Varying anionic functional group density in sulfonate-functionalized polyfluorenes by a one-phase Suzuki polycondensation

Article abstract of DOI:10.1021/ma4004693

A series of anionically functionalized polyfluorene-based conjugated polyelectrolytes were synthesized by the Suzuki polycondensation of the boronic ester 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-di(1-(2-(2- methoxyethoxy)ethoxy)ethyl)fluorene with the ionic and nonionic dihalides 2,7-dibromo-9,9-di(6-sodium sulfonate-hexyl)fluorene and 2,7-dibromo-9,9-di(1- (2-(2-methoxyethoxy)ethoxy)ethyl)fluorene. The latter monomer served as a diluent to control the ionic functional group density. The use of the triethylene glycol monomethyl ether derivatives made possible a one-phase polycondensation in a THF/methanol/Na₂CO₃(aq) mixture using Pd(PPh₃)₄. The one-phase nature of the polymerization was essential to balancing the reactivity of the dihalides so that they were both incorporated into the same polymer and so that the polymer composition became dictated by the monomer feedstock composition. In closely related two-phase reactions, a mixture of polymers, one ionic and one nonionic, was isolated with no control over ionic functional group density possible. Using the one-phase approach, polyfluorene-based polyelectrolytes were synthesized with 2-20 aromatic rings per sulfonate group.

Full text of DOI:10.1021/ma4004693

Article
pubs.acs.org/Macromolecules
Varying Anionic Functional Group Density in Sulfonate-
Functionalized Polyfluorenes by a One-Phase Suzuki
Polycondensation
David Stay and Mark C. Lonergan*
Department of Chemistry and The Materials Science Institute, University of Oregon, Eugene, Oregon 97403, United States
ABSTRACT: A series of anionically functionalized polyfluorene-
based conjugated polyelectrolytes were synthesized by the Suzuki
polycondensation of the boronic ester 2,7-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-9,9-di(1-(2-(2-methoxyethoxy)ethoxy)-
ethyl)fluorene with the ionic and nonionic dihalides 2,7-dibromo-
9,9-di(6-sodium sulfonate-hexyl)fluorene and 2,7-dibromo-9,9-di(1-
(2-(2-methoxyethoxy)ethoxy)ethyl)fluorene. The latter monomer
served as a diluent to control the ionic functional group density. The
use of the triethylene glycol monomethyl ether derivatives made
possible a one-phase polycondensation in a THF/methanol/
Na2CO3(aq) mixture using Pd(PPh3)4. The one-phase nature of
the polymerization was essential to balancing the reactivity of the dihalides so that they were both incorporated into the same
polymer and so that the polymer composition became dictated by the monomer feedstock composition. In closely related two-
phase reactions, a mixture of polymers, one ionic and one nonionic, was isolated with no control over ionic functional group
density possible. Using the one-phase approach, polyfluorene-based polyelectrolytes were synthesized with 2−20 aromatic rings
per sulfonate group.
of an aryl dihalide with an aryl diboronic acid or acid ester is
used in the synthesis of ionically functionalized derivatives.2,7
Other approaches include Ni-catalyzed Colon coupling2,11 and
Pd-catalyzed Sonagashira coupling to form poly(phenylene
ethylenes).2,12−15 Ionic functionality is introduced either by the
direct polymerization of ionic monomers or indirectly through
postpolymerization conversion.
INTRODUCTION
■
Phenylene-based conjugated polymers, such as poly(p-phenyl-
ene) and poly(fluorene), are prototype luminescent conjugated
polymers that have been widely studied for applications ranging
from electroluminescence to biological sensing. In the study
and application of these materials, ionic functionality has
proven useful because it can strongly alter key physical and
chemical properties. Ionic functional groups can impact
solubility, impart ionic or mixed ionic/electronic conductivity,
influence excitons dynamics, provide a mechanism for strong
intermolecular interactions, and alter doping mechanics.1−8
One of the central compositional parameters of conjugated
polyelectrolytes (CPE), also known as ionomers, is the density
of ionic functional groups. In reference to phenylene-based
polymers, we define the parameter χ as the molar ratio of ionic
functional groups to aromatic rings. Despite its importance,
there have been relatively few reports on methods to control χ
in phenylene-based conjugated polymers. The goal of the work
in this paper is the synthesis of a family of soluble, anionically
functionalized poly(fluorene)s with variable ionic density and
similar backbone electronic structure. The synthesis developed
to achieve this goal utilizes the direct polymerization of ionic
monomers, permits for the facile control over the ionic density
in the polymers, and through judicious selection of monomer
and solvent pairs avoids complications introduced by the two-
phase nature of typical Suzuki cross-coupling reactions.9
In the direct route, an ionic monomer A containing one or
more ionic functional groups is coupled with a nonionic
monomer B resulting in an alternating copolymer poly(AB). In
the indirect route, a monomer A′ containing precursors to ionic
functional groups (e.g., esters or amines) is coupled to a
nonionic monomer B resulting in an alternating nonionic
copolymer poly(A′B) that can be converted into an ionically
functionalized polymer. Within either the direct or the indirect
route, χ can in principle be controlled by synthesizing
analogous monomers with varying ionic functional group
density or by the introduction of a third diluent monomer C to
yield poly(AB−CB) or poly(A′B−CB). With the indirect route,
it is also possible to control ionic functional group density by
only converting a fraction of the A′ moieties to ionic groups.
The most widely explored method for controlling χ has been
the indirect postpolymerization route. Using this route, χ has
been controlled in a single poly(A′B) polymer by the extent of
conversion postpolymerization16 or through the addition of a
Metal-catalyzed cross-coupling reactions are the most
commonly employed reactions for synthesizing phenlyene-
based conjugated polymers.2,9,10 Typically, the Suzuki coupling
Received: March 4, 2013
Revised: May 14, 2013
© XXXX American Chemical Society
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diluent monomer C with the same functionality (boronic ester
or arylhalide) as the monomer to be later converted to an ionic
group.11,17,18 If complete conversion of the precursor monomer
can be achieved, the ionic functional group density is controlled
by the polymerization step rather than the conversion step.
