Synthesis and properties of radiopaque polymer hydrogels II: Copolymers of 2,4,6-triiodophenyl- or N-(3-carboxy-2,4,6-triiodophenyl)- acrylamide and p-styrene sulfonate

Article abstract of DOI:10.1016/S0022-2860(01)00710-4

In order to pursue a possibility of application of radiopaque polymer hydrogels to vascular embolization, studies were made on synthesis of iodine containing copolyanions and properties of their hydrogels with polycation via formation of polyion complexes (PIC). Acrylamide derivatives having triiodophenyl groups were synthesized and copolymerized with sodium styrene sulfonate (SS) under several conditions. It was found that N-(3-carboxy-2,4,6-triiodophenyl)-acrylamide (CIPA) and 2,4,6-triiodophenylacrylamide (TIPA) monomers are effectively copolymerized with SS, while N-allyl-2,3,5-triiodobenzamide (ATIBA) are hardly copolymerized. Hydrogels were prepared by mixing aqueous solutions of polyanions, i.e. the copolymers (PCIPA and PTIPA) and polyallylamine (PAAn). ¹³C NMR spectra of PCIPA/PAAn and PTIPA/PAAn hydrogels gave peaks for both polyanion and polycation. This means that there remained free anionic and cationic monomer units, which did not form ion pairs because of spatial hindrance. Time dependence of ¹H T₂ showed quick increment and plateau for PSS/PAAn and gradual increments for PCIPA/PAAn. Therefore, PIC containing the radiopaque copolymer retains the hydrogel state for a long time. Embolization was examined by injection of PCIPA/PAAn hydrogels into the vein of a removed porcine kidney as a preliminary test for transcatheter arterial embolization (TAE). X-ray radiograms of the embolized organ were reasonably explained based on the structure and mobility of hydrogels.

Full text of DOI:10.1016/S0022-2860(01)00710-4

Journal of Molecular Structure 602±603 ꢀ2002)17±28
www.elsevier.com/locate/molstruc
Synthesis and properties of radiopaque polymer hydrogels II:
copolymers of 2,4,6-triiodophenyl- or N-ꢀ3-carboxy-2,4,6-
triiodophenyl)- acrylamide and p-styrene sulfonateq
Masahiko Okamuraa, Takeshi Yamanobea, Tomohiro Araia, Hiroki Ueharaa,
a,
Tadashi Komoto , Seiichi Hosoi , Tatsuo Kumazaki
b
c
*
aDepartment of Chemistry, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-376-8515, Japan
bHosoi Clinic, 1-2-5 Hamamatsu-cho, Gunma 376-0007, Japan
cNippon Medical School, 1-1-5 Sendagi, Bunkyo-ku, Tokyo 113-8603, Japan
Received 27 November 2000; accepted 18 December 2000
Abstract
In order to pursue a possibility of application of radiopaque polymer hydrogels to vascular embolization, studies were
made on synthesis of iodine containing copolyanions and properties of their hydrogels with polycation via formation of
polyion complexes ꢀPIC). Acrylamide derivatives having triiodophenyl groups were synthesized and copolymerized with
sodium styrene sulfonate ꢀSS)under several conditions. It was found that N-ꢀ3-carboxy-2,4,6-triiodophenyl)-acrylamide
ꢀCIPA)and 2,4,6-triiodophenylacrylamide ꢀTIPA)monomers are effectively copolymerized with SS, while N-allyl-2,3,5-
triiodobenzamide ꢀATIBA)are hardly copolymerized. Hydrogels were prepared by mixing aqueous solutions of polyanions,
i.e. the copolymers ꢀPCIPA and PTIPA)and polyallylamine ꢀPAAn). 13C NMR spectra of PCIPA/PAAn and PTIPA/PAAn
hydrogels gave peaks for both polyanion and polycation. This means that there remained free anionic and cationic monomer
1
units, which did not form ion pairs because of spatial hindrance. Time dependence of H T2 showed quick increment and
plateau for PSS/PAAn and gradual increments for PCIPA/PAAn. Therefore, PIC containing the radiopaque copolymer
retains the hydrogel state for a long time. Embolization was examined by injection of PCIPA/PAAn hydrogels into the
vein of a removed porcine kidney as a preliminary test for transcatheter arterial embolization ꢀTAE). X-ray radiograms of the
embolized organ were reasonably explained based on the structure and mobility of hydrogels. q 2002 Elsevier Science B.V.
All rights reserved.
Keywords: Polymer hydrogels; Polyion complexes; Transcatheter arterial embolization
1. Introduction
Recently, the transcatheter arterial embolization
ꢀTAE)has attracted attention as a treatment for hepa-
tocellular carcinoma ꢀHCC)[1]. The effectiveness of
TAE treatment has been evaluated by the same
methods for diagnosis of HCC as computed tomo-
graphy ꢀCT), magnetic resonance imaging ꢀMRI),
q
Dedicated to Professor Graham A. Webb on the occasion of his
65th birthday.
* Corresponding author. Tel.: 181-277-30-1330; fax: 181-277-
30-1333.
E-mail address: komoto@chem.gunma-u.ac.jp ꢀT. Komoto).
0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-2860ꢀ01)00710-4

