Synthesis and characterization of radiopaque poly(ether urethane) with iodine-Containing diol as chain extender

Article abstract of DOI:10.1002/pola.24649

Novel radiopaque iodinated poly(ether urethane) (IPEU) was prepared by using iodine-containing diol as chain extender in a normal two-step condensation polymerization process. This new iodine-containing diol was synthesized by iodination of terephthalic a

Full text of DOI:10.1002/pola.24649

Synthesis and Characterization of Radiopaque Poly(ether urethane)
with Iodine-Containing Diol as Chain Extender
WEIQIANG QU,1 WEIJUAN XIA,1 CHAO FENG,1 XINLIN TUO,1 TENG QIU2
1Department of Chemical Engineering, Laboratory for Advanced Materials, Tsinghua University, Beijing 100084,
People’s Republic of China
2College of Materials Science and Engineering, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education,
Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China
Received 9 November 2010; accepted 24 February 2011
DOI: 10.1002/pola.24649
Published online 22 March 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Novel radiopaque iodinated poly(ether urethane)
(IPEU) was prepared by using iodine-containing diol as chain
extender in a normal two-step condensation polymerization
process. This new iodine-containing diol was synthesized by
iodination of terephthalic acid and then reaction with 3-amino-
propanol. The chemical structure of the diol chain extender
and IPEU was characterized, and the basic properties of IPEU
were measured and compared with PEU. X-ray images showed
that 15 wt % iodine-containing IPEUs were highly radiopaque,
and radiopacity did not decrease after 6-week oxidative degra-
dation treatment. Experimental results showed that IPEUs
possessed good thermal stability, favorable mechanical proper-
ties, and noncytotoxicity. These results reveal that it is an
effective route for the synthesis of biological polyurethane with
radiopacity by using iodine-containing diol as chain extender.
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2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
49: 2191–2198, 2011
KEYWORDS: biomaterials; polycondensation; polyurethanes
INTRODUCTION Polyurethane (PU) is an important biomate-
rial used in implantable cardiovascular and orthopedic
devices for the excellent mechanical properties including
flexibility, strength, and wear resistance as well as favorable
biocompatibility.1–5 However, the radiolucent property of
PU limits its further applications particularly in minimally
invasive operation and the noninvasive evaluation of the
implanted material or device. X-radiography is a simple, low
cost technique, which is widely used in clinic operation. As a
consequence, attempt to prepare the radiopaque PU is well
desired.
of the reaction units of diisocyanates and linear difunctional
chain extenders and soft segments of polydiols. As most of
the iodination reaction is based on oxidation mechanism,
incorporation of iodine in isocyanate-containing component
and polydiol is not a feasible choice. Jayakrishnan and co-
workers9,10 once developed a coupling method to attach the
tri- or penta-iodine-containing derivatives to PU based on
the reaction of chloride with the active ANHA groups in PU
backbone. Recently, this research group reported using a io-
dinated chain extender to synthesize radiopaque PU.11 It is
noteworthy that several patents have used bromine-contain-
ing diols as chain extenders to confer some degree of radio-
pacity on PU.12,13 However, iodinated compound has more
advantages than brominated one for X-ray imaging. First, io-
dine has greater mass attenuation coefficient than bromine,
which makes iodine more effective in radiopacity. Second,
the synthesis and the related toxicity of the iodinated aro-
matic molecules have been extensively investigated as mod-
ern X-ray contrast media and intermediates. Consequently,
iodo-substituted polymer would presumably lessen the
unknown risk for patient’s health.
Several approaches have been reported on the radiopaque
PUs. One commonly used method is to blend some radio-
paque additives such as barium sulfate, zirconium dioxide,
or bismuth halides during the PU polymerization proce-
dure.6–8 Most of the commercial radiopaque PUs are pre-
pared in such manner. However, these blends have some in-
herent disadvantages. For example, some additives would
inevitably damage the mechanical properties of bulk PU, and
some salt particles would be leached out of PU matrix and
be absorbed by the body. A promising and feasible alterna-
tive is to incorporate heavy atoms such as iodine into PU
through covalent bond. This method could offer PUs with
radiopacity and maintain the excellent properties of PUs.
