Synthesis of α,β-unsaturated esters of perfluoropolyalkylethers (PFPAEs) based on hexafluoropropylene oxide units for photopolymerization

Article abstract of DOI:10.1039/c8ra06354k

α,β-unsaturated esters are usually synthesized for polymer applications. However, the addition of maleate (cis-configuration) to a fluorinated moiety is challenging due to its potential isomerization during esterification. Various synthetic routes were attempted and led to very low conversion or side-products. The immiscibility of both reagents combined with an easy isomerization or attack on the double bond were potential explanations. In this paper, the synthesis of maleates oligo(hexafluoropropylene oxide) is reported by Steglich esterification and the reaction conditions are discussed depending on the molecular weight of the fluorinated moieties. After UV-curing, hydrophobic polymers were obtained by copolymerization with vinyl ethers by electron acceptor-donor systems.

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PAPER
Synthesis of a,b-unsaturated esters of
perfluoropolyalkylethers (PFPAEs) based on
hexafluoropropylene oxide units for
photopolymerization†
Cite this: RSC Adv., 2018, 8, 32664
b
Celine Bonneaud,
Melanie Decostanzi,b Julia Burgess,a Giuseppe Trusiano,c
´
´
Trevor Burgess,a Roberta Bongiovanni,
and Chadron M. Friesen
Christine Joly-Duhamelb
c
a
*
a,b-unsaturated esters are usually synthesized for polymer applications. However, the addition of maleate
(cis-configuration) to a fluorinated moiety is challenging due to its potential isomerization during
esterification. Various synthetic routes were attempted and led to very low conversion or side-products.
The immiscibility of both reagents combined with an easy isomerization or attack on the double bond
were potential explanations. In this paper, the synthesis of maleates oligo(hexafluoropropylene oxide) is
reported by Steglich esterification and the reaction conditions are discussed depending on the molecular
weight of the fluorinated moieties. After UV-curing, hydrophobic polymers were obtained by
copolymerization with vinyl ethers by electron acceptor–donor systems.
Received 28th July 2018
Accepted 6th September 2018
DOI: 10.1039/c8ra06354k
rsc.li/rsc-advances
friendly chemical process. The copolymerization of vinyl ethers
with maleates was accomplished under UV-light by electron
Introduction
acceptor–donor (AD) systems. These systems proved to form an
alternated copolymer by utilizing [AD] properties and had the
appropriate stoichiometric ratios.8,9 Under UV-curing, maleates
usually showed a faster reactivity than the more thermody-
namically stable fumarates.2 Nonetheless, the opposite effect
was observed10 and another study will be dedicated to the
comparison of their kinetics. In comparison to the common
(meth)acrylic systems, their polymerization is less sensitive to
oxygen.11,12 The use of a,b-unsaturated esters with uorinated
chains is almost absent from the literature.13–15 However, novel
outstanding properties such as high hydrophobicity and oleo-
phobicity can be obtained thanks to the uorine atoms. The aim
of our work was to bond monoalkyl maleates to long uorinated
moieties. Peruoropolyalkylethers (PFPAEs) are non-toxic uo-
rinated long chains16 which demonstrate interesting properties
in many high technology areas such as aerospace, aeronautic
(seals, gaskets) or automotive industries.17
a,b-unsaturated esters are very useful in polymer applica-
tions.1–3 Two congurations of a,b-unsaturated esters are
conceivable: the cis-conguration (maleate) and the trans-
conguration (fumarate) (Fig. 1). In this paper, we focused on
the mechanism and synthesis of maleates containing uori-
nated moieties. Indeed, the synthesis of the corresponding
fumarates can then easily be transposed but the maleates are
inclined towards isomerization and consequently their
synthesis is more challenging. In the past, maleate monomers
have been successfully copolymerized with a variety of mono-
mers such as vinyl acetate,1,4,5 vinyl ether,2 epoxy3 or even
hydrophobic types.6 They have been seen in applications such
as latex lms,1,5 polyelectrolytes6 or cooperative complexation of
cyclodextrins.7 It is also well known that photopolymerization is
