Polymer International
Polym Int 56:1142–1146 (2007)
First identification of biradicals during
thermal [2π + 2π ] cyclopolymerization of
trifluorovinyl aromatic ethers
Nicolas Mifsud,† Veronique Mellon,† Jianyong Jin,‡ Chris M Topping,
Luis Echegoyen and Dennis W Smith, Jr∗
Department of Chemistry and Center for Optical Materials Science and Engineering Technologies (COMSET), Clemson University,
Clemson, SC 29634, USA
Abstract: The thermal cyclopolymerization of three trifluorovinyl aromatic ether monomers 1,1,1-tris
(4-trifluorovinyloxyphenyl)ethane (1), 4,4 -bis(4-trifluorovinyloxy)biphenyl (2) and 2,2-bis(4-trifluorovinyloxyphenyl)-1,1,1,3,3,3-hexafluoropropane (3) were monitored in situ using time-resolved electron paramagnetic
resonance spectroscopy over a temperature range of 150–210 ◦ C. The signals observed during the early stages
of perfluorocyclobutyl polymer production suggest the formation of a triplet state corresponding to the proposed
biradical intermediate with a strong dipole–dipole interaction. A doublet splitting shows the presence of hyperfine
coupling due to fluorine. The characterization of radical species formed during the polymerization of monomer 1
using model compounds and polymerization kinetics of monomer 2 are also presented.
 2007 Society of Chemical Industry
Keywords: cyclopolymerization; perfluorocyclobutyl polymer; trifluorovinyl ethers; EPR
INTRODUCTION
Perfluorocyclobutyl (PFCB)-containing aromatic
ether polymers represent a unique family of fluoropolymers which, due to specific properties such
as melt processability, low dielectric constant, thermal stability, high glass transition temperature (Tg )
and optical transparency, have applications in optical devices.1 – 4 PFCB polymers are synthesized by the
thermal cyclopolymerization of aryl trifluorovinyl ether
(TFVE) monomers (Scheme 1). Typically, the polymerization is accomplished thermally above 150 ◦ C
where viscosity, molecular weight and polydispersity
can be precisely controlled by varying reaction time
and temperature.
The thermal cyclopolymerization of aromatic TFVE
monomers does not require catalysts or initiators.
Essentially quantitative conversion is obtained by simply heating between 150 and 200 ◦ C. The [2 + 2]
cycloaddition between two trifluorovinyl aryl ethers is
symmetry-forbidden, and thus a biradical mechanism
has been proposed and accepted for several decades.5
However, there has been no direct spectroscopic evidence of these biradical intermediates which form
the cyclobutane dimer. Three different monomers,
1,1,1-tris(4-trifluorovinyloxyphenyl)ethane (1), 4,4 bis(trifluorovinyloxy)biphenyl (2) and bis(trifluorovinyloxyphenyl)hexafluoro-isopropylidene (3), were
studied as shown in Scheme 1. Here we describe the
first detection and identification of radical species
during the thermal [2π + 2π ] cyclopolymerization
of trifluorovinyl aromatic ether monomers using
time-resolved electron paramagnetic resonance (EPR)
spectroscopy.6 Information about the nature and
quantity of the radical intermediates involved in the
polymerization environment, in addition to characterization using model compounds and preliminary
polymerization kinetics, is also presented.
EXPERIMENTAL
Monomers 1, 2 and 3 were prepared as described
previously.3,4 They are commercially available from
Tetramer Technologies LLC and distributed by Oakwood Chemicals Inc., Columbia, SC (http://www.
oakwoodchemical.com). Triphenylmethane (Ph3 CH)
and 1,1,1-triphenylethane (Ph3 CCH3 ) were purchased from Aldrich and used as received.
EPR samples were placed into Wilmad quartz
tubes, stored in a dry box for more than 30 min,
purged with nitrogen and then closed with a plastic
cap but not sealed before each experiment. All EPR
measurements were recorded using a Bruker EMX XBand spectrometer. First-derivative EPR spectra were
recorded with a modulation amplitude of 15 gauss,
a frequency of 100 kHz and a microwave frequency
around 9.5 GHz.
∗
Correspondence to: Dennis W Smith, Jr, Department of Chemistry and Center for Optical Materials Science and Engineering Technologies (COMSET),
Clemson University, Clemson, SC 29634, USA
E-mail: [email protected]
†
Research internship from ESCPE Lyon, Villeurbanne 69100, France.
‡
Current address: Tetramer Technologies LLC, Pendelton, South Carolina, USA.