Complete conversion is rare postpolymerization, as conversions
of 80−90% are commonly cited, and it has been reported that
this approach is not necessarily reproducible.19 To date,
postpolymerization conversion routes to controlling ionic
density have nearly all involved the quaternization of an
amine to form a cationic polymer. There are far fewer examples
of anionic polymer formation, with these involving the
hydrolysis of an ester to form a carboxylate, phosphate, or
sulfate.20−22 It is noted that in studies reporting phenlyene-
based CPEs with varying ionic functional group density such
variation was not typically the primary focus. As a result, some
report only two different ionic functional group densities
(including the parent AB polymer) or use a third monomer not
really intended to dilute ionic functionality, but to serve a
different purpose such as being an electron acceptor.
the χ indicates the molar ratio of sulfonate functional groups to
aromatic rings in the monomer feedstock and hence represents
an idealized polymer composition. As will be shown later, the
initial feedstock corresponds well to the final composition.
RESULTS AND DISCUSSION
■
The Suzuki polycondensation of 9,9-dihexylfluorene-2,7-dibor-
onic acid bis(1,3-propanediol) ester (1) and sulfonated
fluorene derivative, 2,7-dibromo-9,9-di(6-sodium sulfonate-
hexyl)fluorene (2), was initially explored in the synthesis of
SPFs. The initial choice of the hexyl-substituted boronic ester
monomer was motivated by previous reports on the successful
polymerization of closely related monomers.2 The dihexyl-
fluorene monomer is commercially available and has been
widely used in the synthesis of nonionic polymers because the
flexible alkyl side chains promote solubility. The monomer 2
was synthesized by the sulfonation of 2,7-dibromo-9,9-di(6-
bromohexyl)fluorene (3) in water using sodium sulfite with
cetyltrimethylammonium bromide (CTAB) and 2 as phase
transfer agents (Scheme 1). The 3 was synthesized according to
a previously published procedure.18
Although less explored, the direct polymerization of ionic
monomers is of interest because the polymer is synthesized in a
single step, it eliminates possible issues with the intentional or
unintentional incorporation of unconverted precursor groups,
and it is potentially applicable to a wider range of ionic
functional groups. While in theory both changing monomer
structure and dilution using a three-monomer system can be
applied in the direct route, only the former has been reported.2
For instance, Kim et al. demonstrate the direct polymerization
of an anionic monomer with a phenyl diboronic ester and a
different reaction with biphenyl diboronic ester to give two
densities of anionic monomer along the backbone of the
polymer.23 Regardless, this approach requires the synthesis of a
new monomer for each desired ionic functional group density.
One possible reason that control of ionic functional group
density of phenylene-based polyelectrolytes has not been
reported using the direct polymerization of ionic monomers
in the presence of a diluent monomer is the two-phase nature
of many Suzuki cross-couplings.9 In the synthesis of nonionic
polymers, the two monomers A and B are typically dissolved in
an organic phase with the base catalyst in an aqueous layer.
With ionic monomers, the situation is somewhat different
because the ionic monomer, say A′, will partition into the
aqueous layer while the nonionic monomer B stays in the
organic layer. The addition of a nonionic monomer C to
compete with A′ in the coupling reaction will, in many cases,
partition into the nonaqueous layer. As demonstrated more
fully herein, this will favor the formation of a poly(BC) because
both of these monomers are in the same phase rather than the
desired poly[(A′B)(CB)]. The preferential coupling of the
monomers in the same phase over those in different phases is
referred to as the “two-phase problem” herein.
Scheme 1. Synthesis of the Sulfonated Fluorene Monomer 2
The use of 2 as a phase transfer agent in its own synthesis of
course required its initial synthesis by another means. This
alternate means and the motivation for using 2 in its own
synthesis developed out of a number of attempted approaches
to 2 as summarized in Scheme 2. Sulfonation of the 3 alkyl
halide chains using sodium sulfite and CTAB alone as a phase
transfer agent was unsuccessful in water, water/methanol
mixtures, and dimethyl sulfoxide/water mixtures. Sulfonation
of the iodo-functionalized analogue 4 (2,7-dibromo-9,9-di(6-
Scheme 2. Reactions Explored during the Original Synthesis
a
of the Sulfonated Fluorene Monomer 2
By choosing amphiphilic monomers and carefully selecting
solvent systems, we were able to overcome the two-phase
problem and take advantage of the control afforded by direct
polymerization in the synthesis of a family of soluble,
phenylene-based, anionically functionalized, conjugated poly-
mers with variable ionic density. Specifically, we report a series
of sulfonate functionalized poly(fluorene-co-alt-fluorene)s
(SPFs) polymers with varying χ incorporating either hexyl
(to illustrate the two-phase problem) or triethylene glycol
monomethyl ether (TEG) functionality. The polymers in this
series are referred to as SPFHχ ex and SPFχTEG, respectively, where
a
Key (i) Na2SO3, CTAB; (ii) NaI in acetone; (iii) thiourea in EtOH,
reflux, 16 h; (iv) NaOH in H2O, reflux, 3 h; (v) H2SO4; (vi) boiling
HNO3, (vii) 5% 6, Na2SO3, CTAB in water.
B
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Hex
0.5
Scheme 3. Synthesis of SPF
iodohexyl)fluorene) synthesized by halide exchange was also
not successful under similar conditions. An alternative approach
to introducing the sulfonate group functionality is through the
oxidation of a precursor such as thiol or dithiol. Toward this
end, the thiol-functionalized 2,7-dibromo-9,9-di(6-thiolhexyl)-
fluorene (5) was synthesized via the diisothiuronium salt. The
sulfur was introduced through nucleophilic attack by thiourea
in refluxing ethanol, which was then hydrolyzed with sodium
hydroxide and neutralized with sulfuric acid. The resulting thiol
5 was then oxidized using refluxing nitric acid to give the
desired sulfonate functional group, but unfortunately, the
fluorene monomer was also nitrated yielding monomer 6. More
gentle oxidation conditions (30% hydrogen peroxide in acetic
acid) only oxidized to the dithiol. Regardless, we suspected that
the monomer 6 would be a good phase transfer agent, and so it
was tried as such in the direct substitution of 3 with sodium
sulfite. When used together with CTAB, near complete
conversion was observed. Some of the newly synthesized 2
along with CTAB was then used successfully as a phase transfer
reagent resulting in 2 in >80% yield. The inclusion of ∼5% of 2
was needed, and without it, the reaction did not proceed even
when refluxed for several days. All 2 used in the synthesis of
SPFs was synthesized with 2 as a catalyst, not 6.