a fast and eco-friendly process; using UV-light at room
temperature with maleates contributes to this environmentally
These PFPAEs based on structural units such as –(CF2O)–,
–(CF2CF2O)–, –(CF2CF2CF2O)– and –(CF(CF3)CF2O)– showed
high chemical and thermal inertness, low surface energy and
ammability, and excellent ageing and weather resistances.18
Oligo(hexauoropropylene) (oligo(HFPO)) products obtained by
the anionic ring-opening reaction of hexauoropropylene oxide
with cesium uoride were used in this work. Two different
molecular weights of oligo(HFPO) were studied (Mw ꢀ 1250 g
molꢁ1 and Mw ꢀ 2000 g molꢁ1). Due to the high withdrawing
effect of the uorine atoms on its long chain, the nucleophilicity
aTrinity Western University, Department of Chemistry, 7600 Glover Road, Langley,
British Columbia V2Y 1Y1, Canada. E-mail: chad.friesen@twu.ca
b
´
´
Ingenierie et Architectures Macromoleculaires, Institut Charles Gerhardt, Ecole
´
Nationale Superieure de Chimie de Montpellier (UMR5253-CNRS), 240 Avenue Prof
Emile Jeanbrau, 34296 Montpellier Cedex 5, France
cDepartment of Applied Science and Technology, Politecnico di Torino, 10128 Torino,
Italy
† Electronic supplementary information (ESI) available: Copies of 1H, 13C, IR,
GC-MS spectra for compounds 1a–f, 2a–f, 3a–b, 4a, 19F, TGA, DSC spectra for
2a–f, 3a–b, 4a and MALDI-TOF for 2a–f. IR spectra for the kinetics of
photopolymerization. See DOI: 10.1039/c8ra06354k
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of these oligomers is particularly low. They also inherently have by using a pipette and a polypropylene lm (6 mm) was used as
the characteristic of being insoluble in many organic solvents air protector. The sample was irradiated during 600 s. 2-
and are only soluble in specic uorinated solvents. Conversely, Hydroxy-2-methylpropriophenone was added as photoinitiator
the highly polar maleates are soluble in many organic solvent (4% w/w). The following of the kinetics was made by following
types but are insoluble in uorinated solvents. Aer the the disappearance of the band at 1622 cmꢁ1 for the vinyl ether.
synthesis of the monoalkyl maleates with different substituents The calculated conversion rates were made based on the
was achieved, different synthetic methods were conceivable and average of four repeats for the same experiment. The calcula-
are discussed for the esterication of the uorinated alcohols to tions were made by using the univariate method and conrmed
the remaining carboxylic acid group of the maleate. Aer their by the peak deconvolution method. Some photopolymerization
synthesis, the photopolymerization was carried out to create experiments were also performed on an UV production curing
highly hydrophobic polymers.
unit Fusion UV and LC6B Bench-top Conveyor equipped with
a microwave lamp (linear power output of 1200 W cmꢁ2). The
sample was placed on a conveyor and passed repeatedly under
Experimental
the UV lamp at a speed of travel belt of 1 m minꢁ1
.
Materials
Gas chromatography (GC) mass spectrometry (MS). An Agi-
lent Technologies 6890N GC was coupled with an Agilent
Technologies 7638B series injector and Agilent Technologies
5975B inert mass spectrometer (MSD) was employed with
electron impact (EI) as the mode of ionization. The GC was
equipped with a Zebron ZB-5ms column, 30 m ꢂ 0.18 mm
internal diameter (ID), 0.18 mm lm thickness (df). The detector
Maleic anhydride, methanol, ethanol, propan-2-ol, pentanol,
benzyl alcohol, phenol, tert-butyl alcohol, tert-amyl alcohol,
thionyl
chloride,
triethylamine,
carbonyldiimidazole,
Amberlyst-15 hydrogen form (strongly acidic, cation exchanger,
dry), dicyclohexylcarbodiimide, 4-dimethylamino pyridine, 2-
hydroxy-2-methylpropiophenone and dichloromethane were
purchased from Sigma Aldrich. Triethylamine and thionyl
chloride were distilled before use. Triethylamine and tri-
uorotoluene were kept under activated molecular sieves (3A).
The different alcohols were previously dried by using MgSO4.
1,1,1,3,3-pentauorobutane was purchased from Alfa Aesar.
The 1250 g molꢁ1 oligo(HFPO) methylene alcohol was prepared
from Krytox® acyl uoride. The 1250 Krytox® acyl uoride and
the 2000 g molꢁ1 Krytox® methylene alcohol were kindly
provided by E. I. du Pont de Nemours and Company.
