Novel perfluoropolyethers containing 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole blocks: synthesis and characterization

Article abstract of DOI:10.1016/j.jfluchem.2009.07.007

Peroxidic perfluoropolyethers (PFPEs) are industrial intermediates used by Solvay Solexis for the preparation of different classes of (per)fluoropolyethers (Fomblin, Galden, Solvera, Fluorolink). The chemistry o

Full text of DOI:10.1016/j.jfluchem.2009.07.007

Journal of Fluorine Chemistry 130 (2009) 933–937
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Novel perfluoropolyethers containing 2,2,4-trifluoro-5-trifluoromethoxy-1,3-
dioxole blocks: synthesis and characterization
b
a
a
M. Avataneo a, , W. Navarrini , U. De Patto , G. Marchionni
*
a Solvay-Solexis, R&D Centre, viale Lombardia 20, 20021 Bollate Milan, Italy
b Dipartimento di Chimica, Materiali e Ingegneria Chimica ‘‘Giulio Natta’’, Politecnico di Milano, via Mancinelli 7, 20131, Milan, Italy
A R T I C L E I N F O
A B S T R A C T
Article history:
Peroxidic perfluoropolyethers (PFPEs) are industrial intermediates used by Solvay Solexis for the
preparation of different classes of (per)fluoropolyethers (Fomblin1, Galden1, Solvera1, Fluorolink1).
The chemistry of these peroxidic compounds has been recently exploited for the synthesis of novel PFPE
block copolymers. In the present work we report the synthesis, the structural and physical–chemical
characterization of block copolymers obtained by the reaction of peroxidic PFPEs with 2,2,4-trifluoro-5-
trifluoromethoxy-1,3-dioxole, a cyclic homopolymerizable perfluoroolefin. These block copolymers
combine the most attractive properties of the PFPEs, like the excellent lubrication, the high thermal
stability and the optical transparency, with new specific properties which are related to the
perfluorodioxolenic blocks.
Received 28 May 2009
Received in revised form 15 July 2009
Accepted 16 July 2009
Available online 23 July 2009
Keywords:
Peroxidic perfluoropolyether
Fluorinated copolymer
2,2,4-trifluoro-5-trifluoromethoxy-1,3-
dioxole
ß 2009 Elsevier B.V. All rights reserved.
Optical transparency
1. Introduction
[3], perfluoroalkylsulfonyl vinyl ethers [4] and perfluorodioxoles
[5]. For example, by using 2,2,4-trifluoro-5-trifluoromethoxy-1,3-
Perfluoropolyethers (PFPEs) are synthetic fluids largely used as
high quality lubricants and also used in heat transfer applications
[1]. Their superior performances can be explained on the basis of
their unique properties like: chemical and thermal stability,
electrical resistance, wide operating temperature range, non-
flammability, non-toxicity and zero-ozone-depletion potential.
Among the several routes reported in literature for the
synthesis of PFPEs, the process developed by Solvay Solexis is
based on the low temperature oxy-polymerization of perfluoro-
olefins. In particular, in the case of tetrafluoroethylene (TFE), the
product of this reaction is a peroxidic PFPE with the following
structure:
dioxole (TTD) as comonomer, a peroxidic PFPE with functional
repeating units of the type –CF(OCF2OC(O)CF3)O- is obtained [5].
The chemistry of the peroxidic PFPEs has been recently
exploited for the synthesis of a new class of ‘‘PFPE-based’’ block
copolymers that can be prepared by decomposing the peroxidic
units of (I) in the presence of free radical homopolymerizable
fluoro-olefins [6]. These new block copolymers are represented by
the formula (II):
CF3O-½ðAÞðBÞꢀt-CF3
(II)
The A block is a perfluoropolyether moiety composed of –CF2O–
and –CF2CF2O– units, whereas B is a perfluoropolyolefinic block
whose length depends on the reactivity of the olefins and on the
reaction conditions.
