Application of Liquid-Phase Direct Fluorination: Novel Synthetic Methods for a Polyfluorinated Coating Material and a Monomer of a Perfluorinated Polymer Electrolyte Membrane

A new polyfluorinated anti-staining coating material CF3O(CF2CF2O)xCF2-CONHCH2CH2CH2Si(OCH3)3 has been developed by utilizing the PERFECT method, which employs a liquid-phase direct fluorination reaction with elemental fluorine as a key step. Direct fluorination of a partially-fluorinated ester, which was prepared from a non-fluorinated poly(ethylene glycol) and a perfluorinated acyl fluoride, followed by methanolysis, gave the perfluorinated corresponding compound, which was led to the coating material for surface treating agents, and the methyl ester of the starting perfluorinated acyl fluoride. Application to the synthesis of a new perfluorinated bifunctional sulfonate monomer CF2=CFOCF2CF2CF2OCF(CF2SO2F)2 for polymer electrolyte membranes (PEMs) of fuel cells was also developed.


Introduction
Nowadays, organofluorine compounds are essential materials, especially in recent IT, electronics, and medical applications [1].For example, the low surface energy of an organofluorine compound leads to water repellent and oil repellent properties, so that it is utilized as surface treating agents for water-and-oil repellent film.Therefore, it is expected that an anti-staining coating material for glass, for example, for automobiles or displays (a liquid crystal display, a CRT display, a projection display, a plasma display, an EL display or the like), could be achieved by an organofluorine compound.One such compound 1 may be a candidate [2,3].
When the compound 1 is applied to the surface of a substrate, the hydrolyzable groups are hydrolyzed by hydroxyl groups of the substrate or moisture on the substrate to form silanol groups.Once the silanol groups are bonded to the substrate, perfluoroalkyl groups will be oriented on the atmosphere side, so that the coating film made of the compound 1 exhibits water repellency [4].
However, as perfluoroalkyl groups are highly crystalline and stiff, the coating film made of the compound 1 does not have enough efficiency for the removal of oil-and-fat stains.To solve the problem, a compound having a perfluoropolyether group 2 was reported [5].
The perfluoropolyether (PFPE) group has a flexible structure, because stiff perfluoroalkyl groups are separated by oxygen atoms and C-O-C bonding have high mobility, so that the coating film made of the compound 2 is excellent in its efficiency in removing oil-and-fat stains, and also has water and oil repellency.
However, the mobility of the PFPE groups in 2 cannot be regarded as sufficient, and the efficiency for removal of oil-and-fat stains is not enough, because the ratio of oxygen atoms to carbon atoms in the PFPE groups is still small.From the viewpoint of molecular design, -(CF 2 CF 2 O)-as a repeating unit is ideal.However, there has not been a synthetic method for creating such a molecule, while it is possible to utilize the industrial intermediate of fluorinated oil to synthesize 2.
An organofluorine compound also shows chemical and thermal stability, derived from the nature of a C-F bond.Although highly polarized, the C-F bond gains stability from the resultant electrostatic attraction between C and F atoms [6].Thus, because of their high thermal and chemical stability and conductivity, perfluorinated sulfonic acid ionomers are often used in polymer electrolyte membranes (PEMs) of fuel cells [7],.Their performance is, however, still not enough.
To achieve much higher conductivity, it is necessary to increase the number of sulfonyl groups per unit.On the other hand, PEMs should have a certain mechanical strength to assemble membrane electrode assemblies.In the event that conventional monosulfonated monomers are increasingly used to raise this number, the copolymer obtained will have many branches, and the mechanical strength of the PEM from the copolymer decreases.A bifunctional monomer is expected to overcome this dilemma.However, there has not been a feasible synthetic method to realize such a molecule.
On the other hand, we have reported an entirely new synthetic method for perfluorinated molecules, the PERFECT (PERFluorination of an Esterified Compound then Thermolysis) process [8], which makes it possible to create new fluorinated compounds because it utilizes organic synthesis in hydrocarbon molecules.For example, a nonfluorinated primary alcohol can be converted to the corresponding perfluorinated acyl fluoride (Scheme 1).PFPE lubricants for hard disk drive (HDD) [9] Afluid ® , PFPE surfactants [10] and perfluoroalkanesulfonyl fluorides for ion-exchange membranes [11] have all been synthesized by employing the PERFECT process.Thus, it was considered to be possible to synthesize the molecules mentioned above by utilizing this process.Scheme 1.The PERFECT process.
As for the PFPE derivative 3, poly(ethylene glycol) monomethyl ether ( 5) is employed as a starting material in Scheme 1.The target, perfluorobis(alkanesulfonyl) monomer 4, is considered to be synthesized by utilizing the PERFECT process according to the retrosynthesis shown in Scheme 2. Scheme 2. Retrosynthetic analysis of perfluorobis(alkanesulfonyl) monomer.

