Polar, non-coordinating ionic liquids as solvents for the alternating copolymerization of styrene and CO catalyzed by cationic palladium catalysts

Article abstract of DOI:10.1039/b203367d

The palladium-catalyzed copolymerization of styrene and CO in an ionic liquid solvent, 1-hexylpyridinium bis(trifluoromethanesulfonyl)imide, gave improved yields and increased molecular weights compared to polymerizations run in methanol.

Full text of DOI:10.1039/b203367d

Polar, non-coordinating ionic liquids as solvents for the alternating
copolymerization of styrene and CO catalyzed by cationic palladium
catalysts†
Marc A. Klingshirn, Grant A. Broker, John D. Holbrey, Kevin H. Shaughnessy* and Robin D.
Rogers*
The Department of Chemistry and Center for Green Manufacturing, The University of Alabama, Box
870336, Tuscaloosa, AL 35487-0336, USA. E-mail: kshaughn@bama.ua.edu; rdrogers@bama.ua.edu
Received (in Columbia, MO, USA) 4th April 2002, Accepted 8th May 2002
First published as an Advance Article on the web 30th May 2002
The palladium-catalyzed copolymerization of styrene and
CO in an ionic liquid solvent, 1-hexylpyridinium bis(tri-
fluoromethanesulfonyl)imide, gave improved yields and
increased molecular weights compared to polymerizations
run in methanol.
initial studies on the suitability of ILs as solvents for the
palladium-catalyzed, alternating copolymerization of styrene
and CO (eqn. (1)).5,15
Ionic liquids (ILs) have received significant interest recently
due to their potential application as green replacements for
volatile organic solvents.1 Based on this promise, ILs have been
successfully applied as solvents in a wide range of synthetic
reactions, particularly those employing homogeneous metal
catalysts.2,3 The common imidazolium and pyridinium based
ionic liquids have polarities similar to those of acetonitrile and
methanol,4 yet due to their weak donor anions, such as
hexafluorophosphate or bis(trifluoromethanesulfonyl)imide
(NTf2), they act as non-coordinating solvents. The low volatility
and adjustable solvent parameters of ILs also make them
attractive solvents for recyclable catalytic systems.
Insertion polymerizations of alkenes are typically catalyzed
by cationic metal complexes with weakly coordinating ani-
ons.5–7 Polar, non-coordinating ILs are attractive solvents for
these reactions, since they may stabilize the solvent separated
ion pairs that are necessary for high activity. Chloroaluminate
ILs have been applied to cationic8–10 and metal-catalyzed11,12
oligomerization and polymerization of alkenes. Chloroalumi-
nate ILs are air- and moisture-sensitive, thus they have been
supplanted by second generation ILs incorporating stable
anions (i.e., [BF4]2 or [PF6]2). To date the only examples of
alkene polymerization in air-stable ILs are a report of ethylene
oligomerization13 and a brief patent claiming ethylene polymer-
ization, both in [C4mim][PF6] (Fig. 1).14 Herein we report our
(1)
The productivity of a standard catalyst system consisting of
LPd(OAc)2 (L = 2,2A-bipyridine and 1,10-phenanthroline),
excess ligand, benzoquinone and p-toluenesulfonic acid was
tested in IL solvents.16 Pyridinium based ILs were screened
initially due to the known formation of Pd–carbene complexes
from imidazolium ILs.17 The weakly coordinating NTf2 anion
was chosen over [BF4]2 or [PF6]2 to minimize coordination of
the IL anion to the cationic palladium catalyst.18 Copoly-
merization of CO and styrene in [C6pyr][NTf2] (Fig. 1) gave
styrene homopolymer as the major product (Table 1, run 1).
Addition of methanol (10% v/v to IL) gave high yields of
copolymer, while suppressing polystyrene formation (run 2).
Methanol reacts with LnPd2+ and CO to form
[LPdC(O)OCH3]+, which initiates the copolymerization.5
Lower yields were obtained in an imidazolium based IL
([C4mim][NTf2]),19 while no copolymer was formed in a
phosphonium ionic liquid (run 4).
Copolymerization activity was dependent on both
[C6pyr][NTf2] and methanol volumes. Decreasing the volume
of IL to 1 mL gave a slight increase in productivity, but higher
yields were obtained upon increasing the volume to 4 mL (runs
5 and 6). The molecular weight of the copolymer approximately
doubled as the volume was increased from 1 to 4 mL. Increasing
the amount of methanol resulted in a more significant increase
in productivity and molecular weight (runs 7 and 8).
The reaction parameters were further optimized using the
[C6pyr][NTf2]/methanol solvent system (Table 2). Decreasing
the reaction temperature from 70 to 50 °C resulted in a decrease
in the TON from 1.1 to 0.7 kg CP (g Pd)21 (Table 2, runs 1–2),
while polystyrene was formed at 90 °C. In reactions run at 70
Fig. 1
† Electronic supplementary information (ESI) available: experimental
details. See http://www.rsc.org/suppdata/cc/b2/b203367d/
Table 1 Solvent effects on the copolymerization of styrene and COa
b
b
Run
IL/mL
MeOH/mL
TON/kg CP (g Pd)21
Mw
Mn
PDIc
2.1
1
2
3
4
5
6
7
8
[C6pyr][NTf2] (2)
[C6pyr][NTf2] (2)
[C4mim][NTf2] (2)
[C14H29P(C6H13)3][NTf2] (2)
[C6pyr][NTf2] (1)
[C6pyr][NTf2] (4)
[C6pyr][NTf2] (4)
[C6pyr][NTf2] (4)
0
PSd
1.1
0.7
0
1.2
1.7
1.4
2.7
0.2
0.2
0.2
0.2
0.2
0.1
0.4
9700
4700
6000
10000
7900
3100
6200
4200
1.9
1.6
1.9
1.3
34000
25300
a T = 70 °C, CO = 40 bar. See Electronic Supplementary Information (ESI†) for procedure. b Determined by GPC relative to polystyrene standards. c Mw/Mn
d Isolated polymer was largely polystyrene.
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CHEM. COMMUN., 2002, 1394–1395
This journal is © The Royal Society of Chemistry 2002

