Preparation of cobaltocenium-labeled polymers by atom transfer radical polymerization

Article abstract of DOI:10.1021/ma202725c

Cobaltocenium-labeled polymers were prepared by atom transfer radical polymerization (ATRP) using cobaltocenium-containing α-haloesters as initiators. The locations of the cobaltocenium moiety in the polymer chains (either at the end or in the middle) were dictated by the chemical structures of initiators. Kinetic studies showed that polymerizations of styrene, tert-butyl acrylate, and methyl methacrylate monomers followed a controlled/living manner, except that polymerization of methyl methacrylate with the aid of cobaltocenium monoinitiator displayed significant termination. All resultant polymers were redox-active and exhibited characteristic UV-vis absorption from the cobaltocenium moiety.

Full text of DOI:10.1021/ma202725c

Article
pubs.acs.org/Macromolecules
Preparation of Cobaltocenium-Labeled Polymers by Atom Transfer
Radical Polymerization
Lixia Ren, Jiuyang Zhang, Christopher G. Hardy, Deon Doxie, Barbara Fleming, and Chuanbing Tang*
Department of Chemistry and Biochemistry and USC Nanocenter, University of South Carolina, 631 Sumter Street, Columbia, South
Carolina 29208, United States
ABSTRACT: Cobaltocenium-labeled polymers were pre-
pared by atom transfer radical polymerization (ATRP) using
cobaltocenium-containing α-haloesters as initiators. The
locations of the cobaltocenium moiety in the polymer chains
(either at the end or in the middle) were dictated by the
chemical structures of initiators. Kinetic studies showed that
polymerizations of styrene, tert-butyl acrylate, and methyl
methacrylate monomers followed a controlled/living manner, except that polymerization of methyl methacrylate with the aid of
cobaltocenium monoinitiator displayed significant termination. All resultant polymers were redox-active and exhibited
characteristic UV−vis absorption from the cobaltocenium moiety.
extremely high structure integrity stability toward oxidation
(the half-wave potential (E1/2) of the Co(III)/Co(IV) couple
falling at ca. 2.7 V vs ferrocene).16,26 These properties make
cobaltocenium a desirable candidate as a redox-active probe for
use in polymeric materials. In addition, cobaltocenium has
characteristic optical absorption in the UV region. We have
recently developed a class of side-chain cobaltocenium-
containing polymers through both postpolymerization mod-
ification and direct polymerization.40−44 A few other groups
have developed cobaltocenium-containing dendrimers and
main-chain cobaltocenium-containing polymers.45−49 However,
cobaltocenium-labeled polymers have not been reported in the
literature.
There are three major pathways to prepare labeled
polymers:5 (1) Postpolymerization modification by labeling
probes through covalent attachment27 or supramolecular
interactions.50 However, this strategy requires highly efficient
postpolymerization modification reactions. Additionally, the
number of probe units is rather difficult to control. (2) Direct
polymerization using a probe-labeled functional monomer as a
comonomer.51 However, the accurate position of probes is
difficult to measure. (3) With the development of controlled
and living polymerization,52−56 probes can be also integrated
into initiators,13,57 transfer agents,39 or mediating agents,3,58
which can be used to prepare polymers with precise
information about location and number of probes. These
probes can be located at the end of or in the middle of polymer
chain, or placed at the side chain or the backbone of polymers,
as shown in Scheme 1.
