First ring-opening metathesis polymerization in an ionic liquid. Efficient recycling of a catalyst generated from a cationic ruthenium allenylidene complex

Article abstract of DOI:10.1039/b205920g

Ring-opening metathesis polymerization (ROMP) of norbornene was carried out in a biphasic medium (consisting of the ionic liquid [bdmim][PF₆] and toluene with a cationic ruthenium allenylidene precatalyst. The ionic liquid contained the rutheni

Full text of DOI:10.1039/b205920g

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First ring-opening metathesis polymerization in an ionic liquid.
Efficient recycling of a catalyst generated from a cationic
ruthenium allenylidene complex
a
Szilard Csihony, Cedric Fischmeister, Christian Bruneau, Istvan T. Horvath and
a
a
b
´
´
´
´
Pierre H. Dixneuf*a
a
´
Institut de Chimie (CNRS UMR 6509), Universite de Rennes, Campus de Beaulieu, 35042,
Rennes, France. E-mail: pierre.dixneuf@univ-rennes1.fr
Department of Chemical Technology and Environmental Chemistry, Eo¨tvo¨s University,
b
´
Pazmany Peter setany 1/A, H-1117, Budapest, Hungary
´
´
´ ´
Received (in Montpellier, France) 19th June 2002, Accepted 16th July 2002
First published as an Advance Article on the web 7th October 2002
Ring-opening metathesis polymerization (ROMP) of norbornene was carried out in a biphasic medium
consisting of the ionic liquid [bdmim][PF6] and toluene with a cationic ruthenium allenylidene precatalyst.
The ionic liquid contained the ruthenium allenylidene complex and toluene dissolved the formed polymer.
Both the catalyst and the ionic liquid were reused several times and led to very good polymer yields.
Alkene metathesis has become a powerful tool for the cleavage
Results and discussion
=
and formation of C C bonds to produce fine chemicals,
macrocycles or polymers.1 Indeed, the recent discovery of
well-defined metal alkylidene catalysts2–6 has resulted in the
rapid development of various selective metathesis reactions
of alkenes and enynes.7 In particular, ruthenium based preca-
Several considerations were taken into account for the choice
of the anionic as well as the cationic part of the ionic liquid.
The [PF6] anion was chosen since [PF6] containing ionic liquids
are usually hydrophobic and they allow the dissolution of
ruthenium based salts. The degradation of [bmim] type (1-
butyl-3-methylimidazolium) ionic liquids is supposed to be
due to proton abstraction at the 2-position with formation
of a carbene species.14 Another possibility to consider is the
oxidative addition of the imidazolium cation to the metal as
reported for nickel, platinum, and palladium complexes.15
To avoid these processes the [bdmim] (1-butyl-2,3-dimethyli-
midazolium) cation in which the proton at the 2-position is
replaced by a methyl group was used. Furthermore, it must
be emphasized that the [bdmim][PF6] synthesis pathway is
absolutely identical to that of [bmim][PF6] without extra cost.
The only limitation in using [bdmim][PF6] is its melting point
that is above room temperature, thus requiring a slight heat-
ing. Typically, our experiments were performed at 40 ꢀC.
=
talysts selectively transform C C bonds with high tolerance
toward most functional groups, thus presenting a high poten-
tial for the ROMP of functional cyclic olefins. One major pro-
blem for both fine chemistry and ROMP polymerization is the
recovery or recycling of the alkene metathesis catalyst to
decrease both cost and metal residues. An elegant solution to
this problem is the use of ionic liquids (IL), which are known
to be nonvolatile, reusable and compatible with many organic
and catalytic reactions.8 Despite the increasing number of cat-
alytic processes performed in ionic liquids after the pioneering
work by Chauvin,9 only a few very recent reports deal with ole-
fin metathesis.10–13
Among metathesis catalysts the cationic ruthenium allenyli-
dene salts5 [(p-cymene)RuCl(PCy3)( C C CPh2)]X (X ¼
OTf, PF6 , BF4), shown in Scheme 1, are valuable candidates
for applications in ionic liquids due to their ionic character.
Recently these complexes have been used for the ring-closing
metathesis (RCM) of various alkenes in [bmim][PF6].13
Although it was possible to reuse the catalyst the yield
decreased quickly after the second recycling.13
=
= =
We now report for the first time the use of ruthenium alleny-
lidene catalysts for the ring-opening metathesis polymerization
of cyclic olefin in the ionic liquid [bdmim][PF6] (1-butyl-2,3-
dimethylimidazolium hexafluorophosphate) and we especially
show the remarkable high capacity of the system to be
recycled.
