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Polymers from 2,5-difluoroterephthalic acid. I. Polyesters
REACTIVE & FUNCTIONAL POLYMERS Reactive & Functional Polymers 30 (1996) 141-147
Polymers from 2,Sdifluoroterephthalic
acid. I. Polyesters
David V. Person, John W. Fitch *, Patrick E. Cassidy, Keiji Kono, V. Sreenivasulu Reddy Polymer Research
Group, Depurrrnent qf’Chemistry, Southwest Texas State Universig, San Marcos. TX 78666. USA Received 20 September
1995; accepted
18 December
1995
Abstract A new synthetic route to 2,.Sdifluoroterephthalic acid (DFTA) was devised and from this several new series of polymers were prepared by polycondensation. This inaugural paper will cover polyesters while subsequent papers will report on keto polyethers, polyamides and other backbones derived from DFTA. DFTA was synthesized from 2.5difluorotoluene, which was acylated and subsequently converted to the diacid by a several step oxidation process. Various polyesters were prepared by reactin g it with numerous bisphenols and diola by solution condensation to give a series of difluoroterephthalate polyesters with viscosities ranging from 0.16 to 0.61 dl/g. Thermogravimetric analyses showed that the polyesters had thermal stabilities up to 480°C in nitrogen: melting temperatures ranged from I27 to 3 18°C. Keywords:
2,SDifluoroterephthalic
acid; Crystallinity;
Fluorinated
1. Introduction
Significant efforts have been made toward the synthesis, characterization and evaluation of fluorine-containing condensation polymers. A renewed interest in fluoropolymers has led to the synthesis and characterization of condensation polymers from 25difluoroterephthalic acid (DFTA). DFTA presents a relatively new template for polymers either with aromatic fluorine or for use of the fluorine as a site for polymerization. Polyarylates make up an important class of engineering plastics by virtue of their several uses [l-2]. Unfortunately, they also present processing problems due to their high glass-transition temperatures and very poor solubilities. Several studies have been done correlating halogen substitution on terephthalic acid and its effects on * Corresponding author. 1381-5148/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved .SSDl 1381-5148(95)001 18-2
polyesters
the solubility and thermal behavior of various polyamides and polyarylates [3-71. Results from these studies indicate that the addition of at least one halogen onto terephthalic acid greatly increases a polymer’s solubility, with the greatest increase found in the perfluoro species. Our early work on fluorinated polyarylates focused on the use of an aromatic dicarboxylic acid containing the hexafluoroisopropylidene unit with bisphenol A or bisphenol AF to produce polyesters with 12, 6 or no fluorines [S]. Further, the backbone with six fluorines could have them in the acid or the diol portion. The products formed clear, colorless, tough films which had quite good mechanical properties and thermal stabilities. Perhaps most interesting was the discovery that a content of six fluorines was sufficient for maximum thermal stability provided they were properly placed (in the bisphenol portion of the repeating unit). In a study by Nagata et al. [3] overall perfor-
142
D.V Person et al. /Reactive & Functional Polymers 30 11996) 141-147
mance seemed to be maximized for polyamides derived from difluoroterephthalic acid. The difluoro species produced a marked increase in solubility and retained glass-transition temperatures lower than most analogous polyamides having various halogen substituents (F, Cl, Br) and/or degrees of substitution on the terephthalic acid. Our earlier work [9] on fluorinated poly(ether ketones) (12-F PEK) [9a] and aromatic polyethers with aliphatic (CF3) or aromatic fluorines [9a-c], led to several extensions including polymers from DFTA. High strength, toughness, good electrical properties, thermooxidative stability and solvent resistance make these polymers excellent candidates for advanced materials. This paper reports the synthesis and characterization of several aromatic polyesters prepared by polycondensation of DFTA with various diol monomers. Polyamides, keto polyethers and other backbones derived from DFTA will be reported in subsequent papers. 2. Experimental 2. I. Materials The 2,2-bis(4-hydroxyphenyl)propane (Bis A; Hoechst Aktiengesellschaft), 2,2-bis(4-hydroxyphenyl)-l,l, 1,3,3,3-hexafluoropropane (Bis AF; Hoechst Aktiengesellschaft) and 1,4-benzenedimethanol were obtained from Aldrich Chemical Co. and purified via multiple sublimations. The 4,4’-oxydiphenol (Pfaltz and Bauer) and 4,4’-sulfonyldiphenol were also obtained from Aldrich Chemical Co. and purified by repeated recrystallizations from toluene and ethanol/water, respectively, followed by single sublimations. The diol, 1,3-bis (2-hydroxy hexafluoro-2-propyl)benzene (1,3-HFAB) was obtained from Central Glass and was distilled. All aliphatic diols were obtained from the Aldrich Chemical Co. and were dried by heating the diol at reflux with a 5% molar equivalent of sodium followed by vacuum distillation onto activated sieves (4 A). The 4-dimethylaminopyridine (DMAP; Aldrich Chemical Co.) was obtained in 98% purity and stored over phosphorous pentoxide prior to use. Tetrahydrofuran (THF) was obtained anhydrous, 99%+ pure, in
