Synthesis and Solid State 13C and 19F NMR Characterization of Aliphatic and Aromatic Carboxylic Acid Fluoride Polymers

Article abstract of DOI:10.1021/ac9703663

A new chemical/spectroscopic couple that differentiates aromatic from aliphatic carboxylic acid polymers was developed. The method is complementary to more traditional methods of identification (IR) and is applicable to the analysis of complex mixtures wh

Full text of DOI:10.1021/ac9703663

Anal. Chem. 1997, 69, 3950-3958
1
3
1 9
S yn t h e s is a n d S o lid S t a t e C a n d F NMR
Ch a ra c t e riza t io n o f Alip h a t ic a n d Aro m a t ic
Ca rb o x ylic Ac id Flu o rid e P o lym e rs
Edw a rd W. Ha ga m a n,* Suk Kyu Le e , a nd Phillip F. Britt
Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6201
A new chemical/ spectroscopic couple that differentiates
aromatic from aliphatic carboxylic acid polymers was
developed. The method is complementary to more tra-
ditional methods of identification (IR) and is applicable
to the analysis of complex mixtures where IR determina-
tions are complicated by extensive vibrational band over-
lap. The method entails (1) conversion of carboxylic acids
into acid fluorides and (2 ) specific detection of the
resonances of the acid fluoride carbon and of the carbon
directly attached to the carbonyl carbon by solid state 19F-
functionality in complex macromolecules, and (4) the generation
of model acid fluorides for characterization by solid state 19F and
13C NMR. Synthetic efficiencies and the spectroscopic data are
required for the interpretation of more complex spectra generated
by fluorinating acid-containing organic mixtures.5 The polymers
1
were characterized by solid state proton dipolar decoupled [ H],
1
9
F magic angle spinning (MAS)/ NMR, concurrent H and 19F
5
1
dipolar decoupled [ H, F], cross polarization (CP)/ MAS 13C
NMR utilizing H- C and F- C cross polarization, and IR
1
19
6
1
13
19
13
7
spectroscopy.
1
3
C cross polarization (CP )/ MAS 1 3 C NMR. The assign-
ment of the chemical shift of the latter resonance to either
the sp2 or sp3 carbon resonance manifold specifies the
nature of the acid functionality. The preparation of the
acid fluoride derivatives of several polymers containing
aliphatic and aromatic carboxylic acid functionality was
evaluated using sulfur tetrafluoride (SF ), diethylamino-
4
sulfur trifluoride (DAST), cyanuric fluoride, and thionyl
fluoride. Room temperature reactions using DAST in
EXPERIMENTAL SECTION
Instrumentation. Solid state NMR spectra were recorded
on a Bruker MSL 100 spectrometer (2.35 T). 19F NMR spectra
were measured at 94.216 MHz using single-pulse excitation
employing a 3.2 µs 90° pulse and 4.0 s recycle delay. Free
induction decays (FIDs) were acquired with quadrature detection
(
256 data points, 100 kHz spectral width). The FIDs were zero-
filled to 8K and smoothed by convolution with a decreasing
exponential function (94 Hz line broadening), giving transformed
4
methylene chloride or neat SF gave the acid fluorides in
yields g9 0 % for the acids studied. Aromatic acid fluoride
yields were essentially quantitative. Aliphatic acid fluo-
rides were contaminated with the anhydride of the acid.
The acid fluoride polymers were characterized by solid
state 1 3 C and F MAS/ NMR.
1
060 ppm wide spectra with digital resolution of 24 Hz/ point. All
F measurements incorporated H dipolar decoupling at H1H
1
9
1
)
5
5 kHz during preparation, evolution, and acquisition periods, as
measurements
were made using an on-resonance 90-τ-acquire sequence that
provided T
* values.8
C NMR experiments on fluorine-containing organics were
well as magic angle spinning at 6050 ( 50 Hz. T
2
1 9
2
The production of acid fluoride functional groups in polymers
containing aliphatic and aromatic carboxylic acid groups has been
investigated with poly(methacrylic acid) (PMA), poly(4-vinylben-
zoic acid) (PVBA), and 13C-labeled monocarboxy-terminated
13
1
19
13
performed on a Doty H- F- C triple-resonance probe operating
at 25.178 MHz. FIDs were acquired with quadrature detection
(
1K data points, 15 kHz spectral width), zero-filled to 8K, and
13
2
polystyrene (PS- CO H). The conversion of PMA and PVBA into
smoothed (25 Hz line broadening). The 600 ppm frequency
domain spectra have digital resolution of 3.7 Hz/ point.
Experiments on fluorinated products required concurrent H
and 19F dipolar decoupling during the signal acquisition period.6
poly(methacrylic acid fluoride) (PMAF) and poly(4-vinylbenzoic
acid fluoride) (PVBAF), respectively, was explored using sulfur
1
1
2
tetrafluoride (SF
4
), diethylaminosulfur trifluoride (DAST), cya-
).4 The impetus for the
3
nuric fluoride, and thionyl fluoride (SOF
2
19
13
F- C cross polarization experiments were accomplished using
synthesis of acid fluoride polymers included (1) the development
of a new chemical/ spectroscopic method for functional group
analysis of polymers and heterogeneous, macromolecular mix-
tures, (2) the determination of the most efficient synthetic method
for the conversion of acids into acid fluorides, (3) the evaluation
of significant reaction channels that may limit the use of the acyl
fluoride functional group as an accurate reporter of carboxylic acid
a direct difference double cross polarization (DCP) pulse se-
quence with 5 kHz phase modulation of H1F during the 13C-19F
cross polarization interval. Modulation was used to suppress
oscillations in the magnetization transfer process which occur in
9
(
(
(
5) Hagaman, E. W.; Lee, S. K. Energy Fuels 1 9 9 5 , 9, 727-734.
6) Hagaman, E. W. J. Magn. Reson. 1 9 9 3 , 104A, 125-131.
7) Hagaman, E. W.; Burns, J. H. Fuel 1 9 9 3 , 72, 1239-1243.
(
1) Boswell, G. A., Jr.; Ripka, W. C.; Scribner, R. M.; Tullock, C. W. Org. React.
9 7 4 , 21, 1-124.
(8) Farrar, T. C.; Becker, E. D. Pulse and Fourier Transform NMR; Academic
Press: New York, 1971.
1
(
(
(
2) Hudlicky, M. Org. React. 1 9 8 8 , 35, 513.
3) Olah, G. A.; Nojima, M.; Kerekes, I. Synthesis 1 9 7 3 , 487-488.
4) Roberts, H. L. British Patent 908177, 1963; Chem. Abstr. 1 9 6 3 , 58, 5579.
(9) (a) Stejskal, E. O.; Schaefer, J.; McKay, R. A. J. Magn. Reson. 1 9 8 4 , 57,
471-485. (b) Schaefer, J.; Stejskal, E. O.; Garbow, J. R.; McKay, R. A. J.
Magn. Reson. 1 9 8 4 , 59, 150-156.
