Synthesis of 3,3,3-trifluoropropene telomers and their modification into fluorosurfactants

Abstract

Original fluorinated surfactants based on 3,3,3-trifluoropropene (TFP) as alternatives to perfluorooctanoic acid (PFOA) were synthesized in three to five straightforward steps in good overall yields. First, the radical or thermal telomerization of TFP in the presence of perfluoroisopropyl iodide as the chain transfer agent led to various TFP telomers of different molecular weights. They were further chemically modified into various ionic and non-ionic surfactants. To obtain anionic surfactants, TFP telomers bearing iodine end-group was converted into allylic group on which thioglycolic acid was added under a radical or photochemical process. Non-ionic surfactants were obtained by esterification of the anionic acid with poly(ethylene glycol)monomethylether. Cationic surfactants were obtained by ethylene end-capping of the TFP telomers followed by a nucleophilic substitution by either triethylamine or pyridine. All surfactants showed good inertness to bases and acids, and satisfactory surface properties. They exhibit interesting critical micellar concentration values comparable to that of PFOA (0.06, 4.10, and 3.20 versus 3.00 g L⁻¹).

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Introduction

Fluorinated polymers exhibit exceptional properties leading to high tech applications.¹ Most of these specialty polymers are prepared by radical (co)polymerization of fluorinated monomers in aqueous media (suspension or emulsion polymerizations).^1–3 However, these hydrophobic monomers require the use of a surfactant (preferentially fluorinated) to favor the incorporation of these unsaturated derivatives into the micelles, and to enable the synthesis of fluoropolymers. It is well known that most efficient surfactants involved in such processes are amphiphilic,⁴ and are composed of a polar head and one or more perfluorinated chains,^5,6 mainly C₇F₁₅ or C₈F₁₇,^7–12 such as perfluorooctanoic acid C₇F₁₅CO₂H (PFOA) or the perfluorooctane sulfonic acid, C₈F₁₇SO₃H (PFOS). These fluorosurfactants are involved in more than 200 applications including detergents, firefighting foams, coatings, cosmetics, emulsifiers, adhesives, electroplating, antifogging, and antistatic agents…¹³ However, recent studies directed by the U.S. Environmental Protection Agency (EPA)¹⁴ showed that these surfactants are bioaccumulable^13,15,16 (due to the perfluorinated chain that is too stable, for example perfluorooctane sulfonic acid's half-life is ca. 3.26 years in human blood^17,18), toxic, and persistent,^19,20 and that decreasing their production by as much as 95% was scheduled by the end of 2010, followed by definitive stop of their production by 2015. Numerous analyses^13,21 across the planet have demonstrated that these compounds are still present in many rivers,²² seas,²³ and even on the ice cap²¹ (moreover, traces were found in the livers of polar bears), and as an example, according to a German study, the Bavarian Environmental Agency¹⁵ has shown that both these perfluorinated surfactants can be found at concentrations of 22.3 mg L⁻¹ (for PFOS) and 6.8 mg L⁻¹ (for PFOA) in the plasma of the studied population.

At that point, the main companies (Arkema, Asahi Glass, Ciba, Clariant, Daikin, 3M/Dyneon, Dupont and Solvay Solexis) involved in the production of derivatives and fluorinated polymers have been joining efforts since 2005 in a consortium called the “Stewardship Program”²⁴ that aims to reflect the future, and to seek for new alternatives to PFOA and PFOS. Indeed, these PFOA or PFOS have been synthesized from tetrafluoroethylene (TFE) telomers.^1,8 Although short perfluorinated chains (C4)²⁵ or oligo(hexafluoropropylene oxide)^26–29-based surfactants have already led to original non-bioaccumulable alternatives to PFOA, it was of interest to find new derivatives.

The purpose of this article is to deal with the synthesis of cationic, anionic and non-ionic surfactants by simple and efficient modifications of fluorinated telomers obtained by iodine transfer polymerization. The concept is illustrated using the radical telomerization of 3,3,3-trifluoropropene and the chemical modifications of the resulting TFP telomers in 2–4 steps to obtain the various types of surfactants.

