Sulfonamide as an activating group for the synthesis of poly(aryl ether sulfonamide)s by nucleophilic aromatic substitution

Article abstract of DOI:10.1021/ma201154b

Poly(aryl ether sulfonamide)s were produced utilizing the sulfonamide moiety as a new activating group for nucleophlic aromatic substitution polymerization. The activated monomer, 2,4-difluoro-N,N- dimethylbenzenesulfonamide, was synthesized in high yield

Full text of DOI:10.1021/ma201154b

ARTICLE
pubs.acs.org/Macromolecules
Sulfonamide as an Activating Group for the Synthesis of Poly(aryl
ether sulfonamide)s by Nucleophilic Aromatic Substitution
Nathaniel T. Rebeck and Daniel M. Knauss*
Renewable Energy Materials Research Science and Engineering Center, Department of Chemistry and Geochemistry,
Colorado School of Mines, Golden, Colorado 80401, United States
ABSTRACT: Poly(aryl ether sulfonamide)s were produced
utilizing the sulfonamide moiety as a new activating group for
nucleophlic aromatic substitution polymerization. The acti-
vated monomer, 2,4-difluoro-N,N-dimethylbenzenesulfon-
amide, was synthesized in high yields through a simple substitution reaction between 2,4-difluorobenzenesulfonyl chloride and
dimethyl amine. The reactivity of this monomer in a nucleophilic aromatic substitution mechanism was estimated using
semiempirical calculations at the PM3 level, as well as by 1H, 13C and 19F NMR spectroscopy. The data support the sulfonamide
as sufficiently electron-withdrawing to activate aryl fluorides for substitution by phenoxide nucleophiles. Model reactions with tert-
butylphenol confirm that the aryl fluorides of 2,4-difluoro-N,N-dimethylbenzenesulfonamide can be displaced in quantitative yields
and suggest the process is suitable for forming polymers. A variety of novel polymers were produced employing bisphenol A, 4,40-
biphenol, bisphenol AF and hydroquinone as comonomers. The polymers were characterized by gel permeation chromatography,
solution viscometry, NMR spectroscopy, thermogravimetric analysis, and differential scanning calorimetry. The high molecular
weight poly(aryl ether sulfonamide)s exhibit moderate to high glass transition temperatures ranging from 163 to 199 °C. The
polymers were shown to be thermally stable with 5% weight loss occurring from 398 to 442 °C in a nitrogen atmosphere.
’ INTRODUCTION
strong activating groups.16,17 Substitution at the ortho position,
while supported by resonance, is found to be slower due to steric
hindrance.18,19 The difference in reactivity has even been
exploited to produce exclusively para substitution in some
monomers.18 Most poly(aryl ether)s are formed with 1,4-aryl
linkages, forming polymers with rigid linear backbones. Polymers
with kinked backbones, formed by reaction at ortho-substituted
aryl halides, have been less well investigated; however, a few
examples exist. Poly(aryl ether)s with pendent sulfones and
ketones have been synthesized from sulfone and ketone activated
2,4 and 2,6-difluoroaromatics and a variety of bisphenols.20
Poly(pyridine ether)s have also been synthesized in the bulk
through the substitution of 2,6-difluoropyridine by various
silylated bisphenols.21 Multiple research groups have produced
aromatic polyethers with pendent nitriles from 2,4 and 2,6-
dinitro or dihalobenzonitriles using the more strongly activating
nitrile group.22ꢀ27 These studies show that pendent activating
groups must be strongly activating to overcome the decreased
reactivity from the steric crowding at the ortho position.
The sulfonamide moiety has also been shown to be a stable
group that can stand up to an array of reaction conditions and has
led to the extensive use of sulfonamides as protecting groups for
amines28ꢀ30 and sulfonic acids31 during multistep syntheses.
