Fluorinated monomers and polymers with specific properties for integrated optics and photonics

Full text of DOI:10.1134/S0012500812090066

ISSN 0012ꢀ5008, Doklady Chemistry, 2012, Vol. 446, Part 1, pp. 183–187. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © S.M. Igumnov, V.I. Sokolov, V.K. Men’shikov, O.A. Mel’nik, V.E. Boiko, V.I. Dyachenko, L.N. Nikitin, E.V. Khaidukov, G.Yu. Yurkov, V.M. Buznik,
2012, published in Doklady Akademii Nauk, 2012, Vol. 446, No. 3, pp. 288–293.
CHEMISTRY
Fluorinated Monomers and Polymers
with Specific Properties for Integrated Optics and Photonics
S. M. Igumnov, V. I. Sokolov, V. K. Men’shikov, O. A. Mel’nik,
V. E. Boiko, V. I. Dyachenko, L. N. Nikitin, E. V. Khaidukov,
G. Yu. Yurkov, and Academician V. M. Buznik
Received March 23, 2012
DOI: 10.1134/S0012500812090066
Nowadays, fluorinated polymeric materials are range from a few to several tens of microns. Third,
increasingly used in integrated optics owing to their monomers with high and low refractive indices
high functionality and manufacturability [1–4]. As intended to be the components of the compositions for
distinct from hydrocarbon polymers, fluorinated polyꢀ the lightꢀguiding core and the waveguide cladding
mers have lower absorption in all three telecom waveꢀ should readily copolymerize with each other.
length ranges near 0.85, 1.3, and 1.55 µm.
This work is aimed at synthesizing new fluorinated
monomers exhibiting the above set of useful properties
and at studying the possibility of using them for fabriꢀ
cation of polymer waveguides.
This is caused by the fact that the stretching vibraꢀ
tion overtones of the C–F bonds are shifted toward
longer wavelengths as compared with the C–H overꢀ
tones responsible for the absorption in the given specꢀ
tral ranges [5]. Yet another specific feature of fluoriꢀ
nated monomers is the low refractive index (nD). The
copolymerization of fluorinated and nonꢀfluorinated
monomers makes it possible to vary the refractive
index of the composition in wide ranges, which is
important for fabrication of waveguides with a speciꢀ
fied numerical aperture. Finally, fluorinated polymers
are more thermostable and more resistant to degradaꢀ
tion, color change, etc. This is due to the fact that the
C–F bond energy is much higher than the C–H bond
energy.
As is known, perfluoroolefins 1ꢀ1, unlike the nonꢀ
fluorinated analogues, react with KF to form metastaꢀ
ble reactive carbanions 1ꢀ2. This is attested by a mulꢀ
titude of chemical transformations on their basis; in
addition, they have been detected by 19F NMR [6].
The primary addition of the fluoride ion to the double
bond of a perfluoroalkene followed by stabilization of
the resulting carbanion with the potassium cations
occurs smoothly in polar solvents with high solvation
ability (Scheme 1).
R
F
Rf1
F
RRf1
K+
f1
f1
+ F–
K+
C–
F
R
Rf
R
f
f22
F
R
Monomers for producing integrated optical devices
(for example, optical waveguides) should have some
specific properties. First, they should have high optical
transparency in the working range of the spectrum.
Second, monomers should be rather active in the proꢀ
cess of radical photopolymerization since waveguides
are fabricated by UV photolithography. This is associꢀ
ated with the fact that the crossꢀsectional dimensions
of waveguides are very small: their height and width
F
2
f2
1ꢀ1
1ꢀ2
Scheme 1.
It has been shown that the perfluoroalkyl carbanion
can be sometimes stabilized with an organic carbocaꢀ
tion rather than an inorganic counterion (K+, Сs+
,
etc.) [7]. Reactions with intermediate formation of
polyfluorocarbanions occupy a prominent place in the
chemistry of fluoroolefins [8–10]. Their in situ generꢀ
ation in the reaction mixture in the presence of various
electrophilic agents, such as diazo compounds,
αꢀ
Nesmeyanov Institute of Organoelement Chemistry, Russian
Academy of Sciences, ul. Vavilova 28, Moscow 119991 Russia
Institute of Problems of Laser and Information Technologies,
Russian Academy of Sciences, ul. Svyatoozerskaya 1,
Shatura, Moscow oblast, 140700 Russia
oxides, sultones, isocyanates, sulfenyl chlorides, and
alkyl halides, enables the synthesis of new organofluoꢀ
rine compounds and monomers for polymer chemisꢀ
try [11].