CF O½ðCF OÞnðCF CF OÞmðOÞ ꢀCF
(I)
v
3
2
2
2
3
These block copolymers combine the most attractive properties
of PFPEs and, in particular, the excellent lubrication, the high
thermal stability and the optical transparency, with new specific
properties which are strictly related both to the composition and to
the length of the perfluoropolyolefinic blocks.
In the present work we report the synthesis, the structural and
physical–chemical characterization of PFPE–TTD block copolymers
obtained by the reaction of the peroxide (I) with TTD, a cyclic
perfluoro-olefin of formula (III) that can be prepared in good
yields through the hypofluorite technology [7]. TTD is already used
for the preparation of amorphous and high molecular weight
The peroxidic units (O) of the structure (I), randomly
v
distributed along the chain, can be easily removed by thermal
treatment or under UV exposure, thus obtaining a highly stable
PFPE, commercially known as Fomblin1 Z [2].
Photo-oxidation is a very versatile technology that can also be
applied for the preparation of functionalized PFPEs by introduction
of specific fluorinated co-monomers like 1,3-perfluorobutadiene
* Corresponding author. Tel.: +39 02 38356672; fax: +39 02 38356355.
E-mail address: marco.avataneo@solvay.com (M. Avataneo).
0022-1139/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2009.07.007

934
M. Avataneo et al. / Journal of Fluorine Chemistry 130 (2009) 933–937
polymers and copolymers (Hyflon1 AD) whose typical properties
are the solubility in fluorinated solvents, high glass transition
temperature (Tg), high thermal stability, good optical clarity, and
low dielectric constant [8,9].
(III)
These new structures show a low UV absorption, in particular in
the region of interest for microlithography, an excellent thermal
stability and a glass transition that is tunable with the TTD content
in the copolymer. As reported below, these interesting and original
properties are due to a combination of the properties typical of TTD
copolymers with those of perfluoropolyethers.
2. Results and discussion
2.1. Synthesis of PFPE–TTD block copolymers
Four samples of PFPE–TTD block copolymers were synthesized
by exposing a solution of peroxidic PFPE in TTD, kept under
stirring, to UV light for some hours (details are reported in the
Section 4). In these conditions, the peroxidic links of (I), here
indicated as RCF2–O–OCF2R where R is a PFPE moiety, homo-
litically decompose and generate two alkoxy radicals [10–13]:
R ꢁ CF2O ꢁ O ꢁ CF2RUVꢁ!light2R ꢁ CF2Oꢂ
(1)
Scheme 1. Reaction mechanism of the synthesis of PFPE–TTD copolymer.
These alkoxy radicals can then undergo
followed by the formation of new alkyl radicals and the evolution
of a molecule of carbonyl fluoride:
b
-scission reactions
Table 2 below reports the intrinsic viscosity of the PFPE–TTD
block copolymers measured in Galden1 D80, an appropriate
perfluoropolyether solvent. The Mark–Houwink–Sakurada equa-
R-CF2Oꢃ ! Rꢃ þ COF2
(2)
a
tion ([
h
] = KM ) is generally used to correlate the intrinsic viscosity
[h
] of homopolymers and statistical copolymers to their molecular
weight (M). The parameter and of this equation are
experimentally determined and are dependent on the polymer-
In absence of TTD, the recombination of these radicals leads to a
highly stable and non-peroxidic PFPE, whose repeating units are –
CF2O–, –CF2CF2O– and, in minor quantity, –CF2CF2CF2O– and –
CF2CF2CF2CF2O–. In our synthetic experimental approach, how-
ever, as TTD is present in the reaction medium, a portion of the
radicals formed in (Eq. (2)) react with this olefin and initiate its
oligomerization. The reaction mechanism can be described by
referring to the classic kinetic scheme of radical polymerizations as
reported in Scheme 1:
a
K
solvent system. For the same solvent, the value of
moving from flexible to rigid polymers.
a increases by
In the case of the here described PFPE–TTD block copolymers,
the Mark–Houwink–Sakurada equation is not applicable to fit the
intrinsic viscosity data due to the different structure of the four
samples in terms of number and length of the TTD blocks.