Table 1. Molecular Weight and Photophysical Properties of
Sulfonate-Functionalized Poly(fluorene-co-alt-fluorene)s
c
d
e
MW
ex λmax
(nm)
em λmax
(nm)
a
b
CPE
χmonomer χpolymer
(kDa)
Hex
SPF
SPF
SPF
SPF
SPF
SPF
0.5
0.55
0.51
0.21
0.18
0.17
0.048
17
14
19
15
23
12
380
380
380
380
380
380
425
425
425
425
425
425
0.50
TEG
0.50
TEG
0.25
TEG
0.20
TEG
0.16
0.50
0.25
0.20
0.17
0.050
TEG
0.05
a
b
From initial monomer concentration. From 1H NMR of the purified
c
polymer. Apparent molecular weights as determined by GPC vs
polystyrenesulfonate standards in 0.1 M LiNO3/25% water−75%
d
e
DMSO. λmax of the excitation spectrum. λmax of the emission
spectrum.
Hex
0.5
Hex
0.0
them to be SPF and SPF , which are the polymers formed
by reaction of monomer 1 with 2 and 7, respectively.
It is believed that a mixture of polymers was obtained in the
attempt to dilute ionic functionality because of the two-phase
nature of the polymerization. Nearly all Suzuki polycondensa-
tions are two-phase systems. In the synthesis of nonionic
polymers, both the aryl halide and boronic ester partition into
the organic layer, thereby providing optimal contact between
The sulfonated fluorene monomer 2 was reacted with 1 to
yield SPFH0.5ex as shown in Scheme 3. The sulfonated monomer 2
and 1 were coupled using Pd(PPh3)4 in a mixture of THF,
methanol, and 2 M K2CO3(aq). Precipitation of a polymer
from the two-phase mixture was observed after 48 h and
continued over the full course of the reaction (5 days). The
needed reaction times were longer than typically required for
the Suzuki polycondensation of nonionic monomers, but they
were consistent with other polymerizations involving ionic
Hex
0.5
monomers. In the synthesis of ionic polymers, as with SPF
,
the aryl halide and boronic ester are in separate phases leading
to poorer contact, which requires longer polymerization times.
In the copolymerization of boronic ester 1 with ionically
functionalized dibromide 2 and the nonionically functionalized
dibromide 7, it is possible for polymerization to occur both
within the organic layer and across the organic/aqueous
interface. It is hypothesized that the 1 preferentially reacts
with the 7 over the 2, which is within a separate phase, leading
monomers.2,9 The structure of the isolated polymer SPF was
Hex
0.5
to the rapid formation of SPFH0.0ex, with the slower formation of
1
confirmed by H NMR, and the polymer was found to be
Hex
SPF at the organic/aqueous interface.
0.5
soluble in DMSO to a level of >5 mg/mL. The apparent
molecular weight of the polymer was determined to be 17 kDa
by gel permeation chromatography (GPC). The GPC was
carried out using a Waters Styragel HR4 column with a nominal
molecular weight range of 5 × 103−6 × 105 Da with 0.1 M
LiNO3 in H2O/DMSO 25/75 (v/v) as an eluent. The sodium
salts of polystyrenesulfonate standards were used to calibrate
the column and solvent system. The 0.1 M LiNO3 was used to
screen the charge of the polymers from the charge on the
column. Because of the differences in the structure of the
standards and polymers, the reported molecular weights should
be considered apparent (see Table 1).
The two-phase problem was not unique to the copoly-
merization of solely fluorene-based monomers. During the
course of this study, we also explored an analogous sulfonate-
functionalized poly(fluorene-co-alt-phenylene) (SPFP) system.
The polymerization of 1 with 2,5-dibromobenzylsulfonate (8)
to yield a SPFP was successful, but as in the SPF system,
polymerization of 1 with both 8 and dibromo-p-xylene as a
diluent monomer resulted in a mixture of polymers. This was
particular evident when the polymerization was carried out in
acetonitrile. In this solvent, the 1/dibromo-p-xylene polymer is
observed to precipitate immediately upon addition of catalyst.
Hex
0.33
No 1/8 polymer SPFP is observed at these early stages of
In an effort to control the functional group density, the
commercially available dihalide analogue of 1, 2,7-dibromo-9,9-
dihexylfluorene (7), was incorporated as a diluent monomer. As
with SPFH0.5ex, the polymerization of 1 with 2 and 7 resulted in
the precipitation of a yellow solid from the reaction mixture.
This solid, however, could be separated into two polymers with
differing solubilities. Analysis of the separate polymers revealed
the reaction, and the 8 can be nearly quantitatively recovered
by simple separation of the aqueous layer. At later stages,
whether or not the 1/dibromo-p-xylene was separated out, the
polymer of 1 and 8 (SPFPH0.3e3x) was observed.
Returning to the SPF system, a variety of approaches were
pursued in an effort to better balance the activity of 2 and 7 in
the attempted synthesis of SPFHχex. Mixtures of polymers, rather
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Scheme 4. Synthesis of TEG-Functionalized Monomers 9 and 10
than the desired three component polymers, were obtained for
polymerizations under a range of conditions: solvents were
varied (CH3CN/H2O, MeOH/THF/H2O); phase transfer
agents, such as CTAB and tetrabutylammonium bromide,
were added in an attempt to emulsify the reaction; and
Pd(OAc)2 was used as an alternate catalyst because of its
greater solubility in polar solvents relative to Pd(PPh3)4.
Ultimately, the very different solubilities of the nonionic
monomers (1 and 7) and the ionic monomer (2) could not be
overcome through modification of reaction conditions.