Using the commercially available perfluoroolefin
1,1,1,3,4,4,5,5,5ꢀnonafluoroꢀ2ꢀ(trifluoromethyl)ꢀ2ꢀ
Baikov Institute of Metallurgy, Leninskii pr. 49, Moscow,
119991 Russia
183

184
IGUMNOV et al.
chemical composition of waveguides with specific
properties.
Indeed, the reaction of olefin 2ꢀ1 with bromide 2ꢀ2
in the presence of anhydrous KF in dry dimethylformꢀ
amide yields ( )ꢀ1ꢀbromoꢀ6,6,7,7,8,8,8ꢀheptafluꢀ
oroꢀ5,5ꢀbis(trifuloromethyl)ꢀ2ꢀoctene (2ꢀ3). Howꢀ
ever, despite the longꢀterm boiling, the yield of 2ꢀ3
does not exceed 45%. It turned out that the introducꢀ
tion of 5–10 mol % CsF considerably decreases the
reaction time (15–16 h) and increases the yield of the
target product to 70%. It is likely that cesium fluoride
favors the formation of perfluorocarbanion В1, which
is the intermediate of this reaction.
n
1.42
1.40
1.38
1.36
1.34
E,Z
3
1
2
F CF3
C2F5
C–
400
600
800
1000
F CF3
Wavelength, nm
B1
Fig. 1. Refractive index
3ꢀ3, and ( 4ꢀ3 in the telecom wavelength ranges near
0.85, 1.3, and 1.55 m.
(n) of fluorinated monomers (1) 2ꢀ6,
We faced difficulties when we tried to accomplish
next steps of Scheme 2. The dehydrobromination of
compound 2ꢀ3 in an alkaline medium is accompanied
by a dominating side reaction. In particular, the reacꢀ
tion of 2ꢀ3 with KOH yields a mixture of products
(2)
3)
μ
pentene (2ꢀ1) and 1,4ꢀdibromoꢀ2ꢀbutene (2ꢀ2), we
attempted to develop the general methodology of synꢀ
thesis of previously unknown 1ꢀperfluoroalkylꢀ1,3ꢀ
butadienes (2ꢀ6) (Scheme 2). It is worth noting that,
as distinct from acrylates, butadiene and its derivatives
have not been heretofore used for creating optical
waveguides. Nevertheless, the suggested synthetic
scheme promised the synthesis of new highly fluoriꢀ
rather than the desired (
oroꢀ5,5ꢀbis(trifluoromethyl)ꢀ1,3ꢀoctadiene
E,
Z
)ꢀ6,6,7,7,8,8,8ꢀheptafluꢀ
(2ꢀ6).
According to chromatography mass spectrometry and
1H NMR data, the major product in this mixture is
(E,Z)ꢀ6,6,7,7,8,8,8ꢀheptafluoroꢀ5,5ꢀbis(trifluoromeꢀ
thyl)ꢀ2ꢀoctenꢀ1ꢀol (2ꢀ4). It is evident that, under the
given conditions, nucleophilic substitution of OH
group for the bromide ion occurs instead of dehydroꢀ
nated monomer 2ꢀ6 meeting the requirements on the bromination.
C2F5
F
CF3
CF3
Br
+
Br
2ꢀ1
2ꢀ2
CF3
CF3
C2F5
F
CF3
C2F5
F
KOH
KF
–KBr
–KBr
F
F
CF3
OH
Br
2ꢀ3
2ꢀ4
NEt3
CF3
CF3
C2F5
F
CF3
C2F5
F
Et
F
KOH
–KBr, –H2O, –NEt3
N+ Et
Et
Br–
F
CF3
2ꢀ5
2ꢀ6
Scheme 2.
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FLUORINATED MONOMERS AND POLYMERS WITH SPECIFIC PROPERTIES
185
1.3
1
3
2
0.1
1
0.01
0.2 mm
800
1000
1200
Wavelength, nm
1400
1600
Fig. 2. Absorption factor
2ꢀ6, ( 3ꢀ3, and (
ranges near 0.85, 1.3, and 1.55
α
(
λ
) of fluorinated monomers
4ꢀ3 in the telecom wavelength
m.
(1)
2)
3
)
Fig. 3. SEM image of the surface of the copolymer of octaꢀ
diene 2ꢀ6 and of diacrylate 4ꢀ3 (5 mol %).
μ
Therefore, we forced to use an alternative route for arisen from nine fluorine atoms at –82.89 ppm (CF3
)
accomplishing the dehydrobromination of 2ꢀ3 and,
thus, somewhat changed the planned scheme of synꢀ
thesis. It is known that the treatment of quaternary
ammonium salts with aqueous solutions of alkali
and –65.35 ppm (2CF3).
