Tacticity influence on the electrochemical reactivity of group transfer polymerization-synthesized PTMA

Article abstract of DOI:10.1021/jp301207v

Spectroscopic, thermal, and electrochemical characterization results are presented for the redox active polymer poly(2,2,6,6-tetramethyl-1- piperinidyloxy-4-yl methacrylate) or PTMA, synthesized by group transfer polymerization (GTP), and its precursors 4-hydroxy-tetramethylpiperidine-N-oxyl (HO-TEMPO) and 4-methacryloyloxy-tetramethylpiperidine-N-oxyl (MO-TEMPO). DSC analysis of synthesized PTMA showed that the glass transition temperature (T_g) of the polymer structure occurs at 155 °C, corroborated by dynamic mechanical analysis (DMA), which is higher when compared with T _g data for PTMA synthesized by other methods. Also, the amount of radical species present in PTMA synthesized by GTP reactions (100%) is higher than the values typically upon synthesizing PTMA by radical polymerization. Electrochemical and spectroelectrochemical-electron spin resonance studies in acetonitrile revealed two redox events in the PTMA polymer, one of which is reversible, accounting for ca. 80% of the spins in the polymer and giving rise to the battery behavior. The other redox event is irreversible, accounting for the remaining ca. 20% of spins, which has not previously been reported. These two redox events are linked to a structural property associated with the tacticity of the polymer, where the reversible feature (responsible for cathode behavior) is the dominant species. This corresponds to a number of isotactic domains of the polymer (determined by high temperature ¹H NMR). The second feature accounts for the three-line impurity observed in the ESR, which has been reported previously but poorly explained, associated to the number of heterotactic/syndiotactic triads.

Full text of DOI:10.1021/jp301207v

Article
pubs.acs.org/JPCB
Tacticity Influence on the Electrochemical Reactivity of Group
Transfer Polymerization-Synthesized PTMA
†
dez-Munoz, Bernardo A. Frontana-Uribe, Felipe J. Gonzal
́
ez,‡
‡
§,⊥
Hugo A. Lop
Ignacio Gonzal
†
‡
́
ez-Pen
́
̃
a, Lindsay S. Hernan
́
̃
†
,‡,∥
,†
ez, Carlos Frontana,* and Judith Cardoso*
Universidad Auton
Centro de Investigacion
Col. San Pedro Zacatenco, C. P. 07360. Mex
́
oma Metropolitana-Iztapalapa Av., San Rafael Atlixco 186, Col. Vicentina 09340, Mex
y Estudios Avanzados, Departamento de Química, Av. Instituto Politecnico Nacional No. 2508.,
ico, DF., Mexico
en Química Sustentable, UNAM-UAEM, Km. 14.5 Carretera Toluca-Atlacomulco,
C. P. 50200, Toluca, Estado de Mexico, DF, Mexico
Academic of the Instituto de Química, UNAM, Mex
S Supporting Information
́
ico, DF, Mexico
́
́
́
§
Centro Conjunto de Investigacion
́
́
⊥
́
ico, DF, Mexico
*
ABSTRACT: Spectroscopic, thermal, and electrochemical characterization
results are presented for the redox active polymer poly(2,2,6,6-tetramethyl-1-
piperinidyloxy-4-yl methacrylate) or PTMA, synthesized by group transfer
polymerization (GTP), and its precursors 4-hydroxy-tetramethylpiperidine-N-
oxyl (HO-TEMPO) and 4-methacryloyloxy-tetramethylpiperidine-N-oxyl
(
MO-TEMPO). DSC analysis of synthesized PTMA showed that the glass
transition temperature (T ) of the polymer structure occurs at 155 °C,
corroborated by dynamic mechanical analysis (DMA), which is higher when
g
compared with T data for PTMA synthesized by other methods. Also, the
g
amount of radical species present in PTMA synthesized by GTP reactions
(
100%) is higher than the values typically upon synthesizing PTMA by radical
polymerization. Electrochemical and spectroelectrochemical-electron spin
resonance studies in acetonitrile revealed two redox events in the PTMA
polymer, one of which is reversible, accounting for ca. 80% of the spins in the polymer and giving rise to the battery behavior.
The other redox event is irreversible, accounting for the remaining ca. 20% of spins, which has not previously been reported.
These two redox events are linked to a structural property associated with the tacticity of the polymer, where the reversible
feature (responsible for cathode behavior) is the dominant species. This corresponds to a number of isotactic domains of the
1
polymer (determined by high temperature H NMR). The second feature accounts for the three-line impurity observed in the
ESR, which has been reported previously but poorly explained, associated to the number of heterotactic/syndiotactic triads.
INTRODUCTION
Design and construction of lithium-ion batteries, working as
operative rechargeable batteries is an active field of study, due to
their use as power sources for high performance electronic devices.
Development of materials providing the basis for performance-
improvement is therefore a key issue during active research, and in
this context, several materials have been tested; this is a critical
point when considering the underlying conflict between power
capability and energy density output in the battery.