INTRODUCTION
■
Labeled polymers are a class of important functional
materials1,2 which have found applications in molecular
imaging,3−6 (bio)sensing,5,7−9 targeting,10−14 electrode-deco-
rating,15−18 etc. Based on their specific applications, labeled
polymers are designed to be optically active,3,5,19−23 redox-
active,8,15,17,18,24−28 or radioactive.29−31 By labeling the polymer
chain with detectable probes, the study of polymer behavior in
solution, especially single polymer chains, would be more
accurate. For example, molecular weight of a chromophore-
labeled polymer can be precisely determined by analyzing the
chromophore probe using UV−vis spectroscopy.23 Among
diverse probes, redox-active probes have attracted special
attention due to their high sensitivity, rapid response, and
easy detection via appropriate analytical tools,24 which can
make them “visible” in solution. Metallocenes, which are widely
used as sensors and catalysts,8,32,33 are often used as redox
probes due to their reversible electrochemical oxidation and
reduction as well as their notable environmental stability.27,34
Metallocenes have been used to label natural biopolymers
including DNA,35,36 proteins,25 and drugs,26 and synthetic
polymers such as poly(ethylene oxide)37 and polyamides.27
Assembly of ferrocene-labeled streptavidin on biotinylated
electrodes results in a reproducible unidirectional current flow
in the presence of electron donors in solution.15 Ferrocene-
labeled polymers can be mapped via electrochemical atomic
force microscopy.18 A ferrocene-labeled poly(ethylene glycol)
was used to study the chain dynamics of a poly(ethylene glycol)
matrix via quantitative electrochemical diffusion measure-
ments.21,37,38 Recently, Zhu et al. prepared ferrocene-
containing transfer agents for reversible addition−fragmenta-
tion transfer polymerization.39
Herein, we report the preparation of novel cobaltocenium-
labeled polymers by atom transfer radical polymerization
(ATRP), one of the most widely used controlled radical
Cobaltocenium, an 18-electron cationic metallocene, has
been used as a redox probe due to its one-electron reversible
reduction (to cobaltocene) and high sensitivity (detection limit
up to 10−8 M solution).16 In addition, cobaltocenium has
Received: December 16, 2011
Revised: February 7, 2012
Published: February 16, 2012
© 2012 American Chemical Society
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Scheme 1. Labeled Polymers with a Probe at Diverse Locations of Polymer Chains
Scheme 2. Synthesis of Cobaltocenium-Containing α-Haloesters and Their Use as Initiators To Prepare Cobaltocenium-
Labeled Polymers by ATRP
polymerizations to prepare functional materials.54,55,59−61
Specifically, we designed two cobaltocenium-based α-haloest-
ers, monosubstituted and 1,1′-disubstituted bromoisobutyrate
cobaltoceniumcarboxylate, which were used as ATRP initiators.
With the aid of these initiators, polymers from three different
classes of monomers (acrylate, methacrylate, and styrene) with
Synthesis of 2-Hydroxyethyl 2-Bromoisobutyrate (HEBiB).
HEBiB was synthesized following reported procedures.62−64 Ethylene
glycol (40.0 mL, 7.2 × 10−1 mol) and triethylamine (6.0 mL, 4.3 ×
10−2 mol) were added into a dry flask followed by adding 2-
bromoisobutyryl bromide (5.4 mL, 4.3 × 10−2 mol) dropwise over 1 h
at 0 °C. The reaction mixture was kept at 60 °C overnight, followed by
adding water (300 mL) and extracting with dichloromethane (3 × 200
cobaltocenium as the optically active redox-active probe were
prepared. As shown in Scheme 2, all polymers have one
cobaltocenium probe per chain with a predetermined location,
either at the chain end or in the middle of chain. Kinetic studies
using both initiators were conducted. The optical and redox
properties of resultant cobaltocenium-containing polymers
were studied by UV−vis spectroscopy and cyclic voltammetry,
respectively.
mL). The organic layer was collected and dried over anhydrous
magnesium sulfate. The product was purified by passing through a
silica column with dichloromethane as eluent. Yield: 85%. H NMR
1
(CDCl3) δ: 3.84 (t, 2H, HOCH2CH2); 4.30 (t, 2H, HOCH2CH2);
1.92 (s, 6H, CH3). 13C NMR (CDCl3) δ: 171.7 (Br(CH3)2CCO); 66.8
(HOCH2CH2); 60.1 (HOCH2CH2); 50.9 (Br(CH3)2C); 32.8 (CH3).
Synthesis of Cobaltocenium Di-initiator. Freshly prepared
cobaltocenium 1,1′-diacyl chloride (0.56 g, 1.2 × 10−3 mol) and
HEBiB (1.50 g, 7.2 × 10−3 mol) were added into a dry round-bottom
flask with 100 mL of dry THF. Triethylamine (1.0 mL, 7.4 × 10−3
mol) was added slowly. The reaction mixture was stirred at room
temperature for 5 days. Afterward, solvents were evaporated and the
crude product was dissolved in dichloromethane followed by
sequential extraction with aqueous NaPF6 solution and water. The
organic layer was dried over anhydrous magnesium sulfate and
EXPERIMENTAL SECTION
■
Materials. Ethylene glycol (anhydrous, Aldrich), 2-bromoisobu-
tyryl bromide (Aldrich), Cu(I)Br (99.999%, Aldrich), N,N,N′,N″,N″-
pentamethyldiethylenetriamine (PMDETA, Aldrich), sodium hexa-
fluorophosphate (NaPF6, 99%, Alfa Aesar), and tetra-n-butylammo-
nium hexafluorophosphate (TBAPF6, 98%, Acros) were used as
received. Triethylamine (Et3N, 99%, Aldrich) and tetrahydrofuran
(THF, Alfa Aesar) were distilled before use. Cobaltocenium monoacyl
chloride and diacyl chloride were synthesized according to our early
reports.40,41 Styrene (St, Aldrich), tert-butyl acrylate (tBA, Aldrich),
and methyl methacrylate (MMA, Aldrich) were freshly distilled before
use. All other chemicals were obtained from commercial sources and
used as received unless otherwise mentioned.
