Catalytic activity of N-heterocyclic carbenes in ring opening polymerization of cyclic siloxanes

Article abstract of DOI:10.1016/j.jorganchem.2006.10.006

Taking advantage of the strong nucleophilic properties of N-heterocyclic carbene (NHC), the efficient catalytic ring opening polymerization (ROP) of cyclotetrasiloxane D₄ was achieved under mild conditions.

Full text of DOI:10.1016/j.jorganchem.2006.10.006

Journal of Organometallic Chemistry 692 (2007) 705–708
www.elsevier.com/locate/jorganchem
Communication
Catalytic activity of N-heterocyclic carbenes in ring
opening polymerization of cyclic siloxanes
a
a
a
b
´
´
Marta Rodriguez , Sebastien Marrot , Tsuyoshi Kato , Sebastien Sterin ,
c
a,
*
Etienne Fleury , Antoine Baceiredo
a
Laboratoire He´te´rochimie Fondamentale et Applique´e (UMR 5069), Universite´ Paul Sabatier, 118, route de Narbonne, F-31062 Toulouse Cedex 9, France
b
`
RHODIA RECHERCHES, 85 avenue des Freres Perret, BP 62, F-69192 Saint-Fons, France
Laboratoire des Mate´riaux Macromole´culaires (UMR 5627), Domaine Scientifique de la doua, INSA de Lyon, F-69621 Villeurbanne Cedex, France
c
Received 29 September 2006; accepted 2 October 2006
Available online 12 October 2006
Abstract
Taking advantage of the strong nucleophilic properties of N-heterocyclic carbene (NHC), the efficient catalytic ring opening polymer-
ization (ROP) of cyclotetrasiloxane D4 was achieved under mild conditions.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Polymerization; Silicon; Organocatalysis; N-heterocyclic carbenes
1. Introduction
very important materials because of their unique properties
that are attractive for many industrial applications [12].
Recently, two new systems combining an alcohol and phos-
phazene [13] or phosphorus ylide [14] bases were shown to
be extremely fast initiators in the ROP of cyclic siloxanes.
Indeed, these strong bases react readily with alcohols to
generate alcoholates with very soft and voluminous organic
counterions that confer high solubility and an inherently
low tendency for ion-pair association which enhances the
initiation and propagation steps. Although they are very
efficient initiators, they are not catalysts for the process.
We herein report that N-heterocyclic carbenes, such as
1a, are efficient catalysts for the ROP of cyclotetrasiloxane
D4 (Scheme 1).
The transformation of organic compounds catalyzed by
neutral organic molecules is becoming a very important
research area due to the significant structural diversity pos-
sible for the catalysts, the mild reaction conditions
employed, their low cost and the absence of metal contam-
inant [1]. The use of stable strongly r-electron donating N-
heterocyclic carbenes (NHCs) [2] not only as ligands for the
preparation of organometallic catalysts [3] but on their
own as organic catalysts [4] has grown considerably [5–9].
Hedrick et al. recently reported a well-controlled ring open-
ing polymerization (ROP) of cyclic esters using NHC cata-
lysts [10], which is one of the very few organocatalytic
systems known for nucleophilic ROP [11].
Since all the reported catalytic reactions initiated by
NHCs involve the transformations of carbonyl com-
pounds, we decided to expand the scope of this family of
organocatalytic systems to inorganic polymerization. Poly-
meric siloxanes such as poly(dimethylsiloxane) (PDMS) are
2. Results and discussion
Polymerization reactions were carried out by adding, to
a mixture of D4 and alcohol initiator (three types of alcohol
were surveyed), a filtered THF solution of NHC 1a
(0.2 M), which was prepared by the deprotonation of the
corresponding imidazolium triflate with potassium hexam-
ethyldisilazide (KHMDS) [15]. After heating the mixture at
*
Corresponding author. Tel.: +33 5 61 55 62 86; fax: +33 5 61 55 82 04.
E-mail address: baceired@chimie.ups-tlse.fr (A. Baceiredo).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.10.006

