Synthesis and characterization of trifluoromethyl substituted styrene polymers and copolymers with methacrylates: Effects of trifluoromethyl substituent on styrene

Article abstract of DOI:10.1016/j.polymer.2010.12.045

2-Trifluoromethyl styrene (2TFMS), 2,5-bis(trifluoromethyl) styrene (25BTFMS), and 3,5-bis(trifluoromethyl) styrene (35BTFMS) were synthesized. These styrenes were readily polymerized in bulk and also in solution using AIBN as a free radical initiator. The polymerization rate of these trifluoromethyl substituted styrenes and other monomers such as styrene (St), pentafluorostyrene (PFS) and 4-trifluoromethyl-tetrafluorostyrene (TFMTFS) were measured in benzene and dioxane by monitoring the ¹H NMR spectra of the double bond hydrogen. The order of polymerization rates was TFMTFS > 35BTFMS > 25BTFMS > PFS > 2TFMS > St. T_gs of styrene polymers with CF₃ substituted on the ortho position of the phenyl ring were much higher than those of the meta and para substituted styrenes due to the steric hindrance of the bulky CF₃ group close to the polymer main chain, which resulted in a decrease in the segment mobility of the polymer chains and an increasing T_g of the polymers. The copolymers of 2TFMS with methyl methacrylate (MMA) and also 25BTFMS with trifluoroethyl methacrylate (TFEMA) were prepared. T_gs of the copolymers were in the range of 120-145 °C and the copolymers were transparent and thermally stable. The copolymer films were flexible and exhibited high transmittance as the homopolymers of MMA and TFEMA. Thus, these copolymers may be utilized as novel optical materials.

Full text of DOI:10.1016/j.polymer.2010.12.045

Polymer 52 (2011) 949e953
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and characterization of trifluoromethyl substituted styrene polymers
and copolymers with methacrylates: Effects of trifluoromethyl substituent on
styrene
,
,
,
,
,c,
Hongxiang Teng a c, Liping Lou a c, Kotaro Koike a b, Yasuhiro Koike b c, Yoshi Okamoto a
*
a Polymer Research Institute, Polytechnic Institute of New York University, 6 Metrotech Center, Brooklyn, NY 11201, United States
b Faculty of Science and Technology, Keio University, Yokohama 223-8522, Japan
c Koike Photonics Polymer Project, ERATO-SORST, Japan Science and Technology Agency, Kawasaki 212-0032, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
2-Trifluoromethyl styrene (2TFMS), 2,5-bis(trifluoromethyl) styrene (25BTFMS), and 3,5-bis(tri-
fluoromethyl) styrene (35BTFMS) were synthesized. These styrenes were readily polymerized in bulk
and also in solution using AIBN as a free radical initiator. The polymerization rate of these trifluoromethyl
substituted styrenes and other monomers such as styrene (St), pentafluorostyrene (PFS) and 4-tri-
fluoromethyl-tetrafluorostyrene (TFMTFS) were measured in benzene and dioxane by monitoring the 1H
Received 12 October 2010
Received in revised form
10 December 2010
Accepted 20 December 2010
Available online 30 December 2010
NMR spectra of the double bond hydrogen. The order of polymerization rates was TFMTFS
>
35BTFMS > 25BTFMS > PFS > 2TFMS > St. Tgs of styrene polymers with CF3 substituted on the ortho
position of the phenyl ring were much higher than those of the meta and para substituted styrenes due
to the steric hindrance of the bulky CF3 group close to the polymer main chain, which resulted in
a decrease in the segment mobility of the polymer chains and an increasing Tg of the polymers. The
copolymers of 2TFMS with methyl methacrylate (MMA) and also 25BTFMS with trifluoroethyl methac-
rylate (TFEMA) were prepared. Tgs of the copolymers were in the range of 120e145 ꢀC and the copol-
ymers were transparent and thermally stable. The copolymer films were flexible and exhibited high
transmittance as the homopolymers of MMA and TFEMA. Thus, these copolymers may be utilized as
novel optical materials.
