Efficient synthesis of well-defined, high molecular weight, and processible polyanilines under mild conditions via palladium-catalyzed amination [8]

Full text of DOI:10.1021/ja001718h

7606
J. Am. Chem. Soc. 2000, 122, 7606-7607
Efficient Synthesis of Well-Defined, High Molecular
Weight, and Processible Polyanilines under Mild
Conditions via Palladium-Catalyzed Amination
Xiao-Xiang Zhang, Joseph P. Sadighi,
Thomas W. Mackewitz, and Stephen L. Buchwald*
Department of Chemistry
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
Figure 1. 1H (a) and 13C (b) NMR spectra in THF-d8 (*, solvent peaks;
×, H2O) and GPC chromatograph in THF (c) for BOC-PANI.
ReceiVed May 18, 2000
Scheme 1
Polyaniline (PANI) ranks among the most potentially useful
conducting organic polymers, due to its environmental stability
and tunable electrical conductivity; many industrial applications
have been demonstrated.1 PANI is generally prepared by the
chemical or electrochemical oxidation of aniline.2 The polymer
thus obtained may be processed in the neutral emeraldine base
(EB) form, as a solution in N-methylpyrrolidinone (NMP);3 the
conductive salts of emeraldine with bulky organic counterions,
particularly camphorsulfonate, are typically processed from
m-cresol.4 Because the standard syntheses of PANI give rise to a
certain level of regioerrors, and produce some insoluble materials,5
alternative methods have been designed. Most notably are the
polycondensation of 1,4-phenylenediamine with a hydroquinone
surrogate,6 and the recent enzymatic oxidation of aniline in the
presence of a polyanionic template.7
The palladium-catalyzed amination of aryl halides and triflates,8
an increasingly important method for the synthesis of arylamines
in many applications,9 holds great promise for the construction
of the PANI framework. Polymerizations based on this reaction
would be regiospecific and could afford access to modified
polymers with otherwise inaccessible substituent patterns. Pal-
ladium catalysis has been used to prepare a number of arylamine
polymers.10 However, no synthesis of the conductive p-PANI by
this route has yet been reported, possibly because aryl halides
bearing the N-H group in the para position are generally poor
substrates for the aryl amination process. Previously, we reported
a general route to controlled-length and end-functionalized oligo-
anilines, using the Pd/BINAP catalyst system.11 New-generation
catalysts, using bulky, electron-rich phosphinobiphenyl ligands,
display far higher activity toward electron-rich substrates and
allow the amination of aryl chlorides.12 We now report the
efficient synthesis of regiopure BOC-protected PANI of high
molecular weight. These PANI precursors are soluble in common,
low-boiling organic solvents, and are readily and quantitatively
converted to the parent PANI.
The simplest palladium-catalyzed synthesis of PANI would
involve the polymerization of 4-bromoaniline, or the copolym-
erization of 1,4-dibromobenzene with 1,4-phenylenediamine.
Initial studies showed that these reactions only produced low
molecular weight polymers of poor solubility. In contrast, the
BOC-protected monomer 1, prepared by selective deprotection
of the fully protected dimer bromide 211 at the terminal amine
position, underwent efficient step-growth polymerization (Scheme
1). The ab-type monomer may be prepared on large scale and
purified by crystallization; no chromatography is necessary.
Polymerization of 1 using a combination of Pd2(dba)3 and
phosphine ligand 2-(di-tert-butylphosphino)biphenyl (L/Pd ) 3)
as the catalyst affords partially BOC-protected polyaniline (BOC-
PANI). A typical polymerization was carried out in THF between
25 and 80 °C, with 1.4 equiv of the base, NaOt-Bu, per halide.
During a reaction time of 24 h, the initially clear solution became
a gel. The crude product was washed by sonication in ether, water,
and methanol. The remaining solid was dissolved with sonication
in THF and reprecipitated by addition of methanol. Isolated yields
of the pale olive or pale gray polymers range from 80% to 96%,
depending on the reaction conditions. The elemental analyses of
these materials are consistent with the expected structures.
The polymers can be dissolved, with the aid of sonication, in
common organic solvents such as THF and CHCl3, permitting
(1) For reviews on polyaniline, see: (a) MacDiarmid, A. G.; Epstein, A.
J. Faraday Discuss. Chem. Soc. 1989, 88, 317. (b) Heeger, A. J. Synth. Met.
1993, 55-57, 3471. (c) MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1995,
69, 85. (d) MacDiarmid, A. G. Synth. Met. 1997, 84, 27.
(2) (a) MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Somasiri, N. L.
D.; Epstein, A. J. In Conducting Polymers; Alcacer, L., Ed.; Reifel: Dordrecht,
The Netherlands, 1987; pp 105-120. (b) Gospodinova, N.; Terlemezyan, L.
Prog. Polym. Sci. 1998, 23, 1443.