18
M. Okamura et al. / Journal of Molecular Structure 602±603 02002) 17±28
angiography and so on. Of these methods, the angio-
graphy using radiopaque materials has been the
predominant method for HCC. The angiography was
achieved by introducing contrast media to the aimed
vessel via catheter and successive monitoring of the
embolizing state by X-ray radiogram. Requirements
for radiopaque materials for TAE are ꢀ1)no damage to
veins, ꢀ2)¯uidity enough to pass through a catheter,
ꢀ3)embolization of veins for a given period and ꢀ4)
radiopacity to monitor embolization by X-ray radio-
graphy. So far, a mixture of lipiodol which is a
commercial, hydrophobic, low molecular weight
X-ray contrast medium and cisplatin [2], which is a
gelatin sponge combined with lipiodol [3], and a poly-
saccharide solution with anticancer drugs [4] have
principally been used. Microspheres of synthetic poly-
mers with or without radiopacity were also used for
embolization [5±8]. However, for long term analyses
of embolizing states and continual treatments, the
following dif®culties are found; the introduced
contrast medium over¯ows via blood ¯ow to kidney
and then to urinary bladder and ®nally expelled as
urine out of the body and the handling of contrast
media gives great burden to both patients and
doctors.
removed porcine kidney. Although PIC hydrogel
does not ¯ow out through the capillaries to the
peripheral tissues at all, a diffuse X-ray radiogram
taken at 24 h after the injection, where the X-ray
contrast in the region of capillaries disappeared,
showed draining of the contrast medium into the
peripheral tissues.
In the previous paper [10], we have adapted radio-
pacity topolymers byusing triiodophenylacrylamide as
a monomer. Acrylamide derivatives having triiodo-
phenyl and carboxyl groups were synthesized and
copolymerized with sodium styrene sulfonate ꢀSS)
at various molar ratios of initiator to monomer and
temperatures ꢀPCIPA). The hydrogels prepared
from the copolycation obtained under particular
conditions gave high X-ray contrasts of the vein
and remained there, though copolycations with
low molecular weights had a tendency to drain
out through the capillaries to the peripheral tissues.
In this paper, the effects of molecular weight and
iodine content on the capability of embolization and
radiopacity are examined based on the molecular
mobility of water in the hydrogels as studied by
NMR measurements.
We have anticipated that hydrogels of radiopaque
polyion complexes ꢀPIC)formed by mixing aqueous
solutions of polycation and polyanion will be available
by their transcatheter injection into feeding veins
instead of hydrophobic lipiodol for embolic therapy,
in order to embolize the target vessels or organs with
easy handling and control. This is because such polymer
hydrogels are so ¯uid as to ¯ow in intravascular injec-
tion, while they are viscous enough to embolize periph-
eral veins without damaging the tissues.
As a preliminary study [9], we reported on a
polymer hydrogel consisting of synthetic PIC, which
was prepared by mixing a sodium polyꢀstyrene sulfo-
nate)ꢀPSS)in an aqueous commercial X-ray contrast
medium ꢀOmnipaque 350)and that of polyallyla-
mine hydrochloride ꢀPAAn)in the same medium,
where PSS and PAAn are not radiopaque polymers.
The hydrogel thus prepared was transparent and so
viscous like albumen that it has to be picked up with
tweezers. It was found that the hydrogel was suitable
for handling in transcatheter injection and ¯owed
deep into the capillaries of the vein as was evident
from the clearly contrasted X-ray radiogram of a
2. Experimental section
2.1. Materials
All chemicals obtained from commercial sources
ꢀAldrich, Wako, Tokyo Kasei)were used without0
further puri®cation, unless otherwise described. 2,2 -
Azobisisobutyronitrile ꢀAIBN)as an initiator was
recrystallized from methanol solution before use.
Tetrahydrofuran ꢀTHF)used in polymerization was
distilled. Polyallylamine hydrochloride ꢀPAAn)was
also used without puri®cation.
2.1.1. Synthesis of N-03-carboxy-2,4,6-triiodophenyl)
acrylamide 0CIPA)
A mixture of 3-amino-2,4,6-triiodobenzoic acid
ꢀ10 g, 18.4 mmol)and acryloyl chloride ꢀ25 ml,
30.7 mmol)was reacted with stirring at 80 8C for
90 min. After cooling, the resultant precipitate
collected by ®ltration was washed with diethyl ether
and recrystallized from ethanol solution ꢀyield:
70.8%). Chemical shift d ꢀppm)of 1H NMR: 5.80,