a Sure/SealTMbottle from the Aldrich Chemical co. 2.2. Measurements Infra-red spectra of the monomers and polymers were obtained from a Perkin-Elmer Model 1600 spectrophotometer and were analyzed as KBr wafers. Inherent viscosities of the polymers were measured at a concentration of 0.25 g/d1 at 25°C using a Cannon-Fenske type viscometer. The polymers were dissolved into THF, DMF, or a mixture of o-cresol, m-cresol and sym-tetrachloroethane (33% each). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were both performed on a DuPont model 9900 thermal analyzer at Texas Research Institute, Austin, Texas, or NASA, Langley Research Center, Hampton, Virginia. The heating rate was lO”C/min. in air or nitrogen for the TGA. DSC was performed at a heating rate of 20”C/min. The melting points of the monomers were determined using a Laboratory Devices USA Mel-Temp II equipped with a Fluka 51 kJ thermocouple digital thermometer. Elemental analysis of the monomers and polymers were performed by Desert Analytics, Tucson, Arizona. Molecular modeling was done on a Silicon Graphics Iris workstation using Biosym software. All wide angle X-ray diffraction (WAXD) analyses were done at NASA Langley Research Center, Hampton, Virginia on a Philips Model 3600 diffractometer using PC-APD diffraction software. WAXD traces were obtained by a stepscanning method. The step width and fixed time were programmed for steps of 0.02 deg every 2 s, respectively. 2.3. Methods 2.3.1. Synthesis of 2,5-dijboro4-methylacetophenone (I) Carbon disulfide (100 ml), acetylchloride (18 g, 230 mmol), 2,5-difluorotoluene (25 g, 200 mmol) and aluminum chloride (34.6 g, 260, mmol) were placed into a 500-ml, round-bottom flask which was equipped with a condenser connected to a calcium chloride drying tube. The
D.V Person et al. /Reactive
& Functional Po1ynzer.s 30 (1996) 141-147
entire mixture was stirred for 3 h, subsequently heated at reflux for an additional 4 h, then cooled to room temperature, and slowly poured into a mixture of concentrated HCl and ice. The solution was extracted several times with diethyl ether. The ether extract was dried over anhydrous magnesium sulfate and evaporated to leave a dark brown solid. The solid was recrystallized from ethanol and water to give 1 (27.2 g, 82% yield, mp = 43°C). ‘H NMR (acetone-de): 6 2.32 (s, 3H); 2.55 (s, 3H); 7.36 (m, 2H). 2.3.2. Synthesis qf 2,5-dijuoro-4-methylhenzoic acid (2) In a 300-ml Erlenmeyer flask was placed 2,5-difluoro-4-methylacetophenone (28 g, 165 mmol) along with iodine (46 g, 181 mmol) and pyridine (85 ml). The mixture was heated at 100°C for several hours, after which the reaction mixture was allowed to cool to room temperature to leave a black solid. The solid was washed with diethyl ether followed by water. The solid was added to a NaOH solution (6 M) and allowed to heat for 1 hour. The mixture was filtered, and the filtrate was acidified with dilute HzS04, to precipitate 2 (17.3 g, 61% yield, mp = 173°C). ‘H NMR (acetone-d6): 6 2.33 (s, 3H); 4.00 (broad, lH), 7.18 (m, IH), 7.56 (m, 1H). 2.3.3. Synthesis of 2,5-dijuoro4-(a-bromomethyljbenzoic acid In a l-l, round-bottom flask was placed 2,5difluoro-4-methyl benzoic acid (23 g, 134 mmol) along with NBS (118 g, 663 mmol), benzoyl peroxide 1.5 g, and carbon tetrachloride, 400 ml. The flask was then equipped with a condenser and heated. The mixture was heated at reflux for 48 hours, after which the hot mixture was filtered and washed successively with hot carbon tetrachloride. The organic solution was then extracted with 6 M HCl several times, dried over anhydrous magnesium sulfate, filtered and evaporated. The remaining brown solid was dried and used in next reaction without purification. The solid was a mixture of mono-, di- and tribrominated compounds, the molecular weight and yield (83%) was calculated as an average based on ‘H NMR analysis.
2.3.4. Synthesis of 2,5-di@oroterephthalic
143
acid
(3) In a l-l, three-neck, round-bottom flask was placed HzS04 (200 ml, 9 M) along with 2,5difluoro-4-(c;l-bromomethyl)benzoic acid (36.8 g, 111 mmol). The brominated compound was a mixture of mono-, di-, and tribrominated species; the molecular weight was estimated as an average from ‘H NMR. The flask was heated to 130°C then sodium dichromate (30 g, 100 mmol) was slowly added, over a 2 h period. When the addition was complete, the mixture was heated for an additional 20 min. It was subsequently cooled to room temperature, stoppered and placed in a freezer for several hours. The cold mixture was filtered and the precipitate washed with water. The solid was recrystallized from glacial acetic acid to leave white crystals of 3 which were dried in vacua at room temperature. Yield = 65%, mp = 323-353°C [lit. [8] mp = 325-327”C]. 2.3.5. Synthesis of 2,5-dijuoroterephthaloyl chloride (DFTAC) The 2,5-difluoroterephthalic acid chloride (DFTAC) was prepared by heating 3 at reflux in thionyl chloride with a drop of DMF for 3 h. Excess thionyl chloride was removed by distillation, and the acid chloride was purified by sublimation, mp 64-65°C [lit. [3] 61-62”C]. IR (KBr): u 1767 cm-’ (C=O). ‘H NMR (acetonedh): 6 7.79. 7.89, 7.98 (s, 2H). Anal. Calculated for CSH~F~O~C~~: C, 40.21%; H, 0.84%. Found: C, 40.47%; H, 0.77%. 2.3.6. Synthesis ofpolyesters A typical polymer synthesis is illustrated using polymer PE-2 as an example. In a lOOml, three-neck, round-bottom flask was placed DMAP (0.1534 g, 1.256 mmol) along with triethylamine (1.75 ml, 12.55 mmol) and THF (20 ml). The flask was equipped with a condenser, argon inlet valve and a glass stopper. The flask was assembled hot and purged with argon prior to and during all additions. To the above solution was added Bis AF (2.1103 g, 6.276 mmol) and the mixture was stirred. Once the mixture was homogeneous, DFTAC (1.5000 g, 6.276 mmol) was added along with THF (10 ml), to ensure quanti-
144
D.K Person et aLlReactive & Functional Polymers 30 (1996) 141-147
tative transfer, and the entire mixture was heated at reflux in an oil bath. After 48 h, the flask was cooled to room temperature and to it was added 1 M HCl (50 ml) and the polymer was filtered through a medium glass frit. The white polymer was washed with a sodium bicarbonate solution (4.0 g NaHCOs in 75 ml H20), hot methanol and, finally, acetone; it was then air dried and subsequently dissolved into hot DMF and precipitated into methanol to yield 3.01 g (96%). IR (KBr): u 1740 cm-’ (C=O). Anal. Calculated for [C2sHtc04Fsln : C, 55.00%; H, 2.00%. Found: C, 55.40%; H, 2.15%.
verting the diacid to its functional derivative acid chloride by reaction with thionyl chloride. Yields for this step were approximately 85-90%. Purification of the DFI’PC was accomplished by an initial recrystallization from dry hexane followed by sublimation. Once sublimation was completed, transfer and storage of the diacid chloride was performed in an argon-filled dry box. The diacid chloride and the intermediates involved in the preparation were satisfactorily characterized by IR, ‘H NMR and elemental analyses. Elemental analyses showed good agreement with the calculated value.
