Living ring-opening metathesis-polymerization synthesis and redox-sensing properties of norbornene polymers and copolymers containing ferrocenyl and tetraethylene glycol groups

Article abstract of DOI:10.1021/om5006897

The controlled synthesis of monodisperse, redox-active metallopolymers and their redox properties and functions, including robust electrode derivatization and sensing, remains a challenge. Here a series of polynorbornene homopolymers and block copolymers containing side-chain amidoferrocenyl groups and tetraethylene glycol linkers were prepared via living ring-opening metathesis polymerization initiated by Grubbs' third-generation catalyst (1). Their molecular weights were determined using MALDI-TOF mass spectra, size exclusion chromatography (SEC), end-group analysis, and the empirical Bard-Anson electrochemical equation. All polymerizations followed a living and controlled manner, and the number of amidoferrocenyl units varied from 5 to 332. These homopolymers and block copolymers were successfully used to prepare modified Pt electrodes that showed excellent stability. The modified Pt electrodes show excellent qualitative sensing of ATP^2- anions, in particular those prepared with the block copolymers. The quantitative recognition and titration of [n-Bu₄N]₂[ATP] was carried out using the CH ₂Cl₂ solution of the homopolymers, showing that two amidoferrocenyl groups of the homopolymers interacted with each ATP^2- molecule. This stoichiometry led us to propose the H-bonding modes in the supramolecular polymeric network.

Full text of DOI:10.1021/om5006897

Article
pubs.acs.org/Organometallics
Living Ring-Opening Metathesis−Polymerization Synthesis and
Redox-Sensing Properties of Norbornene Polymers and Copolymers
Containing Ferrocenyl and Tetraethylene Glycol Groups
Haibin Gu,†,§ Amalia Rapakousiou,† Patricia Castel,† Nicolas Guidolin,‡ Noel Pinaud,† Jaime Ruiz,†
̈
and Didier Astruc*,†
†ISM, UMR CNRS No. 5255, University of Bordeaux, 33405 Talence Cedex, France
‡LCPO, UMR CNRS No. 5629, University of Bordeaux, 33607 Pessac Cedex, France
S
* Supporting Information
ABSTRACT: The controlled synthesis of monodisperse, redox-active
metallopolymers and their redox properties and functions, including
robust electrode derivatization and sensing, remains a challenge. Here a
series of polynorbornene homopolymers and block copolymers
containing side-chain amidoferrocenyl groups and tetraethylene glycol
linkers were prepared via living ring-opening metathesis polymerization
initiated by Grubbs’ third-generation catalyst (1). Their molecular
weights were determined using MALDI-TOF mass spectra, size exclusion
chromatography (SEC), end-group analysis, and the empirical Bard−
Anson electrochemical equation. All polymerizations followed a living
and controlled manner, and the number of amidoferrocenyl units varied
from 5 to 332. These homopolymers and block copolymers were
successfully used to prepare modified Pt electrodes that showed excellent stability. The modified Pt electrodes show excellent
qualitative sensing of ATP2− anions, in particular those prepared with the block copolymers. The quantitative recognition and
titration of [n-Bu4N]2[ATP] was carried out using the CH2Cl2 solution of the homopolymers, showing that two amidoferrocenyl
groups of the homopolymers interacted with each ATP2− molecule. This stoichiometry led us to propose the H-bonding modes
in the supramolecular polymeric network.
especially well-defined polymers and block copolymers
synthesized by controlled and living polymerization such as
living anionic polymerization (LAP)39 and ring-opening
metathesis polymerization (ROMP),40 as well as controlled
and living radical polymerization (CRP) techniques41,42
including atom transfer radical polymerization (ATRP),43−45
reversible addition−fragmentation chain transfer polymer-
ization (RAFT),46 and nitroxide-mediated polymerization
(NMP).47 These techniques allow the preparation of polymers
with predetermined molecular weight, low polydispersity, high
functionality, and diverse architectures.12,13
Ring-opening metathesis polymerization (ROMP), a varia-
tion of the olefin metathesis reaction, has emerged as a
particularly powerful method for synthesizing polymers with
tunable sizes, shapes, and functions.48 It has found a
tremendous utility for the synthesis of materials having specific
biological, electronic, and mechanical properties. In 1992, the
living ROMP was first applied to prepare well-defined side
chain ferrocene containing polymers and block copolymers by
Schrock and co-workers, who used the molybdenum-based
metathesis catalyst [Mo(CH-t-Bu)(NAr)(O-t-Bu)2] (Figure
1. INTRODUCTION
The past several decades have witnessed the rapid development
of metallocene-containing macromolecules, especially with
ferrocenyl groups,1−23 owing to their multielectron redox
properties and wide applications such as catalysts,24 bio-
sensors,25 virus-like receptors,26 models of molecular bat-
teries,27 colorimetric sensors,28 etc. Among the polymers, there
are two major classes of materials: (i) main chain ferrocene
containing polymers in which the ferrocenyl group is an integral
part of the polymer backbone29 and (ii) side chain ferrocene
containing polymers in which the ferrocenyl moiety is a
pendant group.12,13 For the side chain ferrocene containing
polymers, early studies focused mainly on vinylferrocene and
ferrocene containing acrylate and methacrylate that were
polymerized by conventional techniques such as free radical,
cationic, and anionic polymerization. The polymers that were
prepared using these methods often had low molecular weight
(<10000) and lacked control of the molecular weight and
molecular weight distribution.30−38 Therefore, the synthetic
challenges have halted further interest in the exploration of side
chain ferrocene containing polymers prepared by such
conventional polymerization techniques.
Recently, significant attention has been paid again to the first
originally developed side chain ferrocene containing polymers,
Received: July 1, 2014
Published: August 5, 2014
© 2014 American Chemical Society
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Figure 1. Catalysts successively used for ROMP syntheses of ferrocenyl-containing polymers since 1992 (from left to right).