1
precipitated from diethyl ether to give the product. Yield: 90%. H
NMR (CDCl3) δ: 6.25 (broad, 4H, Cp); 6.15 (broad, 4H, Cp); 4.56
(dt, 8H, OCH2CH2O); 1.92 (s, 12H, CH3). 13C NMR (CDCl3) δ:
171.5 (Br(CH3)2CCO); 162.5 (CpCO); 85−89 (carbon from Cp
ring); 64.2 and 65.0 (OCH2CH2O); 56.2 (Br(CH3)2CCO); 30.2
(CH3). MS: theoretical m/z 569.94; found m/z 569.85.
Synthesis of Cobaltocenium Monoinitiator. The preparation
was similar to the synthesis of cobaltocenium di-initiator. Cobaltoce-
nium monoacyl chloride (0.48 g, 1.2 × 10−3 mol) and HEBiB (0.75 g,
3.6 × 10−3 mol) were mixed in a THF (100 mL) solution, followed by
adding triethylamine (0.5 mL, 3.7 × 10−3 mol) slowly. The reaction
mixture was stirred at room temperature for 5 days. The product was
purified using the same procedure as the synthesis of cobaltocenium
Characterization. 1H NMR (400 MHz) and 13C NMR (100
MHz) spectra were recorded on a Varian Mercury 400 spectrometer
with tetramethylsilane (TMS) as an internal reference. Gel permeation
chromatography (GPC) was performed in dimethylformamide (DMF,
containing 0.1% LiBr) at a flow rate of 0.8 mL/min at 50 °C on a
Varian system equipped with a ProStar 210 pump and a Varian 356-
LC RI detector and three 5 μm phenogel columns (Phenomenex Co.)
with narrow dispersed polystyrene as standards. High-resolution mass
spectrometry (MS) was conducted on a Waters Micromass quadro-
pole time-of-flight mass spectrometer, using electrospray ionization in
a positive ion mode. UV−vis spectra were recorded on a Shimadzu UV
2450 spectrophotometer. Cyclic voltammetry (CV) characterization
was carried on a BAS CV-50W voltametric analyzer. The samples were
dissolved in 0.1 M TBAPF6 solution in DMF at a concentration of 0.5
mM. The samples were then purged with nitrogen gas for 10 min
before running CV. The samples were scanned at a rate of 100 mV/s
at different potential ranges vs Ag/AgCl electrode.
1
di-initiator. Yield: 90%. H NMR (CDCl3) δ: 6.20 (broad, 2H, Cp);
5.95 (broad, 2H, Cp); 5.82 (broad, Cp, 5H); 4.61 (d, broad, 4H,
OCH2CH2O); 1.91 (s, 6H, CH3). 13C NMR (CDCl3) δ: 171.7
(Br(CH3)2CCO); 163.6 (CpCO); 85−89 (carbon from Cp ring); 62.9
and 65.0 (OCH2CH2O); 55.7 (Br(CH3)2CCO); 30.2 (CH3). MS:
theoretical m/z 424.98; found m/z 424.89.
Polymerization Procedure. The polymerization of all three
monomers was very similar. A typical procedure was as follows. Cu(I)
Br (2 equiv) and cobaltocenium monoinitiator (1 equiv) or
cobaltocenium di-initiator (1 equiv of bromide) were charged into a
Schlenk flask and degassed under nitrogen. Degassed monomer (400
equiv) and PMDETA (2 equiv) were then transferred into the Schlenk
flask under the protection of nitrogen. The polymerization was
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conducted at 90 °C for St or tBA, while the polymerization of MMA
was conducted at room temperature. Samples were taken out at
predetermined intervals to monitor the reaction conversion before
stopping the reaction. The final polymers were obtained by
precipitation in methanol.