3. Results and discussion
3.2. Polymers
3.1. Monomer synthesis
The polyesters were synthesized by solution polymerization (THF) of the diols or bisphenols with DFTAC in the presence of triethylamine and DMAP (Scheme 2). Yields and inherent viscosities along with composition and designations for the difluoroterephthalate polyesters are given in Table 1. Yields for the polyarylates ranged from 6097 percent. Attempts to prepare polydifluoroterephthalates from either 1,4-bis(hexafluoro2-hydroxy-2-propyl)benzene or 4,4’-sulfonyldiphenol yielded very low molecular weight polymer (n = 0.10 dl/g). However, PE-4, from 1,3bis(hexafluoro-2-hydroxy-2-propyl)benzene was obtained in 67% yield with n = 0.61 dl/g, suggesting that the failure with the synthesis of the 1,4-isomer is not due to the low nucleophilicity of the diol. Quite possibly both 4,4-sulfonyldiphenol and 1,4-bis(hexafluoro-2-hydroxy-2-propyl)benzene yield highly crystalline difluoroterephthalates which become insoluble early in the reaction. WAXD data for the low molecular weight oligomers obtained with these monomers support this suggestion indicating greater than 50% crystallinity in both samples. PE-3, from oxydiphenol, was insoluble in all solvents tested and was also highly crystalline by WAX diffraction (45%, Table 1). However, a high degree of crystallinity did not always cause solubility problems. Both PE-1 and PE-10 are also highly crystalline (Table l), but are soluble in several solvents (Ta-
Although the preparation of 2,5_difluoroterephthalic acid is known [ 10,l l] the method described herein is new and proved to be more reliable, albeit more lengthy, in our hands than the literature methods. Scheme 1 illustrates the route employed in the synthesis of 2,5-difluoroterephthalic acid from commercially available difluorotoluene. The overall reported yield of difluoroterephthalic acid from the difluorotoluene is about 2530%. However, this value represents a minimum routine value; overall yields as high as 60% have been obtained. The final step incorporates con-
0 Br,H,C
&OH
Scheme 1.
D.1/: Person et al/Reactive
145
& Functional Polymers 30 (1996) 141-147
F 0
0
&Cl
Cl-Z
HO-X-OH
+
F
I
PE-1: X =
PE-3: X =
PE-5: X = ~-7:
W
X = C’&b,
PE-6: X FE-8 X = (CH2)4,
= (CH2)2, PE-9: X = (CH&
, PE-11): x = (CH2)6
Scheme 2
Table 1 Results of polyesters Bisphenol
or dial
Bis A Bis AF 4,4’-Oxydiphenol
1,3-HFAB 1,4-Benzenedimethanol Ethylene glycol 1,3-Propandiol
1,4-Butanediol 1,5-Pentanediol 1,6-Hexanediol
derived from 2,5-difluoroterephthalic Polymer code
Yield (%)
_ (dhz)
PE-1 PE-2 PE-3 PE-4
87 96 97 67
0.32” 0.17b insol. 0.61’
PE-5 PE-6 PE-7 PE-8 PE-9 PE-10
80
71 67 60 66 16
‘hh
0.16a 0.38’ 0.20 a 0.23’ 0.20b 0.26 b
acid
Crystallinity (%) by WAXD >50 3-5 45 _ 15-20 35-40 _ 20 _ >50
“Measured at a concentration of 0.25 g/d1 in a mixture of ocresollm-cresollsym-tetrachloroethane (33% each) at 25°C. bMeasured at a concentration of 0.25 g/d1 in N,N-dimethylformamide at 25°C. ’ Measured in a mixture of DMAc and LiCl (7%).
ble 2). Polymers PE-8,9, and 10 are not included in Table 2; PE-9 and PE-10 are both highly soluble in all solvents tested and PE-8 is at least moderately soluble in all of the solvents. Viscosities for the polyester series ranged from 0.16 to 0.6 1 dl/g. In general, the polymers derived from bisphenols had lower viscosities than the polymers derived from the aliphatic di-
Table 2 Solubility
profile of difluoroterephthalate
Solvent a
1234567
Dimethylformamide DMAc/LiCl (7%) m-Cresollo-Cresol (50/50) o-Chlorophenol Dimethylsulfoxide sym-Tetrachloroethane N-Methyl-2-pyrrolidone Chloroform Chloroform/TFAA (SO/SO)
polyesters
(PE)
_ ++ ---+-+-
-
-
-
-
-
+ ++ -
+
+
+
+
-
+ --+++ -
_
_
_
+
-
-
-
-
+
++
-
-
-
-
f i ++
++: Soluble cold; +: soluble hot; f: swells; -: insoluble. a DMAc: N,N-dimethylacetamide, TFAA: trifluoroacetic acid.
