Poly(para -phenylene ethynylene)s functionalized with Gd(III) chelates as potential MRI contrast agents

Article abstract of DOI:10.1139/V10-156

A poly(para-phenylene ethynylene) with water-solubilizing groups and Gd(III) chelates conjugated to the polymer backbone was designed and synthesized. Pre-and post-polymerization functionalization approaches were explored and the pre-polymerization approa

Full text of DOI:10.1139/V10-156

47
Poly(para-phenylene ethynylene)s functionalized
with Gd(III) chelates as potential MRI contrast
agents
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Katelyn M. Atkins, Francisco M. Martınez, Ali Nazemi, Timothy J. Scholl, and
Elizabeth R. Gillies
Abstract: A poly(para-phenylene ethynylene) with water-solubilizing groups and Gd(III) chelates conjugated to the poly-
mer backbone was designed and synthesized. Pre- and post-polymerization functionalization approaches were explored and
the pre-polymerization approach for the introduction of the Gd(III) chelate was found to be more successful. The UV–vis
absorption and fluorescence emission properties of the protected polymers were characterized and were found to be consis-
tent with the results expected for this class of polymers. Removal of the protecting groups followed by chelation of Gd(III)
led to a water-dispersible polymer. Relaxivity measurements were performed on this polymer with the aim of evaluating
its potential as a new MRI contrast agent, and an r1 of 1.37 L mmol–1 s–1 at 310 K and 20 MHz was determined. These re-
sults, along with dynamic light scattering analyses, suggested that the polymers formed micrometre-sized assemblies in
aqueous solution. Although the relaxivity was relatively modest, these results provide important insights into the assembly
properties of this new class of polymers and into the design criteria for future agents.
Key words: poly(para-phenylene ethynylene), magnetic resonance imaging, contrast agent.
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Resume : On a developpe et synthetise un poly(para-phenyleneethylene) portant des groupes solubles dans l’eau et des
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chelates de Gd(III) conjugues au squelette du polymere. On a explore des approches de fonctionnalisations avant et apres
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les polymerisations et on a trouve que l’approche prepolymerisation est meilleure pour l’introduction du chelate de Gd(III).
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On a caracterise les proprietes d’absorption UV–visible et d’emission de fluorescence des polymeres proteges et on a
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trouve qu’ils sont en accord avec les resultats attendus pour cette classe de polymeres. L’enlevement des groupes protec-
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teurs, suivi par la chelation du Gd(III) conduit a l’obtention d’un polymere qui peut etre disperse dans l’eau. On a effectue
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des mesures de relaxation sur ce polymere dans le but d’evaluer son potentiel comme un nouvel agent de contraste en ima-
–1 –1
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gerie de resonance magnetique (IRM) et on a determine que sa valeur de r1 est egale a 1,37 L mmol s , a 310 K et a
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20 MHz. Ces resultats, avec les analyses de dispersion dynamique de la lumiere, suggerent qu’en solution aqueuse les po-
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lymeres forment des assemblees de la taille du micron. Meme si la relaxation est relativement faible, ces resultats fournis-
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sent connaissances importantes dans les proprietes d’assemblage de cette nouvelle classe de polymere et dans les criteres
de conceptualisation des futurs agents.
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Mots-cles : poly(para-phenyleneethylene), imagerie de resonance magnetique (IRM), agent de contraste.
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[Traduit par la Redaction]
molecule agents have enabled significant advancements in
MRI, they possess several limitations.
Introduction
Over the past few decades, magnetic resonance imaging
(MRI) has emerged as a powerful noninvasive diagnostic
tool in medicine.1–3 It provides excellent spatial resolution
and soft tissue contrast for conveying anatomical structure.
In cases where the inherent soft tissue contrast is insufficient
to distinguish diseased tissues from normal tissues, contrast
agents are routinely used.2,3 These agents are typically stable
chelates of the lanthanide ion Gd(III). While these small-
First, the relaxivities of the commercially available agents
are only a few percent of the theoretically predicted values.4
The low sensitivity of these agents imparts a requirement for
gram-scale doses to obtain the required contrast. This raises
concerns regarding the toxicity of these agents, particularly
in patients with kidney disease.5 Furthermore, there is in-
creasing interest in the development of contrast agents tar-
Received 20 August 2010. Accepted 21 October 2010. Published on the NRC Research Press Web site at canjchem.nrc.ca on
21 December 2010.
K.M. Atkins, A. Nazemi, and E.R. Gillies.1 Department of Chemistry, The University of Western Ontario, 1151 Richmond Street,
London, ON N6A 5B7, Canada.