1
(1) Polymer PMMA-Cc-PMMA: H NMR (CDCl3, δ): 3.35−3.75
(broad, OCH3), 2.15−2.40 (broad, CH2−C(CH3)), 1.20−1.95 (broad,
CH2−C(CH3)). GPC Mn: 120 100 g/mol; PDI: 1.20.
(2) Polymer PtBA-Cc-PtBA: 1H NMR (CDCl3, δ): 2.00−2.35
(broad, CH2−CH), 1.65−1.80 (broad, CH2−CH), 1.00−1.65 (broad,
−C(CH3)3). GPC Mn: 22 500 g/mol; PDI: 1.45.
1
(3) Polymer PSt-Cc-PSt: H NMR (CDCl3, δ): 6.00−7.2 (broad,
aromatic), 0.80−2.00 (broad, CH2−CH). GPC Mn: 123 000 g/mol;
PDI: 1.20.
1
(4) Polymer Cc-PMMA: H NMR (CDCl3, δ): 3.35−3.75 (broad,
OCH3), 2.15−2.40 (broad, CH2−C(CH3)), 1.20−1.95 (broad, CH2−
C(CH3)).
1
(5) Polymer Cc-PtBA: H NMR (CDCl3, δ): 2.00−2.35 (broad,
CH2−CH), 1.65−1.80 (broad, CH2−CH), 1.00−1.65 (broad, −C-
(CH3)3). GPC Mn: 135 000 g/mol; PDI: 1.18.
(6) Polymer Cc-PSt: 1H NMR (CDCl3, δ): 6.00−7.2 (broad,
aromatic), 0.80−2.00 (broad, CH2−CH). GPC Mn: 134 200 g/mol;
PDI: 1.35.
Kinetic Study of Monomers Initiated by Cobaltocenium Di-
initiator and Monoinitiator. All kinetic studies were conducted
similarly as aforementioned. Here, the St polymerization is described
as an example. Cobaltocenium di-initiator (4.1 mg, 5.0 × 10−6 mol)
and Cu(I)Br (2.8 mg, 2.0 × 10−5 mol) were charged into a Schlenk
flask and purged with nitrogen for 30 min. Degassed St (0.9 mL, 9.0 ×
10−3 mol) and PMDETA (5 μL, 2.0 × 10−5 mol) were transferred into
the Schlenk flask. The flask was placed in an oil bath set at 90 °C.
Samples were taken at different intervals under the protection of
nitrogen. The conversion of the monomers was calculated by 1H NMR
analyses. The kinetic studies of all three monomers initiated by
cobaltocenium monoinitiator were also conducted similarly to those
using the cobaltocenium di-initiator. It should be noted that the kinetic
studies of MMA monomers were conducted at room temperature.
1
Figure 1. (A) H NMR and (B) 13C NMR spectra of cobaltocenium
di-initiator in CDCl3.
cobaltocenium-containing ATRP initiators were successfully
synthesized by simple esterification reactions with high yields.
We first conducted polymerization of tBA in solution.