M. Okamura et al. / Journal of Molecular Structure 602±603 02002) 17±28
19
6.24 ꢀd,2H,yCH2), 6.41ꢀq,1H,yCH), 8.36 ꢀs,1H,Ar±
H), 10.12 ꢀs,1H,±OH).
2.1.3. Synthesis of N-allyl-2,3,5-triiodobenzamide
0ATIBA)
Thionyl chloride ꢀ1.4 ml, 20 mmol)was added
dropwise to a solution of 2,3,5-triodobenzamide
ꢀ5 g, 10 mmol)in THF ꢀ200 ml)and the mixture
was stirred at 808C for 2 h. Then the reaction mixture
was concentrated under a reduced pressure. After
diluting with THF, triethyl amine solution in THF
was added to the solution to remove HCl as triethyl-
amine hydrochloride. Allylamine ꢀ7.6 ml, 10 mmol)
was added to the ®ltrate, stirring the mixture at
room temperature for 1 h. Product ATIBA thus
obtained was recrystallized from water ꢀyield:
71.6%).
2.1.2. Synthesis of 2,4,6-triiodophenyl acrylate 0TIPA)
Acryloyl chloride ꢀ1.9 ml, 22 mmol)dissolved in
dichloromethane ꢀ70 ml)was added dropwise in
30 min to a mixture of 2,4,6-triiodophenol ꢀ10 g,
19 mmol)and triethylamine ꢀ2.64 ml, 19 mmol)
dissolved in dichloromethane ꢀ100 ml)with stirring
below room temperature and kept for 3 h. The resul-
tant TIPA was collected from the organic and water
phases, extraction being carried out from the latter
phase with cyclohexane. TIPA obtained from the
cyclohexane extract was washed with 10 wt%
aqueous citric acid, 10 wt% aqueous sodium bicarbo-
nate and 10 wt% aqueous sodium chloride for three
times and then dried over anhydrous sodium sulfate
ꢀyield: 73.3%). Chemical shift d ꢀppm)of 1H NMR:
6.28, 6.65 ꢀd,2H, yCH2), 6.47 ꢀq,1H, yCH), 8.20
ꢀs,1H, Ar)
2.2. Copolymerization of radiopaque monomers
0CIPA, TIPA and ATIBA) and sodium styrene
sulfonate 0SS)
Copolymerizations of radiopaque monomers
ꢀCIPA, TIPA and ATIBA)and SS were carried out,
initiated by AIBN in dimethyl sulfoxide ꢀDMSO)with
stirring at 808C for 20 h in a glass ampule sealed in
vacuo. The reaction mixture was poured into acetone/
methanol ꢀ9/1)mixture. The precipitate was ®ltered,
washed with acetone and dried. Copolymers of CIPA,
TIPA and ATIBA with SS are abbreviated as PCIPA,
PTIPA and PATIBA, respectively.
2.3. NMR measurements
1H NMR spectra PSS and PAAn solutions and
hydrogels of PSS and copolymers with PAAn were
measured with a JEOL Alpha 500 NMR spectrometer