In the initiation step (Eq. (3)) an alkyl PFPE radical (Rꢃ) reacts
with TTD, preferably on the position 4 which is the most favourite
for both electronic and steric reasons. In fact the carbon radical in
(Eq. (3)) is highly stabilized by the presence of two oxygen atoms in
alpha position [14,15]. The radical formed in (Eq. (3)) can then
grow by addition of other molecules of TTD (Eq. (4)) until the
kinetic chain ends by coupling with another radical of the same
type (Eq. (5)) or with a PFPE primary radical (Eq. (6)).
Table 1
Structural characterization of the PFPE–TTD block copolymers (IV) determined by
19F-NMR.
Sample
Mn
n
m
a
t
TTD (% by
weight)
1
2
3
4
15,000
19,000
35,000
85,000
90
88
46
1.7
4.0
7.2
8.0
8.4
19
37
58
42
45
54
105
277
13.4
14.2
252
12.0
2.2. Characterization of PFPE–TTD block copolymers
The structure of the four samples of the PFPE–TTD block
copolymers was determined by 19F-NMR and it can be summarized
by the following general formula (IV):
Table 2
Intrinsic viscosity of the PFPE–TTD block copolymers measured at 30 8C in Galden1
D80. ‘‘a’’ is the number of TTD units per each dioxolenic block and ‘‘t’’ is the number
of dioxolenic blocks per chain.
CF3O-½ðCF2OÞnðCF2CF2OÞmðCF2-ðTTDÞa-CF2OÞtꢀCF3
(IV)
a
Sample
Mn
TTD (% by
weight)
a ꢄ t
Intrinsic viscosity
(dl/g)
In the structure (IV), (CF2–(TTD)a–CF2O) represents the
dioxolenic blocks randomly distributed along the chain, ‘‘a’’ is
the number of TTD units per each dioxolenic block and ‘‘t’’ is the
number of dioxolenic blocks per chain. The structural data of
samples 1–4 are reported in Table 1 (see also Section 4.3).
1
2
3
4
15,000
19,000
35,000
85,000
19
37
58
42
14
34
0.051
0.060
0.069
0.096
96
170

M. Avataneo et al. / Journal of Fluorine Chemistry 130 (2009) 933–937
935
Fig. 2. Glass transition temperature of the PFPE–TTD block copolymers as a function
Fig. 1. Intrinsic viscosity of the PFPE–TTD block copolymers as a function of average
number of TTD units per chain (a ꢄ t).
of the TTD content.
However, the correlation between the intrinsic viscosity and the
molecular weight and between the intrinsic viscosity and the
rigidity of the structure is still valid and can be used to discuss the
experimental data: in the four samples, the intrinsic viscosity
increases with both the molecular weight (Mn) and the TTD
content, since the TTD blocks are more rigid, due to the presence of
the cyclic units, than the perfluororopolyether blocks. In particular,
from sample 1 to sample 2, the increase of intrinsic viscosity seems
mainly related to the variation of TTD content as Mn does not
increase dramatically. On the contrary, from sample 3 to sample 4,
the increase of intrinsic viscosity can be attributed to the variation
of Mn as the TTD content decreases. An interesting and empirical
correlation is reported in Fig. 1 where the intrinsic viscosity is
plotted as a function of the average number of TTD units per chain
(column ‘‘a ꢄ t’’ of Table 2). Since the parameter ‘‘a ꢄ t’’ takes into
consideration both the TTD content and its block length, there is a
direct dependence of the intrinsic viscosity from this parameter.