General
All boiling points were not corrected.IR spectra were recorded on a Nicolet Impact 410 spectrometer.NMR spectra were obtained on a JEOL AL-300 or EX-400 (tetramethylsilane as internal standard for 1 H and 13 C, and trichlorofluoromethane for 19 F).High resolution mass spectra were obtained on a JEOL SX-102A coupled to a HP 5890 with a 60 m capillary column, J&W DB1301.
Average molecular weight (Mn) number and average molecular weight (Mw) weight were measured by gel permeation chromatography (GPC).A solvent mixture of R225 (Dichloropentafluoropropane, ASAHIKLIN ® AK-225G, available from Asahi Glass Co., Ltd.) and hexafluoroisopropyl alcohol (HFIP) (AK-225G/HFIP=99/1 volume ratio) was used as a mobile phase.As the column for analysis, one having two PL-gel MIXED-E (Polymer Laboratories Ltd.) connected in series was used.As a detector, an evaporation light scattering detector (ELSD, Shimazu Co.) was used, and GPC was measured at a column temperature of 37 °C at a mobile phase flow rate of 1.0 mL/min.
As standard samples for measuring the molecular weight, five types of perfluoropolyethers having molecular weight distributions (Mw/Mn) of less than 1.1 and different molecular weights between 1,300 and 10,000.
Elemental fluorine was generated by Fluorodec TM 30, Fluoro Gas (UK).Elemental fluorine is a highly toxic and corrosive gas, and may cause an explosion on contact with organic compounds in the vapor-phase.Extreme care must be taken when handling!Hydrogen fluoride (bp.19.5 °C), which evolved in both the liquid and vapor phases during the reaction, are also highly corrosive and cause severe burns on skin contact.Care must be taken!Prior to use, all hydrocarbon greases must be removed and apparatus gradually passivated with elemental fluorine.
1,1,2-trichlorotrifluoroethane (R113: CCl 2 FCClF 2 ) was used as a solvent in the fluorination reaction on a small scale.Although the use of R113 is regulated, we give experimental examples with it for convenience, because it is still much more cheaply available (Aldrich) than other solvents.Care must be taken in order to avoid environment emission!Poly(ethylene glycol) monomethyl ether (5, Uniox M-400: CH 3 O-(CH 2 CH 2 O) n -H average Mn 400.NOF Co.) was employed as the starting alcohol for the PFPE derivative 3. Other reagents were obtained from Kanto Chemicals (Japan).Commercially obtained materials were used as received unless otherwise noted.All reactions sensitive to oxygen and/or moisture were conducted under nitrogen atmosphere with magnetic stirring.For large scale column chromatography, a Biotage Flash 150M pre-packed column (KP-Sil silica, 15 cm ID × 30 cm long) was used.

Typical Procedure for the Anti-staining Coating Material
2.2.1.Synthesis of the Anti-staining Coating Material (Scheme 3) (7) Prior to use, all glass flasks were oven dried at 120 °C.A mixture of 5 (25.0 g, 62.5 mmol), R225 (20.0 g, 98.5 mmol), NaF (1.20 g, 28.6 mmol) and pyridine (1.60 g, 20.2 mmol) was vigorously stirred in a flask at under 10 °C.Subsequently, perfluoro(2,5-dimethyl-3,6-dioxanonanoyl) fluoride (6, 46.6 g, 93.5 mmol) was added to the flask over a period of 3.0 h while maintaining the internal temperature to be no higher than 5 °C.After completion of the addition, the mixture was stirred at 50 °C for 12 h and at room temperature for 24 h.The crude liquid was filtered under reduced pressure and dried for 12 h in a vacuum drier at 50 °C.The crude liquid was then dissolved in R225 (100 mL) and washed three times with a saturated sodium hydrogen carbonate aqueous solution (1,000 mL).To the organic phase, magnesium sulfate (1.0 g) was added, followed by stirring for 12 h, and removed by filtration under pressure.From the recovered liquid, R225 was evaporated to obtain 7 (56.1 g, 62.0 mmol, 99.0%, average value of n: 7.3) which was liquid at room temperature; Prior to use, all hydrocarbon greases must be removed and the apparatus must be gradually passivated with elemental fluorine.Into a 3,000 mL hastelloy autoclave, R113 (1,560 g, 8.32 mol) was stirred at 25 °C.At a gas outlet of the autoclave, a condenser held at 20 °C, a NaF pellet-packed layer and a condenser held at −20 °C were connected in series.Further, a liquid-returning line was installed to return a liquid condensed by the condenser held at −20 °C to the autoclave.
The molecular weight (Mn) of obtained 9 was 1718, and the molecular weight distribution was 1.11.
In a 100 mL round-bottomed flask, a mixture of 9 (33.1 g, 31.6 mmol) and 3-trimethoxysilylpropylamine (10, 5.80 g, 32.3 mmol) were stirred at room temperature for 2 h.Then, unreacted 10 and by-product methanol were distilled off under reduced pressure to obtain 3 (32.3g, 27.0 mmol) which was liquid at room temperature.From the results of the NMR analysis of the compound, it was confirmed that 98.0 mol% of -CF 2 C(O)OCH 3  The molecular weight (Mn) of 3 was 1072, and the molecular weight distribution was 1.33.

