TEMPO Containing Radical Polymonothiocarbonate Polymers with Regio- and Stereo-Regularities: Synthesis, Characterization, and Electrical Conductivity Studies

Article abstract of DOI:10.1002/anie.202108041

We report the synthesis of a (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl) (TEMPO) appended polymonothiocarbonates through the ring-opening copolymerization of (4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (GTEMPO) in the presence of carbonyl sulfide under ambient conditions. We have prepared the atactic and isotactic versions of this polymer, using enantiopure R or S forms of the GTEMPO monomer in the latter instances. Cyclic voltammetry studies revealed both oxidation and reduction events that were characteristic of TEMPO radicals. Electrical conductivity of these polymers was measured as solid-state films after annealing the samples above their glass transition temperatures. At room temperature the isotactic polymer shows much greater conductivity (ca. 10^?4 S cm^?1) than the atactic (ca. 10^?7 S cm^?1), attributed to the well-defined stereochemistry and regulated charge transport pathway of isotactic polymer chains in contrast to the irregular structure of the atactic counterpart.

Full text of DOI:10.1002/anie.202108041

Angewandte
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How to cite:
International Edition: doi.org/10.1002/anie.202108041
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Copolymerization
Hot Paper
TEMPO Containing Radical Polymonothiocarbonate Polymers with
Regio- and Stereo-Regularities: Synthesis, Characterization, and
Electrical Conductivity Studies
Gulzar A. Bhat, Ahmed Z. Rashad, Xiaozhou Ji, Manuel Quiroz, Lei Fang,* and
Donald J. Darensbourg*
Abstract: We report the synthesis of a (2,2,6,6-tetramethylpi-
peridin-1-yl)oxidanyl) (TEMPO) appended polymonothio-
carbonates through the ring-opening copolymerization of (4-
glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (GTEMPO)
in the presence of carbonyl sulfide under ambient conditions.
We have prepared the atactic and isotactic versions of this
polymer, using enantiopure R or S forms of the GTEMPO
monomer in the latter instances. Cyclic voltammetry studies
revealed both oxidation and reduction events that were
characteristic of TEMPO radicals. Electrical conductivity of
these polymers was measured as solid-state films after anneal-
ing the samples above their glass transition temperatures. At
room temperature the isotactic polymer shows much greater
conductivity (ca. 10À4 ScmÀ1) than the atactic (ca. 10À7 ScmÀ1),
attributed to the well-defined stereochemistry and regulated
charge transport pathway of isotactic polymer chains in
contrast to the irregular structure of the atactic counterpart.
co-workers in 1967.[22] Later, Kishita and co-workers reported
that the nitroxide groups in these redox active polymers were
reversibily oxidized to oxiammonium cations, which following
two decades of research have resulted in their use as the
positive electrode (cathode) in rechargeable batteries.[23–26]
The early electrical conducting materials explored were
polymers with p-conjugation along their molecular back-
bones, which were later doped to achieve higher conductiv-
ities.[27,28] Subsequently, it was evident that most of these
polymers have several drawbacks, such as a lack of optical
transparency in the visible region, complicated synthesis, poor
processability and low stability as the result of different
dopants. These issues were addressed by Boudouris and co-
workers who examined the solid state electrical conductivities
of charge neutral radical polymers having open shell stable
pendant radical moieties.[29] The initial studies of these type of
radical polymers culminated in 2018 of polymers with an
electrical conductivity of 28 SmÀ1.[30] The pathway for charge
transport in these organic radical polymers is thought to be
electron or hydrogen hopping from radical site to radical site
with concomitant sequential motion of the polymer chains.[31]
It was further deduced from these studies that the backbone
structure of these radical polymers can significantly affect
their conductivity. Figure 1 lists a comparable group of
TEMPO based polymers with various polymer backbones.
Despite these significant advances made in this field, it is still
a great challenge to develop feasible synthetic methodologies
for these radical-pendant polymers with full control of the
regiochemistry.