3950 Analytical Chemistry, Vol. 69, No. 19, October 1, 1997
S0003-2700(97)00366-1 CCC: $14.00 © 1997 American Chemical Society

isolated spin pairs. The large magnetic moment of the 19F nucleus
and the typical fluorine densities in these materials result in
Ta b le 1 . P ro d u c t Dis t rib u t io n o f t h e Re a c t io n o f P MA
w it h Flu o rin a t in g Re a g e n t s
homonuclear spin-spin relaxation rates, 19F 1/ T
of 1 kHz to several kilohertz. Spin systems for which the
2
, on the order
functional
a
group, area %
1
9
13
exponential time constant for the F- C cross polarization
preparation
reaction conditions
COF anhyd
acid
1
9
2
process (TCF), the MAS rotor period (Trot), and the F T are the
1
2
3
4
5
6
7
8
9
SF4, -40 °C, 1 h
45
80
90
65
77
55
20
10
32
22
>95
40
9
18
same order of magnitude show complicated cross polarization
dynamics in the absence of this modulation. The measured TCF
in phase-modulated DCP experiments are longer than those found
SF4, 0 °C, 1 h
b
SF , RT, 1 h
4
SF4/ KF, RT, 1 h
SF4/ KF, RT, 16 h
3
1
in conventional X- C CP experiments (X ) spin /
2
nucleus).10
1
3
1
cyanuric fluoride, CH3CN
SOF2, 120 °C, 17 h
DAST, RT, 1 h, CH2Cl2
DAST, RT, 1 h, C6H6
DAST, -78 °C, 2 h, CH2Cl2
trace
60
Relative rates remain valid parameters for qualitative structure
assessment. 13C chemical shifts of polymers are reported relative
91
82
66
to TMS using the methyl resonance of hexamethylbenzene (δ )
1
0
32
3
1
7.3 ppm) as secondary standard.
1H and 19F solution state NMR spectra were recorded on a
a
Determined by the peak area of each functional group in IR or
1 1 13 13
19
[
F, H] H- C CP/ MAS C NMR spectra. Single determinations.
RT, room temperature.
Bruker MSL 400 spectrometer. Chemical shifts are reported in
b
1
ppm from TMS and CFCl
3
for H and 19F NMR, respectively.
IR spectra were recorded on a Bio-Rad FT-IR spectrophotom-
eter using KBr pellets. Just the IR signals used to asses the
relative concentrations of carboxyl functional groups are listed
cyanuric fluoride were purchased from Pfaltz & Bauer, Inc. Sulfur
tetrafluoride was purchased from Matheson Gas Products.
Safety P recautions. Some syntheses described in this paper
involve the toxic compounds sulfur tetrafluoride and hydrogen
fluoride. The toxicity of sulfur tetrafluoride is about twice that of
phosgene. Sulfur tetrafluoride is easily hydrolyzed, forming
hydrogen fluoride and thionyl fluoride. Anhydrous hydrogen
fluoride and even dilute aqueous hydrofluoric acid cause painful,
slow-healing wounds to skin tissue. All work with sulfur tetrafluo-
ride and anhydrous hydrogen fluoride must be carried out in an
efficient fume hood. It is imperative to protect the hands with
undamaged rubber gloves and shield the face with a protective
shield of transparent plastic in all operations involving these
compounds. Practitioners should be knowledgeable of treatments
for inhalation and skin contact injuries.11
for each product. The pertinent wavenumbers for PMA-based
polymers are as follows: acid fluoride at ∼1830 cm-1, except for
the COF group, which is shifted to lower frequency, 1791 cm-1;
1
3
acid at ∼1700 cm ; and anhydride at ∼1760 and 1800 cm-1.
-1
Molecular weight distributions were determined by gel per-
meation chromatography (GPC) using a Waters chromatography
system consisting of a 600E system controller and pump, 717
autosampler, and 410 differential refractometer interfaced to a
computer running Waters Millennium 2010 software for GPC
analysis. The column (two Waters ultrastyrogel linear beds) was
calibrated using five molecular weight standards (polystyrene,
MW 2 000 000, 900 000, 400 000, 90 000, and 4000) at 32 °C in
THF at a flow rate of 1.00 mL/ min.
Mass spectra were obtained at 70 eV on a Hewlett-Packard
5
972 GC/ MS.
General Considerations. The acid fluoride group is remark-
ably stable, being able to survive brief cold water contact without
significant hydrolysis. It is not impervious to hydrolysis, however,
and a slow reaction proceeds over the span of weeks, in nominally
dried, stored derivatives, to liberate HF. This reaction is most
noticeable in complex heterogeneous amorphous mixtures, e.g.,
coal, which retain inherent sources of acidic protons (phenols,
etc.) which may assist in the slow hydrolysis. The reaction may
also proceed with adsorbed water gained during normal chemical
manipulations in the open air. Timely analysis of freshly prepared
samples avoids complications from this reaction.
Chemicals. Styrene (Aldrich, 99+%) was purified by vacuum
fractional distillation under argon. The middle cuts (bp 42-43
C at 25 mmHg) were degassed and stored over lithium hydride
°
in the dark at -20 °C until use (purity >99.9% by gas chromato-
graphic analysis). Benzene (J.T. Baker Resi-Analyzed) was
distilled from LiAlH
4
and stored over activated 4 Å molecular
sieves until use. Tetrahydrofuran (J.T. Baker reagent) was
distilled from sodium benzophenone ketyl before use. Analysis
by gas chromatography showed that the THF was free from
benzophenone. Hexanes (J.T. Baker Resi-Analyzed) was refluxed
over sodium for 1 day and distilled. Diphenylacetic acid was
Reaction of P MA with Sulfur Tetrafluoride: General
recrystallized from benzene and dried over P
2
O
5
under vacuum.
P rocedure for SF
4
Reactions. See Table 1 for specific time and
n-Butyllithium (Aldrich, 1.6 M in hexanes) was diluted ap-
proximately 4-fold in dry hexanes. The molarity was determined
by titration of diphenylacetic acid in THF at 0 °C. Perchloric acid
temperature data for reactions with PMA. Powdered PMA (0.5-
1.5 g) was placed in a Monel pressure vessel, and the vessel was
evacuated (0.1 mmHg). The vessel was cooled in a dry ice/
(
EM Science, 70%) was diluted 1:1 with water. Carbon-13-labeled
4
acetone slurry, and SF (18-21 g) was condensed into the vessel
barium carbonate was obtained from MSD Isotopes and was 98.3
atom % C. Methanol (J.T. Baker reagent) was used as received.
PMA was purchased from Scientific Polymer Products, Inc. and
through a Teflon-lined stainless steel transfer line. The reaction
vessel was sealed and warmed to reaction temperature (Table 1).
After standing at the final temperature for the time listed in Table
1, excess sulfur tetrafluoride and gaseous byproducts were vented
through 2 M KOH traps, and the pressure vessel was opened.
The gray-black solid polymer turned a faint yellow color upon
addition of acetone (20 mL). The mixture was diluted with water
13
95:5 and 75:25 copolymers of poly(methyl methacrylate) (PMMA)
and PMA were purchased from Polysciences, Inc. 4-Vinylbenzoic
acid was purchased from MTM Research Chemicals. DAST was
purchased from Aldrich Chemical Co., Inc. Thionyl fluoride and
(
10) Hagaman, E. W.; Ho, P. C.; Brown, L. L.; Schell, F. M.; Woody, M. C. J.
Am. Chem. Soc. 1 9 9 0 , 112, 7445-7450.
