Synthesis, Characterization, and Unusual Surface Activity of a Series

A series of water-dispersible, surface-active poly(fluorinated oxetane)s was prepared by ring-opening polymerization of fluorinated oxetane monomers u...
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Langmuir 2002, 18, 5933-5938

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Synthesis, Characterization, and Unusual Surface Activity of a Series of Novel Architecture, Water-Dispersible Poly(fluorooxetane)s Charles M. Kausch, Jane E. Leising, Robert E. Medsker,* Vernon M. Russell, and Richard R. Thomas* OMNOVA Solutions, Inc., 2990 Gilchrist Road, Akron, Ohio 44305-4489

Aslam A. Malik Aerojet Fine Chemicals, P.O. Box 1718, Rancho Cordova, California 95714 Received February 22, 2002. In Final Form: May 8, 2002 A series of water-dispersible, surface-active poly(fluorinated oxetane)s was prepared by ring-opening polymerization of fluorinated oxetane monomers using Lewis acid catalysis. The fluorinated oxetane monomers are made by phase-transfer catalytic reaction of a fluorinated alcohol with 3-bromomethyl3-methyloxetane. Water dispersibility was introduced by conversion of the diol-terminated R,ω-(dihydroxy)poly(fluorinated oxetanes) into diammonium salts of R,ω-sulfate esters. The poly(fluorinated oxetane) salts exhibit unusually low surface tensions for materials based on a pendant trifluoro- or pentafluoroalkyl group. At a critical micelle concentration of ∼10-5 mol/L (∼10-3 wt %), surface tensions of ∼20-30 mN/m are obtained. The novel architecture of the poly(fluorinated oxetane) salts is thought to be responsible for the anomalous surface activity.

Introduction Fluorosurfactants have found widespread use in many aqueous coatings as wetting, flow, and leveling agents. Typically, fluorosurfactants are relatively small molecules, possessing a long perfluoroalkyl group as the hydrophobe and an ionic or nonionic alkoxylate group as the hydrophile. In most cases, the perfluoroalkyl (Rf) group is of the generic formula F(CF2)n(CH2)m-. The value of n is maximized to ∼8, and m ) 0-2. The length of the perfluoroalkyl group (value of n) is chosen to produce a material with the lowest practical surface tension for this type of molecule. For wetting and repellency applications, low surface tension is necessary.1 However, there is mounting evidence concerning possible persistence, bioaccumulation, and/or toxicity (PBT) of these types of fluorosurfactants in the environment. Recent findings suggest that these types of fluorosurfactants are bioaccumulative in many types of flora and fauna.2,3 Largely, this is due to the length of the perfluoroalkyl chain. With a value of n ≈ 8, these molecules are relatively lipophilic and concentrate in certain cell structures such as fatty tissue and blood serum. In addition, small molecules are of concern generally due to their bioavailability. In an effort to prepare suitable fluorosurfactants having lower PBT concerns, a search was begun for candidates that are capable of reducing surface tension effectively and efficiently without the need for long Rf groups. The focus of the synthetic program was to design and synthesize molecules that appear nearly opposite in architecture to fluorosurfactants used currently in commerce. The synthetic protocol chosen focuses on polymeric fluorosurfactants with short-chain perfluoroalkyl groups (n * To whom correspondence should be addressed. (1) Thomas, R. R. In Fluoropolymers. 2. Properties; Hougham, G., Cassidy, P. E., Johns, K., Davidson, T., Eds.; Kluwer/Plenum: New York, 1999; Chapter 4. (2) Giesy, J. P.; Kannan, K. Environ. Sci. Technol. 2001, 35, 1339. (3) Giesy, J. P.; Kannan, K.; Jones, P. D. Sci. World 2001, 1, 627.

e 2). There are examples in the literature discussing the effect of architecture on a variety of surfactant properties such as surface tension effectiveness and efficiency and critical micelle concentration using structured surfaceactive molecules of the bolaamphiphilic4,5 and/or gemini types.6-8 Bolaamphiphilic surface-active agents are characterized by two hydrophiles, often in R,ω positions on the molecule, joined by a single hydrophobe. Gemini surfactant types can be described as a “twin” combination of a hydrophobe and hydrophile pair. Typically, this type of surface-active molecule would have a matching number of hydrophilic and hydrophobic groups, often with two of each in a given molecule. The fluorinated poly(oxetanes) described here represent a perturbation on the bolaamphiphilic and gemini surfactant theme. The architecture of the fluorinated poly(oxetanes) can be described as a polyether backbone with Rf groups emanating as tines off of the main polymer backbone. In this report, the synthesis and characterization of a novel series of short-chain Rf-fluorinated, surface-active, water-dispersible poly(oxetanes) will be demonstrated and discussed. Surface activity was confirmed by measurement of surface tension isotherms in aqueous methanol solution. Experimental Section Materials and Instrumentation. Water was distilled and deionized prior to use. 2,2,2-Trifluoroethanol was purchased from Halocarbon Products, Inc. (River Edge, NJ). Boron trifluoride tetrahydrofurate was purchased from Honeywell, Inc. (Morristown, NJ). 3-Bromomethyl-3-methyloxetane was obtained from Chemada (Nir Itzhak D. N. HaNegev, Israel). Tetrabutylammonium bromide (TBAB), dichloromethane, neopentylglycol, pyridine, sulfamic acid, trifluoroacetic anhydride, and sodium (4) Muzzalupo, R.; Ranieri, G. A.; La Mesa, C. Langmuir 1996, 12, 3157. (5) Wang, X.; Shen, Y.; Pan, Y.; Liang, Y. Langmuir 2001, 17, 3162. (6) Luchetti, L.; Mancini, G. Langmuir 2000, 16, 161. (7) Rosen, M. J. CHEMTECH 1993, 23, 30. (8) Song, L. D.; Rosen, M. Langmuir 1996, 12, 1149.