R
ecently, interest in redox active radical polymers have
garnered significant attention as functional materials in the
fields ranging from organic electronics,[1–5] energy storage,[6–9]
high performance conductors,[10,11] high-efficiency solar
cells,[12,13] light-emitting devices,[14,15] electrochromism,[16]
organocatalysis,[17] biological imaging agents[18–20] and other
energy related modules. This is primarily due to these
polymers intrinsic potential to offer chemical and electro-
chemical tunable solutions for applications which are tradi-
tionally dominated by metals. Among the diverse macro-
molecular redox active polymers, 2,2,6,6-tetramethylpiperi-
din-1-yl)oxyl (TEMPO) containing polymers are well studied
because of their high innate stability, fast and reversible redox
kinetics, and ease of functionalization.[21] The first radical
polymer, (2,2,6,6-tetramethyl piperidinyl oxymethacrylate)
(PTMA) was synthesized by functionalizing polymethacrylate
(PMA) with stable radical TEMPO moieties by Griffith and
Inspired by the recent report by Savoie, Boudouris and co-
workers on the polyether produced by the ROP of an epoxide
containing a TEMPO substituent (GTEMPO),[30] we have
employed this epoxide to synthesize monothiocarbonate
polymers featuring pendant radical TEMPO moieties. This
was accomplished using the ring-opening copolymerization
(ROCOP) methodology employing the well-defined binary
[*] G. A. Bhat, A. Z. Rashad, X. Ji, M. Quiroz, L. Fang, D. J. Darensbourg
Department of Chemistry, Texas A&M University
College Station, TX 77843 (USA)
E-mail: fang@chem.tamu.edu
djdarens@chem.tamu.edu
L. Fang
Department of Material Science and Engineering, Texas A&M
University
College Station, TX 77843 (USA)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202108041.
Figure 1. Comparison of radical polymers having different backbones.
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catalytic system of (salen)CrCl/PPNCl. A major advantage of
this procedure is that it provides an uncomplicated synthetic
route to both regio- and stereo-regular radical polymers. That
is, using rac- or (R or S)-epichlorohydrin it is possible to
prepare large quantities of a TEMPO containing epoxide
monomer in a single step. This coupled with the fact that the
ring-opening of the TEMPO epoxide during the copolymer-
ization with carbonyl sulfide (COS) occurs highly regioselec-
tively at the less-hindered methylene carbon, affords stereo-
regularity in the copolymer consistent with that of the epoxide
monomer. In this manner, we have prepared a regioregular
atactic copolymer from rac-GTEMPO and COS, as well as the
isotactic versions using the enantiopure R and S forms of the
GTEMPO monomer. Furthermore, this process can be easily
scalable to the kilogram level. Herein, we have examined the
solid-state electrical conductivities of these radical copoly-
mers, and have shown that the polymers with controlled
stereochemistries (R or S) display much better conductivity
than their stereoirregular analogues.
Scheme 1. Synthesis of radical containing copolymers using COS and
GTEMPO.
coupling of COS and GTEMPO is highly selective at room
temperature for copolymer formation, at 708C both cyclic and
polymeric monothiocarbonate are produced. Table 1 contains
the copolymers obtained at different monomer loading,
where the molecular weights were controlled by the amount
of trace water or hydrolyzed epoxide (CTA) in the epoxide
monomer.
The epoxide monomer, GTEMPO was synthesized as
previously described from epichlorohydrin and 4-hydroxy-
2,2,6,6-tetramethylpiperidin-N-oxy.[32] Attempts at copoly-
merizing GTEMPO with CO2 were not successful in the
presence of well-defined (salen)CoCl and (salen)CrCl binary
catalyst systems at ambient temperature. Instead, when the
coupling reaction of GTEMPO and CO2 was carried out at
808C using the (salen)CrCl/PPNCl catalyst, a cyclic organic
carbonate product was obtained. The compound was fully
characterized spectroscopically (SI) and crystallographically
(Figure 2). As we have previously noted, epoxides which are
unreactive with CO2 under these conditions are often reactive
with carbonyl sulfide. Indeed, this was shown to be the case as
described below.
Copolymerization of rac-GTEMPO and COS was per-
formed in a 1:1 solvent of dichloromethane/toluene in
a stainless steel reactor at ambient temperature in the
presence of (salen)CrCl/PPNCl catalyst at a pressure of
1.0 MPa (Scheme 1). This ROCOP reaction pathway repre-
sents the first reported preparation of a polymer containing
pendant TEMPO groups utilizing this strategy. Although the
Table 1: Summary of the copolymerization of GTEMPO and COS using
different equivalents of the GTEMPO monomer.[a]
Entry
Monomer/Cat/Co-Cat
Mn [kgmolÀ1
]
PDI
1
2
3
250:1:1
500:1:1
750:1:1
10.8
11.1
11.3
1.6
1.77
1.76
[a] Reaction conditions: ambient temperature in DCM/toluene at
1.0 MPa for 24 h. The polymers are atactic. Molecular weight is
controlled by water in monomer.