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F.M. Martınez and T.J. Scholl. Department of Medical Biophysics, The Schulich School of Medicine and Dentistry, The University of
Western Ontario, 1151 Richmond Street, London, ON N6A 5C1, Canada.
1Corresponding author (e-mail: egillie@uwo.ca).
Can. J. Chem. 89: 47–56 (2011)
doi:10.1139/V10-156
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Can. J. Chem. Vol. 89, 2011
geted to specific diseases via binding to receptors in the tar-
get tissues. Considering a stoichiometric 1:1 binding of the
agent to the target, the target must be present at a concentra-
tion of approximately 125 mmol L–1.3 Unfortunately, many
targets are not present at this relatively high concentration
in vivo.
process that would potentially release toxic free Gd(III) in
vivo.39 In addition, although the conjugation of the modified
DTPA chelate to the phenols of the polymer backbone
would be expected to provide the highest relaxivity, this
conjugation was not efficient in preliminary work. There-
fore, an ethyl spacer was incorporated between the polymer
backbone and the chelate to introduce a more nucleophilic
amine group for the conjugation to the acid-functionalized
DTPA.
To address these limitations, significant efforts have been
aimed at enhancing the relaxivities of Gd(III)-based contrast
agents. It has been predicted by Solomon–Bloembergen–
Morgan theory that increases in the rotational correlation
times of the current agents can result in significantly increased
relaxivities.6,7 As macromolecules have slower tumbling rates
in solution, much research has involved the conjugation of
Gd(III) chelates such as diethylenetriaminepentaacetic acid
(DTPA) and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic
acid (DOTA) to macromolecules. Early efforts involving
conventional linear macromolecules led to relatively disap-
pointing results.8–14 However, the conjugation of these
chelates to macromolecules such as proteins15,16 and den-
drimers17–25 provided much greater enhancements in relaxivity.
These results highlight the importance of macromolecular
structure and architecture, as flexible backbones allow the
Gd(III) chelates to tumble in solution in a manner similar to
the small molecules, while those structures possessing well-
defined and rigidified conformations provide longer rota-
tional correlation times and thus higher relaxivities.
With the importance of molecular rigidity in mind, our
group was interested in investigating poly(para-phenylene
ethynylene) (PPE) as a backbone for the conjugation of
Gd(III) chelates. This polymer backbone, comprising alter-
nating phenyl and alkyne moieties, exists in an elongated
linear conformation, with backbone conjugation leading to
high levels of fluorescence.26–31 This fluorescence and its
sensitivity to its environment have been exploited for the de-
velopment and commercialization of sensors.29–31 More re-
cently, water-soluble analogues of these polymers have
been developed and have been investigated as sensors for
DNA,32–34 proteins,35 and bacteria.36,37 However, to the best
of our knowledge PPEs have never been investigated as
MRI contrast agents. We proposed that the rigidified, elon-
gated conformation of PPE would lead to significant en-
hancements in relaxivity of conjugated Gd(III) chelates.
Furthermore, the fluorescence may enable the optical detection
of these agents in cells and tissues, thus facilitating biologi-
cal studies of the agents and potentially providing a dual
modality imaging probe, perhaps even with sensor capabil-
ities. Described here is the synthesis of a new PPE with con-
jugated Gd(III) chelates and the characterization of its
optical properties and relaxivity.
Although the chelates were expected to impart some aque-
ous solubility to the PPEs, water-solubilizing oligo(ethylene
glycol) moieties were also incorporated in the design. PPEs,
like other conjugated polymers having a rigid and hydropho-
bic backbone, have inherently very poor water solubility that
results in aggregation and subsequent fluorescence quench-
ing.40–42 Carboxylic acid terminated oligo(ethylene glycol)
moieties were chosen because they have been previously
demonstrated by Wosnick et al. to impart aqueous solubility
to PPEs.37 Indeed, preliminary work in our laboratory dem-
onstrated that neutral tri(ethylene glycol) monomethyl ether
derivatives were insufficient to impart aqueous solubility.
PPEs are typically prepared from diiodophenylene and di-
alkyne monomers via Sonogashira–Hagihara copolymeriza-
tions.43,44 Based on this strategy, the proposed design
incorporated the modified DTPA chelates into the diiodo-
phenylene monomer 2 and the tetra(ethylene glycol) moi-
eties into the dialkyne monomer 3 (Fig. 1). In addition, it
was of interest to explore the postpolymerization functional-
ization of the PPE backbone as an alternative means of pre-
paring the target polymers. This would potentially provide a
versatile synthetic method that would allow easy incorpora-
tion of targeting ligands in future generations of the agent.
Therefore, polymer 4 was also designed as a target, which
would be prepared from monomers 3 and 5.