10.1021/la020196b CCC: $22.00 © 2002 American Chemical Society Published on Web 06/27/2002

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bicarbonate were purchased from Aldrich Chemical Co. (Milwaukee, WI). 2,2,3,3,3-Pentafluoropropanol was provided graciously by Daikin Industries Ltd. (Osaka, Japan). Trifluoromethyl benzene was purchased from Occidental Petroleum Co. (Los Angeles, CA). The telomer alcohol Fluowet EA-812-AC was obtained from Clariant (Muttenz, Switzerland). This alcohol was approximately 50-60 wt % HOCH2CH2(CF2)8F according to company literature and analyzed at 54 wt % with the remainder being of a distribution of other perfluoroalkyl chain lengths. All chemicals and reagents were used as received unless specified otherwise. NMR spectroscopy was performed using a Varian Unity 400 spectrometer with a probe frequency of 399.945, 100.575, and 376.282 MHz for 1H, 13C, and 19F observation, respectively. Chemical shifts were reported against an internal standard of tetramethylsilane for 1H and 13C and hexafluorobenzene for 19F (-162.1 ppm vs σ CFCl3 ) 0). 13C NMR spectra were acquired with gated 1H decoupling. Infrared spectroscopy was performed using a Bio-Rad Laboratories FTS 175 FTIR spectrometer. Combustion analysis was performed by Galbraith Laboratories, Inc. Water content was determined by Karl Fisher titration using a Mettler Toledo DL38 titrator. GPC was performed on a Waters system with AM GPC gel columns (1000 and 500 Å, from American Polymer Standards Corp., Mentor, OH) using Viscotek DV model 100 differential viscometric and Knauer DRI detectors. Mn and Mw/Mn were determined as averages from three different samples. Molecular weights reported are from a universal calibration curve using polystyrene standards corrected for intrinsic viscosities. Synthesis of Ammonium Salts of Sulfate Esters of Rf Alcohols. Using a modified literature procedure,9 a 500 mL, three-necked flask with paddle stirrer, thermometer, and inert gas inlet and outlet was charged with 25.0 g (0.25 mol) of 2,2,2trifluoroethanol, 40.0 g (0.50 mol) of pyridine, and 49.0 g (0.50 mol) of sulfamic acid. The flask was heated to 115 °C with inert gas purging for 2 h to yield a creamy, white solution. After the reaction mass was cooled to room temperature, 94.6 (0.75 mol) of 28 wt % aqueous ammonium hydroxide was added. The reaction was allowed to stir for 15 min, after which time a precipitate was formed and filtered. The product, an ammonium salt of monosulfate ester of trifluoroethanol (ammonium 2,2,2-trifluoroethoxy sulfate), was isolated in 77% yield based on 2,2,2-trifluoroethanol. The product can be purified by recrystallization from ethanol. The other derivatives were prepared in similar fashion. The products analyzed satisfactorily by spectroscopic methods. Synthesis of 3-Methyl-3-[(2,2,2-trifluoroethoxy)methyl]oxetane (1a) and 3-Methyl-3-[(2,2,3,3,3-pentafluoropropoxy)methyl]oxetane (1b). A three-necked, 10 L jacketed vessel with heater/chiller bath, thermometer, stir bar, condenser, addition funnel, and inert gas inlet and outlet was preheated to 85 °C. An aqueous solution (12.2 wt %, 1.97 kg, 0.751 mol) of the phase-transfer catalyst, TBAB, was added to the reaction vessel and allowed to stir until catalyst was dissolved. 3-Bromomethyl3-methyloxetane (5.04 kg, 30.6 mol) and 2,2,2-trifluoroethanol (3.0 kg, 30.0 mol) were added to the vessel and purged with nitrogen. The addition funnel was charged with a 45% aqueous solution of KOH (4.11 kg, 32.967 mol). The KOH solution was added to the vessel as fast as possible without allowing the reaction exotherm to increase the solution temperature >100 °C. The reaction was allowed to proceed for 4 h. Water (∼30 g) was added when the solution temperature has cooled to 20-60 °C. Bromophenol blue indicator can be added to enhance the visibility of the phase split. The two phases separate in approximately 2 h, after which time the aqueous layer was separated and discarded. A total of 5.84 kg of crude monomer was isolated. Heptane (600 g) was added to the reaction mass, and the organic phase was isolated by azeotropic vacuum distillation (82-86 °C, ∼28 mmHg). The product was isolated as a clear, low-viscosity liquid in 90% yield. The purity of monomer 1a was determined by 1H NMR spectroscopy. Monomer 1b was prepared in identical fashion using 2,2,3,3,3-pentafluoropropanol at 90% yield. Polymerization of monomers 1a and 1b is sensitive to water. Water content was determined by Karl Fisher titration. If the water level is too high (>0.20%), the monomer can be distilled at 82-86 °C, ∼28 mmHg. (9) Burwell, R. L., Jr. J. Am. Chem. Soc. 1949, 71, 1769.