Polymers bearing TEMPO or N-oxy radical species act as
alternative materials, due to the reversible properties of their
electrochemical conversion, rendering them usable as anode
materials in operating battery systems.
methyl-1-piperinidyloxy-4-yl methacrylate) or PTMA (3) is
particularly useful as a cathode material for lithium-ion
rechargeable batteries,
efficiencies have been achieved. This polymer is built from
repeated nitroxide radical moieties, which present reversible
oxidation processes forming stable oxoammonium cations.
Synthetic procedures previously described for PTMA (3)
generate nitroxide radicals at the last stage of the process by
oxidizing the polymer obtained, from either 4-methacryloyloxy-
■
1
,2
2
2
,2,6,6-tetramethylpiperidine or 4-methacryloyloxy-N-hydroxy-
,2,6,6-tetramethylpiperidine, via a free radical polymerization
13
reaction. Unfortunately, this oxidation reaction does not pro-
ceed with a 100% oxidation yield and affords PTMA containing
only 65% to 81% of the theoretically possible amount of redox
active nitroxide groups. An alternative synthetic method for
PTMA from compound 2 (Scheme 1A) is to employ group
transfer polymerization (GTP) reactions; this method has the
advantage of yielding a higher number of repetitive units con-
taining the redox radical species per mass unit of the polymer,
leading to an increase of charge capacity of the corresponding
3,4
5
14
6−9
Poly(2,2,6,6-tetra-
14
device.
9
−12
in which high charge/discharge
5
Received: February 7, 2012
Revised: April 13, 2012
Published: April 17, 2012
©
2012 American Chemical Society
5542
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The Journal of Physical Chemistry B
Article
Scheme 1. (A) Synthesis of Poly(4-methacryloyloxy-2,2,6,6-
for quenching the nitroxide moiety. The deconvolution procedure
is presented in the Supporting Information.
tetramethylpiperidine-N-oxyl) (PTMA, 3): (a) HPWO , H O ,
4
2
2
(
b) Methacryloyl Chloride, NEt , PhCH , (c) 1-Methoxy-2-
Mass spectra were recorded on a JEOL JMS-AX 5005HA
spectrometer using electron impact (EI). Elemental analysis
(Elemental Perkin-Elmer 2400 CHNS/O Series II) was used
to verify the chemical composition of the polymer. Molecular
weight was determined at room temperature using a gel per-
meation chromatography (GPC) apparatus with a light
scattering detector Dawn-EOS (Wyatt Tech.) at λ = 6900 Å
3
3
methyl-1-trimethylsililoxy-propene, TBAF, THF; (B) Reduction
and Oxidation Processes for Nitroxide Radicals
−1
in THF with a flow rate of 0.5 mL min . Refractive index
increment dn/dc for the polymer was calculated as 0.109 mL g−
in both the same solvent and wavelength using a Brice-Phoenix
Visual Refractometer. Thermal behavior was determined
using a MDSC2920 Modulated Differential Scanning
Calorimeter manufactured by TA Instruments (Newcastle,
Delaware, USA). Modulated DSC scans were carried out at
1
Interestingly, only scarce attention has been devoted to the
5
°C/min with amplitude of ±0.5 °C, a period of 40 s, and a
microstructure present in the final synthesized PTMA
structure,
−1
7
−12
50 mL min nitrogen flow within the range of −50 to 180 °C.
A similar analysis was performed using a Thermogravimetric
Analyzer Pyris 1 TGA, Perkin-Elmer, at a heating rate of 10 °C
which could be related to the change in charge
capacity of the polymer structure, due to the different types of
arrangements that the radical species would present as a result
of the polymerization process. These effects can also influence
the energy required to oxidize these radical structures, which
must be related to the high charge/discharge potentials attain-
able during normal operation of batteries. For this purpose, in
this work, PTMA (3) and two precursor molecules 1 and 2
were extensively characterized using NMR, in particular
polymer tacticity, and mass spectrometry; also the stability of
PTMA was evaluated using thermal analysis, and finally,
electrochemical and spectroelectrochemical-electron spin reso-
nance studies in acetonitrile solution provide a systematic
evaluation of the structural factors determining the reactivity in
the final polymeric structure.
−1
−1
min under a 50 mL min nitrogen flow within the range of
5 to 700 °C. Dynamic mechanical analysis (DMA) was
4
performed using a stress-controlled CVO rheometer (Malvern
Instruments). A temperature scan was performed in shearing
mode at 1 Hz ranging from 120 to 210 °C; using a heating rate
−1
of 2 °C min . Measurements of the magnetic susceptibility of
PTMA were performed with direct current SQUID equipment
(IIM-UNAM).
Voltammetric and Electrochemical-Electron Spin
Resonance Measurements. Acetonitrile (CH CN) Merck
3
Spectroscopic grade, distilled from P O and kept
2
5
under molecular sieves (3 Å, Merck), was used as solvent.