In our and others previously reported studies, the
copolymerization reaction of mono-substituted epoxides
with bulky electron-donating substituents, ring-opening
occurs highly selectively at the methylene carbon with COS
insertion with metal sulfur bonding, resulting in formation of
tail-to-head copolymers as depicted in Scheme 2.[33] Hence,
upon utilizing enantiopure R or S versions of the GTEMPO
monomer (synthesized from R- or S-epichlorohydrin) should
afford isotactic polymonothiocarbonate copolymers (see
below).
Formation of these radical copolymers was confirmed by
1
(IR, H and 13C NMR, EPR, MALDI-ToF) spectroscopies.
That is, the FTIR spectrum shows the characteristic mono-
thiocarbonate peak at 1718 cmÀ1 in addition to the peaks due
to the GTEMPO monomer, and the 1H NMR spectrum of the
radical quenched sample clearly supports ROCOP of the
GTEMPO monomer (see SI for details). In the radically
quenched 13C NMR spectrum of the T-H atactic copolymer
two 13C signals are evident, whereas, as anticipated only one
13C signal is observed for the isotactic polymers (Figure 3a).
1H NMR showed no quantifiable ether linkages in the
copolymer, and there were no O/S exchange processes
noted in the ambient temperature copolymerization process
as indicated by FTIR and 13C NMR spectroscopies.
Further characterization of the copolymers provided by
the polymerization of COS and GTEMPO was obtained by
MALDI-ToF spectroscopy. In Figure 3b is shown the
MALDI spectrum of the low molecular weight portion of
the copolymer prepared (entry 1 of Table 1), which depicts
Figure 2. X-ray crystal structure of the cyclic carbonate derived from
[35]
À
GTEMPO and CO2. N O distance of 1.285 ꢁ.
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Scheme 2. Mechanistic pathway for the formation of tail-to-head copolymer.
spectroscopic studies in solution. The EPR spectra of these
copolymers exhibit a single signal centered around 3280 G
rather than showing hyperfine splitting owing to nitrogen
atoms (I = 1) (Figure S15). This has also been seen in the
literature for high molecular weight radical polymers, where
the exchange interaction between closely spaced radicals
quenches the hyperfine-induced triplet line shape of the
TEMPO molecule in solution.[34] The g-value in these
TEMPO based polymers is quite similar to that of the free
electron (g = 2.0023).
The thermal stability of both atactic and isotactic copoly-
mers were determined by thermal gravimetric analysis
(TGA). These measurements revealed that these copolymers
are thermally stable, with the temperature at which 50% of
the polymer remains (Td50value) is around 2708C as indicated
in Figure 4. DSC curves of all the copolymers showed Tgs of
around 588C.
To gain insight into the redox behavior of these radical
copolymers, cyclic voltammograms of polymer samples were
recorded in DMF solutions. Measurements were carried out
at ambient temperature under argon using 0.1 M [tBu4N][PF6]
as the supporting electrolyte, and referenced to Fc+/0 (E1/2
=
0.0 V) as an internal standard. Commencing in the anodic
direction, both atactic and isotactic copolymers have a rever-
sible oxidation at almost identical potentials ranging from
0.25 to 0.28 V, which are assigned to the oxidation of the
TEMPO moieties (Figure 5).
These values are similar to that of the pure monomer
(GTEMPO). Upon scanning in the opposite direction these
radical copolymers exhibit a reversible reduction that is
associated with a one electron event on the TEMPO moieties
with essentially identical potentials ranging from À1.95 to
À1.98 V. As observed in the pure monomer, the irreversibility
of this reduction event was confirmed upon increasing the
scan rates. Full scans and scan rate dependence of GTEMPO,
isotactic (R and S), and atactic copolymers are indicated in
Figure 3. a) Comparison of carbonyl 13C NMR region of the isotactic
and atactic radical copolymers. b) MALDI-ToF spectrum of radical
copolymer produced in entry 1 of Table 1.
a single series of peaks with a separation of 288 m/z which
corresponds to the mass of the repeat unit GTEMPO/COS.
More detailed analysis reveal the hydrolyzed GTEMPO
monomer serves as the chain-transfer agent.