Monomer syntheses
The synthesis of the dialkyne monomer 3 was performed
following a protocol similar to that reported by Wosnick
et al. for slightly different oligo(ethylene glycol) lengths
(di(ethylene glycol) and penta(ethylene glycol)).37 As shown
in Scheme 1, 2,5-diiodo-1,4-hydroquinone (6)30 was reacted
with the tosylate 7,45 providing 8. The alkyne moieties were
then introduced by a Sonogashira reaction with trimethylsi-
lylacetylene (TMSA) in the presence of catalytic copper(I)
iodide and bis(triphenylphosphine)palladium(II) chloride. Fi-
nally, the trimethylsilyl protecting groups were removed by
treatment of 9 with tetrabutylammonium fluoride (TBAF),
providing 3.
The synthesis of monomer 2 began with the reaction of
the tosylate 1046 with 2,5-diiodo-1,4-hydroquinone to give
the diazide 11 (Scheme 2). The azides were then reduced to
amines under Staudinger conditions, providing 12. Finally,
the amines were conjugated to the previously reported
DTPA derivative 1338 using dicyclohexylcarbodiimide (DCC)
in the presence of 4-(dimethylamino)pyridine (DMAP) and 4-
(dimethylamino)pyridinium p-toluenesulfonate (DPTS) to
provide monomer 2. For the preparation of monomer 5,
tert-butyl carbamate (Boc) protected 2-bromoethylamine
(14)47 was reacted with 2,5-diiodo-1,4-hydroquinone as
shown in Scheme 3.
Results and discussion
Polymer design
The structure of the target copolymer 1 is shown in
Fig. 1. A modified DTPA38 was selected as the Gd(III) che-
late in this polymer, as the extra carboxylate incorporated in
the aspartic acid linker allows 8 chelation sites of the ligand
to be preserved for binding to the metal ion. Direct use of
one of the carboxylates of DTPA for conjugation to the poly-
mer backbone would lead to decreased complex stability,
thus increasing the chances of transmetallation by Zn(II), a
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Atkins et al.
49
Fig. 1. Design of two alternative routes to a PPE with Gd(III) chelates and water-solubilizing side chains.
Scheme 1. Synthesis of monomer 3.
Polymer syntheses and characterization
(SEC) (Fig. 2), 15 was found to have a weight-average
molecular weight (Mw) of 31 300 g mol–1 and a polydisper-
sity index (PDI) of 2.6 as determined by a conventional cal-
ibration relative to polystyrene standards. Following
determination of the refractive index increment (dn/dc) for
Monomers 2 and 3 were copolymerized under Sonogashira–
Hagihara conditions43,44 to provide the protected polymer 15,
which was purified by dialysis in N,N-dimethylformamide
(DMF) (Scheme 4). Using size exclusion chromatography
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50
Can. J. Chem. Vol. 89, 2011
Scheme 2. Synthesis of monomer 2.
As shown in Fig. 3, both polymers 15 and 4 exhibited
red-shifted absorption maxima (lmax) at 430 nm and
406 nm, respectively, relative to that of the dialkyne mono-
mer 3 at 327 nm. This bathochromic shift can be attributed
to the significantly increased conjugation of the polymers
relative to the monomer and is typical of PPEs.26,29 Con-
comitantly, the emission maxima also shifted to 465 nm
and 460 nm for polymers 15 and 4, respectively, relative to
the monomer’s emission lmax of 372 nm (Fig. 4). These
emission maxima are similar to those of other previously re-
ported PPEs.37,48
Polymer deprotection and Gd(III) chelation
The tert-butyl ester groups on both the tetra(ethylene gly-
col) and DTPA derivatives of polymer 15 were easily re-
moved by treatment of the polymer with 1:1 trifluoroacetic
acid (TFA):CH2Cl2 to provide polymer 16 (Scheme 5). The
resulting polymer was then dissolved in pure water and the
pH was adjusted to 7.4 prior to the introduction of Gd(III) in
the form of GdCl3Á6H2O, giving the target polymer 1. Fol-
lowing purification by extensive dialysis, the Gd(III) content
was measured by inductively coupled plasma mass spec-
trometry. Based on these measurements, Gd(III) was suc-
cessfully introduced into each chelation site and no extra
Gd(III) was detected.
Scheme 3. Synthesis of monomer 5.