However, it is not easy to incorporate iodine into PU in prac-
tical synthesis. Generally, PU is composed of hard segments
In this article, a new iodinated diol of N,N0-bis(3-hydroxy-
propxyl)-2,3,5,6-tetraiodoterephthalamide (HPTDP) was syn-
thesized and used as chain extender to make PU radiopaque.
Several tetraiodoterephthalic acid-based compounds have
been investigated as X-ray contrast and these studies have
Correspondence to: X. Tuo (E-mail: tuoxl@mail.tsinghua.edu.cn)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 2191–2198 (2011) 2011 Wiley Periodicals, Inc.
SYNTHESIS AND CHARACTERIZATION OF IPEU, QU ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthesis of N,N0-bis(3-hydroxypropxyl)-2,3,5,6-tetraiodoterephthalamide.
demonstrated the medical safety of tetraiodoterephthalic
acid-based compounds.14,15 In this sense, using tetraiodo-
terephthalic acid-based diol as radiopaque medium in PU
is a secure choice to patient’s health. The whole synthesis
procedure was characterized in detail, and the radiopacity of
the iodine-containing PU were investigated briefly. Further,
the biostability of the iodinated PU was briefly studied.
Experimental results show that the iodinated PU is a promis-
ing material as X-ray opacifier with excellent mechanical
properties and biostability.
nol (Rizhao Lideshi Chemical Co.), terephthalic acid, iodine,
sodium bisulfite, chloroform, triethylamine, thionyl chloride
and other materials (Tianjin Kermel Chemical Reagent Co.)
were used as received.
Preparation of N,N0-Bis(3-hydroxypropxyl)-2,3,5,6-
tetraiodoterephthalamide
HPTDP was synthesized as follows (Scheme 1): 2,3,5,6-tet-
raiodobenzen-1,4-dioic acid was prepared from terephthalic
acid by iodination, using 50% oleum as solvent, then
acylated by thionyl chloride in the presence of DMF to
obtain 2,3,5,6-tetraiodobenzene-1,4-dioyl dichloride,16 which
reacted with 3-aminopropanol using DMAC as solvent and
triethylamine as acid binding agent to get the solid product
HPTDP. 1H nuclear magnetic resonance (NMR) (The spec-
trum was taken in DMSO-d6, Fig. 1.): d ¼ 8.41 (NH), 4.42
(OH), 3.51 (CH2O), 3.23 (CH2N), 1.70 (CH2). IR (thin film):
3420, 3290 cmꢁ1 (NH, OH), 2925 cmꢁ1 (CH2), 1639 cmꢁ1
(C¼¼O), 1560 cmꢁ1 (Ar), 1261 cmꢁ1 (CAN), 1043 cmꢁ1
(CH2-OH).
EXPERIMENTAL
Materials
4,40-Diphenylmethane diisocyanate (MDI; Sigma-Aldrich, St.
Louis, MO) was purified by hot filtration; poly(tetramethy-
lene glycol) (PTMG, Mn ¼ 650, BASF China Co.) was dried
for 24 h at 80ꢀC under vacuum prior to use; tetrahydrofuran
(THF), dimethylformamide (DMF), and dimethylacetamide
(DMAC) (Tianjin Kermel Chemical Reagent Co.) were vacuum
distilled and kept over 4 Å molecular sieves; 3-aminopropa-
FIGURE
1
1H NMR spectrum of
HPTDP.
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SCHEME 2 Synthesis of iodine-
containing poly(ether urethanes)
(IPEUs).