Kausch et al. For 1a: 1H NMR (CDCl3) δ 1.32 (-CH3, s, 3H), 3.69 (-CH2O-, s, 2H), 3.88 (-O-CH2-, q, J19F-1H ) 8.0 Hz, 2H), 4.42 (ring -CH2-, AB q, J1H-1H ) 5.6 Hz, 4H); 13C NMR (CDCl3) δ 20.7 (-CH3, s), 68.6 (-O-CH2-, q, J19F-13C ) 33.6 Hz), 77.6 (-CH2O, s), 79.4 (ring -CH2-, s), 124.0 (-CF3, q, J19F-13C ) 279 Hz). For 1b: 1H NMR (CDCl3) δ 1.32 (-CH3, s, 3H), 3.69 (-CH2O-, s 2H), 3.96 (-O-CH2-, t of q, 2H, J19F-1H ) 13.2, 1.2 Hz), 4.43 (ring -CH2-, AB q, 4H, J1H-1H ) 5.6 Hz); 13C NMR (CDCl3) δ 20.8 (-CH3), 39.9 (backbone -C-), 68.0 (-O-CH2-, t, J19F-13C ) 26.8 Hz), 78.1 (-CH2-O-), 79.5 (ring -CH2-), 113.3 (-CF2-, t of q, J19F-13C ) 36.4 and 256 Hz), 118.7 (-CF3, q of t, J19F-13C ) 34.2 and 286 Hz). Synthesis of r,ω-(Dihydroxy)poly(3-methyl-3-[(2,2,2-trifluoroethoxy)methyl]oxetane) (2a) and r,ω-(Dihydroxy)poly(3-methyl-3-[(2,2,3,3,3-pentafluoropropoxy)methyl]oxetane) (2b). A three-necked, 10 L jacketed vessel with heater/ chiller bath, thermometer, stir bar, condenser, addition funnel, and inert gas inlet and outlet was charged with dried neopentyl glycol (329.92 g, 3.17 mol), BF3‚THF (177.26 g, 1.27 mol) catalyst, and CH2Cl2 (1.86 kg, 21.84 mol) solvent cooled at 25-30 °C. The neopentyl glycol was dried by dissolution in toluene and removal of solvent under reduced pressure. The initiator and catalyst solution was allowed to stir for 30 min at room temperature under a positive pressure nitrogen purge. Monomer 2a (3.50 kg, 19.01 mol) was then added to the catalyst/initiator solution at a rate of ∼50 g/min using a pump while the reaction temperature was maintained at 35 ( 10 °C. The reaction was allowed to stir for 2 h. Extra CH2Cl2 was added (2.8 kg, 32.97 mol). Residual BF3‚THF was removed by washing with 2.5 wt % sodium bicarbonate and a water rinse at 40 °C. Solvent was then removed under reduced pressure at 80 °C. Polymer 2a was obtained as a clear, viscous liquid in 95+% yield. The degree of polymerization was determined using 1H NMR spectroscopic end group analysis and found to be 7 ( 0.1. The 1H NMR methodology involved conversion of the R,ω-dihydroxy groups on the respective polymer to trifluoroacetate derivatives by reaction with trifluoroacetic anhydride. The methylene groups of the R,ω-dihydroxy units have 1H NMR resonances that overlap substantially with the methylene moiety of the pendant fluorinated groups (e.g., -OCH2(CF2)nF, n ) 1 for 2a or 2 for 2b) at ∼3.8 ppm (see Supporting Information). Conversion of the R,ω-dihydroxy groups to trifluoroacetate derivatives shifts the resonance of the methylene groups downfield to ∼4.3 ppm. Degree of polymerization is calculated by integration of the two methylene signals using the formula, degree of polymerization ) 2I-OCH2(CF2)nF/I-CH2OC(dO)CF3, where I is the integrated signal of the respective methylene group and the subscripts refer to the pendant fluorinated and terminal trifluoroacetate groups, respectively. There is some NMR spectroscopic evidence for the presence of the neopentyl glycol initiator fragment occupying a terminal position in the polymer; however, trifluoroacetic anhydride derivatization could not differentiate easily between methylene groups arising from an oxetane monomer or the neopentyl glycol initiator. By gel permeation chromatographic analysis, Mn ) 1700 Da and Mw/Mn ) 1.18. Polymer 2b was prepared similarly in 95+% yield, Mn ) 1900 Da and Mw/Mn ) 1.95. For 2a: 1H NMR (CDCl3) δ 0.86-0.95 (-CH3, 3H), 3.20 (backbone -CH2-, 4H), 3.43-3.48 (-CH2O-, 2H), 3.72-3.81 (-OCH2-, 2H); 13C NMR (CDCl3) δ 17.1-17.3 (-CH3), 41.041.4 (backbone -C-), 69.0 (-OCH2-, q, J19F-13C ) 33.6 Hz), 75.375.5 (ring -CH2-), 76.0 (-CH2O-), 124.1 (-CF3, q, J19F-13C ) 281 Hz). For 2b: 1H NMR (CDCl3) δ 0.85-0.95 (-CH3, 3H), 3.20 (backbone -CH2-, 4H), 3.43-3.44 (-CH2O-, 2H), 3.81-3.93 (-OCH2-, 2H); 13C NMR (CDCl3) δ 17.0-17.3 (-CH3), 41.041.4 (backbone -C-), 68.2 (-OCH2-, d of t, J19F-13C ) 5.6 and 24 Hz), 75.5 (-CH2O-), 73.8-74.1 (backbone -CH2-), 113.2 (-CF2-, t of q, J19F-13C ) 37.4 and 255 Hz), 118.8 (-CF3, q of t, J19F-13C ) 33.3 and 287 Hz). Synthesis of r,ω-(Diammonium disulfato)poly(3-methyl3-[(2,2,2-trifluoroethoxy)methyl]oxetane) (4a) and r,ω(Diammonium disulfato)poly(3-methyl-3-[(2,2,3,3,3-pentafluoropropoxy)methyl]oxetane (4b). A 10 L jacketed reactor with heating and cooling capability was charged with R,ω-(dihydroxy)poly(3-methyl-3-[(2,2,2-trifluoroethoxy)methyl]oxetane) diol (2a) (3.52 kg, 2.46 mol) and 200 g of THF. The solution temperature was lowered to 0 °C. Fuming sulfuric acid