20
M. Okamura et al. / Journal of Molecular Structure 602±603 02002) 17±28
using DMSO-d6 either as a solvent or as internal refer-
ence. 13C NMR spectra were recorded with a JEOL
EX-270W NMR at room temperature using deuter-
reactivities of CIPA and TIPA with SS seem to be
similar. Conversion of both polymers was 98%, indi-
cating that these two radiopaque monomers almost
completely copolymerized with SS. Iodine contents
of these polymers are more than 300 mg/g-polymer.
Assuming a radiopaque hydrogel from 30 wt%
polyanionic and polycationic solutions, where the
concentration of the resultant polyanion is 15 wt%,
the iodine content will be 45 mg/g in the gel.
Compared with the iodine concentration of commer-
cial radiopaque compounds, in the range of 24±
500 mg/g, the above estimated concentration of
45 mg/g in solution is in the low level range. There-
fore, these polymers may give radiopacity for TAE.
In comparison with PCIPA and PTIPA, the copo-
lymer composition and iodine content of PATIBA
was 1/27.7 and 61 mg/g-polymer, respectively.
These values are extremely lower than those of
PCIPA-1 and PTIPA. Conversion was also as low as
56%. Inspite of the same copolymerizaiton condition
as for PCIPA-1 and PTIPA, the reactivity of ATIBA
with SS was so low that only SS seems to polymerize.
This is consistent with a fact that PATIBA is very
soluble in water, though the monomer, ATIBA is
hydrophobic.
Although conversion of PCIPA-2 was 69% which
is lower than those of PCIPA-1 and PTIPA, copo-
lymer composition of PCIPA-2 was the same as the
monomer composition in feed ꢀTable 1). Therefore,
CIPA is effectively copolymerized with SS even at a
monomer ratio of one. As a result, this polymer has
the highest iodine content ꢀ508 mg/g)of all the CIPA
polymers, being expected to give good X-ray contrast.
Water solubility of this polymer, however, was lower
than the others, probably due to lower SS content in
the copolymer chain.
1
ated oxide as a solvent. H T2 measurements were
carried out in a JEOL MU-25 NMR at room tempera-
ture by CPMG pulse sequence for the hydrogels as a
function of time after mixing the aqueous solutions of
polycation and polyanion, each concentration being
15 wt %.
2.4. Out¯ow time measurement
Time for out¯ow of the hydrogels through a glass
tube with a size of inner diameter of 3 mm and length
of 60 mm was measured under atmospheric pressure
as a function of aging time to estimate a change in
hydrodynamic properties of the gels in which the
concentration of polyanion and polycation was
15 wt% each.
2.5. Embolization
Prior to embolization experiment, we examined the
texture of the PIC hydrogels prepared by mixing 10 or
30 wt% of aqueous solutions of the copolymer and
polyallylamine hydrochloride ꢀPAAn)in a glass
vessel. Polyanion and polycation were mixed under
a condition that the number of cation is equal to that of
anion. Embolization experiments were made by
injecting the gel prepared from the 30 wt% solutions
through a catheter into the vein of a removed porcine
kidney. X-ray radiograms were taken immediately
and 24 h after the injection.
3. Results and discussion
PCIPA-3 was obtained by copolymerization with
SS at CIPA/SS of 1/4 and AIBN/monomer ratio of
1/1000. Copolymer composition of this polymer was
also comparable to the molar ratio in monomer feed. It
is evident that this polymer has the highest molecular
weight of all the PCIPAs as estimated from relative
viscosity.
It was also found that PTIPA has a copolymer
composition close to that of the charged monomers
and a relatively high viscosity. It is, therefore,
expected that both PCIPA and PTIPA can give PIC
hydrogels with PAAn.
3.1. Molecular properties of radiopaque copolymers
and their hydrogels
In Table 1 are shown conversion of the copolymer-
ization of radiopaque monomer and SS, copolymer
composition as the molar ratio of radiopaque
monomer to SS, and iodine content, solubility in
water and relative viscosity of the copolymer. Copo-
lymer composition of PCIPA-1 and PTIPA was 1/3.2
and 1/3.5, respectively. Since ratios of radiopaque
monomer to SS and AIBN to monomer in feed are
1/4 and 1/100, respectively, for both polymers, the

Table 1
Copolymerization and properties of the resultant copolymers
a
Radiopaque
SS ꢀmmol)AIBN
Conversion ꢀ%)Composition
Iodine content
Water solubilityb
Relative viscosity
Relaxation
time ꢀms)
monomer ꢀmmol)
ꢀmg/g-polymer)
PCIPA-1
PCIPA-2
PCIPA-3
PTIPA
4
8
4
4
4
16
8
1/100
98
69
72
98
56
1/3.2
1/1.0
1/3.4
1/3.5
1/27.7
317
508
306
305
61
A
B
A
B
A
1.38
1.19
3.58
1.58
1.43
1280
614
1/100
1/1000
1/100
1/100
16
16
16
714
1630
978
PATIBA
a
1
Copolymer composition as a mole ratio of radiopaque monomer/SS. Determined by H NMR.
b
Soluble at room temperature ꢀA)and 80 8C ꢀB).

