
Ar'' =
(a)
(b)
CHCH3
CH2CH3
(c)
Structural characterization of PEIs
CH2CH2SCH3
(d)
The chemical structure and purities of the resulting PEIs
1
were analyzed by elemental analysis, H-NMR and FT-IR
Scheme 3 Mechanistic representation of polycondensation reaction
of aromatic diol 5 with different amino acid based diacids
spectroscopy. The measured result in elemental analyses
closely corresponded to the calculated ones, indicating that
the expected compound was obtained. The FT-IR spectrum
of PEI 6a showed carbonyls stretching of imide and ester
groups around 1,780 and 1,725 cm-1, respectively. All of
these PEIs exhibited imide characteristic ring vibration at
1,380 and 727 cm-1 which confirmed the presence of the
5 was added, and the whole solution was maintained at an
elevated temperature for several hours. Polycondensation
was carried out at various reaction times and the molar
ratio of TsCl to diacid, and the optimum conditions were
obtained (Table 1). The aging time and reaction tempera-
ture were used according to the optimized conditions
reported before (Mallakpour and Kowsari 2005). The PEIs
were obtained under optimized conditions in good yields
(74–96%) and had inherent viscosity values ranging
between 0.14 and 0.19 dL g-1 (Table 2). The incorporation
of a chiral unit into the polymer backbone was obtained by
1
imide heterocycle in these macromolecules. The H-NMR
spectrum (500 MHz) of the polymer 6a is shown in Fig. 1.
The assignments of proton are also shown in the figure and
all the NMR spectroscopic data are consistent with the
proposed molecular structure. The aromatic protons (H5,
H6) appeared in the region of 6.75–7.19 ppm. The other
123

Synthesis and properties of optically active nanostructured polymers
2191
Fig. 1 1H-NMR (500 MHz) spectrum of PEI 6a in DMSO-d6 at RT
Fig. 2 X-ray diffractogram of PEI 6a
aromatic protons (H7, H8, H9) were observed at
8.07–8.49 ppm. The peak in the region of 5.64 ppm is
assigned to the chiral carbon proton in the main chain of
the polymer. The peaks of diastereotopic protons of CH2
(H3, H3’) appeared at 3.46 and 3.63 ppm, respectively.
One of the challenges polymer scientists face is
molecular weight determination of their materials. Though
membrane osmometry, gel permeation chromatography,
viscosity analysis and mass spectrometry are usually used
for molecular weight determination, the techniques can be
time consuming and require specialized instrumentation.
The ends of polymer chains sometimes consist of groups
different from the monomer units that make up the body of
the polymer molecule. A wide range of chemical and
physical techniques have been employed to the determi-
nation of functional groups as well as the end groups. End
group analysis by 1H-NMR offers an easy, convenient, and
rapid method for determination of molecular weights of
polymers using an instrument commonly found in many
analytical labs (Sandler et al. 1998).
The morphology of the obtained polymers containing
different amino acids was studied by FE-SEM and is shown
in Fig. 3. As is observed from Fig. 3a, b, PEI 6a (based on
L-phenylalanine as chiral moiety diacid monomer) presents
relatively spheroid surface. Compared with smooth and
layered surface of PEI 6b with L-leucine building blocks
(Fig. 3c, d), PEI 6c (based on L-isoleucine) exhibits a
cylindrical-like nanostructure with a diameter of a 100 nm
(Fig. 3e, f). The surface morphology of the PEI 6d with
L-methionine moieties shows nanostructures with a spher-
ical shape of approximately 170 nm as shown in Fig. 3g, h.
Based on these images it is evident that each polymers have
nanostructure morphology and the morphology of obtained
polymers was significantly affected by variation of amino
acid building blocks. Nanostructured polymers could have
attractive properties that are different from those of the
bulk materials, depending on their shape, size and com-
position (Dai 2004).
Solubility of polymers
Herein with increasing the intensity of peaks, one peak
appeared at 9.66 ppm. If we assume to have phenolic proton
as two end groups, the calculated molecular weights via end
group analysis and using of Eq. 1 for PEI 6a and for PEI 6d
are obtained at 7000 and 6800 g/mol, respectively.
The solubility of the PEIs was experienced in several organic
solvents at a concentration of 0.5 g dL-1 at room tempera-
ture. Bulky t-butyl groups situated on the diol segment sig-
nificantly increase the disorder in polymer chains and cause
less chain packing and facilitate the distribution of solvent
molecules among the macromolecules chains, consequently
increase the solubility of these polymers. Therefore all of the
prepared PEIs were easily soluble in polar aprotic solvents
like DMF, DMSO, NMP, N,N-dimethylacetamide and sul-
furic acid at room temperature and are insoluble in some
other solvents such as chloroform, methylene chloride,
cyclohexane, tetrahydrofuran and water.
MnðpolymerÞ ¼ ½ðI1=I2Þ ꢂ MWðrepeating unitÞꢁ
þ MW end group
ð1Þ
where, I1 is the integral intensity of peak corresponding to
two protons and I2 is the integral intensity of end group.
Morphology of polymer
The crystallinity of PEI 6a was examined by means of
X-ray diffraction scans. As seen in Fig. 2, PEI 6a showed
an amorphous nature. Existence of the non-coplanar and
twisted units into the polymer architecture decreases the
intermolecular forces between the polymer chains, causing
a decrease in the crystallinity.
Thermal behavior of PEIs
The thermal behavior of the resulting PEIs was determined
by means of TGA at a heating rate of 20°C/min, under a
123