Scheme 1. Synthesis of Amidoferrocenyl-Containing Homopolymers 6 by ROMP
Scheme 2. Synthesis of Amidoferrocenyl-Containing Block Copolymers 10 by ROMP
1).49−51 Since then, the groups of Mirkin,52−57 Abd-El-
Aziz,58−66 and Luh67−76 prepared a series of side chain
ferrocene containing polynorbornene homopolymers and
block copolymers by ROMP. The most frequently used
catalysts were ruthenium-based Grubbs first- and second-
generation catalysts (Figure 1).77 Furthermore, although most
of the obtained polymers showed low polydispersity, they are
often oligomers or polymers with a relatively small number of
pendant ferrocenyl units (no more than 30). Up to now, only
Tew and co-workers78 used Grubbs’ third-generation catalyst
(1), shown in Figure 1, a very active catalyst that has a much
faster initiation (by at least 3 orders of magnitude) than
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Grubbs’ first- and second-generation catalysts.79,80 Tew’s group
prepared a series of metal-containing block−random copoly-
mers composed of an alkyl-functionalized homo block (C16)
and a random block of cobalt carbonyl (alkyne) units (Co) and
ferrocenyl-functionalized (Fe) units via ROMP. These
copolymers showed excellent monodispersities (PDI < 1.1)
and had the largest theoretical number of ferrocene units of 75.
Therefore, these successful results obtained with alkylferrocenyl
units opened the route to more work involving functional
ferrocenyl units and large polymers and the exploration of their
properties and applications.
10.7 mmol). The mixture was stirred overnight under a nitrogen
atmosphere at room temperature and then washed with saturated
NaHCO3 solution (1 × 100 mL) and distilled water (3 × 100 mL).
The organic solution was dried over anhydrous sodium sulfate and
filtered, and the solvent was removed in vacuo. The product was
purified by column chromatography with DCM/methanol (1% →
20%) as the eluent and obtained as a brown sticky oil. Yield: 0.234 g,
1
71.8%. H NMR of 5 (300 MHz, CDCl3): δppm 1.20 (d, J = 9.9 Hz,
1H, CH2-bridge), 1.32 (d, J = 9.9 Hz, 1H, CH2-bridge), 2.52 (d, J =
0.9 Hz, 2H, CO-CH), 3.09 (t, J = 3.1 Hz, 2H, CH-CH), 3.41−3.55
(m, 16H, 4 × CH2CH2), 4.05 (s, 5H, free Cp), 4.18 (t, J = 3.4 Hz, 2H,
sub. Cp), 4.61 (t, J = 3.8 Hz, 2H, sub. Cp), 6.12 (t, J = 3.6 Hz, 2H,
CHCH), 6.59 (t, J = 9.9 Hz, 1H, NHCO). 13C NMR of 5 (50 MHz,
CDCl3): δppm 177.841 (CON), 170.25 (CONH), 137.71 (CHCH),
70.375, 70.24, 70.11, 69.975, 69.745, 69.65, 68.20
(−OCH2CH2OCH2CH2OCH2−, sub. Cp and free Cp), 66.76
(-CH2NH), 47.68 (CO-CH), 45.14 (CH-CH), 42.615 (CH2-
bridge), 39.20 (CH2−NCO), 37.665 (−CH2CH2-NCO). MS (ESI,
m/z): calcd for C28H34N2O6Fe, 550; found, 573.2 (M + Na+).
2.4. General Procedure for the Synthesis of Polymeric N-[3-
(3′,6′,9′-Trioxaundecyl-11′-ferroceneformamido)]-cis-5-nor-
bornene-exo-2,3-dicarboximide (6) via ROMP. The desired
amount of 1 was placed in a small Schlenk flask, flushed with
nitrogen, and dissolved in a minimum amount of dry DCM. A known
amount of monomer 5 in dry DCM (1 mL per 100 mg of monomer 5)
was added to the catalyst solution under a nitrogen atmosphere with
vigorous stirring. The reaction mixture was stirred vigorously for 1 h
and then quenched with 0.2 mL of ethyl vinyl ether (EVE). The yellow
solid polymers 6 were purified by precipitating in methanol five times
In this work, the very active Grubbs’ third-generation ROMP
catalyst 1 is used as the initiator (Figure 1).
We present the syntheses and some applications of side chain
amidoferrocenyl containing homopolymers (Scheme 1) and
block copolymers (Scheme 2) by controlled and living ROMP.
Tetraethylene glycol (TEG) was chosen as the linker between
the norbornene moiety and amidoferrocenyl units to improve
the solubility of macromolecules81,82 and their biocompatibility
that also involves enhanced permeation and retention
effects.82,83 The molecular weights of these new polymers
have been well characterized by end-group analysis, MALDI-
TOF mass spectra, size exclusion chromatography (SEC), and
the Bard−Anson electrochemical method.84,85 These homo-
polymers and block copolymers showed an excellent potential
in electrode modification resulting from the large polymer sizes
and in electrochemical sensing of the ATP2− anion provided by
the presence of the amido group on the ferrocenyl moiety that
forms efficient hydrogen bonding with oxoanions.86−88
1
and dried in vacuo until constant weight. H NMR of 6 (300 MHz,
CDCl3): δppm 7.23−7.44 (m, phenyl and CDCl3), 6.65 (broad, 1H,
NHCO), 5.75 and 5.53 (double broad, 2H, CHCH), 4.76 (s, 2H,
sub. Cp), 4.35 (s, 2H, sub. Cp) (Cp = η5-C5H5), 4.22(s, 5H, free Cp),
3.51−3.67 (broad, 16H, −CH2(CH2OCH2)3CH2−), 3.26 (broad, 
CH-CH), 2.71 (broad, CHCHCHCH2), 2.13 (broad, CO−CH),
1.61 (broad, CHCHCHCH2).
2. EXPERIMENTAL SECTION
2.1. General Data. For general data including solvents,
apparatuses, compounds, reactions, spectroscopy, CV, and SEC, see
the Supporting Information.