ꢃ
ꢃ
and the injector temperatures were 200 C and 280 C, respec-
tively. The temperature program started from 50 ꢃC with a 2 min
hold then the heating rate was 25 ꢃC minꢁ1 until reaching
250 ꢃC and holding at 250 ꢃC for 2 min. The total pressure 108
kPa, total ow was 25.9 mL minꢁ1, column ow 0.74 mL minꢁ1
,
purge ow 3 mL minꢁ1, linear velocity 38.2 cm sꢁ1, and a split
injection of 30 : 1. The sample was previously diluted in
methoxyperuorobutane (3M’s Novec™ HFE-7100) in a GC vial.
Nuclear Magnetic Resonance (NMR) spectroscopy. The
structure of the products was determined by NMR spectroscopy
at room temperature (25 ꢃC) except if specied. NMR spectra
were recorded on a Bruker AVANCE III 400 MHz spectrometer
instruments using CDCl3 and C6D6 capillaries as internal
references for the oligo(HFPO) products. The experimental
conditions were accomplished by using TopSpin 3.5 operating
at 400.13 (1H), 376.46 (19F), 100.62 (13C) MHz. The letters s, d, t,
q, quint, sext and spt stand for singlet, doublet, triplet, quartet,
quintuplet, sextet and septuplet respectively.
Analysis
Fourier-transform (FT) – real time infrared spectroscopy
(RTIR). Real-time infrared spectroscopy and photo-
polymerization kinetics were performed on a Nicolet Nexus
apparatus by using OMNIC soware. The UV light came from an
OmniCure S2000 (Mercury lamp) equipment and the light
output was controlled by OmniCure R2000 Radiometer. The real
intensity provided to the sample was found to be 1 mW cmꢁ2
thanks to a radiometer from Solatell. The product was dropped
Matrix assisted laser desorption ionization time-of-ight
mass spectrometry (MALDI-TOF-MS). The homologue distri-
butions of the products were determined with a Bruker Auto-
exTM MALDI-TOF/TOF-MS spectrometer equipped with a 1
kHz smartbeam-II laser and reector in positive ionization. For
sample preparation, a 1 drop sample of oligo(HFPO) was added
to a 1 mL solution of 50 : 50 1% LiCl in MeOH and 2% per-
uorocinnamic
acid
dissolved
in
50 : 50
MeOH : methoxynonauorobutane (3 M HFE-7100). A 1 mL
solution was then pipetted on to a ground steel plate, dried, and
irradiated for a minimum of 5000 shots.
Thermogravimetric analyses (TGA). The degradation
temperatures were determined with a NETZSCH TG209F1 at
a heating rate of 20 ꢃC minꢁ1. Approximately 8 mg of the sample
were placed in an alumina crucible and heated from room
temperature to 600 C under inert atmosphere (40 mL minꢁ1).
ꢃ
Differential scanning calorimetry (DSC). The glass transition
Fig. 1 Cis- and trans- configurations of a,b-unsaturated esters.
temperatures were determined with a NETZSCH DSC200F3
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Synthesis of maleate oligo(HFPO) Mw ꢀ 1250g molꢁ1
calorimeter. Constant calibration was performed using indium, n-
octadecane and n-octane standards. 10–15 mg were placed in
pierced aluminium pans and the thermal properties were recorded
between ꢁ150 ꢃC and 100 ꢃC at 20 ꢃC minꢁ1. The glass transition
temperatures were measured at the second heating ramp and are
the onset values. Nitrogen was used as the purge gas.
Contact angle measurements. The hydrophobicity was deter-
mined thanks to a contact angle system OCA20 coupled with
a CCD-camera from DataPhysics Instrument using the soware
SCA20 4.1. The measurements were made in air at room temper-
ature by the sessile drop technique with distilled water. Three
repeats were made on three different samples previously irradi-
ated. Their difference in the average value was no more than 3ꢃ.
To a solution of oligo(HFPO) alcohol (Mw ¼ 1250 g molꢁ1
,
938 mg, 0.75 mmol), monoalkyl maleate (2 eq.) and DMAP (0.1
eq.) in 1,1,1,3,3-pentauorobutane (10 mL) and DCM (30 mL),
2.2 eq. of DCC in DCM (10 mL) were added dropwise at 0 C
ꢃ
during 20 min. Aer 5 min of reaction, the ice bath was
removed. The conversion of the reaction was followed by 19F
NMR. Aer 15 min, the reaction was stopped. The reaction
mixture was ltrated and concentrated under vacuum. Aer
ltration onto silica and then onto Celite®, the solvents were
removed under vacuum to afford clear colorless oil products.