Keywords:
Trifluoromethyl styrenes
Homopolymer
Copolymer
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
such as in automobile and airplane require optical polymers with
higher Tg (>120 ꢀC) [5e8].
Polystyrene (PS) is a commercially important polymer with
good light transmittance. The favorable physical properties of PS
coupled with ease of processing make it useful as optical device
materials [1]. However, high losses in the visible and near infrared
region are dominated by the combinations of stretch and defor-
mation vibrations of CeH bonds [2,3]. When heavier atoms such as
deuterium or fluorine replace the hydrogen atoms in CeH bond, the
band vibration energy is decreased and thus the absorption band
is minimized. Thus, we have previously synthesized fluorine
containing polystyrene such as poly(2,3,4,5,6-pentafluorostyrene)
and poly(4-trifluoromethyl 2,3,5,6-tetrafluorostyrene) [4]. They
are thermally stable and transparent, but their glass transition
temperatures (Tg) were about 110 ꢀC. However, some applications
Bomer and Hagemann have synthesized various CF3 substituted
styrenes and their homopolymers. They reported that when CF3
group substituted on the ortho position of polystyrene, the Tg was
considerably higher (175 ꢀC) than those of CF3 on the meta and para
positions whose Tgs were 63 and 101 ꢀC, respectively [9]. This is due
to the steric hindrance of the bulky CF3 group (the Van der Waals
volume of CF3 is 42.6 Å3 compared to CH3 with 16.8 Å3) [10] at the
ortho position, which results in a decrease in the segmental
mobility of the polymer chains and greatly increases the Tg of the
polystyrene. Thus, we have further investigated the CF3 substituted
polystyrenes for possible optical polymer materials with higher Tg.
Various fluorine containing styrene monomers (Fig. 1) were
synthesized and the polymerization rates of these styrene mono-
mers in benzene and dioxane solutions were measured. Also
reported here are the thermal and optical properties of the tri-
fluoromethyl substituted styrenes homopolymers as well as their
copolymers with methyl methacrylate and 2,2,2-trifluoroethyl
methacrylate.
* Corresponding author. Polymer Research Institute, Polytechnic Institute of New
York University, 6 Metrotech Center, Brooklyn, NY 11201, United States.
E-mail address: yokamoto@poly.edu (Y. Okamoto).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.12.045

950
H. Teng et al. / Polymer 52 (2011) 949e953
anhydrous THF. After slowly adding acetaldehyde, the reaction
CH2 CH
F
CH2
F
CH
CH2
CH
mixture was treated with diluted hydrochloric acid solution. The
resulted alcohol was subjected to distillation under vacuum.
The styrene monomers were obtained as a colorless liquid by the
elimination of water from alcohols with phosphorous pentoxide,
followed by the distillation under vacuum.
F
F
F
F
F
F
CF3
F
Pentafluorostyrene
(PFS)
4-trifluoromethyl-
tetrafluorostyrene
(TFMTFS)
Styrene
(St)
2-trifluoromethyl styrene (2TFMS): b.p. 65e66 ꢀC/30 mmHg
with a yield of 57%. 1H NMR (CDCl3): 5.30 (dd, J1 ¼ 1.1 Hz,
J2 ¼ 11.0 Hz,1H), 5.66 (d, J ¼ 17.2 Hz,1H), 7.00 (m,1H), 7.30e7.70 (m,
4H). 19F NMR (CDCl3): ꢂ59.5 (s, 3F).
CH2
CH
CH2
CH
CH2 CH
2,5-bis(trifluoromethyl) styrene (25BTFMS): b.p. 70e71 ꢀC/
30 mmHg with a yield of 60%. 1H NMR (CDCl3): 5.43 (dd, J1 ¼ 0.6 Hz,
J2 ¼ 11.1 Hz,1H), 5.80 (d, J ¼ 17.2 Hz,1H), 7.00 (m,1H), 7.60e7.90 (m,
3H). 19F NMR (CDCl3): ꢂ60.2 (s, 3F), ꢂ63.3 (s, 3F).