In Table 3 and in Fig. 2 the glass transition temperatures (Tg) of
the PFPE–TTD block copolymers are listed. For each sample, the
differential scanning calorimetry (DSC) shows only one glass
transition whose value is between those of the PFPE and of the TTD
blocks. This is an indication of the absence of segregated phases, for
example micro-domains of TTD blocks, being the TTD block
perfectly dissolved into the PFPE blocks and vice versa.
uniform mixing, as evidenced by the presence of a unique glass
transition, the Tg of the copolymer is intermediate between about
ꢁ130 8C and 192 8C and it increases with the content of TTD. By
extrapolating the available data, we can extimate a Tg of these
copolymer higher than room temperature for the TTD content
above 70% by weight.
The results of the thermogravimetric analyses, both in nitrogen
and in air, of the four synthesized copolymers are indicated in
Table 4. The onset of the decomposition curve (2% weight loss) is
comprised between 254 and 346 8C (in nitrogen) for the four
samples (this broad range of temperature can be explained by
supposing the presence of residual peroxidic units in the
copolymers, traces of solvent and oligomeric structures), with
very small differences between the values obtained in nitrogen and
those obtained in air. The perfluorinated structure of these
copolymers and the absence of ‘‘weak points’’ in the chain, like
hydrogen atoms, double bonds, etc, make the PFPE–TTD block
copolymers highly stable in the presence of oxygen at high
temperature. As a consequence, the difference between the
starting degradation temperature in nitrogen and in air is
negligible. Also at temperature higher than 300 8C, the effect of
oxygen (TGA in air) on the degradation kinetic is slight. The 50%
weight loss is substantially identical for all the block copolymers
showing a very good thermal stability well above 400 8C.
In Fig. 3 the UV absorption spectra of two PFPE–TTD block
copolymers (sample 1 and sample 4) are reported in the range
between about 155 and 180 nm. For comparison, we have also
reported in the same figure the spectra of:
The glass transition at infinite molecular weight (Tg1) of the
perfluoropolyethers is related to the overall oxygen content
through the following equation [1]:
Tg1ðKÞ ¼ 200 ꢁ 80ðO=CÞ
(7)
ꢃ
Fomblin1 Z 25, a commercial perfluoropolyether whose formula
is CF3(CF2O)n(CF2CF2O)mCF3 with m/n = 0.5 and Mn = 9500. The
repeating units of this perfluoropolyether are the same of that of
the PFPE blocks in the PFPE–TTD copolymers;
In Eq. (7), the O/C parameter is the numerical ratio between
oxygen and carbon in the PFPE chain. Hence, the higher the oxygen
content, the lower is the glass transition of the perfluoropolyether.
By using Eq. (7), for the PFPE blocks of the four block copolymers
synthesized, we can estimate a Tg1 equal to ꢁ133 8C for the
samples 1 to 3 (O/C = 0,75) and a Tg1 equal to ꢁ127 8C for the
sample 4 (O/C = 0,68). The TTD homopolymer is completely
amorphous and has a Tg = 191 8C [16], well above the room
temperature. As the two type of blocks display an intimate and
ꢃ
ꢃ
PTTD homopolymer (Tg = 191 8C, Intrinsic viscosity = 20 cc/g)
Two copolymers of tetrafluoroethylene (TFE) and TTD, one with a
molar composition TFE/TTD equal to 20/80 (Mn = 500,000) and
one equal to 70/30 (Mn = 700,000).
Table 4
TGA analyses of the PFPE–TTD block copolymers in nitrogen and in air (data in
brackets).
Table 3
Tg and
DCp of the PFPE–TTD block copolymers determined
by DSC analysis.
Sample
Temperature (8C) versus percentage weight loss
2%
10%
50%
Sample
Tg (8C)
DCp (J/g 8C)
1
2
3
4
295 (293)
254 (250)
275 (277)
346 (331)
377 (364)
373 (346)
375 (367)
389 (375)
434 (420)
424 (410)
416 (404)
430 (415)
1
2
3
4
ꢁ70.3
ꢁ37.7
+9.7
0.181
0.168
0.119
0.087
ꢁ42.6

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M. Avataneo et al. / Journal of Fluorine Chemistry 130 (2009) 933–937
OCF2OO–). The composition of the polymer and the
reaction yields, generally higher than 90%, depend on the
temperature and on the tetrafluoroethylene concentration.