Polymer 4 could also be fully deprotected by treatment
with 1:1 TFA/CH2Cl2 to give 17 (Scheme 6). The next step
towards the target was the introduction of the Gd(III) che-
lates by reaction of 17 with an N-hydroxysuccinimidyl ester
derivative of 13 or the commercially available p-isothiocya-
natobenzyl derivative of DTPA. Unfortunately, although 17
was soluble in organic solvents such as DMF and CHCl3
when the amines were present as their TFA salts, all at-
tempts to neutralize or basify the solution or to dissolve the
polymer in aqueous buffer for the conjugation led to rapid
polymer precipitation. The use of other polar solvents such
as dimethylsulfoxide or methanol did not result in enhanced
solubility, so this polymer could not be further functional-
ized.
Fig. 2. Size exclusion chromatography traces for polymers 15 and
4.
Relaxivity measurements
The longitudinal relaxivity (r1) of polymer
1 in
100 mmol L–1 pH 7.4 phosphate buffer was measured at
298 and 310 K between 0.01 and 35 MHz using a field cy-
cling relaxometer. The results are shown in Fig. 5. When
calculated on a per Gd(III) ion basis, the results corre-
sponded to an r1 of 1.37 0.03 L mmol–1 s–1 at 310 K and
20 MHz. This result was unexpectedly low relative to the
value of 4.1 L mmol–1 s–1 measured for Gd-DTPA under
the same conditions.15 In addition, although 1 appeared
‘‘soluble’’ upon first inspection, it was noted that after pro-
longed standing some material settled from solution, result-
ing in even lower relaxivities. This material could be
redispersed by mixing, resulting in recovery of the relaxivity
to the original value. This behaviour, along with a complete
lack of polymer fluorescence, suggested that the polymer
may have assembled into micrometre-sized particles in solu-
tion.
the polymer, its absolute molecular weight was also deter-
mined by multi-angle light scattering (MALS). This meas-
urement provided an Mw of 40 500 g mol–1 and a PDI of
1.9. The values obtained by the conventional calibration
and light scattering were therefore in relatively good agree-
ment.
Monomers 3 and 5 were polymerized under the same con-
ditions described above, leading to polymer 4, which was
also purified by dialysis in DMF. This polymer was found
by SEC (Fig. 2) to have an Mw of 2640 g mol–1 and a PDI
of 2.4. The lower Mw of this polymer in comparison with 15
can be attributed at least in part to the lower average mono-
mer molecular weight but was also likely due to a lower de-
gree of polymerization for unknown reasons.
The suspension of polymer 1 was investigated by dy-
namic light scattering. As shown in Fig. 6, micrometre-sized
particles were indeed detected. Optical microscopy was also
Published by NRC Research Press

Atkins et al.
51
Scheme 4. Synthesis of polymer 15.
Fig. 3. UV–vis absorption spectra of polymers 15 and 4 and
monomer 3 (in CH3OH).
Fig. 4. Fluorescence emission spectra of polymers 15 and 4 and
monomer 3 (in CH3OH).
mmol–1 s–1, while those on the interior were much lower.
For example, based on a mean particle size of 1.5 mm and
assuming that water can freely penetrate the particle to a
depth of 10 nm, the relaxivity on a per Gd(III) ion basis
would be approximately 35 L mmol–1 s–1, much higher than
that for Gd-DTPA. However, this estimation is clearly very
dependent on the value used for the depth of water penetra-
tion and the result ranges from 115 L mmol–1 s–1 for a pen-
etration depth of 3 nm to 3.9 L mmol–1 s–1, very close to the
relaxivity of Gd-DTPA, if a penetration depth of 100 nm is
assumed (see Supplementary data).
used to verify the size and revealed that the particles were
solid and approximately spherical (Fig. 7). By extensive
centrifugation it was possible to remove essentially all of
the polymer from the suspension. Combined, these results
suggest that the relaxivity exhibited by polymer 1 arose pri-
marily through the interaction of water molecules with the
Gd(III) ions at the surface of the particles, while the Gd(III)
ions on the interior would have very poor access to water, a
property that is known to substantially decrease the relaxiv-
ities of Gd(III) agents.49–51 As the r1 value was calculated
based on the total concentration of Gd(III) ions in the sam-
ple, it is likely that the relaxivities of the complexes on or
near the particle surface were much higher than 1.37 L
Overall, the significant aggregation of polymer 1 was sur-
prising given the solubilizing carboxylic acid terminated
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52
Can. J. Chem. Vol. 89, 2011
Scheme 5. Synthesis of the target polymer 1.