CF3
CF3
CF3
CF3
CF3
3,5-bis(trifluoromethyl) styrene (35BTFMS): b.p. 72e73 ꢀC/
30 mmHg with a yield of 55%. 1H NMR (CDCl3): 5.49 (d, J ¼ 11.0 Hz,
1H), 5.90 (d, J ¼ 17.6 Hz, 1H), 6.77 (dd, J1 ¼11.0 Hz, J2 ¼ 17.6 Hz, 1H),
7.70e7.90 (m, 3H). 19F NMR (CDCl3): ꢂ63.2 (s, 6F).
3,5-bis(trifluoromethyl)
styrene
(35BTFMS)
2-trifluoromethyl
styrene
(2TFMS)
2,5-bis(trifluoromethyl)
styrene
(25BTFMS)
Fig. 1. Chemical structures of styrene monomers investigated.
4-trifluoromethyl-2,3,5,6-tetrafluorostyrene (TFMTFS): b.p.
36e37 ꢀC/20 mmHg with a yield of 58%. 1H NMR (CDCl3): 5.87 (d,
J ¼ 11.8 Hz, 1 H), 6.24 (d, J ¼ 17.9 Hz, 1H), 6.71 (dd, J ¼ 11.8 Hz,
J ¼ 17.9 Hz, 1H). 19F NMR (CDCl3): ꢂ56.37 (m, 3 F), ꢂ141.86 (m, 4F).
2. Experimental
2.1. Materials
2.4. Polymerization
All solvents, methyl methacrylate (MMA), styrene (St) and 2,20-
azobisisobutyronitrile(AIBN)werepurchased from Aldrich. 2,3,4,5,6-
Pentafluorostyrene (PFS), 2-trifluoromethyl-bromobenzene, 2,5-bis
(trifluoromethyl)-bromobenzene, 3,5-bis(trifluoromethyl)-bromo-
benzene and 4-trifluoromethyl-2,3,5,6-tetrafluoro-bromobenzene
were purchased from SynQuest Co. AIBN was purified through the
recrystallization from methanol. All monomers were distilled before
use.
Polymerizations were carried out in bulk for the general char-
acterization purpose, or in benzene and dioxane solutions for
kinetics study. AIBN was used as the free radical initiator in all the
polymerizations. Monomers and initiator were transferred into
a glass tube, which was subject to three times of the freeze-
epumpethaw cycles and followed by sealing with flame under
vacuum. After polymerizing in a water bath at 60e80 ꢀC for
20e30 h, the tube was opened carefully, and the content was
purified by the precipitation from THF solution to a large amount of
methanol or hexane. The polymer collected was dried in vacuum
oven at 60 ꢀC for 48 h.
2.2. Characterization
1H NMR and 19F NMR spectra were recorded by a Bruker AC 300
spectrometer at 300 and 282 MHz, respectively. Chemical shifts are
reported in ppm from internal TMS or CFCl3. The differential
scanning calorimetry (DSC) measurement was performed on a DSC
2920 module in conjunction with the TA Instruments 5100 system
at a scan rate of 10 ꢀC/min under a nitrogen atmosphere. The
instrument was calibrated using indium and zinc as calibration
standards for the temperature and enthalpy changes. The midpoint
of the heat capacity transition was taken as Tg. Thermogravimetric
analysis (TGA) was performed on Hi-Res Modulated TGA 2950
thermogravimetric analyzer under nitrogen at a heating rate of
20 ꢀC/min. The extrapolation of break in the TGA curve was taken as
the decomposition temperature (Td). The molecular weights of
polymer samples were determined by gel permeation chromatog-
raphy (GPC) (Waters 510) using THF as the eluent at a flow rate of
1.0 mL/min. The molecular calibration curves were obtained using
polystyrene standards. The refractive indices were measured using
a Metricon model 2010 prism coupler, and the measurement
accuracy is ꢁ0.0005. The transmittance of polymer films was
measured with a UV/VIS spectrometer (Lambda 800, Perkin Elmer,
Inc.) at a scanning rate of 100 nm/min. The film thickness was in the
2.5. Kinetics
The polymerization rate can be expressed as following equa-
tions [11]:
ꢀ
ꢁ
1=2
fkd
kt
Rp ¼ kp
½Mꢃ0
½Iꢃ1=2½Mꢃ
(1)
ln
¼ k½Iꢃ1=2t
(2)
½Mꢃ
where Rp is the polymerization rate, kp is the propagation rate
coefficient, f is the initiator decomposition efficiency, kd is the
range of 110e200 mm.