Table 2 Optimization of the copolymerization of styrene and CO in [C6pyr][NTf2]a
b
b
Run
[C6pyr][NTf2]/mL
MeOH/mL T/°C
PCO/bar TON/kg CP (g Pd)21
Mw
Mn
PDIc
1
2
3
4
5
6
7
8
9
2
2
2
4
4
4
4
4
4
4
0.2
0.2
0.2
0.4
0.4
0.4
0.4
0.4
0.4
0.4
50
70
90
70
70
70
70
70
70
70
40
40
40
20
40
60
40
40
40
40
0.7
1.1
1.2
2.5
2.7
1.8
2.9d
2.3e
0.006f
0g
20800
9700
3700
12200
34000
8400
23000
11000
46800
8000
4700
1500
7200
25300
5000
14100
6700
23300
2.6
2.1
2.4
1.7
1.3
1.7
1.6
1.6
2.0
10
a See Electronic Supplementary Information (ESI†) for procedure. b Determined by GPC relative to polystyrene standards. c Mw/Mn d (phen)Pd(OAc)2 (0.026
mmol). e 0.1% (w/w) [C6pyr][Br]. f 0.5% (w/w) [C6pyr][Br]. g 1% (w/w) [C6pyr][Br]
°C, high yields of copolymer were obtained at 20 and 40 bar, but
the yield decreased at higher pressures (runs 4–6). The highest
molecular weight values were obtained for copolymers pre-
pared at 40 bar. Excess ligand ( > 15 eq./Pd) and benzoquinone
(75+1 benzoquinone+Pd) were necessary for copolymer forma-
tion. In their absence only polystyrene was produced, while
under these conditions thermal polymerization of polystyrene
was surpressed. Using 1,10-phenanthroline as ligand in place of
2,2A-bipyridine resulted in a small increase in productivity, but
the copolymer had a lower molecular weight (run 7).
Under the optimal conditions of 4 mL of [C6pyr][NTf2], 0.4
mL methanol, 40 bar CO and 70 °C, an average of 2.7 ± 0.2 kg
CP (g Pd)21 was obtained over several runs. The productivity
was higher than is typically observed with this catalyst system
in methanol under similar conditions (0.6–2.2 kg CP (g
Pd)21).16,20 Copolymers produced in [C6pyr][NTf2] also had
higher molecular weights (Mn = 25000) than are obtained in
methanol.20,21 The polydisperities of the copolymers were
narrow (1.3–2.5) suggesting a single-site catalyst. The isolated
copolymers were pale yellow rather than the grey polymer
produced in MeOH. Characterization of polymer samples by 1H
and 13C NMR and IR was consistent with a syndiotactic
alternating copolymer structure.5
difficulties, we have shown the potential for catalyst recycling
using IL solvents.
In conclusion, we have shown that [C6pyr][NTf2] is an
effective solvent for the palladium-catalyzed copolymerization
of styrene and CO. Catalyst productivity in [C6pyr][NTf2]
approaches that obtained in polar, non-coordinating solvents
such as 2,2,2-trifluoroethanol,21 while opening the possibility of
catalyst recycling. In addition higher molecular weights and
improved catalyst stability are observed in [C6pyr][NTf2]
compared with methanol. Methanol acts as a chain transfer
agent and a reductant for Pd(II) catalysts to inactive Pd(0)
clusters. By replacing methanol with [C6pyr][NTf2], chain
transfer and catalyst decomposition appear to be inhibited.
Increased activity could be due to improved catalyst stability,
increased rate of propagation, or both.
This research has been supported by the U.S. Environmental
Protection Agency’s STAR program through grant number R-
82825701-0 and by the National Science Foundation (Grant
EPS-9977239). We also wish to thank Cytec, Inc. for a gift of
trihexyl(tetradecyl)phosphonium chloride.
Notes and references
Since IL purity is difficult to determine, the effects of small
amounts of residual bromide on copolymerisation activity was