2.5. N-[3-(3′,6′,9′-Trioxadecyl)]-cis-5-norbornene-exo-2,3-di-
carboximide Monomer (8). To a solution of freshly prepared 2-(2-
(2-methoxyethoxy)ethoxy) ethylamine (7; 1.99 g, 12.21 mmol, 5.0
equiv) in toluene (20 mL) was added dropwise a solution of 2 (0.4 g,
2.44 mmol, 1 equiv) in toluene (25 mL) at room temperature in 0.5 h
with vigorous stirring. Then, triethylamine (0.2 mL, 1.43 mmol, 0.59
equiv) was added dropwise. The obtained mixture was refluxed for 12
h with a Dean−Stark apparatus before the solvent as well as residual
triethylamine were removed via vacuum distillation. Purification was
achieved by column chromatography with DCM/methanol (1% →
50%) as eluent, and the product was obtained as a colorless oil. Yield:
0.65 g, 86.3%. 1H NMR of 8 (300 MHz, CDCl3): δppm 1.37 (d, J = 9.6
Hz, 1H, CH2-bridge), 1.49 (d, J = 9.6 Hz, 1H, CH2-bridge), 2.69 (d, J
= 3.6 Hz, 2H, CO-CH), 3.27 (d, J = 1.8 Hz, 2H, CH-CH), 3.38 (s, J
= 4.3 Hz, 3H, CH3), 3.536−3.683 (m, 12H, 6 × CH2), 6.23 (t, J = 3.8
Hz, 2H, CHCH). 13C NMR of 8 (75 MHz, CDCl3): δppm 177.45
(CO-N), 137.60 (CC), 71.657, 70.223, 69.612, 66.53
(−OCH2CH2OCH2CH2O−), 58.64 (−CH3), 47.49 (CO-CH), 45.00
(CH-CH), 42.464 (CH2-bridge), 37.47 (N-CH2CH2). MS (ESI, m/
z): calcd for C16H23NO5, 309; found, 332.2 (M + Na+).
2.2. N-[11′-Amine-3′,6′,9′-trioxahendecyl]-cis-5-norbor-
nene-exo-2,3-dicarboximide (4). To a solution of freshly prepared
3 (2.49 g, 12.97 mmol, 5.3 equiv) in toluene (25 mL) was added a
solution of 2 (0.4 g, 2.44 mmol, 1 equiv) in toluene (25 mL) dropwise
at room temperature over 0.5 h with vigorous stirring. Then,
triethylamine (0.2 mL, 1.43 mmol, 0.59 equiv) was added dropwise.
The obtained mixture was refluxed for 12 h with a Dean−Stark
apparatus before the solvent as well as residual triethylamine were
removed via distillation in vacuo. Purification was achieved by column
chromatography with dichloromethane (DCM)/methanol (1% →
60%) as eluent, and the product was obtained as a pale yellow oil.
Yield: 0.56 g, 68%. 1H NMR of 4 (300 MHz, CDCl3): δppm 1.35 (d, J =
10.1 Hz, 1H, CH2 bridge), 1.48 (d, J = 10.1 Hz, 1H, CH2-bridge), 1.89
(s, br, 3H, −NH2 + H2O), 2.68 (d, J = 1.1 Hz, 2H, CO-CH), 2.85 (t, J
= 10.6 Hz, 2H, CH2-NH2), 3.26 (t, J = 3.4 Hz, 2H, CH-CH), 3.49
(t, J = 10.3 Hz, 2H, CH2CH2NH2), 3.57−3.71 (m, 12H, 6 × CH2),
6.28 (t, J = 3.6 Hz, 2H, CHCH). 13C NMR of 4 (75 MHz, CDCl3):
δppm 178.01 (CO-N), 137.79 (CC), 73.07 (−CH2CH2NH2), 70.53,
70.47, 70.205, 69.86 (−OCH2CH2OCH2CH2O−), 66.88
(−CH2NH2), 47.78 (CO-CH), 45.235 (CH-CH), 42.675 (CH2-
bridge), 41.45 (CH2-N-CO), 37.74 (-CH2CH2-N-CO). MS (ESI, m/
z): calcd for C17H26N2O5, 338; found, 339.19 (M + H+).
2.3. N-[11′-Ferroceneformamido-3′,6′,9′-trioxahendecyl]-
cis-5-norbornene-exo-2,3-dicarboximide Monomer (5). To a
suspension of ferrocenecarboxylic acid (0.5 g, 2.17 mmol) in dry DCM
(40 mL) was added dropwise triethylamine (0.1 mL, 0.72 mmol) at
room temperature under a nitrogen atmosphere. Then, oxalyl chloride
(0.7 mL, 8.2 mmol) was added dropwise at 0 °C. The obtained
mixture was stirred overnight at room temperature and dried in vacuo.
The residual red solid of crude chlorocarbonyl ferrocene (FcCOCl)
was dissolved in dry DCM (20 mL) and added dropwise to a DCM
solution (20 mL) of 4 (0.2 g, 0.59 mmol) and triethylamine (1.5 mL,
2.6. General Procedure for the Synthesis of the Block
Copolymers 10 by ROMP. The desired amount of 1 was placed in a
small Schlenk flask, flushed with nitrogen, and dissolved in a minimum
amount of dry DCM. Known amounts of monomers 8 and 5 were
placed in two small glass tubes, respectively, and dissolved in dry DCM
(1 mL per 100 mg of monomers). First, the monomer 8 was
transferred to the flask containing 1 via a syringe. The reaction mixture
was stirred vigorously for 8 min, and a known amount of the reaction
solution was taken out and quenched with 0.1 mL of ethyl vinyl ether
(EVE) for 1H NMR analysis. Then, the solution containing monomer
5 was transferred to the reaction flask via a syringe. The
polymerization was allowed to continue for 60 min and quenched
with 0.2 mL of ethyl EVE. The copolymers 10 were purified by
precipitation in diethyl ether five times and dried in vacuo to constant
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1
Figure 2. H NMR spectra of 4 (A), monomer 5 (B), and polymer 6 (C) in CDCl3.
weight. 1H NMR of polymers 9 (300 MHz, CDCl3): δppm 7.190−7.354
(m, phenyl and CDCl3), 5.751 and 5.508 (broad doublet, 2H, CH
CH), 3.521−3.595 (broad, 12H, −CH2(CH2OCH2)2CH2−), 3.355 (s,
3H, OCH3), 3.043 (broad, CH-CH), 2.683 (broad, CH
CHCHCH2), 2.070 (broad, CO-CH), 1.578 (broad, CH
200, 300, 400, 500, and 600 mV s−1. Redox recognition was conducted
in two different ways. (a) In solution via titration: the CVs were
recorded upon addition of 0, 0.25, and 0.5 equiv of [n-Bu4N]2[ATP].