(11) Hudlicky, M. Chemistry of Organic Fluorine Compounds, 2nd (rev.) ed.; Ellis
Horwood PTR Prentice Hall: Chichester, 1992; pp 13-14.
Analytical Chemistry, Vol. 69, No. 19, October 1, 1997 3951

(
300 mL), and the polymer was collected by filtration and dried
under an argon atmosphere, and a solution of 2,2′-azobis(2-
methylpropionitrile) (25 mg, 0.15 mmol) in benzene (1 mL) was
added dropwise. The reaction mixture was heated to reflux for 2
h, cooled, filtered, and dried in vacuum to give 2.4 g of white
in vacuum overnight.
Reaction of P MA with Sulfur Tetrafluoride/ KF. For reac-
tions with SF
4
/ KF, anhydrous KF (5.0 g) was mixed with PMA
polymer. IR: 1696 cm-1. 13C NMR (solid): δ 172.5 (CO
(C(4)), 130.0 (CH + C(1)), 42.2 (backbone CH + CH
H), 151.7
2
).
(
1.5 g). The mixture was placed in a Monel pressure vessel. SF
4
2
was added as described above, and the vessel was allowed to stand
at room temperature for 1 h (16 h in a second run). The acetone-
insoluble product was washed with water and dried in vacuum to
give a white polymer, 1.5 g (1.4 g). IR: 1834, 1805, 1762, 1710
cm-1. 13C NMR (solid): δ 166.0 (COF), 171.4 (anhydride), 51.9
Reaction of P VBA with Sulfur Tetrafluoride. A Monel
pressure vessel was charged with PVBA (0.5 g) and evacuated at
room temperature. The vessel was cooled to -78 °C, and sulfur
tetrafluoride (5.0 g) was condensed into the chamber. The
reaction vessel was warmed to room temperature, heated to 100
°C for 18 h, and cooled to room temperature. Excess sulfur
tetrafluoride and gaseous byproducts were vented through 2 M
KOH traps, and the pressure vessel was opened. The black
product turned yellow upon addition of acetone (20 mL). The
solid was collected by filtration, washed with water (100 mL), and
dried in vacuum to give 460 mg of polymer. IR: 1809 cm-1. 13C
NMR (solid): δ 157.3 (COF), 154.3 (C4), 131.6 (CH), 123.2 (C1),
(
CH
2
), 45.0 (C), 20.8-17.9 (CH
3
).
Reaction of P MA with Cyanuric Fluoride. To a solution of
PMA (291 mg) and pyridine (267 mg) in acetonitrile (3.5 mL)
was added a solution of cyanuric fluoride (450 mg) in acetonitrile
(
sphere. The reaction mixture was stirred for 18 h at room
temperature and cooled to 0 °C, and then water (20 mL) was
added. The precipitate was filtered and dried in vacuum to give
3.5 mL) dropwise at room temperature, under an argon atmo-
20 mg of solid. IR: 1802, 1762 cm-1
.
41.4 (backbone CH + CH ).
2
2
Reaction of P VBA with Thionyl Fluoride. PVBA (200 mg)
Reaction of P MA with Thionyl Fluoride. A Monel pressure
and thionyl fluoride (4.0 g) were placed in a Monel pressure vessel
and heated at 120 °C for 17 h, as described in the last paragraph.
The procedure returned the starting acid as a major product
contaminated with a trace of acid fluoride. IR: 1810, 1700 cm-1.
Reaction of P VBA with Cyanuric Fluoride. To a slurry of
PVBA (500 mg) in pyridine (267 mg) and acetonitrile (3.5 mL)
was added a solution of cyanuric fluoride (450 mg) in acetonitrile
vessel was charged with PMA (1.0 g) and evacuated. The vessel
was cooled to -78 °C, and thionyl fluoride (3.9 g) was condensed
into the vessel. The vessel was heated to 120 °C for 17 h and
cooled to room temperature. Excess thionyl fluoride and gaseous
byproducts were vented through 2 M KOH traps, and the pressure
vessel was opened. The white product was washed with water
100 mL) and dried in vacuum. IR: 1802, 1754, 1704 cm-1
(
.
(
3.5 mL) dropwise at room temperature under argon. The
Reaction of P MA with DAST. DAST (1.77 g) was added to
reaction mixture was stirred for 18 h at room temperature, and
cold water (20 mL) was added. The precipitate was filtered,
washed with methanol (10 mL), and dried in vacuum to give 460
mg of solid. IR: 1809 cm-1.
Reaction of P VBA with DAST. DAST (838 mg) in dichlo-
romethane (3 mL) was added dropwise to a cooled (0 °C), stirred
solution of PVBA (700 mg) in dichloromethane (15 mL). The
reaction mixture was warmed to room temperature and stirred
for 1 h. Cold water (5 mL) was added, and the solvent was
removed under reduced pressure. The solid was washed with
water (100 mL) and methanol (20 mL) and dried to give 680 mg
of solid. IR: 1809 cm-1.
a cooled (0 °C), stirred solution of PMA (860 mg) in dichlo-
romethane (10 mL) or benzene (10 mL). The resulting solution
was warmed to room temperature and stirred for 1 h. After the
addition of cold water (5 mL), the solvent was removed under
reduced pressure. The soft solid was washed with water (100
mL) and dried in vacuum to give 800 (780) mg of solid. The dried
product was dissolved in acetone and poured into rapidly stirred
water. The precipitated polymer was collected by filtration and
redried in vacuum. IR: 1832, 1802, 1756 cm-1. 19F NMR (DMSO-
6
d ): φ 36.09, 34.96, 33.61. The multiple fluorine resonances are
singlets and reflect the tacticity of the polymer.
Reaction of 9 5 :5 P MMA/ P MA Copolymer with DAST. To
a stirred solution of 95:5 poly(methylmethacrylate) (PMMA)/ PMA
P reparation of 4 -Vinylbenzoic Acid Fluoride. A solution
of DAST (3.6 g, 22 mmol) in dichloromethane (10 mL) was added
dropwise to a cooled (0 °C) stirred solution of 4-vinylbenzoic acid
(
(
2.0 g) in dichloromethane (15 mL) was added a solution of DAST
210 mg) in dichloromethane (2 mL) dropwise at room temper-
(
2.96 g, 20 mmol) in dichloromethane (30 mL). The reaction
mixture was stirred for 3 h at room temperature, and cold water
10 mL) was added. The organic layer was separated, washed
with water, dried (MgSO ), concentrated, and purified by column
chromatography on silica gel using benzene as an eluent to give
colorless liquid: bp 47.5 °C (1 mmHg). 1H NMR (CDCl
): δ 7.97
d, J ) 8.2 Hz, 2H), 7.5 (d, J ) 8.2 Hz, 2H), 6.75 (d of d, J ) 17.6
and 10.9 Hz, 1H), 5.92 (d, J ) 17.6 Hz, 1H), 5.47 (d, J ) 10.9 Hz,
ature. The reaction mixture was stirred for 3 h and cooled to 0
C, and then cold water (3 mL) was added. The solvent was
°
(
removed under reduced pressure, and the solid material was
washed with water (100 mL) and dried in vacuum to give 1.8 g of
4
copolymer. 19F NMR (DMSO-d
cm-1.