Sulfonamides have also been shown to activate aryl halides at the
para position for nucleophilic displacement reactions under very
mild conditions.32ꢀ36 The mild reaction conditions suggest that
the sulfonamide is strongly activated. Surprisingly, to the best of
Poly(aryl ether)s have continued to be researched as an
important class of polymers due to their excellent thermal
stability, good mechanical properties, and ease of functionaliza-
tion. The majority of poly(aryl ether)s are synthesized by
reacting an activated diaryl halide and a bisphenol through a
nucleophilic aromatic substitution (SNAr) mechanism. SNAr
provides a flexible, inexpensive route to these high performance
materials and can be accomplished when an electron-withdraw-
ing group activates an aryl halide or aryl nitro for substitution by a
nucleophile. The reaction occurs through a two-step mechanism
in which the nucleophile first attacks and forms the tetrahedral
intermediate Meisenheimer complex, and then upon loss of the
halide leaving group, aromaticity is regained and the product
ether is formed. The electron-withdrawing group promotes this
reaction in two ways. First, the electron-withdrawing nature of
the activating group removes electron density from the carbon
ipso to the fluorine, increasing the rate of attack by the
nucleophile.1,2 Second, the electron-accepting character of the
activating group lowers the activation energy by stabilizing the
Meisenheimer complex.3,4 A variety of groups have been used to
activate aryl fluorides for substitution by phenoxide nucleophiles
and to provide an assortment of characteristics to the polymer.
The most commonly used and most highly activating groups for
polymerization have been sulfones4,5 and ketones,4,6 but many
other groups have also been shown to be activating for polymeri-
zation, including amides,7,8 azomethine,9 sulfide,10 thianthrene,11,12
and phosphine oxides.13ꢀ15
Activating groups are typically positioned to activate the ortho
and para positions for nucleophilic substitution, although some
examples of meta substitution have been demonstrated for
Received: May 19, 2011
Revised:
July 13, 2011
Published: August 11, 2011
r
2011 American Chemical Society
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ARTICLE
our knowledge, sulfonamides have never been used to activate for
SNAr polymerization.
The semiempirical calculations were performed with Spartan 04 software
at the PM3 level.
Synthesis of 2,4-Difluoro-N,N-dimethylbenzenesulfona-
mide (1). 2,4-Difluorobenzenesulfonyl chloride (120.9 mmol, 25.71 g)
was added to a 500 mL Erlenmeyer flask followed by 150 mL of
dichloromethane. Dimethylamine (271.4 mmol, 47.05 g) was added to
the flask with 200 mL of deionized water (DI H2O). The mixture was
stirred at room temperature for 3 days. The organic layer was separated,
and the aqueous layer was extracted two times with dichloromethane.
The combined organic layer was washed with 5% hydrochloric acid,
water, 0.1 M sodium hydroxide solution, and water. The dichloro-
methane was removed by rotary evaporation, and the resulting oil
crystallized upon cooling. The product was recrystallized twice from
65/35 ethanol/water and dried under vacuum overnight. Pure yield:
62%; mp: 30.1ꢀ31.2 °C. 1H NMR (CDCl3 with respect to TMS at 0.00,
δ): 2.84 (s, 6H), 6.94ꢀ7.04 (m, 2H), and 7.85ꢀ7.93 (q, 1H). Elem.
Anal. Calculated for C8H9F2NO2S: C, 43.43%; H, 4.11%; F, 17.18%; N,
6.33%; O, 14.46%, S, 14.49%. Found: C, 43.56%; H, 4.15%; F, 17.18%;
N, 6.50%; S, 14.79%.
Sulfonamides allow for the incorporation of a new functional
group into poly(aryl ether)s leading to a variety of new polymers.
Sulfonamide activation of fluorines at both the ortho and para
positions of a ring for substitution can produce polymers with
pendent sulfonamides, making available a latent functionality
that could be converted into a sulfonic acid. The work presented
here shows the utility of the sulfonamide moiety in producing
new functional poly(aryl ether)s.
’ EXPERIMENTAL SECTION
Materials. 2,4-Difluorobenzenesulfonyl chloride (Sigma-Aldrich,
97%), dimethylamine (Eastman Kodak, 26% solution in water), and
dimethylformamide (Mallinckrodt Chemical, ChromAR) were used as
received. 4-tert-Butylphenol (Sigma-Aldrich, 99%) was recrystallized
from petroleum ether. 4,40-Isopropylidenediphenol (Bisphenol A)
(Sigma-Aldrich, 97%) and 4,40-(hexafluoroisopropylidene)diphenol
(Bisphenol AF) (Sigma-Aldrich, g98%) were twice recrystallized from
toluene. Bisphenol AF was further purified by sublimation under
reduced pressure. 4,40-Biphenol (Sigma-Aldrich, 97%) was twice recrys-
tallized from acetone then sublimed under reduced pressure. Hydro-
quinone (Fisher Chemical, purified) was recrystallized from acetone.