However, the polymerization was very slow, as only 30%
conversion was reached after 24 h. Hence, bulk polymerization
was carried out for all monomers initiated by cobaltocenium-
containing initiators reported in this paper. Both tBA and St
polymerizations were carried out at 90 °C, while MMA
polymerization was employed at room temperature. Bulk
polymerizations of monomers including tBA, St, and MMA
were first carried out with the aid of cobaltocenium-containing
di-initiator. The catalyst system consisted of copper(I) bromide
and PMDETA. The molar ratio of catalyst to initiator was 4:1
(2:1 for copper bromide to the bromide in the initiator). As
reactions continued, all reaction mixtures clearly turned more
RESULTS AND DISCUSSION
■
Synthesis of Cobaltocenium-Ended and Cobaltoce-
nium-Centered Polymers by ATRP. As shown in Scheme 2,
cobaltocenium-containing ATRP initiators were synthesized via
esterification reactions between cobaltocenium acyl chloride
and 2-hydroxyethyl 2-bromoisobutyrate (HEBiB), in the
presence of triethylamine. The use of 1,1′-diacyl chloride and
monoacyl chloride produced cobaltocenium di-initiator and
monoinitiator respectively, with yields at ∼90%. Both initiators
were characterized by proton and carbon NMR. As shown in
1
Figure 1A, the H NMR spectrum of the di-initiator showed
1
viscous. Figure 3A shows the H NMR spectrum of PMMA
peaks at 5.97−6.33, 4.37−4.73, and 1.75−2.10 ppm, corre-
sponding to characteristic Cp protons, ethylene protons, and
methyl protons, respectively. The integration ratios of each
group were in great agreement with theoretical ones. 13C NMR
polymers initiated by the di-initiator. The vinyl protons (5.0−
6.0 ppm) from the monomers disappeared completely, while
broad peaks from 1.0 to 2.5 ppm appeared, corresponding to
protons from the PMMA backbone. More importantly, for a
polymer with relatively lower molecular weight, two peaks were
clearly observed at lower field from 6.05 to 6.23 ppm, which
originated from Cp protons of the cobaltocenium moiety from
the initiator in the middle of polymer chain. Similarly, PtBA
and PSt prepared by ATRP with the cobaltocenium di-initiator
showed their respective signals in the NMR spectra (Figure
3B,C). Figure 4 shows the GPC traces of the polymers, PtBA-
Cc-PtBA, PSt-Cc-PSt, and PMMA-Cc-PMMA, synthesized
using the cobaltocenium di-initiator. In all cases, symmetric
monomodal peaks were observed. The polydispersity indexes
(PDI) of PSt-Cc-PSt and PMMA-Cc-PMMA were ∼1.20 while
the PtBA-Cc-PtBA had a higher PDI of 1.45.
1
agreed very well with the H NMR result, and each signal was
clearly assigned (Figure 1B). The identity of di-initiator was
further confirmed by high-resolution MS, which showed a
mass/charge (m/z) ratio of 569.85, consistent with the
theoretical m/z of 569.94. Compared to the di-initiator,
monoinitiator showed slight shift to the lower field for both
the Cp protons (5.75−6.25 ppm) and the ethylene protons
(4.50−4.70 ppm), as shown in Figure 2A. Three distinct peaks
were observed for the Cp protons. The 13C NMR spectrum
showed similar chemical shifts for all characteristic protons to
those of the di-initiator (Figure 2B). High-resolution MS
showed that the m/z ratio was 424.89, in a good agreement
with the theoretical ratio of 424.98. Therefore, both
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1
Figure 2. (A) H NMR and (B) 13C NMR spectra of cobaltocenium
monoinitiator in CDCl3.
In order to verify whether the polymerization was living and
controlled, kinetic studies of all monomer polymerization were
1
carried out. The conversion was determined from in situ H
NMR analysis by comparing the integration areas between the
monomers and polymers in the reaction system. Figure 5A
shows the semilogarithmic plots of ln([M0]/[M]) vs reaction
time for polymerization of monomers initiated by the
cobaltocenium di-initiator. These plots exhibited linear kinetics,
indicating that all polymerizations followed a controlled and
living manner. While the rates of polymerization of St and tBA
were very close, the polymerization of MMA was much faster.
The conversion of MMA reached 50% within 2.5 h, while it
took 10 h for St to reach a comparable conversion. The
evolution of molecular weight vs conversion was also
monitored in the polymerizations of tBA, St, and MMA.
Clearly the molecular weight increased linearly with the
conversion, another signature of controlled and living polymer-
izations (Figure 5B).
1
Figure 3. H NMR spectra of (A) PMMA, (B) PtBA, and (C) PSt
prepared by ATRP with the use of cobaltocenium di-initiator in
CDCl3.
The cobaltocenium monoinitiator was then used to conduct
ATRP of tBA, St, and MMA under similar conditions. Figure 6
shows the 1H NMR spectra of Cc-PtBA, Cc-PSt, and Cc-PMMA
polymers, which showed similar characteristic peaks to those
observed from polymers prepared by the cobaltocenium di-
with the PDI as low as 1.18. Although Cc-PSt had a PDI of
1.35, a shoulder appeared at the higher molecular weight end,
suggesting that some bimolecular termination may occur. The
Cc-PMMA polymer exhibited an asymmetrical trace with a
rough baseline, indicating the polymerization was not
controlled.