The potentials of the new wave were measured using [FeCp*2] as an
internal reference. (b) With modified electrodes: the CVs were
recorded upon addition of [n-Bu4N]2[ATP] to an electrochemical cell
containing a Pt-modified electrode.
1
CHCHCH2). H NMR of copolymers 10 (300 MHz, CDCl3): δppm
7.26−7.36 (m, phenyl and CDCl3), 6.47 (broad, NHCO), 5.75 and
5.50 (broad doublet, CHCH), 4.71 (s, sub. Cp), 4.32 (s, sub. Cp),
4.19 (s, free Cp), 3.52−3.60 (broad, −CH2(CH2OCH2)2CH2− and
−CH2(CH2OCH2)3CH2−), 3.355 (s, OCH3), 3.04 (broad, CH-
CH), 2.68 (broad, CHCHCHCH2), 2.08 (broad, CO-CH), 1.575
(broad, CHCHCHCH2).
3. RESULTS AND DISCUSSION
3.1. Synthesis and ROMP of the Amidoferrocenyl-
Containing Monomer 5. As shown in Scheme 1, the new
amidoferrocenyl-containing monomer 5 was prepared by an
amidation reaction between ferrocenylcarbonyl chloride and
the key intermediate N-[11′-amine-3′,6′,9′-trioxahendecyl]-cis-
5-norbornene-exo-2,3-dicarboximide (4). This compound 4 was
prepared from cis-5-norbornene-exo-2,3-dicarboxylic anhydride
(2) in the presence of 1,11-diamine-3,6,9-trioxaundecane (3),
whose method of synthesis is well described in the Supporting
2.7. Electrochemistry, Modified Electrodes, and Redox
Sensing. All electrochemical measurements were recorded under a
nitrogen atmosphere. Conditions: solvent, dry dichloromethane;
temperature, 20 °C; supporting electrolyte, [nBu4N][PF6] 0.1 M;
working and counter electrodes, Pt; reference electrode, Ag; internal
reference, FeCp*2 (Cp* = η5-C5Me5); scan rate, 0.200 V s−1. The
number of electrons involved in the oxidation wave of ferrocenyl
polymers was calculated by the Bard−Anson equation: np = (idp/Cp)/
(idm/Cm)(Mp/Mm)0.275 (see text and the Supporting Information). The
experiments were conducted by adding a known amount of polymer
(see the Supporting Information) in 3 mL of dry DCM, and then a
known amount of [FeCp*2] (see the Supporting Information) in 2 mL
of DCM was added to the solution. After the CVs were recorded, the
intensities of the oxidation waves of the polymers and of the internal
reference [FeCp*2] were measured. The values were introduced in the
above equation, giving the final number of electrons (np). The
modified electrodes were prepared after approximately 25 adsorption
cycles around the ferrocenyl potential on Pt electrodes. Their
electrochemical behavior was checked in 5 mL of a DCM solution
containing only [nBu4N][PF6] 0.1 M at various scan rates: 25, 50, 100,
1
Information. Figure 2A shows the H NMR spectrum of the
intermediate 4. The peak at 6.28 ppm corresponds to the
olefinic protons, while the two double peaks at 1.36−1.37 and
1.46−1.50 ppm originate from the characteristic bridge-
methylene protons of the cis-norbornene structure. As shown
1
in the H NMR spectrum of monomer 5 (Figure 2B), the
appearance of the amido proton at 6.59 ppm and the three
characteristic cyclopentadienyl (Cp) protons at 4.61, 4.18, and
4.05 ppm, respectively, demonstrate the success of the
amidation reaction. The methylene protons of the TEG linker
are concentrated at 3.41−3.53 ppm, which is different from the
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the peak at 6.13 ppm corresponding to the olefinic protons of
monomer 5 and the appearance of new two broad peaks at 5.53
and 5.75 ppm that arise from the olefinic protons of polymers 6
indicate the successful polymerization of the monomer 5.
Furthermore, the other peaks of the cis-norbornene backbone
in polymers 6 change into broad signals that are very different
from the sharp signals of the monomers.
Table 1. Molecular Weight Data for the Amidoferrocenyl-
Containing Polymer 6
a
[M5]:[C]
5:1
>99
16:1
>99
50:1
>99
100:1
>99
400:1
83
b
conversn (%)
c
np1
5
1
16
2
1
1
50
34
47
5
2
3
95
51
94
5
332
64
d
np2
4.2 0.4
0.1
14
3
5
3
In this study, a series of amidoferrocenyl-containing
homopolymers 6 were synthesized with molar feed ratios of
e
np3
4
15
336
8
f
1
Mn
2854
2878.7
1417
1.09
8904
27604
55104
182704
monomer to catalyst from 5:1 to 400:1. In situ H NMR
g
Mn
8930.2
4239
1.08
analysis of the crude reaction mixture indicated that the
monomer conversions, which were calculated by comparing the
1H NMR signals of the olefinic protons between monomer 5
(6.13 ppm) and the polymers 6 (5.53 and 5.75 ppm), were
nearly 100% within 60 min when the molar feed ratio was less
than 50. It was necessary to extend the polymerization time in
order to obtain the larger polymers. For instance, when the
molar feed ratio was 100:1, the monomer conversion was only
50% after 60 min but improved to nearly 100% after overnight
stirring. For the largest molar feed ratio of 400:1, the monomer
conversion only reached 83% even after 4 days. The
amidoferrocenyl-containing homopolymers 6 are not soluble
in organic solvents such as acetone, acetonitrile, methanol, and
diethyl ether, unlike the monomer 5, but they are soluble in
dichloromethane, chloroform, tetrahydrofuran (THF), and
strongly polar solvents such as DMF and dimethyl sulfoxide
(DMSO). The smaller polymers have better solubilities than
the larger polymers. For instance, the polymer 6 with a molar
feed ratio of 50:1 is partially soluble in THF, but when the
h
Mn
5508
1.03
h
PDI
a
b
[M5]:[C] is the molar feed ratio of monomer 5 and 1. Monomer
c
conversion determined by 1H NMR. Degree of polymerization
d
1
obtained from H NMR using conversion of monomer 5. Degree of
polymerization determined via end-group analysis by 1H NMR
e
spectroscopy in CD2Cl2. Degree of polymerization determined by
f
the Bard−Anson electrochemical method. MWs obtained by 1H
g
NMR using conversion of monomer 5. MWs (+Na+) determined via
h
MALDI-TOF mass spectroscopy. Obtained from SEC using
polystyrenes as standards.
dispersed distribution in intermediate 4. All of the other peaks
are clearly assigned. 13C NMR and mass spectroscopy (Figures
S6 and S7, Supporting Information) further confirm the
structure of the monomer 5.