22
M. Okamura et al. / Journal of Molecular Structure 602±603 02002) 17±28
In this ®gure, all the peaks are assigned to PSS and no
signals due to PAAn were found, indicating very low
chain mobilities of PAAn. Since the monomer ratio of
SS/AAn is 4/1, i.e. the amount of AAn is lean in the
PCI, PAAn is thought to be almost incorporated in ion
pairs. On the other hand, excess SS units have enough
mobility to give NMR peaks with intensities similar to
those for PSS solution.
Fig. 2ꢀb)is a 13C NMR spectrum of the PIC gel
prepared from PCIPA-1 and PAAn. In this ®gure,
very weak signals assigned to CIPA units appeared
at about 170±180 ppm ꢀCyO)and 90±100 ppm ꢀC±
I), while SS and AAn peaks are all strong, though they
are boarder than those shown in Figs. 1 and 2ꢀa).
Existence of SS and AAn peaks means that some SS
and AAn units, which do not form ion pairs are so
mobile to give strong NMR peaks. However, the line
widths of the SS and AAn peaks are larger than those
in Figs. 1 and 2ꢀa), suggesting that these SS and AAn
groups are less mobile than those in the solution state.
The low mobility of AAn groups may be accounted
for, by a weak interaction to be formed between the
carboxyl group of CIPA unit and the free amino group
of PAAn. On the other hand, the low mobility of SS
units may be due to a restriction in chain mobility in
their vicinity where ion pairs and/or weak interchain
interactions are formed.
A similar trend was observed for PTIPA PIC gel
ꢀFig. 2ꢀc)). Comparing Fig. 2ꢀb) and ꢀc), peaks in Fig.
2ꢀc)are much broader than those in Fig. 2ꢀb.) The
broad signals of both SS and AAn units in the
PTIPA PIC gel are thought to be due to the nature
of TIPA unit, which has no polar side groups and
accordingly is insoluble in water. Therefore, a hydro-
phobic interaction between spatially close TIPA
groups should decrease the chain mobility, giving
rise to more broadening of the NMR signals.
Fig. 1. 13C NMR spectra of PSS and PAAn in D2O solutions.
3.2. Structure of radiopaque copolymer hydrogels as
measured by 13C NMR
In order to characterize the structure of the PICs,
13C NMR spectra of aqueous solutions of the copoly-
mers and their PIC hydrogels were measured. In Fig. 1
is shown 13C NMR spectra of aqueous PSS and PAAn
solutions ꢀ15 wt%)as reference data prior to mixing.
Peaks for PSS between 120 and 150 ppm are assigned
to phenyl carbons. Methylene and methine carbons of
PSS appeared at about 45 and 40 ppm, respectively.
Peaks at 42, 33 and 29 ppm for PAAn are assigned to
NCH2, CH and CH2 carbons, respectively. Sharp
peaks for all the carbons mean that PSS and PAAn
chains have enough mobility to average out the
dipolar interaction in aqueous solutions.
Fig. 2 shows 13C NMR spectra of PIC hydrogels as
measured soon after mixing the solutions where the
molar ratios of SS/AAn are all 4/1. Fig. 2ꢀa)is a 13C
NMR spectrum of PIC prepared from PSS and PAAn.
Fig. 3 shows 13C NMR spectra of PIC hydrogels as
measured soon after mixing the solutions in which
molar ratios of SS/AAn are all 1/1. Fig. 3ꢀa)is a
spectrum of PIC gel prepared from PSS and PAAn.
Sharp signals for both PSS and PAAn appeared as
compared with that in Fig. 2ꢀa), where the SS/AAn
ratio was 4/1. The appearance of sharp peaks of both
SS and AAn units may indicate that these free units
are rather mobile in the gel state and accordingly that
the fraction of the ion-paired SS and AAn units are
rather small.
Fig. 2. 13C NMR spectra of PIC hydrogels with monomer mole
ratios SS/AAn of 4/1: ꢀa)PSS/PAAn, ꢀb)PCIPA-1/PAAn and ꢀc)
PTIPA/PAAn.