Recrystallized tetrabutylammonium hexafluorophosphate from
methanol/ethyl acetate (Fluka Chemika, electrochemical grade,
NBut PF ) was used as the supporting electrolyte. All the
EXPERIMENTAL SECTION
■
Synthesis of Poly(4-methacryloyloxy-2,2,6,6-tetrame-
thylpiperidine-N-oxyl) (PTMA, 3). 4-Hydroxy-2,2,6,6-tetra-
methylpiperidine-N-oxyl (HO-TEMPO, 1) was obtained from
4
6
solutions were purged with high purity argon (Praxair, grade
5.0) for 25 min before each series of experiments. Cyclic
voltammetry experiments were performed with an AUTOLAB
PGSTAT 100 potentiostat/galvanostat. IR Drop correction was
4
-hydroxy-2,2,6,6-tetramethylpiperidine oxidation according to
15
the procedure described by Brier
̀
e, employing a reaction time
1
7,18
of 8 h (Scheme 1A). The final compound was recrystallized
performed during all of the experiments.
A three electrode
15
from hexane: mp 72 °C (Lit. 72 °C). 4-Methacryloyloxy-
,2,6,6-tetramethyl piperidin-N-oxyl (MO-TEMPO, 2) was
cell was used to carry out these experiments: a glassy carbon
2
2
microelectrode (surface, 0.07 cm ) was used as the working
obtained from compound 1 and methacryloyl chloride
following the procedure described by Nesvadba and co-
workers. The obtained compound was recrystallized from
hexane, mp 88−89 °C (mp 91−92 °C). PTMA (3) was
synthesized according to the GTP reaction described earlier,
using 1-methoxy-2-methyl-1-trimethylsililoxy-propene as initia-
tor and tetrabutylammonium fluoride (TBAF) as catalyst in
tetrahydrofurane (THF). The reaction was stirred for 48 h with
electrode. Prior to use, it was polished with 0.05 μm alumina
powder (Buehler), sonicated in distilled water for 10 min, and
̈
1
4
rinsed with acetone; rinsing with acetone was also performed
between each voltammetric run. A platinum mesh was used as
an auxiliary electrode. Potential values were obtained versus a
commercial Saturated Calomel Electrode (SCE) separated from
the solution by a salt bridge and referenced to the ferricinium/
16
14
+
ferrocene couple (Fc /Fc) according to the IUPAC recom-
3
additions of the catalyst (1:1250 with respect to the amount
19
mendation. The potential value of this redox couple versus
the reference electrode was 0.40 ± 0.01 V.
of monomer used) during this period.
Characterization of PTMA. Chemical structures for
1
13
ESR spectra were recorded in the X band (9.85 GHz), using
a Bruker EMX Plus instrument with a rectangular TE102 cavity.
A commercially available spectroelectrochemical cell (Wilmad)
molecules 1, 2, and 3 were analyzed by H and C NMR
(
δ = ppm, J = Hz), (Bruker DMX 500 MHz) in 5−10% weight
solution of CDCl with tetramethylsilane (TMS) as an internal
3
2
was used. A platinum mesh (0.7 cm ) was introduced into the
reference.
1
flat path of the cell and used as a working electrode. A platinum
PTMA tacticity was determined by employing H NMR
2
(
1
δ = ppm, J = Hz), (Varian MR 400 MHz) at 160 °C in a
wire was used as an auxiliary electrode (2.5 cm ). Ag/AgNO
3
6.7 mg mL− solution in a mixture of solvents (41.7%
1
−1
−1
0.01 mol L + 0.1 mol L NBu
PF
in acetonitrile was
4
6
o-dichlorobenzene, and 58.3% deuterated DMSO, which was
employed as the reference electrode. Potential control was per-
formed with an AUTOLAB PGSTAT 100 potentiostat.
−3
−1
used as reference). PhNHNH (10.1 × 10 mg mL ) was used
2
5
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The Journal of Physical Chemistry B
Article
RESULTS AND DISCUSSION
determined by modulated differential scanning calorimetry
■
(
(
MDSC) and confirmed by dynamic mechanical analysis
DMA, see below). MDSC trace for reversible heat obtained
Synthesis and Characterization of PTMA. Synthesis of
PTMA was carried out by employing a group transfer
polymerization (GTP) reaction (Scheme 1) according to
from the second heating is shown in Figure 2, from which the
1
4
1
13
Bugnon et al. A complete description of H and C NMR
spectra for compounds 1, 2, and 3 is presented in the
Supporting Information for this article. Elemental analysis
for the synthesized PTMA structure, C (64.97%); H (9.23%);
N (5.83), corrected due to water presence in the sample
(
see below), is in agreement with percentages calculated
for C H NO (240.33): C (62.88%); H (9.22); N (5.64%),
13
22
3
confirming also the structure proposed. The obtained polymer
presented a molecular weight of 36 240 g mol−1 with a
polydispersity M /M of 1.532 (M and M represent the
w
n
w
n
molecular weight and number averaged molecular weight,
respectively), indicating a mean content of 151 repetitive units.
PTMA was soluble in tetrahydrofurane, chloroform, and
acetonitrile.