The synthesis of 3,3,3-trifluoropropene (TFP) was pioneered by Haszeldine⁴³ in the early 1950s from dehydroiodination of CF₃–CH₂–CH₂I and is now commercially available (from Great Lakes-Chemtura now DuPont). TFP has been used in telomerization with several telogens (Table 1) using various type of initiation: redox, UV, γ-radiation, thermal, or peroxides. Results show that perfluoroalkyl iodides R_FI are the best telogens.¹

Table 1 Telomerization of 3,3,3-trifluoropropene (TFP) with various telogens^a

Telogen	Initiation	Product obtained	Ref.
a BPO: dibenzoyl peroxide, DTBP: di-tert-butyl peroxide.
H3COCOC(CH3)2H	Peroxides	H3COCOC(CH3)2, [CH2CH(CF3)]nH	30
(CH3)2C(OH)H	γ-rays (T < 90 °C)	(CH3)2C(OH), [CH2CH(CF3)]nH (n = 1, 2)	31–34
C6H5CH2–Cl	Fe(CO)5	C6H5CHX [CH2CH(CF3)]nY (n = 1, 2; X = Cl, H; Y = H, Cl)	35
CCl4	Fe(CO)5	Cl3C[CH2CH(CF3)]nCl (n = 1–3)	36
CCl4	CuCl2/ICl	Cl3C[CH2CH(CF3)]nCl (n = 1–7)	37
C6H5CH2–Br	Fe(CO)5	C6H5CHX [CH2CH(CF3)]nY (n = 1, 2; X = Br, H; Y = H, Br)	38,39
Br2CH–Br	Fe(CO)5	Br2CH[CH2CH(CF3)]nBr (n = 1–3)	40
		CF3CBrHCH2CBr2[CH2CH(CF3)]2H
Br2CH–Br	Peroxides	XCBr2[CH2CH(CF3)]nY (Y = X = H or Br; n = 1, 2)	40
Br2CH–Br	Fe(CO)5	BrCH2[CH2CH(CF3)]nBr (n = 1, 2)	40
CBr4	BPO	Br3C[CH2CH(CF3)]nBr (n = 1–3)	41
CF3I	UV, 5 days	CF3[CH2CH(CF3)]nI (n = 1, 2)	42
CF3I	225 °C, 36 h	CF3[CH2CH(CF3)]nI (n = 1–3)	42
CF3I	UV, various T	Normal and reverse monoadducts, few amounts of diadducts	43
C6F13I	Δ, UV, Fe^3+/benzoin or peroxides	C6F13[CH2CH(CH3)]nI (n = 1, 2)	44
(CF3)2CFI	DTBP	(CF3)2CF[CH2CH(CH3)]nI (n = 1, 2)	44
Cl3SiH	UV	Cl3Si[CH2CH(CF3)]nH (n = 1, 2)	45
(C2H5O)2P(O)H	DTBP, 130 °C	(C2H5O)2P(O)(TFP)nH (n = 1–5)	46–48
THF	DTBP, 140 °C	Monoadduct (48%)	49
2-Me-1,3-dioxolane	Fe(CO)5	2,4-bis(TFP)-2-Me-dioxolane	50
CH3SSCH3	UV	CH3SCH2CH(CF3)SCH3	51
HBr	UV	CF3CH2CH2Br	42
cyclopentadiene	180 °C, 72 h	exo : endo = 62 : 38	52

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Experimental section

Materials

All reagents were used as received unless stated.

3,3,3-Trifluoropropene (CH₂=CH–CF₃, TFP) and 2-iodoperfluoropropane ((CF₃)₂CF–I), and perfluorooctanoic acid (C₇F₁₅–COOH, PFOA) were kindly supplied by Great Lakes Chemical Corporation, now Chemtura (West Lafayette, USA). 2-Iodoperfluoropropane was treated with sodium thiosulfate and then distilled prior to use to remove impurities and molecular iodine. 1,1,1,3,3-Pentafluorobutane (Solkane® 365mfc, CF₃–CH₂–CF₂–CH₃) was kindly supplied by Solvay S.A. (Tavaux, France). tert-Butyl peroxypivalate in solution of isododecane (Trigonox® 25-C75, tBuOOC(O)tBu, TBPPi) (purity 75 wt.%), di-tert-butyl peroxide (Trigonox® B, DTBP) (purity 99%), and azobisisobutyronitrile (AIBN) (purity 99%) were gifts from Akzo Nobel (Châlons-sur-Marne, France). Ethylene (CH₂=CH₂, E) was purchased from Air Liquide (France).

Allyl acetate (CH₂=CH–CH₂–O–CO–CH₃), triethylamine (N(C₂H₅)₃), pyridine, zinc dust, polyethylene glycol monomethylether (n = 13), thioglycolic acid (HSCH₂CO₂H), magnesium sulfate (MgSO₄), hydrochloric acid, and sodium thiosulfate (Na₂S₂O₃) were purchased from Sigma-Aldrich (Saint Quentin-Fallavier, France). Tert-butanol, 2-butanone, dichloromethane, pentane, all of analytical grade, were purchased from SDS (France). Deuterated acetone d₆ was purchased from Euroiso-top (Grenoble, France) (purity >99.8%).