6
): φ 34.53, 31.84. IR: 1829, 1733
3
(
Reaction of 7 5 :2 5 P MMA/ P MA Copolymer with DAST.
A solution of DAST (1.05 g) in dichloromethane (2 mL) was added
dropwise at room temperature to a stirred solution of 75:25
PMMA/ PMA (2.0 g) in dichloromethane (20 mL). The reaction
mixture was stirred for 3 h and cooled to 0 °C, and then cold
water (3 mL) was added. The solvent was removed under reduced
pressure, and the solid material was washed with water (100 mL)
and dried in vacuum to give 1.8 g of solid. IR: 1830, 1734 cm-1.
P reparation of P oly(4 -vinylbenzoic acid).12 A solution of
1
9
1
1
H). F NMR (CDCl
3
): φ +17.30. IR: 3080, 1807, 1605, 1406,
257, 1179, 1035, 1006, 924, 856, 773, 702 cm- . GC/ MS (70 eV),
m/ z (relative intensity): 151 (M + 1, 27), 150 (M , 100), 131 (26),
1
+
123 (22), 122 (100), 121 (57), 103 (52), 102 (41), 101 (56), 96 (64),
7
7 (56), 76 (29), 75 (51), 74 (34).
P reparation of P oly(4 -vinylbenzoic acid fluoride) by Free
Radical P olymerization. A solution of 4-vinylbenzoic acid
4
-vinylbenzoic acid (2.5 g, 16.9 mmol) in benzene (10 mL) was
(
12) Gander, R. J.; Buck, C. J.; Sipos, T. U.S. Patent 4,364,924, 1983; Chem. Abstr.
1 9 8 3 , 98, 113546Z.
purged with argon for 30 min. The solution was heated to reflux
3952 Analytical Chemistry, Vol. 69, No. 19, October 1, 1997

fluoride (1.2 g, 8.0 mmol) and 2,2′-azobis(2-methylpropionitrile)
12 mg, 0.07 mmol) in benzene (5 mL) was degassed with three
2
(CH), 41.0 (backbone CH + CH ). A 9:1 acid fluoride:anhydride
distribution in the product was determined from peak deconvo-
(
freeze-pump-thaw cycles and sealed. The solution was heated
to 80 °C for 16 h and cooled to room temperature. The white
precipitate that formed was collected by filtration, washed with
benzene, and dried in vacuum to give 600 mg of benzene-insoluble
polymer. IR: 1809 cm-1.
lution of the overlapping carbonyl resonances in the NMR
spectrum.
P reparation of P S-1 3 COF with SF
. PS- CO
13
H (0.5 g) and
4
2
4
SF (6 g) were placed in a Monel pressure vessel (see general
procedure) and allowed to stand for 1 h at room temperature.
The solid product was removed from the reactor, triturated with
benzene (50 mL), filtered, and dried to give 0.46 g of a light yellow
solid. IR: 1791 cm-1.
P reparation of Monocarboxy-Terminated P olystyrene. All
glassware was oven dried (140 °C) overnight, assembled hot, and
cooled under a stream of dry argon. Benzene (80 mL) was
transferred under argon via cannula to a round-bottom flask (100
mL) with a sidearm stopcock. The benzene was dried with
n-butyllithium (5 mL, 1.6 M), and 50 mL was vacuum transferred
to a Schlenk reaction tube (i.e., sidearm stopcock and female joint
on the side of the tube, with a male joint on top), which was fitted
with an adapter with a sidearm stopcock. Styrene (5.5 mL, 52.9
mmol), vacuum transferred from the lithium hydride to a round-
bottom flask with a sidearm stopcock, was added to the Schenk
reaction tube via syringe. The contents of the flask were degassed
with three freeze-pump-thaw cycles (10-4 Torr) and, on the final
cycle, thawed under argon. Immediately after the benzene had
melted, n-butyllithium (0.2 mL, 0.38 M in hexanes) was added
with stirring until a pale yellow color persisted (i.e., pretitrated
the impurities). An additional portion of n-butyllithium (1.17 mL,
RESULTS AND DISCUSSION
P olymer Derivatization. Table 1 summarizes the effective-
ness of four fluorinating reagents in forming the acid fluoride from
the free carboyxlic acid residues of PMA. As previously reported,
neat SF
of anhydride.13 In our experiments, however, the reaction between
SF and PMA produced PMAF in variable yield, always ac-
companied by a significant amount of the unwanted anhydride.
4
cleanly converts PMA into PMAF without the formation
4
The results using neat SF , preparations 1-3, show that formation
4
of the acid fluoride is favored as the reaction temperature
increases. The use of still higher temperatures is not likely to
increase product selectivity since even gentle warming with
prolonged reaction times leads to the conversion of the acid
fluoride into the trifluoromethyl moiety.1 Performing the fluorina-
tion in the presence of finely divided KF, an HF scavenger by
virtue of the formation of potassium bifluoride (KF‚HF), did not
prevent the formation of the anhydride (preparations 4 and 5). IR
spectra of the reaction products of preparations 1-5 in Table 1
0
.38 M, 0.44 mmol) was added to the reaction mixture. THF (1.0
mL) was added to the yellow reaction mixture, and it immediately
turned reddish orange. The reaction mixture was warmed to
room temperature and stirred for 30 min. 13CO
2
was generated
(35%, 10
3
(1.0 g, 5.0 mmol). The gas was passed through
under a helium atmosphere by the addition of HClO
4
-
1
3
had absorptions at 1800 and 1760 cm 1, the signature of the
mL) to Ba CO
anhydride, in addition to the 1832 cm- resonance of the acyl
fluoride group. A low concentration of free carboxylic acid (1700
cm-1) was also observed in the preparations made with KF.
The reaction of PMA with cyanuric fluoride did not produce
the acid fluoride. The material isolated from this reaction was
almost exclusively the anhydride. The resolution of the carbonyl
bands of the anhydride function in the IR spectrum of preparations
1
a coiled tube containing Drierite and collected in a round-bottom
flask containing a sidearm stopcock cooled in liquid nitrogen. The
flask containing the 13CO
2
was placed under argon and quickly
attached to the female joint on the side of the Schlenk tube. The
reaction mixture was quickly poured onto the 13CO
2
, and the color
was immediately discharged. The polymer solution was poured
into excess cold (0 °C) methanol (500 mL), and the solid polymer
was isolated by filtration. The polymer was dissolved in warm
benzene (50-60 mL) and precipitated two times by the slow
addition of cold methanol (600 mL). The polymer was isolated
by precipitation into cold methanol (600 mL) and dried under
vacuum at 50 °C to yield 4.10 g of a white powder. The polymer
has a molecular weight of ∼10 000 and a polydispersity of <1.1.
1
-5 was the same as in preparation 6.
In the reactions of PMA with DAST, the acid fluoride was the
major product. The distribution of acid fluoride and anhydride
products was solvent- and temperature-dependent (preparations
8
-10). The room temperature reaction of PMA with DAST in
dichloromethane produced the acid fluoride group in >90% yield.
Every preparation in Table 1 contains the anhydride as a major
or minor reaction product, showing the facility with which this
product forms from PMA. By virtue of its 1,5-dicarbonyl structure,
PMA can adopt conformations that optimally predispose the
molecule for formation of the intramolecular cyclic anhydride.