Preparation of Iodine-Containing Poly(ether urethanes)
The product iodine-containing poly(ether urethane) (IPEUs)
based on PTMG as soft segment and MDI/HPTDP as hard
segment were synthesized using normal two-step process
(Scheme 2). The MDI and PTMG were taken in a three-
necked round-bottom flask. The reaction took place at 70–
80 ꢀC over 2.5 h in an inert atmosphere of nitrogen with a
mechanical stirrer. Then HPTDP dissolved in anhydrous
DMAC was added. The reaction mixture was stirred about 9–
12 min at 90–100 ꢀC. DMAC (15–20 mL) was added in
batches during the process. The polymer was precipitated
from methanol, washed with water. Then the viscous mixture
was cast onto a Teflon plate and vacuum dried at 75 ꢀC for
6–8 h and stored at room temperature in dark. IR (thin film,
Fig. 2): 3277 cmꢁ1 (NH), 2939cmꢁ1 (CH2), 1728 cmꢁ1
(C¼¼O), 1632 cmꢁ1 (NHAC¼¼O), 1099 cmꢁ1 (CAOAC),
769 cmꢁ1 (O¼¼CAO).
Characterization
NMR spectra were measured on a JEOL JNM-ECA600 300
MHz instrument with tetramethylsilane as an internal stand-
ard at the ambient temperature. Fourier transform infrared
spectra (FTIR) were traced with a Nicolet 560-IR spectrome-
ter. The molecular weights and their distributions were
determined by using a gel permeation chromatography
(GPC) apparatus at room temperature with THF as eluent (1
mL/min). The instrument was equipped with a refractive
index detector (Wyatt Optilab rEX) and fitted with a PLgel 5
lm mixed-D column. The column was calibrated with linear
polystyrene standards. Overall surface morphology and
energy dispersive spectrometer (EDS) analysis were charac-
terized by emission scanning electron microscope (SEM).
Samples of SEM were placed on brass disks and were sput-
ter coated with a gold target under vacuum in an argon
atmosphere. Coated specimens were examined on a JSM-
6301F at an accelerating voltage of 5 kV. Differential scan-
ning calorimetry (DSC) analyses were performed on a DSC-
2910 (TA instruments, New Castle, DE) in nitrogen
In a similar manner, PEU was also prepared from the same
reactants in the molar ratio of MDI/PTMG ¼ 1/1, without
chain extender for comparative studies.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 2 13C NMR spectrum of
HPTDP.
atmosphere with a heating rate of 10 K/min. Thermogravi-
metric analyses (TGA) were performed on a Q5000 (TA
Instruments) in nitrogen atmosphere with a heating rate of
10 K/min. Elemental analysis (EA) of HPTDP was performed
by Service Institute of Chemistry, Chinese Academy of Scien-
ces, Beijing, China.
Cytotoxicity Studies
Cytotoxic evaluation of IPEU was carried out by the direct
contact and test on extract assay with a monolayer of
L929 mouse fiberblast cells as mentioned elsewhere. Cyto-
toxicity of IPEU was quantitatively assessed further by MTT
(3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide)
assay.17
Mechanical Properties
Radiopaque Properties
The mechanical properties of IPEUs were measured on a
Universal Testing Machine (GT-TS-2000, Taiwan) according to
GB528-76 with a speed of 50 mm/min at 25 ꢀC to obtain
the tensile strength and the breaking elongation. The sam-
ples were casted from THF solutions and shaped to dumb-
bell products. The thickness and width of the specimens
were 3.0 and 3.2 mm, respectively. The length of the sample
between the two pneumatic grips of Testing Machine was
12 mm. Five measurements were conducted for each sample,
and the results were averaged to obtain a mean value.
X-radiographs were obtained using a standard clinical X-ray
machine (General Electric, XR/A) equipped with 2.5 alumi-
num filtration set at 60 kV with 10 mA current for 0.2 s. The
square IPEU samples with 2 mm thickness were cast from
30% THF solutions. PEU was used as control following the
same method. The aluminum wedge with 2 mm thickness
was kept as standard for visual comparison.