Characterization

Nuclear magnetic resonance (NMR). The NMR spectra were recorded on Bruker AC 400 instruments, using deuterated acetone, as the solvent and tetramethylsilane (TMS) (or CFCl₃) as the references for ¹H (or ¹⁹F) nuclei. Coupling constants and chemical shifts are given in hertz (Hz) and part per million (ppm), respectively. The experimental conditions for recording ¹H [or ¹⁹F] NMR spectra were as follows: flip angle 90° [or 30°], acquisition time 4.5 s [or 0.7 s], pulse delay 2 s [or 2 s], number of scans 128 [or 512], and a pulse width of 5 μs for ¹⁹F NMR. All the intermediates and surfactants synthesized were characterized by ¹H, ¹⁹F and ¹³C NMR spectroscopy.

Surface tension. The surface tension of the various surfactants was measured at different concentrations to assess the critical micellar concentration (CMC) of the surfactant in water at 25 °C. The surface tension was determined with a Spectra-Physics Tensiometer DCAT21 using the Wihlemy plate method. Each sample was prepared at least 24 h prior to measurement. The solution was allowed to equilibrate in the apparatus and the surface tension was considered stable when the difference was less than 0.03 mN m⁻¹. The value for the surface tension was assessed as the average of the last 50 points. The CMC was calculated as the intersection between the two straight lines emerging from high and low concentrations.

Synthesis

Telomerization of TFP with R_FI. The preparation of TFP telomers is the first step common to all 3 surfactants. In a 160 or 300 mL Hastelloy Parr autoclave equipped with a manometer, a magnetic stirrer, and safety valve (3000 psi), R_FI and the initiator (di-tert-butyl peroxide) (in amounts depending on initial molar ratios R₀ = [Telogen]₀/[TFP]₀ and C₀ = [Initiator]₀/[TFP]₀) were introduced and purged with argon for 15 min. The reactor was closed and checked for the leaks at 30 bar of nitrogen. The reactor was placed in acetone/liquid nitrogen to cool the content. Then, 5–7 nitrogen/vacuum cycles were applied to remove oxygen from the liquid. The required amount of TFP, depending on the targeted R₀ was condensed in the autoclave. The autoclave was heated up to 150 °C and the telomerization was carried out for 4 h. After reaching a maximum pressure of 15–35 bars (according to the amount of TFP), a sharp decrease in the pressure during the first hour and then only small pressure changes were observed. After the reaction stopped, the autoclave was cooled to room temperature and placed in an ice bath. The unreacted monomer was expelled by purging and the conversion of telomerization was determined by double weighing (ca. 90–95%). The crude product was analyzed by GC and then washed with a saturated solution of Na₂S₂O₅ in aqueous NaOH to remove the iodine produced. The reaction mixture was distilled to separate the various telomeric adducts.

The adducts were already characterized by Kostov et al.⁴⁴

Diadduct: (CF₃)₂CF(C₃H₃F₃)₂-I; 2-iodo-2H,3H,3H,4H,5H,5H-4,6-bis(trifluoromethyl)perfluoroheptane (normal adduct), 1-iodo-1H,1H,2H,3H,4H,4H-2,3,5-tris(trifluoromethyl) perfluorohexane (reverse adduct). b.p. = 68–71 °C/20 mm Hg.

¹⁹F NMR (CDCl₃): −69.6, −70.9, −71.5, −72.9 (assigned to CF₃ of both TFP (normal and reverse adducts), 6 F); −77.3, −78.5, −78.8 (t, (CF₃)₂, 6 F); −185.5, −187.2 (dt, C(F), 1 F).

¹H NMR (CDCl₃): 1.9–2.3 (m, *C–CH₂–C*, 2 H); 2.3–2.7 (m, R_F–CH₂, 2 H); 2.6–3.2 (m, *CH(CF₃)), 25% of one diastereoisomer overlapping with R_F–CH₂protons and 75% in the range of 2.75–3.2; negligible reverse adduct (absence of signal at 3.5, 1 H)); 4.15–4.45 (m, *CH(CF₃)I, 1 H).