The preparation of amidoferrocenyl-containing polymers 6
by ROMP was carried out in dry DCM at room temperature
using catalyst 1. As shown in Figure 2C, the disappearance of
Figure 3. MALDI-TOF MS spectrum of polymer 616. The molar feed ratio of monomer 5 to 1 is 16:1. The red dotted lines correspond to the
difference between molecular peaks of a value of 550 1 Da (MW of 5).
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Figure 4. Electrochemical properties of monomer 5 and polymer 650. The molar feed ratio of monomer 5 to 1 is 50:1. (A) CV of monomer 5 in
CH2Cl2: internal reference, FeCp*2; reference electrode, Ag; working and counter electrodes, Pt; scan rate, 0.4 mV/s; supporting electrolyte, [n-
Bu4N][PF6]. The wave at 0.0 V is that of the reference [FeCp*2]. (B) CV of the polymer 650 in CH2Cl2: internal reference, [FeCp*2]; reference
electrode, Ag; working and counter electrodes, Pt; scan rate, 0.2 mV/s; supporting electrolyte, [n-Bu4N][PF6]. The wave at 0.0 V is that of the
reference [FeCp*2]. (C) Progressive adsorption of the polymer 650 upon scanning around the ferrocenyl area. (D) Pt electrode modified with the
polymer 650 at various scan rates in CH2Cl2 solution (containing only the supporting electrolyte). (E) Intensity as a function of scan rate (linearity
shows the expected behavior of the absorbed polymer).
Table 2. Redox Potentials and Chemical (ic/ia) and Electrochemical (Epa − Epc = ΔE) Reversibilities for Monomer 5, Polymers
6, and Corresponding Modified Electrodes
modified electrode
a
a
compd
E1/2 (ΔE) (mV)
ic/ia
E1/2 (ΔE) (mV)
Γ (mol/cm2)
Γ (mol/cm2) (ferrocenyl sites)
monomer 5
polymer 616
polymer 650
polymer 6100
polymer 6400
680 (70)
680 (30)
680 (40)
680 (30)
680 (40)
1.0
2.2
3.1
2.2
2.5
660 (0)
660 (0)
660 (0)
660 (0)
5.52 × 10−11
4.53 × 10−11
3.11 × 10−11
1.30 × 10−11
8.27 × 10−10
2.13 × 10−9
2.92 × 10−9
4.40 × 10−9
a
Surface coverage on the modified Pt electrode obtained after approximately 25 adsorption cycles.
molar feed ratio is increased to 100:1, the polymer 6 is
insoluble in THF.
group analysis, and the Bard−Anson electrochemical meth-
od84,85 were used to investigate the MWs of the amidoferro-
cenyl-containing polymers 6.
3.2. Molecular Weight Analysis of the Polymers 6.
Molecular weights (MWs) can be measured via a variety of
techniques, including gel permeation chromatography (GPC),
osmometry, static light scattering, matrix-assisted laser
desorption-ionization time-of-flight mass spectrometry
(MALDI-TOF MS), viscometry, small-angle X-ray scattering,
small-angle neutron scattering, ultracentrifugation, cryoscopy,
ebulliometry, and end-group analysis.86 Each method has its
respective advantages and disadvantages, and the most suitable
methods also depend on the polymer type. In this study, size
exclusion chromatography (SEC), MALDI-TOF MS, end-
As shown in Table 1, the theoretical MWs and polymer-
ization degrees of the polymers 6 were calculated according to
the molar feed ratios and the corresponding monomer
1
1
conversions from H NMR. End-group analysis by H NMR
of the polymers 6 in CD2Cl2 (see Figure S10, Supporting
Information) was conducted by comparing the five protons of
end-group phenyls (7.20−7.43 ppm) with amido protons (6.59
ppm), olefinic protons (5.56 and 5.77 ppm), Cp protons (4.23,
4.37, and 4.74 ppm), and linker protons (3.55−3.65 ppm),
respectively. For the small polymers in which theoretical MWs
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Figure 5. CVs for the titration of [n-Bu4N]2[ATP] with polymer 650 in
CH2Cl2 at 20 °C by adding the salt of the anion to the polymer
solution: (A) before addition of [n-Bu4N]2[ATP]; (B) during the
titration with 0.25 equiv of [n-Bu4N]2[ATP]; (C) with 0.5 equiv of [n-
Bu4N]2[ATP].
Figure 7. CVs for the titration of [n-Bu4N]2[ATP] by the modified Pt
electrode with polymer 650 in CH2Cl2 at 20 °C: (A) before addition of
[n-Bu4N]2[ATP]; (B) during titration of [n-Bu4N]2[ATP]; (C) after
addition of excess [n-Bu4N]2[ATP].
monomer 5 unit. There is a peak at 8930.2 Da that corresponds
to the molecular weight of (C6H6)(C28H34N2O6Fe)16(C2H2)-
Na. On the other hand, the MWs obtained by SEC were always
smaller than the theoretical values, which may result from the
obvious structural difference between the polystyrene standards
and the amidoferrocenyl-containing polymers 6. However,
none the polydispersity indexes (PDI) obtained by SEC traces
were larger than 1.1, which demonstrated a controlled
polymerization.