Thermal stability of 3 obtained by GTP was studied by
employing a combined analysis using TGA and derivative thermo-
gravimetric analysis (DTGA). Synthesized PTMA was stable
up to 200 °C (Figure 1), and the decomposition temperature
Figure 2. Variations of reversible heat flow (solid line) and reversible
Cp (dashed line) employing MDSC analysis (third scan plotted).
Arrows indicate the beginning of Tg.
Tg value occurred at 153 °C. This value is significantly higher
than the previously reported value of 76 °C for PTMA
22
synthesized by radical polymerization. As the T is a quasi-
g
reversible property, its value could also be confirmed by
evaluating the reversible C curve in the thermogram obtained
p
by modulated DSC. This shows a variation in the reversible Cp
starting at 153 °C. Also, no changes in the reversible C value
p
were observed at the temperature range around the T value
g
23
(
76 °C) reported by Kim and co-workers, thus confirming
that PTMA obtained by GTP shows different properties to that
obtained by radical polymerization. The glass transition
temperatures were further verified by carrying out dynamic
mechanical analysis (DMA). The storage shear modulus (G′)
and the viscous shear modulus (G″) as a function of temper-
ature for PTMA are shown in Figure 3. In dynamic mechanical
Figure 1. Thermogravimetric and differential thermogravimetric
analysis plots for PTMA.
(
corresponding to 10% of weight loss) was observed at 270 °C,
20
close to a previously reported value (263 °C). A weight loss
of 3% that approximately ends at 170 °C (Figure 1) was
associated with water elimination of the sample. Thermal
degradation of PTMA showed two degradation steps starting
around 200 and 400 °C. However, a closer inspection of DTGA
curves of PTMA showed that the first degradation step is
composed of two overlapped peaks, one sharp signal at 275 °C
and another wide and ill-defined signal around 300 °C. The
first decomposition step, which includes the two overlapping
peaks mentioned, corresponds to 67% of weight loss and is
−1
associated with 161.02 g mol of the total molar mass of each
repetitive unit, corresponding to the elimination of a piperidine
ring derivative (154 g). The second step (16% of weight loss,
−1
corresponding to 38 g mol considering the molar mass of
each repetitive unit) is associated to decarboxylation of the
resulting polymer after the first decomposition in accordance
−1
Figure 3. Storage (■) and loss shear (Δ) moduli at 1 rad s plotted
against temperature for PTMA. Onset occurs at 162 °C.
21
with previous reports.
Differential scanning calorimetry (DSC) and thermogravi-
metric analysis (TGA) were used to determine the glass transi-
analysis, the glass transition temperature can be measured,
while the temperature is scanned, although it is not uniquely
defined because it is dependent on the frequency applied, and it
is also conventionally determined by either the onset in the
tion temperature (T ) and thermal stability of the polymer,
g
respectively. The glass transition temperature T for PTMA was
g
5
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storage modulus G′ (Tg,G′) or the temperature at maximum in
the loss modulus G″ (Tg,G″
́
). The peak at 167 °C is attributed to
the α mechanism associated with the transition from glasslike
2
4,25
to rubberlike consistence.
The value of 162 °C obtained in
onset of G′ is close to the T value obtained by DSC (Figure 2).
g
This small difference in T values is usually obtained upon
g
26
comparing results from DMA and DSC techniques.
The high T value found in this work for PTMA could be
g
associated to the ability of the nitroxide (NO) groups to act as
acceptors of hydrogen bonds and the possibility of the forma-
20,27−30
tion of hydrogen-bonded networks.
SQUID measure-
ments revealed that the magnetic susceptibility of PTMA differs
from the expected behavior for a paramagnetic material at
low temperatures (Figure 4), revealing a certain ferromagnetic
1
Figure 5. H NMR of poly(4-methacryloyloxy-TEMPO) (PTMA, 3)
5
−10% weight in solution of CDCl and chemically reduced with an
added excess of phenylhydrazine. The spectrum was obtained at room
temperature.
3
Figure 6. 13C NMR of poly(4-methacryloyloxy-TEMPO) (PTMA, 3)
Figure 4. Variations of molar magnetic susceptibility (cM) as a
function of temperature. Inset represents the variations for T > 81 K.
5−10% weight in solution of CDCl and chemically reduced with an
3
added excess of phenylhydrazine. The spectrum was obtained at room
temperature.
31
nature of this polymer and supporting the proposal referred
above. Regardless of the nature and strength of these
interactions, aggregates must be favored by increased radical
concentration because PTMA has a radical per repetitive unit,
so that above a certain concentration level, several kinds of aggre-
signal at 43.1, and a quaternary carbon appeared at 44.7 ppm.
Both gem dimethyl carbons are fused in a large singlet (31.8 ppm)
and the methyl group of the backbone chain was assigned to the
signal at δ = 20.08. DEPT experiment of the reduced PTMA
(Figure SI-12, Supporting Information) confirmed that the signal
at δ = 67.3 ppm corresponds to the oxygen base methyne, and the
methylene in the chain is located at δ = 58.6 ppm. All these signals
confirm the PTMA structure.
32
gates may coexist. These aggregates and interactions diminish
the flexibility of the chain of polymer and then T must
g
increase.