(Received 20 September 2006; accepted 8 December 2006)
Published online 21 February 2007; DOI: 10.1002/pi.2250
 2007 Society of Chemical Industry. Polym Int 0959–8103/2007/$30.00
Identification of biradicals during cyclopolymerization of trifluorovinyl aromatic ethers
F
F
F
F
F
O Ar
O
F
F
F
F
ArO
∆
F
F
F
O
F
F
F
OAr
F
F
F
O Ar
n
Ar =
F3C
CH3
2
O
F
F
F
CF3
3
1
Scheme 1. Cyclopolymerization of aryl trifluorovinyl aromatic ether monomers.
Polymerization was carried out in situ by heating
monomer samples to 150, 180, 195 and 210 ◦ C
with a microwave frequency of 9.5 GHz and a field
modulation frequency of 100 kHz. The temperature
was controlled using WINEPR acquisit software
and maintained with a precision of 0.1 K. Model
compounds were used with a 50/50 weight ratio.
RESULTS AND DISCUSSION
The time-resolved EPR spectra of monomer 2 during
thermal polymerization at 195 ◦ C are shown in
Fig. 1 as a function of time. Initially, between 1
and 7 min, the signal observed shows characteristic
features that can be assigned to those arising from
a randomly oriented axially symmetric triplet state.7
Clear polarization is observed for the first and last
transitions representing the characteristic absorption
and emission of triplet states corresponding to the
resonant field value when the magnetic field is oriented
along the Z axis. The transitions for the perpendicular
orientations are not very clear, but discernable (see
arrows in Fig. 1(a)). Very similar observations have
been reported for the case of photoexcited zinc
phthalocyanine, where the transitions for the X and Y
orientations were also barely observed.7 After 7 min,
the signal appears as a doublet due to the presence
of fluorine. At 150 ◦ C, polymerization is much slower
but the triplet state (EPR data not shown) remains
for 60 min prior to the appearance of the doublet.
These spectra suggest the presence of a diradical
intermediate, in which two unpaired electrons have a
strong dipole–dipole interaction. Thus, experimental
measurement of the zero-field splitting between the
absorption and emission peak maxima (2D) provides
an approximate separation between the two electrons
of R = 8.71 Å.
The EPR parameter values of monomer 2 polymerization were determined using Bruker Simfonia
and Bruker Winepr software. Analysis of the spectrum
shows different couplings; however, due to the broadness of the spectra obtained, precise measurement of
Polym Int 56:1142–1146 (2007)
DOI: 10.1002/pi
Table 1. Parameter values of monomer 2 polymerization EPR
spectrum
2D (gauss)
a1 (gauss)
a2 (gauss)
g factor
≈84
≈34
≈59
2.0012
the coupling constants was not possible (Table 1). The
couplings may be assigned to α-fluorine = 59 gauss
and β-fluorine = 34 gauss according to Iwasaki.8
Polymerization of monomer 1 seems to follow a
mechanism that includes slightly different radical
species. Upon heating, the EPR signal from 1 is similar
to that from 2. Yet after 15 min, a new signal appears
and grows with time. The EPR spectrum of monomer
1 heated after 55 min at 210 ◦ C is shown in Fig. 2.
A new radical is cleanly observed and may be due to
hydrogen atom abstraction from the methyl group on
monomer 1, which is not present in monomer 2.
To test this hypothesis, model compounds Ph3 CH
and Ph3 CCH3 were added to the polymerization of
monomer 2. Polymerization of 2 in the presence of
Ph3 CH was carried out by heating at 210 ◦ C and
followed by EPR analysis (Fig. 3). Although not an
exact model, it appears that the new radical from 1
is due to hydrogen atom abstraction from the methyl
group. The linewidth of 1 (16 gauss) is larger than of
the Ph3 CH (11 gauss), due to the α-coupling of the
two aliphatic protons in monomer 1. Only one peak
is observed because of the high modulation amplitude
(Fig. 3(b)).
Polymerization of 2 in the presence of closer model
compound Ph3 CCH3 at 210 ◦ C was followed using
EPR (Fig. 4). Initially the signal observed is very broad
but two different lines can be distinguished. One signal
appears to originate from monomer 2 (linewidth of 85
gauss) and the other has a linewidth of 16 gauss. With
time, the largest signal tends to disappear whereas the
other one grows as in the case of 1. It appears that
the neat polymerization of 1 and the polymerization
of 2 in the presence of Ph3 CCH3 both involve the
1143
N Mifsud et al.
1 210 °C 55 min
2000
100
2 195 °C 3 min
Intensity (a.u.)
50
Intensity (a.u.)
1000
0
0
-1000
-50
-2000
3350
-100
3300
3350
(a)
3400
3450
3500
Magnetic Field (G)
3400
3450
3500
Magnetic Field (G)
3550
Figure 2. EPR spectrum of monomer 1 heated at 210 ◦ C after 55 min.