The mass spectrum of 2ꢀ6 (a Finnigan MAT
INCOS 50 quadrupole mass spectrometer, direct inserꢀ
tion probe, electron impact ionization, 70 eV) shows the
molecular ion peak at 372 [M]+ and the characteristic
peaks of the products of destruction of compound 2ꢀ6 at
metal hydroxides on moderate heating (to 60°С)
smoothly leads to the elimination of trialkylammoꢀ
nium bromide to give the double bond [12]. To syntheꢀ
size the starting quaternary salt, heptafluorobroꢀ
m/z (
= 203+ CH2=CH–CH=CH–С(СF3)2, 169+
moalkene
reaction of these reagents in dry acetone at room temꢀ
perature gave crystalline )ꢀ ꢀtriethylꢀ
2ꢀ3
was reacted with triethylamine. The (С3F7), and 69+ (CF3)
.
To obtain copolymers with octadiene 2ꢀ6 of differꢀ
ent composition, we also synthesized highly fluoriꢀ
nated acrylates (Schemes 3 and 4).
(E,Z N,N,N
6,6,7,7,8,8,8ꢀheptafluoroꢀ5,5ꢀbis(trifluoromethyl)ꢀ
2ꢀocteneꢀ1ꢀammonium bromide (2ꢀ5) in high yield.
Salt 2ꢀ5 thus obtained was treated with a 10% KOH
F
F F F F F
OH
solution at 60°C, which resulted in elimination of the
+
HO
H
ammonium group with simultaneous double bond
isomerization. After the treatment of the reaction mixꢀ
ture and fractionation of the resulting distillate, diene
2ꢀ6 was obtained in 75% yield. The overall yield of 2ꢀ
O
F
F F F F
3ꢀ2
F
3ꢀ1
O
6
6
, as calculated for initial 2ꢀ1, was 41%. Compound 2ꢀ
is a colorless liquid with bp 146–147°C/760 mmHg,
F
F F F F F
(CF3CO)2O
+ 2CF3CO2H
O
H
density d 20 = 1.528, and refractive index 20 = 1.3403.
nD
F
F F F F F
The liquid is insoluble in water and soluble in organic
solvents.
3ꢀ3
3ꢀ4
Scheme 3.
It is worth noting that 2,2,3,3,4,4,5,5,6,6,7,7ꢀ
dodecafluoroheptyl acrylate 3ꢀ3 and
2,2,3,3,4,4,5,5ꢀoctafluoroꢀ1,6ꢀhexadiyl diacrylate
4ꢀ3) have been previously synthesized for purposes
The structure of 2ꢀ6 has been confirmed by 1Н and
19F NMR, mass spectrometry, and elemental analysis.
The 1H NMR spectrum of compound 2ꢀ6 in CDCl3 (a
Bruker AM300 spectrometer operating at 300.13 MHz,
chemical shifts were referenced to TMS) shows the
signals of CH protons (5.80, 6.50, and 6.91 ppm) and
СН2 protons (5.52 ppm). The 19F NMR spectrum of
octadiene 2ꢀ6 in CDCl3 (the same spectrometer operꢀ
ating at 282.4 MHz, chemical shifts were referenced to
СFCl3) shows the characteristic signals of two CF2
(
)
(
unrelated to application in fiber optic electronics [13, 14].
However, the described method of their synthesis did
not ensured the purity required for monomers for fabꢀ
rication of light guides. Therefore, we needed to find a
new method of their synthesis. Applying the procedure
suggested in [15] for synthesis of other acrylates, where
groups at –125.14 and –110.75 ppm and the signals trifluoroacetic anhydride was used as the condensing
DOKLADY CHEMISTRY Vol. 446
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2012

186
IGUMNOV et al.
250
µ
m
(а)
Lightꢀguiding cores
250 µm
Cladding layer
Lightꢀguiding cores
Buffer layer
(b)
Fig. 4. Photograph of the array of polymer waveguides on a silicon substrate fabricated by contact UV lithography. (a) Top view
before covering the waveguides with a cladding layer and (b) end view after the formation of the cladding layer.
agent, we obtained target acrylate 3ꢀ3 and diacrylate acrylate 3ꢀ3
4ꢀ3 of purity ~99% (Scheme 4). absorption factors of monomers 2ꢀ6
1.5 m are 0.12, 0.33, and 0.61 dB/cm, respectively
(Fig. 2).
(
2
), and 1.3892 for diacrylate 4ꢀ3
(3). The
,
3ꢀ3, and 4ꢀ3 near
μ
F
F F
F
OH
OH
2
+
HO
The synthesized monomers were used for preparaꢀ
tion of copolymers of different composition. Before
proceeding to a laborious process of fabricating
O
F
F F
4ꢀ2
F
4ꢀ1
waveguides from monomers 2ꢀ6, 3ꢀ3, and 4ꢀ3, we carꢀ
O
F
F F F
ried out their bulk freeꢀradical copolymerization and
studied the optical properties of the resulting copolyꢀ
mers.