706
M. Rodriguez et al. / Journal of Organometallic Chemistry 692 (2007) 705–708
R = Cy
1a
1b
N
N
R =
tBu
R
R
Si
O
O
O
Si
O
n
Si
Si
Si
Si
H
Si
O
O
O
O
R'O
R'OH
n
Si
Scheme 1. ROP of D4 catalyzed by NHC 1a,b.
80 ꢁC for 16 h, GPC analysis of the resulting polymers
obtained showed that the monomer conversions were
around 85% in all cases (Table 1).
ing a fast initiation step compared with the propagation
step, which results in a more efficient catalytic system.
In order to rule out the possibility that small residual
traces of trifluoromethanesulfonate salts were initiating
ROP, polymerization tests were carried out using purified
NHCs [crystallized 1a, or sublimed 1b (R = t-Bu)] [16],
and benzyl alcohol as initiator. Exactly the same trends
were observed as those reported in Table 1, which strongly
support the catalytic role of the NHC for the ROP of D4
(Table 2).
Furthermore, the nucleophilicity of the NHC is inti-
mately linked to catalytic activity. Indeed NHC 1c
(R = mesityl) with bulky and p-accepting substituents on
the nitrogen atoms is no longer active for the polymeriza-
tion of D4.
The catalytic properties of 1a were clearly indicated by
the gradual reduction of the molecular weight of polysilox-
anes with increasing the quantity of initiator relative to that
of 1a. From these data it is clear that more efficient molec-
ular weight control is obtained using primary alcohols
(MeOH and BnOH), as indicated by the inverse depen-
dence of molecular weight upon the amount of alcohol
used. In the case of the hindered t-BuOH, only a slight
decrease in molecular weights was observed, even in the
presence of a large excess of the initiator. These observa-
tions are consistent with the less hindered alcohols provid-
Table 1
Results of ROP of D4 catalyzed by 1a with different types of alcohols:
[cat.] = 1000 ppm, 80 ꢁC, 16 h
Table 2
Results of ROP of D4 using purified NHCs (1a,b): I = BnOH,
Initiator
[I]/[cat.]
t-BuOH
Mn [103]
MeOHb
BnOH
[cat.] = 1000 ppm, 80 ꢁC, 16 h.
Ip
Mn[103]
Ip
Mn [103]
Ip
a
a
a
[I]/[cat.]
1aa
1bb
1
3
5
10
20
70
a
216
190
146
144
138
114
1.59
1.64
1.61
1.62
1.58
1.59
218
164
128
122
72
1.64
1.61
1.61
1.54
1.53
1.53
211
157
125
75
49
17
1.65
1.70
1.61
1.56
1.68
1.53
1
5
20
1
5
20
Mn (103)
Mw (103)
Ip
Yield
a
186
304
1.63
85
98
61
91
1.48
85
161
266
1.65
88
114
183
1.61
88
47
74
1.57
87
150
1.53
86
28
Polydispersity index.
50 ꢁC, 16 h.
Purified by recrystallization in pentane.
Purified by sublimation.
b
b
R
N
Si
O
O
Si
N
O
O
O
O
n
Si
Si
Si
Si
Si
O
R
O
Si
3
N
N
R'OH
R
R
N
N
R
O
R
H
O
R'O
R'O
O
O
O
O
O
OH
Si
Si
Si
Si
Si
Si
Si
Si
5
4
R' = alkyl, alkyl-(O-SiMe2)n-
Scheme 2. Proposed mechanism of ROP of D4.