150
2 195 °C 10 min
600
100
2/Ph3CH (1/1) 210 °C 25 min
Intensity (a.u.)
Intensity (a.u.)
300
50
0
-50
-300
-100
-600
-150
3300
0
3350
3350
3400
3450
3500
3400
3450
3500
3550
Magnetic Field (G)
(a)
Magnetic Field (G)
(b)
600
60
2 195 °C 120 min
20
0
0
-300
-20
-600
-40
3350
-60
3300
(c)
3350
3400
3450
3500
Magnetic Field (G)
Figure 1. EPR spectra of monomer 2 heated at 195 ◦ C after (a) 3 min,
(b) 10 min and (c) 120 min.
same intermediate radical species. For comparison,
neat Ph3 CCH3 was heated at 210 ◦ C and analyzed
using EPR (Fig. 5). Only one line was observed with a
linewidth of 15 gauss. This species does not appear to
be stable and the signal disappears after 10 min. The
EPR line in Fig. 5 was most likely generated by autooxidation. The model study of Ph3 CCH3 mixed with
1144
2/Ph3CH (1/1) 210 °C 60 min
300
Intensity (a.u.)
Intensity (a.u.)
40
(b)
3400
3450
3500
3550
Magnetic Field (G)
Figure 3. EPR spectra of monomer 2 + Ph3 CH heated at 210 ◦ C after
(a) 25 min and (b) 60 min.
2 indicates a new radical originating from the aliphatic
portion of monomer 1. In the case for monomer 3, the
same signal shape as that observed for monomers 2 and
1 was observed during early stages of polymerization;
however, this signal remains noisy likely due to the low
concentration of radicals.
PFCB polymerization kinetics have been previously
reported using other methods such as Raman
spectroscopy.9 Preliminary EPR kinetic plots for the
Polym Int 56:1142–1146 (2007)
DOI: 10.1002/pi
Identification of biradicals during cyclopolymerization of trifluorovinyl aromatic ethers
100
2/(Ph)3CCH3 (1/1) 210 °C 26 min
polymerization of 2
210 °C
195 °C
Intensity (a.u.)
Intensity (a.u.)
50
0
-50
-100
3350
3400
(a)
3450
3500
3550
Magnetic Field (G)
400
0
Intensity (a.u.)
40
60
80
100
120
Time (min)
2/(Ph)3CCH3 (1/1) 210 °C 97 min
Figure 6. Kinetics of monomer 2 polymerization at 195 and 210 ◦ C.
however, at 195 ◦ C, an EPR signal remains visible after
2 h (Fig. 1(c)). Molecular weights of these resulting
PFCB polymers were determined using 19 F NMR
spectroscopy to be 3462, 5017 and 6924 after heating
for 60 minutes at 180, 195 and 210 ◦ C, respectively.
200
0
-200
-400
3400
20
3420
3440
3460
3480
Magnetic Field (G)
(b)
Figure 4. EPR spectra of monomer 2 + Ph3 CCH3 heated at 210 ◦ C
after (a) 26 min and (b) 97 min.
200
Ph3CCH3 210 °C 5 min
CONCLUSIONS
EPR techniques have been successfully used for the
first time to study the TFVE cyclodimerization to
PFCB polymers. Initial signals correspond to an axially
symmetric triplet state. Thus, the signal describes
the presence of a diradical showing a reasonably
strong dipole–dipole interaction between the two
electrons. Hydrogen atom abstraction, where possible,
was confirmed using model compounds, and it appears
possible to collect kinetic information using the EPR
technique.
Intensity (a.u.)
100
0
-100
-200
3400
3440
3480
Magnetic Field (G)
ACKNOWLEDGEMENTS
We are grateful to the Defense Advanced Research
Projects Agency (DARPA), the National Textiles
Center, NASA Space Grant, SC EPSCoR, and the
NSF Chemistry division for grant (CHE 0135786)
for financial support. We also thank B Elliott,
N Abayasinghe, S Chen and C Ligon (Clemson
University) for helpful discussions and running GPC
and NMR. DS is a Cottrell Scholar of Research
Corporation.
Figure 5. EPR spectrum of Ph3 CCH3 heated at 210 ◦ C after 5 min.
polymerization of monomer 2 at 195 and 210 ◦ C are
shown in Fig. 6. The concentration of radical species
increases dramatically in the first 10 min of heating.
After this initial period, the radical concentration and
thus rate of polymerization begin to decrease. After
60 min at 210 ◦ C, the EPR signal is very broad and
tends to disappear (indicating near-zero reaction rate);
Polym Int 56:1142–1146 (2007)
DOI: 10.1002/pi
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Polym Int 56:1142–1146 (2007)
DOI: 10.1002/pi