End-group analysis and MALDI-TOF MS are not reliable for
the large polymers, however. The SEC traces of the large
polymers 6 could not be obtained in THF because of solubility
problems. SEC measurements were also attempted in CHCl3,
but no signal was observed, probably because of the strong
adsorption of the large polymers 6 on the column stationary
Figure 6. Hydrogen-bonding interactions between ATP2− and two
amidoferrocenyl groups of polymers 6.
were less than 10000 Da, the MWs by NMR conversion and
end-group analysis were in good agreement, which was further
confirmed by MALDI-TOF MS results (Figure 3 and Figure
S13 (Supporting Information)). As shown in Figure 3, the
MALDI-TOF mass spectrum of polymer 616 showed well-
defined individual peaks for polymer fragments that are
separated by 550 1 Da corresponding to the mass of one
1
phase. From the DOSY H NMR spectra of the polymers 6
(Figure S14−S16, Supporting Information), the hydrodynamic
diameters of polymers 6 can be calculated using the Stokes−
Einstein equation (see Supporting Information). A progressive
increase of the hydrodynamic diameters was observed upon
increasing the molar feed ratio of monomer 5 to 1 from 50:1 to
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1
Figure 8. H NMR spectra of monomer 8 (A), polymer 9 (B), and copolymer 10 (C) in CDCl3.
to the anode during the electrochemical experiment. This
number np is estimated by employing the Bard−Anson
empirical equation84,85 previously derived for conventional
polarography, where id, M, and C are the CV wave intensity of
the diffusion current, molecular weight, and concentration of
the monomer (m) and polymer (p), respectively:
Table 3. Molecular Weight Data of the Amidoferrocenyl-
Containing Block Copolymers 10
a
[M8]:[M5]:[C]
6:3:1
>99
20:10:1
100:50:1
100:100:1
b
conversn (%)
>99
10
>99
50
>99
100
c
np1
3
3
0.275
d
⎛
⎞
(idp/Cp) Mp
np2
0.3
10
9
1
44
44
3
3
82
98
5
3
e
np =
⎜
⎟
⎠
np3
2.7 0.3
3608
1
(idm/C ) M
⎝
f
m
m
Mn
11784
58504
86004
g
Mn
3633.9
2585
As shown in Table 1, the estimated values of electrons (np3)
for all of the polymers 6 showed excellent consistency with the
h
Mn
7139
1.06
25454
1.14
22325
1.11
h
PDI
1.10
1
polymerization degree (np1) obtained from H NMR, which
a
b
c
[M8]:[M5]:[C]: molar feed ratio of monomer 8, monomer 5, and 1.
further demonstrated the controlled characteristic for the
ROMP of the amidoferrocenyl-containing monomer 5. For
example, for the polymer 6400, the largest polymer prepared in
this study, the calculated polymerization degree (np1) from the
conversion rate is 332, and the value of np2 from end-group
analysis is 64 3, but the np3 value from the above formula is
336 8, which is very close to the theoretical result. Thus, the
Bard−Anson electrochemical method is a valuable tool to check
the np and MW values of amidoferrocenyl containing polymers
6.
Monomer conversion of monomer 5 determined by 1H NMR.
1
Degree of polymerization obtained from H NMR using conversion
d
of the amidoferrocenyl-containing monomer 5. Degree of polymer-
ization for the amidoferrocenyl-containing block determined via end-
e
1
group analysis by H NMR spectroscopy. Degree of polymerization
for the amidoferrocenyl-containing monomer 5 determined by the
f
Bard−Anson electrochemical method. MWs obtained for copolymers
g
10 by 1H NMR using conversion of monomers 8 and 5. MWs
h
(+Na+) determined by MALDI-TOF mass spectroscopy. Obtained
from SEC using polystyrenes as standards.
3.3. Redox Properties of Polymers 6 and Electro-
chemical Sensing of ATP2−. The new ferrocenyl monomer 5
and the side chain amidoferrocenyl containing homopolymers 6
have been studied by CV87−90 using decamethylferrocene
[FeCp*2] as the internal reference.90 The CVs have been
recorded in DCM (Figure 4 and Figures S20 and S21
(Supporting Information)), and the E1/2 data (measured vs
[FeCp*2]) are gathered in Table 2. For monomer 5 and all of
the polymers 6, a single oxidation wave is observed for all the
ferrocenyl groups, and this single wave is marred by adsorption
of the polymer onto the electrode. For the monomer 5, the
FeIII/II oxidation potential of the ferrocenyl redox center is
around 680 mV, whereas for polymers 6 the potentials are also
around 680 mV, although the precise value is to a certain extent
not as precise due to the adsorption (Figure 4B).
400:1, which indicates a concomitant increase of MWs.
Although the DOSY results cannot quantitatively characterize
the polydispersity of polymers, the low deviation values of the
diffusion coefficient (D) from different DOSY H NMR peaks
show that these polymers should have a narrow molecular
weight distribution.
In order to further characterize the polymers 6, especially the
large ones, we have used the Bard−Anson electrochemical
method,84,85 in which the compared intensities in the cyclic
voltammograms (CVs) of the polymers and monomer were
used. The total number of electrons transferred in the oxidation
wave for the polymer (np) is the same as that of monomer units
in the polymer, because only one electron from FeII (ferrocene)
to FeIII (ferrocenium) is transferred from each monomer unit
1
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Figure 9. MALDI-TOF MS spectrum of the copolymer 106/3. The molar feed ratio of monomers 8 and 5 to 1 is 6:3:1. The dotted red and blue lines
correspond to the difference between molecular peaks of 550 1 (MW of 5) and 309 1 Da (MW of 8), respectively.
Figure 10. Electrochemical properties of the copolymer 10100/50. The molar feed ratio of monomers 8 and 5 to 1 is 100:50:1. (A) CV of the
copolymer in DCM: internal reference, [FeCp*2]; reference electrode, Ag; working and counter electrodes, Pt; scan rate, 0.4 mV/s; supporting
electrolyte, [n-Bu4N][PF6]. (B) Pt electrode modified by the copolymer at various scan rates in DCM solution containing only the supporting
electrolyte. (C) Intensity as a function of scan rate (the linearity shows the expected behavior of the adsorbed polymer).
There was no adsorption phenomenon during CV for
monomer 5, but for all the polymers 6 obvious and strong
adsorption onto electrodes was observed, as shown in Figure
4C, upon scanning around the oxidation potential of the
amidoferrocenyl group. The progressive adsorption onto
electrodes is an advantage for the facile formation of robust
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solution. Addition of [n-Bu4N]2[ATP] to an electrochemical
cell containing a solution of polymer 650 in DCM led to the
appearance of a new wave at a potential less positive than the
initial wave, the intensity of which decreased while that of the
new wave increased (Figure 5). Indeed, the interaction of the
anions with redox groups releases electron density, rendering
oxidation of the amidoferrocenyl group easier. The difference in
amidoferrocenyl redox potential between the initial wave and
the new wave (ΔE) is 70 mV. The equivalence point is reached
when 0.5 equiv of [nBu4N]2[ATP] has been added (Figure
5C), which is in accord with the double negative charge of this
anion and signifies that the ATP2− anion is quantitatively
recognized by the polymer 650 in DCM solution and that two
amidoferrocenyl groups are interacting with each ATP2−.