Scheme 6. Deprotection of polymer 4.
rigidity of the polymer backbone. Two synthetic routes to
the target were explored, one involving the polymerization
of a DTPA-functionalized monomer and the other involving
a postpolymerization modification. While the postpolymeri-
zation modification was not successful owing to poor poly-
mer solubility, the former route successfully led to a
polymer having two Gd(III) complexes on alternating phe-
nyl rings. Unexpectedly, this polymer assembled into micro-
particles in aqueous solution with a relaxivity of 1.37 L
mmol–1 s–1 on a per Gd(III) ion basis. This relaxivity is
likely an average of Gd(III) complexes on the microparticle
surface with high relaxivities and complexes on the particle
interior with low relaxivities due to poor water accessibility.
tetra(ethylene glycol) units, the inherently water-soluble
Gd(III) complexes, and the previous reports of water-soluble
PPEs bearing similar densities of charged moieties.32,34,37
Considering the multiple competing effects that the incorpo-
ration of the polymeric complexes into the micrometre-sized
aggregates would have on the relaxivity, it is not possible to
confirm definitively that increased relaxivity would result
from the rigidified polymer backbone. Thus, the unexpected
assembly of polymer 1 introduces a new form of Gd(III)
where each particle carries many Gd(III) ions along conju-
gated polymer backbones. However, to further investigate
the effects of the PPE backbone on relaxivity and to exploit
its fluorescence properties it will be necessary to design poly-
mers that do not aggregate in water.
Experimental
Conclusions
General procedures and materials
All chemicals were purchased from commercial suppliers
and used without further purification unless otherwise noted.
Anhydrous N,N-dimethylformamide (DMF), tetrahydrofuran
In summary, a PPE with conjugated water-solubilizing
groups and DTPA derivatives was designed with the aim of
obtaining Gd(III) complexes with high relaxivity due to the
Published by NRC Research Press

Atkins et al.
53
Fig. 5. Longitudinal relaxivity (r1) of polymer 1 in pH 7.4 buffer as
a function of field strength and temperature.
Fig. 7. Optical microscopy image of assemblies formed by polymer
1.
ternational QM-4 SE spectrofluorometer. Size exclusion
chromatography (SEC) was performed in THF using a
Waters 515 HPLC pump, Wyatt Optilabrex RI and mini-
DAWN-TREOS detectors, and a ResiPore (300 mm Â
7.5 mm) column from Polymer Laboratories. Column cali-
bration was performed using polystyrene standards from
Polymer Laboratories. Dynamic light scattering was per-
formed on a Zetasizer Nano ZS instrument from Malvern
Instruments. Optical microscopy was performed using an
Olympus model IX71S8F-3 microscope equipped with a
20Â (NA 0.50) objective. Dialyses were performed using
Fig. 6. Size distribution of aggregates formed by polymer 1 in
pH 7.4 phosphate buffer, as measured by dynamic light scattering.
Spectra/Por regenerated cellulose membranes with
a
molecular weight cutoff of 3500 Da. The inductively
coupled plasma mass spectrometry (ICP-MS) analysis was
performed at the Environmental Analytical Laboratories of
the Saskatchewan Research Council. Relaxation rate meas-
urements were performed on a Stelar Spinmaster FFC2000
1T C/DC relaxometer. The concentration of the polymer
was 0.17 mg mL–1 in a 100 mmol L–1 pH 7.4 phosphate buf-
fer solution.
Synthesis of compound 8
To a flame-dried flask were added 2,5-diiodo-1,4-hydro-
quinone (0.64 g, 1.8 mmol, 1.0 equiv.), the tosylate 7
(1.9 g, 4.4 mmol, 2.5 equiv.), potassium iodide (0.15 g,
0.88 mmol, 0.50 equiv.), potassium carbonate (1.5 g,
11 mmol, 6.0 equiv.), and 18-crown-6 (0.92 g, 3.5 mmol,
2.0 equiv.). DMF (30 mL) was added and the resulting solu-
tion was heated to 100 8C for 48 h under an argon atmos-
phere. The solvent was removed in vacuo and the resulting
residue was dissolved in a small amount of CH2Cl2, concen-
trated to dryness on a small amount of silica gel, and puri-
fied by column chromatography on silica gel (20:80 ethyl
acetate – hexanes eluent) to yield compound 8 (0.71 g,
46%). IR (cm–1, thin film from CH2Cl2): 3088, 2967, 2866,
(THF), and toluene were obtained from a solvent purifica-
tion system. CH2Cl2 and NEt3 were distilled over CaH2. Ul-
trapure water was obtained from a Barnstead EASYpure II
system. Column chromatography was performed using silica
1
gel (0.063–0.200 mm particle size, 70–230 mesh). H NMR
spectra were obtained at 400 MHz and 13C NMR data were
obtained at 100 MHz. Chemical shifts are reported in ppm
and are calibrated against residual solvent signals of CDCl3
(d 7.26, 77.2) or D2O (d 4.75). High-resolution mass spec-
trometry (HRMS) was performed using a Micromass LCT
(electrospray time-of-flight (ES+)) mass spectrometer or a
Finnigan MAT 8200 mass spectrometer in time-of-flight
ES+ mode. UV–vis absorption spectroscopy was performed
on a Varian Cary 300 Bio UV–vis spectrophotometer. Emis-
sion spectra were obtaining using a Photon Technology In-
1
1726. H NMR (400 MHz, CDCl3, d): 1.44 (s, 18 H), 2.49
(t, J = 6.63 Hz, 4 H), 3.57–3.80 (m, 20 H), 3.83–3.91 (m, 4
H), 4.04–4.12 (m, 4 H), 7.23 (s, 2 H). 13C NMR (100 MHz,
CDCl3, d): 170.8, 153.1, 123.4, 86.4, 80.4, 71.1, 70.7, 70.5,
70.4, 70.2, 69.6, 66.9, 36.24, 28.1. HRMS calcd. for [M]+
(C32H52I2O12): 882.1548; found: 882.1546.