The paramagnetic nature of these radical copolymers was
established by electron paramagnetic resonance (EPR)
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Figure 4. TGA and DSC curves of the atactic and isotactic radical
copolymers.
Figure 6. Comparison of solid-state electrical conductivity of isotactic
and atactic radical copolymers measured as solid-state films on gold
electrodes.
isotactic polymer chains. In the temperature-dependent
measurements, the polymer samples were heated in
a vacuum probe station and held at the desired temperature
for 10 min before collecting conductance data. For both
isotactic and atactic polymers, conductivity increased above
their Tg values as a result of thermal activated transport.
However, no obvious correlation between temperature and
conductivity was noted within the high temperature region
(Figure S21) as the electrical behavior can be interfered with
by multiple factors, such as defects and impurities.
Figure 5. a) Cyclic voltammogram plot of atactic polymer. b) Scan rate
dependence for the reversible redox couple of atactic polymer.
Herein, we have synthesized regio-regular TEMPO con-
taining radical polymers by the (salen)CrCl/PPNCl catalyzed
copolymerization of a TEMPO substituted epoxide and
carbonyl sulfide. In a similar manner, isotactic copolymers
were prepared from the enantiopure (R and S) versions of the
epoxide. These copolymers were fully characterized by
spectroscopic (IR, 1H and 13C NMR, EPR, MALDI) and
thermal (TGA, DSC) techniques. Electrical conductivity
measurments of films of these copolymers deposited on
gold electrodes were determined at ambient temperature,
clearly indicating that the isotactic polymers displayed an
electrical conductivity value of about 10À4 ScmÀ1 which is
much greater than that of its atactic analogue (ca.
10À7 ScmÀ1). To put this in perspective, PTMA is reported
to have an electrical conductivity of about 1 ꢀ 10À6 ScmÀ1.[29]
Figures S17–S19, respectively. Scan rate dependent CVs
confirmed diffusion-controlled electrochemical processes by
exhibiting a linear correlation between the cathodic peak
currents at the TEMPO reduction and the square root of the
scan rate.
Of most importance to our motivation for synthesizing
these radical polymonothiocarbonate polymers is an exami-
nation of their electrical conductivity properties. In particular,
to evaluate any difference in electron mobility in proceeding
from atactic to isotactic polymer structures. Among the
numerous factors influencing electron transport along the
polymer chains in these radical polymers should be the
distance between radical centers. This specific separation
would be expected to be much greater for atactic vs. isotactic
polymers. Other considerations are the film morphology, free
volume between chains, and percolation pathways, which are
all expected to differ for atactic and isotactic polymers.
Herein, we measured the electrical conductivities of these
radical polymers as solid-state films deposited on gold
electrodes. The solid-state devices were annealed above the
Tg values of the polymers before performing measurements.
Both atactic and isotactic polymers are fully amorphous as
revealed by PXRD. At ambient temperature, the isotactic
polymers exhibited greater conductivity (ca. 10À4 ScmÀ1) than
the atactic analog ( ꢀ 10À7 ScmÀ1; Figure 6). This can be
attributed to the higher packing order originating from the
Acknowledgements
The authors gratefully acknowledge the financial support of
this work through research grants by the Welch Foundation
(A-0923) to D.J.D. and L.F. (A-1898). We also thank Dr.
Joseph Reibenspies for his help with X-ray crystallography
and Peiran Wei for helping in collecting some DSC data.
4
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The authors declare no conflict of interest.
Keywords: carbonyl sulfide · copolymerization ·
electrical conductivity · epoxides · TEMPO
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Version of record online: && &&, &&&&
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Communications
Copolymerization
The preparation of atactic and isotactic
radical polymonothiocarbonates by the
copolymerization of a TEMPO derived
epoxide and carbonyl sulfide (COS) in the
presence of the metal catalyst
G. A. Bhat, A. Z. Rashad, X. Ji, M. Quiroz,
L. Fang,*
D. J. Darensbourg*
&&&& — &&&&
(salen)CrCl/PPNCl is reported. It was
uncovered that there is a significant en-
hancement of the electrical conductivity
of the isotactic copolymer (ca.
TEMPO Containing Radical
Polymonothiocarbonate Polymers with
Regio- and Stereo-Regularities: Synthesis,
Characterization, and Electrical
Conductivity Studies
10À4 ScmÀ1) as compared to that of the
atactic copolymer (10À7 ScmÀ1).
6
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