The α and β phosphates near the ribose are those that were
found by the group of Hampe and Kappes using infrared
multiple photon dissociation and photoelectron spectroscopy
to bear the two negative charges of ATP2−.94 Accordingly, the
stoichiometry of the titration that corresponds to two
amidoferrocenyl units per ATP2− is dictated by the interactions
of these two negatively charged α and β phosphates with the
NH groups of amidoferrocenyl units. In the oxidized
ferrocenium form generated at the anode, the interaction of
the oxygen anions involves an NH group of considerably
increased acidity due to the positive charge that is delocalized
over the amidoferrocenium moiety. The H bond is then
strengthened, and the synergy between this H bond and the
electrostatic bond between the cation and the anion is
sufficiently strong to significantly modify the ferrocenyl redox
potential. The two negatively charged phosphates are very
different from each other (Figure 6): the β and γ phosphates
form a favorable chelating double H bond with an
amidoferrocenyl group of polymers 6 (“intramolecular H
bonding”), whereas the α phosphate can only form a single H
bond with another amidoferrocenyl group. This group also
forms another H bond between its carbonyl group and another
ATP2− molecule (“intermolecular” H bonding), as shown in
Figure 6.
Figure 11. CVs for the titration of [n-Bu4N]2[ATP] by the Pt
electrode modified with the copolymer 10100/50 in DCM at 20 °C: (A)
before addition of [n-Bu4N]2[ATP]; (B) during addition of [n-
Bu4N]2[ATP]; (C) after addition of excess [n-Bu4N]2[ATP].
metallopolymer-modified electrodes upon scanning around the
amidoferrocenyl potential zone.87−90 Modification of electrodes
using polymers 6 with various MWs has been successful,
resulting in detectable electroactive materials. The electro-
chemical behavior of the modified electrodes was studied in
DCM containing only the supporting electrolyte (Figure 4D).
A well-defined symmetrical redox wave is observed that is
characteristic of a surface-confined redox couple, with the
expected linear relationship of peak current with potential
sweep rate (Figure 4E).87 The modified electrode is stable, as
repeated scanning does not modify the CVs. Furthermore, no
splitting between oxidation and reduction peaks is observed
(ΔE = 0 mV), which suggests that no structural change takes
place within the electrochemical redox process.85,87 These Pt
electrodes modified by polymers 6 are durable and reprodu-
cible, as no loss of electroactivity is observed after scanning
several times or after standing in air for several days. The
surface coverages of the electroactive amidoferrocenyl sites of
the modified electrodes for all the polymers are given in Table
2.
The Pt electrode modified with the polymer 650 was also
used for its recognition in DCM solution containing only [n-
Bu4N][PF6] as the supporting electrolyte, and a similar trend
was observed. As shown in Figure 7, the addition of [n-
Bu4N]2[ATP] to an electrochemical cell containing the
modified Pt electrode in DCM caused the appearance of a
new wave at a potential less positive than that for the initial
wave. The intensity of the initial wave decreased, while that of
the new wave increased. The difference in ferrocenyl redox
potential between the initial wave and the new wave (ΔE) is
130 mV: i.e., 60 mV larger than that observed with polymer 650
in solution. The larger ΔE value signifies a rather strong
interaction of the amidoferrocenium group on the modified Pt
electrode with the ATP2− anions. Consequently, the modified
Pt electrode with polymer 650 is a good candidate for the
qualitative recognition of ATP2− anions.91−93
3.4. Synthesis of the Amidoferrocenyl Block Copoly-
mers 10. As shown in Scheme 2, first the new monomer N-[3-
(3′,6′,9′-trioxadecyl)]-cis-5-norbornene-exo-2,3-dicarboximide
(8) was synthesized by reaction between 2 and 2-(2-(2-
methoxyethoxy)ethoxy)ethylamine (7). Figure 8A shows the
1H NMR spectrum of the monomer 8. The peak at 6.30 ppm
corresponds to the olefinic protons, and two doublet peaks at
1.36−1.39 and 1.47−1.51 ppm originate from the bridge-
methylene protons of the cis-norbornene structure. Further-
Oxoanion sensing is a key field of molecular recogni-
tion,91−93 in particular because DNA fragments include
adenosine triphosphate anion (ATP2−), an important coen-
zyme that transports chemical energy to cells for metabolism.
Here electrochemical recognition of ATP2− by the redox-active
polymers was studied first in dichloromethane (DCM) solution
using the n-butylammonium salt [n-Bu4N]2[ATP] and then
using a modified electrode that was derivatized by adsorption of
the polymers. Let us first examine the redox recognition in
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more, the protons of the methyl group of the side chain are
found at 3.37 ppm.
which is very close to the calculated value of 3633.4 Da. For
polymers 9, the MW from SEC analysis (Figures S32−S34,
Supporting Information) is also close to the theoretical values
obtained by 1H NMR conversion. For the corresponding
copolymers 10, as for the homopolymers 6, the MWs obtained
by SEC are always smaller than the calculated values.
Fortunately, the PDI values for all the copolymers 10 are less
than 1.15, which shows the good monodispersity of the
copolymers.
3.6. Redox Properties and Electrochemical Sensing of
ATP2− for the Block Copolymers 10. The side chain
amidoferrocenyl containing block copolymers 10 were studied
by CV using [FeCp*2] as the internal reference. The CVs were
recorded in DCM (Figure 10 and Figures S44−S46
(Supporting Information)), and the E1/2 data (measured vs
[FeCp*2]) are gathered in Table S3 (Supporting Information).