Published by NRC Research Press

54
Can. J. Chem. Vol. 89, 2011
Synthesis of compound 9
Synthesis of compound 12
Compound 8 (0.69 g, 0.78 mmol, 1.0 equiv.) was added
to a flame-dried flask and put under argon atmosphere. Dry
THF (6 mL) was added, followed by trimethylsilylacetylene
(0.76 g, 7.8 mmol, 10 equiv.) and triethylamine (5.4 mL,
39 mmol, 50 equiv.). Copper(I) iodide (4.5 mg, 23 mmol,
0.029 equiv.) and bis(triphenylphosphine)palladium(II)
chloride (11 mg, 16 mmol, 0.020 equiv.) were added to the
solution under a flow of argon. The mixture was stirred at
room temperature for 48 h, and then the solvent was re-
moved in vacuo. The resulting residue was purified by col-
umn chromatography (10:90 ethyl acetate – hexanes to 40:60
ethyl acetate – hexanes eluent gradient) to afford compound
9 (0.54 g, 96%). IR (cm–1, thin film from CH2Cl2): 3010,
The diazide 11 (0.25 g, 0.58 mmol, 1.0 equiv.) and triphenyl-
phosphine (0.76 g, 2.9 mmol, 5.0 equiv.) were dissolved in
THF (10 mL). Water (5 mL) was added and the resulting
mixture was heated at 60 8C for 2 h. The solvent was then
removed in vacuo and the residue was purified by column
chromatography (90:10 dichloromethane–methanol to 100%
methanol eluent gradient) to yield compound 12 (0.19 g,
88%). IR (cm–1, KBr pellet): 3360, 3038, 2925, 2869, 1693,
1
1526. H NMR (400 MHz, CDCl3, d): 1.51 (br s, 4 H), 3.10
(t, J = 4.88 Hz, 4 H), 3.99 (t, J = 4.88 Hz, 4 H), 7.21 (s, 2
H). 13C NMR (100 MHz, CDCl3, d): 152.8, 123.5, 86.3,
69.8, 45.3. HRMS calcd. for [M]+ (C10H14I2N2O2):
447.9145; found: 447.9158.
1
2954, 2867, 2151, 1729, 1495. H NMR (400 MHz, CDCl3,
d): 0.17 (s, 18 H), 1.37 (s, 18 H), 2.42 (t, J = 6.64 Hz, 4 H),
3.46–3.75 (m, 20 H), 3.80 (t, J = 4.88 Hz, 4 H), 4.05 (t, J =
4.88 Hz, 4 H), 6.85 (s, 2 H). 13C NMR (100 MHz, CDCl3,
d): 170.8, 153.8, 117.7, 114.1, 100.8, 100.3, 80.4, 71.1,
70.7, 70.5, 70.3, 69.6, 69.4, 66.8, 36.2, 28.0, –0.1. HRMS
calcd. for [M + Na]+ (C42H70O12NaSi2): 845.4304; found:
845.4329.