RESULTS AND DISCUSSION
Preparation of IPEUs
In Vitro Degradation
Terephthalic acid was iodinated according to the procedure
reported elsewhere.18 The method is based on electrophilic
substitution of protons by iodide ions. In the typical experi-
ment, when 4 g of terephthalic acid was reacted with 2.5
equiv. of iodine, the yield of the product was 14.5 g, which
was the expected yield if tetra-iodination took place. NMR
analysis confirmed that the reaction resulted in tetra-iodina-
tion of terephthalic acid. Further, the possible steric hin-
drance due to the presence of bulky iodine atoms in the ring
might have limited the extent of iodination of the ring. EA of
HPTDP showed about 65% of iodine content in the product,
which corresponded to tetra-iodination. The melting point
The IPEUs were dissolved in THF and cast to a thickness of
ꢂ0.5 mm on Teflon substrates. Films were peeled from the
substrates after being dried in a vacuum oven at 60 ꢀC and
were cut into pieces with a size ꢂ1 ꢃ 5 cm2. H2O2 (10%)
and 0.05 M CoCl2 were used as oxidants for our oxidative
pretreatment. IPEU samples were immersed at 37 ꢀC for
6 weeks. The solution was changed every 3 or 4 days. After
the treatment, the specimens were removed, rinsed
thoroughly in deionized water, and vacuum dried prior to
examination. The transparency of the samples did not show
apparent change visually, and the surface morphology was
observed by SEM.
ꢀ
(obtained from DSC) of HPTDP was found to be 327 C.
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stoichiometry of the reactants used for reaction and the
abbreviation of the resulting polymers were listed in Table 1.
It could be observed that the molecular weight of IPEU
slightly decreased with the addition of HPTDP, which is
attributed to the rigidity of HPTDP in chemical structure.
Thermal Properties of IPEUs
Thermal characterization of PUs was studied by DSC and
TGA. TGA curves of IPEUs showed two decomposition stages
(Fig. 4). The first decomposition stage between 200 and
305 ꢀC is a depolycondensation process that is associated
with the urethane hard segment. The second decomposition
stage between 305 and 450 ꢀC is the decomposition of the
soft segment by polyol depolycondensation (PTMG). Further-
more, segmented PU in the decomposition stage depends on
the hard-segment content.19 Figure 4 reveals that the TGA
curves in the second decomposition stage shifted to higher
weight remained region as hard-segment content was
increased. PEU was stable up to 343ꢀC and 50% weight loss
FIGURE 3 FTIR spectrum of IPEU-4.
ꢀ
1H NMR spectrum of HPTDP is shown in Figure 1. The sec-
ondary amine group and hydroxyl group are indentified at d
¼ 8.41 and 4.22 ppm, respectively. Three characteristic shifts
for CH2 protons appeared between d ¼ 1.70 and 3.51 ppm.
The singlet signal indicated the absence of protons on the
benzene ring as a result of the successful substitution of the
protons by iodine, which was also confirmed by 13C NMR
(Fig. 2). Thus, it can be safely stated that the reaction used
here is leading to pure product isolation in excellent yield.
occurred at 386 C. Hence it was clear that the introduction
of iodine-containing chain extender accelerated the advent of
decomposition of IPEUs. However, IPEUs still possess good
thermal properties as only 5% weight loss occurred at
ꢀ
300 C, which is very meaningful for guiding the processing
of the polymers.
The results of DSC studies are presented in Figure 5. PUs
are known to exhibit two glass transitions in the hard and
soft segment domains, respectively.20 However, for all the
samples, only one glass transition was detected. The results
suggested that hard and soft segments in PEUs were well
compatible in the amorphous phase. Compared with the
glass transition temperature (Tg) of PEU, Tgs of IPEUs shifted
to higher temperature region, as exhibited by DSC curves.
MDI as hard segments usually formed densely packed regu-
lar structures close to a crystalline order.21 Whereas, DSC
curves did not display distinct cold crystallization and melt-
ing peaks, which should result from the disturbance of chain
regularity of MDI molecules owing to the introduction of
HPTDP as chain extender. This disturbance also suggests
that IPEUs did not form microphase separation.