Spectroscopic Characterization of PTMA and Its
Precursors. The precursors of PTMA were characterized by
NMR and mass spectrometry corroborating the proposed
structures (1 and 2 in Scheme 1) (Supporting Information
shows the spectroscopic characterization of the PTMA pre-
3
2,33
Bovey and co-workers
have found that the signals of
1
interest for the H NMR microstructural elucidation of metha-
crylic polymers are those corresponding to α-methyl and β-
methylene. In the case of PTMA, we have found some
difficulties in the experimental NMR determination of tacticity
due to the fact that signals corresponding to axial and equatorial
protons, at positions 3 and 5 of piperidine ring, overlap with
signals of methylene groups in the polymer backbone. Similarly,
signals of methyl groups at positions 2 and 6 of piperidine ring
overlap with those of α-methyl. Polymers confer a high
viscosity to their solutions through entanglement and through
entrapment of solvent molecules. If molecular motion is
allowed to take place, i.e., by raising the temperature, a narrow-
ing of the resonance line will be observed due to the relaxation
33
cursors). PTMA (3) obtained by GTP reactions was charac-
1
13
terized by H and C NMR. Because of the paramagnetic effect
of the N−O radical, the spectrum showed poor resolution,
1
lacking also of signals corresponding to the TEMPO unit ( H
and 13C NMR, Figure SI-11A and Figure SI-11B, Supporting
Information respectively). Therefore, treatment of the polymer
with phenylhydrazine allowed obtaining the spectra of the
corresponding hydroxylamine of 3 with the complete number
of NMR signals of hydrogen and carbon atoms in PTMA.
1
Figure 5 shows the H NMR spectrum of PTMA after
33
reduction with an excess of phenylhydrazine. Polymer
formation was deduced as only aliphatic protons are exclusively
observed.
time increased, and therefore, deconvolution is easier. In
order to solve such overlapping and quantify the areas of the
1
relevant signals H NMR spectra were obtained at 160 °C in a
13C NMR spectrum (Figure 6, signal assignment is shown
in the spectra) showed the presence of the ester function at
δ = 175.6 and the presence of two methylene groups as a broad
solvent mixture of o-DCB and d -DMSO; a deconvolution of
6
the NMR signals at these spectra was carried out, and
experimental NMR determination of tacticity was achieved.
5
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After deconvolution (see Supporting Information for the
deconvolution strategy), signals where assigned as shown in
Figures 7 and 8.
great overlapping of signals, the match between calculated and
experimental quantities could be considered as satisfactory and
proves that signal assignment after deconvolution is reasonable.
The facts described above validate the assertion that PTMA
synthesized by GTP, at the experimental conditions used in this
work, is a polymer with 84% of isotactic triads, 12% of hetero-
tactic triads, and 4% of syndiotactic triads (the combined per-
centages of hetero- and syndiotactic triads is 16% of the total
triads).
Electrochemical and Spectroelectrochemical-ESR Study
of the Nitroxide Containing Monomers and PTMA. In
order to understand the effect that structural modifications have
on the stability of the nitroxide radical, an electrochemical and
spectroelectrochemical-electron spin resonance (ESR) study of
compounds 1 and 2 and the polymer 3 was performed.
For compound 1, the corresponding cyclic voltammogram
(Figure 9A) shows the reversible monoelectronic process (peaks
1
Figure 7. Methylenic portion of the H NMR spectrum for PTMA, 3
chemically reduced with phenylhydrazine in o-dichlorobenzene and
deuterated DMSO solution. Assignments are shown in the spectrum.
The spectrum was obtained at 160 °C.
Figure 9. Cyclic voltammograms for (A) 1 × 10 mol L−1 1 and (B) 1 ×
−
3
−
3
−1
−1
2
10
mol L 2 in 0.1 mol L Bu NPF /CH CN. WE, GC (0.07 cm );
4 6 3
−1
v = 100 mV s . All voltammetric signals are indicated.
0
+
Ic and Ia, E = 0.26 V vs Fc /Fc, ΔE = 62 mV). A shoulder
p
1
Figure 8. High-field portion of the H NMR spectrum for PTMA (3)
signal was observed after this oxidation process (peak Ia′),
which did not affect the reversible behavior of the system Ia/Ic.
This signal is probably related to an impurity present in the
original commercial sample.
chemically reduced with phenylhydrazine in o-dichlorobenzene and
deuterated DMSO solution. Assignments are shown in the spectrum.
The spectrum was obtained at 160 °C.
In the case of 2, a reversible one-electron transfer (peaks Ia
0
+
In fact, signals labeled as an AB quartet in Figure 7 are
centered at 1.98 ppm, and the AB quartet has a coupling con-
stant of 15.46 Hz. Both facts strongly correlate with evidence
found by Bovey et al. for PMMA of an AB quartet centered at
and Ic; E = 0.32 V vs Fc /Fc, ΔE = 71 mV, Figure 9B), in a
p
similar fashion as occurred for compound 1 (Figure 9A) was
observed. This oxidation process is the only one appearing in the
electrochemical window of the employed medium. The difference
32−34
0
2
.09 ppm and a coupling constant of 14.9 Hz.