Synthesis of monomer 2
To a flame-dried flask were added the diamine 12
(0.038 g, 0.10 mmol, 1.0 equiv.), DTPA derivative 13
(0.18 g, 0.25 mmol, 2.5 equiv.), DMAP (0.012 g,
0.10 mmol, 1.0 equiv.), and DPTS (0.029 g, 0.10 mmol, 1.0
equiv.). The flask was placed under nitrogen and dry
CH2Cl2 (5 mL) was added. Once the reagents had dissolved,
DCC (0.12 g, 0.60 mmol, 6.0 equiv.) was added and the re-
action mixture was stirred for 3 h at room temperature. The
reaction mixture was then filtered through cotton to remove
the dicyclohexylurea by-product and the solvent was re-
moved in vacuo. The resulting material was purified by col-
umn chromatography (20:80 ethyl acetate – cyclohexane
eluent) to afford compound 2 (0.074 g, 40%). IR (cm–1, thin
film from CH2Cl2): 3310, 3060, 2974, 2925, 1729, 1663,
Synthesis of monomer 3
To a solution of compound 9 (0.51 g, 0.71 mmol, 1.0
equiv.) in methanol (4 mL) was added 1 mol L–1 tetrabutyl-
ammonium fluoride in THF (1.8 mL, 2.5 equiv.). The result-
ing solution was stirred for 12 h, and then the solvent was
removed in vacuo and the resulting residue was purified by
column chromatography (50:50 ethyl acetate – hexanes to
80:20 ethyl acetate – hexanes eluent gradient) to afford
monomer 3 (0.36 g, 87%). UV–vis absorption (CH3OH)
lmax: 327 nm. Emission lmax: 372 nm. IR (cm–1, thin film
from CH2Cl2): 3242, 3010, 2972, 2866, 2092, 1727, 1495.
1H NMR (400 MHz, CDCl3, d): 1.42 (s, 18 H), 2.48 (t, J =
6.64 Hz, 4 H), 3.34 (s, 2 H), 3.51–3.77 (m, 20 H), 3.84 (t, J =
5.08 Hz, 4 H), 4.12 (t, J = 4.88 Hz, 4 H), 6.97 (s, 2 H). 13C
NMR (100 MHz, CDCl3, d): 170.9, 154.0, 118.2, 113.5,
82.8, 80.4, 79.5, 71.0, 70.6, 70.5, 70.4, 69.6, 69.5, 66.8,
36.2, 28.1. HRMS calcd. for [M]+ (C36H54O12): 678.3615;
found: 678.3605.
1
1535, 1455. H NMR (400 MHz, CDCl3, d): 1.36–1.56 (m,
90 H), 2.55 (br s, 2 H), 2.72 (br s, 2 H), 2.80 (br s, 16 H),
3.38–3.47 (m, 16 H), 3.61–3.81 (m, 4 H), 3.88 (br s, 2 H),
3.94–4.14 (m, 4 H), 7.23 (s, 2 H), 7.83 (br s, 2 H). 13C
NMR (100 MHz, CDCl3, d): 178.4, 171.2, 170.4, 152.8,
123.2, 86.3, 81.5, 81.0, 68.9, 60.9, 55.8, 53.1, 50.1, 41.9,
29.6, 28.21, 28.17. HRMS calcd. for [M
(C82H141I2N8O24): 1875.8148; found: 1875.8074.
+
H]+
Synthesis of monomer 5
To a flame-dried flask was added 2,5-diiodo-1,4-hydro-
quinone (0.56 g, 1.5 mmol, 1.0 equiv.), bromide 14 (0.86 g,
3.8 mmol, 2.5 equiv.), potassium iodide (0.13 g, 0.76 mmol,
0.50 equiv.), potassium carbonate (1.3 g, 9.2 mmol, 6.0
equiv.), and 18-crown-6 (0.81 g, 3.1 mmol, 2.0 equiv.).
DMF (30 mL) was added and the resulting solution was
heated at 100 8C for 24 h under an argon atmosphere. The
solvent was removed in vacuo and the resulting residue was
dissolved in a small amount of dichloromethane, concen-
trated to dryness on a small amount of silica gel, and puri-
fied by column chromatography on silica gel (10:90 ethyl
acetate – hexanes eluent) to yield compound 5 (0.39 g,
40%). IR (cm–1, thin film from CH2Cl2): 3356, 3052, 2974,
Synthesis of compound 11
To
a flame-dried flask were added 2,5-diiodo-1,4-
hydroquinone (3.3 g, 9.0 mmol, 1.0 equiv.), the tosylate 10
(5.1 g, 23 mmol, 2.5 equiv.), potassium iodide (0.60 g,
3.6 mmol, 0.40 equiv.), potassium carbonate (5.0 g,
36 mmol, 4.0 equiv.), and 18-crown-6 (2.4 g, 9.2 mmol, 1.0
equiv.). DMF (30 mL) was added and the resulting solution
was heated at 100 8C for 48 h under an argon atmosphere.
The solvent was removed in vacuo and the resulting residue
was dissolved in a small amount of dichloromethane, con-
centrated to dryness on a small amount of silica gel, and
purified by column chromatography (10:90 ethyl
acetate – hexanes to 30:70 ethyl acetate – hexanes eluent gra-
dient) to yield compound 11 (1.3 g, 32%). IR (cm–1, KBr
1
1688, 1532. H NMR (400 MHz, CDCl3, d): 1.47 (s, 18 H),
1
3.56 (dt, J = 5.08, 5.07 Hz, 4 H), 4.01 (t, J = 5.07 Hz, 4 H),
5.08 (br s, 2 H), 7.19 (s, 2 H). 13C NMR (100 MHz, CDCl3,
d): 164.6, 152.77, 123.2, 86.5, 71.5, 69.9, 40.8, 28.42.
