Triphenylene-Enchained Perfluorocyclobutyl Aryl Ether Polymers: A Modular Synthetic Route to Processable Thermoplastics Approaching Upper Limit Tgand Photostability

Article abstract of DOI:10.1021/acs.macromol.1c01043

Triphenylene-containing trifluorovinyl ether monomers prepared from 2,3-disubstituted-bis-1,4-(p-bromophenyl)triphenylene core building blocks undergo thermal step-growth polymerization (Ph2O, 180 °C), affording perfluorocyclobutyl polymers with unprecede

Full text of DOI:10.1021/acs.macromol.1c01043

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Article
Triphenylene-Enchained Perfluorocyclobutyl Aryl Ether Polymers: A
Modular Synthetic Route to Processable Thermoplastics
Approaching Upper Limit Tg and Photostability
Behzad Farajidizaji, Ernesto I. Borrego, Sumudu Athukorale, Mehdi Jazi, Bruno Donnadieu,
Charles U. Pittman, Jr., and Dennis W. Smith, Jr.*
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ABSTRACT: Triphenylene-containing trifluorovinyl ether monomers prepared from 2,3-disubstituted-bis-1,4-(p-bromophenyl)-
triphenylene core building blocks undergo thermal step-growth polymerization (Ph2O, 180 °C), affording perfluorocyclobutyl
polymers with unprecedented glass-transition temperatures (up to 295 °C), excellent high thermal-oxidative stabilities, and solution
processability. The modular synthetic route provides access to a series of triphenylene monomers from a common
cyclopentadienone derivative and variably substituted alkynes, which polymerize thermally to solution-processable, tough,
transparent films with bright blue solid-state photoluminescence (λem = ∼400−470 nm). Conversion was monitored by 19F NMR
end-group analysis and gel permeation chromatography to reasonably high molecular weights (Mn = 45−93 kDa). Remarkably,
photoemission persists at 250 °C in air for 24 h with negligible changes in absorbance and emission wavelengths after cooling to
room temperature.
class due to their ease of synthesis and wide ranging structural
variety.12 Thermal [2 + 2] cycloaddition polymerization of
aromatic TFVE groups does not require catalysts or initiators
and proceeds free from mobile impurities, condensates, or
other byproducts.10,13 In addition, the TFVE group is highly
tolerant to a wide variety of functional group transformations14
including Grignard,15 Suzuki,16 and Sonogashira coupling14
and Huisgen’s 1,3-dipolar cycloadditions.17,18
INTRODUCTION
■
Fluoropolymers are outstanding specialty materials due to their
superb chemical inertness, thermal and oxidative stability, high
gas permeability, and good dielectric properties.1,2 In addition
to traditional highly crystalline perfluorinated polymers,
ongoing research in semi-fluorinated polymers has provided
viable alternatives with improved processability, optical
properties, and mechanical performance, further enabling
emerging technologies such as separation membranes,3 fuel
cells,4 and biomedical devices.5 To expand material diversity,
we have developed a platform of semi-fluorinated aryl ether
polymers via step-growth polymerization of fluoroalkenes
including perfluorocyclobutyl (PFCB),6 fluorinated arylene
vinylene ether (FAVE),7 and perfluorocycloalkene (PFCA)8
aromatic ether polymers.9 Prepared by thermal cyclopolyme-
rization of aromatic trifluorovinyl ether (TFVE) monomers,
PFCB aryl ether-containing thermoplastic and thermosetting
polymers, originally developed by The Dow Chemical
Company,10,11 represent the most successful family in this
Here, we report the first synthesis and polymerization of
TFVE monomers (M1−M4) containing triphenylene cores
(Figure 1), affording amorphous semi-fluorinated thermo-
Received: May 11, 2021
Revised: July 22, 2021
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Figure 1. Modular synthesis of monomers M1−M4 and polymers P1−P4.