This AB
in E values for both compounds is associated to the inductive
quartet is associated with an isotactic microstructure, and the
signals at the center of the quartet are related with a syndio-
effect of the methacryloyloxy group at position C-4, acting as an
electron-withdrawing group, leading compound 2 to become less
3
1−33
0
tactic one.
The former is related to meso dyads and the
prone to oxidation, increasing its E value compared to that of
3
1,32
36,37
later with racemo ones.
compound 1.
This effect is relevant, as the molecule lacks π
Figure 8 shows three signals at 1.31, 1.27, and 1.23 ppm. The
chemical shifts of these signals strongly resemble the values of
conjugation to consider this effect as resonant, and would therefore
be regarded as a field-inductive effect.
37
1
.33, 1.21, and 1.1 ppm found for PMMA by Bovey et al. for
Compounds 1 and 2 presented paramagnetic activity,
characterized by the presence of an organic radical signal
with hyperfine couplings related with the interaction between
the electronic spin of the unpaired electron and the nuclear
spin of the adjacent nitrogen atom (Figure 10; 1, HFCC =
15.66 G; 2, HFCC = 15.55 G) generating a triplet structure.
These signals disappear progressively with time by setting the
applied potential in a spectroelectrochemical cell at 0.34 and
3
2,33
iso-, hetero-, and syndiotactic triads, respectively.
Integra-
tion of the areas of the α-methyl peaks allowed the quan-
tification of their normalized frequencies as 0.84, 0.12, and 0.04
for (mm), (mr), and (rr), respectively. Use of these values and
well-known mathematical relationships between dyads and
triads yield frequencies of 0.9 and 0.1 for (m) and (r) dyads,
3
3,35
respectively.
Experimental determination of (m) and (r)
+
produces values of 0.95 and 0.05, respectively. Considering the
0.44 V vs Fc /Fc, respectively (Figure 10A,B). This result
5
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The Journal of Physical Chemistry B
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its diffusion coefficient must be the lowest. This result indicates
that the only factor that contributes to increase the current
intensity for PTMA until 30 μA is the electron number, which
must be close to 151 (36 240/240). This value corresponds to
the number of methacryloyloxyl-TEMPO units present in the
polymer structure, which are oxidized at the level of peak Ia
shown in Figure 11.
PTMA shows paramagnetic activity (g ≈ 2), characterized by
the presence of a broad central signal (Linewidth around 10 G).
This signal did not improve its resolution by adjusting both the
power of irradiated microwaves or modulation amplitude
(
dashed line, Figure 12). Because of the significant broadness of
−3
Figure 10. Spectroelectrochemical-ESR spectra of (A) 1 × 10 mol
−
1
−3
−1
−1
L
1 and (B) 1 × 10 mol L 2 in 0.1 mol L Bu NPF /CH CN.
4 6 3
2 +
WE, Pt (0.7 cm ); applied potential, A, 0.34 V vs Fc /Fc and B, 0.44 V
vs Fc /Fc. Arrows indicate increasing electrolysis times that provoke
+
signal disappearance.
indicates that the observed voltammetric peaks correspond to
the monoelectronic oxidation of the nitroxide radical generating
the corresponding oxoammonium cation (Scheme 1B).
PTMA shows a different electrochemical and spectral
3
8
behavior. Cyclic voltammograms for PTMA show the pre-
sence of signal Ia/Ic (Figure 11); as a reversible electron
Figure 12. Spectroelectrochemical-ESR spectra of a solution
−
1
containing 2.5 mg of PTMA, 3, in 4 mL of 0.1 mol L Bu NPF /
4
6
2
+
CH CN. WE, Pt (0.7 cm ). Applied potential, 0.6 V vs Fc /Fc. Dashed
3
line indicates initial spectrum. Solid line indicates spectrum obtained
after 50 min of electrolysis. Arrow indicates variations of the spectra
upon increasing electrolysis times.
the central signal, it can be presumed that the amount of radical
species related to the central signal is high as the ESR signals
would show coalescence due to fast spin exchange mecha-
39
nisms. This spectral pattern, obtained in solution, is similar to
that reported in a previous study where solid composite electro-
10
des were prepared with PTMA, suggesting that spin density
distribution is not affected by the medium in which the polymer
is analyzed. It has also been reported that this broadening is
related to interchain interaction within the polymer structure,
which support the proposed fast spin exchange process sug-
gested.
Figure 11. Cyclic voltammograms for a solution containing 2.5 mg
−
1
of PTMA, 3, in 10 mL of 0.1 mol L Bu NPF /CH CN. WE, GC
4
6
3
2
−1
(
0.07 cm ); v = 100 mV s . Different inversion potential conditions
are depicted. All voltammetric signals are indicated.