Water-Dispersible Poly(fluorooxetane)s (854.7 g, 9.11 mol) was then added to the flask. At this stage, the free acid form of R,ω-(dihyrogen disulfato)poly(3-methyl-3-[(2,2,2trifluoroethoxy)methyl]oxetane) (3a) is formed. The reaction was followed by end-group analysis, performed by 1H NMR spectroscopy and ammonium hydroxide titration to a bromothymol blue endpoint. Once conversion exceeded 90%, the acid end-groups and excess acid were neutralized by addition to 25.2 wt % aqueous ammonium hydroxide (708.2 g, 5.09 mol). The solution pH was followed by pH paper or meter to a pH of 7-8. The solution was allowed to stir at 0 °C for 2 h to allow for product formation. Excess salts were removed by vacuum filtration. Solvent was removed by rotary evaporation to yield a clear, viscous oil. The synthesis of 4b was conducted identically using 2b. Yields were typically >95% for 4a and 4b. The degree of polymerization was checked by 1H NMR spectroscopy and found to be unchanged from the respective macrodiol. For 4a: 1H NMR (acetone-d6) δ 0.8-1 (-CH3, 3H), 3.14-3.36 (backbone -CH2-, 2H), 3.52-3.61 (-CH2-O-, 2H), 3.91-4.05 (-O-CH2-, 2H); 13C NMR (acetone-d6) δ 17.2-17.8 (-CH3), 41.2-42.4 (backbone -CH2-), 75.4-76.2 (-CH2-O-), 69.4 (-OCH2-, q, J19F-13C ) 33 Hz), 125.5 (-CF3, q, J19F-13C ) 279 Hz); 19F NMR (acetone-d6) δ -72.5 (-CF3, m); IR (neat) -OH str 3458 cm-1, NH4+ str 3065 cm-1, NH4+ bend 1458 cm-1, -CF3 str 11001370 cm-1, -CH2-O-CH2 str 1065-1162 cm-1; Mn ≈ 2675 g/mol and Mw/Mn ≈ 1.26. Anal. Calcd for C54H95N2O22F21S2: C, 40.1; H, 6.0; N, 1.76; O, 22.2; F, 25.1; S, 4.04. Found: C, 35.2; H, 7.2; F, 17.3; S, 4.1. For 4b: 1H NMR (acetone-d6) δ 0.89-0.99 (-CH3, 3H), 3.20 (backbone -CH2-, 4H), 3.54-3.59 (-CH2O-, 2H), 4.00-4.12 (-OCH2-, 2H); 13C NMR (CDCl3) δ 17.2-17.5 (-CH3), 41.442.1 (backbone -C-), 68.6 (-OCH2-, t, J19F-13C ) 26.5 Hz), 74.5 (-CH2O-), 73.8-74.1 (backbone -CH2-), 114.4 (-CF2-, t of q, J19F-13C ) 36 and 252 Hz), 118.4 (-CF3, q of t, J19F-13C ) 35 and 286 Hz); 19F NMR (acetone-d6) δ -81.8 (-CF3), -121.5 (-CF2-); Mn ≈ 3492 g/mol and Mw/Mn ≈ 1.16. Anal. Calcd for C61H95N2O22F35S2: C, 37.8; H, 5.0; N, 1.4; O, 22.2; F, 34.3; S, 3.3. Found: C, 29.6; H, 7.1; N, 1.76; F, 21.1; S, 2.7. Surface Tension Isotherms. Solutions of 4a and 4b were prepared in deionized, distilled water using an aqueous pH 8 buffer (Aldrich, pHydrion)/methanol (83.3/16.7, v/v) mixture. Surface tensions were measured by bubble tensiometry using The Tracker from ThetaDyne Instruments Corp. at 21 ( 0.2 °C. The surface tension was evaluated directly from the bubble profile by solution of the Laplace equation. A bubble was formed, and the surface was allowed to relax to equilibrium, after which time the surface tension was recorded for isotherm data. The surface tension of the aqueous methanol stock solution was determined to be 53.7 ( 0.2 mN/m. The measured surface tensions had a precision (0.5 mN/m.