01s. This result is expected, due to the aliphatic chain of the diols reducing the rigidity of the polymer backbone, thus increasing the solubility for that series and thereby increasing molecular weight and the viscosity. This behavior was readily apparent in the fact that during polymerizations the aromatic series precipitated out of solution within five to ten minutes after combination, while the aliphatic series stayed in solution for a minimum of two hours and in most cases the duration of the polymerization. Polymers PE-9 and PE-10 are both highly soluble in all solvents. All of the aromatic polyesters
146
D.I! Person et al. /Reactive
& Functional Polymers 30 (1996) 141-147
Table 3 Thermal properties for the difluoroterephthalate polyester series Polymer
TGA in N2 (“C)
TGA in air (“C)
% Char yield at 6OO’C (Nz/air)
T, (“C)
r,, (“C)
PE-1 PE-2 PE-3 PE-4 PE-5 PE-6 PE-8 PE-10
328 480 342 358 380 370 339 334
332 335 360 331 314
44/O 52 48/O 37 1l/l3 01 0 71 3
160 123 -
260-65 219 233 221 196 203 127, 180
formed brittle films, most likely due to their low molecular weights and somewhat crystalline nature, while the aliphatic series, for the most part, formed flexible films. The thermal behavior of the new polymers is as suspected (Table 3). With the exception of PE-2 which is stable to 480” in nitrogen, the polyesters undergo 10% weight loss between 328” and 380°C in nitrogen. Char yields at 600°C for PE-1, 2, 3, and 5, all of which contain the benzene ring in the diol moiety, are 44, 52, 48 and 37%, respectively. They are much lower, O1 1%, for the polymers derived from the aliphatic diols. The thermal behavior of the poly(difluoroterephthalate)s derived from the aliphatic diol series was compared to that of analogous poly(terephthalate)s to assess the effect of T, of the 2,5-difluoro substitution (Table 4). As can
be seen there is a consistent lowering of T, in the difluoro series. Interestingly, PE-10 shows two melting endotherms by DSC which might indicate the existence of a mesophase in this polymer. This possibility is currently under investigation. 4. Conclusions A new, more reliable, synthetic route to 25 difluoroterephthalic acid (DFTA) was developed and from this a new series of polyesters was prepared and characterized. (Polyamides, keto polyethers and others will be reported in the future.) This was done with the goal to evaluate the effect of aromatic fluorines on the reactivity of terephthalic acid and on the properties of subsequent polymers. Viscosities of the DFTA-based polyester series ranged from 0.16 to 0.61 dl/g. These polyesters are thermally stable up to 480°C in nitrogen, but aliphatic versions suffered, as anticipated. The aromatic-aliphatic polyesters had sharp melt transitions around 200°C and comparison to nonfluorinated analogues showed fluorine substitution to lower 7’,‘,. The aromatic analogues provided stiff, brittle films while the inclusion of an aliphatic diol provided flexible films. Also, as the length of the aliphatic link increased, the TGA stability decreased, all readily predictable. Although previous reports indicated that the 12F-diol [a bis(hexafluoroisopropanol-substituted) benzene] that decreased nucleophilicity may inhibit its re-
Table 4 Comparison of aliphatic-aromatic difluoroterephthalates to analogous aliphatic-aromatic terephthalates Diol d-s
Diacid a
T, b (“C)
p-Benzenedimethanol p-Benzenedimethanol d Ethylene glycol Ethylene glycol e 1,CButanediol 1,4-Butanediol’ 1,6-Hexanediol I ,6-Hexanediol s
TPC DFTPC TPC DFTPC TPC DFTPC TPC DPTPC
265” 221 265” 200 232l’ 203 154” 127 (180)
TGAC (“C)
Crystallinity
_ 380 248” 360 _ 334 314
15-20% 35-40% 20% 250%
a TPC: terephthaloyl chloride; DFTPC: 2,5-difluoroterephthaloyl chloride. b Determined by DSC or DTA at a heating rate of 20”Clmin. c Temperature at which 10% weight loss occurred; determined by TGA at a heating rate of 2S”Clmin in nitrogen. d PE-5; c PE-6; f PE-8; s PE-10.
D.V Person et al. /Reactive
& Functional
activity, this study revealed no lack of this diol to form polyesters. Acknowledgements A great appreciation is extended to the National Aeronautics and Space Administration (grant number NAG-l-63 l), the Robert A. Welch Foundation, Houston, TX (Grant AI-0524) and to the Texas Coordinating Board (advanced research grant number 003615-010) for their financial support of this research. Appreciation is also extended to Mr. Rock Rushing, Texas Research Institute, for performing thermal analyses.
Polymers 30 (1996) 141-147
[3] [4] [5] [6] [7] [8] [9]
References [lo] [l] G. Bier, Polymer, 15 (1974) 527. [2] B.D. Dean, M. Matzner and J.M. Tibbitt, in: G. Allen and J.C. Bevington (Ed%), Polyarylates, Comprehensive
[ 1l]
147
Polymer Science, Vol. 5. Permagon Press, Oxford. 1989, p. 317. M. Nagata, N. Tsutsumi and T. Kiyotsukuri, J. Polym. Sci., Polym. Chem. Ed., 26 (1988) 235. T. Kiyotsukuri. N. Tsutsumi, K. Okada and K. Asai, J. Polym. Sci., Polym. Chem. Ed., 26 (1988) 2225. Y. Oishi, S. Harada, M. Kakimoto and Y. Imai, J. Polym. Sci., Polym. Chem. Ed., 27 (1989) 1425. M. Nagata. N. Uchino and T. Kiyotsukuri, Sen-i Gakkaishi, 34 (1978) T-493. T. Kiyotsukuri, M. Nagata, K. Nakashita and T. Ishi, Sen-i Gakkaishi. 40 (1984) T-381. K. Kane, L. Wells, P. Cassidy, High Perform. Polym., 3 (1991) 191. (a) G.L. Tullos, P.E. Cassidy and A.K. St. Clair, Macromolecules, 24 (1991) 6059; (b) J.A. Irvin, A.R. Lee, J.W. Fitch and P.E. Cassidy, PMSE Division Preprints, 69, 558. American Chemical Society National Meeting, Chicago, IL, Fall 1993; (c) J. Irvin. C.J. Neef, K. Kane and P.E. Cassidy. Polym. Sci., Part A: Polym. Chem., 30 (1992) 1675. S. Sugawara, N. Ishikawa, Kogyo Kagaku Zasshi. 73 (1970) 972. G. Valkanas. German Patent 1,I 16,208 (1959); Chem. Abs. 56, (1962) 722Og.