As shown in Figure 10A, a single oxidation wave is observed for
the ferrocenyl groups of the copolymer 10100/50, and this single
wave shows better reversibility and less adsorption than that of
6, which is taken into account by the solubilizing property of
the TEG chains in 10. Some adsorption is still observable,
however, as characterized by an intensity ratio ia/ic (0.9) that is
lower than 1 and a ΔE value that is lower (0.020 V) that the
Nernstian value of 0.059 V at 25 °C. The anodic and cathodic
CV waves are also slightly broader than those of the monomer
5, which is probably due to the nonequivalence of all the
ferrocenyl groups in the polymer chain. The FeIII/II oxidation
potential of the ferrocenyl redox center is found around 680
mV as well.
The block copolymers 10 were synthesized by chain
extension of monomer 8 to the second amidoferrocenyl-
containing monomer 5 via a one-pot two-step sequential
ROMP. The preparation of the first block, polymer 9, was
accomplished with nearly 100% monomer conversion in 8 min,
which was demonstrated by the disappearance of the peak at
6.30 ppm corresponding to the olefinic protons of monomer 8
and the appearance of new two broad peaks at 5.51 and 5.75
ppm corresponding to the olefinic protons of polymers (Figure
8B). The SEC results (Figures S32−S34, Supporting
Information) show the good monodispersity (PDI < 1.1) of
polymers 9 and demonstrate the controlled polymerization of
monomer 8. Full characterization of polymers 9 is detailed in
1
the Supporting Information. Figure 8C shows the H NMR
spectrum of the block copolymer 10. The protons of the Cp of
the ferrocenyl groups are located at 4.71, 4.32, and 4.19 ppm,
respectively. The peak at 6.47 ppm corresponds to the proton
of the amido group in the amidoferrocenyl block. The presence
of the above new peaks indicates the successful preparation of
the block copolymers 10.
Similarly, a series of amidoferrocenyl-containing copolymers
10 were synthesized with various molar feed ratios of monomer
8 and 5 to catalyst 1. The polymerization of monomer 8 is
finished at nearly 100% conversion within 8 min, even when the
molar feed ratio of monomer 8 to 1 was increased to 200:1.
However, for the second block, reaction times longer than 60
min (48 h in this study) were necessary when the feed ratio of
monomer 5 to 1 was increased to 100:1. The most obvious
difference in structure between monomers 5 and 8 is the
presence of the amidoferrocenyl moiety in 5. Thus, it is
believed that the polymerization is slowed down by the
presence of the amidoferrocenyl moiety due to steric constraint
of the linked ferrocenyl bulk.95 Furthermore, the block
copolymers 10 show a better solubility than the homopolymers
6. All of the prepared copolymers are soluble in DCM, CHCl3,
THF, DMF, and DMSO, and the small copolymers are even
soluble in acetone, acetonitrile, and ethyl acetate.
3.5. Molecular Weight Analysis of Block Copolymers
10. The MWs of polymers 9 and block copolymers 10 were
characterized by end-group analysis, MALDI-TOF MS, and
SEC, respectively. The polymerization degrees of the first,
polymers 9, were first obtained by end-group analysis using the
1H NMR spectra of polymers 9 in CD3CN (Figure S27,
Supporting Information). Then, the polymerization degrees of
the second block, polymers 10, were calculated by comparing
the integration of the methyl proton (3.355 ppm) with that of
the protons of the amido group (6.472 ppm) and Cp rings
(4.710, 4.318, and 4.189 ppm), respectively. As shown in Table
3, the polymerization degrees from end-group analysis (np2) are
very close to that obtained using the 1H NMR conversion (np1).
The number of amidoferrocenyl units in the copolymers 10 was
also determined using the Bard−Anson electrochemical
method. The estimated values of electrons (np3) for all of the
copolymers showed a good consistency with the value of np1, as
well. As shown in Figure 9, the MALDI-TOF MS of the small
copolymer 106/3, in which the molar feed ratio of monomer 8
and 5 to 1 is 6:3:1, shows well-defined individual peaks for
polymer fragments that are separated by 550 Da (MW of
monomer 5) and 309 Da (MW of monomer 8), respectively.
The accessibility of modified electrodes85−90 has also been
explored. Indeed, upon scanning around the oxidation potential
of the amidoferrocenyl group, the copolymers are adsorbed
onto electrodes (see Figure S46B). Thus, modification of
electrodes using the copolymers 10 has been successful. Figure
10B and Figure S46C show the electrochemical behavior of
modified electrodes in DCM containing only the supporting
electrolyte. A well-defined symmetrical redox wave that is
characteristic of a surface-confined redox couple is observed,
including the expected linear relationship of peak current with
potential sweep rate. Furthermore, repeated scanning does not
change the CVs, which indicates that the modified electrode is
stable. There is no structural change during the electrochemical
redox process, as no splitting between the oxidation and
reduction peaks is observed (ΔE = 0 mV).
Finally, electrochemical recognition of [n-Bu4N]2[ATP] by
the copolymer 10 was also found to be possible. As shown in
Figure 11, the addition of [n-Bu4N]2[ATP] to an electro-
chemical cell containing the Pt electrode modified with
copolymer 10100/50 in DCM provoked the appearance of a
new wave at a potential less positive than the initial wave. The
intensity of the initial wave decreased, while that of the new
wave increased. The difference in amidoferrocenyl redox
potential between the initial wave and the new wave (ΔE) is
150 mV: i.e., 20 mV larger than that obtained using the
modified Pt electrode with polymer 650. This might possibly be
the consequence of encapsulation by the triethylene glycol
branch network of the amidoferrocene−ATP interaction.
Consequently for the qualitative recognition of ATP2− anions
the Pt electrode modified with the copolymer 10 shows a better
effect in comparison to that modified with the homopolymer 6.
T h e
M W
f o u n d
f o r
( C H ) -
6 6
(C16H23NO5)6(C28H34N2O6Fe)3(C2H2)Na is 3633.9 Da,
4333
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4. CONCLUSION
Article
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AUTHOR INFORMATION
■
Present Address
§On sabbatical leave from the Key Laboratory of Leather
Chemistry and Engineering of Ministry of Education, Sichuan
University, Chengdu 610065, People’s Republic of China.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Science Foundation of
China (21106088), the Ph.D. Program Foundation of the
Ministry of Education of China (20110181120079), the
University of Bordeaux, the Centre National de la Recherche
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