To test whether the proclivity of PMA to form the anhydride
is unique or is common to a broader class of aliphatic carboxylic
acids, the conversion of monocarboxy-terminated polystyrene (PS-
1
3
C NMR (solid): δ 181.9 (CO
backbone CH + CH ).
P reparation of P S- COF with DAST. A solution of DAST
2
H), 145.9 (C), 128.5 (CH), 41.0
(
2
1
3
(
0.2 mL) in dichloromethane (20 mL) was added dropwise (1 h)
1
3
2
to a cooled (0 °C), stirred solution of PS- CO H (1.02 g) in
dichloromethane (40 mL). The solution was stirred 1 h and
warmed to room temperature. The organic solution was extracted
twice with water (30 mL), and the polymer was precipitated from
dichloromethane by the slow addition of methanol (100 mL). The
rubbery polymer was transferred to a Buchner filter funnel,
washed with water, dried under vacuum, and pulverized to yield
CO
In PS-CO
2
H) into its acid fluoride derivative with DAST was performed.
H, the carboyxlic acid functions cannot form an
2
intramolecular anhydride, and the acid functionality is chemically
dilute. The low concentration of carboxylic acid groups in this
polymer and NMR sensitivity considerations required the syn-
thesis of the acid to be performed with 13C-labeled carbon dioxide
6
30 mg of white polymer. IR: 1791 cm-1 (carbonyl stretch of
13COF). The carbonyl resonance is weak by virtue of chemical
dilution and appears in the region of the spectrum with the
aromatic overtone bands. It was detected in the IR difference
spectrum with the acid precursor by nulling the overtone bands.
13C NMR (solid): δ 172 (anhydride), 163.9 (COF), 146.1 (C), 128.2
13
(
98 atom % C, see Experimental Section). The reaction of PS-
(
13) Khardin, A. P.; Protopopov, P. A.; Solomina, T. I. Macromol. Synth. 1 9 8 5 ,
9, 65-67.
Analytical Chemistry, Vol. 69, No. 19, October 1, 1997 3953

13CO
2 2
H with DAST in CH Cl
produces the acid fluoride, PS-13-
2
COF, in 90% yield, the remainder of the material occurring as the
anhydride. The assignment of the minor reaction product to the
anhydride is based on the 172 ppm chemical shift of the minor
peak, which is similar to the anhydride resonance in the PMA
13
series. The spectrum of PS- COF is discussed in the next section.
These results show that the formation of the anhydride from an
aliphatic carboyxlic acid is not a sensitive function of polymer
4
structure. Since the product forms in both the SF and DAST
reactions and is repressed by faster reaction rates, the anhydride
production may be occurring through the reaction of initially
formed acid fluoride with unreacted acid.
The conversion of PVBA into PVBAF was examined using the
same suite of fluorinating reagents listed in Table 1. The reaction
4
of PVBA with SF , cyanuric fluoride, or DAST generated the acid
fluoride exclusively, as judged by the 1810 cm-1 resonance in the
IR spectrum. The reaction of PVBA with thionyl fluoride at 120
°
C for 17 h gave only a trace of the acid fluoride. Thus, the
production of the aromatic acid fluoride in high yield from the
free acid is effected with any of several fluorinating agents. The
lack of anhydride production in these reactions can be explained
by the reduced positive charge at the carbonyl carbon of the
resonance-stabilized aromatic acid fluorides, relative to aliphatic
acid fluorides. In the reaction of an acid fluoride with an acid
residue, postulated above for the formation of the anhydride, the
lower electrophilicity of the carbonyl carbon is expected to result
in a slower reaction rate.
Fig u re 1 . (a,b) [ H,19F] 1H-13C CP/MAS 13C NMR spectra of
1
PVBAF. (a) The conventional spectrum with all carbon resonances
exhibiting intensities that reflect atomic ratios. (b) H- C dipolar
dephased spectrum (40 µs dephasing interval) showing the three
1
13
PVBAF was also prepared by free radical polymerization of
1
19
19
13
-vinylbenzoyl fluoride. The IR and 19F NMR spectra of the
nonprotonated resonances of the polymer. (c-e) [ H, F] F-
C
4
CP/MAS 13C MMR spectra of PVBAF, illustrating the growth of carbon
insoluble polymer from this reaction are identical to that of the
19
13
resonances as a function of F- C cross polarization contact time,
t. (c) t ) 100 µs; the spectrum is dominated by the resonance of the
acid fluoride carbon, the only carbon directly bonded to fluorine and
hence showing rapid cross polarization. (d) t ) 800 µs; the
resonances of carbons in the second and third bonding spheres, with
slower CP rates, grow into the spectrum. (e) t ) 5 ms; at t
comparable to TCF for carbons in the second and third bonding
spheres, these carbons and the fluorocarbon resonances show strong
intensity.
polymer obtained by the reaction of PVBA with SF
fluoride, or DAST (vide infra).
4
, cyanuric
1
3
C NMR: P oly(vinylbenzoic acid fluoride) (P VBAF). The
[1
spectra of PVBAF are shown in Figure 1a and b, respectively.
The backbone carbons (CH + CH ) appear at 41.4 ppm, and five
of the six aromatic carbons are represented by the two peaks at
23.2 and 131.6 ppm. The acyl fluoride carbon, C(R), and the
H, F] H- C CP/ MAS and corresponding H dipolar dephased
19
1
13
1
2
1
ring carbon para to the acid fluoride function, C(4), appear as a
closely spaced pair of resonances near 155 ppm (see Figure 1 for
PVBAF structure and numbering). The dipolar dephased spec-
tion, DCF, of 26 kHz.7 Fluorocarbons exhibit a fast cross polariza-
tion rate, 1/ TCF, characterized by an exponential time constant,
T
CF, which is on the order of tens to hundreds of microseconds.
2
trum displays the three nonprotonated sp carbon resonances. The
19
13
Fluorocarbon resonances will dominate F- C CP spectra
acquired with cross polarization contact times on the order of TCF
for bonded pairs, as illustrated in Figure 1c. The contact time
for this spectrum is 100 µs. Only the carbon directly bonded to
fluorine has strong signal intensity in the spectrum. The 157.3
ppm resonance is assigned to C(R) from this spectrum. With a
longer contact time (0.8 ms, Figure 1d), the resonances of carbons
more distant from the fluorine have accrued intensity. Specifically,
the 123.2 ppm resonance evolves as the next most intense
resonance, confirming the C(1) assignment made in the dipolar
dephased spectrum. Carbons two bonds removed from fluorine
have C-F internulcear distances that are weakly dependent on
structure and cluster around 0.24 nm, for which the maximum
dipolar interaction, DCF, is 4 kHz.
ring carbon bound to the carboxyl group, C(1), is assigned to
the 123.2 ppm resonance. The two low-field resonances are
resolved and assigned by the 19F- C CP spectra.
13
The suite of 19F- C CP spectra of PVBAF shown in Figure
13
1
c,d,e illustrate the subspectral editing capability of this experi-
ment. Resonance selection is based on the strength of the 19F-
13C dipolar interaction and the experimentally adjusted 19
F- C
13
cross polarization contact time. The major trend in the series, c
f d f e, is the accrual of resonance intensity of two aromatic
carbon resonances relative to the 157.3 ppm resonance. At long
contact time, weak signals due to the remote backbone carbon
sites also appear in the spectrum. It is probable that intermo-
lecular F-C interactions (g0.35 nm) contribute more to this signal
than intramolecular interactions (∼0.6 nm, minimum).