Polymers from 2,Sdifluoroterephthalic
acid. I. Polyesters
David V. Person, John W. Fitch *, Patrick E. Cassidy, Keiji Kono, V. Sreenivasulu Reddy Polymer Research
Group, Depurrrnent qf’Chemistry, Southwest Texas State Universig, San Marcos. TX 78666. USA Received 20 September
1995; accepted
18 December
1995
Abstract A new synthetic route to 2,.Sdifluoroterephthalic acid (DFTA) was devised and from this several new series of polymers were prepared by polycondensation. This inaugural paper will cover polyesters while subsequent papers will report on keto polyethers, polyamides and other backbones derived from DFTA. DFTA was synthesized from 2.5difluorotoluene, which was acylated and subsequently converted to the diacid by a several step oxidation process. Various polyesters were prepared by reactin g it with numerous bisphenols and diola by solution condensation to give a series of difluoroterephthalate polyesters with viscosities ranging from 0.16 to 0.61 dl/g. Thermogravimetric analyses showed that the polyesters had thermal stabilities up to 480°C in nitrogen: melting temperatures ranged from I27 to 3 18°C. Keywords:
2,SDifluoroterephthalic
acid; Crystallinity;
Fluorinated
1. Introduction
Significant efforts have been made toward the synthesis, characterization and evaluation of fluorine-containing condensation polymers. A renewed interest in fluoropolymers has led to the synthesis and characterization of condensation polymers from 25difluoroterephthalic acid (DFTA). DFTA presents a relatively new template for polymers either with aromatic fluorine or for use of the fluorine as a site for polymerization. Polyarylates make up an important class of engineering plastics by virtue of their several uses [l-2]. Unfortunately, they also present processing problems due to their high glass-transition temperatures and very poor solubilities. Several studies have been done correlating halogen substitution on terephthalic acid and its effects on * Corresponding author. 1381-5148/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved .SSDl 1381-5148(95)001 18-2
polyesters
the solubility and thermal behavior of various polyamides and polyarylates [3-71. Results from these studies indicate that the addition of at least one halogen onto terephthalic acid greatly increases a polymer’s solubility, with the greatest increase found in the perfluoro species. Our early work on fluorinated polyarylates focused on the use of an aromatic dicarboxylic acid containing the hexafluoroisopropylidene unit with bisphenol A or bisphenol AF to produce polyesters with 12, 6 or no fluorines [S]. Further, the backbone with six fluorines could have them in the acid or the diol portion. The products formed clear, colorless, tough films which had quite good mechanical properties and thermal stabilities. Perhaps most interesting was the discovery that a content of six fluorines was sufficient for maximum thermal stability provided they were properly placed (in the bisphenol portion of the repeating unit). In a study by Nagata et al. [3] overall perfor-
142
D.V Person et al. /Reactive & Functional Polymers 30 11996) 141-147
mance seemed to be maximized for polyamides derived from difluoroterephthalic acid. The difluoro species produced a marked increase in solubility and retained glass-transition temperatures lower than most analogous polyamides having various halogen substituents (F, Cl, Br) and/or degrees of substitution on the terephthalic acid. Our earlier work [9] on fluorinated poly(ether ketones) (12-F PEK) [9a] and aromatic polyethers with aliphatic (CF3) or aromatic fluorines [9a-c], led to several extensions including polymers from DFTA. High strength, toughness, good electrical properties, thermooxidative stability and solvent resistance make these polymers excellent candidates for advanced materials. This paper reports the synthesis and characterization of several aromatic polyesters prepared by polycondensation of DFTA with various diol monomers. Polyamides, keto polyethers and other backbones derived from DFTA will be reported in subsequent papers. 2. Experimental 2. I. Materials The 2,2-bis(4-hydroxyphenyl)propane (Bis A; Hoechst Aktiengesellschaft), 2,2-bis(4-hydroxyphenyl)-l,l, 1,3,3,3-hexafluoropropane (Bis AF; Hoechst Aktiengesellschaft) and 1,4-benzenedimethanol were obtained from Aldrich Chemical Co. and purified via multiple sublimations. The 4,4’-oxydiphenol (Pfaltz and Bauer) and 4,4’-sulfonyldiphenol were also obtained from Aldrich Chemical Co. and purified by repeated recrystallizations from toluene and ethanol/water, respectively, followed by single sublimations. The diol, 1,3-bis (2-hydroxy hexafluoro-2-propyl)benzene (1,3-HFAB) was obtained from Central Glass and was distilled. All aliphatic diols were obtained from the Aldrich Chemical Co. and were dried by heating the diol at reflux with a 5% molar equivalent of sodium followed by vacuum distillation onto activated sieves (4 A). The 4-dimethylaminopyridine (DMAP; Aldrich Chemical Co.) was obtained in 98% purity and stored over phosphorous pentoxide prior to use. Tetrahydrofuran (THF) was obtained anhydrous, 99%+ pure, in
a Sure/SealTMbottle from the Aldrich Chemical co. 2.2. Measurements Infra-red spectra of the monomers and polymers were obtained from a Perkin-Elmer Model 1600 spectrophotometer and were analyzed as KBr wafers. Inherent viscosities of the polymers were measured at a concentration of 0.25 g/d1 at 25°C using a Cannon-Fenske type viscometer. The polymers were dissolved into THF, DMF, or a mixture of o-cresol, m-cresol and sym-tetrachloroethane (33% each). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were both performed on a DuPont model 9900 thermal analyzer at Texas Research Institute, Austin, Texas, or NASA, Langley Research Center, Hampton, Virginia. The heating rate was lO”C/min. in air or nitrogen for the TGA. DSC was performed at a heating rate of 20”C/min. The melting points of the monomers were determined using a Laboratory Devices USA Mel-Temp II equipped with a Fluka 51 kJ thermocouple digital thermometer. Elemental analysis of the monomers and polymers were performed by Desert Analytics, Tucson, Arizona. Molecular modeling was done on a Silicon Graphics Iris workstation using Biosym software. All wide angle X-ray diffraction (WAXD) analyses were done at NASA Langley Research Center, Hampton, Virginia on a Philips Model 3600 diffractometer using PC-APD diffraction software. WAXD traces were obtained by a stepscanning method. The step width and fixed time were programmed for steps of 0.02 deg every 2 s, respectively. 2.3. Methods 2.3.1. Synthesis of 2,5-dijboro4-methylacetophenone (I) Carbon disulfide (100 ml), acetylchloride (18 g, 230 mmol), 2,5-difluorotoluene (25 g, 200 mmol) and aluminum chloride (34.6 g, 260, mmol) were placed into a 500-ml, round-bottom flask which was equipped with a condenser connected to a calcium chloride drying tube. The