Spectroelectrochemical-ESR measurements applying poten-
tial values more positive than peak Ia lead to the disappearance
of the central signal (Figure 12). In this process, another
spectral structure becomes visible, showing a triplet hyper-
fine coupling structure (HFCC = 15.86 G, g = 2.0104, solid
line, Figure 12). This result indicates that the original spec-
trum (dashed line, Figure 12) is the sum of two overlapping
spectra arising from two different organic radical species.
Upon applying potential values more positive than peak IIa
0
+
transfer (E = 0.37 V vs Fc /Fc, ΔE = 15 mV). Also, a second
p
irreversible oxidation signal, peak IIa, occurs at more positive
potential values (EpIIa = 1.6 V vs Fc /Fc). It should be noticed
that the presence of peak IIa has not been presented during
previous studies with this polymer.
because the previous studies have been performed in solvents
with a potential window shorter than that of acetonitrile.
It is noticeable that the voltammograms of both PTMA
+
5
−12
This is probably
+
(1.7 V vs Fc /Fc), the second radical structure is consumed
(
Figure 11) and compound 2 (Figure 9B) show a similar value
(Figure 13). This behavior, along with the spectral structure
for this signal, is analogous to that reported for nitroxide
radicals proceeding from TEMPO species, considering only
of current intensity for peaks Ia. This similarity can be ration-
alized by considering that, at a constant scan rate, the current
peak is proportional to the concentration of electroactive
species, the diffusion coefficient and the electron number. In
both experiments, the solutions were prepared with a similar
amount of these compounds: 2.4 and 2 mg, respectively.
However, considering that the molecular weight of com-
1
0
the first signal (peak Ia, Figure 11). However, the corres-
ponding voltammetric signal is not reversible for the case of
PTMA (peak IIa, Figure 11), indicating that the correspond-
ing formation of the cationic species is followed by a coupled
chemical process, probably with the nucleophilic solvent,
such as in the case of the Ritter reaction in which carboca-
−1
pound 2 is too low (240 g mol ) with respect to the polymer
(
−1
40
36240 g mol ), the molar concentration of PTMA as well as
tions react with acetonitrile.
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The Journal of Physical Chemistry B
Article
for each polymer chain being oxidized, thus rendering sites marked
B as more difficult to be oxidized. B-labeled sites would then be
oxidized at the second peak following the reaction
+
−
+
+
(
A ) − (B) – me ↔ (A ) − (B )
(Peak IIa)
n
m
n
m
+
These B species undergo a coupled chemical reaction to generate
non-electrochemically active species C
+
+
+
(
A ) − (B ) → (A ) − (C)
n
m
n
m
+
Sites A are not affected in this chemical process as peak Ic still
appears during the inverse scan (Figure 11). This suggests that,
even though the charging of the polymer determines the potential
values at which the secondary radical is being oxidized, it does not
determine specific reactions on each site. It should be noticed that,
within the polymer structure, an irregular distribution of the sites is
expected, but in the overall, the proportions are consistent with the
experimental tacticity. Because of the charging of the chain
occurring at both peaks, it is possible that specific solvation of the
cationic species can determine the potential values of these signals,
an issue that is currently under study.
Figure 13. Spectroelectrochemical-ESR spectra of a solution
containing 2.5 mg of PTMA, 3, in 4 mL of 0.1 mol L−1 Bu NPF /
4
6
+
CH CN after being electrolyzed for 50 min at 0.6 V vs Fc /Fc. WE, Pt
3
2
+
(
0.7 cm ); applied potential, 1.7 V vs Fc /Fc. Dashed line, initial
spectrum; solid line, spectrum obtained after 50 min of electrolysis.
Arrow indicates variations of the spectra upon increasing electrolysis
times.
To determine the amount of spins related to each particular
radical, double integration of the experimental spectra obtained
both before performing electrochemical oxidation of the sample
and later by oxidizing the first radical species (dashed and solid
As these two radical species are present in the structure of
PTMA, their properties and relative proportions could be
20
related to the microstructure present in the entire chain. It
would be reasonable to correlate this variety of configurations
to different chemical environments for nitroxide radicals within
iso-, hetero-, and syndiotactic triads. In fact, the finding that
38
lines, Figure 12) was performed. With this procedure, a total
18
of 5.29 × 10 spins was determined for the sum of both radical
species present in PTMA (2 mg of the polymer). Considering
the molecular weight of PTMA, this spin count is close to the
value expected considering a mean proportion of 151 repetitive
7
8% of radical species (peak Ia, Figure 11) within PTMA
behaving in a very different way that the remaining 22% (peak
IIa, Figure 11), from spectroelectrochemical studies, and the
quantification of 84% of isotactic triads and 16% of
heterotactic/syndiotactic triads (determined by signal deconvo-
18
units in the structure of the polymer (∼5.18 × 10 spins).
