Thermally polymerized rylene nanoparticles

Article abstract of DOI:10.1021/ma2000866

Rylene dyes functionalized with varying numbers of phenyl trifluorovinyl ether (TFVE) moieties were subjected to a thermal emulsion polymerization to yield shape-persistent, water-soluble chromophore nanoparticles. Perylene and terrylene diimide derivativ

Full text of DOI:10.1021/ma2000866

ARTICLE
pubs.acs.org/Macromolecules
Thermally Polymerized Rylene Nanoparticles
Trisha L. Andrew and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge,
Massachusetts 02139, United States
S Supporting Information
b
ABSTRACT: Rylene dyes functionalized with varying numbers
of phenyl trifluorovinyl ether (TFVE) moieties were subjected
to a thermal emulsion polymerization to yield shape-persistent,
water-soluble chromophore nanoparticles. Perylene and terry-
lene diimide derivatives containing either two or four phenyl
TFVE functional groups were synthesized and subjected to
thermal emulsion polymerization in tetraglyme. Dynamic light
scattering measurements indicated that particles with sizes
ranging from 70 to 100 nm were obtained in tetraglyme, depending on monomer concentration. The photophysical properties
of individual monomers were preserved in the nanoemulsions, and emission colors could be tuned between yellow, orange, red, and
deep red. The nanoparticles were found to retain their shape upon dissolution into water, and the resulting water suspensions
displayed moderate to high fluorescence quantum yield.
’ INTRODUCTION
reaction to generate perfluorocyclobutane (PFCB) derivatives
(Figure 1A).9 Smith and co-workers have synthesized numerous,
high molecular-weight polymers by thermally polymerizing mono-
mers containing multiple TFVE moieties.10 Moreover, because of
the availability of a key synthetic intermediate (1, Figure 1B), the
straightforward incorporation of phenyl TFVE moieties into a
variety of chromophore skeletons is possible.11 In the past decade,
TFVE-containing chromophore thermosets have been explored as
thermally stable nonlinear optical polymers,12 optical wavegui-
des,13 and hole-transporting materials in organic light-emitting
diodes.14
Chromophore and conjugated polymer (CP) nanoparticles
and nanocomposites1 have recently found prominence in various
applications, including optoelectronics2 and biological imaging
and sensing.3 In certain cases, CP nanoparticles displayed desir-
able properties that were either absent (water solubility) or not as
prominent (two-photon absorption cross section) in CP thin
films or solutions.3,4 Additionally, there is considerable interest in
using CP nanoparticles to improve or control the nanoscale
composition of CP blends to improve the efficiency of polymer
LEDs and photovoltaics.2,5 Fluorescence energy transfer in small
molecule chromophore-containing nanoparticles has also been
investigated for live cell imaging.3e,6
CP nanoparticles are predominantly fabricated by micropreci-
pitation methods, where a small aliquot of a dilute solution of the
CP in a good solvent (such as tetrahydrofuran) is added to a poor
solvent (such as water) with sonication.4 In the case of small
molecule chromophores, emulsions of monomers containing
polymerizable functional groups are first formed (sometimes in
the presence of surfactants), and then the monomers are polym-
erized within the microcapsules (using either radical initiators or
metal catalysts) to yield shape-persistent chromophore or CP
nanoparticles.7 Rylene dyes, particularly 3,4,9,10-perylene tetra-
carboxidiimides (PDIs), are frequently used as the chromophore
component owing to their brilliant colors, large extinction coeffi-
cients, near-unity fluorescence quantum yields, and remarkable
photostability.8 Recent examples of rylene-containing nanoparti-
cles almost always utilize methacrylate-functionalized chromo-
phores, which are then polymerized in microemulsions by
established controlled radical polymerization processes.3c-e
Aryl trifluorovinyl ethers (TFVEs) are a unique class of
molecules thathavebeenshownto undergo a thermal dimerization
Figure 1. (A) Thermal dimerization reaction of aryl trifluorovinyl
ethers (TFVEs), shown for phenyl TFVE. (B) Structure of 4-bromo-
phenyl TFVE (1).