D.V Person et al. /Reactive
& Functional Po1ynzer.s 30 (1996) 141-147
entire mixture was stirred for 3 h, subsequently heated at reflux for an additional 4 h, then cooled to room temperature, and slowly poured into a mixture of concentrated HCl and ice. The solution was extracted several times with diethyl ether. The ether extract was dried over anhydrous magnesium sulfate and evaporated to leave a dark brown solid. The solid was recrystallized from ethanol and water to give 1 (27.2 g, 82% yield, mp = 43°C). ‘H NMR (acetone-de): 6 2.32 (s, 3H); 2.55 (s, 3H); 7.36 (m, 2H). 2.3.2. Synthesis qf 2,5-dijuoro-4-methylhenzoic acid (2) In a 300-ml Erlenmeyer flask was placed 2,5-difluoro-4-methylacetophenone (28 g, 165 mmol) along with iodine (46 g, 181 mmol) and pyridine (85 ml). The mixture was heated at 100°C for several hours, after which the reaction mixture was allowed to cool to room temperature to leave a black solid. The solid was washed with diethyl ether followed by water. The solid was added to a NaOH solution (6 M) and allowed to heat for 1 hour. The mixture was filtered, and the filtrate was acidified with dilute HzS04, to precipitate 2 (17.3 g, 61% yield, mp = 173°C). ‘H NMR (acetone-d6): 6 2.33 (s, 3H); 4.00 (broad, lH), 7.18 (m, IH), 7.56 (m, 1H). 2.3.3. Synthesis of 2,5-dijuoro4-(a-bromomethyljbenzoic acid In a l-l, round-bottom flask was placed 2,5difluoro-4-methyl benzoic acid (23 g, 134 mmol) along with NBS (118 g, 663 mmol), benzoyl peroxide 1.5 g, and carbon tetrachloride, 400 ml. The flask was then equipped with a condenser and heated. The mixture was heated at reflux for 48 hours, after which the hot mixture was filtered and washed successively with hot carbon tetrachloride. The organic solution was then extracted with 6 M HCl several times, dried over anhydrous magnesium sulfate, filtered and evaporated. The remaining brown solid was dried and used in next reaction without purification. The solid was a mixture of mono-, di- and tribrominated compounds, the molecular weight and yield (83%) was calculated as an average based on ‘H NMR analysis.
2.3.4. Synthesis of 2,5-di@oroterephthalic
143
acid
(3) In a l-l, three-neck, round-bottom flask was placed HzS04 (200 ml, 9 M) along with 2,5difluoro-4-(c;l-bromomethyl)benzoic acid (36.8 g, 111 mmol). The brominated compound was a mixture of mono-, di-, and tribrominated species; the molecular weight was estimated as an average from ‘H NMR. The flask was heated to 130°C then sodium dichromate (30 g, 100 mmol) was slowly added, over a 2 h period. When the addition was complete, the mixture was heated for an additional 20 min. It was subsequently cooled to room temperature, stoppered and placed in a freezer for several hours. The cold mixture was filtered and the precipitate washed with water. The solid was recrystallized from glacial acetic acid to leave white crystals of 3 which were dried in vacua at room temperature. Yield = 65%, mp = 323-353°C [lit. [8] mp = 325-327”C]. 2.3.5. Synthesis of 2,5-dijuoroterephthaloyl chloride (DFTAC) The 2,5-difluoroterephthalic acid chloride (DFTAC) was prepared by heating 3 at reflux in thionyl chloride with a drop of DMF for 3 h. Excess thionyl chloride was removed by distillation, and the acid chloride was purified by sublimation, mp 64-65°C [lit. [3] 61-62”C]. IR (KBr): u 1767 cm-’ (C=O). ‘H NMR (acetonedh): 6 7.79. 7.89, 7.98 (s, 2H). Anal. Calculated for CSH~F~O~C~~: C, 40.21%; H, 0.84%. Found: C, 40.47%; H, 0.77%. 2.3.6. Synthesis ofpolyesters A typical polymer synthesis is illustrated using polymer PE-2 as an example. In a lOOml, three-neck, round-bottom flask was placed DMAP (0.1534 g, 1.256 mmol) along with triethylamine (1.75 ml, 12.55 mmol) and THF (20 ml). The flask was equipped with a condenser, argon inlet valve and a glass stopper. The flask was assembled hot and purged with argon prior to and during all additions. To the above solution was added Bis AF (2.1103 g, 6.276 mmol) and the mixture was stirred. Once the mixture was homogeneous, DFTAC (1.5000 g, 6.276 mmol) was added along with THF (10 ml), to ensure quanti-
144
D.K Person et aLlReactive & Functional Polymers 30 (1996) 141-147
tative transfer, and the entire mixture was heated at reflux in an oil bath. After 48 h, the flask was cooled to room temperature and to it was added 1 M HCl (50 ml) and the polymer was filtered through a medium glass frit. The white polymer was washed with a sodium bicarbonate solution (4.0 g NaHCOs in 75 ml H20), hot methanol and, finally, acetone; it was then air dried and subsequently dissolved into hot DMF and precipitated into methanol to yield 3.01 g (96%). IR (KBr): u 1740 cm-’ (C=O). Anal. Calculated for [C2sHtc04Fsln : C, 55.00%; H, 2.00%. Found: C, 55.40%; H, 2.15%.
verting the diacid to its functional derivative acid chloride by reaction with thionyl chloride. Yields for this step were approximately 85-90%. Purification of the DFI’PC was accomplished by an initial recrystallization from dry hexane followed by sublimation. Once sublimation was completed, transfer and storage of the diacid chloride was performed in an argon-filled dry box. The diacid chloride and the intermediates involved in the preparation were satisfactorily characterized by IR, ‘H NMR and elemental analyses. Elemental analyses showed good agreement with the calculated value.