First the ROMP of norbornene was performed in neat
[bdmim][PF6] with 0.3 mol % of the ruthenium complex [(p-
=
= =
cymene)RuCl(PCy3)( C C CPh2)][OTf] (Scheme 1). The dis-
solution of the complex in the ionic liquid was complete within
a few minutes to give a homogeneous dark violet solution.
After addition of norbornene the reaction was stirred at
40 ꢀC for 2 h and the polymer extracted from the ionic liquid
Scheme 1 ROMP of norbornene.
DOI: 10.1039/b205920g
New J. Chem., 2002, 26, 1667–1670
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Table 1 Norbornene polymerization: recycling of the catalyst in
[bdmim][PF6]a
Table 3 Norbornene polymerization: reuse of the ionic liquid with
reloaded catalysta
c
Entry
Recycling
Time/h
Polymer yield/%
Polymer
Mn
/
Entry Recycling Time/h yield/% trans %b g molꢁ1 PDid
1
2
3
0
1
2
2
2
2
98
61
10
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
0.5
0.5
0.5
0.5
0.5
0.5
0.5
95
98
91
89
87
57
19
99
87
86
86
85
87
86
86
86
106 400
142 300
145 100
n.d.e
1.9
2.1
2.1
a
Reactions were performed at 40 ꢀC, with 4 mmol of norbornene and
n.d.
n.d.
n.d.
2.0
=
= =
C CPh2)][OTf] in 2.5 mL
0.3 mol % of [(p-cymene)RuCl(PCy3)(
of [bdmim][PF6].
C
n.d.
n.d.
166 500
141 500
24
2.0
with toluene and precipitated from methanol in 98% yield
(Table 1). The possibility to reuse the catalyst was tested by
addition of a new portion of norbornene. After 2 h at 40 ꢀC
the polymer was obtained in a lower 61% yield and the second
recycling yielded only 10% of polymer.
a
Reactions were performed at 40 ꢀC with 4 mmol of norbornene and
=
= =
C CPh2)][OTf]
in 2.5 mL of [bdmim][PF6] used in the previous experiments (Table
additional 0.3 mol % of [(p-cymene)RuCl(PCy3)(
C
2) and 10 mL of toluene. Determined by 1H NMR. Determined
b
c
by GPC measurements in THF, calibrated with monodisperse poly-
d
styrene standards. PDi ¼ Mw/Mn . Not determined.
e
It was postulated that the loss of activity could be due to a
loss of catalyst during the extraction step, either by removal or
decomposition. Indeed, the time necessary to extract the poly-
mer from the ionic liquid phase was exceedingly long. Thus, a
biphasic system made of [bdmim][PF6] and toluene was used
with the aim to solubilize the polymer in the organic phase
during the polymerization reaction. It was found that this
biphasic solvent mixture was very appropriate since the poly-
mer could be recovered almost quantitatively by pouring the
very viscous toluene upper phase into methanol. One fast sub-
sequent washing with toluene was then enough to complete the
polymer extraction. It was also found that 30 min at 40 ꢀC was
an appropriate duration for complete polymerization. With
these new parameters it has been possible to reuse the catalyst
three times with very good polymer yields (Table 2). Even after
the 6th recycling a good polymer yield could be obtained pro-
vided a longer reaction time was used (Table 2, entry 7). It was
established from the GPC results of the first three reaction
cycles that the average molecular weight (Mn) increased with
the number of cycles, denoting a loss of active catalytic species
able to initiate a polymer chain. Interestingly, the polydisper-
sities as well as the trans/cis ratio remained constant during
the recycling. These results clearly show the possibility to reuse
the catalyst with good yields. This catalyst recycling is much
more efficient than that obtained in fine chemistry,11,13 prob-
ably because of the shorter reaction time and lower tempera-
ture required for polymerization.
purification. The polymer properties were also comparable to
those obtained in the first set of experiments but without sig-
nificant influence on the Mn values after each successive run.
Two other non-ionic metathesis catalysts, the alkylidene
ruthenium Grubbs catalysts a3 and b4 have been tested in the
same conditions.