As an alternate strategy, the nonionic monomers were
redesigned to include triethylene glycol monomethyl ether
(TEG) functionality promoting better solubility in polar
solvents. In particular, the TEG-functionalized dibromofluorene
2,7-dibromo-9,9-di(1-(2-(2-methoxyethoxy)ethoxy)ethyl)-
fluorene (9) and its boronic ester derivative 2,7-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-di(1-(2-(2-
methoxyethoxy)ethoxy)ethyl)fluorene (10) were synthesized
(see Scheme 4). The target polymer in this case for χ = 0.5 is
similar to that reported by Zhu et al., but control over ionic
functional group density was not reported.24 The monomer 9 is
an oligoether derivative of commercially available 2,7-
dibromofluorene. Functionalization was accomplished using
LDA to deprotonate the 9-position of fluorene followed by the
addition of excess Br(CH2CH2O)3CH3. The monomer 10 is
the boronic acid ester of 9, functionalized by lithium halide
exchange of 9 using n-butyllithium at −78 °C followed by
quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane.
using Pd(PPh3)4 as a catalyst. With the organic solvent being a
mixture of equal volumes of THF and methanol, the reaction
was single phase over the full range of monomer compositions.
The reactions all proceeded with the formation of a yellow-
orange precipitate that began forming around 18 h. The
resulting polymer was washed with CHCl3, THF, and water.
The organic washes were found to contain unreacted monomer
and reaction byproducts but no polymer. It is noted that the
polymer of 9 and 10 is known to be soluble in both CHCl3 and
THF.25 The water wash was found to contain a small amount
of unreacted ionic monomer but again no polymer. The
isolated polymers were all found to be soluble in DMSO to a
TEG
0.25
level of at least 5 mg/mL, and SPFT0.E5G and SPF
were soluble
in methanol. These solubility characteristics strongly argue
against the formation of a mixture of polymers. In particular, no
component of the polymers with lower ionic concentration was
soluble in either methanol or THF, whereas the end point
polymers from the reaction of 10 with either 2 or 9 are soluble
in methanol or THF, respectively.
The ionic density in the SPFTχ EG series was determined by 1H
NMR of the purified polymers in DMSO-d6 using the integral
of two sets of resonances. The first set is from four equivalent
methylene protons on the sulfonate side chains (the fifth
carbon from the sulfonate). The second integral is the total
number of aromatic protons. Table 1 and Figure 1 compare the
polymer compositions from 1H NMR to the idealized
compositions based on the monomer feedstock. The polymer
composition was successfully varied between χ = 0.05 and χ =
0.5. As can be seen, there is a good correlation between input
monomer ratio and the ratio of monomers incorporated into
The polymerization of 2, 9, and 10 using the Suzuki
polycondensation reaction was carried out with monomer
TEG
compositions targeting the following polymers SPFT0.E5G, SPF
,
0.25
TEG
TEG
SPFT0.E2G, SPF , and SPF
(see Scheme 5). With the TEG-
0.16
0.05
functionalized monomers, it was possible to identify conditions
where the reaction did not break into two phases, which
ultimately led to the successful synthesis of copolymers with
varying χ. All of these polymerizations used an 8:1 organic
solvent to aqueous base ratio, instead of the more common 3:2,
Scheme 5. Synthesis of Sulfonate-Functionalized
Polyfluorenes (SPFTχ EG) with Varying Ionic Density
Figure 1. Correlation between χmonomer and χpolymer for SPFTχ EG (red
Hex
0.33
Hex
0.50
squares), SPFP (black circle), and SPF (green triangle).
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the CPEs. The percent yields for the polymerizations were
between 50 and 75%. The agreement between monomer
feedstock and polymer composition at less than 100%
conversion is consistent with random incorporation of the
monomers into the polymer.
Gel permeation chromotography on all of the polymers in a
0.1 M LiNO3 DMSO/H2O 75/25 (v/v) eluent revealed a
single broad peak. The apparent molecular weights are reported
in Table 1. There comes a point where the ionic density is so
low that it is statistically improbable, or impossible, for each
polymer chain to contain an ionic monomer. The solubility
characteristics of the isolated polymers and the absence of any
substantial quantity of the nonionically functionalized polymer
9/10 in the CHCl3 and THF washes used during purification
argue that this point was not reached in any of the polymers. A
molecular weight of 9.2 kDa is needed for the lowest
TEG
0.05
concentration of SPF
to have on average one ionic
functional group. This argues that the polymer molecular
weight is greater than 10 kDa, which is consistent with the
apparent molecular weight of 12 kDa determined in relation to
polystyrenesulfonate standards.
Excitation, emission, and/or absorption spectra were
collected for the synthesized SPFTχ EG polymers to understand
how ionic functional groups affect the optical properties and
solution aggregation of these materials. Figure 2 shows a
Figure 3. Normalized excitation (filled), emission (open), and
TEG
TEG
absorbance (lines) spectra for the SPF
(black squares), SPF
(blue down triangles),
0.05
0.5
0.25
TEG
TEG
(red circles), SPF
(green triangles), SPF
0.16
Hex
and SPF
(orange lozenges) polymers. The SPFTχ EG excitation
0.5
spectra were collected while observing at 425 nm, and emission spectra
were recorded while the polymer was excited at 380 nm.
380 nm range). The overlap of the emission and absorption
spectra indicates that the species in solution doing the majority
of the absorption is also doing the majority of emission. The
emission spectra exhibit the vibronic structure typical of
fluorene-based polymers, but there is some difference in the
relative strengths of the emission peaks for the various
polymers. The two polymers where the lowest energy peak of
the vibronic progression is the most intense are also the two
polymers with the lowest molecular weights. A similar
dependence on molecular weight has been previously observed
by Gao et al.26
One of the goals in the work herein was to make a family of
soluble luminescent CPEs with similar backbone electronic
structures. This was indeed achieved with the SPFTχ EG series as
evidenced by the very similar excitation and emission spectra
across the family (see Figure 3). This is perhaps not surprising
because the bridging carbon on fluorene-based polymers tends
to help lock in planarity. Further, the straight-chain oligoethers
used in this family of polymers also minimize steric bulk close
to the backbone, relative to often used branched side chains.
The SPFχTEG CPEs exhibit solvatofluorochromism as
illustrated by comparing the polymers’ emission in pure
methanol to its binary mixtures with dichloromethane and
water. A 12 nm bathochromic shift is observed for all of the
polymers in going from methanol to a more polar methanol/
water mixture, so-called positive solvatofluorochromism. This
TEG
Figure 2. Three-dimensional total luminescence plot for SPF
.
0.5
representative total luminescence spectrum for SPFT0.E5G. As can
be seen, the intensity of the emission changes with excitation
wavelength, but the shape of the emission spectrum does not.
This was also the case for the other polymers studied. More
conventional excitation and emission spectra correspond to
slices through the total luminescence spectrum at both a
particular emission and excitation wavelength, respectively.