Potassium carbonate (Fisher Chemical) was dried overnight under vacuum.
N-Methylpyrrolidinone (NMP) (Mallinckrodt Chemical, ChromAR)
was twice distilled from phosphorus pentoxide under reduced
pressure. 1,3-Dimethyl-3,4,5,6-tetrahydro-2-(1H)-pyrimidinone (DMPU)
(Sigma-Aldrich, 98%) was distilled from calcium hydride under reduced
pressure.
Synthesis of 2,4-(4-tert-Butylphenoxy)-N,N-dimethylben-
zenesulfonamide (2). 1 (4.521 mmol, 1.000 g), 4-tert-butylphenol
(9.107 mmol, 1.368 g), potassium carbonate (6.79 mmol, 0.939 g),
DMPU or NMP (9 mL), and toluene (10 mL) were added to a dry, two-
neck 50 mL round-bottom flask. The flask was equipped with a stirbar, a
DeanꢀStark trap, a condenser, and a nitrogen inlet. Water was
azeotropically distilled with the toluene at 140 °C for about 6 h in a
thermostat controlled oil bath. The temperature of the oil bath was
increased to 165 °C for 20 h. The reaction was monitored by thin layer
chromatography (TLC) in 80/20 hexanes/ethyl acetate. The comple-
tion of the reaction was determined by the formation of a single product
spot and the disappearance of 1. The product (2) was precipitated in
300 mL of DI H2O. The off-white precipitate was extracted with
dichloromethane, washed with DI H2O four times, and dried over
magnesium sulfate. The solvent was removed by rotary evaporation. The
product was purified by flash chromatography in 80/20 hexanes/ethyl
acetate and then recrystallized from 80/20 ethanol/water. The product
was dried under vacuum overnight. Pure yield: 74%; mp: 114.0ꢀ
Characterization. Thermogravimetric analysis (TGA) was per-
formed on a Seiko TGA/DTA320 at a heating rate of 10 °C/min under a
nitrogen atmosphere. The glass transition temperatures (Tg’s) were
measured on a Perkin-Elmer Pyris 1 differential scanning calorimeter
running Pyris software. Measurements were carried out at a heating rate
of 10 °C/min under a nitrogen purge. Tg was taken at one-half Cp
extrapolated. 1H, 13C, and 19F NMR measurements were performed on a
QE-300 NMR spectrometer with a Techmag upgrade or a JEOL ECA-
1
118.6 °C. H NMR (CDCl3 with respect to TMS at 0.00, δ): 1.31ꢀ
1
500 NMR spectrometer. H and 13C NMR spectroscopy were per-
1.32 (d, 18H), 2.89 (s, 6H), 6.52ꢀ6.57 (m, 2H), 6.91ꢀ7.01 (dd, 4H),
7.34ꢀ7.39 (t, 4H), and 7.86ꢀ7.89 (d, 1H). Elem. Anal. Calculated for
C28H35NO4S: C, 69.82%; H, 7.32%; N, 2.91%; O, 13.29%, S, 6.66%.
Found: C, 70.01%; H, 7.31%; N, 2.96%; S, 6.79%.
formed on dilute solutions in CDCl3 or DMSO-d6 and referenced to
tetramethylsilane at δ 0.00. 19F NMR spectroscopy was performed on
dilute solutions in DMSO-d6. The 19F chemical shifts were referenced to
CFCl3 at δ 0.00. Intrinsic viscosity measurements were performed with a
Cannon-Ubbelohde viscometer in a 30 °C thermostat controlled water
bath. Each data point was an average of at least three measurements. The
intrinsic viscosity was taken as an average of the intercepts of the linear
extrapolations of the reduced and inherent viscosities versus concentra-
tion plots. Average molecular weights and molecular weight distribu-
tions were determined by gel permeation chromatography (GPC) on a
Viscotek GPCmax VE-2001 chromatograph equipped with Viscotek
Model 270 Series differential viscometer/low angle laser light scattering
detectors and a refractive index detector Model 3580. Elutions were
performed with two ViscoGel I-Series columns (I-M and I-H) in series at
55 °C. Dimethylformamide (DMF) with ammonium acetate (0.02 M)
was used as the eluent with a flow rate of 1 mL/min. Molecular weight
data analysis was performed using OmniSec software. Matrix-assisted
laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectro-
metry was performed on a PerSeptive Biosystems Voyager DE instru-
ment. Data Explorer software was used for data manipulation. The
samples were prepared by depositing a 3:1:1 (by volume) mixture of a
1 mg/mL solution of polymer in DMSO, a 13.6 mg/mL solution of
1,8,9-anthracentriol (dithranol) in THF, and a 1 mg/mL solution of
sodium trifluoroacetate in THF. The samples were dried in vacuo.