When (PCy3)2RuCl2(=CHPh) (a) was used as a catalyst a
rapid loss of activity was observed since no polymer was
formed during the second recycling (Table 4, entry 3). In this
case the catalyst was completely extracted in the organic sol-
vent or decomposed since it has not been possible to get a poly-
mer even with a longer reaction time (Table 4, entry 4). Better
results were obtained with the more robust catalyst b as very
good yields were reached in the first three experiments (Table
5) but a significant drop of activity was observed in the 3rd
recycling.
As observed with the Ru-allenylidene complex a longer reac-
tion time allowed us to recover a very good polymer yield
(Table 5, entry 6). It is also noteworthy that the use of the cat-
alyst b gave a polymer with a very low trans/cis ratio. The
recycling data are summarized in Fig. 1 and show the better
possibilities offered by the cationic ruthenium allenylidene
complex.
The ionic liquid used in the previous seven cycles was
a
reloaded with
new portion of [(p-cymene)RuCl-
=
= =
(PCy3)( C C CPh2)][OTf] and a new set of norbornene poly-
merization started. Here again the polymer was obtained
in very good yields until the 5th recycling (Table 3).
Consequently, the ionic liquid can be reused without any
Table 2 Norbornene polymerization in [bdmim][PF6]–toluene bipha-
sic system: recycling of the catalysta
c
Time/ Polymer
h
Mn
/
Table 4 Norbornene polymerization: recycling of catalyst aa
Entry Recycling
yield/% trans %b g molꢁ1 PDid
c
1
2
3
4
5
6
7
0
1
2
3
4
5
6
0.5
0.5
0.5
0.5
0.5
0.5
2
96
99
98
97
64
27
82
87
86
86
86
86
86
86
113 800
168 100
206 700
n.d.e
1.9
1.9
1.8
n.d.
n.d.
1.5
1.6
Polymer
Mn
/
Entry Recycling Time/h yield/% trans %b g molꢁ1 PDid
1
2
3
4
0
1
2
3
0.5
0.5
0.5
2
90
66
0
85
79 000 1.2
211 000 1.7
n.d.
303 000
260 400
86
n.d.e
n.d.
n.d.
n.d.
n.d.
n.d.
0
a
a
Reactions were performed at 40 ꢀC with 4 mmol of norbornene and
Reactions were performed at 40 ꢀC with 4 mmol of norbornene and
0.3 mol % of catalyst a in 2.5 mL of [bdmim][PF6] and 10 mL of
=
= =
C C CPh2)][OTf] in 2.5 mL
b
0.3 mol % of [(p-cymene)RuCl(PCy3)(
of [bdmim][PF6] and 10 mL of toluene. Determined by 1H NMR.
toluene. Determined by 1H NMR. Determined by GPC measure-
ments in THF, calibrated with monodisperse polystyrene standards.
b
c
c
Determined by GPC measurements in THF, calibrated with mono-
d
e
d
e
disperse polystyrene standards. PDi ¼ Mw/Mn . Not determined.
PDi ¼ Mw/Mn . Not determined.
1668
New J. Chem., 2002, 26, 1667–1670

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Table 5 Norbornene polymerization: recycling of catalyst ba
1-Butyl-2,3-dimethylimidazolium hexafluorophosphate, [bdmim]-
[PF6]. 1-Butyl-2,3-dimethylimidazolium chloride (50,0 g, 265
mmol) and KPF6 (51.4 g, 268 mmol) were stirred in 75 mL
of acetone at room temperature for 4 days. The mixture was
filtered on alumina. The acetone was evaporated and the crude
product was dissolved in 50 ml of dichloromethane. This solu-
tion was extracted 8 times with water to remove the chloride
residues. The organic phase was dried over MgSO4 , the sol-
vent removed under reduced pressure and the ionic liquid dried
for 5 h at 70 ꢀC in vacuo. Yield: 51.5 g, (173 mmol, 65%) as a
c
Polymer
Mn
/
Entry Recycling Time/h yield/% trans %b g molꢁ1 PDid
1
2
3
4
5
6
0
1
2
3
4
5
0.5
0.5
0.5
0.5
0.5
99
97
94
31
7
42
42
41
41
41
42
73 700
99 800
109 300
231 700
n.d.e
1.7
2.0
1.9
1.6
n.d.
n.d.
24
99
n.d.