Thus far, the thermal, radical initiator-free fabrication of
shape-persistent nanoparticles has not been demonstrated. We
anticipated that the thermal reactivity of TFVEs would allow
Received: January 13, 2011
Revised:
February 15, 2011
Published: March 15, 2011
r
2011 American Chemical Society
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access to such nanoparticles. Herein we describe the synthesis of
rylene dyes functionalized with two or more phenyl TFVE
moieties and the fabrication of rylene nanoparticles via a thermal
emulsion polymerization.
1F), -126.69 (dd, J = 109, 98 Hz, 1F), -134.47 (dd, J = 109, 60 Hz, 1F).
UV-vis (CHCl3): λmax (log ε) = 400 (3.8), 558 (4.0), 592 (4.3). HRMS
(ESI, m/z): [M þ H]þ calcd for C72H55F12N2O8, 1303.3767; found,
1303.3769.
M4. Isolated in 74% as a greenish-blue solid from 6 following the
1
procedure described above. H NMR (400 MHz, CDCl3, δ): 8.37 (s,
’ MATERIALS AND METHODS
4H), 7.98 (s, 4H), 7.56 (d, J = 8.4 Hz, 8H), 7.27 (t, J = 4.8 Hz, 2H), 7.23
(d, J = 8.4 Hz, 8H), 7.01 (d, J = 4.8 Hz, 4H), 2.68 (m, 4H), 1.08 (s, 24H).
13C NMR (100 MHz, CDCl3, δ): 163.3, 155.0, 153.2, 147.8, 145.8,
134.6, 132.1, 132.0, 129.7, 129.2, 129.2, 129.0,128.9, 128.4, 127.4, 127.3,
126.1, 125.8, 125.5, 124.1, 123.4, 122.7, 122.0, 119.3, 34.7, 29.2, 21.6. 19F
NMR (376 MHz, CDCl3, δ): -119.00 (dd, J = 98, 60 Hz, 1F), -126.69
(dd, J = 109, 98 Hz, 1F), -134.47 (dd, J = 109, 60 Hz, 1F). UV-vis
(CHCl3): λmax (log ε) = 410 (3.0), 610 (3.1), 665 (3.7), 720 (3.9).
HRMS (ESI, m/z): [M þ H]þ calcd for C90H59F12N2O8, 1523.4080;
found, 1523.1520.
General Procedure for the Synthesis of M3 and M5. A flame-
dried 50 mL Schlenk flask was charged with the appropriate rylene
bromide, 5 (1.1 equiv per bromine substituent), and anhydrous post-
assium carbonate (1.1 equiv per bromine substituent) under a positive
flow of argon. Dry, degassed N-methyl-2-pyrrolidone (10 mL) was
introduced via cannula addition, and the resulting mixture was heated at
80 °C for 12 h. The reactionwas cooledto room temperature, diluted with
1 M HCl, andextractedwithCHCl3 (3 ꢀ 30 mL). Theorganiclayers were
combined and dried over magnesium sulfate, and the solvent evaporated
under reduced pressure. The resulting residue was purified by flash
column chromatography using 60/40 hexanes/dichloromethane as the
eluent.
M3. Isolated in 85% as a deep red solid from 4 following the
procedure described above. 1H NMR (400 MHz, CDCl3, δ): 9.47 (d,
J = 8.4 Hz, 2H), 8.57 (d, J = 8.4 Hz, 2H), 8.23 (s, 1H), 7.18 (m, 8H), 4.04
(m, 4H), 1.87 (m, 2H), 1.57 (s, 4H), 1.28 (m, 22H), 0.88 (m, 16H). 13C
NMR (100 MHz, CDCl3, δ): 163.7, 155.3, 139.4, 132.1, 130.5, 129.3,
129.0, 125.3, 124.1, 123.9, 122.4, 121.3, 118.3, 44.5, 38.1, 30.9, 28.9, 24.2,
23.2, 14.3, 10.8. 19F NMR (376 MHz, CDCl3, δ): -119.00 (dd, J = 98,
60 Hz, 1F), -126.69 (dd, J = 109, 98 Hz, 1F), -134.47 (dd, J = 109, 60
Hz, 1F). UV-vis (CHCl3): λmax (log ε) = 400 (3.8), 461 (3.8), 502
(4.1), 536 (4.4). HRMS (ESI, m/z): [M þ H]þ calcd for
C56H49F6N2O8, 991.3393; found, 991.3398.