M. Rodriguez et al. / Journal of Organometallic Chemistry 692 (2007) 705–708
707
The polymerization reaction can be realized easily
quenched by heating (150 ꢁC, 5 h) the resulting polymer
which decomposes the NHC catalyst. Without this treat-
ment, the carbene catalyst remains active.
agreement with the deactivation of NHC catalyst 1 by rep-
rotonation with the excess of alcohol or by forming more
acidic silanols 4 [18]. However, all attempts to observe
the NHC–D4 adduct 3 failed (Scheme 3).
From a mechanistic point of view, a similar process to
the ROP of cyclic esters proposed by Hedrick et al. can
be considered here. Polymerization is initiated by nucleo-
philic attack of the NHC at the silicon centre of D4 induc-
ing ring opening of the cyclosiloxane monomer and
subsequent formation of the zwitter-ionic silonate 3, which
reacts with alcohol initiator to form silanol 4 (Scheme 2).
This proposed mechanism is consistent with the suscep-
tibility of the initiation step upon the nucleophilicity and
steric bulk of the NHC catalyst, and the necessity for an
alcohol initiator. Analysis of the polymer endgroups con-
firmed the initiation by alcohols. Indeed, signals character-
istic of –SiMe2OH and –SiMe2OBn endgroups were
observed in the 29Si NMR spectrum at À11 and
À12 ppm, respectively [17]. Moreover, kinetic studies show
that the polymerization is sensitive to the amount of NHC
catalyst (Fig. 1). Under the stoichiometric conditions ([I]/
[cat.] = 1) the polymerization is extremely fast with
3000 ppm compared to 500 ppm of NHC 1a.
3. Conclusion
In conclusion, the nucleophilicity of NHC 1a can be
exploited to bring about the first efficient catalytic ROP
of cyclotetrasiloxane D4, which was achieved under mild
conditions. Interestingly, with this NHC/ROH system the
molecular weight of the silicone polymers can be regulated
simply by varying the quantity of the alcohol initiator.
Because of the neutral condition, the only by-products
are a catalytic amount of moisture sensitive NHC and vol-
atile alcohol which can be easily removed. Detailed mech-
anistic studies of this new ROP and the use of other
stable singlet carbenes are under active investigation.
4. Experimental
All manipulations were performed under an inert atmo-
sphere of argon using standard Schlenk techniques. Dry,
oxygen-free solvents were employed. D4 was dried by aze-
otropic distillation with small amount of toluene and was
The fact that the rate of the polymerization is only accel-
erated by increasing the amount of NHC, and not on
increasing the amount the alcohol, suggests that the rate-
determining-step could be the nucleophilic attack of the
NHC on D4. Interestingly, the use of an excess of BnOH
initiator slows down the polymerization reaction rate, in
˚
kept over 4 A molecular sieves. t-BuOH, MeOH and
BnOH were dried under Ar by distillation over small
amount of Na metal. H, 13C and 29Si NMR spectra were
1
recorded on Bruker AC200, WM250 or Avance 300 spec-
trometers. 1H, 13C and 29Si NMR chemical shift were
reported in ppm relative to Me4Si as external standard.
D4 was obtained from Rhodia Silicones. NHCs (1b, 1c)
were prepared according to a previously reported method.
The degree of conversion and molecular weight distribu-
tions were obtained from gel-permeation chromatography
(GPC) using an HP 1050 Series instrument. The instrument
was fitted with two Waters styragel HR GPC columns
(7.8 · 300 mm, HR4 and HR4E) heated to 35 ꢁC. Toluene
was used as the eluent. Sample detection was performed at
35 ꢁC by a Waters 410 differential refractometer. The
molecular weights of the silicones prepared are uncorrected
and reported relative to the polystyrene standards (Poly-
mer Laboratories) used for calibration.
[cat.] = 500ppm, [I]/[cat.] = 1
[cat.] = 3000ppm, [I]/[cat.] = 1
[cat.] = 500ppm, [I]/[cat.] = 20
[cat.] = 3000ppm, [I]/[cat.] = 20
4.1. Synthesis of NHC (R = Cy) (1a)
To a solid mixture of imidazolium triflate (1.5 g,
3.9 mmol) and potassium t-butoxide (0.44 g, 3.9 mmol)
was added at À78 ꢁC 5 mL of THF. The solution was then
stirred at RT for 1 h. After removal of the potassium salts
by filtration, and removal of THF under vacuum, the res-
idue was extracted with pentane. Carbene 1a was obtained
as colourless crystals from a concentrated pentane solution
Fig. 1. Reaction time vs. % monomer conversion; [cat.] = 500 ppm or
3000 ppm. cat. = 1a, I = BnOH, [I]/[cat.] = 1or 20, 80 ꢁC.
R
N
HOR'
H
O
R'
N
N
+
R
R
1
N
at À30 ꢁC (yield, 0.59 g, 65%). Mp 56–57 ꢁC. H (C6D6,
R
300 MHz): d = 0.9–1.4 (m, 6H; CH2), 1.5–1.65 (m, 2H;
CH2), 1.65–1.9 (m, 8H; CH2), 2.1–2.3 (m, 4H; CH2), 4.22
(tt, J = 3.7 Hz, J = 11.7 Hz, 2H; NCH), 6.73 (s, 2 H;
1
6
Scheme 3. Acid/base equilibrium of NHC and alcohols.

708
M. Rodriguez et al. / Journal of Organometallic Chemistry 692 (2007) 705–708
HC@CH). 13C (C6D6, 75 MHz): d = 24.9 (s; CH2), 25.1 (s;
CH2), 34.3 (s; CH2), 59.1 (s; NCH), 115.0 (s; C@C), 211.3
(s broad; NCN).
[7] (a) G.A. Grasa, R.M. Kissling, S.P. Nolan, Org. Lett. 4 (2002) 3583–
3586;
(b) G.W. Nyce, J.A. Lamboy, E.F. Connor, R.M. Waymouth, J.L.
Hedrick, Org. Lett. 4 (2002) 3587–3590.
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4.2. Typical procedures for D4 polymerization
(b) N.T. Reynolds, J.R. de Alaniz, T. Rovis, J. Am. Chem. Soc. 126
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Method A: To a mixture of two solids, imidazolium tri-
flate (0.38 g, 1 mmol) and potassium hexamethyldisilazide
(0.20 g, 1 mmol) was added, at RT, 5 mL of THF. The for-
mation of carbene 1a was monitored by 1H and 13C NMR
spectroscopies. After removal of KOTf by filtration, a
THF solution of 1a was obtained. To a THF solution of
1a (0.2 M) were added at RT the appropriate quantities
of D4 and alcohol. Then the mixture was heated at 80 ꢁC
for 16 h. Method B: To purified NHC in the solid state
(1a or 1b) were added, at RT, the appropriate quantities
of D4 and alcohol. Then the mixture was heated at 80 ꢁC
for 16 h. In both cases, the obtained silicones were analyzed
[9] (a) J.J. Song, Z. Tan, J.T. Reeves, F. Gallou, N.K. Yee, C.H.
Senanyake, Org. Lett. 7 (2005) 2193–2196;
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1
by H NMR and gel-permeation chromatography (GPC)
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(c) R.C. Pratt, B.G.G. Lohmeijer, D.A. Long, R.M. Waymouth,
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Acknowledgements
Acknowledgment is made to Rhodia corporation and
the CNRS for financial support of this work and to the
FSE for a grant to (S.M.).
(b) T.C. Kendrick, B.M. Parbhoo, J.M. White, in: G. Allen, J.C.
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