Carbons three bonds from a fluorine substituent are the first
carbons that can exhibit C-F internuclear distances with strong
configurational and conformational dependence. Internuclear
distances typically lie in the range of 0.28-0.38 nm. These
distances correspond to a maximum DCF of 1300-500 Hz. In
1
9
13
F- C cross polarization rates are inversely proportional to
the sixth power of the carbon-fluorine internuclear distance. The
carbon directly bonded to fluorine in PVBAF has an internuclear
distance, rCF, of ∼0.13 nm and a maximum dipole-dipole interac-
3954 Analytical Chemistry, Vol. 69, No. 19, October 1, 1997

PVBAF, one aromatic carbon, three bonds distant from the
fluorine, C(2), is estimated to lie near the minimum distance from
the fluorine by virtue of the coplanarity of the acyl fluoride group
and aromatic ring.14 This resonance, 131.6 ppm, accrues intensity
at about half the rate of the C(1) resonance. Unlike the distinction
between carbons one and two bonds removed from the fluorine,
no “snapshot” spectrum exists that will discriminate carbon
centers two and three bonds removed from the fluorine.
The direct difference double cross polarization (DCP) experi-
ment has the advantage of monitoring the 13C- F cross polariza-
tion process directly. The detected magnetization does not
19
1
depend on relaxation in the H spin system because this
9
dependence is eliminated in the differencing protocol. Phase
modulation of H1F during the 19F- C cross polarization interval
is normally used to defeat oscillatory magnetization behavior
expected for isolated spin pair interactions. This case does not
strictly apply for 19F- C cross polarization since the fluorine
13
13
nuclei are not isolated spins but show significant T
2
relaxation.
Direct 19F- C cross polarization intensity does show strong,
complex dipolar oscillations. Phase modulation, implemented in
the DCP measurements, promotes an exponential cross polariza-
tion response with measured values of (TCF)- longer than those
estimated from direct cross polarization experiments (see Experi-
mental Section).
13
1
Fig u re 2 . (a-c) [ H, F] H-13C CP/MAS 13C NMR spectra of the
carbonyl region of PMA and two PMAF preparations. (a) PMAF
sample prepared at room temperature is predominantly acid fluoride
(X ) F). The raised low-field shoulder of the acid fluoride resonance,
1
19
1
T
CF for the C(R), C(1), and C(2) resonances in PVBAF are
0
.20, 5.6, and 10.5 ms, respectively. Using 10% as the estimated
relative error in these rates, the rate ratio for the C(R) and C(1)
carbons, TC(1)F/ TC(R)F, is 29 ( 6. The corresponding distance ratio,
(r
C(R)F)-6/ (rC(1)F)-6, is 36. (Distance measurements from PVBAF
are not available. The distances used in this comparison come
from the crystal structure of methyl-4-carbonyl fluoride ben-
166 ppm, indicates the presence of anhydride. (b) Impure PMAF
sample prepared at -40 °C shows acid fluoride and anhydride
carbonyl resonances (X ) F, OCOR) in near equal concentrations.
(c) PMA spectrum displays the heterogeneously broadened carboxylic
acid resonance (X ) OH). The small resonance at 206 ppm in (a)
1
4
zoate. ) If it is assumed that the only varying parameter in the
cross polarization process between 19F and 13C(R) or 13C(1) is
and (b) is from acetone occluded in the polymer during precipitation.
1
19
1
13
13
(d) Complete [ H, F] H- C CP/MAS C NMR spectrum of an
impure PMAF preparation, containing both anhydride and acid fluoride
internuclear distance, then [(rC(R)F)-6/ (rC(1)F)-6]/ (TC(1)F/ TC(R)F) )
functions. (e,f) [1H,19F] F- C CP/MAS C NMR spectra of the
same material shown in (d) acquired with (e) t ) 800 µs and (f) t )
200 µs. The latter two spectra show that the acid fluoride resonance
is derived from an aliphatic carboxylic acid, as the carbon resonance
in the second bonding sphere, cross polarizing in hundreds of
microseconds, is in the aliphatic carbon resonance manifold.
19
13
13
1
. The value, 36/ (29 ( 6) ) 1.2 ( 0.3, validates the assumption
within the error in the rate measurements. High discrimination
in signal intensity between carbons in these environments is
achieved using a cross polarization contact time, t, less than TCF
of the CF bonded pair. The signal intensity ratio, C(R)/ C(1), in
the spectrum acquired with t ) 100 µs is 15 (Figure 1c).
The analogous rate and distance ratios for C(1) and C(2) of
PVBAF are 1.9 ( 0.3 and 2.7, respectively. The ratio [(rC(1)F)-6/
appear at 18.6, 45.8, 56.3, and 182.2 ppm, respectively. The
aliphatic resonances in the spectrum of PMA and PMAF are
similar, though subtle differences are apparent. The carbonyl
-6
C(2)F
(r ) ]/ (TC(2)F/ TC(1)F) ) 1.5 ( 0.3, misses unity within the stated
region of [ H, F] H- C CP/ MAS 13C NMR spectra of PMA and
1
19
1
13
error. This comparison is more complicated than the previous
one. The evaluation of the rates for these carbons requires long
t, where measured signal intensity for C(2) will represent not only
this carbon resonance but also contain contributions from other
aromatic methines. Nonspecific signal intensity originates from
intra- and intermolecular sources and is maximized in this example
since all aromatic methines have degenerate chemical shifts. The
qualitative trend is correct: the C(1) resonance is consistently
more intense than the C(2) signal for t e 2TCF, but the minimal
difference in DCF for carbons two and three bonds removed from
the fluorine, ∼2.7 kHz, is not great enough to allow a quantitative
discrimination of carbons in the second from the third bonding
spheres based on (TCF)-1 data, especially when these resonances
two PMAF samples, prepared with SF at different reaction
temperatures, reveal dramatic changes in the carboxyl region of
4
the spectrum (Figure 2a-c). The 182 ppm resonance of the free
carboxylic acid residue of PMA (Figure 2c) is absent in the SF -
4
treated polymer spectra (Figure 2a,b). The spectrum of the low-
temperature preparation (Figure 2b) shows partially resolved
carbonyl resonances of similar intensity at 166 and 172 ppm. The
spectrum of the PMAF sample prepared at room temperature is
dominated by the 166 ppm resonance (Figure 2a). On a chemical
shift basis, the 172 ppm resonance arises from an ester or
anhydride. The IR spectrum distinguishes between these choices
in favor of the anhydride.
1
13
The H- C CP spectra of PMAF preparations shown in Figure
overlap.
2a,b exhibit a resonance at 206 ppm. This resonance and another
P oly(methacrylic acid fluoride) (P MAF). The methyl,
quaternary, methylene, and carboxyl carbon resonances of PMA
at 30 ppm (not shown) arise from acetone, physically occluded in
the polymer network during purification by precipitation from
acetone solution. These signals do not appear in the 19F- C CP
13
(
14) Burns, J. H.; Hagaman, E. W. Acta Crystallogr. 1 9 9 3 , C49, 1393-1396.