General Considerations. Synthetic manipulations were carried
out under argon using dry solvents and standard Schlenk techniques. All
solvents were of ACS reagent grade or better unless otherwise noted.
1,4-Dioxane and 1,2-dimethoxyethane were purified by distillation over
activated alumina. Silica gel (40-63 μm) was obtained from SiliCycle
Inc. Pd2(dba)3 CHCl3 and S-Phos were purchased from Strem Chem-
3
icals and used without further purification. Tetraglyme was purchased
from VWR and purified by passing through a plug of activated neutral
alumina. Compounds 2,15 3,9c 4,16 5,9c and 617 were synthesized
following published procedures. Ultraviolet-visible absorption spectra
were measured with an Agilent 8453 diode array spectrophotometer and
corrected for background signal with a solvent-filled cuvette. Fluores-
cence spectra were measured on a SPEX Fluorolog-τ3 fluorimeter
(model FL-321, 450 W xenon lamp) using right-angle detection.
Fluorescence lifetimes were measured via frequency modulation using
a Horiba-Jobin-Yvon MF2 lifetime spectrometer equipped with a
365 nm laser diode and using the modulation of POPOP as a calibration
reference. Mesitylene/tetraglyme biphasic mixtures were emulsified
with either an IKA Ultra-Turrax T25 Basic high-shear disperser (at a
shear rate of 24/min) or a Misonix Microson ultrasonic cell disruptor.
DLS measurements were performed at the MIT Biophysics Instrumen-
tation Facility using a Wyatt Technologies DynaPro Titan dynamic light
scatterer equipped with a 830 nm diode laser. Data were fitted to a
globular protein model, taking into account solvent refractive indices
and viscosities (CHCl3: 0.57 cP at 20 °C; toluene: 0.59 cP at 20 °C;
tetraglyme: 4.1 cP at 20 °C; water: 1.00 cP at 20 °C).
General Procedure for the Synthesis of M1, M2, and M4. A
flame-dried 50 mL Schlenk flask was charged with the appropriate rylene
bromide, 3 (1.5 equiv per bromine substituent), Pd2(dba)3 CHCl3 (0.05
3
equiv), S-Phos (0.2 equiv), and anhydrous postassium phosphate (20
equiv) under a positive flow of argon. Dry, degassed 1,2-dimethoxyethane
(15 mL) was introduced via cannula addition, and the resulting mixture
was heated at 60 °C for 12 h. The reaction was cooled to room
temperature and passed through a Celite plug, and the solvent evaporated
under reducedpressure. Theresulting residuewas purifiedbyflash column
chromatography using 50/50 hexanes/dichloromethane as the eluent.
M1. Isolated in 70% as a deep red solid from 2 following the
procedure described above, with the substitutions of 1,4-dioxane for
1,2-dimethoxyethane and 3.0 M aqueous potassium phosphate for
anhydrous potassium phosphate. 1H NMR (400 MHz, CDCl3, δ):
8.56 (s, 2H), 8.18 (d, J = 8.4 Hz, 2H), 7.80 (d, J = 8.4 Hz, 2H), 7.56 (d, J =
8.0 Hz, 4H), 7.23 (d, J = 8.0 Hz, 4H), 4.11 (m, 4H), 1.92 (m, 2H), 1.55
(s, 6H), 1.32 (m, 20H), 0.91 (m, 14H). 13C NMR (100 MHz, CDCl3,
δ): 163.8, 139.9, 139.4, 136.8, 136.1, 135.0, 131.1, 130.3, 130.1, 129.8
128.6, 122.5, 121.4, 117.8, 44.5, 38.1, 30.9, 28.9, 24.2, 23.2, 14.3, 10.8. 19F
NMR (376 MHz, CDCl3, δ): -119.00 (dd, J = 98, 60 Hz, 1F), -126.69
(dd, J = 109, 98 Hz, 1F), -134.47 (dd, J = 109, 60 Hz, 1F). UV-vis
(CHCl3): λmax (log ε) = 400 (3.8), 475 (3.2), 519 (4.0), 553 (4.4).
HRMS (ESI, m/z): [M þ H]þ calcd for C56H49F6N2O6, 959.3495;
found, 959.3493.