2.3. Synthesis of styrene monomers
Except for those purchased, styrene monomers were prepared
via the dehydration of the related alcohols, which were synthesized
by the reaction of corresponding Grignard reagents with acetal-
dehyde (Scheme 1). In a typical procedure, the Grignard agent was
prepared from the related bromide compound and magnesium in
Scheme 1. Synthesis routes for the preparation of substituted styrene monomers.

H. Teng et al. / Polymer 52 (2011) 949e953
951
Table 1
initiator decomposition rate coefficient, kt is the termination rate
Polymerization rate constants of styrene monomers at 50 ꢀC.
coefficient, [I] is the initiator concentration, [M] is the monomer
concentration, k is the observed polymerization rate coefficient,
and t is the polymerization time. The change of the monomer
concentration could be monitored by 1H NMR measurements of the
double bond protons (Fig. 2).
k (benzene)
k (dioxane)
Tg(ꢀC)
(ꢄ10ꢂ5 L1/2/mol1/2 S)
(ꢄ10ꢂ5 L1/2/mol1/2 S)
St
PFS
TFMTFS
2TFMS
25BTFMS
35BTFMS
2.4
4.8
8.7
3.1
7.7
6.1
4.0
9.8
16
6.1
10
105
108
112
165
160
116
2.6. Reactivity ratio
15
The reactivity ratios of each pair of comonomers in a copolymeri-
zation were calculated according to the MayoeLewis method [12].
It can be seen that the more the electron-drawing substitution,
the more active the monomer is. The polymerization rates of
fluorinated styrene are faster than that of regular styrene because
of this polarity effect [20]. The CF3 substitution at the para position
of TFMTFS has much stronger polarity effect when comparing with
the F substitution at the same position in PFS. When X is CF3 group,
3. Results and discussion
The CF3 substituted styrenes were readily polymerized in bulk
and also in solutions using AIBN as a free radical initiator. To study
the effect of the F and CF3 substitution on the activity of related
styrene monomers, the polymerization kinetics was carried out in
nonpolar solvent benzene and polar solvent dioxane. To obtain
accurate results, the solvents used in kinetics study were purified
carefully with the traditional methods. The polymerization rate was
monitored by measuring the double bond protons in monomers
(Fig. 2). The observed polymerization rate coefficient k was calcu-
lated according to Eqn. (2) (Table 1). It was found that the poly-
merization is faster in a polar solvent such as dioxane than in a less
polar solvent such as benzene, which is agreeing to the literature
results [13].
the Hammett constant sp is 0.54, where
r was calculated as 0.6.
While X is F atom only, the Hammett constant sp is 0.062 [21]. It can
be seen that there is a big difference between CF3 and F substitu-
tions at the para position of the benzene ring, which could explain
the big difference between the polymerization activities of PFS and
TFMTFS. The polymerization rate of 25BTFMS is much slower than
that of 35BTFMS due to the steric effect of the CF3 group at the ortho
position of 25BTFMS.
Tgs of 2TFMS, 25BTFMS and 35BTFMS homopolymers were 165,
160 and 116 ꢀC [22], respectively, and Tgs of PFS and TFMTFS
homopolymers were 108 and 112 ꢀC, respectively (Table 1). As can
be seen, Tgs of polymers with CF3 substituted at the ortho position
of the phenyl ring are much higher in comparison with P35BTFMS.