Evaluation of Anti-staining Performance
After synthesis, 3 was carried out to assess anti-staining performance compared with a conventional anti-staining material C 6 F 13 C 2 H 4 Si(OCH 3 ) 3 (1a).Both 3 and 1a were applied on glass by dip coating in a dilute solution, as described below.
Glass substrate was cleaned in acetone with ultrasound for 10 minutes and dried at 60 °C for 10 min.It was dipped in a 0.05% solution of the material at room temperature for 10 min.ASAHIKLIN ® AC6000 (C 6 F 13 C 2 H 5, available from Asahi Glass Co., Ltd.) was used as a dilute solvent.Finally, the glass was cured at room temperature under a relative humidity between 40% and 60% for 24 h.After coating, contact angle, friction coefficient and the abrasion resistance were evaluated.
An abrasion test on the surface of a coating film of an article with a flannel cloth was carried out according to Japanese Industrial Standards JIS L0849 under conditions of a load of 1 kg and abrasion times of 1,000 reciprocations.The water contact angle was measured after various intervals to obtain the relationship with abrasion cycles.

Synthesis and Evaluation of the Anti-staining Coating Material
On the other hand, the PERFECT method enables it by employing a partially-fluorinated compound as the substrate, synthesized from a non-fluorinated alcohol [9].According to the typical PERFECT procedure [8], firstly, poly(ethylene glycol) monomethyl ether (5) was reacted with a perfluoroacyl fluoride 6 to obtain partially-fluorinated ester 7. Next, perfluorination was achieved by liquid-phase direct fluorination with elemental fluorine to give the perfluorinated ester 8. Injection of a diluted solution of benzene after substrate addition was effective for complete perfluorination, because benzene reacts with elemental fluorine to generate many fresh fluorine radicals.Instead of the thermal elimination in the typical PERFECT procedure, methanol was added to perfluorinated ester 8 to provide the desired methyl ester 9 of the PFPE and the methyl ester of the starting perfluoroacyl fluoride 6, which was removed from the mixture by distillation.Finally, 9 was treated with 10 to afford the desired 3. The overall yield from the starting material 5 was 83%.
Thus, the target PFPE derivative 3 for an anti-staining coating material was successfully synthesized by employing the PERFECT process as a key step.

Evaluation of Water and Oil Repellency
Water and n-hexadecane contact angles and friction coefficient are summarized in Table 1.As can be seen in Table 1, 3 showed higher water and oil (n-hexadecane) contact angles and lower friction coefficient than the conventional anti-staining material 1a.This is because 3 has more fluorine contents in a molecule and a higher degree of molecular mobility, due more to an ether bond than 1a.Higher hydrophobic and oleophobic properties will give a better anti-staining performance.Moreover, a lower friction coefficient predicts that 3 will have both better stay-clean and easy-to-clean characteristics.The effect of the abrasion resistance is also a key parameter when considering the use of 3 coating on surfaces subjected to physical wear.An abrasion test of the treated surface is a method commonly used to predict the effective life of the surface modification.The results are shown in Figure 1.As can be seen from Figure 1, there was a significant difference in the resistance to abrasion as measured by water contact angle.After 200 cycles the conventional 1a material had a drastic reduction in water contact angle, while 3 maintained high contact angle even after 1,000 cycles.It is considered that both longer molecular chain and lower friction coefficient of 3 contribute to this higher abrasion property.
The overall yield of 4 was as low as 2%, the net result of several low yield steps.The single lowest yield was the fluorination step.C-S bond disconnection occurred as in the case of the fluorination of other C-S bond containing substrates [11].The typical yields of the fluorination of the substrates with one C-S bond have been 60-70%.The substrate 16, however, has two C-S bonds so that there is a high chance of the disconnection compared with substrates with one C-S bond.The introduction of sulfur into fluorinated molecules should be reconsidered.
The yields of the steps after the fluorination were also low.They could be improved after optimizing the reaction conditions.

Conclusions
Applying the PERFECT process, a perfluoropoly(ethylene glycol) derivative has been synthesized, and proved to be a surface treating agent, which is excellent in water-and-oil repellency and efficiency for the removal of oil-and-fat stains.An industrial application to stain-proof glass is now being studied.Furthermore, the monomer having two sulfonyl groups has been synthesized by utilizing the PERFECT process.Polymerization with it and application to PEMs of fuel cells are items to be studied.The polymer made from the monomer is expected to show good performance and enhanced durability when used for PEMs of electropower systems.
Thus, the PERFECT process provides the synthetic method of entirely new functional fluorinated molecules for leading-edge industries.

3. 1 . 1 .
Synthesis of a PFPE which Possesses -(CF 2 CF 2 O)-as a Repeating Unit (Scheme 3) It has so far been quite limited to synthesize a perfluorinated poly(ethylene glycol) structure.It has only been achieved by liquid-phase direct fluorination.Lagow et al. reported this synthesis by utilizing liquid-phase direct fluorination with elemental fluorine

Table 1 .
Contact angle and friction coefficient of coating glass.