Elemental analysis was performed by Huffman Laboratories (Golden, CO).
Polymerization. All polymerizations were carried out in a similar
manner. The following is a detailed example polymerizing 1 with
bisphenol A. 1 (9.051 mmol, 2.002 g) was weighed into a dry two-neck
50 mL round-bottom flask. The flask was equipped with a stirbar, a
DeanꢀStark trap, a condenser, and a nitrogen inlet. The flask was
charged with bisphenol A (9.051 mmol, 2.066 g), potassium carbonate
(13.62 mmol, 1.882 g), NMP (13 mL), and toluene (10 mL). Water was
azeotropically removed with the toluene at 140 °C for 6 h, after which
the toluene was removed. The temperature was increased to 165 °C for
16 h. The viscous mixture was diluted with 25 mL of NMP and
precipitated in methanol. The polymer (3) was boiled in DI H2O for
30 min then dried under vacuum at 80 °C overnight. Polymerizations
using 4,40-biphenol, bisphenol AF, and hydroquinone to produce
polymers 4, 5, and 6, respectively, were performed in a similar manner.
’ RESULTS AND DISCUSSION
A significant amount of research on new activating groups for
nucleophilic aromatic substitution polymerization has been done
to design the properties of poly(aryl ether)s for applications. The
incorporation of a variety of functional groups into the backbone
has produced polymers with a range of unique properties.7,9,11,13,37ꢀ40
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Scheme 1. Monomer Synthesis
Table 1. Semiempirical Calculations and NMR Spectroscopy
Results for 1 and Other Aryl Fluorides
charge at charge 13C NMR 19F NMR
compound
ipso C
at F
(CDCl3) (DMSO-d6)
1 (CꢀF ortho to sulfonamide) 0.199 ꢀ0.078 159.93
1 (CꢀF para to sulfonamide) 0.144 ꢀ0.080 165.79
ꢀ104.82
ꢀ103.08
bis(4-fluorophenyl) sulfone
4,40-difluorobenzophenone
bis(4-fluorophenyl)amide
4-fluorobenzene
0.117a ꢀ0.085a 165.30a ꢀ104.08a
0.096a ꢀ0.088a 165.27b ꢀ106.01b
0.095a ꢀ0.086a 164.04a ꢀ108.55a
0.065a ꢀ0.093a 162.29a ꢀ112.77a
a Data taken from ref 2. b Data taken from ref 1.
molecule. The charge at the carbon para to the sulfonamide was
found to be 0.144. This value is significantly higher than strongly
activated monomerssuchassulfoneor ketone,2 suggestingthat the
sulfonamide is highly activating toward substitution.
NMR chemical shift values have been shown to correlate well
with the reactivity of aryl fluorides with respect to electron-
withdrawing groups. The chemical shift of a proton ortho to a
substituent can be used to estimate the inductive electron-
withdrawing capabilities.42 1H NMR spectroscopy of 1 reveals
a chemical shift of δ 7.89 for the proton ortho to the sulfonamide,
suggesting the monomer will have a reactivity similar to that of
4,40-difluorobenzophenone, which has a shift of δ 7.9.42
Although 1H NMR spectroscopy directly explores the electron-
withdrawing character of a moiety, it is an indirect method of
examining the effect of an electron-withdrawing group on the
reaction sites of a monomer. A more direct probe of the reaction
site is 13C NMR spectroscopy. 13C NMR spectroscopy of 1
reveals the carbon para to the sulfonamide to have a chemical
shift of δ 165.79, indicating it is more activated than sulfone
(δ 165.30) and ketone (δ 165.27).1 19F NMR also suggests that
the para position is activated for polymerization with a chemical
shift of δ ꢀ103.08. This value is similar to that found for bis-
(4-fluorophenyl) sulfone (δ ꢀ104.08).1 1H, 13C, and 19F NMR
results all suggest that 1 is activated for SNAr polymerization.