studied. Addition of 0.1% (w/w) of [C6pyr][Br] to
[C6pyr][NTf2] resulted in no change in activity or molecular
weight (Table 2, run 8), while increasing the amount of bromide
to = 0.5% resulted in complete inhibition of the copoly-
merization reaction. Halides coordinate strongly to the open
coordination site inhibiting catalyst activity. Therefore, ensur-
ing that IL solvents are halide free is critical to achieve high
activity and molecular weight.
An initial attempt was made to recycle the IL catalyst
solution. An IL solution recovered from a copolymerization
reaction was washed with hexane to remove spent organic co-
catalysts. Copolymerization with the recovered catalyst solution
gave a lower yield, although the TON was still 1.9 kg CP (g
Pd)21 (Table 3, cycle 2). Recycling the IL catalyst solution a
third time gave a significantly lower yield and the polymer
obtained was orange rather than pale yellow. Decreased activity
is most likely due to Pd precipitation during the polymerization
reaction or work up. In addition, extraction of the IL catalyst
solution results in some mechanical loss. Despite these
1 T. Welton, Chem. Rev., 1999, 99, 2071–2083.
2 P. Wasserscheid and W. Keim, Angew. Chem., Int. Ed., 2000, 39,
3772–3789.
3 R. Sheldon, Chem. Commun., 2001, 2399–2407.
4 S. N. V. K. Aki, J. F. Brennecke and A. Samanta, Chem. Commun.,
2001, 413–414.
5 E. Drent and P. H. M. Budzelaar, Chem. Rev., 1996, 96, 663–681.
6 S. D. Ittel, L. K. Johnson and M. Brookhart, Chem. Rev., 2000, 100,
1169–1203.
7 H. G. Alt and A. Köppl, Chem. Rev., 2000, 100, 1205–1221.
8 P. W. Ambler and N. J. Stewart, Eur. Pat., EP 0558187, 1993.
9 A. A. K. Abdul-Sada, P. W. Ambler, P. K. G. Hodgson, K. R. Seddon
and N. J. Stewart, World Pat., WO 9521871, 1995.
10 V. Murphy, World Pat., WO 0032658, 2000.
11 R. T. Carlin, R. A. Osteryoung, J. S. Wilkes and J. Rovang, Inorg.
Chem., 1990, 29, 3003–3009.
12 R. T. Carlin and J. S. Wilkes, J. Mol. Catal., 1990, 63, 125–129.
13 P. Wasserscheid, C. M. Gordon, C. Hilgers, M. J. Muldoon and I. R.
Dunkin, Chem. Commun., 2001, 1186–1187.
14 G. G. Hlatky, World Pat., WO 0181436, 2001.
15 During the preparation of this manuscript a report of styrene/CO
copolymerization in [C6pyr][NTf2] was published: C. Hardacre, J. D.
Holbrey, S. P. Katdare and K. R. Seddon, Green Chem., 2002, 4,
143–146.
16 E. Drent, Eur. Pat. Appl., 229,408, 1987.
Table 3 Recycling of the catalyst [C6pyr][NTf2] solutiona
17 L. Xu, W. Chen and J. Xiao, Organometallics, 2000, 19, 1123–1127.
18 A. Macchioni, G. Bellachioma, G. Cardaci, M. Travaglia, C. Zuccaccia,
B. Milani, G. Corso, E. Zangrando, G. Mestroni, C. Carfagna and M.
Formica, Organometallics, 1999, 18, 3061–3069.
b
b
Cycle TON/kg CP (g Pd)21
Mw
Mn
PDIc
1
2
3
2.9
1.9
0.8
26300
51900
19200
12700
32700
11400
2.1
1.6
1.7
19 V. R. Koch, C. Nanjundiah and R. T. Carlin, World Pat., WO 9702252,
1997.
20 R. Santi, A. M. Romano, R. Garrone, L. Abbondanza, M. Scalabrini and
G. Bacchilega, Macromol. Chem. Phys., 1999, 200, 25–30.
21 B. Milani, G. Corso, G. Mestroni, C. Carfagna, M. Formica and R.
Seraglia, Organometallics, 2000, 19, 3435–3441.
a See Electronic Supplementary Information (ESI†) for procedure. b De-
termined by GPC relative to polystyrene standards. c Mw/Mn
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