M5. Isolated in 87% as a deep blue solid from 6 following the
1
procedure described above. H NMR (400 MHz, CDCl3, δ): 9.47 (s,
4H), 8.27 (s, 4H), 7.40 (d, J = 8.4 Hz, 8H), 7.27 (t, J = 4.8 Hz, 2H), 7.09
(d, J = 8.4 Hz, 8H), 7.01 (d, J = 4.8 Hz, 4H), 2.68 (m, 4H), 1.08 (s, 24H).
13C NMR (100 MHz, CDCl3, δ): 163.3, 155.0, 153.2, 147.8, 145.8,
134.6, 132.1, 132.0, 129.7, 129.2, 129.2, 129.0,128.9, 128.4, 127.4, 127.3,
126.1, 125.8, 125.5, 124.1, 123.4, 122.7, 122.0, 119.3, 34.7, 29.2, 21.6. 19F
NMR (376 MHz, CDCl3, δ): -119.00 (dd, J = 98, 60 Hz, 1F), -126.69
(dd, J = 109, 98 Hz, 1F), -134.47 (dd, J = 109, 60 Hz, 1F). UV-vis
(CHCl3): λmax (log ε) = 410 (3.2), 569 (3.8), 623 (4.0), 680 (4.2).
HRMS (ESI, m/z): [M þ H]þ calcd for C90H59F12N2O12, 1587.3876;
found, 1587.3877.
Nanoparticle Synthesis. 0.5 mL of a solution of the appropriate
TFVE-functionalized rylene diimide monomer in mesitylene (0.2-1.6
mg/mL) was added to 5.0 mL of tetraglyme. The resulting biphasic
mixture was either homogenized with a high-shear disperser or sonicated
to yield a homogeneous emulsion, which was then heated to 190 °C
under argon for 12 h.
M2. Isolated in 50% as a deep red solid from 2 following the
1
procedure described above. H NMR (400 MHz, CDCl3, δ): 8.29 (s,
’ RESULTS AND DISCUSSION
4H), 7.56 (d, J = 8.0 Hz, 8H), 7.23 (d, J = 8.0 Hz, 8H), 4.11 (m, 4H), 1.92
(m, 2H), 1.55 (s, 6H), 1.32 (m, 20H), 0.91 (m, 14H). 13C NMR (100
MHz, CDCl3, δ): 163.7, 139.8, 139.4, 136.8, 135.8, 135.0, 131.1, 130.3,
130.1, 128.6, 122.5, 121.4, 117.8, 44.5, 38.1, 30.9, 28.9, 24.2, 23.2, 14.3,
10.8. 19F NMR (376 MHz, CDCl3, δ): -119.00 (dd, J = 98, 60 Hz,
Monomer Synthesis. PDIs were bay-functionalized with
phenyl TFVE moieties starting from either tetrabromide 215 or
dibromide 416 (Scheme 1). Following the previously reported
synthesis and CsF/Ag2O-mediated tetraarylation of 2,15 we
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Scheme 1. Synthesis of Perylene Diimide-Containing
Thermoset Monomers
Scheme 2. Synthesis of Terrylene Diimide-Containing
Thermoset Monomers
Table 1. Photophysical Properties (in CHCl3) of Phenyl
TFVE-Containing Rylene Diimides
compound
λmax/nm (log ε)
λem/nm
Φ
τ/ns
M1
M2
M3
M4
M5
553 (4.4)
592 (4.3)
536 (4.4)
720 (3.9)
680 (4.2)
602
640
563
765
700
0.88a
0.61a
0.81a
0.08b
0.17b
8.7
9.8
5.8
5.7
2.4
initially aimed to perform a 4-fold Suzuki-Miyaura coupling
between 2 and 3. However, the TFVE moiety was found to be
slightly susceptible to nucleophilic addition of fluoride in the
presence of the CsF/Ag2O base system, and the resulting hydro-
1,2,2-fluoroethane-containing (-O-CHF-CF3) side products
could not be separated from the reaction mixture. Instead, a
4-fold Suzuki-Miyaura cross-coupling under the modified
Buchwald conditions was pursued. When aqueous potassium
phosphate was employed as a base, tetrabromide 2 was partially
dehalogenated and monomer M1, with two phenyl TFVE
moieties, was isolated in 70% yield. Upon switching the base to
rigorously anhydrous potassium phosphate, tetraarylated mono-
mer M2 was isolated in 50% yield. Taking advantage of the
electron-deficient nature of the PDI skeleton, monomer M3 was
synthesized via an SNAr reaction between 4 and 5.17
The synthesis of TFVE-functionalized terrylene diimide (TDI)
monomers is shown in Scheme 2. Similar to M2, the tetraarylated
TDI monomer M4 was synthesized starting from tetrabromide 6
via a modified Suzuki-Miyaura cross-coupling reaction with
anhydrous potassium phosphate. Tetraaryloxy monomer M5
was synthesized by an SNAr reaction between 6 and 5.