Studies of the reactive site ortho to activating groups have not
been explicitly investigated for comparison. In the case of 1, the
ortho site is assumed to be reactive because of the strong
activation estimated for substitution at the para position.
The reactivity of 1 with phenolic nucleophiles was examined
through a model reaction with 4-tert-butylphenol in DMPU or
NMP as a solvent using potassium carbonate as a weak base
(Scheme 2). Poly(aryl ether)s are commonly synthesized using a
weak base to generate the nucleophile in situ.7,11,13ꢀ15,38,43,44
Thin layer chromatography was used to monitor the reaction. A
single product was formed after 24 h at 165 °C, indicating a
complete reaction. The product was purified and identified by
1H NMR, mass spectrometry, and elemental analysis. The
complete conversion of 1 in a short amount of time and at a
moderate temperature suggests that 1 is suitable for SNAr
polymerization.
Polymers with pendent functionality offer an alternative method
to produce desired properties. In order to incorporate the
pendent functional group in poly(aryl ether)s, a separate func-
tionality is typically included in one of the monomers in addition
to the activating group or the polymer is functionalized post-
polymerization. These routes can lead to side reactions and a lack
of control over functionalization. Also, the polymers contain
multiple functionalities that can have an adverse effect on the
properties of the polymers. Incorporating a sulfonamide to activate
two positions on the same aromatic ring activates the aryl fluorides
for substitution and incorporates the desired functionality into the
polymer without interfering with the polymerization.
In order to study the activating nature of the sulfonamide
moiety, a sulfonamide functional difluorobenzene (1) was
synthesized through a simple substitution reaction (Scheme 1).
The reaction was carried out in an interfacial mixture, from which
the product was easily recovered and purified by recrystallization.
1
The structure of 1 was confirmed by H, 13C, and 19F NMR
spectroscopy, mass spectrometry, and elemental analysis.
The effectiveness of an electron-withdrawing group to activate
aryl fluorides toward nucleophilic substitution has been studied
by two methods, each focusing on para-fluoro compounds. The
first method is computational and examines the net charge on the
ipso carbon as a measure of the potential reactivity via a SNAr
mechanism, with a larger positive charge at the ipso carbon
indicating an increased reactivity with a nucleophile in a SNAr
mechanism. Semiempirical molecular orbital calculations have
been used to gauge reactivity of activated aryl fluorides.1,2,40,41
The second method to assess the activating nature of an electron-
withdrawing group for the nucleophilic displacement of aryl
1
fluorides is through NMR spectroscopy. The H, 13C, and 19F
NMR chemical shifts are used to estimate reactivity based on
correlations derived from known activated molecules for SNAr
polymerization.
Semiempirical calculations were performed at the PM3 level
to estimate the reactivity of 1 with nucleophiles. The results are
shown in Table 1 along with literature results1,2 for other aryl
fluorides. Bis(4-fluorophenyl) sulfone and 4,40-difluorobenzophe-
none are compared as commonly used monomers for polymeri-
zation. Bis(4-fluorophenyl)amide is compared as a similar func-
tional group. Fluorobenzene is included to depict an unreactive
The polymerization of 1 with various bisphenols is depicted
in Scheme 3 following the same reaction conditions used in
the model reaction. NMP was preferred over DMPU due to
solubility issues. A highly viscous solution indicative of high
molecular weight polymer was formed within 16ꢀ24 h at 165 °C.
Polymers formed from bisphenol A and bisphenol AF exhibit
partial solubility in tetrahydrofuran and chloroform and com-
plete solubility in DMF, DMSO, and NMP. The polymers
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Scheme 2. Synthesis of Model Compound 2
Scheme 3. Polymerization of 1 with Various Bisphenols
incorporating 4,40-biphenol and hydroquinone were soluble in
DMF, DMSO, and NMP.
1H NMR spectroscopy was used to characterize the repeat
unit structure of each of the polymers. The spectrum obtained
from polymer 3 in DMSO-d6 is shown as an example in Figure 1,
and the chemical shift assignments are depicted in the figure.