1
pale-yellow liquid that solidifies upon storage. H NMR (200
a
Reactions were performed at 40 ꢀC with 4 mmol of norbornene and
0.3 mol % of catalyst b in 2.5 mL of [bdmim][PF6] and 10 mL of
MHz, CD3COCD3): d 7.39 (d, 1H, J 1.83 Hz, CH3–N–
CHCH–N), 7.36 (d, 1H, J 1.83 Hz, CH3–N–CHCH–N), 4.14
(t, 2H, J 7.32 Hz, NCH2CH2CH2CH3), 3.81 (s, 3H, CH3–N),
2.62 [s, 3H, N–C(CH3)–N], 1.79 (m, 2H, NCH2CH2CH2CH3),
1.37 (m, 2H, NCH2CH2CH2CH3), 0.92 (t, 3H, J 7.32 Hz,
NCH2CH2CH2CH3); 13C NMR (75 MHz, CD3COCD3):
d 146.2 [N–C(CH3)–N], 123.7 (CH3–N–CHCH–N), 122.3
(CH3–N–CHCH–N), 49.5 (NCH2CH2CH2CH3), 35.9 (NCH2-
CH2CH2CH3), 33.0 (CH3–N), 20.6 (NCH2CH2CH2CH3), 14.6
(NCH2CH2CH2CH3), 10.0 [N–C(CH3)–N].
b
c
toluene. Determined by 1H NMR. Determined by GPC measure-
ments in THF, calibrated with monodisperse polystyrene standards.
d
e
PDi ¼ Mw/Mn . Not determined.
Conclusion
As a conclusion, we have demonstrated that the cationic
ruthenium allenylidene precatalyst [(p-cymene)(PCy3)RuCl-
=
= =
( C C CPh2)][OTf] leads to a recyclable catalyst for the
ring-opening metathesis polymerisation of norbornene in the
ionic liquid [bdmim][PF6] (1-butyl-2,3-dimethylimidazolium
hexafluorophosphate). The best recycling results were obtained
by using a biphasic medium made of [bdmim][PF6] and
toluene. The ionic liquid can be reused after the 6th recycling
for the same polymerization reaction without any treatment
simply by reloading a new portion of the cationic allenylidene
complex. Finally we have shown that the allenylidene complex
displays better recycling capabilities than the alkylidene cata-
lysts probably because of its ionic character.
Typical procedure for the preparation of polynorbornene
in the two phase ionic liquid–toluene system
1-Butyl-2,3-dimethylimidazolium hexafluorophosphate (2.5
mL) was melted and transferred using a pre-heated syringe.
The ruthenium allenylidene complex (11 mg, 12 mmol) was
added and the reaction mixture stirred at 40 ꢀC for 10 min.
Freshly distilled toluene (10 mL) and 0.38 g (4 mmol) of nor-
bornene were added. After 30 min the stirring was stopped and
the two phases were separated. The toluene upper phase was
collected and the ionic liquid phase washed with 10 mL of
toluene. The toluene phase was then poured into 300 mL of
methanol containing 0.1 mol % of BHT (2,6-di-tert-butyl-p-
cresol) resulting in the precipitation of the polymer as a white
solid. The polymer was collected and dried. A new portion of
toluene and norbornene was added to the ionic liquid and stir-
red for 30 min at 40 ꢀC. The treatment procedure was repeated
several times.
Experimental
All the reactions were carried out under an inert atmosphere of
argon using Schlenk tube techniques. Toluene was dried by
distillation over sodium wires. Other solvents (p.a. grade) were
used without purification. 1,2-Dimethylimidazole (98%) and
norbornene (99%) were obtained from Acros Organics and
used as received.
=
= =
[(p-cymene)RuCl(PCy3)( C C CPh2)][OTf] was prepared
as described in the literature.5 Catalysts a and b were prepared
according to literature procedures.3,4
Acknowledgements
The authors are grateful to the CNRS, to the EU ‘‘POLY-
CAT’’ network, HPRN-CT-2000-00010 for financial support
and for a pre-doc grant to Sz. Csihony in Rennes, and to the
COST chemistry action WG D17/0003/00.
Preparations
1-Butyl-2,3-dimethylimidazolium chloride, [bdmim]Cl. 1,2-
Dimethylimidazole (28.3 g, 295 mmol) and 1-chlorobutane
(27.6 g, 298 mmol) were stirred at 75 ꢀC for 48 h under argon.
The flask was cooled to room temperature and the excess of
1-chlorobutane removed under reduced pressure. The pale-
yellow molten product solidified in the flask and was used
without further purification. Yield: 50.0 g (265 mmol, 90%).
References
1
Alkene Metathesis in Organic Synthesis, ed. A. Furstner, Springer,
¨
Berlin, 1998. For recent reviews on alkene metathesis, see: A.