¹³C NMR (CDCl₃): 130.8 (130.6), 128.0 (127.8), 125.2 (125.0), 122.4 (122.3) (q of d (coupling of two diastereoisomers), CF₃ of TFP between two CH₂groups in normal adduct, ¹J_CF = 279.7, 1 C); 128.4 (128.16), 125.66 (125.42), 122.92 (122.68), 120.18 (119.94) (q of d, CF₃ adjacent to I in normal adduct, ¹J_CF = 276.7, 1 C); 125.04 (124.94), 122.27 (122.10), 119.39 (119.26), 116.55 (116.43), 124.79 (124.66), 121.95 (121.82), 119.11 (118.99), 116.27 (116.15) (two q of d, CF₃, ¹J_CF = 285.7, ²J_CF = 28.17, 6 C); 92.63–89.02 (d of sept, C(F), ¹J_CF = 207.3, ²J_CF = 32.2, 1 C); 38.10, 37.8, 37.6, 37.3, 37.0, 36.7 (sext, C*H(CF₃) of TFP between two CH₂groups, ²J_CF = 27.17, 1 C); 35.56, 33.29 (d, CH₂ of TFP-I, 1 C); 27.70, 27.51, 27.29, 27.10 (two d, CH₂, ²J_CF = 19.1, 1 C); 19.53–18.55, 17.23–16.30 (d of q (tr/er), *CH(CF₃)I, ²J_CF = 29.2, 1 C).

Preparation of surfactant A.
Synthesis of R_F(C₃H₃F₃)_nCH₂CHICH₂OC(O)CH₃. The TFP telomer (n = 1 or 2, 0.18 mol) and a 1.2-fold excess of allyl acetate (0.215 mol) were introduced to a 250-ml three-neck round-bottom flask equipped with a double condenser and a thermometer. Then the mixture was heated up to 82 °C and was stirred. When the temperature reached 80 °C, AIBN (C₀ = 0.015–0.050) was introduced stepwise over the 10 h reaction time. No exotherm was observed. After 10 h, the reaction was stopped; the crude product was cooled to room temperature, filtered and analyzed by GC. The reaction mixture was distilled to purify the iodoacetate (yield = 75–80%).

(CF₃)₂CF(C₃H₃F₃)₂CH₂CH(I)CH₂OCOCH₃; 8,9,9,9-tetrafluoro-2-iodo-4,6,8-tris(trifluoromethyl)nonyl acetate. b.p. = 78–80 °C/0.1 mm Hg

¹⁹F NMR (CDCl₃): no signal at −67 (no reverse product); −70.5 to −72.77 (m, 2 × CF₃ of TFP, 6 F); −77.2 to −78.75 (m, (CF₃)₂, 6 F); −186.4, −187.14, −187.73 (t, C(F), 1 F)

¹H NMR (CDCl₃): 1.6–1.9 (m, *C–CH₂–*C, 2 H); 2.02–2.25 (CH₃OCO, 3 H); 2.25–2.4 (CH₂–CHI, 2 H); 2.3–2.5 (R_F–CH₂, 2 H); 2.6–3.0 (*CH(CF₃), 1 H); 4.2 (m, 1 H of CH(I) + 1 H of *CH(CF₃)); 4.4 (m, ester CH₂OCOCH₃, 2 H).

Synthesis of R_F(C₃H₃F₃)_nCH₂CH=CH₂. Zn dust (2 equivalents about iodoacetate) was activated with 1.2 g of a mixture of acetic acid/acetic anhydride (1 : 1). Then, methanol (40 mL) was added, and the mixture was introduced into 250-ml two-neck round-bottom flask with a reverse condenser and a magnetic stirrer. The temperature was increased to 65 °C while stirring. The iodoacetate (0.12 mol) from the previous step dissolved in methanol (30 mL) was added dropwise within 3 h at reflux to a zinc slurry with vigorous stirring. Upon complete addition, the stirring was maintained for additional 2 h and a colorless product was obtained. The zinc complex was filtered off, the filtrate was diluted with dichloromethane (1 : 1 vol./vol.), washed with 10% hydrochloric acid aqueous solution (100 ml) and washed again with distilled water. The organic phase was added dropwise to anhydrous magnesium sulfate under stirring to eliminate traces of water, filtered and twice distilled (quantitative conversion, yield after distillation 75%). The product obtained is a colorless liquid.

(CF₃)₂CFCH₂CH(CF₃)CH₂CH(CF₃)CH₂CH=CH₂; 8,9,9,9-tetrafluoro-4,6,8-tris(trifluoromethyl)non-1-ene. b.p. = 62–65 °C/20 mmHg.

¹⁹F NMR (CDCl₃): −71.7, −72.2, −72.5 (t, 2 × CF₃ in TFP, 6 F); −77.7 to −78.9 (dt, (CF₃)₂, 6 F); −186.9, −187.4 (m, C(F), 1 F).