Carbonyl difluoride and tetrafluoroethylene oxide are the main
by-products.
4.2. Synthesis of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole
TTD was synthesized according to a multi steps methodology
based on the addition of CF3OF to 4,5-dichloro-2,2-difluoro-1,3-
dioxole at low temperature, followed by dechlorination with zinc
in anhydrous DMF. Details of this preparation methodology are
reported in [7].
Fig. 3. UV absorption data of some fluorinated polymers.
4.3. Typical procedure for the synthesis of the PFPE–TTD block
copolymers
Electronic transitions between the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)
are responsible for the absorption in this UV region. For example,
The syntheses of the PFPE–TTD block copolymers were carried
out in a photo-chemical reactor which includes an immersion
high-pressure Hanau mercury lamp (150 W), a quartz jacket
cooled by a cryostat with Galden1 HT110 fluid and an outer Pyrex
cylindrical reactor (volume = 70 cm3). This reactor was provided
with magnetic stirring, adjustable cooling system (water),
thermocouple, inlet tubes for addition of nitrogen.
A solution of peroxidic PFPE and TTD (boiling point = 26 8C) in
Galden1 HT55 as solvent (the amounts of these compounds are
reported in Table 5) is charged into the reactor and cooled, under
stirring, at about ꢁ30 8C, while a flow of nitrogen (2.0 Nl hꢁ1) is fed
into the reactor in order to remove the dissolved oxygen. After
about 30 min the UV lamp is switched on and the nitrogen flow is
lowered at 0.5 Nl hꢁ1. The reaction is carried out, under stirring, in
the temperature range between +10 and +15 8C. At the end the UV
lamp is switched off and the solution is discharged from the
reactor. The copolymeric product is recovered after distillation of
the solvent and of the unreacted monomer. The residual peroxidic
content present in the crude products is removed by heating the
reaction products under nitrogen at 230 8C for 2 h.
the perfluorinated polymers having
a backbone made of a
sequence of C–C bonds are not transparent since the difference
of energy between HOMO and LUMO corresponds to energy
associated to this UV region: as evidenced in Fig. 3, the PTTD
homopolymer and the TFE-TTD copolymers, being constituted by a
chain of C–C bonds, show a very low UV transparency. On the
contrary, linear perfluoropolyethers, like Fomblin1 Z, are known
for their exceptional UV transparency: in these compounds the
presence of oxygen atoms, every one or two perfluoromethylene
groups, significantly increases the energy in the HOMO–LUMO
transition, thus lowering the wavelength at which the absorption
takes place [17]. The PFPE–TTD copolymers, having both the PFPE
and the TTD blocks, shows a behaviour intermediate between that
of Fomblin1 Z and the TTD homopolymer.
3. Conclusion
Peroxidic perfluoropolyether are useful intermediates for the
preparation of a new class of PFPE based block copolymers. This
versatile synthesis has been applied to the TTD monomer and four
different samples of PFPE–TTD copolymers have been prepared
and characterized. The two blocks (PFPE and TTD) are perfectly
soluble into each other and form a structure that is homogeneous
at molecular level, as evidenced by the presence of a unique Tg.
The presence of TTD blocks affects some of the typical properties of
the perfluoropolyethers by increasing, for example, the viscosity,
the glass transition and the UV absorption.
4.3.1. 19F-NMR analysis
All NMR spectra were recorded at 25 8C, on a Varian INOVA
400 MHz using CFCl3 as internal standard. Analyses were carried
out on solution of PFPE–TTD block copolymers (10% by weight) in
hexafluorobenzene. The main signals and their correspondent
chemical shifts are reported in Table 6. Since the signals of the –
OCF3 end groups are partially superimposed to the signals of the –
OCF2O– and OCF3 groups of the dioxolenic units, for the
determination of the molecular weight (Mn) of these copolymers
we have assumed that the number of –OCF3 in the final products is
equal to the number of –OCF3 groups in the correspondent
peroxidic precursor (where the –OCF3 groups are easily detect-
able).