Results and Discussion Synthesis and Characterization of Surface-Active, Fluorinated Poly(oxetane) Salts. The study of poly(oxetanes), unlike their close relatives poly(oxiranes), is an area of polymer chemistry remaining largely unexplored.10-20 Compared to poly(oxiranes), poly(oxetanes) have found little practical utility. Several reports have appeared in the literature that detail studies of fluorinated (10) Bednarek, M.; Kubisa, P.; Penczek, S. Macromolecules 2001, 34, 5112. (11) Cowling, S. J.; Toyne, K. J.; Goodby, J. W. J. Mater. Chem. 2001, 11, 1590. (12) Dreyfuss, P.; Dreyfuss, M. P. Polym. J. 1976, 8, 81. (13) Dreyfuss, M. P.; Dreyfuss, P. In Encyclopedia of Polymer Science and Engineering; Kroschwitz, J., Ed.; Wiley: New York, 1987; Vol. 10, p 653. (14) Kim, Y.; Lee, J.; Ji, Q.; McGrath, J. E. Polym. Mater. Sci. Eng. 2001, 84, 349. (15) Li, C. J. Solid Rocket Technol. 1997, 20, 44. (16) Magnusson, H.; Malmstro¨m, E.; Hult, A. Macromolecules 2001, 34, 5786. (17) Malik, A. A.; Archibald, T. G. U.S. Patent 5,807,977, 1998. (18) Malik, A. A.; Archibald, T. G. U.S. Patent 6,037,483, 2000. (19) Rose, J. B. J. Chem. Soc. 1956, 542. (20) Takeuchi, D.; Aida, T. Macromolecules 1996, 29, 8096.

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Figure 1. Synthesis of fluorinated oxetane monomers.

Figure 2. Polymerization of fluorinated oxetane monomers.

oxetane monomers21 and polymers17,18 and the use of fluorinated R,ω-(dihydroxy)poly(oxetanes) as monomers in urethane-type polymers.14 One of the main commercial uses of oxetane polymers is as a rocket fuel binder.15 To our knowledge, there have been no reports on the preparation of water-dispersible poly(oxetanes) or the conversion to surface-active materials. The oxetane monomer used in the current study was prepared by reaction of a short-chain fluorinated alcohol such as 2,2,2-trifluoroethanol or 2,2,3,3,3-pentafluoropropanol with 3-bromomethyl-3-methyloxetane and KOH. This is shown schematically in Figure 1. Using tetrabutylammonium bromide (TBAB) as the phase-transfer catalyst, typical reaction yields for 3-methyl-3-[(2,2,2trifluoroethoxy)methyl]oxetane (1a) and 3-methyl-3[(2,2,3,3,3-pentafluoropropoxy)methyl]oxetane (1b) are >90%. The products 1a and 1b are clear, slightly viscous oils and have been characterized satisfactorily by 1H and 13C NMR spectroscopy. Spectra for 1b are similar, except for additional 19F and 13C NMR signals due to the longer perfluoroalkyl group with first- and second-order J19F-13C coupling. Cationic ring-opening polymerization of oxetane monomers was performed at 40 ( 5 °C in methylene chloride solvent using an initiating system composed of neopentyl glycol and boron trifluoride tetrahydrofurate (BF3‚THF). Repeat units of the cationic ring-opening polymerization products of the fluorooxetane monomers, R,ω-(dihydroxy)poly(3-methyl-3-[(2,2,2-trifluoroethoxy)methyl]oxetane) (2a) and R,ω-(dihydroxy)poly(3-methyl-3-[(2,2,3,3,3-pentafluoropropoxy)methyl]oxetane) (2b), are shown in Figure 2. Note that each polymer unit contains a single neopentyl glycol initiator fragment. The neopentyl glycol initiator fragment can be distributed statistically along the main polymer chain. Other glycols, such as 1,4-butanediol, could be used as initiators, but it was discovered that neopentyl glycol eliminated a polymerization induction period observed with a number of linear diols. The elimination of the induction period allows for better molecular weight control of the polymer. Polymerization of fluorooxetane monomers proceeds exothermically, producing quantitative yields of clear viscous oils. The viscosity of the poly(oxetane) is determined by the degree of polymerization. The degree of polymerization can be controlled readily by (21) Brey, W. S.; Brey, M. L. J. Fluorine Chem. 2000, 102, 219.