¹H NMR (CDCl₃): 1.55–2.2 (m, CH₂ from *C–CH₂–C*, 2 H); 2.0–2.2 (m, allylCH₂, 2 H); 2.3–2.5 (m, 2 H of (CF₃)₂CF–CH₂ + 25% of diastereoisomer *CH(CF₃) from the side of (CF₃)₂CF–); 2.9 (m, 1 H of *CH(CF₃) from the side of the allyl); 5.20 (m, =CH₂, 2 H); 5.7 (m, –CH=, 1 H).

¹³C NMR (CDCl₃): 133.21 (132.89) (d, CH₂=CH-, 1 C); 131.76 (131.61), 128.98 (128.83), 126.20 (126.05), 123.43 (123.28) (q, CF₃ of TFP on the side of (CF₃)₂CF–, ¹J_CF = 279.7, 1 C); 131.18 (131.00), 128.40 (128.22), 125.62 (125.45), 122.84 (122.67) (q, CF₃ of TFP adjacent to allyl, ¹J_CF = 279.7, 1 C); 125.10 (125.06), 124.82 (124.79), 122.26 (122.23), 121.98 (121.95), 116.59 (116.55), 116.30 (116.26) (two qd, (CF₃)₂, ¹J_CF = 285.7, ²J_CF = 28.2, 2 C); 188.29 (118.24) (d, -CH=CH₂, 1 C); 92.74–88.77 (d of sept, (CF₃)₂CF, ¹J_CF = 206.25, ²J_CF = 32.2, 1 C); 40.37–39.38 (qn, *CH(CF₃) adjacent to (CF₃)₂CFCH₂–, ²J_CF = 25.15, 1 C); 35.85–34.68 (qn of d, *CH(CF₃) adjacent to allyl, ²J_CF = 27.16, 1 C); 32.78–32.75 (d, CH₂ adjacent to vinyl, 1 C); 32.10, 32.08, 31.91 (t, CH₂ between 2*C, 1 C); 28.94, 27.70 (two d, CH₂-CF(CF₃)₂, ²J_CF = 19.12, 1 C).

Synthesis of R_F(C₃H₃F₃)_n(CH₂)₃SCH₂COOH. The allylic TFP telomer (0.05 mol) and thioglycolic acid (0.06 mol) in acetonitrile (50 mL) were introduced to a 250-ml three-neck round-bottom flask equipped with a condenser and a thermometer. Then the mixture was heated up to 75 °C while stirring. At that temperature, tert-butyl peroxypivalate was introduced (3 mol% with respect to allylic TFP telomer). After 3 h, the reaction was stopped. The crude product was cooled to room temperature, and the solvent rotavaped under vacuum. R_F(TFP)₂(CH₂)₃SCH₂COOH was obtained in 73% yield. The same procedure was also carried out at room temperature without any radical initiator but photoinitiated (P = 50 W, wide spectral range of wavelength of the UV lamp located at 15 cm from the reaction medium). The obtained yield was slightly higher (81%) than that noted above.
Synthesis of R_F(C₃H₃F₃)_n(CH₂)₃SCH₂COO(CH₂CH₂O)_nCH₃. In a 100 mL two-neck round-bottom flask equipped with a Dean–Stark apparatus and a condenser, polyethylene glycol monomethylether (HO(CH₂CH₂O)₁₃CH₃) (17.6 × 10⁻³ mol) was dissolved in toluene (50 mL) at 140 °C, and the solution of the previous product (17.6 × 10⁻³ mol) was added. Methane sulfonic acid (CH₃SO₃H) was used as the catalyst (5 mol% with respect to PEGMe). Water generated by the esterification was eliminated by azeoptropic distillation, and the progress of the reaction was monitored by FTIR (vanishing of the OH band at 3500 cm⁻¹). After evaporating the toluene under vacuum, surfactant A was obtained in 74% yield. NMR (Fig. 1 and 2); FTIR (Fig. 3).

Fig. 1 ¹⁹F NMR spectrum of R_F-TFP-I telomer and its derivatives (A, B, and C surfactants) recorded in CDCl₃.

Fig. 2 ¹H NMR spectrum of (CF₃)₂CF(C₃H₃F₃)_n(CH₂)₃SCH₂COO(CH₂CH₂O)_nCH₃Surfactant A recorded in CDCl₃.

Fig. 3 FTIR spectra during the esterification reaction of (CF₃)₂CF(C₃H₃F₃)_n(CH₂)₃SCH₂COOH with PEO–OH leading to (CF₃)₂CF(C₃H₃F₃)_n(CH₂)₃SCH₂COO(CH₂CH₂O)_nCH₃ after 3 h (upper spectrum) and 8 h (lower spectrum).