Methyl maleate oligo(HFPO) 3a (yield ¼ 72%): 1H NMR (400
MHz, C6D6, 25 ꢃC, d): 3.77 (s, –OCH3, 3H), 4.76–4.89 (m, HFPO–
3
CH2O–, 2H), 6.32 (dd, –CH ¼ CH-cis, 2H, JH–H ¼ 11.9 Hz and
3
34.3 Hz), 6.87 (dd, –CH ¼ CH-trans, 2H, JH–H ¼ 15.9 Hz and
Synthesis of the monoalkyl maleates
28.7 Hz, 4%), 13C NMR (100 MHz, C6D6, 25 ꢃC, d): 51.0
(CH3OCO–), 60.0 (–COOCH2Rf), 127.5 (–CH ¼ CHCOOCH2–),
131.5 (–CHCOOCH2Rf), 163.3 (–COOCH3), 165.0 (RfCH2COO–),
19F NMR (376 MHz, C6D6, d): ꢁ135.3 (dR-Sq, –CF(CF3)CH2Rh),
GC-MS, 70 eV, m/z: 69.1 (32), 85.1 (13), 99 (12), 100.1 (12), 113.1
(100), 150.1 (11), 169 (54), FT-IR (ATR) nmax (cmꢁ1): 979.6,
1117.8, 1226.6, 1645.8, 1742.3.
In a general procedure, 10 mmol of maleic anhydride (981 mg)
were dissolved in 20 mmol of the corresponding alcohol (2 eq.).
The mixture was stirred at room temperature or heated between
45 ꢃC and 55 ꢃC. The different reaction times and temperatures
are reported for each compound. The alcohol was then removed
under high vacuum or the product was puried by ash chro-
matography if specied.
1
Monomethyl maleate 1a (R.T., 5 h, yield ¼ 96%): H NMR
Synthesis of vinyl ether oligo(HFPO) Mw ꢀ 1250g molꢁ1
(400 MHz, CDCl3, 25 ꢃC, d): 3.90 (s, CH3OCO, 3H), 6.43 (dd, –CH
¼ CHCOOH–, 2H, 3JH–H ¼ 12.7 Hz and 35.2 Hz), 13C NMR (100
MHz, CDCl3, 25 ꢃC, d): 53.8 (CH3OCO–), 129.0 (–CH ¼
CHCOOH–), 137.0 (–CHCOOH), 164.3 (CH3CH2OCO–), 168.4
(–COOH), GC-MS, 70 eV, m/z: 41.1 (17), 43.1 (34), 45 (34), 54 (24),
55 (10), 59 (11), 72 (17), 99 (100), 100 (16), FT-IR (ATR) nmax
(cmꢁ1): 819.6, 856.2, 1165.6, 1222.5, 1439.6, 1633.1, 1727.90.
To a solution of Krytox Acid Fluoride (2 g) in 5 mL of previously
dried triuorotoluene with 363 mL of dry triethylamine (1.5 eq.),
155 mL of ethylene glycol vinyl ether (1 ꢃeq.) in 5 mL of dry tri-
uorotoluene was added dropwise at 0 C. Aer 5 min, the ice
bath was removed and the reaction mixture was le to stir at
room temperature during 14 h. The solvent and volatiles were
removed under vacuum. The crude was then washed with water
(5ꢂ) and acetone (3ꢂ). The solvent traces were then removed
under vacuum.
Synthesis of maleate oligo(HFPO) Mw ꢀ 2000 g molꢁ1
4 (yield ¼ 38%) 1H NMR (400 MHz, C6D6, 25 ꢃC, d): 3.86 (br,
–CH2CH2OCH ¼ CHaHb, 2H), 3.94 (dd, –CH2CH2OCH ¼
To a solution of oligo(HFPO) alcohol (Mw ¼ 2000 g molꢁ1, 2 g, 1
mmol), monoalkyl maleate (2 eq.) and DMAP (0.1 eq.) in
1,1,1,3,3-pentauorobutane (15 mL), 2.2 eq. of DCC in DCM (10
mL) were added dropwise at 0 ꢃC during 30 min. Aer 5 min of
reaction, the ice bath was removed. The conversion of the
reaction was followed by 19F NMR. Aer 20 min, the reaction
was stopped. The reaction mixture was ltrated and concen-
trated under vacuum. A ash column chromatography by solid
deposit was performed. The solvents were removed under
vacuum to afford oil products. The products were analyzed by
NMR, GC-MS, MALDI-TOF and IR.