a Measured against Rhodamine B in ethanol (Φ 0.71). b Measured
against zinc phthalocyanine in 1% pyridine in toluene (Φ 0.30).
and emission maxima and fluorescence quantum yields of mono-
mers M3 and M5 were similar to phenyloxy-substituted PDI18a and
TDI,18b respectively. Monomer M2 displayed similar absorp-
tion and emission maxima to the previously reported tetra-
phenyl PDI. Consistent with similar observations for biphenyl-
containing fluorophores,19 phenyl substitution was found to
increase the otherwise-small Stokes’ shift of rylene dyes and
decrease their fluorescence quantum yields. Moreover, the
excited-state lifetimes of the phenyl-substituted rylenes, M1,
M2, and M4, were found to be significantly longer than their
parent rylene diimides (4.5 ns for N-(2-ethylhexyl)-PDI and
3.5 ns for N-(2,6-diisopropylphenyl)-TDI). These differences
in photophysical properties are most likely caused by excited-
state planarization across the biphenyl linkage.
Nanoparticle Synthesis. Rylene nanoparticles were synthe-
sized by adding mesitylene solutions of monomers M1-5 to
tetraglyme and either homogenizing or sonicating the resulting
biphasic system. This homogeneous mixture was then thermally
cross-linked by heating to 190 °C for 12 h (Scheme 3). Scanning
Monomer Photophysics. The photophysical properties of
monomers M1-5 (in CHCl3 solutions) are summarized in
Table 1. In general, the TFVE moieties were not observed to signifi-
cantly affect the optical properties of rylene dyes. The absorption
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Scheme 3. Thermal Formation of Shape-Persistent Rylene
Nanoparticles
Figure 3. (A) Typical distribution of hydrodynamic radii in tetraglyme
for the thermally polymerized mixtures fabricated with M1-5, as
measured by dynamic light scattering (DLS); the particle size distribu-
tion for NP2 is shown (1.6 mg/mL starting monomer concentration in
mesitylene). (B) Average hydrodynamic radii of particles obtained by
varying the concentration of rylene monomer in the starting mesitylene
solution (shown for NP2).
Figure 2. SEM micrograph of the thermally polymerized reaction
mixture obtained from M2.
TFVE moiety9c were not observed (see Supporting Information).
Additionally, undesired side products from the addition of water
or other nucleophiles to the TFVE moiety were not evident in the
19F spectrum of NP2. Nanoparticles NP2, NP4, and NP5 were
observed to retain their shape upon extraction into organic
solvents (toluene and CHCl3); however, the hydrodynamic radii
of these particles were observed to increase by ∼30% in organic
solvents (see Figure 4). This is most likely due to swelling of the
cross-linked nanoparticles by the organic solvents.
Nanoparticle Photophysics. The absorption and emission
spectra of colloidal NP1-5 in tetraglyme (Figure 5A) were,
overall, similar to those of their respective monomers in chloro-
form or toluene solutions, with two notable differences: the
absorption and emission bands of the rylene nanoparticles were
broadened and their emission maxima were hypsochromically
shifted by ∼15 nm relative to their corresponding monomers.
We tentatively ascribe these observations to aggregation of the
rylene chromophores within individual nanoparticles, which is
induced by the presence of trapped tetraglyme. Accordingly, the
fluorescence quantum yields of the nanoparticles in tetraglyme
were also slightly lower than those of the starting monomers—a
ca. 5% loss in fluorescence quantum yield was generally observed
after thermal emulsion polymerization in tetraglyme.
electron micrograph (SEM) images of the drop-cast reaction
mixture thus obtained from M2 revealed the presence of poly-
disperse, submicrometer particles (see Figure 2). Dynamic light
scattering (DLS) measurements on the thermally polymerized
mixtures fabricated from M1-5 indicated that particles with
hydrodynamic radii between ca. 70 and 100 nm in tetraglyme
were obtained (see Figure 3A). A significant difference was not
observed in the size of the nanoparticles obtained from perylene
(NP1-3) versus terrylene (NP4,5) diimide monomers. The size
of the dye particles in tetraglyme could be controlled within the
70-100 nm range by varying the concentration of monomer in
the starting mesitylene solution (see Figure 3B). DLS measure-
ments indicated that NP1-5 did not coagulate in tetraglyme for
at least 6 months (the stability of the nanoparticles as colloids in
tetraglyme was only monitored for 6 months).