M. Okamura et al. / Journal of Molecular Structure 602±603 02002) 17±28
23
1
Fig. 4. Dependence of H relaxation time T2 for PIC hydrogels on
aging time after preparation. ꢀa)PSS/PAAn, ꢀb)PCIPA-1/PAAn, ꢀc)
PCIPA-2/PAAn and ꢀd)PCIPA-3/PAAn.
Fig. 3. 13C NMR spectra of PIC hydroges with monomer mole ratios
SS/AAn of 1/1: ꢀa)PSS/PAAn, ꢀb)PCIPA-1/PAAn and ꢀc)PTIPA/
PAAn.
1
in relevant to application of the PIC gels to TAE. H
Fig. 3ꢀb)and ꢀc)shows spectra for PCIPA-1/PAAn
and PTIPA/PAAn gels. It is of interest that the relative
peak intensities for PAAn in these gels are higher than
that in Fig. 3ꢀa), while those of PSS are lower. This
may indicate that the mobility of SS units decrease
probably due to hydrophobic interactions of CIPA or
TIPA units in the vicinity of the SS units. It is also
noted that the line width broadening for these gels is a
similar tendency in the case of SS/AAn of 4/1 ꢀFig.
2ꢀb) and ꢀc)).
T2 measurements were carried out to estimate a
change in chain mobility with time after mixing the
polycation and polyanion solutions.
1
Fig. 4 shows a dependence of H T2 for PIC hydro-
gels on aging time after their preparations. T2 for
PSS/PAAn increased rapidly with time and became
almost constant after about 12 h. On the other
hand, T2 increased gradually with aging time for
the other PIC hydrogels. The increament of T2 indi-
cates an increase in mobility of water molecules.
When polycation and polyanion are mixed, the
network gel structure of PIC is thought to grow
with time.
First, the primary complex is formed by ion
pairings between cation and anion units which are
spatially close to each other soon after the mixing.
In this stage, the complex will be formed at the inter-
face of the polycation and polyanion phases which are
thought to be dispersed heterogeneously just after
mixing the solutions. As this structure should be a
metastable state, complex formation gradually
proceeds via mutual diffusion of the polymer chains
through the interface to attain to a thermodynamically
stable state, i.e. the secondary complex formation.
This process may accompany changes in polymer
chain conformation such that neutralization of the
1
3.3. Relaxation time H T2 of hydrogels
When the polycation and polyanion solutions are
mixed, a large fraction of ionic units did not form ion
pairs and these residual, free ionic groups may contri-
bute to swelling in water, though the network
structure of the gel is formed ꢀprimary complex
formation)[11]. It is anticipated that the free SS and
AAn units tend to form ion pairs by their mutual colli-
sion with time due to chain mobility in the gel state
ꢀsecondary complex formation). As the spectra in Fig.
3 show the initial stage of PIC gels, most SS and AAn
units with no ion pairs maintain high chain mobilities.
This should arise from a spatial hindrance of these
units to collide with each other. It is of interest to
pursue the rate of the secondary complex formation