plastics with unprecedented thermal performance (Tg ∼ 300
°C) and solid-state emission after prolonged heating at 250 °C
in air. Triphenylene is a well-studied blue light-emitting
chromophore19−23 and mesogen, which forms discotic liquid
crystalline (LC) phases that act as one-dimensional charge
carriers.24 The triphenylene unit has been incorporated into
star polymers to serve as a discotic core,25 linear conjugated
polymers by Suzuki cross-coupling,22 and conjugated den-
drimers by Diels−Alder reaction.23 Triphenylene moieties have
also been photocured by incorporating methacrylates26 and
copolymerized by the Sonogashira reaction.27 Unfortunately,
triphenylene derivatives typically lack solubility in common
organic solvents, requiring the incorporation of alkyl or alkoxy
groups. Moreover, solution polymerizations carried out by
metal-catalyzed coupling strategies produced low monomer
conversion and limited molecular weight, primarily due to
solubility limits.22
Cyclopolymerization of TFVE monomers to stereo-random
1,2-disubstituted PFCB repeat units is famous for solubilizing
extended aromatic cores such as triphenylene for both
monomer and growing polymer. Previously reported accounts
of PFCB polymers enchained with polycyclic aromatic
hydrocarbons (PAHs) include hexabenzocoronene,28 naph-
thalene,29,30 and most recently, acenaphthoquinone6 homo-
and co-polymers. In addition, high aspect ratio penta-p-
phenylene PFCB polymers have provided unique semi-
fluorinated LC materials.31 The broad synthetic versatility
developed over many years has provided direction to prepare
extreme PAH targets with controlled photonic properties and
phase behavior. In addition to the largest PAH enchained in a
PFCB thermoplastic, we present a straightforward modular
synthesis to TFVE functional triphenylene monomers in a few
steps from readily available starting materials.
none intermediate 5 in 77% yield. Triphenylene core
formation was completed via a Diels−Alder/decarbonylation
reaction of 5 with a variety of monosubstituted and
disubstituted alkynes, providing a modular route to 2,3-
disubstituted-bis-1,4-(p-bromophenyl)triphenylene building
blocks. Thus, reacting 5 with phenylacetylene, diphenylacety-
lene, dimethyl acetylenedicarboxylate, and 4-ethynyl-N,N-
diphenylaniline in mesitylene at 210 °C gave dibromo
compounds (6−9) in 89, 56, 88, and 76% yields, respectively
(Figure 1).
A commercial intermediate, p-bromo(trifluorovinyloxy)-
benzene, was converted to the corresponding boronic ester
(10) via a Suzuki−Miyaura cross-coupling reaction in 68%
yield. It should be noted that several failed attempts were made
to cross-couple 5 and boronic ester 10, utilizing different
conditions. Subsequent Suzuki−Miyaura cross-coupling of
substituted dibromotriphenylenes 6−9 with 10 (Figure 1)
successfully extended the π-system to give monomers M1−M4
in moderate to good yields (68−93%). Interestingly, M2 is a
candidate for oxidative aromatization (Scholl reaction) to
produce a TFVE containing coronene moiety as a future
target.28
The structures of monomers M1−M4 were determined by
1H NMR, 13C NMR, and 19F NMR, FTIR, and HR-MS, for
which detailed data can be found in the SI. Single crystals of
M3 and M4 suitable for X-ray structure analysis (Figure 2)
Figure 2. X-ray crystal structures of M3 and M4 illustrating twisted
triphenylene cores.
RESULTS AND DISCUSSION
■
Monomer synthesis included preparation of the intermediate,
1,3-bis(4-bromophenyl)-2-propanone (3), via a modified
literature procedure,32 where Claisen self-condensation of
ester 1 in the presence of NaH in anhydrous toluene at 70 °C
produced the corresponding beta-keto ester (2). Non-isolated
intermediate 2 was then hydrolyzed and decarboxylated under
acidic conditions to obtain ketone 3 in 66% overall conversion
from 1 to 3 (Figure 1). Knoevenagel condensation of 3 with
1,9-phenanthraquinone (4) gave key dibromo cyclopentadie-
were obtained by the ultra-slow evaporation of a 1:2 ratio of
dichloromethane:hexane solution. The triphenylene cores are
not planar but twisted, with torsional angles of 30° (M3) and
38° (M4). The bent phenanthrene moiety of the triphenylene
core decreases the intermolecular π−π stacking and enhances
the solubility. The biphenyl moieties containing the TFVE
groups exhibit a dihedral angle between the phenyl rings of
34−37°, typical of biphenyl groups. The distance between the
B
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TFVE end-groups is ∼24.7 Å. Despite the strain in the
triphenyl amine substituent of M4, the nitrogen’s geometry is
found to be nearly planar and compares well with the crystal
structure of triphenylamine.33
The thermal [2 + 2] cyclopolymerizations of TFVE
monomers M1−M4 were observed by DSC (10 °C/min),
whereby M1 and M4 gave sharp melting points (191 and 246
°C, respectively) beyond the known onset of exothermic
TFVE polymerization (∼150 °C) under these conditions
(Figure 3 and see Figure S16 in the SI for more details).13
heating cycle. These Tg values are the highest reported for any
PFCB or triphenylene-based polymer.24 They are among the
highest reproducible glass-transition temperatures ever meas-
ured for any organic thermoplastic without degradation. With
rare exception, organic thermoplastics undergo degradation
and/or other reactions affecting properties like Tg when cycling
to 300 °C. The Tg value for each was reproducible over
subsequent heating cycles, and no further calorimetric events
were detected. Molecular weight by gel permeation chroma-
tography (GPC) and other spectroscopic data further confirms
no significant changes in the polymer structure after heating.