HRMS calcd. for [M]+ (C20H30I2N2O6): 648.0193; found:
648.0176.
pellet): 3043, 2920, 2867, 2106, 1692. H NMR (400 MHz,
CDCl3, d): 3.65 (t, J = 5.08 Hz, 4 H), 4.11 (t, J = 5.08 Hz, 4
H), 7.23 (s, 2 H). 13C NMR (100 MHz, CDCl3, d): 153.1,
123.7, 85.9, 67.8, 49.2. HRMS calcd. for [M]+
(C10H12I2N6O2): 499.8955; found: 499.8964.
Published by NRC Research Press

Atkins et al.
55
Synthesis of polymer 15
of the paramagnetic Gd. ICP-MS: mass of polymer ana-
lyzed: 0.2 mg; mass of Gd(III) expected: 35 mg; mass of
Gd(III) found: 34.8 mg.
Monomer 2 (61 mg, 33 mmol, 1.0 equiv.) and monomer 3
(19 mg, 33 mmol, 1.0 equiv.) were added to a flame-dried
two-neck round-bottom flask. The monomers were put under
a nitrogen atmosphere and dry toluene (2 mL) was added,
followed by dry NEt3 (0.23 mL, 1.6 mmol). To this solution
were added bis(triphenylphosphine)palladium(II) chloride
(1.0 mg, 1.4 mmol, 0.040 equiv.) and copper(I) iodide
(1.0 mg, 5.2 mmol, 0.16 equiv.). The reaction mixture was
heated at 50 8C for 48 h. The solvent was removed in vacuo
and the resulting residue was dissolved in DMF (2 mL).
This solution was then dialyzed against DMF for 24 h to
yield polymer 27 (71 mg, quantitative yield). UV–vis ab-
sorption (CH3OH) lmax: 430 nm. Emission lmax: 465 nm.
1H NMR (400 MHz, CDCl3, d): 1.23 (s, 18 H), 1.42 (s, 90
H), 2.29–2.46 (m, 10 H), 2.69–2.94 (m, 16 H), 3.06–4.15
(m, 52 H), 7.00 (br s, 2 H), 7.42 (br s, 2 H). SEC: Mw (con-
ventional calibration) = 31 300 g mol–1, PDI = 2.6; Mw
(MALS; dn/dc = 0.138) = 40 500 g mol–1, PDI = 1.9.
Deprotection of polymer 4
In a flame-dried flask under a nitrogen atmosphere, poly-
mer 4 (0.28 g, 0.29 mmol, 1.0 equiv.) was dissolved in di-
chloromethane (1 mL) and TFA (1 mL) was added. The
resulting solution was stirred for 2 h at room temperature
and then the solvent was removed in vacuo and the resulting
residue was dissolved in DMF (2 mL). The solution was
then dialyzed against DMF for 24 h to yield the deprotected
1
polymer 17 (0.118 g, 62%). H NMR (400 MHz, D2O, d)
2.44 (br s, 4 H), 3.10–4.33 (m, 36 H), 7.01 (br s, 4 H).
Supplementary data
Supplementary data for this article are available on the
journal Web site (canjchem.nrc.ca).
Acknowledgements
Synthesis of polymer 4
We thank the Natural Sciences and Engineering Research
Council of Canada and the Ontario Institute for Cancer Re-
search for funding. The Lagugne-Labarthet group is thanked
Monomer 3 (0.37 g, 0.63 mmol, 1.0 equiv.) and monomer
5 (0.41 g, 0.63 mmol, 1.0 equiv.) were added to a flame-
dried two-neck round-bottom flask. The monomers were put
under a nitrogen atmosphere and dry toluene (5 mL) was
added, followed by dry NEt3 (4.4 mL, 32 mmol). To this
solution were added bis(triphenylphosphine)palladium(II)
chloride (8.0 mg, 13 mmol, 0.020 equiv.) and copper(I)
iodide (4.0 mg, 19 mmol, 0.030 equiv.). The reaction mix-
ture was heated at 50 8C for 48 h. The solvent was then re-
moved in vacuo and the resulting residue was dissolved in
DMF (2 mL). This solution was then dialyzed against DMF
for 24 h to yield polymer 4 (0.29 g, 47%). UV–vis absorp-
´
for assistance with optical microscopy.
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