24
M. Okamura et al. / Journal of Molecular Structure 602±603 02002) 17±28
charged SS and AAn units gives rise to a contraction
of the initial, expanded polymer coils.
of 50 mol%)and PCIPA-3 ꢀa high relative viscosity of
3.58)ꢀTable 1), respectively, the PCIPA-1 molecules
should have a less interchain interaction due to repul-
sive forces among SS units and a high molecular chain
mobility. This gives rise to a high molecular mobility
of water occluded in interstices of entangled PCIPA-1
chains and accordingly T2 value as high as 1280 ms.
On the other hand, PCIPA-2 chains seem to have a
high interchain interaction due to a comparable hydro-
phobic interaction between CIPA units ꢀbecause of a
low SS content). This should decrease the chain
mobility and subsequently the mobility of water
occluded therein, giving rise to a low T2 value
ꢀ614 ms). Although the solution viscosity of PCIPA-
2 was lowest of all, the interchain interaction between
CIPA units is thought to play a role in restriction of
chain mobility. The low T2 value for PCIPA-3 can be
interpreted by its high molecular weight as estimated
by the highest solution viscosity. This is because the
higher the chain length is, lower the molecular
mobility is, due to a large amount of crosslinks
which arise from chain entanglements.
T2 value for PSS/PAAn increased up to ca. 12 h and
then leveled off. This indicates that the mutual diffu-
sions of PSS and PAAn chains and maximum ion
pairings were accomplished within 12 h. No further
PIC formation is thought to take place thereafter. On the
other hand, T2 for PCIPA-1, 2 and 3 increased gradually
with time and did not become constant even after 40 h.
The gradual increase of T2 within the time range
measured means that PIC formation proceeded for a
very long time during which conformational changes
of these copolymer chains due to a change in inter-
chain interactions of the CIPA groups took place.
Many polycation ꢀor polyanion)chains should be
mutually interpenetrated or entangled, based on
random coil concept, in such a concentrated solution
before mixing as used in this study. Water molecules
in the concentrated polymer solution are in a low
entropy state, irrespective of their hydration to ionic
groups of polymer chains, as compared with those in
its pure liquid where they have a high mobility. When
ion pairings take place between the oppositely
charged polymer chains and subsequently the chains
are contracted, the interaction between water mole-
cules and polymer chains decreases. This gives rise
to an increase in molecular motion of water and hence
1
an increase in H T2 value [12].
T2 values were much larger for PSS/PAAn and
PCIPA-1/PAAn than for PCIPA-2/PAAn and
PCIPA-3/PAAn. The difference in T2 values between
the two groups was far beyond each T2 change over
the whole range of measuring time. It is worthy to
compare T2 values in these mixtures with those for
the polyanion solutions before mixing. T2 value was
1280, 614 and 714 ms for PCIPA-1, PCIPA-2 and
PCIPA-3 solutions, respectively. These values are
very close to those measured just after mixing ꢀat
time ˆ 0 h in Fig. 4)for the corresponding mixtures.
It can be said, therefore, that the molecular motion of
water is more restricted in the concentrated PCIPA-2
and PCIPA-3 solutions than in PCIPA-1 one.
T2 value for PSS solution itself was 978 ms which is
much lower than 1250 ms for PSS/PAAn just after
mixing ꢀFig. 4ꢀa)). This indicates that the mobility
of water molecules is restricted to some extent in
PSS solution and abruptly increases as soon as the
PSS solution is mixed with PAAn solution. This can
be accounted for by a high rate of ion pair formation
between SS and AAn units in case of PSS/PAAn as
compared with the cases of the copolymers/PAAn, the
SS content being as low as ca. 76 mole% in the copo-
lymers. The relative viscosity of 1.43 in DMSO for
PSS, which is comparable to that for PCIPA-1 may
support a high mobility of PSS chains so that they can
easily undergo their conformational change and diffu-
sion when being mixed with PAAn solution.
3.4. Hydrodynamic properties of hydrogels
In order to examine the hydrodynamic properties of
PCIPA hydrogles, out¯ow time of PIC hydrogels,
where they ¯ow out through a glass tube ꢀ3 mm
diameter and 6 cm long)under atmospheric pressure,
was measured as a function of aging time after
preparation. Fig. 5 shows the time dependence of
out¯ow time for the PIC hydrogels. It is found that
out¯ow time increased with aging time in all the
cases. This indicates that the out¯ow time is consis-
tent with hydrogel viscosity. Namely, the network
structure of PCIPA hydrogel developed with aging
Taking into consideration the molecular properties
of PCIPA-1 with a SS content of ca. 76 mol% and a
low viscosity as compared with PCIPA-2 ꢀSS content

M. Okamura et al. / Journal of Molecular Structure 602±603 02002) 17±28
25
Fig. 5. Time dependence of out¯ow time of PIC hydrogels through a
glass tube with a diameter of 3 mm and a length of 60 mm under
atmospheric pressure: ꢀa)PCIPA-3/PAAn, ꢀb)PCIPA-1/PAAn and
ꢀc)PCIPA-2/PAAn.
time. The out¯ow time at a given aging time was in
the order PCIPA-3=PAAn . PCIPA-1=PAAn .
PCIPA-2=PAAn: This tendency is in accord with the
relative viscosities of the copolymers: PCIPA-3 .
PCIPA-1 . PCIPA-2: This indicates that three-
dimensional networks of polyion complexes devel-
oped with aging time. It is also found that the mobility
of water molecules occluded in the gel networks is not
dependent on the gel viscosity but on the local inter-
chain interactions.
Fig. 6. X-ray radiograms of a removed porcine kidney embolized
with a PIC hydrogel prepared by mixing 30 wt % aqueous solutions
of PCIPA-1 and PAAn: Immediately ꢀa)and 24 h ꢀb)after injection.
As for the ¯uidity of the hydrogels, it is noted that
they did not ¯ow at all through a glass tube with a
diameter less than 1 mm which is much larger than the
diameter of capillaries of veins. This suggests that the
hydrogels prepared in this study do not ¯ow out
through the capillaries of veins but reach there
through a catheter under a moderate pressure. In addi-
tion, polyion complexes did not precipitate over the
whole range of measurement. This behavior means
that the hydrogels from PCIPA copolymers are stable
and suitable for TAE.
ments were prepared by mixing 30 wt % aqueous
solutions of PCIPA-1, 2 and 3 and that of PAAn.
Fig. 6 shows X-ray radiograms of a removed porcine
kidney, into the vein of which a radiopaque hydrogel
prepared from PCIPA-1 and PAAn was injected
through a catheter. In Fig. 6ꢀa), the hydrogel intro-
duced into the vein is clearly seen and a weak,
wide-spread and gradational X-ray absorption
contrast is also seen due to the thickness of the kidney.
As capillaries of the vein are slightly seen in the
periphery of the kidney, the hydrogel seems to be
introduced deep into capillaries. This PIC hydrogel
has enough ¯uidity to be introduced into both thick
branches and capillaries. The X-ray contrast of the
3.5. Embolization by radiopaque hydrogels as studied
by X-ray radiogram
All the hydrogels used for embolization experi-