1
initiator. A representative H NMR spectrum of Cc-PtBA with
relatively lower molecular weight clearly showed typical signals
in the range of 5.85−6.45 ppm, corresponding to the Cp
protons from the ended cobaltocenium moiety. Figure 7 shows
the GPC traces of the polymers synthesized using the
cobaltocenium monoinitiator. Cc-PtBA had a symmetric trace
Kinetic studies were conducted using the cobaltocenium
monoinitiator and the same procedures as those with the
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Figure 4. GPC traces of PtBA-Cc-PtBA, PSt-Cc-PSt, and PMMA-Cc-
PMMA prepared by ATRP with the use of cobaltocenium di-initiator.
1
Figure 6. H NMR spectra of (A) PtBA, (B) PSt, and (C) PMMA
prepared by ATRP with the use of the cobaltocenium monoinitiator.
Figure 5. (A) Semilogarithmic plots and (B) number-average
molecular weight vs conversion for bulk polymerizations of tBA, St,
and MMA initiated by the cobaltocenium di-initiator.
cobaltocenium di-initiator. As shown in Figure 8, bulk
polymerizations of both St and tBA exhibited linear kinetics,
while the molecular weight had a linear relationship with
conversion. All these results indicated the polymerizations of
tBA and St were controlled and living. In addition, the rates of
polymerization for tBA and St were also comparable. However,
the kinetic plot of polymerization of MMA showed a distinct
curvature, a signature of significant termination. Nevertheless,
at conversions below 50% the polymerization of MMA was still
much faster than that of St and tBA. Repeated experiments
showed similar kinetics. This is in sharp contrast with the living
characteristics of MMA initiated by the cobaltocenium di-
initiator. Given that both initiators have similar functionalities,
this behavior was quite peculiar. It is not very clear at this stage
what could induce the significant termination reactions.
Figure 7. GPC traces of Cc-PtBA, Cc-PSt, and Cc-PMMA prepared by
ATRP with the use of the cobaltocenium monoinitiator.
cobaltocenium can undergo exchange with other anions. With
the use of chloride-based anionic exchange resin, hexafluor-
ophosphate ions can be readily exchanged to chloride ions. This
early work indicated that a similar ion exchange between
hexafluorophosphate ions in the cobaltocenium initiators and
Ion-Exchange Effect on the Polymerization. In our
early study,41,43 we have demonstrated that the counterions of
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indicated that these initiators were somehow not stable,
ultimately leading to the decomposition of the cobaltocenium
moiety. More investigations on this matter are currently in
progress.
The ion-exchange process was further confirmed with ATRP
experiments on cobaltocenium-containing monomers. We have
failed to polymerize any cobaltocenium hexafluorophosphate-
containing vinyl monomers by ATRP. Most likely, the reaction
system was involved with the ion-exchange process between
cobaltocenium hexafluorophosphate in the vinyl monomers and
catalyst copper(I) bromide, as shown in Scheme 4. Since the
molar ratio of monomer to catalyst is usually very high (i.e.,
100:1), even a small fraction of monomers (i.e., 1%)
participating the ion exchange would cause all catalysts to
lose activity.
Optical and Redox Properties. Using the cobaltocenium-
containing ATRP initiators, single cobaltocenium-labeled
polymers PtBA, PSt, and PMMA were obtained. The presence
1
of cobaltocenium was quantitatively verified from H NMR
analysis discussed above. In our early report,41 we have shown
that side-chain cobaltocenium-containing polymers exhibited
characteristic UV−vis absorption from their cobaltocenium
units. Therefore, UV−vis spectroscopy was first used to
monitor the cobaltocenium moiety in these labeled polymers
prepared by ATRP. Figure 9 showed the representative UV−vis
absorption spectra of cobaltocenium-containing initiators and
their polymers. Both cobaltocenium mono- and di-initiators
exhibited two characteristic absorptions at 229 and 276 nm,
which were assigned to π−π* or n−π* transitions of
cobaltocenium, very similar to transitions in ferrocene studied
by Scott and Becker.65 A weak absorption at 410 nm
corresponded to the d−d* transition of cobaltocenium. All
cobaltocenium-labeled polymers showed a typical absorption at
ca. 230 nm, similar to the initiators. In addition, these polymers
also showed an absorption peak in the range of 260−300 nm.
The peak at ∼400 nm was much weaker in most
cobaltocenium-labeled polymers. The change of peak intensity
and the variation of peak positions were most likely related with
the change of surroundings of cobaltocenium, as these polymer
chains have different diffusion dynamics. In contrast, no distinct
UV−vis absorptions were observed from polymers synthesized
via ATRP without the cobaltocenium moiety, further
confirming the presence of cobaltocenium group in the
polymers.