4. Experimental section
Peroxidic PFPEs and TTD are not commercial products and they
were synthesized according to the descriptions reported below.
Galden1 HT55 and Galden1 D80 are perfluoropolyethers with a
boiling point of, respectively, 55 and 80 8C produced and
commercialized by Solvay Solexis.
A typical spectrum of PFPE–TTD copolymer is reported in Fig. 4.
The numerical average number of TTD units per blocks (‘‘a’’)
was calculated on the basis of the ratio between the area of the
signal C and the signal D.
4.1. Synthesis of a peroxidic PFPEs
The peroxidic PFPE was obtained by oxidative polymerization of
tetrafluoroethylene at low temperature in a fluorinated solvent
where elemental fluorine is an efficient free radicals initiator at low
temperature [10]. A peroxidic polymeric precursor is formed as
result of the polymerization reaction:
Table 5
Amount of reactants used for the synthesis of the PFPE–TTD block copolymers. The
perodixic PFPE (I) used for sample 1, 2 and 3 has Mn = 13,000 and 26.0 peroxidic
units per chain (2.0 mmol/g). The peroxidix PFPE (I) used for sample
Mn = 43,000 and 29.6 peroxidic units per chain (0.69 mmol/g).
4 has
Sample
Peroxidic PFPE (g)
TTD (g)
Galden1 HT55 (g)
Reaction time (h)
F2
1
2
3
4
6.3
19.8
10.0
10.5
15.8
45.0
117.9
75.2
0.0
5
15
15
15
CF2 ¼ CF2 þ O2ꢁ!ðIÞ
(8)
130.0
129.5
The perfluoroether repeating units (–CF2O– and –CF2CF2O–)
are interspaced with peroxidic units (–OCF2CF2OO– and
0.0
–

M. Avataneo et al. / Journal of Fluorine Chemistry 130 (2009) 933–937
937
Table 6
meric samples were cooled at ꢁ170 8C, and after 3 min at ꢁ170 8C
were heated up to 70 8C at the heating rate of 20 8C minꢁ1. The
scans were carried out twice and the Tg was determined as the
midpoint of the specific heat (Cp) step with a repeatability of
ꢅ0.5 K.
Signal assignment and correspondent chemical shift of the PFPE–TTD block
copolymers.
Signal
Structure
Chemical shift (ppm)
A
B
–OCF2O–
–OCF3
ꢁ52 $ ꢁ56
ꢁ56 $ ꢁ58
ꢁ88 $ ꢁ92
The thermogravimetric analyses were carried out with the
instrument PYRIS 1 TGA – PERKIN ELMER. The samples were
heated from 30 8C to 750 8C at 10 8C/min.
–OCF2CF2O–
C
D
ꢁ105 $ ꢁ125
ꢁ75 $ ꢁ85
ꢁ48 $ ꢁ65
4.3.3. UV absorption
Experimental data were obtained by a series of transmittance
measurements on a DUV spectrometer operating in vacuum
(10ꢁ1 torr) in the spectral range 140–180 nm. Atmospheric gases
were removed by applying several cycles of freeze (between 80 and
300 K) under vacuum.
E
F
4.3.4. Viscosities
Solution viscosities were measured at 30 ꢅ 0.18C with
a
Ubbelhode viscosimeter
Acknowledgment
We thank Renzo Bartolini for the synthesis of the PFPE–TTD
copolymer, Mattia Bassi for the help in the VUV absorption
measurements, Emma Barchiesi for the NMR characterization and
Solvay Solexis for the permission to publish this work.
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4.3.2. Calorimetric analysis
The glass transition temperature of the block-copolymers were
obtained using the differential scanning calorimeter Perkin Elmer
DSC 2C (calibrated with Indium and cyclohexane). The copoly-