3. Results and discussion
3.2. Polymers
3.1. Monomer synthesis
The polyesters were synthesized by solution polymerization (THF) of the diols or bisphenols with DFTAC in the presence of triethylamine and DMAP (Scheme 2). Yields and inherent viscosities along with composition and designations for the difluoroterephthalate polyesters are given in Table 1. Yields for the polyarylates ranged from 6097 percent. Attempts to prepare polydifluoroterephthalates from either 1,4-bis(hexafluoro2-hydroxy-2-propyl)benzene or 4,4’-sulfonyldiphenol yielded very low molecular weight polymer (n = 0.10 dl/g). However, PE-4, from 1,3bis(hexafluoro-2-hydroxy-2-propyl)benzene was obtained in 67% yield with n = 0.61 dl/g, suggesting that the failure with the synthesis of the 1,4-isomer is not due to the low nucleophilicity of the diol. Quite possibly both 4,4-sulfonyldiphenol and 1,4-bis(hexafluoro-2-hydroxy-2-propyl)benzene yield highly crystalline difluoroterephthalates which become insoluble early in the reaction. WAXD data for the low molecular weight oligomers obtained with these monomers support this suggestion indicating greater than 50% crystallinity in both samples. PE-3, from oxydiphenol, was insoluble in all solvents tested and was also highly crystalline by WAX diffraction (45%, Table 1). However, a high degree of crystallinity did not always cause solubility problems. Both PE-1 and PE-10 are also highly crystalline (Table l), but are soluble in several solvents (Ta-
Although the preparation of 2,5_difluoroterephthalic acid is known [ 10,l l] the method described herein is new and proved to be more reliable, albeit more lengthy, in our hands than the literature methods. Scheme 1 illustrates the route employed in the synthesis of 2,5-difluoroterephthalic acid from commercially available difluorotoluene. The overall reported yield of difluoroterephthalic acid from the difluorotoluene is about 2530%. However, this value represents a minimum routine value; overall yields as high as 60% have been obtained. The final step incorporates con-
0 Br,H,C
&OH
Scheme 1.
D.1/: Person et al/Reactive
145
& Functional Polymers 30 (1996) 141-147
F 0
0
&Cl
Cl-Z
HO-X-OH
+
F
I
PE-1: X =
PE-3: X =
PE-5: X = ~-7:
W
X = C’&b,
PE-6: X FE-8 X = (CH2)4,
= (CH2)2, PE-9: X = (CH&
, PE-11): x = (CH2)6
Scheme 2
Table 1 Results of polyesters Bisphenol
or dial
Bis A Bis AF 4,4’-Oxydiphenol
1,3-HFAB 1,4-Benzenedimethanol Ethylene glycol 1,3-Propandiol
1,4-Butanediol 1,5-Pentanediol 1,6-Hexanediol
derived from 2,5-difluoroterephthalic Polymer code
Yield (%)
_ (dhz)
PE-1 PE-2 PE-3 PE-4
87 96 97 67
0.32” 0.17b insol. 0.61’
PE-5 PE-6 PE-7 PE-8 PE-9 PE-10
80
71 67 60 66 16
‘hh
0.16a 0.38’ 0.20 a 0.23’ 0.20b 0.26 b
acid
Crystallinity (%) by WAXD >50 3-5 45 _ 15-20 35-40 _ 20 _ >50
“Measured at a concentration of 0.25 g/d1 in a mixture of ocresollm-cresollsym-tetrachloroethane (33% each) at 25°C. bMeasured at a concentration of 0.25 g/d1 in N,N-dimethylformamide at 25°C. ’ Measured in a mixture of DMAc and LiCl (7%).
ble 2). Polymers PE-8,9, and 10 are not included in Table 2; PE-9 and PE-10 are both highly soluble in all solvents tested and PE-8 is at least moderately soluble in all of the solvents. Viscosities for the polyester series ranged from 0.16 to 0.6 1 dl/g. In general, the polymers derived from bisphenols had lower viscosities than the polymers derived from the aliphatic di-
Table 2 Solubility
profile of difluoroterephthalate
Solvent a
1234567
Dimethylformamide DMAc/LiCl (7%) m-Cresollo-Cresol (50/50) o-Chlorophenol Dimethylsulfoxide sym-Tetrachloroethane N-Methyl-2-pyrrolidone Chloroform Chloroform/TFAA (SO/SO)
polyesters
(PE)
_ ++ ---+-+-
-
-
-
-
-
+ ++ -
+
+
+
+
-
+ --+++ -
_
_
_
+
-
-
-
-
+
++
-
-
-
-
f i ++
++: Soluble cold; +: soluble hot; f: swells; -: insoluble. a DMAc: N,N-dimethylacetamide, TFAA: trifluoroacetic acid.