Solution polymerizations were monitored by FTIR and 19F
NMR spectroscopy. A small but distinctive FTIR signal at
1832 cm−1 for M1−M4 corresponds to the FCCF2
stretching (Figure S19). After cyclopolymerization, this
TFVE peak is diminished and a new characteristic band
emerges at 960 cm−1, attributed to the PFCB ring breathing
mode.13,15,34 Complete monomer and polymer H NMR, 13C
1
NMR, and 19F NMR characterizations are reported in the SI.
The representative monomer, M2, and polymer, P2, 19F NMR
spectra are shown in Figure 4. Each fluorine on the TFVE
group exhibits a dd at −119.6 (FA, trans to O), −126.6 (FM, cis
to O), and −133.8 (FX, gem to O) ppm, represented by an
AMX pattern. Upon thermal polymerization, the well-resolved
end-group signals diminish, while the cyclobutyl fluorine
signals emerge as multiplets between −127 and −132 ppm for
the stereo-random 1,2-disubstituted ring. Number-averaged
molecular weight (Mn) data for P1−P4 were calculated by
quantitative integration of FA vs cyclobutyl fluorine signals
(Table 1) and gave Mn values ranging from 25 to 31 kDa.
Conversion by 19F NMR was also consistent with 13C NMR
data, where, for example, the vinyl carbons of M2 exhibit a ddd
pattern at 133.0 and 146.6 ppm. These patterns vanish upon
their thermal polymerization to appear as sp3 cyclobutyl
carbons displaying a complex multiplet pattern from 105 to
116 ppm due to 13C/19F couplings (Figures S12 and S15).
Moreover, the diastereomer ratio, expressed as cis/trans
isomers, for the PFCB rings of P1−P4 was estimated by
integrating the corresponding ipso and ortho carbon signals
under quantitative 13C NMR conditions. As expected, the
results were in agreement with previous measurements and
with ab initio studies that predicted a near equal cis/trans
isomer distribution.35 GPC analysis of solution polymerized
(Ph2O, 180 °C) P1−P4 in THF (relative to polystyrene)
exhibited polydispersity indices that were characteristic of step-
growth polymerization (Table 1).
Figure 3. Monomers M1−M4 melting and polymerization by DSC
(10 °C/min in N2) and reproducible glass transitions (Tg) from the
fourth heating cycle for PFCB polymers P1−P4 (inset).
Monomer M3 does not exhibit a melting transition yet a clear
exothermic event around 150 °C is evident, presumably due to
a disorder−order transition and not an ester related reaction as
the ester signals are intact in the IR spectrum of P3 after
polymerization at 180 °C (Figure S19). Due in part to these
extraordinarily high melting monomers, exothermic polymer-
ization events were not easily detected and did not follow the
broad DSC profile observed for typical TFVE polymerization.
Instead, a dramatic exotherm is detected immediately upon
melting for M3 and M4, resulting in a broad exothermic peak
range (e.g., Texo max = 242−268 °C, Table 1).
Larger-scale polymerization of monomers M1−M4 was
performed in diphenyl ether solutions at 180 °C for 24−48 h
(Figure 1). Polymers P1−P4 produced from neat monomers
by DSC and from solution both gave the same Tg values of
295, 254, 227, and 285 °C, respectively, after the fourth
Like other PFCB polymers, P1−P3 are soluble in common
organic solvents at room temperature, especially in aromatic,
Table 1. Selected Properties of Triphenylene PFCB Homopolymers P1−P4
temp at 5% wt
b
b
loss (°C)
char yield 800 °C (%)
molecular weight (kDa)
c
d
NMR
GPC
a
a
a
a
entry monomer Tm (°C)
onset Texo (°C)
max Texo (°C)
Tg (°C)
air
N2
N2
Mn
Mn
Mw
Mw/Mn
P1
P2
P3
P4
191
none
252
246
166
188
250
247
244
268
265
253
295
254
227
285
459
428
513
349
508
505
523
528
73
68
62
76
29
26
27
31
45
77
93
90
191
227
428
214
1.6
1.9
1.9
2.4
a
b
DSC (10 °C/min) in nitrogen, onset upon melting, and Tg after the fourth heating cycle. TGA at 10 °C/min in nitrogen and air from 0 to 800
c
d
°C. Number average molecular weight of solution-polymerized (180 °C, Ph2O) P1−P4 determined by 19F NMR end-group analysis. Solution
polymerized P1−P4 vs polystyrene standards in THF.