Figure 3 show these spectra for the family of SPFTχ EG polymers
in DMSO. Also shown are the absorption spectra of the
polymers. The shapes of the absorption, excitation, and
emission spectra were observed to be similar to that observed
for the analogous nonionic polymers that have been reported in
the literature.26 Both the absorption and excitation spectra are
characterized by a single broad peak with λmax in the UV (350−
bathochromic shift suggests increasing polymer aggregation as
TEG
0.5
the nonsolvent water is added. Interestingly, all but SPF
show a bathochromic shift upon going from methanol to a less
polar methanol/dichloromethane mixture, so-called negative
solvatofluorochromism. This shift is smaller (∼4 nm), but it is
again consistent with an increase in aggregation due to the
addition of a nonsolvent, in this case dichloromethane. It is
somewhat puzzling that the polymer with the greatest ion
TEG
0.5
content SPF
does not show any shift as the solvent polarity
is decreased. Other than this observation, these data seem to
suggest little dependence of the fluorescence and aggregation
on ion density. These data, however, only reflect end points of
solvent polarity.
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A more comprehensive study of solvatofluorchromism was
conducted for three compositions of the SPFTχ EG family by
dissolving the polymers in methanol and then adding either
water or dichloromethane in small increments. Controls were
run to ensure that any change in wavelength was not due to
dilution, and the spectra did not change upon filtering the
solution through a 0.2 μm filter, as evidence that gross
precipitation had not occurred. The polarity of the solvent
mixture was calculated using the ET(30) scale. The ET(30) is an
an empirical polarity scale for quantifying the polarity of a
binary mixture, and it is defined by the equation
higher solvent polarity. Also significant is the nature of the
nonionic functionality on the polymer. When comparing
TEG
Hex
SPF
with SPF , the solvent polarity at which aggregation
0.5
0.5
Hex
occurs for SPF is much lower than solvent polarity at which
0.5
TEG
0.5
SPF
aggregation occurs even though they have the same
value of χ. This is consistent with the greater polarity of the
TEG side chains, which help promote solubility in more polar
solvents.
The solvatofluorochromism of fluorene-based polymers
somewhat complicates direct comparison of the CPEs
synthesized in this work with their nonionic counterparts.
Nevertheless, the excitation and emission of all of the CPEs
c
⎛
⎜
⎞
p
0
⎟
studied herein are comparable to other fluorene-based
ET(30) = ED ln
+ 1 + ET(30)
⎝
⎠
Hex
0.5
polymers. The family of SPFTχ EG CPEs and SPF
all have
(1)
c*
where E0T(30) is the ET(30) value of the pure, less polar
component, cp is the molar concentration of the more polar
component, and ED and c* are parameters determined
experimentally when the equation was developed.27 ED is a
measure of the sensitivity of the ET(30) scale to changes in cp
while c* is used to divide the equation into a linear and
logarithmic portion. The c* term is the threshold value of cp, at
which the transition from a linear to a logarithmic relationship
between the two solvents occurs.27
very similar excitation and emission spectra, which correspond
very well to the literature values for other ionically function-
alized PFs.28 Note that in these comparisons the data in
methanol were used because of the most limited aggregation in
this solvent.
The tuning of ionic functional group density over the range
achieved in the SPFTχ EG system has not been demonstrated in
other luminescent ionically functionalized conjugated polymers.
Most typically, an ionic monomer (or its precursor) is coupled
with either another ionic monomer or a nonionic monomer in
a one-to-one ratio leading to relatively high ionic functional
group densities. This is the case with anionic poly(fluorene)
CPEs where carboxylate,20 sulfonate,21 and phosphonate22
examples are known, with χ = 0.5, 0.5, or 1, respectively. More
broadly speaking, there are very few examples of CPEs with
ionic functional group densities much lower than this or where
deliberate variation has been demonstrated. The most notable
examples come from the nonexhaustive quaternization of
amine-derivatized conjugated polymers to yield cationic CPEs.
For instance, Liu et al. have demonstrated using this approach
the synthesis of polyfluorene-phenylenes with χ = 0.2, 0.4, and
0.53. In their work, the varying level of quaternization was
achieved by control over reaction conditions, including solvent,
temperature, and time.16 Quaternization yields in similar
reactions have also been reported by Mikroyannidis et al. to
be very sensitive to such conditions.19
Figure 4 shows the change in λmax of emission as a function
of ET(30). The end points of each curve in Figure 4 illustrate
SUMMARY
■
The synthesis and characterization of soluble, sulfonate-
functionalized poly(fluorene)s with varying ionic density were
reported. The study reveals the challenges of using traditional
two-phase Suzuki polycondensations in the competitive
polymerization of a nonionic boronic ester with ionic and
nonionic dihalides. The partitioning of the dihalides into
separate phases leads to vastly different reactivities, thereby
preventing their incorporation into a single polymer. A single-
phase synthesis was developed through the use of amphiphilic,
oligoether-functionalized monomers and a carefully selected
solvent system. The synthesis utilizes direct polymerization of
ionic with nonionic monomers, allows for easy control over
ionic density through monomer feedstock composition, and
avoids the vastly different reactivities encountered in two-phase
Suzuki polycondensations involving ionic and nonionic
monomers. The synthesized SPFTχ EG family included CPEs
with χ values varying between 0.5 and 0.05, a range not seen in
other luminescent anionic CPEs. This illustrates the possibility
of using direct polymerization in the presence of a diluent
monomer to control ionic functional group density in
phenylene-based polymers, an approach which in principle is
Figure 4. λmax of the fluorescence emission spectra of SPF polymer
solutions as a function of the polarity of the solvent. The value in pure
methanol is indicated by the vertical red dashed line and has an ET(30)
value of 55.4 kcal nm mol−1.
the same trends in solvatofluorochromism as discussed earlier.