3
CHaHb, 1H, JH–H ¼ 6.7 and 2.1 Hz), 4.09 (d, –CH2CH2OCH ¼
CHaHb, 1H, 3JH–H ¼ 14.5 Hz), 4.52 (br, –CH2CH2OCH ¼ CHaHb,
2H), 6.35 (dd, –CH2CH2OCH ¼ CHaHb, 1H, 3JH–H ¼ 6.7 and 14.4
Hz), 13C NMR (100 MHz, C6D6, 25 ꢃC, d): 64.0 (–CH2CH2OCH ¼
CH2), ꢁ 65.9 (–CH2CH2OCH ¼ CH2), 85.8 (–CH2CH2OCH ¼
CH2), 150.7 (–CH2CH2OCH ¼ CH2), 158.6 (–COOCH2CH2OCH ¼
CH2), 19F NMR (376 MHz, C6D6, 25 ꢃC, d): ꢁ133.1 (dR-Sq,
–CF(CF3)CH2Rh), FT-IR (ATR) nmax (cmꢁ1): 979.1, 118.8, 1227.1,
1622.0, 1788.4.
Methyl maleate oligo(HFPO) 2a (puried by ash chroma-
1
tography 10 : 90 EtOAc : pentane, yield ¼ 55%): H NMR (400
Results and discussion
Synthesis of the starting monoalkyl maleates
MHz, C6D6, 25 ꢃC, d): 3.63 (s, –COOCH3, 3H), 4.64–4.78 (m,
HFPO–CH2O–, 2H), 6.20 (dd, –CH ¼ CH-cis, 2H, 3JH–H ¼ 11.9 Hz
and 30.7 Hz), 13C NMR (100 MHz, C6D6, 25 ꢃC, d): 50.7 The different monoalkyl maleates were synthesized by the ring-
(CH3OCO–), 59.8 (–COOCH2Rf), 127.5 (–CH ¼ CHCOOCH2–), opening reaction of maleic anhydride with the corresponding
131.2 (–CHCOOCH2Rf), 163.1 (–COOCH3), 164.6 (RfCH2COO–), alcohol. Poor-nucleophilic alcohols such as phenol, tert-
19F NMR (376 MHz, C6D6, 25 ꢃC, d): ꢁ135.2 (dR-Sq, –CF(CF3) butanol, tert-amyl alcohol did not permit the ring-opening
CH2Rh), GC-MS, 70 eV, m/z: 68.9 (55), 84.9 (14), 99.9 (16), 113 reaction under mild conditions as expected. Indeed, the with-
(100), 118.9 (14), 146.9 (17), 149.9 (26), 168.9 (73), MALDI-TOF, drawing mesomeric effect of the phenol or the high steric
[M + Li]+: 1765.5, 1931.8, 2098.0, 2263.2, 2429.5, FT-IR (ATR) hindrance of the tert-butyl and tert-amyl explained their low
nmax (cmꢁ1): 982.4, 1126.4, 1230.2, 1646.0, 1743.7.
nucleophilicity. Thus, benzyl alcohol was used as an alternative
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alcohols were used as model molecules: methanol, 1-pentanol,
1-octanol and 2,2,3,3,4,4,5,5-octauoro-1-pentanol. In any case,
a complete conversion of the starting maleate was observed.
However, side-products were also present (Table 1). The pres-
ence of the trans-product was reported in small quantity.
Indeed, in the literature, it was reported for this maleate
product a chemical shi for the two protons of the double bond
of 6.24 ppm23,24 whereas the chemical shi for the fumarate
product was 6.85 ppm.24,25 As the cis and trans-products did not
show the same reactivity in copolymerization,2,10 a control of the
reaction time is essential.
Scheme 1 Synthesis scheme of the monoalkyl maleates.
to create a hindered bulk on one side of the maleate. All the
products were synthesized with a good yield (Scheme 1).
A product from the addition of the alcohol onto the double
bond was also observed. It was clearly identied thanks to the
presence of three doublets of doublets at 2.43, 3.51 and
4.32 ppm. The three signals were correlated in 1H–1H COSY (as
observed for the carbonyldiimidazole reaction). Nonetheless,
the lower nucleophilicity of 2,2,3,3,4,4,5,5-octauoro-1-
pentanol underwent a low percentage of this side-product
(Fig. 2 & Table 1). Three doublets of doublets were also
observed at 2.85, 3.0 and 4.04 ppm and were correlated in
1H–1H COSY. However, these signals did not correspond to the
addition onto the a-position of the acid as assumed at the
beginning but to the formation of another side product: 1,3,5-
trisubstituted hydantoin.26,27 This product was prepared by an
intramolecular reaction from the in situ activated carboxylic
acid (I) due to the electrophilic centers of the double bond.