(3) Angelopoulos, M.; Asturias, G. E.; Ermer, S. P.; Ray, A.; Scherr, E.
M.; MacDiarmid, A. G.; Akhtar, M.; Kiss, Z.; Epstein, A. J. Mol. Cryst. Liq.
Cryst. 1988, 160, 151.
(4) (a) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1992, 48, 91. (b) Xia,
Y.; Wiesinger, J. M.; MacDiarmid, A. G.; Epstein, A. J. Chem. Mater. 1995,
7, 443.
(5) (a) Adams, P. N.; Monkman, A. P. Synth. Met. 1997, 87, 165. (b) Beadle,
P. M.; Nicolau, Y. F.; Banka, E.; Rannou, P.; Djurado, D. Synth. Met. 1998,
95, 29.
(6) Wudl, F.; Angus, R. O.; Lu, F. L.; Allemand, P. M.; Vachon, D. J.;
Nowak, M.; Liu, Z. X.; Heeger, A. J. J. Am. Chem. Soc. 1987, 109, 3677.
(7) Wei, L.; Kumar, J.; Tripathy, S.; Senecal, K. J.; Samuelson, L. J. Am.
Chem. Soc. 1999, 121, 71.
full characterization by NMR and GPC. The H and 13C NMR
spectra (Figure 1a,b) for BOC-PANI in THF-d8 were well resolved
and the polymer endgroups are not discernible. The H NMR
spectrum exhibits two doublets for the aryl protons and a singlet
for the BOC protons, in the ratio of 2:2:9. The N-H resonance
appears as a singlet at 7.35 ppm, corresponding to one proton by
integration. The 13C spectrum shows resonances for a carbonyl
group, four types of aromatic carbons, and the quaternary and
primary carbons of the tert-butyl group. Analysis by 31P NMR
1
1
(8) For reviews on palladium-catalyzed amination, see: (a) Wolfe, J. P.;
Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc. Chem. Res. 1998, 31, 805.
(b) Hartwig, J. F. Angew. Chem., Int. Ed. Engl. 1998, 37, 2046. (c) Yang, B.
Y.; Buchwald, S. L. J. Organomet. Chem. 1999, 576, 125.
(9) (a) Harwood: E. A.; Sigurdsson, S. T.; Edfeldt, N. B. F.; Reid, B. R.;
Hopkins, P. B. J. Am. Chem. Soc. 1999, 121, 5081. (b) Liang, L.-C.; Schrock,
R. R.; Davis, W. M.; McConville, D. H. J. Am. Chem. Soc. 1999, 121, 5797.
(c) Wagaw, S.; Yang, B. H.; Buchwald, S. L. J. Am. Chem. Soc. 1999, 121,
10251.
(11) (a) Singer, R. A.; Sadighi, J. P.; Buchwald, S. L. J. Am. Chem. Soc.
1998, 120, 213. (b) Sadighi, J. P.; Singer, R. A.; Buchwald, S. L. J. Am.
Chem. Soc. 1998, 120, 4960.
(12) (a) Old, D. W.; Wolfe, J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1998,
120, 9722. (b) Wolfe, J. P.; Buchwald, S. L. Angew. Chem., Int. Ed. Engl.
1999, 38, 2413. (c) Wolfe, J. P.; Tomori, H.; Sadighi, J. P.; Yin, J.; Buchwald,
S. L. J. Org. Chem. 2000, 65, 1158.
(10) Goodson, F. E.; Hauck, S. I.; Hartwig, J. F. J. Am. Chem. Soc. 1999,
121, 7527 and references therein.
10.1021/ja001718h CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/21/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 31, 2000 7607
Figure 2. IR (A) and UV-vis (B) spectra of BOC-PANI (a) and its deprotection products: 185 °C, Ar (b); 185 °C, air (c); and H+, air (d).