To achieve the goal of obtaining radiopaque PU with desira-
ble properties for medical applications, the strategy adopted
here was to incorporate the radiopacifying molecule as the
chain extender in the PEU synthesis. We expected the result-
ant PEU to be sufficiently radiopaque with HPTDP as chain
extender as it contained four iodine atoms per molecule.
IPEU was synthesized using MDI, PTMG and HPTDP as chain
extender. The FTIR spectrum of IPEU is shown in Figure 3,
which demonstrates the presence of the expected functional
groups. The FTIR spectrum of IPEU shows ANH peak at
3277 cmꢁ1, C¼¼O peak at 1728 cmꢁ1, CAOAC peak at 1099
cmꢁ1, respectively. The peak at 1632 cmꢁ1 related to the
NHAC¼¼O stretching proves the successful incorporation of
HPTDP to the PU backbone during copolymerization. IPEUs
were obtained in 90% yield, typical for PU synthesis. HPTDP
was incorporated into the prepolymer in the second stage of
the two-step process. To study the influence of the addition
of iodine-containing diol, four IPEUs were synthesized. The
Mechanical Properties of IPEUs
Mechanical properties of polymer are also important with
respect to practical applications. The tensile properties of
the IPEU samples are summarized in Table 2. There were
many factors which can affect the physical behaviors of the
synthesized PEUs, such as molecular weight, chemical
TABLE 1 Composition and Molecular Weight of Resulting IPEUs
Molar Ratio
Hard
Feeded Chain
Extender
(wt %)
of MDI/Chain
Extender/PTMG
Segment
(wt %)
Sample
Mn (ꢃ104)
PDI
IPEU-1
IPEU-2
IPEU-3
IPEU-4
1.1:0.1:1
1.2:0.2:1
1.3:0.3:1
1.4:0.4:1
35.24
38.51
46.30
50.53
7.81
14.83
19.43
23.87
4.61 6 2.77
3.74 6 2.25
3.19 6 1.92
3.38 6 2.03
1.9 6 0.2
2.0 6 0.2
2.0 6 0.2
2.1 6 0.2
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 The Tensile Mechanical Properties of the Samples
Breaking Strength
(MPa)
Breaking Elongation
(%)
Sample
IPEU-1
IPEU-2
IPEU-3
IPEU-4
28.8 6 0.5
26.6 6 1.2
24.0 6 1.0
25.1 6 1.8
1670 6 25
1334 6 32
1450 6 62
1150 6 18
sharpness of the image was compared with aluminum wedge
of 2 mm thickness (Fig. 7). It can be seen from the X-ray
images that the images due to IPEUs were much sharper
than the image of PEU. Increasing the iodine content from
IPEU-1 to IPEU-4 clearly leads to improved contrast. Com-
pared with aluminum wedge of same thickness, IPEU-4
showed image darker than that of aluminum wedge, which
demonstrated good radiopaque property.
FIGURE 4 Thermogravimetric traces of IPEUs and PEU.
Biodegradation Research of IPEUs
composition, and glass transition temperature. A trend of
decreasing of breaking strength was observed with decrease
of molecular weight, and the elongation at break exhibited
reduction with the increase of HPTDP, which should result
from the huge bulk of iodine-hindered molecular movement.
As IPEUs obtain good radiopaque efficiency, desired thermal
properties and mechanical properties, it is expected that the
radiopacity acting under oxidative degradation treatment
would stabilize. Also, this is the first time that radiopaque
performance research of iodine-containing PU after biodegra-
dation by this method is reported.
Cytotoxicity of IPEUs
To confirm the nontoxic nature of IPEU, a preliminary cyto-
toxicity evaluation using L929 mouse fibroblast cells was
carried out. Neither IPEU nor its extract induced any mor-
phological changes to the cells confirming its noncytotoxic
response to fibroblast cells at an extract radio of 0.2 g/mL.
The morphology of the cells growing on the surface (scored
as zero) is depicted in Figure 6.