2192
S. Mallakpour, S. Soltanian
Fig. 3 FE-SEM micrographs of
a, b PEI 6a, c, d PEI 6b, e, f PEI
6c, g, h PEI 6d
nitrogen atmosphere. Table 3 shows the data for the ther-
mal degradation of the PEIs, including the temperature at
which 5 (T5) and 10% degradation occurs (T10), char yield
at 800°C and also limiting oxygen index (LOI) (Van
Krevelen and Hoftyzer 1976).
thermal and thermo-oxidative stabilities and did not show
considerable weight loss below 380°C and lost 10% of their
total weight between 402 and 413°C in nitrogen atmosphere.
This is in agreement with the wholly aromatic polymers
backbone which increases the stability of the polymers
toward heating. The residual weight for these polymers at
800°C ranged from 19 to 34% that can be attributed to the
participation of thermally stable aromatic rings and imide
group in their repeating unit and the rigid architecture of
these polymers.
LOI ¼ 17:5 þ 0:4 CR
where CR is the char yield.
There was no significant difference in the thermal sta-
bility curves of the polymers. All of them exhibited good
123

Synthesis and properties of optically active nanostructured polymers
2193
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The objective of this study was to synthesize and char-
acterize the novel chiral PEIs derivatives of several nat-
ural amino acids in high purity and good yields from the
condensation reaction of aromatic diol TTMP (5) with
natural amino acid based diacids using TsCl/DMF/Py as
condensing agent. The influence of the reaction time and
molar ratio of TsCl/diacid on the physical properties of
the resulting polymers was investigated and the optimized
conditions were used for the preparation of PEIs. These
polymers displayed good solubility and high thermal
stability with the decomposition temperature being above
380°C as assessed by their TGA curves. Morphology
study of the obtained polymers revealed that all of the
polymers possess nanoarchitecture morphology. Nano-
structured polymers have interesting biomedical, optical,
electronic, and mechanical properties for use in many
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from amino acids, are biodegradable. Because of the
nano-scale morphology of these polymers, they have high
surface area, therefore, activity of the microorganisms on
these polymers could be significantly increased, and thus
biodegradability and biocompatibility of the obtained
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Acknowledgments We wish to express our gratitude to the
Research Affairs Division, Isfahan University of Technology (IUT),
for partial financial support. Further financial support from National
Elite Foundation (NEF) and Center of Excellency in Sensors and
Green Research (IUT) is gratefully acknowledged.
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