Followed by a rearrangement called O / N acyl migration on
(II), the corresponding 1,3,5-trisubstituted hydantoin (III) was
formed (Scheme 3). It has to be noticed that the intramolecular
reaction is always in competition with the nucleophilic substi-
tution between the nucleophile (ROH, RNH2, RCOOH, etc.) and
the in situ activated carboxylic acid with DCC. By using a,b-
unsaturated carboxylic acids, the team of Volonterio et al.
showed the synthesis of 1,3,5-trisubstituted hydantoins (III) by
a one-pot domino condensation/aza-Michael addition/O / N
acyl migration of symmetric and asymmetric carbodiimides.
Then a part of the alcohol did not react and remained in the
nal mixture. The hydantoin products were conrmed thanks
to the three doublets of doublets as well as the triplets of triplets
from the hydrogens of the cyclohexyl in a-position of the amines
(Fig. 2). As the nucleophilicty of 2,3,3,4,4,5,5-octauoro-1-
pentanol is lower, more hydantoin product was formed. Con-
cerning the octanol, the higher nucleophilicity allowed more
formation of the desired product instead of hydantoin without
a large amount of addition product to the a-ester.
Synthesis of the disubstituted maleates
Four different synthetic strategies were attempted: by direct
esterication, by using thionyl chloride, by using carbon-
yldiimidazole (CDI) and by Steglich esterication. No conver-
sion or very low conversion was observed by direct esterication
even in the presence of catalysts. Isomerization was highlighted
by using thionyl chloride. In addition, the bonding of the in situ-
formed imidazole into the double bond of the maleate by using
CDI prohibited the use of any of these synthetic routes (See ESI†
for further information).
By Steglich esterication
Carbonyldiimidazole is a “greener” method for esterication of
the uorinated alcohols to the maleates, but due to the nucle-
ophilicity of the in situ-formed imidazole, a non-nucleophilic
base, dicyclohexylcarbodiimide (DCC), was used with 4-dime-
thylaminopyridine (DMAP) as catalyst to carry out the Steglich
esterication instead (Scheme 2).
In a common procedure, 2–4 equivalents of alcohol were
used to achieve good yields.19 More recent procedures used only
a slight excess of the carboxylic acid or the alcohol with a high
efficiency of reactivity.20–22 Before working on the long uori-
nated oligo(HFPO) methylene alcohol, experiments were carried
out on hydrogenated or partially uorinated alcohols to deter-
mine if the desired product could be obtained. Different
When the similar reaction was carried out in absence of any
alcohol (Scheme 3), the 1,3,5-trisubstituted hydantoin was
conrmed. Another product was found and the chemical shis
were typical of the diester maleate: one triplet from the CH3–,
one quadruplet from the –CH2– and one singlet from the
hydrogen of the double bonds (See ESI†). The assumed product
was most likely the compound I, which was stable enough to be
recovered. DMAP could also have attacked the carbonyl by
following the Steglich mechanism (Scheme 3) and that would
explain the low quantity of dicyclohexyl urea found. Besides, in
presence of 1 equivalent of DMAP instead of using it as catalyst
(0.1 eq.), no hydantoin was formed aer 24 h and dicyclohexyl
Scheme 2 Scheme synthesis of the esterification of maleate by
Steglich esterification.
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Table 1 Summary of the molar % products generated from the Steglich esterifications reaction using (DCC/DMAP)a
Methanol
(%mol)
1-pentanol
(%mol)
1-octanol
(%mol)
2,2,3,3,4,4,5,5-octauoropentanol
(%mol)
Product
Structure
Cis-product
65
8
70
4
71
6
70
4
Trans-product
1,3,5-Trisubstituted hydantoin
Addition on a-ester
18
9
15
11
10
13
22
4
Conversion of the starting maleate 1b
100
100
100
100
a
3 h of reaction (calculated using benzophenone as internal reference).
urea was formed in larger quantities. Moreover, for the previous formation of hydantoin in presence of a poor nucleophilic
reactions, even if a slight excess (1.1 eq.) of the starting alcohol alcohol or the formation of the a-ester for a good nucleophile.
was used, a signicant peak of this reagent was present at the In our case, oligo(HFPO) alcohol was not assumed to attack
end of the reaction in comparison to the complete conversion of onto double bond due to the very low nucleophilicity of the long
the maleate. Furthermore, even by taking into account the uorinated chain. However, the 1,3,5-trisubstituted hydantoin
percentage of non-used alcohol due to the hydantoin reaction, is highly expected to be formed if our alcohol does not show fast
the percentage of alcohol was still higher than expected. By enough reaction kinetics.