Furstner, Angew. Chem., Int. Ed., 2000, 39, 3012; M. Schuster
¨
and S. Blechert, Angew. Chem., Int. Ed. Engl., 1997, 36, 2036;
R. H. Grubbs and S. Chang, Tetrahedron, 1998, 54, 4413; T. M.
Trnka and R. H. Grubbs, Acc. Chem. Res., 2001, 34, 18. For a
review on metathesis polymerization, see: M. R. Buchmeiser,
Chem. Rev., 2000, 100, 1565.
2
3
4
R. R. Schrock, J. S. Murdzek, G. C. Bazan, J. Robbins, M.
DiMare and M. O’Regan, J. Am. Chem. Soc., 1990, 112, 3875;
R. R. Schrock, Tetrahedron, 1999, 55, 8141.
P. Schwab, M. B. France, J. W. Ziller and R. H. Grubbs, Angew.
Chem., Int. Ed. Engl., 1995, 34, 2039; P. Schwab, R. H. Grubbs
and J. W. Ziller, J. Am. Chem. Soc., 1996, 118, 100.
T. Weskamp, W. C. Schattenmann, M. Spiegler and W. A.
Herrmann, Angew. Chem., Int. Ed., 1998, 37, 2490; M. Scholl,
T. M. Trnka, J. P. Morgan and R. H. Grubbs, Tetrahedron Lett.,
1999, 40, 2247; J. Huang, E. D. Stevens, S. P. Nolan and J. L.
Petersen, J. Am. Chem. Soc., 1999, 121, 2674; M. Scholl, S. Ding,
C. W. Lee and R. H. Grubbs, Org. Lett., 1999, 1, 953.
Fig. 1 Recycling comparison using (S) cationic ruthenium alkenyli-
dene complex, (L) catalyst a and (G) catalyst b.
New J. Chem., 2002, 26, 1667–1670
1669

View Article Online
5
A. Furstner, M. Picquet, C. Bruneau and P. H. Dixneuf, Chem.
¨
Commun., 1998, 1315; A. Furstner, M. Liebl, C. W. Lehmann,
¨
H. Olivier-Bourbigou and L. Magna, J. Mol. Catal. A: Chem.,
2002, 182–183, 419.
M. Picquet, R. Kunz, C. Bruneau, D. Touchard and P. H.
Dixneuf, Chem.-Eur. J., 2000, 6, 1847.
9
Y. Chauvin, B. Gilbert and L. Guibard, J. Chem. Soc., Chem.
Commun., 1990, 1715.
6
7
8
J. S. Kingsbury, J. P. A. Harrity, P. J. Bonitatebus and A. H.
Hoveyda, J. Am. Chem. Soc., 1999, 121, 791.
10 C. Gurtler and M. Jautelat (Bayer AG), EP 1035093A2, 2000.
¨
11 R. C. Buijsman, E. van Vuuren and J. G. Sterrenburg, Org. Lett.,
N. Saito, Y. Sato and M. Mori, Org. Lett., 2002, 4, 803; T.
Kitamura, Y. Sato and M. Mori, Chem. Commun., 2001, 1258.
Y. Chauvin and H. Olivier-Bourbigou, CHEMTECH, September
1995, p. 26; K. R. Seddon, J. Chem. Technol. Biotechnol., 1997,
68, 351; T. Welton, Chem. Rev., 1999, 99, 2071; P. Wasserscheid
and W. Keim, Angew. Chem., Int. Ed., 2000, 39, 3772; R.
Sheldon, Chem. Commun., 2001, 2399; J. D. Holbrey and K. R.
Seddon, Clean Product. Process., 1999, 1, 223; D. Zhao,
M. Wu, Y. Kou and E. Min, Catal. Today, 2002, 74, 157;
2001, 3, 3785.
12 K. G. Mayo, E. H. Nearhoof and J. J. Kiddle, Org. Lett., 2002, 4,
1567.
´
13 D. Semeril, H. Olivier-Bourbigou, C. Bruneau and P. H. Dixneuf,
Chem. Commun., 2002, 146.
14 J. G. Huddleston, A. E. Visser, W. M. Reichert, H. D. Willauer,
G. A. Broker and R. D. Rogers, Green Chem., 2001, 3, 156.
15 D. S. McGuinness, K. J. Cavell, B. F. Yates, B. W. Skelton and
A. H. White, J. Am. Chem. Soc., 2001, 123, 8317.
1670
New J. Chem., 2002, 26, 1667–1670