Once thermally polymerized, the chromophore nanoparticles
could be extracted into organic solvents from the tetraglyme
suspension. 19F NMR spectra of the toluene-d8 extract of the
thermally polymerized reaction mixture obtained from M2 in-
dicated that the desired formation of PFCBs had proceeded, as
the three characteristic doublet of doublets arising from the
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The filtered aqueous solutions were homogeneous and nano-
particle precipitation was not evident by eye. DLS measurements
revealed that the hydrodynamic radii of NP1-5 in water
decreased by ca. 35% relative to those measured in tetraglyme
(see Figure 4), most likely due to expulsion of trapped tetraglyme
from within the nanoparticles. These aqueous solutions re-
mained homogeneous by eye, and the measured hydrodynamic
radii of the nanoparticles remained unchanged for at least 3
months. The emission spectra of NP1-5 in water are shown in
Figure 5B. The emission maxima of NP1-5 in water are red-
shifted relative to those in tetraglyme and are very similar to those
of their corresponding monomers in chloroform solutions. In
this case, we posit that the aforementioned expulsion of trapped
tetraglyme from within the nanoparticles effectively reverses the
aggregation-induced hypsochromic shifts observed in tetraglyme
suspensions of NP1-5. Aqueous solutions of the perylene
diimide nanoparticles NP1, NP2, and NP3 displayed good
fluorescence quantum yields (63%, 50%, and 60%, respectively);
however, the fluorescence quantum yields of aqueous solutions
of the terrylene diimide nanoparticles NP4 and NP5 were
relatively low (3% and 11%, respectively).20
Figure 4. Typical changes in the measured hydrodynamic radii for NP2,
NP4, and NP5 with changing solvents. The particle size distribution for
NP2 (fabricated with 1.6 mg/mL starting monomer concentration in
mesitylene) is shown in (A) tetraglyme, (B) toluene, and (C) water.
’ CONCLUSIONS
Rylene dyes functionalized with varying numbers of phenyl
trifluorovinyl ether (TFVE) moieties were synthesized and sub-
jected to a thermal emulsion polymerization to yield shape-
persistent, water-soluble chromophore nanoparticles. The re-
ported thermal emulsion polymerization is unique from previously
reported methods to fabricate chromophore or conjugated poly-
mer nanoparticles, as it does not involve the use of radical initiators
or metal catalysts. Aqueous solutions of perylene diimide-contain-
ing nanoparticles remained homogeneous for at least 3 months
and displayed desirable fluorescence quantum yields.
’ ASSOCIATED CONTENT
S
Supporting Information. Characterization data for the
b
linear polymer of M1, 19F NMR spectra of select systems, and
spectral characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: tswager@mit.edu.
’ ACKNOWLEDGMENT
T.L.A. thanks the Chesonis Family Foundation for a graduate
fellowship. The Biophysical Instrumentation Facility for the
Study of Complex Macromolecular Systems (NSF-0070319
and NIH GM68762) is gratefully acknowledged.
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(16) A mixture of the 1,6- and 1,7-dibrominated PDI isomers was
obtained following published procedures: (a) Zhan, X.; Tan, Z.;
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A mixture of 1,6- and 1,7-dibromo isomers was used to synthesize M3
(see note 17); however, compound 4 is shown as the 1,7-isomer in
Scheme 1 for simplicity.
(17) Following multiple purifications by flash column chromatogra-
phy, a sample that contained ca. 85% of the 1,7-isomer of M3 (shown in
Scheme 1) and ca. 15% of the 1,6-isomer was obtained, as determined
from the 1H NMR spectrum of the final mixture (see Supporting
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dx.doi.org/10.1021/ma2000866 |Macromolecules 2011, 44, 2276–2281