Analytical Chemistry, Vol. 69, No. 19, October 1, 1997 3955

spectra taken with contact times e1 ms. The carbon atoms of
solvent molecules, necessarily having intermolecular distances to
the fluorine site, are too far removed to cross polarize appreciably
in this time. The ability to edit the spectrum in a manner that
excludes resonances from entrained solvents and reagents is a
practical feature of the 19F- C CP experiment that permits the
determination of chemical modifications in complex mixtures or
unknowns that contain reagent and/ or solvent impurities.
13
1
13
Figure 2d displays the H- C CP spectrum, and Figure 2e,f
1
9
13
shows two F- C CP spectra of an impure (acid fluoride plus
anhydride) PMAF preparation. The latter spectra, acquired using
(
e) t ) 800 µs and (f) t ) 200 µs, allow a qualitative assessment
of the bonding in this substance. The 166 ppm resonance shows
fully developed intensity at 200 µs, implying a TCF in the tens of
microseconds, thus identifying the resonance as that of the acid
fluoride carbon. The quaternary carbon resonance (46 ppm)
requires hundreds of microseconds to cross polarize, placing it
in the second bonding sphere. These spectra show the acid
fluoride resonance is from an aliphatic carboxylic acid moiety
since the resonances from the carbons in the second bonding
3
sphere are from sp carbon resonances. This qualitative criterion,
resting entirely on the observation of resonances with either sp2
3
or sp carbon hybridization in the second bonding sphere, is free
of the uncertainties arising from resonance overlap inherent when
using 13C carboxyl resonance chemical shifts or IR carboxyl
resonance bands to differentiate aromatic and aliphatic acid
residues.15
Fig u re 3 . (a) [ H,19F] 19F-13C CP/MAS 13C NMR spectrum of the
1
13
carbonyl region of a PS- COF sample prepared with DAST. The
spectrum show the band shape of the acid fluoride resonance of the
material. (b) [ H, F] H- C CP/MAS C NMR spectrum of the
carbonyl region of PS- COF at high vertical expansion for direct
comparison with (a). The low-field shoulder on the acid fluoride
resonance in (b) arises from the anhydride. A trace of acid at 182
1
3
1
19
1
13
13
C-Enriched Monofluorocarbonyl-Terminated P oly(sty-
13
rene) (P S-1 3 COF). The H- C CP/ MAS 13C NMR spectra of
1
13
13
13
2
PS- COF and of PS- CO H are shown in Figure 3c,d, respectively.
The spectra illustrate the conversion of a simple aliphatic car-
boxylic acid polymer into its acid fluoride, evident from the shift
in the carbonyl resonance from 182 to 164 ppm. For this sample,
the fluorination was performed with DAST.
1
19
1
13
ppm is apparent in this preparation. (c,d) [ H, F] H- C CP/MAS
13
13
C NMR spectra. (c) PS- COF: part of this spectrum is shown at
high gain in (b). This material was used to measure the spectrum in
a). (d) PS-13CO2H.
(
1
3
The spectrum of PS- COF shown in Figure 3c is depicted in
Figure 3b at high vertical gain to magnify the details of the
carbonyl resonances. The 19F- C CP/ MAS 13C NMR spectrum
of the material (Figure 3a) contains just the resonance of the acid
fluoride (the resonances of carbons in the second bonding sphere
are 2 orders of magnitude less intense by virtue of their isotopic
abundance). This spectrum reveals the band shape of just the
acid fluoride resonance. Comparison of the carbonyl band shapes
in these two spectra show that a low-field component to this band,
centered at 172 ppm, occurs in Figure 3b (as does a trace of the
acid at 182 ppm). The 172 ppm component of the resonance band,
by virtue of chemical shift and distance from fluorine, must arise
from the carbonyl resonance of either an ester or an anhydride.
The PMA results discussed above lead us to suggest the latter.
Several conclusions follow from these experiments. The reaction
formation of cyclic anhydrides, as is possible in PMA, but is a
more general phenomenon. Intermolecular association of car-
boxylic acid groups, i.e., hydrogen-bonded dimerization, that
characterizes the structure of carboxylic acids in the solid state,16
orders the acids pairwise, arranging the molecular participants
for possible anhydride formation. We suggest that this energeti-
cally favorable association occurs in these carboxylic acid poly-
mers, facilitating anhydride formation. Under the optimum
reaction conditions for the formation of the acid fluoride (room
13
2 2 4
temperature DAST reaction in CH Cl or room temperature SF
reaction, neat), the anhydride is a minor product (10%) in the cases
examined. Though a reaction producing a quantitative yield is
required of a bona fide titration method, the conversion rate of
9
0% for aliphatic acids is a usable performance mark for a broadly
applicable labeling reaction.2
4
of DAST (and SF ) with polymeric aliphatic carboxylic acids to
One application of the acid fluorination/ NMR experiments
demonstrated in this paper is their use in measuring the relative
contributions of aromatic and aliphatic carboxylic acid functionality
in complex mixtures. The observation of aromatic or aliphatic
resonances in the second bonding sphere from the fluorine is the
basis for the identification and differentiation of aromatic and
aliphatic carboxylic acids. Note that, for most acids, the identifica-
tion of the acid as aromatic or aliphatic does not require the
form the acyl fluoride derivative is generally contaminated with
the anhydride of the acid. The formation of the anhydride may
also occur in nonpolymeric carboxylic acids in solution and not
represent a synthetic limitation, as the acid fluoride yield is high
(
90%) and the product easily purified. Anhydride formation does
not require favorable intramolecular orientation that facilitates the
(
15) Strictly speaking, this criterion does not differentiate olefinic, R,â-unsaturated
carboxylic acids from aromatic acids, both with sp2 carbon in the second
bonding sphere. It gives the sum (aromatic
distinguishes these from the sum (aliphatic + benzylic acids).
+
olefinic acids) and
(16) Nyburg, S. C. X-ray Analysis of Organic Structures; Academic Press: New
York, 1961; pp 184-190.
3956 Analytical Chemistry, Vol. 69, No. 19, October 1, 1997

separate identification of carbons in the second from carbons in
the third bonding sphere. Independent of this distinction, the
assignment is unambiguous for all but one class of acid fluo-
rides: benzylic acid fluorides, e.g., phenylacetic acid or 1,2,3,4-
tetrahydro-1-naphthoic acid fluoride. In these aliphatic acid
derivatives, the potential for confusion arises because the carbon
three bonds removed from the fluorine is aromatic and will, in
the minimum C-F distance conformation, contribute to the short
1
9
13
contact time F- C CP spectra and lead to erroneous conclu-
sions.
1
9
F NMR. The direct observation of the 19F resonance in
fluorine-containing organics by solid state 19F MAS/ NMR is an
efficient, high signal-to-noise experiment that is complementary
to the triple-resonance 13C NMR experiments described above.