01s. This result is expected, due to the aliphatic chain of the diols reducing the rigidity of the polymer backbone, thus increasing the solubility for that series and thereby increasing molecular weight and the viscosity. This behavior was readily apparent in the fact that during polymerizations the aromatic series precipitated out of solution within five to ten minutes after combination, while the aliphatic series stayed in solution for a minimum of two hours and in most cases the duration of the polymerization. Polymers PE-9 and PE-10 are both highly soluble in all solvents. All of the aromatic polyesters
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D.I! Person et al. /Reactive
& Functional Polymers 30 (1996) 141-147
Table 3 Thermal properties for the difluoroterephthalate polyester series Polymer
TGA in N2 (“C)
TGA in air (“C)
% Char yield at 6OO’C (Nz/air)
T, (“C)
r,, (“C)
PE-1 PE-2 PE-3 PE-4 PE-5 PE-6 PE-8 PE-10
328 480 342 358 380 370 339 334
332 335 360 331 314
44/O 52 48/O 37 1l/l3 01 0 71 3
160 123 -
260-65 219 233 221 196 203 127, 180
formed brittle films, most likely due to their low molecular weights and somewhat crystalline nature, while the aliphatic series, for the most part, formed flexible films. The thermal behavior of the new polymers is as suspected (Table 3). With the exception of PE-2 which is stable to 480” in nitrogen, the polyesters undergo 10% weight loss between 328” and 380°C in nitrogen. Char yields at 600°C for PE-1, 2, 3, and 5, all of which contain the benzene ring in the diol moiety, are 44, 52, 48 and 37%, respectively. They are much lower, O1 1%, for the polymers derived from the aliphatic diols. The thermal behavior of the poly(difluoroterephthalate)s derived from the aliphatic diol series was compared to that of analogous poly(terephthalate)s to assess the effect of T, of the 2,5-difluoro substitution (Table 4). As can
be seen there is a consistent lowering of T, in the difluoro series. Interestingly, PE-10 shows two melting endotherms by DSC which might indicate the existence of a mesophase in this polymer. This possibility is currently under investigation. 4. Conclusions A new, more reliable, synthetic route to 25 difluoroterephthalic acid (DFTA) was developed and from this a new series of polyesters was prepared and characterized. (Polyamides, keto polyethers and others will be reported in the future.) This was done with the goal to evaluate the effect of aromatic fluorines on the reactivity of terephthalic acid and on the properties of subsequent polymers. Viscosities of the DFTA-based polyester series ranged from 0.16 to 0.61 dl/g. These polyesters are thermally stable up to 480°C in nitrogen, but aliphatic versions suffered, as anticipated. The aromatic-aliphatic polyesters had sharp melt transitions around 200°C and comparison to nonfluorinated analogues showed fluorine substitution to lower 7’,‘,. The aromatic analogues provided stiff, brittle films while the inclusion of an aliphatic diol provided flexible films. Also, as the length of the aliphatic link increased, the TGA stability decreased, all readily predictable. Although previous reports indicated that the 12F-diol [a bis(hexafluoroisopropanol-substituted) benzene] that decreased nucleophilicity may inhibit its re-
Table 4 Comparison of aliphatic-aromatic difluoroterephthalates to analogous aliphatic-aromatic terephthalates Diol d-s
Diacid a
T, b (“C)
p-Benzenedimethanol p-Benzenedimethanol d Ethylene glycol Ethylene glycol e 1,CButanediol 1,4-Butanediol’ 1,6-Hexanediol I ,6-Hexanediol s
TPC DFTPC TPC DFTPC TPC DFTPC TPC DPTPC
265” 221 265” 200 232l’ 203 154” 127 (180)
TGAC (“C)
Crystallinity
_ 380 248” 360 _ 334 314
15-20% 35-40% 20% 250%
a TPC: terephthaloyl chloride; DFTPC: 2,5-difluoroterephthaloyl chloride. b Determined by DSC or DTA at a heating rate of 20”Clmin. c Temperature at which 10% weight loss occurred; determined by TGA at a heating rate of 2S”Clmin in nitrogen. d PE-5; c PE-6; f PE-8; s PE-10.
D.V Person et al. /Reactive
& Functional
activity, this study revealed no lack of this diol to form polyesters. Acknowledgements A great appreciation is extended to the National Aeronautics and Space Administration (grant number NAG-l-63 l), the Robert A. Welch Foundation, Houston, TX (Grant AI-0524) and to the Texas Coordinating Board (advanced research grant number 003615-010) for their financial support of this research. Appreciation is also extended to Mr. Rock Rushing, Texas Research Institute, for performing thermal analyses.
Polymers 30 (1996) 141-147
[3] [4] [5] [6] [7] [8] [9]
References [lo] [l] G. Bier, Polymer, 15 (1974) 527. [2] B.D. Dean, M. Matzner and J.M. Tibbitt, in: G. Allen and J.C. Bevington (Ed%), Polyarylates, Comprehensive
[ 1l]
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Polymer Science, Vol. 5. Permagon Press, Oxford. 1989, p. 317. M. Nagata, N. Tsutsumi and T. Kiyotsukuri, J. Polym. Sci., Polym. Chem. Ed., 26 (1988) 235. T. Kiyotsukuri. N. Tsutsumi, K. Okada and K. Asai, J. Polym. Sci., Polym. Chem. Ed., 26 (1988) 2225. Y. Oishi, S. Harada, M. Kakimoto and Y. Imai, J. Polym. Sci., Polym. Chem. Ed., 27 (1989) 1425. M. Nagata. N. Uchino and T. Kiyotsukuri, Sen-i Gakkaishi, 34 (1978) T-493. T. Kiyotsukuri, M. Nagata, K. Nakashita and T. Ishi, Sen-i Gakkaishi. 40 (1984) T-381. K. Kane, L. Wells, P. Cassidy, High Perform. Polym., 3 (1991) 191. (a) G.L. Tullos, P.E. Cassidy and A.K. St. Clair, Macromolecules, 24 (1991) 6059; (b) J.A. Irvin, A.R. Lee, J.W. Fitch and P.E. Cassidy, PMSE Division Preprints, 69, 558. American Chemical Society National Meeting, Chicago, IL, Fall 1993; (c) J. Irvin. C.J. Neef, K. Kane and P.E. Cassidy. Polym. Sci., Part A: Polym. Chem., 30 (1992) 1675. S. Sugawara, N. Ishikawa, Kogyo Kagaku Zasshi. 73 (1970) 972. G. Valkanas. German Patent 1,I 16,208 (1959); Chem. Abs. 56, (1962) 722Og.