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Figure 3. Preparation of R,ω-(dihydrogen disulfato)poly(fluorooxetane)s.

the mole ratio of monomer-to-initiator. Increasing the ratio leads to a higher degree of polymerization. Water dispersibility is determined largely by the hydrophilelipophile balance of the molecule. Greater water dispersibility is achieved by either having a smaller degree of polymerization and/or increasing the number of hydrophilic groups (in essence, increasing the charge-to-volume ratio); however, as the degree of polymerization gets smaller, there is a practical limit statistically on the number of hydrophilic groups that can be introduced. Free radical preparation of (meth)acrylate polymers with small degrees of polymerization is difficult and requires generally the addition of substantial amounts of chain-transfer agents. These chain-transfer agents then become a large mole fraction component of polymers with small degrees of polymerization. The chain-transfer agent fragment can then alter the hydrophile-lipophile balance of the polymer in an undesirable way. For this particular study and to increase the water dispersibility of the final product, vide infra, the degree of polymerization (based on fluorinated oxetane monomer) chosen was 7 ( 0.1 for both 2a and 2b. The degree of polymerization was determined through end-group analysis using 1H NMR spectroscopy. Values of Mn (Mw/Mn) were determined by gel permeation chromatography to be 1700 Da (1.18) and 1900 Da (1.95) for 2a and 2b, respectively. Values of Mn are within the expected molecular weight range for a degree of polymerization of 7. Spectra for 1b are similar except for additional 19F and 13 C NMR signals due to the longer perfluoroalkyl group with first- and second-order J19F-13C coupling. To make 2a and 2b water dispersible and increase surface activity, the terminal hydroxyl groups are first converted to the sulfate esters using fuming sulfuric acid with THF as solvent.22 The synthesis of R,ω-(dihydrogen disulfato)poly(3-methyl-3-[(2,2,2-trifluoroethoxy)methyl]oxetane) (3a) and R,ω-(dihydrogen disulfato)poly(3-methyl-3-[(2,2,3,3,3-pentafluoropropoxy)methyl]oxetane) (3b) is shown in Figure 3. Polymers 3a and 3b are formed in high yield as clear, viscous liquids that are not isolated but are converted, vide infra, directly to the diammonium salts. Molecular weight analysis of the sulfate esters using NMR spectroscopy showed no change in the average degree of polymerization, which was interpreted as no detectable main chain degradation. The final products, R,ω-(diammonium disulfato)poly(3-methyl-3-[(2,2,2-trifluoroethoxy)methyl]oxetane) (4a) and R,ω-(diammonium disulfato)poly(3-methyl-3[(2,2,3,3,3-pentafluoropropoxy)methyl]oxetane) (4b), were prepared quantitatively by addition of a THF solution of 3a and 3b to an aqueous solution of ammonium hydroxide. (22) Sandler, S. R.; Karo, W. In Organic Chemistry; Wasserman, H. H., Ed.; Academic Press: New York, 1986; Vol. 12, Chapter III.

Kausch et al.

Figure 4. Preparation of R,ω-(diammonium disulfato)poly(fluorooxetane)s.

Figure 5. 1H NMR spectrum of R,ω-(diammonium disulfato)poly(3-methyl-3-[(2,2,2-trifluoroethoxy)methyl]oxetane) (4a) in acetone-d6. Peak marked by an asterisk are due to solvent. Poly(THF) at 1.6 and 3.4 ppm. Residual THF solvent at 1.8 and 3.65 ppm.

Figure 6. 13C NMR spectrum of R,ω-(diammonium disulfato)poly(3-methyl-3-[(2,2,2-trifluoroethoxy)methyl]oxetane) (4a) in acetone-d6. Peak marked by an asterisk are due to solvent.

This reaction scheme is shown in Figure 4. The salts 4a and 4b were found to be unstable at pH < 8 and elevated temperatures and to decompose rapidly to starting diols 2a and 2b, respectively, via the free acid form of the R,ω(disulfato) esters 3a and 3b, respectively. A sodium bicarbonate solution is added at 1.5% (on a dry weight basis) to the mixture during solvent stripping in vacuo (45-48 °C, 28 in. Hg) to maintain pH > 8. Again, the products are clear, viscous oils. Polymers 4a and 4b analyzed satisfactorily by 1H and 13C NMR spectroscopy and infrared spectroscopy. 1H and 13C NMR spectra for 4a are shown in Figures 5 and 6, respectively. Attempts to determine molecular weights and polydispersities by gel permeation chromatography in THF proved unsatisfactory. Values of Mn determined by gel permeation chromatography were ∼70 and 80% higher than expected for 4a and 4b, respectively, based on atomic composition for a degree of polymerization of 7, determined by NMR spectroscopic end-group analysis. This is due, undoubt-