Previous work has shown that the static line width of the solid
state 19F NMR resonance can vary by more than an order of
magnitude, depending on the nature of the fluorine functional
group.5 The two major orientation-dependent interactions that
contribute to the powder line width are chemical shift anisotropy
(
CSA) and the dipole-dipole interaction. CSA is an intrinsic
property of the functional group reflecting asymmetric electron
density around the fluorine atom. In common organofluorine
functional groups, the magnitude of the interaction is 0-160 ppm
(
0-15 kHz at 2.35 T). The dipole-dipole interaction is propor-
1
9
19
tional to the inverse third power of the F- F internuclear
distance. Unlike the short-range nature of CSA, the dipolar
interaction strength at a particular fluorine may have contributions
from several neighboring fluorine nuclei. The maximum dipolar
interaction for the pair of fluorine nuclei of a difluoromethylene
Fig u re 4 . [1H] 19F MAS spectrum of (a) 95:5 PMMA:PMAF
copolymer and (b) PMAF, both acquired using MAS ) 6050 ( 50
Hz. The spectra depict the acid fluoride resonance of these materials
that have isotropic chemical shifts at (a) 26 and (b) 33 ppm. Side
bands, marked with asterisks, flank the center band at multiples of
the MAS spinning speed. The similarity of the center band/side band
comb in the two materials signifies similar static line widths. The
sharp resonance at -174.4 ppm in (b) is from 2-fluoroadamantane,
added to the sample as a secondary chemical shift standard. Small
impurity peaks at -130 and from -140 to -150 ppm are due to
diethylammonium fluoride and adsorbed HF, respectively.
2
group (-CF -) with internuclear separation of 0.23 nm is 18 kHz.
Thus, the two major interactions that determine the static fluorine
line width can have comparable magnitudes. The relative strength
of these interactions will depend on the nature of the fluorine
functional group and functional group density.
Figure 4 compares the 19F MAS NMR spectra of two acid
fluoride polymer derivatives of PMA. The spectrum in Figure 4a
is of a 95:5 random copolymer of PMMA and PMAF which was
synthesized from commercially available 95:5 PMMA:PMA and
DAST. The spectrum in Figure 4b is of PMAF. In the PMAF
polymer, the fluorine density is high, and the 19F- F internuclear
distance is minimized. In the 95:5 PMMA:PMAF copolymer, the
average 19F- F intramolecular distance is on the order of a few
nanometers. Intermolecular 19F distances are shorter but are also
dilute in the copolymer, relative to PMAF.
polymer attributed to adsorbed HF.5 The fractional center band
area in the spectra of all PVBAF preparations in 63 ( 2% at MAS
)
6050 ( 50 Hz.
The 19F spin-spin relaxation time constants, T
2
, of the PMAF
polymers and of PVBAF were measured with the expectation that
they would be sensitive to differences in the strength of the 19F-
19
19F dipolar interaction. The anticipated trend in T
2
, as judged from
19
the line width, is not observed in PMAF and PMMA/ PMAF
copolymers (Figure 4), where the increase in the average 19F-
19
F internuclear distance in the copolymer is expected to be
The decrease in fluorine density and the corresponding
increase in 19F- F internuclear distance between these two
polymers are not evident in their spectra. In fact, the equivalence
of the two spectra is expressed quantitatively by the equal
fractional centerband area for the acid fluoride resonance of the
polymers, recorded at constant MAS speed (6050 ( 50 Hz). The
equal centerband area (44 ( 2%) and sideband intensity pattern
for both PMAF polymers indirectly indicate that the static line
widths of these materials are similar. From this result, it follows
that the static line width is dictated by CSA and that the dipolar
contribution to the line width is small compared to the magnitude
of CSA, even in the high fluorine density PMAF polymer.
The solid state 19F NMR spectra of PVBAF prepared by free
radical polymerization of 4-vinylbenzoic acid fluoride and by DAST
fluorination of PVBA show no differences, apart from a small
resonance at -150 ppm in the spectrum of the DAST-treated
reflected in narrowing line width as the 19F density decreases.
19
The measured line width of the acid fluoride resonance of PMAF
and PMMA/ PMAF is 1900 ( 50 Hz (T ) 1/ πv ) 168 ( 5 µs).
2
1/ 2
This apparent inconsistency is resolved by a T2 relaxation
measurement which does demonstrate the expected trend: T in
2
PMAF, 160 µs (v1/ 2 ) 1/ πT ) 1990 Hz); T in 95:5 PMMA:PMAF,
2
2
250 µs (ν1/ 2 ) 1/ πT ) 1270 Hz). The change in T corresponding
2
2
to a line width narrowing of 700 Hz is not reflected in the spectra
because heterogeneous broadening (chemical shft dispersion)
occurs in the copolymer and masks the T narrowing. The slight
2
peak splitting of the first upper sideband in Figure 4a relative to
that in Figure 4b hints at the narrower homogeneous line width
inherent in this sample. All preparations of PVBAF have the same
line width, v1/ 2 ) 975 Hz, and T
2
) 1/ πv1/ 2 ) 326 µs. T
2
inferred
from the line width agrees with the T
2
assessed by rate measure-
ment (320 ( 10 µs).
Analytical Chemistry, Vol. 69, No. 19, October 1, 1997 3957

The long 19F T
5:5 PMMA:PMAF copolymer suggests a weak homonuclear
is PVBAF relative to that of the low 19F density
used to advantage to reveal just those carbons in the immediate
vicinity of the 19F spin probe. Sorting these signals qualitatively
by “bond distance” from the fluorine allows the identification and
differentiation of aromatic and aliphatic carboxylic acid functional-
ity. It is anticipated that this chemical/ spectroscopic couple will
be useful in the analysis of complex organic mixtures where more
routine techniques of determining aromatic and aliphatic carboxy-
lic acid content are not applicable.
2
9
dipolar interaction in the former. Greater molecular mobility and
consequent motional narrowing of the dipolar interactions in the
styrene-based polymer, relative to the PMA-based systems, could
account for this ordering. This hypothesis is in agreement with
the narrower powder line width of the 19F resonance in PVBAF
relative to those of the PMAF polymers, as indicated by the higher
fractional center band area in the former when compared at equal
MAS speed.
ACKNOWLEDGMENT
This research was sponsored by the Division of Chemical
Sciences, Office of Basic Energy Sciences, U.S. Department of
Energy, under Contract DE-AC05-96OR22464 with Oak Ridge
National Laboratory, managed by Lockhead Martin Energy
Research Corp. S.K.L. was supported by an appointment to the
ORNL Postdoctoral Research Associate Program administered by
Oak Ridge Institute for Science and Education and ORNL. E.W.H.
CONCLUSION
The chemical studies reported herein indicate that several
4
reagents, notably DAST, cyanuric fluoride, and SF , convert
polymeric aromatic acids into acid fluorides quantitatively. Ali-
phatic carboxylic acids can be converted to the acid fluoride with
4
DAST or SF at room temperature in yields of 90%. The balance
1
3
of the material is found to be the anhydride. Though not an ideal
titration reagent, the error introduced in the chemical derivatiza-
tion is not prohibitive and is of the same order as the errors which
accrue in the solid state NMR measurements.
thanks Ms. Leigh Farrow for the DAST preparation of PS- COF
and PVBAF.
Received for review April 7, 1997. Accepted July 16,
19
13
F- C cross polarization is an excellent diagnostic NMR tool
X
1997.
to identify fluorine-containing functional groups in fluorine-
derivatized organics. Chemical dilution of the fluorine atom and
the distance dependence of the cross polarization process can be
AC9703663
X
Abstract published in Advance ACS Abstracts, September 1, 1997.
3958 Analytical Chemistry, Vol. 69, No. 19, October 1, 1997