To study radiopacity change of IPEU after degradation, an in
vitro oxidative treatment was involved. This degradation
experiment used 10% H2O2 and 0.05 M CoCl2 as oxidants,
and the IPEU films were immersed at 37ꢀC. After 6 weeks,
the IPEU films were removed from the treatment solution
and vacuum dried for 24 h prior to analysis. In vivo, the oxi-
dative H2O2/CoCl2 treatment accurately reproduced the 1-
year effect within 24 days.22 Other studies also have shown
the use of the H2O2/CoCl2 solution for simulating the
Radiopaque Properties of IPEUs
IPEUs and PEU were subjected to X-radiographic examina-
tion to compare their visibility in X-radiogram. Here, IPEUs
and PEU were made into discs of 2 mm thickness, and the
FIGURE 6 Fibroblast cells around IPEU-4. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.
com.]
FIGURE 5 DSC traces of IPEUs and PEU.
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FIGURE 7 X-ray images of IPEU-1 (A), IPEU-2 (B), IPEU-3 (C),
IPEU-4 (D), aluminum wedge (E), and PEU (F) discs.
FIGURE 8 Histogram of molecular weight of IPEUs and IPEUs
degradation. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
chemical and physical characteristics of in vivo degradation
of PEU and for predicting long-term stability.23–25 Moreover,
in vitro studies provided the necessary data to elucidate the
mechanism of soft segment oxidation of PEU.
vivo for long periods have similar change of molecular
weight and polydispersity. Compared to IPEU-1, both the
number and weight average molar mass were less for IPEU-
2, IPEU-3, and IPEU-4. This indicated that, the chain extend-
ing ability of HPTDP was slightly hampered, possibly due to
the presence of bulky iodine atoms in HPTDP.
The IPEU samples before and after oxidative treatment were
measured by GPC chromatograms to detect the variety of the
molecular weight and polydispersity. The visualized change
of molecular weights of IPEUs was also shown in Figure 8.
In contrast to corresponding IPEU, IPEU degradation showed
significantly decreased molecular weight and increased poly-
dispersity. Ebert et al.26 also reported that PEU implanted in
The SEM micrographs of the IPEU and IPEU degradation
in vitro are shown in Figure 9. IPEU films displayed smooth
FIGURE 9 SEM micrographs of:
(a) and (c) IPEUs and (b) and (d)
IPEUs degradation.
SYNTHESIS AND CHARACTERIZATION OF IPEU, QU ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
5 Pierce, B. F.; Brown, A. H.; Sheares, V. V. Macromolecules
and neat surface [Fig. 9(a)]. However, the overall surface of
the IPEU degradation appears to be rougher than that of the
IPEU substrate [Fig. 9(b)]. After 6 weeks, limited amount of
large pits were observed on unstabilized IPEU degradation
specimens. Previously, pitting of the surface was attributed
to extraction of low-molecular weight degradation products
that resulted from chain scission.27
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The presence of iodine in the PEU could be elucidated by
EDS. In the EDS spectrum, both IPEU and IPEU degradation
showed peaks corresponding to carbon, oxygen, and iodine
atoms. Thus the presence of iodine resulting from incorpora-
tion of HPTDP in the PEU and conservation of iodine after
oxidative degradation treatment were confirmed. Comparing
the X-ray absorption images of the IPEUs and IPEUs degra-
dation it can be seen that IPEUs degradation do not show
clearly visible change of sharpness, from which can be con-
cluded that the radiopaque properties of IPEUs maintain
good stability after oxidative degradation treatment in vitro
for 6 weeks.
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2097–107.
Novel PEU with radiopaque property was synthesized by a
normal two-step process using MDI, PTMG, and chain
extender HPTDP. FTIR spectra and GPC indicated that prod-
uct IPEU was successfully synthesized. Moreover IPEU exhib-
ited good radiopacity compared to the same thick aluminum
wedge, and radiopacity did not decrease after oxidative
degradation treatment. Also, it showed good thermal stability
and mechanical properties. The cytotoxicity evaluation
confirmed the nontoxicity of IPEU. The purpose of this work
is to explore a new route for the synthesis of biological PU
with radiopaque properties.
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