This is due to the steric hindrance of the bulky CF3 group near the
polymer main chain, which resulted in a decreased segmental
mobility of the polymer chains and the increased Tg of the
polymers.
Comparing with homopolymers, copolymer systems produce
more variable properties. Generally, the copolymerization can
decrease the crystallinity and increase the flexibility of the polymer
materials. Styrene polymers have higher thermal and chemical
stabilities. While they exhibit poor mechanical properties and are
difficult to prepare thin fibers due to the rigid polymer structure.
Comparatively, acrylate polymers showed excellent processing
properties, while they can be only utilized at a low temperature
environment due to the low Tg. The combination of the positive
properties from styrene polymers and acrylate polymers would be
an ideal result. However, as optical materials, copolymers tend to
have extremely high light scattering loss due to the large hetero-
geneous macromolecular structure and the corresponding
The free radical initiated polymerization of styrene has been
widely investigated [14e17]. The head to tail placement would be
expected to be overwhelmingly predominant, since successive
propagating radicals formed by the attachment of a radical at the
b
carbon of the double bond is more stable due to the resonance
effects of the phenyl group and less steric hindrance. The situation
with substitution on the benzene ring of substituted styrene
monomers should be the same.
As shown in Fig. 2, when X has a more electron-withdrawing
property, such as Cl and CN, the polymerization rate of the mono-
mer was found to be faster than those of X having an electron
donating property, such as CH3 and OCH3 [18]. The polymerization
rates of para substituted styrenes were following a Hammett
relationship [19].
ser
The protons in nonfluorinated styrene appeared in the upper
field in NMR spectrum as comparing to those in fluorosubstituted
styrenes due to the electron-withdrawing property of F and CF3
(Fig. 3). The protons in TFMTFS appeared at a lower field than those
of PFS because the CF3 group is a more electron-withdrawing group
than the F atom. To reveal the structure and activity relationship,
the chemical shifts of the proton H2 were plotted against the
polymerization rate coefficients of these styrene monomers, and
a linear relationship was found as shown in Fig. 4. Besides the
solvent effect as mentioned above, the polymerization rates of
these styrene monomers were in the decreasing order of
TFMTFS > 35BTFMS > 25BTFMS > PFS > 2TFMS > St.
25BTFMS
35BTFMS
PFS
2TFMS
St
TFMTFS
H2
H1
C
H3
X = F and/or CF3 substituent
5.8
5.6
5.4
5.2
X
Chemical shift (ppm)
Fig. 2. Proton identification of the substituted styrene monomers.
Fig. 3. 1H NMR spectra for the H2 protons of the double bonds in styrene monomers.

952
H. Teng et al. / Polymer 52 (2011) 949e953
Table 3
16
14
12
10
8
Thermal analysis and molecular weight results.
dioxane
benzene
Tg (ꢀC)
Td (ꢀC)
Mw
Mn
35BTFMS
PFS
Sample
Composition (wt%)
1
2
3
4
5
6
7
8
9
10
MMA
108
127
136
146
165
75
338
312
341
338
382
280
311
322
332
366
101.000
e
39.500
e
TFMTFS
MMA/2TFMS ¼ 74/26
MMA/2TFMS ¼ 50/50
MMA/2TFMS ¼ 26/74
2TFMS
337.700
e
99.200
e
25BTFMS
193.100
241.600
e
59.700
86.200
e
TFEMA
6
35BTFMS
PFS
TFEMA/25BTFMS ¼ 76/24
TFEMA/25BTFMS ¼ 51/49
TFEMA/25BTFMS ¼ 25/75
25BTFMS
86
2TFMS
103
129
160
344.400
e
89.300
e
4
St
266.300
88.800
2
0
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
Chemical shift of proton H (ppm)
160
140
120
100
80
Fig. 4. Relationship of the polymerization rate constants and the chemical shifts of H2
MMA/TFMS copolymer
protons.
heterogeneity of the refractive index distribution. Consequently,
copolymers have not been the leading candidate for photonic
materials.