Preparation of surfactants B and C.
Synthesis of R_F(C₃H₃F₃)_n(CH₂)₂I. A 160 or 300 mL Hastelloy Paar autoclave equipped with a manometer, a magnetic stirrer, and safety valve (3000 psi), was pressurized with 30 bars of nitrogen to check for leaks. The autoclave was then conditioned for the reaction with several nitrogen/vacuum cycles (10⁻² mbar) to remove any trace of oxygen. TFP telomer, tert-butanol, and tert-buyl peroxypivalate (TBPPi 0.1 mol % with respect to ethylene) were then introduced in the autoclave. Ethylene was introduced by double weighing (1.2 : 1.0 compared to TFP telomer). The ethylenation was carried out at 75 °C for 6 h. A drop of pressure was observed. After the reaction stopped, the autoclave was cooled and placed in an ice bath to purge the unreacted ethylene. After distillation, the ethylenation yielded 75–80% of iC₃F₇(TFP)₂CH₂CH₂I telomer as a colourless liquid.

iC₃F₇(TFP)₂CH₂CH₂I b.p. = 44–46 °C/0.2 mm Hg

¹H NMR (acetone d₆, ppm): 3.6–3.4 (m, central –CH₂C*H(CF₃)–, 1H); 3.4–3.1(m, –CH₂CH₂I, 2H); 2.8–2.7 (m, –CH₂C*H(CF₃)–, 1H); 2.3–2.1, centre 2.2, (m, HA, HB in AB system R_F–CH₂C*H–, 2H), –(C*H–CH₂C*H–, 2H) and (–CH₂CH₂I, 2H).

¹⁹F NMR (acetone d₆, ppm; Fig. 1) δ: −69.2 and −71.6 [2 × CF₃ in –CH₂C*H(CF₃), 6F]; −78.5 (2 × CF₃ end-groups, 6F); −187 (m, [double bond splayed left]CF–, 1F).

¹³C NMR (acetone d₆, ppm) d: 130.8 (130.6); 128.0 (127.8); 125.2 (125.0); 122.4 (122.3); ¹J_CF = 265 Hz; q (coupling of two diastereoisomers) 2 × 1C (CF₃ of TFP between two CH₂groups); 128.4; 125.7; 122.9; 120.1; 125.0; 122.3; 119.4; 116.3; two q of d (¹J_CF = 275.7 Hz; ²J_CF = 28.2 Hz; 2 × 1C from terminal (CF₃)₂– groups); 92.6–89.0 d of hept., 1C [C(F)], ¹J_CF = 207.3 Hz; ²J_CF = 32.2 Hz; 40.8; 40.7; 40.5; 40.3; 35.2; 34.9, 34.3; 34.1; d of q, ²J_CF = 27.2 Hz, 2×1C [C*H(CF₃) of TFP between two CH₂groups]; 32.5; 31.8, d, 1C [–C(F)–CH₂– of TFP, ²J_CF = 27.1 Hz]; 29.0; 28.9; 28.7; 28.5; q, 1C (–CH₂–CH₂I of E); 27.0; 26.8; 26.6; t, 1C (C*–CH₂–C*) ²J_CF = 19.1 Hz; 0.8; 0.0; s, 1C (–CH₂–I, 2 diastereoisomers).

Synthesis of R_F(C₃H₃F₃)_n(CH₂)₂NR⁺I⁻. (NR stands for pyridinium or triethyl ammonium.)

The ethylenated TFP telomer (0.05 mol) was introduced to a 250-ml three-necked round-bottom flask equipped with a condenser and a thermometer. Pyridine or triethylamine (0.1 mol) was added dropwise in DMF or MeOH, respectively (50 mL). Then, the mixture was purged with argon for 15 min and heated up to 40 °C (or room temperature for NEt₃) under stirring. After 10 h, the reaction was stopped. The crude product was cooled, and the solvent rotavaped under vacuum and then the product was precipitated from ether. R_F(C₃H₃F₃)_n(CH₂)₂NR⁺I⁻ (where NR represents a pyridinium or triethyl ammonium function) was obtained in 40–50% yield. ¹⁹F NMR spectra were similar to those of the ethylenated TFP telomers.