Scheme 2
air. Complete removal of the BOC groups takes only 15 to 30
min and the subsequent air oxidation requires several hours. These
processes can be monitored by IR and UV-vis spectroscopy
(Figure 2). The IR spectrum (Figure 2Aa) of the BOC-PANI film
clearly exhibits both N-H (3342 cm-1) and carbonyl stretches
(1707 and 1686 cm-1) as well as aromatic C-H (3035 cm-1
)
and aliphatic C-H stretches (2975 and 2929 cm-1). It has a simple
optical spectrum (Figure 2Ba) with only the benzene π-π*
transition in the UV region (310 nm). Upon thermolysis or
protonolysis, no obvious morphology changes of the films were
observed. The IR spectra (Figure 2Ab, 2Ac, and 2Ad), however,
show the complete disappearances of the carbonyl and aliphatic
C-H stretches and changes in the N-H stretches. The vibrational
bands characteristic of the polymer backbone broaden and shift
systematically to lower energy from BOC-PANI (Figure 2Aa:
1604 and 1510 cm-1) to its deprotected ES product (Figure 2Ad:
1580 and 1494 cm-1), indicating longer effective conjugation
lengths. Consistent with this interpretation, the localized π-π*
transition in the UV region of BOC-PANI became broader, less
intense, and red-shifted, and new bands appeared in the visible
region (Figure 2B). The resulting IR and UV-vis spectra are
similar or identical to those of the corresponding forms of
conventionally prepared PANI.1-3 Notably, in the case of the ES
produced by protonolysis, the IR spectrum shows a broad and
intense band extending from about 1600 cm-1 into the near-IR
region (Figure 2Ad). Correspondingly, a lower-energy broad
absorption band beginning at ca. 500 nm and increasing in
intensity up to the spectrometer’s limit at 1100 nm toward the
near-IR region can be observed in the UV-vis spectrum (Figure
2Bd). These spectroscopic features are characteristic of the
conductive form of PANI, ES. Conductivities of ca. 1 S/cm,
similar to those reported for PANI‚HCl prepared by aniline
oxidation,3 have been measured for films treated with 1.0 M HCl
solution in diethyl ether under air.
showed no detectable peaks after several thousand scans, indicat-
ing that the phosphine ligand was successfully removed during
workup, and incorporation of phosphine into the polymer chain
is minimal.13 The molecular weights (MW) of the polymers were
determined by GPC in THF. The typical GPC trace for the
polymer (Figure 1c) exhibits a monomodal distribution and the
presence of minimal low molecular weight oligomers.
Factors that influence the yield, MW, and MW distribution of
the polymers were examined. The polymerization was typically
carried out for 24 h. Longer reaction times make no significant
difference. Among several solvents used, THF normally gave a
highest yield. Higher monomer concentrations gave rise to
polymers with higher MW and lower PDI. At 1.0 M concentration
in THF with 1.0 mol % Pd2(dba)3, BOC-PANI was obtained at
80 °C in 96% yield with a Mw of 80 400 and a Mn of 19 200.
The reduction of the catalyst loading led to a surprisingly large
effect on MW. A lower catalyst loading led a significant increase
in both Mw and Mn, although a decrease in yield was observed.
At 0.25 mol % Pd2(dba)3 with 1.0 M concentration in THF, a
polymer with a Mw of 109 400 and a Mn of 34 800 was made at
80 °C in 82% yield. At lower reaction temperatures, polymers
were synthesized in better yields, with higher Mw and Mn as well
as lower PDI. For example, the polymerization reaction carried
out at room temperature in THF using 1.0 mol % Pd2(dba)3 at
1.0 M concentration gave the best result, with a Mw of 98 300
and a Mn of 39 500 in 92% yield. These weights correspond to a
PDI of 2.49 and a number-averaged degree of polymerization of
over 140. Since the monomer is derived from an aniline dimer,
the average chain length is greater than 280 aniline units.14
The BOC-protected precursor, soluble in common organic
solvents and easily deprotected, is a conveniently processed form
of PANI. Homogeneous thin films of the protected polymer may
be readily fabricated from solutions in THF or CHCl3. The
colorless or pale gray films may be directly converted to the parent
PANI in different electroactive forms by thermolysis or proto-
nolysis (Scheme 2). Thermolysis of BOC-PANI films at 185 °C
under an inert atmosphere affords PANI in the fully reduced LB
form, whereas thermolysis in air affords the deep blue, partially
oxidized EB form. This process can be completed in 6 to 8 h,
depending on the thickness of the film. Alternatively, the BOC-
PANI films may be directly transformed to the dark-green
conductive ES form simply by immersion in acid solutions under
In conclusion, well-defined, high molecular weight BOC-
protected polyanilines have been synthesized in high yield, under
mild conditions, through step-growth polymerization of a bifunc-
tional dimeric monomer via palladium-catalyzed amination
chemistry. The polyaniline precursor is soluble in common organic
solvents and can be conveniently fabricated into films. The BOC-
PANI films can be directly converted to the parent PANI in
different electroactive forms including the conductive emeraldine
salt in a facile manner. This new approach expands the proces-
sibility and structural variability of polyaniline, potentially
enhancing its practical utility.
Acknowledgment. We are grateful to the Office of Naval Research
for partial support of this research. We thank Ms. Debbie Lightly for
help in conductivity measurements and Prof. Timothy M. Swager for
helpful discussions.
(13) Goodson, F. E.; Wallow, T. I.; Novak, B. M. J. Am. Chem. Soc. 1997,
119, 12441.
(14) Various MW values of polyanilines prepared by aniline oxidation were
reported in the literature. They were typically determined in NMP using
emeraldine base form with addition of LiCl at elevated column temperature.
Many factors can influence the MW determinations. See ref 5 and references
therein for detailed discussions.
Supporting Information Available: Experimental details and char-
acterization data, and enlarged copies of Figures 1 and 2 (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
JA001718H