using 2 equivalents of maleate and DCC, the starting alcohol
was fully consumed as well as the maleate. The use of 2 equiv-
Synthesis of maleate PFPAEs by steglich esterication
alents of maleate increased the conversion of the alcohol to the
desired product. The conversion of the alcohol was also
dependent on its nucleophilicity undergoing either the
Procedure in DCM. Based on the low nucleophilicity of the
long uorinated chain associated with a non-solubility in
Fig. 2 1H NMR spectra of the side-products from Steglich esterification: (1) the addition onto the double bond of the maleate, (2) the formation
of hydantoin (* signal overlapped).
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Scheme 3 Reaction scheme of the formation of hydantoin using DCC in absence of any nucleophilic reagent.26
1
dichloromethane (DCM), a slower conversion rate was expected. products could be analyzed by H, 13C, 19F NMR, GC-MS, FTIR
The correct cis-product was initially observed (3JH–H ¼ 11.6 Hz). and MALDI-TOF to conrm their structure. The doublet of
3
However, aer 48 h of reaction, the trans-product appeared (3JH– doublet around 6.1–6.25 ppm with a JH–H lower than 12 Hz
¼ 15.5 Hz) and an incomplete conversion was observed and conrmed the cis product. In addition, the maleate end group
H
conrmed by 1H and 19F NMR (Scheme 4). Longer reaction was supported by GC/MS with the correct fragments, by IR with
times always led to higher percentage of isomerization as the the C]C band at 1645 cmꢁ1 as well as by MALDI-TOF thanks to
trans-product is thermodynamically favorable.
the desired molecular weights [M + Li]+. However, as they are
Procedure in 1,1,1,3,3-pentauorobutane:DCM mixture. In not soluble in pentane and ethyl acetate except in high dilution
order to increase the solubility of oligo(HFPO) methylene it explains the low yields. Besides, a higher percentage of
alcohol, a mixture of solvents is a convenient procedure. isomerization was detected for longer reaction times as well as
However, depending on the molecular weight, the dilution as a percentage of the starting alcohol. The reaction was then
well as the mixture of solvents slightly differed. Combined with stopped as soon as the product was conrmed by 19F NMR.
a use of excess of the monoalkyl maleate, DCC and DMAP, even
if the hydantoin was formed, a faster reaction was assumed.
On contrary to the longest chain, the oligo(HFPO) (Mw
ꢀ
1250 g molꢁ1) showed a highest percentage of the starting
The rst synthesis was carried out on the highest molecular alcohol aer purication. It was noticed that aer 15 min of
weight (Mw ꢀ 2000 g molꢁ1). This oligomer showed the lowest addition, no starting alcohol was observed. Overtime, a side-
solubility in the organic solvent and was expected to show the reaction caused a reversal of the reaction, leaving the starting
ꢁ1
lowest nucleophilicity. The procedure followed the Steglich alcohol. By using a higher dilution ([CM ꢀ1250 g mol ] ¼ 0.02 g
w
esterication in Scheme 5 to synthesize six different maleates by mLꢁ1 instead of [CM ꢀ2000 g mol ] ¼ 0.08 g mLꢁ1) and a mixture
ꢁ1
w
using a mixture 60 : 40 1,1,1,3,3-pentauorobutane : DCM. The 20 : 80 1,1,1,3,3-pentauorobutane : DCM, no starting alcohol
disappearance of the starting alcohol was followed by 19F NMR was observed aer 48 h (Scheme 6). However, for the same
and conrmed in less than 20 min. These oligo(HFPO) products reaction stopped aer 15 min (aer conrmation by 19F NMR of
have been puried by column chromatography for the rst absence of starting alcohol) and aer column chromatography,
time. The purication mainly permitted the removal of any 20% of the starting alcohol was observed. We believe the
hydrogenated organic molecules such as the excess of DCC,
starting monoalkyl maleate, DMAP, 1,3,5-trisubstituted hydan-
toin and the in situ formed dicyclohexyl urea. Thus, the
Scheme 4 Synthesis scheme of maleate oligo(HFPO) using DCC and Scheme
DMAP.
5 Reaction scheme of Steglich esterification with oli-
go(HFPO) methylene alcohol Mw ꢀ 2000 g molꢁ1
.