Results and discussion

To our knowledge only surfactants based on 3,3,3-trifluoropropene (TFP) and vinylidene fluoride (VDF) has been prepared⁵³ and it was worth investigating the synthesis of cationic surfactants which bear weak points which are able to be metabolized or enzymatically degraded.⁵⁴ In contrast to perfluorinated materials, hydrogenofluorinated compounds can be degraded by first, elimination of HF, yielding a double bond that can be oxidized and cleaved.²⁰ This degradation can be advantageously achieved by the methylene and methynegroup brought by each TFP unit.⁵⁴

The objective of the present article consists in synthesizing original TFP-based surfactants by simple chemical modifications of TFP telomers,^44,55,56 achieved by radical telomerization of TFP with isoperfluoropropyl iodide.⁴⁴

The overall paths for the syntheses of these surfactants is described in Scheme 1. From the radical telomerization of 3,3,3-trifluoropropene (TFP), several surfactants can be obtained (anionic, cationic and non-ionic) in satisfactory yields (50% overall yields from iC₃F₇I). The initial step is the telomerization of 3,3,3-trifluoropropene (TFP) in the presence of perfluoroalkyl iodides ((CF₃)₂CFI) that was reported in patents from Great Lakes^55,56 and by Kostov et al.⁴⁴ The telomerization in bulk as that initiated by organic peroxides such as di-tert-butylperoxide led to TFP telomers in high yields (>80%). TFPhomopolymerization was reported to lead to poly(TFP) homopolymer but the presence of (CF₃)₂CFI as an efficient chain transfer agent drastically lowered DP_n to achieve either monoadduct or diadduct or triadduct for a high amount of TFP in the feed. The degree of polymerization (DP_n = n in (CF₃)₂CF–(TFP)_n-I) could be tuned by the initial molar ratio [TFP]₀/[((CF₃)₂CFI]₀. The ¹⁹F NMR spectra (Fig. 1) enabled to assess DP_n value from the ratio between trifluoromethyl groups of TFP units and those of the perfluoroisopropylgroup. Fig. 1 exhibits the signals of the perfluoroisopropyl end-group (two CF₃ and CF centered at −77.7, −78.5, and −188.9 ppm, respectively). It also displays the signal at −72 ppm assigned to the CF₃group of TFP corresponding to the normal addition of (CF₃)₂CF• onto TFP [(CF₃)₂CF–CH₂CH(CF₃)–] while that assigned to the reverse addition [(CF₃)₂CF–CH(CF₃)CH₂–] expected at −67 ppm was never observed, indicating a selective normal addition, at least for the first adducts.

Scheme 1 Straightforward strategies for the preparation of 3,3,3-trifluoropropene-based cationic and non-ionic surfactants.

The synthesis of the non-ionic surfactants was carried out in three steps. First, the preparation of R_F–(TFP)_x–CH₂–CH=CH₂ allylic derivative was reported,⁴⁴ and consists in the direct radical addition of R_F-(TFP)_x-I onto allyl acetate (Yield = 75–80%) followed by the deiododeacetalization in the presence of zinc and methanol. As for tetrafluoroethylene telomers,⁵⁷ the yields were high showing a suitable electron-withdrawing effect of CHCF₃group to easily generate (CF₃)₂CF–CH₂–CH(CF₃)^• radical. The conversion of iodoacetate was quantitative while allylic TFP telomers were obtained 75% yield after distillation. Second, the allylic derivative was further reacted with thioglycolic acid in acetonitrile to obtain a carboxylic ω-functionalized compound (as an anionic surfactant, PFOA type). That reaction occurred either photochemically (81% yield) or in the presence of a source of radicals arising from tert-butyl peroxypivalate (73% yield). ¹H NMR enabled that reaction to be monitored by the vanishing of both allyl double bond and mercaptogroups centered at 5.0–6.5 and 1.5 ppm, respectively. ¹⁹F NMR spectrum was similar to that of the precursor. Finally, the esterification with an ω-hydroxy poly(ethylene oxide) oligomer (PEO-OH) led to the formation of an original R_F-(TFP)_x–CH₂–CH₂–CH₂–S–CH₂–CO₂–PEO non-ionic surfactant. The ¹H NMR spectrum (Fig. 2) exhibits the characteristic signals centered at 4.0, 3.4, 3.3, and 3.0 assigned to the –SCH₂CO₂– group, ethylene oxide units, methyl end-group, and CHCF₃, respectively. To ensure a complete conversion of acid into ester, the reaction was carried out in toluene and the water-toluene azeotrope was continuously distilled using a Dean–Stark apparatus at 95 °C (b.p. of water and toluene are 100 and 110 °C, respectively). The reaction was monitored by FTIR spectroscopy (Fig. 3). The vanishing of OHgroup frequency was observed at 3600 cm⁻¹ while that at 1750 cm⁻¹, assigned to the carbonyl of the ester, increased. The former frequency disappeared after 8 h reaction, meaning that the esterification was complete, and was confirmed visually by the absence of water-toluene azeotrope.