C
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Figure 4. 19F NMR (CDCl3) spectra for M2 and P2 after 180 °C polymerization in Ph2O and transparent P2 film (left) with creased P2 under UV
radiation at 365 nm (inset).
chlorinated, ether, and formamide solvents. P4, however, is
slightly less soluble and requires heating above room
temperature to dissolve yet remains in solution after cooling.
Tough, free-standing films that are creasable without tearing
and optically transparent were obtained for P1−P3 by solution
casting from THF (see the inset in Figure 4 and Figure S17),
while P4 gave a slightly brittle film. Noteworthy is the
remarkable toughness of these high Tg thermoplastics with
large relatively inflexible aromatic cores. Surface hydro-
phobicities were probed by measuring P1−P4 water contact
angles using the sessile drop method and averaging (Figure 4
and Figure S23). Without spin-coating or surface modification,
the free-standing films had hydrophobic surfaces with a highest
contact angle of 92° (P3) and the lowest at 84° (P1). Surface
morphologies of the solvent-cast films were investigated by
atomic force microscopy (AFM). The films displayed a very
low surface roughness over a 5 μm2 area (Figure S24) as is
consistent with the previous PFCB literature.16,36 Thermal
Figure 5. (a) P1−P4 solutions irradiated at 365 nm and (b) overlay
of the UV−vis extinction (left) and emission (right; normalized)
stabilities of P1−P4 were evaluated in nitrogen and air by
TGA (shown in Table 1and Figure S18). PFCB polymers P1−
spectra of P1−P4 at equal concentrations in THF.
P4 are exceptionally robust with TGA 5 wt % loss
temperatures ranging from 349 to 523 °C in air and 508 to
Table 2. UV−Vis Absorption and Fluorescence Emission of
528 °C in N2. Upon heating to 800 °C in N2, the polymers
Polymers P1−P4 in THF and DCM
produce glassy carbon in relatively high yields ranging from 62
a
b
THF
DCM
to 76% (Table 1 and Figure S18), as somewhat expected
considering that triphenylene is a major component of a
mesophase pitch, a known high carbon yielding material.37
The UV−vis absorption and fluorescence spectra for M1−
M4 were obtained in DCM (Figure S21), and all monomers
exhibit absorption peaks at ∼230, ∼ 260, and ∼298 nm.
Although the maximum absorption values (λmax) of M1−M4
were similar, the emission maxima (λem) for each monomer
varied (λem = 414, 403, 417, and 473 nm). The UV−vis spectra
of P1−P4 were also obtained in THF (Figure 5) as well as in
DCM (Table 2, except for insoluble P4). The initial
photophysical properties reported here compare well with
those obtained from previous studies on triphenylene-based
molecules and polymers.19,22 Similarly, a variety of chromo-
phores have been functionalized with the TFVE moiety and
copolymerized to PFCB polymers with predicted optical
λem
λem
c
c
b
polymer λabs (nm)
(nm)
φsolution
λabs (nm)
(nm)
P1ssss
P2
P3
261, 298
260, 298
262, 298
261, 298
417
399
413
467
0.19 0.07
0.26 0.05
0.19 0.02
0.47 0.02
260, 290
260, 290
260, 290
395
418
399
d
d
P4
N/A
N/A
a
b
c
−6
Concentration = ∼7 × 10
M and temperature = 25 °C.
Concentration = ∼6.6 × 10−6 M and temperature = 25 °C.
d
Emission was obtained using the higher λmax (e.g., ∼290 nm). P4 is
not soluble in DCM. N/A = not applicable. Quantum yields were
determined in THF (λmax = 298 nm) relative to quinine sulfate in
0.1% H2SO4.
performance based on a combination of bare chromophore
properties.29,38,39
D
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Fluorescence quantum yields, obtained in THF upon
excitation at 298 nm and using quinine sulfate as a reference,40
were 0.19, 0.26, 0.19, and 0.47 for P1−P4, respectively. As
expected, these values are lower relative to triphenylene-based
polymers with repeating units, ideally represented by complete
conjugation22 but are still much higher than those reported for
lone triphenylene (∼0.04−0.06).19,41 Other unknown factors,
such as the rigidity and planarity of the triphenylene core, or
other cooperative effects induced by the electron withdrawing
fluorocarbon PFCB ring, may play a role in determining the
competitive rates between radiative and non-radiative decay.