2(CF3CO)2O
O
O
F
F F
4ꢀ3
F
O
We studied the effect of the initiator and copolyꢀ
merization conditions (reaction time, temperature,
initiator concentration) on the yield and properties of
the copolymers. The optimal conditions for the synꢀ
+ 4CF3CO2H
4ꢀ4
thesis were found to be 70
°C, the reaction time of 12–
16 h, and 1.5 wt % of benzoyl peroxide.
Scheme 4.
Octadiene 2ꢀ6 (the fluorination degree 72.2%) and
acrylate 3ꢀ3 (the fluorination degree 66.7%) are transꢀ
parent and infinitely miscible liquids, which is a necꢀ
essary condition for fabricating polymer waveguides by
contact UV photolithography. The ratio of 2ꢀ6 and 3ꢀ3
units was varied from 3 : 7 to 7 : 3. The resulting copolꢀ
ymers are transparent rubbery substances. Their strucꢀ
ture has been confirmed by elemental analysis and IR
spectroscopy. We have demonstrated that the introꢀ
duction of 5 mol % of diacrylate 4ꢀ3 (the fluorination
The optical properties of the synthesized fluoriꢀ
nated monomers in the telecom wavelength ranges
were studied. The refractive indices of monomers 2ꢀ6
(1),
3ꢀ3 ), and 4ꢀ3 ) were measured on an IRF454ꢀB2M
(2
(3
refractometer (Fig. 1) and the absorption factors, on a
Shimadzu UV3600 spectrophotometer (Fig. 2).
Figure 1 shows that the refractive indices have norꢀ
mal dispersion in the visible and nearꢀIR spectral
ranges. The refractive indices (nD) at the wavelength
589.3 nm are 1.3403 for octadiene 2ꢀ6 (1), 1.3400 for degree 44.4%) into the above monomer mixture and
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FLUORINATED MONOMERS AND POLYMERS WITH SPECIFIC PROPERTIES
187
its subsequent copolymerization leads to materials index, absorption factor) have been studied. The bulk
with the density and hardness sufficient for processing freeꢀradical copolymerization of the monomers has
been carried out, and the optimal conditions for copoꢀ
lymerization have been determined. Compositions
with high and low refractive indices capable of photoꢀ
polymerization have been created. Multimode polyꢀ
mer waveguides on a silicon substrate have been fabriꢀ
cated from these compositions by UV photolithoꢀ
graphy.
and the optical transparency in telecom wavelength
ranges.
Scanning electron microscopy (SEM) of the samꢀ
ple surface has shown that the copolymer of octadiene
2ꢀ6 and 5 mol % diacrylate 4ꢀ3 as a crossꢀlinking agent
has a network structure (Fig. 3). The absorption factor
of this copolymer near 0.85
μm is 0.05 dB/cm.
From mixtures of monomers 2ꢀ6
,
3ꢀ3, and 4ꢀ3, we
ACKNOWLEDGMENTS
prepared compositions with high and low refractive
indices capable of photopolymerization. For fabricatꢀ
ing lightꢀguiding cores, composition no. 1 composed
of these monomers taken in the ratio 0.15 : 0.75 : 0.1
We are grateful to S.I. Molchanova for her help in
studying the optical properties of the monomers.
This work was supported by the Russian Foundaꢀ
tion for Basic Research (project no. 11–03–12068ꢀ
ofiꢀm) and the Presidium of the RAS (program
no. P7).
with
layers, composition no. 2 with the monomer ratio
0.25 : 0.65 : 0.1 and D = 1.3466 was used. For initiaꢀ
nD = 1.3569 was used. For the buffer and cladding
n
tion of the radical polymerization, 1.5% of Irgacureꢀ
651 initiator was introduced into the compositions. In
the course of photopolymerization, shrinkage of the
compositions occurs, which is accompanied by some
increase in the refractive index. Inasmuch as the difꢀ
ference between the refractive indices of the lightꢀ
guiding core and the cladding of the waveguide deterꢀ
mines its numerical aperture (which is an important
characteristic), we measured nD of the copolymers
synthesized from compositions no. 1 and no. 2. Meaꢀ
surements were taken with a Metriconꢀ2010 prism
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width is 40
between lightꢀguiding cores is 210
μ
m, the height is 80
μ
m, and the distance
m.
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com wavelength ranged near 0.85, 1.3, and 1.5
μm.
The optical properties of the monomers (refractive
DOKLADY CHEMISTRY Vol. 446
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2012