Water-Dispersible Poly(fluorooxetane)s

Figure 7. Surface tension isotherm for (9) R,ω-(diammonium disulfato)poly(3-methyl-3-[(2,2,2-trifluoroethoxy)methyl]oxetane) (4a) and R,ω-(diammonium disulfato)poly(3-methyl-3[(2,2,3,3,3-pentafluoropropoxy)methyl]oxetane) (4b) (b) in a water (pH 8 buffered)/methanol (83.3/16.7, v/v) mixture.

edly, to aggregation of the ionomers in THF solvent23 and is a subject for further study. By NMR, the degree of polymerization was unchanged from that of the respective macrodiol. Spectra for 1b are similar except for additional 19 F and 13C NMR signals due to the longer perfluoroalkyl group with first- and second-order J19F-13C coupling. The infrared spectrum of 4a is characterized by a broad adsorption at ∼3460 cm-1 due to -OH stretching, and NH4+ stretching and bending at ∼3065 and ∼1458 cm-1, respectively. The spectrum contains a number of bands at ∼990-1370 cm-1. In this region are the expected stretching frequencies of C-F, ether, and ROSO3- groups that are difficult to assign unequivocally. The infrared spectrum of 4b is nearly identical to that of 4a. Surface Activity. Surface activities of polymers 4a and 4b were confirmed by surface tension isotherm measurements in water (pH 8 buffered)/methanol (83.3/ 16.7, v/v). At the high end of the concentration range examined, the solutions exhibit the Tyndall effect due to scattering from aggregates or micelles. Solutions near or below the critical micelle concentration (cmc) are clear. Shown in Figure 7 are the data for the salts R,ω(diammonium disulfato)poly(3-methyl-3-[(2,2,2-trifluoroethoxy)methyl]oxetane) (4a) and R,ω-(diammonium disulfato)poly(3-methyl-3-[(2,2,3,3,3-pentafluoropropoxy)methyl]oxetane) (4b) in aqueous pH 8 buffered methanol solution (83.3/16.7, v/v). Both effectiveness and efficiency are unusual, considering the short perfluoroalkyl (-CF3, -CF2CF3) group and the polymeric nature of the species. For typical small-molecule fluorosurfactants, the Rf group is chosen to be -(CF2)∼8F, resulting in a minimum surface tension in the range 15-20 mN/m for aqueous solutions.24 For example, F(CF2)∼8SO2N(C2H5)CH2CO2-K+ (3 M FC-129) gave a surface tension of 17.9 mN/m (at cmc ≈ 2 × 10-4 mol/L) in the pH 8 buffered methanol solution. Molecules with short Rf groups would not be expected to be particularly surface active and yield a surface tension at saturation in the range 30-60 mN/m depending on the number of -CF2- units per Rf chain. The data in Figure 7 show a relatively sharp cmc ≈ 2.3 × 10-5 mol/L (∼3 × 10-3 wt %) for 4a and ∼9.5 × 10-5 mol/L (∼1 × 10-2 wt %) for 4b, indicating the formation of micelles or aggregates at concentrations greater than the cmc. This observation was confirmed visually by an increase in solution turbidity when the concentration exceeded the cmc determined from the surface tension isotherm data. (23) Holliday, L. In Ionic Polymers; Holliday, L., Ed.; Applied Science Publishers, Ltd.: London, 1975. (24) Kissa, E. Fluorinated Surfactants-Synthesis, Properties and Applications; Surfactant Series 50; Marcel Dekker: New York, 1994.