Average molecular weights were measured by gel permeation
chromatography using low angle light scattering detection. Each
of the polymerizations produced high molecular weight polymers
with varying amounts of low molecular weight material as
depicted in the GPC chromatograms (Figure 2). The average
molecular weights and polydispersities reported are determined
by selecting the entire sample (Table 2). The peaks at high
elution volume are presumably low molecular weight cyclics and
are produced to varying extent depending on the bisphenol
monomer used. The polymerization using 4,4-biphenol (4) and
hydroquinone (6) result in larger amount of low molecular
weight material. 4,40-Biphenol and hydroquinone are very rigid
molecules due to the aromatic rings and lack of free bond
rotation. The rigidity of the monomer coupled with the meta
linkage in the activated monomer is presumed to produce a
significant concentration of low molecular weight cyclics, parti-
cularly a cyclic dimer, thereby broadening the polydispersity.
The MALDI-TOF mass spectra for each of the polymers
(Figure 3) exhibit a series of peaks with molecular weights
matching cyclic species, suggesting that the 2,4 substitution
influences the formation of cyclics. A similar phenomenon has
been seen with difluorobenzophenone, where the 2,4-difluoro-
benzophenone produced a larger amount of cyclics than other
isomers.20 In the spectrum for polymer 4, the peak correspond-
ing to the repeat unit trimer is missing. In the spectra for both
polymer 4 and polymer 6, the distribution is favored toward the
lower cyclics. This data supports the conclusion that the larger
low molecular weight peak in the GPC chromatograms for these
Figure 1. 1H NMR spectrum of polymer 3 in DMSO-d6.
two polymers is due to favored cyclic formation because of the
rigidity of the comonomer.
The polymers were melt-pressed or solution cast into crea-
sable thin films. Solution viscometry was used to measure the
molecular weight to corroborate the high molecular weight
measured for polymer 3. It was hypothesized that the unusually
high molecular weight could be due to aggregation of chains. The
viscosity was measured using a Cannon-Ubbelohde viscometer at
a series of concentrations of each polymer. The reduced and
inherent viscosities were calculated and plotted against concen-
tration. The linear data were extrapolated to zero concentration,
and the intrinsic viscosity was taken as the average of the two
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Figure 2. Gel permeation chromatograms of poly(aryl ether sulfonamide)s.
Table 2. Characterization Results for Poly(aryl ether
sulfonamide)s
[η]
TGA 5%
lossc
a
a
polymer
Mn
Mw
Mw/Mn (dL/g)b Tg (°C)c
3
4
5
6
109 000 446 000
20 100 207 000
4.1
10.3
2.4
0.99
0.30
0.27
0.21
163
199
187
169
435
417
442
398
34 900
82 900
Figure 3. MALDI-TOF mass spectra of poly(aryl ether sulfonamide)s.
7 170
74 400
10.4
a Measured by GPC with light scattering detection. b Measured in DMF
at 30 °C. c Measured under a nitrogen atmosphere.
high glass transition temperatures (Tg) in the range of 163ꢀ199 °C
depending on the bisphenol used. Polymer 4 exhibited the
highest Tg at 199 °C, and this can be attributed to the rigidity
imparted by the 4,40-biphenol in the backbone. Polymer 5 also
displayed a higher Tg than polymers 3 and 6 due to the
stronger interchain interactions of the ꢀCF3 groups. The
thermal stabilities of these polymers were determined using
dynamic TGA in a nitrogen atmosphere at a heating rate of
10 °C/min (Figure 5). All of the polymers exhibited good
thermal stability with 5% weight loss values from 398 to
443 °C depending on the bisphenol used in the polymeriza-
tion (Table 2). Polymers 3 and 4 show some weight loss
starting around 200 °C. This can be attributed to residual
intercepts. Intrinsic viscosities for all four polymers are shown in
Table 2. The plots of reduced and inherent viscosities versus
concentration were very linear with R2 values of 0.999 and 0.993,
respectively (Figure 4). The linearity suggests that there is no
aggregation and the high viscosity is due to the high molecular
weight of the polymer. The data for the other polymers were
similarly linear and the intrinsic viscosities correlated well with
the molecular weight values determined by light scattering.
The thermal properties of the polymers were measured by
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) (Table 2). The polymers exhibited moderately
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Good thermal stabilities were shown for each polymer with 5%
weight loss values at or above 400 °C.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: dknauss@mines.edu.
’ ACKNOWLEDGMENT
This work was supported by the Renewable Energy Materials
Research Science and Engineering Center funded by the National
Science Foundation (DMR-0820518). The NMR spectroscopy
was possible through an NSF MRI grant (CHE-0923537).
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