Therefore, the total number of spins detected by ESR indicates
that the radical content in the polymer structure is about 100%
of the expected value. This calculation was validated by
evaluating the variations of the molar magnetic susceptibility of
PTMA at temperatures above 81 K (Inset, Figure 4), where a
fraction of 94% of spin character was determined per repetitive
1
lution of high temperature H NMR spectra, Figures 7 and 8)
strongly suggests, within experimental error, a correlation
between spectroelectrochemical behavior and microstructural
features. The reversible behavior of peak Ia (Figure 11) would
be related to the oxidation of a high number of structurally
similar radicals in the isotactic microstructure, leading to a
3
4
18
unit. However, only 22% of these spins (∼1.17 × 10 spins)
were related to nitroxide radicals, oxidized at peak IIa, while the
remaining 78% of radical species have a different spin density,
oxidized at peak Ia (Figure 11).
39
broad spectral line width (Figure 12). This broadening can
also be originated from interchain effects by interaction of
radical species within PTMA structures, as has been reported in
The large shift in potential values (EpIa − E ≈ 1.25 V) for
pIIa
10
the oxidation of both types of nitroxide radicals in the polymer
structure can be associated to changes in the electronic charge
occurring for the chain being oxidized, due to the high amount
independent studies. However, the species being oxidized at
peak IIa, corresponds to radical species in the syndiotactic/
heterotactic domain of the polymer. It should be noticed that
the hyperfine structure of the involved radicals (Figure 13)
resembles more so those from the original TEMPO structures
−
38
of electrons being removed at peak Ia (about 118 e ); this
significant decrease in electron density leads to the formation of
a structure that becomes less prone to be oxidized, requiring
more positive potential values to do so (Figure 11, peak IIa).
This effect could be related to the highly positive structure
(
Figure 10), probably due to less pronounced dynamic spin
39
exchange processes as the radical sites are spatially more
separated than those considering isotactic domains. The
presence of both types of radical species has not been
considered in earlier reports, presumably due to the lack of
characterization on extended oxidation potential scales, due to
3
6,37
acting as a strong electron-withdrawing group,
resulting in
the large shift in potential values. Peak IIa has not been
addressed in previous studies concerning PTMA, probably due
to the less positive value of the anodic barrier occurring for the
solvents employed in such works (e.g., propylene carbo-
5
−12
the solvents employed.
In an isotactic triad, the central monomeric unit is flanked by
two monomers with the same configuration; in comparison,
heterotactic triads present the central unit as a monomer with
the same configuration at one side and a monomer with the
opposite configuration at the other side, while, in a syndiotactic
triad, the central monomer is surrounded by two units with
opposite configuration. This would make reasonable to
consider that this variety of configurations would lead to
different chemical environments for nitroxide radicals within
iso-, hetero-, and syndiotactic triads.
5
−10
nate).
By representing the polymer oxidizable sites as A
(
isotactic) and B (hetero/syndiotactic domains), respectively,
the electrochemical processes occurring can be represented
schematically as follows
−
+
(
A) − (B) − ne ↔ (A ) − (B)
(Peak Ia)
n
m
n
m
This reaction is electrochemically reversible, as denoted by the
presence of peak Ic (Figure 11). The product of this reaction
carries an n positive charge, which would be approximately +118
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The Journal of Physical Chemistry B
Article
CONCLUSIONS
ACKNOWLEDGMENTS
■
■
In this work, spectroscopic, thermal, and electrochemical
characterization results are presented for the redox active
polymer poly(4-methacryloyloxy-tetramethylpiperidine-N-
oxyl), PTMA, synthesized by group transfer polymerization
We acknowledge Mrs. Isabel Chav
Perez, Elizabeth Huerta, Ma de los Angeles Pen
Zavala, and Atilano Gutierrez for their technical assistance and
to Dr. Angel Romo-Uribe (Instituto de Ciencias Fısicas
UNAM) for the DMA analysis. Also, we acknowledge the
assistance of Professor Roberto Escudero (IIM-UNAM) for his
help on obtaining magnetic susceptibility measurements.
Financial supports from the CONACyT-Mexico projects No.
́
ez, Rocío Patin
̃
o, Javier
́
̃
a, M. Nieves
́
-
(
GTP) and its precursors 4-hydroxy-tetramethylpiperidine-N-
oxyl (HO-TEMPO) and 4-methacryloyloxy-tetramethylpiper-
idine-N-oxyl (MO-TEMPO). The procedure presented in this
work is helpful to determine the content of redox useful units in
a TEMPO based polymer like PTMA. As a whole, the facts
exposed above validate the assertion that PTMA synthesized by
GTP (known for yielding polymer with 100% of the expected
5
7856, No. 60686, and No. 107037, PAPIIT-UNAM 202011,
and Instituto de Ciencia y Tecnología del DF project
PICCO11-22 are also acknowledged. L.H. and C.F. thank
CONACyT-Mexico for financial support for their Ph.D. and
postdoctoral studies, respectively. H.A.L.-P. thanks SNI Mexico
for financial support for his B. in Sc. studies.
1
4
radical species), at the experimental conditions used in this
work, is a polymer with 84% of isotactic triads, 12% of
heterotactic triads, and 4% of syndiotactic triads (the combined
percentages of hetero- and syndiotactic triads is 16% of the
total triads). In fact, the finding of 78% of radical species within
PTMA behave in a very different way that the remaining 22%
after spectroelectrochemical studies and the quantification, by
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