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Scheme 6 Reaction scheme of Steglich esterification on oligo(HFPO)
alcohol Mw ꢀ 1250 g molꢁ1
.
product reverted back to the starting alcohol due to the pH and
large excess of the silica. Besides, the longer uorinated
chained alcohol created higher steric hindrance, protecting
itself from removal.
Photopolymerization of maleate and vinyl ether oli-
go(HFPO). Methyl maleate oligo(HFPO) (2a and 3a) were used in
copolymerization with vinyl ether oligo(HFPO) 4 (Fig. 3) to form
Fig. 4 Photopolymerization profiles of 3a : 4 and 2a : 4 in presence of
photoinitiator (4% w/w).
with the length of the uorinated chain: around ꢁ75 ꢃC and
ꢁ68 ꢃC, respectively for 1250 g molꢁ1 and 2000 g molꢁ1
.
Compound 2f showed a slightly higher Tg due to the reduced
chain exibility. Regarding the copolymers, only the glass
transition temperature of the uorinated phase was detected
an
electron
acceptor–donor
system.
2-Hydroxy-2-
methylpropiophenone was used as a photoinitiator (PI –
Fig. 3) due to its highly efficient homolytic scission under UV-
light.
ꢃ
(ꢁ69 ꢃC and ꢁ68 C for 3a : 4 and 2a : 4 respectively). Highly
hydrophobic polymers (i.e. 109ꢃ and 111ꢃ for 3a : 4 and 2a : 4
respectively) were obtained, as reported for acrylic photopoly-
mers containing PFPAEs' (see ESI†).28,29
For a stoichiometric amount of maleate/vinyl ether with 4%
w/w of PI, a complete conversion of the vinyl ether was obtained
in less than 40 s. (Fig. 4) No homopolymerization of 4 was
observed under UV-light aer 500 s and in presence of PI (4% w/
w). The IR band (C]C) of the vinyl ether did not overlap with
other structures, therefore, kinetics was done only on the vinyl
ether band at 1622 cmꢁ1. Surprisingly, in comparison with
literature, a partial homopolymerization of 2a was observed in
presence of PI but proved to be much slower than its copoly-
merization (See ESI†). Besides, the vinyl ether 4 showed quan-
titative conversion and since a stoichiometric ratio was used, no
homopolymerization of the maleate could have happened or
only in small quantities within 40 s. No clear effect was observed
due to the chain length (2a and 3a) for the polymerization
kinetics. In absence of photoinitiator, no conversion of the
monomers was observed. In addition, the presence of air was
not a key factor in polymerization as the nal conversion was
reached in the same amount of time (See ESI†). The degradation
temperature T5% was mainly dependent on the chain length (i.e.
1250 g molꢁ1 and 2000 g molꢁ1). Concerning oligo(HFPO) with
Mw ꢀ 1250 g molꢁ1, the addition of a substituent (maleates or
vinyl ether) increased the degradation temperature in compar-
ison to the starting alcohol. However, for the higher molecular
weight, no changes were observed except for the benzyl maleate
2f due to the more stable phenyl group. The copolymers showed
a similar T5% but demonstrated slower degradation kinetics
than the monomers: 174 ꢃC and 200 ꢃC for 3a : 4 and 2a:4
respectively. The glass transition temperatures were also linked
Conclusions
The synthesis of uorinated maleates was successfully carried
out by Steglich esterication. Indeed, the high reactivity of the
a,b-unsaturated esters indicated that other side products are
possible. However, other routes to esterication were not
conceivable. It is noted that long uorinated moieties decreased
the nucleophilicity of the hydroxyl group but reduces the
formation of other side-products. Further considerations would
be required to fully understand the reactivity of the a,b-unsat-
urated esters in presence of DCC, DMAP and a nucleophile. In
addition, thanks to the uorinated groups, highly hydrophobic
polymers were made by UV-curing. An electron acceptor/donor
system (maleate/vinyl ether) showed a complete conversion in
less than 40 s for both tested maleate monomers. Further
studies will be carried out for the optimization of the photo-
polymerization kinetics. The choice of photoinitiator, its
concentration as well as the inuence of the substituent will be
investigated.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This research project has received funding from the European
Union’s Horizon 2020 research and innovation program under
grant agreement No. 690917 – PhotoFluo; Natural Sciences and
Engineering Research Council of Canada (NSERC), Discovery
`
Grants Program RGPIN-2015-05513 and Ministere de l’Educa-
Fig. 3 Structure of the vinyl ether oligo(HFPO) 4 and the photoinitiator
´
tion superieure et de la recherche.
PI.
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