26
M. Okamura et al. / Journal of Molecular Structure 602±603 02002) 17±28
Fig. 7. X-ray radiograms of a removed porcine kidney embolized with a PIC hydrogel prepared by mixing 30 wt % aqueous solutions of
PCIPA-2 and PAAn: Immediately ꢀa)and 24 h ꢀb)after injection.
thick branches and capillaries are deteriorated 24 h
after injection ꢀFig. 6ꢀb)). The X-ray contrast of the
capillaries completely disappeared, though that of the
thick branches remained. The change in X-ray
contrast with aging time can be accounted for by
draining of radiopaque, low molecular weight copo-
lymers, which were not incorporated via ionic bonds
in the three dimensional network gel, through the
capillaries to the peripheral tissues.
Fig. 7 shows the corresponding X-ray radiograms
for PIC hydrogel prepared from PCIPA-2 and PAAn.
As seen in Fig. 7ꢀa), a clear X-ray contrast was
obtained not only in the thick vein but also in the
capillaries just after injection of the hydrogels,
where a relatively weak, wide-spread, gradational
X-ray contrast due to thickness of the organ is also
seen. Contrast of capillaries is enhanced compared
with Fig. 6ꢀa). As the iodine content of PCIPA-2 is

M. Okamura et al. / Journal of Molecular Structure 602±603 02002) 17±28
27
Fig. 8. X-ray radiograms of a removed porcine kidney embolized with a PIC hydrogel prepared by mixing 30 wt % aqueous solutions of
PCIPA-3 and PAAn: Immediately ꢀa)and 24 h ꢀb)after injection.
500 mg/g-polymer, highest of all the copolymers,
details of capillaries are clearly observed. This result
indicates that the contrast of radiogram increases
with iodine concentration. The X-ray contrast of
both the vein and the capillaries turned obscure to
some extent at 24 h after the injection ꢀFig. 7ꢀb)).
This shows draining of some radiopaque copolymer
molecules through the capillaries to peripheral
tissues, similarly to the case of PCIPA-1/PAAn
hydrogel.
branches but not in the capillaries compared with
those for PCIPA-1 and 2. The very slight X-ray
contrast in the capillaries is due to a low ¯uidity of
the hydrogel which may arise from a dif®culty of
change in gel texture to be deformed into small size
of 5±10 mm. The X-ray contrast was found to be kept
to a large extent 24 h after injection as is evident from
Fig. 8ꢀb). This indicates that most radiopaque PCIPA-
3 remained in the vein in the form of hydrogel, but a
small amount of the copolymer which did not join
three-dimensional network of the gel drained out
through the capillaries. As is anticipated from the
highest molecular weight and the longest out¯ow
time of PCIPA-3, it is said that its three dimensional
Fig. 8 shows the corresponding X-ray radiograms
for PCIPA-3 which has the highest molecular weight.
As is seen in Fig. 8ꢀa), X-ray contrast immediately
after hydrogel injection was clearly seen in thick

28
M. Okamura et al. / Journal of Molecular Structure 602±603 02002) 17±28
gel network with PAAn is well grown to be suitable
for embolization.
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4. Conclusion
In order to design the radiopaque embolizing mate-
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