Although the absorption spectra for the monomers and
polymers are similar, P4 exhibits the largest Stokes shift and
the highest quantum yield. These differences may be due to
aggregation-induced emission since P4 is the least soluble of
the polymer series.
thermoplastics, exhibiting extremely high and reproducible
glass-transition temperatures (Tg = 295 °C for P1) and
thermal and oxidative stabilities. The polymers also demon-
strated excellent solubility in common organic solvents, from
which tough, creasable, transparent films were obtained. The
new PFCB polymers are bright blue emitters with high-
temperature photo-survivability in air at 250 °C for 24 h.
ASSOCIATED CONTENT
■
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.macromol.1c01043.
Complete experimental methods, characterization of
monomers and polymers (PDF)
Upon further study, polymer films continued to emit blue
light after heating to temperatures up to 250 °C in air for 4, 8,
and 24 h (Figure 6a). As the films are heated in air, blue
AUTHOR INFORMATION
■
Corresponding Author
Dennis W. Smith, Jr. − Department of Chemistry and MSU
Advanced Composites Institute, Mississippi State University,
Mississippi State, Mississippi 39762, United States;
orcid.org/0000-0003-0410-3341; Phone: +1−662−325-
7813; Email: dsmith@chemistry.msstate.edu
Authors
Behzad Farajidizaji − Department of Chemistry and MSU
Advanced Composites Institute, Mississippi State University,
Mississippi State, Mississippi 39762, United States
Ernesto I. Borrego − Department of Chemistry and MSU
Advanced Composites Institute, Mississippi State University,
Mississippi State, Mississippi 39762, United States
Sumudu Athukorale − Department of Chemistry and MSU
Advanced Composites Institute, Mississippi State University,
Mississippi State, Mississippi 39762, United States
Mehdi Jazi − Department of Chemistry and MSU Advanced
Composites Institute, Mississippi State University, Mississippi
State, Mississippi 39762, United States
Bruno Donnadieu − Department of Chemistry and MSU
Advanced Composites Institute, Mississippi State University,
Mississippi State, Mississippi 39762, United States
Charles U. Pittman, Jr. − Department of Chemistry and MSU
Advanced Composites Institute, Mississippi State University,
Mississippi State, Mississippi 39762, United States
Figure 6. (a) Normalized UV−vis extinction (left) and emission
(right; λex = 298 nm) of P1 in THF and P1 in THF after heating at
250 °C for 4−24 h in air and then cooled to rt. Photographs of
thumbnail size films of P3 irradiated (λex = 365 nm) and heated to
(b) 70 °C in air, (c) 250 °C in air for 2 h, and then (d) cooled to ∼25
°C in air compared to (e) a virgin P3 film.
emission is maintained with slight specular attenuation
throughout the heating cycle (Figure 6c). However, upon
cooling and irradiating at 365 nm, the solid-state emission
intensity appears to recover (Figure 6d). More importantly, the
solution-based absorptions and emissions of the films showed
negligible changes after heating in air at 250 °C for 24 h
(Figure 6a). Therefore, thermal oxidative degradation that can
dramatically alter the optical properties and commonly plagues
π-conjugated systems is not observed here. Few organic
polymers exhibit such persistent photostability above 200 °C
for extended periods of time (e.g., 24 h) in air.42
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.macromol.1c01043
Notes
The authors declare no competing financial interest.
X-ray diffraction data can be found at https://www.ccdc.cam.
ac.uk for M3 (CCDC 2089914) and M4 (CCDC 2089915).
ACKNOWLEDGMENTS
■
CONCLUSIONS
The authors would like to acknowledge the Department of
Chemistry, Mississippi State University and Tetramer Tech-
nologies, L.L.C. for the generous donation of TFVE starting
materials.
■
A modular synthetic preparation of triphenylene-containing
TFVE monomers from simple building blocks and their
thermal step-growth cyclopolymerization to PFCB-linked
polymers were described. Knoevenagel condensation afforded
a key dibromoaryl-cyclopentadienone intermediate for Diels−
Alder decarbonylation with monosubstituted or disubstituted
acetylenes. Catalyst-free and initiator-free thermal polymer-
ization yielded four triphenylene-enchained PFCB aryl ether
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