TEG
0.5
With the exception of SPF
where there is little to no shift,
the λmax for the remaining polymers shift similarly and
continuously upon lowering ET(30) from pure methanol (left
side). However, the shift in λmax upon increasing ET(30) from
pure methanol (right side) occurs over a somewhat more
narrow region of solvent polarity. Further, the region of this
transition depends on the nature of the functionality and ion
concentration. As the ion content increases, so does the ET(30)
TEG
at which the shift happens. SPF
has its shift centering at 58.3
0.05
kcal nm mol−1, SPF
centers at 58.8 kcal nm mol−1, while
TEG
0.25
TEG
0.5
SPF
does not shift until 59.1 kcal nm mol−1. Given the
possibility for strong interactions between water and ions, it is
perhaps not surprising that increasing the ionic functional
group density causes the polymer to stay unaggregated up to a
F
dx.doi.org/10.1021/ma4004693 | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
dried over MgSO4, and the solvent removed under vacuum, resulting
in a yellow oil. Oil was distilled under vacuum to remove excess
dibromohexane (about 100 °C). The remaining yellow oil was run
through a column of silica using chloroform/hexane (1/9), giving a
amenable to a wide variety of ionic functionality. This
development advances the study of CPEs because χ is a central
parameter in determining and understanding their properties.
For instance, the aggregation state of the polymers in solution
was observed to depend on the ionic functional group density
and hence could be controlled through both this quantity and
solvent polarity.
1
white crystalline solid. Yield: 2.93 g (90%). H NMR (d1-CHCl3, 300
MHz): δ (ppm) 0.578 (4H, m), 1.08 (4H, m), 1.20 (4H, m), 1.64
4
(4H, quin), 1.92 (4H, m), 3.29 (4H, t), 7.43 (2H, d, JHH = 1.8 Hz),
7.46 (2H, dd, 3JHH = 8.0 Hz, 4JHH = 1.8 Hz), 7.53 (2H, d,3JHH = 8 Hz).
13C NMR (d1-CHCl3, 70 MHz): δ (ppm) 23.67, 27.52, 27.99, 29.18,
32.72, 32.82, 33.92, 40.27, 55.75, 121.47, 121.80, 126.31, 130.56,
139.27, 152.39.
EXPERIMENTAL SECTION
■
Monomer Synthesis. Sodium 2,5-Dibromobenzylsulfonate (8).
α,2,5-Tribromotoluene (3.29 g, 10 mmol) was added to a solution of
of Na2SO3 (1.26 g, 10 mmol) in 40 mL of H2O. The tribromotoluene
did not dissolve in the water, but as the water heated, the
tribromotoluene melted and formed a puddle on the bottom of the
flask. This biphasic mixture was brought to reflux and refluxed for 60 h.
The starting material was not all reacted as evidenced by a small
puddle of molten α,2,5-tribromotoluene, but the reaction was removed
from heat and from stirring and allowed to cool to room temperature
because it did not seem to be progressing anymore. Product
crystallized from the water upon cooling and was separated by
filtration while washing with ice cold water and ether. The recovered
crystals were the monohydrate of the desired product and dehydrated
by placing them under vacuum and heating to 100 °C for 48 h. Yield =
2.376 g (67.5%). 1H NMR (d2-H2O, 300 MHz): δ (ppm) 4.24 (2H, s),
2,7-Dibromo-9,9-di(6-iodohexyl)fluorene (4). 3 was dissolved in
acetone, and NaI (10 equiv) was added to the reaction flask. The
reaction was brought to reflux and stirred for 12 h. The acetone was
removed under vacuum, leaving a white solid. The solids were
extracted with chloroform. The organic layer was dried with MgSO4.
The organic layer was removed, leaving an off-white solid. Yield: 1.89 g
1
(98%). H NMR (d1-CHCl3, 300 MHz): δ (ppm) 0.575 (4H, quint),
1.11 (8H, m), 1.62 (4H, quint), 1.92 (4H, m), 3.06 (4H, t), 7.43 (2H,
4
3
4
d, JHH = 1.8 Hz), 7.47 (2H, dd, JHH = 8.0 Hz, JHH = 1.8 Hz), 7.52
(2H, d, 3JHH = 8 Hz). 13C NMR (d1-CHCl3, 70 MHz): δ (ppm) 7.46,
28.94, 30.20, 33.53, 40.21, 55.75, 121.51, 121.47, 121.85, 126.32,
130.54, 139.36, 152.46.
2,7-Dibromo-9,9-di(6-thiolhexyl)fluorene (5). 3 (0.650 g, 1.0
mmol) and thiourea (0.166 g, 2.2 mmol) were dissolved in 50 mL
of refluxing ethanol and stirred for 16 h. NaOH (6 mL of 1.0 mL) was
added to the reaction, causing it to become cloudy. The reaction
mixture was refluxed for 3 h during which time the solution cleared.
The total reaction volume was reduced by half, and 6 M H2SO4 was
added dropwise until precipitation of white solid stopped. The
reaction volume was reduced to 20 mL and extracted with ether (3 ×
100 mL). The organic extractions were combined and dried with
MgSO4. Solvent was removed to give a sticky thick colorless oil. Yield:
0.550 g (98%).
2,7-Dibromo-9,9-di(6-sulfonic acid-hexyl)fluorene (6). Concen-
trated nitric acid (15 mL) was added to 5 (0.55 g, 1.0 mmol) and
refluxed during which time the 5 dissolved in the nitric acid. After 12
h, water (50 mL) was added to the reaction mixture. Solvent was
removed under vacuum to give a red oil, which was used without
further purification.
4
3
3
7.30 (1H, dd, JHH = 2.8 Hz, JHH = 9.0 Hz), 7.44 (1H, d, JHH = 9.0
Hz), 7.56 (1H, d, JHH = 2.8 Hz). 13C NMR (d2-H2O, 70 MHz): δ
4
(ppm) 55.6, 120.6, 124.4, 131.6, 134.6, 135.1, 138.2. TOF-MS ES
−
negative mode C7H5Br2SO3 = 328.82.
α,2,5-Tribromotoluene (4). 3 (500 mg, 2.0 mmol), N-bromosucci-
namide (534 mg, 3.0 mmol), benzoyl peroxide (5 mg, 0.02 mmol),
and CCl4 (10 mL) were added to a round-bottom flask and refluxed
overnight. The reaction mixture was then washed with copious
amounts of of H2O, and the organic layer was removed, dried over
MgSO4, and filtered. Silica gel (30 g) was added to the organic layer,
and the solvent was removed in vacuo. The loaded silica was placed in
filter and washed with hexanes until no more material came through.
Solvent was removed leaving an off-white solid. Yield = 644 mg (98%).