To obtain the series of cationic surfactants, first ethylene end-capping of the TFP telomers was achieved. This reaction, initiated by an organic peroxide, quantitatively led to R_F-(TFP)_x–CH₂–CH₂–I in 89% yield with a quantitative conversion of (CF₃)₂CF(TFP)_nI. This successful ethylenation was evidenced by ¹H, ¹³C and ¹⁹F NMR spectroscopy. The chemical shifts of ethylene end-groups were centered at about 29.0, 28.9, 28.7 and 28.5 ppm for –CH₂–CH₂–I, 8.0 and 0.0 for –CH₂–I on the ¹³C NMR spectrum, and 2 ppm (–CH₂–CH₂–I) in the ¹H NMR spectrum (details are given in the experimental section). The nucleophilic substitution of R_F–(TFP)_x–CH₂–CH₂–I by either triethylamine (NEt₃) or pyridine (C₅H₅N) at room temperature or at 40 °C, respectively, led to cationic surfactants bearing ammoniumgroups (Scheme 1) in satisfactory yield (>75%) with a slight dehydroiodination (Fig. 4 and 5). These mild experimental conditions avoid any dehydroiodination.

Fig. 4 ¹H NMR spectrum of (CF₃)₂CF(TFP)_n(CH₂)₂NC₅H₅⁺I⁻ (Surfactant B) recorded in acetone d₆.

Fig. 5 Expansion of the ¹H NMR spectrum (0.8 to 3.9 ppm) of (CF₃)₂CF(TFP)_n(CH₂)₂N(CH₂CH₃)₃⁺I⁻ (Surfactant C) recorded in CDCl₃.

The structures of both cationic surfactants were proved by ¹H NMR spectroscopy. In the ¹H NMR spectrum of surfactant B (Fig. 4) the group of signals present in the 8.1 to 9.6 ppm range (with an integral ratio of 1: 0.5: 1) was assigned to the pyridinium group. In the case of surfactant C, the ¹H NMR spectrum (Fig. 5) obviously does not display any signal in the 7–9 ppm range while the signal of the ethyl group of the triethylammonium are located at 1.4 (CH₃) and 3.15 ppm (CH₂).

Physicochemical properties of surfactants are often searched since they can be used under hostile conditions (e.g. oil wells, acidic buffers for emulsion polymerization of tetrafluoroethylene). These surfactants exhibit good overall inertness to acids and bases especially iC₃F₇(TFP)(CH₂)₃SCH₂CO₂(PEO)CH₃, and a satisfactory solubility in water and methanol but a poor solubility in apolar solvents, and acetone (Table 2).

Table 2 Physicochemical characteristics of A, B, and C surfactants based on 3,3,3-trifluoropropene (TFP)

Media	T/°C	Time (days)	Surfactant
			A	B	C
(1) Base-acid resistance			Weight losses (%)
98% H2SO4	25	7	0.0	<1.2	0.0
60% HNO3	25	7	-	>40.0	<20
37% HCl	25	7	0.0	>15.0	0.0
40% NaOH	25	7	0.0	0.0	0.0
(2) Solubility in selected solvents			Solubility (g/100mL)
Water	25	3	>10	>10	>10
Methanol	25	3	>10	>10	>10
Diethyl ether	25	3	<1	<1	<1
Benzene	25	3	<2	<2	<2
Acetone	25	3	<10	<10	<10
A:
B:
C:

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Surface tensions of the various surfactants were assessed at different concentrations to determine the critical micellar concentration (CMC) of the surfactants in water at 25 °C. The results are displayed in Fig. 6, and the CMC values were found to be 0.06, 4.10, 3.20, and 3.00 g L⁻¹ for surfactants A, B, C, and PFOA, respectively. It can be seen that both cationic surfactants behave like PFOA while the non ionic surfactant (A) has a higher surface tension but a lower CMC.

Fig. 6 Surface tension versus the concentration of TFP-based surfactants compared to that of PFOA.

Conclusions

Original 3,3,3-trifluoropropene telomers were synthesized under radical conditions and were modified into hydrogenofluorosurfactants by simple chemistry in high overall yields. Their physicochemical properties showed good inertness to bases and acids. Their surface tensions were assessed and show interesting CMC values similar to that of PFOA. Nonionic surfactant A should behaves as a very interesting surfactant, because its CMC is quite low (0.06 g L⁻¹). Further work on the biodegradation of these surfactants is under progress. Also surfactants based on vinylidene fluoride with alternating C–H and C–F bonds (which may act as degradation sites), and also containing both TFP and VDF units appear as excellent alternatives to perfluorinated chains. Such compounds are currently under investigation.

Acknowledgements

The authors would like to thank Great Lakes/Chemtura company (Dr S. Brandstater, and V. Sharma) for financial support.

Notes and references

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