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The sharpness of the cmc is unusual for polymeric-type surfactants and is due probably to the relatively small polydispersity of ∼1.2 for poly(oxetane)s 4a and 4b. Another unusual feature is the low surface tension obtained at the cmc, ∼29.4 and 27.4 mN/m for 4a for 4b, respectively. This surface tension value is low compared to those of other, short Rf group-containing materials. For example, the simple ammonium salts of sulfate esters of 2,2,2-trifluoroethanol, 2,2,3,3,3-pentafluoropropanol, and j ≈ 8) were a telomer Rf alcohol (HOCH2CH2(CF2)nF, with n prepared,9 and their surface tensions were examined in water, yielding 60.4 ( 4, 53.7 ( 4, and 23.7 ( 0.3 mN/m at 0.1 wt %, respectively. Comparing these values to surface tension values for the fluorinated poly(oxetane) salts 4a (27.3 mN/m at 3.5 × 10-4 mol/L ≈ 10-2 wt %) and 4b (24.2 mN/m at 5.7 × 10-4 mol/L ≈ 10-2 wt %), a clear advantage in surface activity can be seen as a result of preparing surfactants with an architecture different from that of typical small-molecule fluorosurfactants. A possible explanation for the unusual effectiveness and efficiency is afforded by examination of the architecture of 4a and 4b. There are examples in the literature detailing “anomalous” surface tension effectiveness and efficiency behavior of structured surfactants of the bolaamphiphilic or gemini type.7,8 The unusual behavior of these types of surfactants is thought due to increased packing efficiency at the solvent/air interface. While poly(oxetanes) surfactants, such as 4a and 4b, defy classification into either the bolaamphiphile or gemini type, there are structural similarities in the fact that two hydrophiles and more than two hydrophobic groups are present. Poly(oxetanes) 4a and 4b would be better categorized as an amphiphilic “brush” due to the number of pendant Rf groups emanating as tines from the main polymer backbone. An estimate can be made of molecular area demand by examination of the linear premicellar portion of the surface tension isotherm data, shown in Figure 7, using the Gibbs adsorption equation (Γ ) -(1/nRT)(dγ/d ln c), where Γ is surface excess, γ is surface tension, c is concentration, and n indicates the type of surfactant and is chosen to be 1 in this case).25 Analysis of the data reveals a surface excess of (3.84 ( 0.3) × 10-6 and (3.80 ( 0.6) × 10-6 mol/m2, corresponding to a surface molecular area demand of 43 ( 7 and 44 ( 7 Å2/molecule for 4a and 4b, respectively. The same analysis of the surface tension isotherm for the small-molecule, single Rf chain compound, F(CF2)8SO2N(C2H5)CH2CO2-K+, yielded a surface molecular area demand of 36 ( 2 Å2/molecule. In the extended chain configuration, the end-to-end distances of 4a and 4b are estimated at 42 Å, with a molecular area demand of ∼404 and 582 Å2/molecule, respectively.26 Typical crosssectional areas for Rf groups are ∼30 Å2/molecule,27-29 suggesting that two or three of the Rf groups present with 4a and 4b are organized at the solvent/air interface. Clearly, the small molecular area demand accounts for the anomalous surface tension effectiveness and efficiency observed for molecules 4a and 4b. (25) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley: New York, 1990; Chapter III. (26) The molecular geometry for 4a was determined by using the semiempirical PM3 force field from the application Alchemy 2000 (Tripos Associates). Molecular area demand was estimated by constructing a plane through the Rf groups and projecting that onto a surface. (27) Bernett, M. K.; Zisman, W. A. J. Phys. Chem. 1963, 67, 1534. (28) Hann, R. A. In Langmuir-Blodgett Films; Roberts, G., Ed.; Plenum: New York, 1990; Chapter 2. (29) Acero, A. A.; Li, M.; Lin, B.; Rice, S. A.; Goldmann, M.; Azouz, I. B.; Goudot, A.; Rondelez, F. J. Phys. Chem. 1993, 99, 7214.

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Considering the size of 4a and 4b, this is a small surface molecular area demand and indicates that the molecule is probably distorted substantially from its extended chain configuation at the solvent/air interface by placing both ionic groups into the solvent phase and extending several Rf groups along a loop into the air phase. In essence, there is surface activity synergism that results from placing an ensemble of short Rf chains acting in concert on a polymeric backbone. Currently, surface activity of molecules of the type 4a and 4b, along with other derivatives, is being investigated under a variety of conditions, including the measurement of interfacial rheology. This work will be the subject of a separate report. Conclusions The synthesis of fluorinated oxetane monomers, polymers, and modified polymers has been described. Surface activity in aqueous methanol solution is observed upon conversion of an R,ω-(dihydroxy)poly(fluorinated oxetane) to the diammonium salt of the R,ω-(dihydrogen disulfato)poly(fluorinated oxetane). The perfluoroalkyl groups chosen were -CH2CF3 and -CH2CF2CF3. These surfaceactive poly(oxetane) ionomers have a novel architecture that differs from that of simple and gemini surfactants and bolaamphiphiles and are christened “cristataamphiphiles” due to the “comb” formed by the pendant perfluoroalkyl groups. These molecules give unusually low surface tensions (∼22-28 mN/m) at low critical micelle

Kausch et al.

concentrations (∼10-5 mol/L) in pH 8 buffered aqueous methanol solution. The surface tension measured is approaching that observed typically with much longer perfluoroalkyl chain, small-molecule surfactants. It is thought that there is surface activity synergism through the ensemble of short perfluoroalkyl groups on a flexible polymer backbone. These novel R,ω-(diammonium disulfato)poly(fluorinated oxetane)s have demonstrated useful attributes as wetting, flow, and leveling agents in a variety of applications once limited exclusively to longer perfluoroalkyl chain fluorosurfactants.30,31 Acknowledgment. The authors thank Drs. Ray Weinert, Dann Woodland, and Gary Jialenella for many helpful discussions. The authors acknowledge gratefully suggestions made by the reviewers. Supporting Information Available: 1H and 13C NMR spectra of monomers (1a and 1b) and polymers (2a, 2b, 4a, and 4b) and IR spectra of polymers 4a and 4b. This material is available free of charge via the Internet at http://pubs.acs.org. LA020196B (30) Thomas, R. R.; Medsker, R. E.; Kausch, C. M.; Woodland, D.; Jialenella, G.; Weinert, R.; Leising, J. E.; Robbins, J. U.S. Patent Pending, 2001. (31) The diammonium salts of R,ω-(disulfato)poly(3-methyl-3[2,2,2-trifluoroethoxy)methyl]oxetane) (4a) and R,ω-(disulfato)poly(3methyl-3-[2,2,3,3,3-pentafluoropropoxy)methyl]oxetane (4b) are available commercially as PolyFox TM and PolyFox VM, respectively, from the Performance Chemicals division of OMNOVA Solutions, Inc. (Chester, SC).