1H NMR (d1-CHCl3, 300 MHz): δ (ppm) 4.52 (2H, s), 7.29 (1H, dd,
4JHH = 2.6 Hz, 3JHH = 8.5 Hz), 7.43 (1H, d, 3JHH = 8.5 Hz), 7.80 (1H,
2,7-Dibromo-9,9-di(1-(2-(2-methoxyethoxy)ethoxy)ethyl)-
fluorene (9). A dry three-neck round-bottom flask was charged with
diisopropylamine (1.856 g, 18.35 mmol) and 15 mL of freshly distilled
THF. A magnetic stir bar was added, and the solution was cooled to
−78 °C. After 10 min n-butyllithium (8.08 mL, 20.19 mmol) was
added and stirred for 10 min. While LDA was stirring, 2,7-
dibromofluorene was dissolved in 25 mL of freshly distilled THF
and added dropwise to the now formed LDA solution and stirred at
−78 °C for 30 min. Upon addition, a dark orange solution formed. 1-
(2-(2-Methoxyethoxy)ethoxy)ethyl bromide (5.0 g, 22 mmol) was
added to the orange solution. The orange color lightened, and the
reaction was stirred at −78 °C for 11/2 h and allowed to come to room
temperature overnight, during which time the solution turned green.
Water was added to the solution and stirred for an hour. The organic
layer was removed and washed with water (3 × 100 mL), and then the
aqueous layer was extracted with CHCl3 (2 × 50 mL). The organic
layers were combined and dried over MgSO4 and concentrated to give
an orange yellow oil. The oil was purified by column chromatography
on silica with hexanes until all colored bands (3) moved off the
column. After hexanes, 2:5 ethyl acetate:hexanes was used to give two
4
d, JHH = 2.6 Hz).
2,7-Dibromo-9,9-di(6-sodium sulfonate-hexyl)fluorene (2). The
initial synthesis of 2 was conducted using 6 as a phase-transfer agent,
but subsequent syntheses used 2 as a phase transfer-agent in its own
synthesis. The method in either case was the same, with the specific
procedure for the latter given below. It is noted that 6 was not
detected in the initially isolated 2 and that any remaining trace of this
compound was further diluted with each round of synthesis. 3 (5.5 g,
8.46 mmol), Na2SO3 (10.6 g, 84 mmol), 2 (0.150 g, mmol), and
cetyltrimethylammonium bromide (CTAB) (0.308 g, 0.84 mmol)
were added to 100 mL of H2O and refluxed for 48 h. The solvent was
removed, and the solids were washed with CHCl3 to remove leftover
starting material and CTAB. The white solids were sonicated with 200
mL of methanol. The undissolved solids were filtered off and again
sonicated with 200 mL of methanol, and the solids were again filtered.
Methanol was removed to give a white solid. Yield: 4.97 g (84%) (d4-
CH2OH, 300 MHz): δ (ppm) 0.544 (4H, quint), 1.13 (8H, m), 1.59
(4H, quint), 2.03 (4H, m), 2.67 (4H, m), 7.48 (2H, dd, 3JHH = 8.0 Hz,
4
3
4JHH = 1.6 Hz), 7.56 (2H, d, JHH = 1.6 Hz), 7.66 (2H, d, JHH = 8.0
Hz). 13C NMR (d2-H2O, 125 MHz): δ (ppm) 22.72, 24.44, 28.03,
29.15, 39.01, 51.24, 55.34, 121.12, 121.45, 126.52, 130.12, 138.67,
152.85.
1
bands. The second band is the product. H NMR (d1-CHCl3, 300
MHz): δ (ppm) 2.33 (4H, t), 2.76 (4H, t), 3.20 (4H, m), 3.34 (6H, s),
3
3.39 (4H, m), 3.29 (4H, t), 3.45−3.55 (8H, m), 7.46 (2H, dd, JHH
=
=
8.0 Hz, 4JHH = 1.8 Hz), 7.50 (2H, d,3JHH = 8 Hz), 7.53 (2H, d, 4JHH
2,7-Dibromo-9,9-di(6-bromohexyl)fluorene (3). 50 g of KOH in
100 mL of H2O was heated to 75−80 °C at which point 2,7-
dibromofluorene (1.620 g, 5.0 mmol), dibromohexane (12.20 g, 50
mmol), and tetrabutylammonium bromide (0.161 g 0.5 mmol) were
added and stirred vigorously for 45 min. The reaction was then
extracted with CH2Cl2. The organic layer was washed with dilute HCl
(100 mL), brine (100 mL), and H2O (100 mL). The organic layer was
1.8 Hz). 13C NMR (d1-CHCl3, 125 MHz): δ (ppm) 39.51, 51.90,
59.01, 66.78, 70.07, 70.46, 70.49, 71.00, 121.22, 121.61, 126.72,
130.65, 138.45, 150.98.
2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-di(1-(2-
(2-methoxyethoxy)ethoxy)ethyl)fluorene (10). Monomer 9 (1.5 g,
2.4 mmol) was dissolved in 30 mL of freshly distilled THF and cooled
G
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Macromolecules
Article
to −78 °C. n-Butyllithium (2.44 mL, 6.11 mmol) was added dropwise
to the cooled reaction and stirred at −78 °C for 30 min, resulting in an
orange solution. 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(1.59 g, 8.55 mmol) was added to the solution and stirred for 1.5 h at
−78 °C and then allowed to warm to room temperature overnight,
resulting in an opaque colorless solution. Water was added and stirred
for 30 min. The organic layer was removed and washed with water (2
× 50 mL), and the aqueous layer was washed with ethyl acetate. The
organic layers were combined, dried over MgSO4, and solvent
removed under vacuum to give off-white orange solid. The solids
h. The resulting polymer was washed with CHCl3, THF, and water.
The organic washes were found to contain both unreacted monomer
and reaction byproducts but no polymer. Yields were in the 50−75%
range.
SPFT0E.5G. H NMR (d6-DMSO, 300 MHz): δ (ppm) 0.840 (4H, m),
1
1.22 (12H, m), 2.4−2.9 (16H, m), 3.1 (14H, m), 3.25 (8H, m) 7.65−
8.00 (12H, m).