Figure 8. (A) Semilogarithmic plots of bulk polymerization of tBA, St,
and MMA initiated by cobaltocenium monoinitiator. (B) number-
average molecular weight vs conversion for bulk polymerizations of
tBA and St initiated by cobaltocenium monoinitiator (due to the
nonliving behavior of MMA polymerization, Mn vs conversion was not
monitored).
bromide ions in the copper catalysts should occur. Such
exchange could induce significant undesirable effects on the
activation and deactivation equilibrium of the ATRP process.
This hypothesis was confirmed with the following experiments,
in which the 1:1 molar ratio of the bromide in the initiators and
copper(I) bromide led to the formation of uncontrolled
polymers with very low molecular weight. As shown in Scheme
3, the exchange between hexafluorophosphate ions and
copper(I) bromide would significantly alter the activation and
deactivation equilibrium, as copper hexafluorophosphate is a
very poor catalyst. Therefore, all the reaction systems were
intentionally designed with the 1:2 molar ratio of the bromide
in the initiators and copper(I) bromide. The 1 equiv of excess
of copper(I) bromide would be used as a “buffer reagent” to
digest the hexafluorophosphate ions and therefore minimize the
ion-exchange effect on the control of polymerization. Another
possible strategy is to prepare cobaltocenium initiators with
halide as the counterions. However, our preliminary study
Redox properties of cobaltocenium initiators and cobaltoce-
nium-labeled polymers were investigated with the aid of cyclic
voltammetry.40,41 As shown in Figure 10, the redox process of
cobaltocenium mono- and di-initiators was not reversible. The
oxidation peak current was much higher than the reduction
peak current. The irreversibility was likely due to solubility
Scheme 3. Possible Side Effects of Ion-Exchange between Initiators and Catalysts
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Scheme 4. Possible Side Effects of Ion-Exchange between Monomers and Catalysts
Figure 10. Cyclic voltammetry curves of (A) cobaltocenium
monoinitiator and cobaltocenium di-initiator, (B) polymers prepared
by di-initiator, and (C) polymers prepared by monoinitiator.
Figure 9. UV−vis absorption spectra of (A) cobaltocenium
monoinitiator and di-initiator, (B) polymers prepared by di-initiator,
and (C) polymers prepared by monoinitiator (solvent: dichloro-
methane).
CONCLUSIONS
■
In conclusion, we synthesized cobaltocenium-containing mono-
and dihaloesters, which were used as initiators to prepare
cobaltocenium-labeled polymers by atom transfer radical
polymerization. Three different classes of monomers including
styrene, acrylate, and methacrylate were polymerized by ATRP.
Because of the potential ion exchange between copper catalysts
and counterions of cobaltocenium moiety, excess copper
catalysts (compared to initiators) were used. Most polymer-
ization systems exhibited a living and control kinetics, while
polymerization of MMA by the monoinitiator showed
significant termination. Both cobaltocenium-ended and cen-
tered-polymers showed characteristic UV−vis absorption of
cobaltocenium. All polymers exhibited redox-active behaviors,
change of cobaltocenium mono- and di-initiators under redox
process. Although cobaltocenium mono- and di-initiators are
soluble in DMF, the reduced cobaltocenium-labeled polymers
(cobaltocene-containing mono- and di-initiators) might have
different solubility in DMF. The styrene-based polymers (Cc-
PSt and PS-Cc-PSt) had the similar redox process to the
initiators, while the acrylate polymers showed different redox
behaviors between end- and centered cobaltocenium-labeled
polymers. Both PMMA polymers (Cc-PMMA and PMMA-Cc-
PMMA) exhibited a much more reversible redox process.
Nevertheless, all polymers exhibited redox-active properties.
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indicating the retention of cobaltocenium moiety although the
redox process was not reversible. These optically active and
redox-active cobaltocenium-labeled polymers may find applica-
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AUTHOR INFORMATION
Corresponding Author
*E-mail: tang4@mailbox.sc.edu.
■
(30) Fukukawa, K.-i.; Rossin, R.; Hagooly, A.; Pressly, E. D.; Hunt, J.
N.; Messmore, B. W.; Wooley, K. L.; Welch, M. J.; Hawker, C. J.
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
We acknowledge the University of South Carolina for financial
support.
■
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