The copolymerization reactivity ratios r1 and r2 were found to be
0.84 and 1.55 for MMA and 2TFMS, respectively (Table 2), which
indicate that these two monomers produce copolymers with the
random composition. The refractive indices of poly(2TFMS) and
PMMA are 1.51 and 1.50 at 532 nm, respectively. The very close
refractive indices of the different polymeric units could minimize
the light scattering. Combining the random copolymer chain
structure characteristics, the transparent copolymer could be
produced from the two monomers. The same situation is in the case
of the trifluoroethyl methacrylate (TFEMA) and 25BTFMS system.
The copolymerization reactivity ratios r1 and r2 are 0.64 and 1.18 for
TFEMA and 25BTFMS, respectively. The refractive indices of poly
(25BTFMS and PTFEMA) are 1.45 and 1.44 at 532 nm, which are also
very close to each other. Therefore, these copolymers may exhibit
no light scattering [23]. Thus, the copolymers of MMA with 2TFMS
and also the copolymers of TFEMA with 25BTFMS were prepared
for possible novel optical applications (Table 3).
These copolymers exhibit a single glass transition temperature,
which indicates that they have a random homogeneous macro-
molecular structure (Table 3). Tgs of the copolymers are dependent
on their composition and follow a linear relationship as shown in
Fig. 5. This makes it possible to design the copolymers with
different refractive indices for different applications. In comparison
to the low Tg of methacrylate polymers and the poor mechanical
properties of the styrene polymers, these copolymers offer the
higher Tg (>130 ꢀC) required for special applications of optical
polymers in high temperature environment [5e8], while main-
taining the excellent processability.
TFEMA/25BTFMS copolymer
60
0
20
40
60
80
100
TFM substituted styrene unit in copolymer (wt%)
Fig. 5. Composition dependence of Tgs for MMA/2TFMS and TFEMA/25BTFMS
copolymers.
the gradient index plastic optical fiber (GI-POF), since it is necessary
to add dopants and chain transfer agents in the system during the
preparation, while keeping the good mechanical properties of the
polymer material [24,25].
The refractive indices of the copolymers from methacrylates and
CF3 substituted styrenes were studied at different wavelengths
(532 nm, 633 nm, 839 nm), and the composition dependence of the
refractive index was plotted in Fig. 6. The results show that there is
a linear relationshipbetweenthe refractiveindices of thecopolymers
and their compositions. It suggests that there is no macrophysical
property change such as density, which may occur during the
1.54
1.52
1.50
The copolymers were found to degrade at above 310 ꢀC in TGA
measurements, which means these copolymers are thermally
stable in comparison with MMA and TFEMA homopolymers and are
capable to the general fiber preparation procedure at 200e300 ꢀC.
The data in Table 3 also showed that copolymers have higher
molecular weight in comparison to the homopolymers. This high
molecular weight is precious for the special preparation method for
MMA/2TFMS copolymers
1.48
532nm
633nm
839nm
1.46
1.44
1.42
TFEMA/25BTFMS copolymers
1.40
1.38
Table 2
The monomer reactivity ratios of comonomer pair.
0
20
40
60
80
100
M1/M2
r1
r2
r1r2
Amount of substituted styrene in copolymers (wt%)
MMA/2TFMS
TFEMA/25BTFMS
0.84 ꢁ 0.04
0.64 ꢁ 0.03
1.55 ꢁ 0.08
1.18 ꢁ 0.06
1.30
0.76
Fig. 6. Composition dependence of the refractive index for copolymer systems.

H. Teng et al. / Polymer 52 (2011) 949e953
953
4. Conclusion
Homopolymers of 2TFMS and 25BTFMS, as well as their copol-
ymers with MMA and TFEMA, exhibited relatively high Tgs, high
thermal stability and high transmittance. These polymers may be
utilized as novel optical materials.
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