1
SPFT0E.2G5. H NMR (d6-DMSO, 300 MHz): δ (ppm) 0.820 (4H, m),
1.25 (19H, m), 2.4−2.9 (88H, m), 3.1 (38H, m), 3.25 (56H, m) 7.55−
8.07 (28H, m).
SPFT0E.2G0. H NMR (d6-DMSO, 300 MHz): δ (ppm) 0.819 (8H, m),
1
1
were washed with hexanes to give pure white solid. H NMR (d1-
1.21 (64H, m), 1.49 (16H, m), 1.97 (8H, m), 2.48 (64H, s), 2.79
CHCl3, 300 MHz): δ (ppm) 1.39 (24H, s), 2.44 (4H, t), 2.67 (4H, t),
3.18 (4H, m), 3.33 (6H, s), 3.39 (4H, m), 3.43−3.54 (8H, m), 7.70
(2H, d), 7.81 (2H, d), 7.84 (2H, d). 13C NMR (d1-CHCl3, 125 MHz):
δ (ppm) 24.97, 39.52, 51.02, 58.98, 66.94, 69.99, 70.45, 70.49, 71.86,
83.84, 119.53, 129.25, 134.07, 143.14, 148.59.
1,2-Di-(4,4′-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
phenyl)-1-ethene (13). 15 (1.00 g, 2.95 mmol) was dissolved in dry
THF (30 mL) and cooled to −78 °C. n-BuLi (2.48 mL, 6.21 mmol)
was added via syringe, and the reaction was warmed to 0 °C over 1 h,
during which time the reaction turned red. The reaction was cooled
back to −78 °C, and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxabor-
olane (2.11 g, 6.21 mmol) was added dropwise and allowed to come to
room temperature overnight.
(48H, m), 3.12−3.54 (289H, m), 7.55−8.07 (70H, m).
SPFT0E.1G7. H NMR (d6-DMSO, 300 MHz): δ (ppm) 0.820 (4H, m),
1
1.25 (17H, m), 2.5 (41H, m), 2.7 (14H, m), 3.1−3.4 (104H, m) 7.55−
8.07 (35H, m).
SPFT0E.0G5. H NMR (d6-DMSO, 300 MHz): δ (ppm) 0.819 (4H, m),
1
1.11−1.49 (93H, m), 2.2 (37H, m), 2.79 (53H, m), 3.12−3.54 (315H,
m), 7.55−8.15 (124H, m).
Characterization. NMR spectra were recorded with a Varian
INOVA 300 MHz spectrometer with CP solutions in DMSO-d6.
Visible absorption spectroscopy was performed on either DMSO or
methanol solutions using a Hewlett-Packard 8452A diode array
spectrometer. Gel permeation chromatography was performed on a
Waters chromatography system utilizing a Styragel HR4 size exclusion
column, a 515 pump, and 2410 differential refractometer. The flow
rate of the GPC was 0.1 mL/min.
Water was added until fizzing stopped, and the layers were
separated. The organic layer was washed with saturated KCl brine and
twice with H2O. The water layers were extracted with CHCl3. The
organic layers were combined, dried over MgSO4, and evaporated in
vacuo. Solids were recrystallized from hot hexanes to give white
AUTHOR INFORMATION
1
■
needles. Yield = 523 mg (41%). H NMR (d1-CHCl3, 300 MHz): δ
3
Corresponding Author
(ppm) 1.35 (24H, s), 7.18 (2H, s), 7.52 (4H, d, JHH = 8.3 Hz), 7.80
3
(4H, d, JHH = 8.3 Hz). 13C NMR (d1-CHCl3, 70 MHz): δ (ppm)
*E-mail lonergan@uoregon.edu; phone (541) 346-4748; Fax
(541) 346-4643 (M.C.L.).
24.8, 83.8, 125.9, 129.5, 135.1, 135.2, 135.3, 139.8.
Polymer Synthesis. SPFPH0.e3x3. Monomer 8 (352 mg, 1 mmol) and
K2CO3 (1.382 g, 10 mmol) were dissolved in 5 mL of H2O. 9,9-
Dihexylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester (502
mg, 1 mmol) was dissolved in THF (20 mL). The organic and
aqueous solutions were both placed in a three-neck round-bottom
flask along with a magnetic stir bar and fitted with a water cooled
condenser. The mixture of solutions was deoxygenated by bubbling N2
through the solutions and heated to 85 °C. After purging with N2 for
10 min, the catalyst was added, and the reaction was stirred for 72 h.
The reaction was poured into water, and the solids were collected via
centrifugation. The solids were washed with chloroform, dissolved in
methanol, and precipitated into water. This washing and precipitating
process was repeated three times. 1H NMR (d4-methanol, 300 MHz):
δ (ppm) 0.5−1.5 (22H, m), 2.1 (4H, m), 4.25 (2H, s), 7.4−8.2 (9H,
m).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was funded by the Division of Chemical Sciences,
Geosciences, and Biosciences, Office of Basic Energy Sciences
of the U.S. Department of Energy through Grant DE-FG02-
07ER15907. This project made use of equipment in the
SuNRISE Photovoltaic Laboratory supported by the Oregon
Built Environment and Sustainable Technologies (BEST)
signature research center.
REFERENCES
SPFH0.e5x. Monomer 2 (524 mg, 0.5 mmol) and 9,9-dihexylfluorene-
2,7-diboronic acid bis(1,3-propanediol) ester (251 mg, 0.5 mmol)
were dissolved in 8 mL of THF/methanol 50/50 (v/v). 1 mL of 2 M
Na2CO3 and the organic solution were both placed in a round-bottom
flask fitted with a magnetic stir bar and condenser. The mixture of
solutions was deoxygenated by bubbling N2 through the solutions and
heated to 85 °C. After purging with N2 for 10 min, the Pd(PPh3)4
(0.05 equiv) catalyst was added, and the reaction was stirred for 48−72
h. The reaction proceeded with the formation of a yellow-orange
precipitate that began forming around 18 h. The resulting polymer was
washed with CHCl3, THF, and water. 1H NMR (d6-DMSO, 300
MHz): δ (ppm) 0.682 (8H, m), 1.10 (22H, m), 1.33 (4H, m), 2.4
(289H, m), 7.55−8.07 (70H, m).
■
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dx.doi.org/10.1021/ma4004693 | Macromolecules XXXX, XXX, XXX−XXX