A dual organic/organometallic approach for catalytic ring-opening polymerization

Article abstract of DOI:10.1039/c1cc13915k

A dual catalytic system combining an original cationic zinc complex and a tertiary amine is shown to promote efficiently the polymerization of lactide under mild conditions.

Full text of DOI:10.1039/c1cc13915k

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COMMUNICATION
A dual organic/organometallic approach for catalytic ring-opening
polymerizationw
Estefanı
´
a Piedra-Arroni,ab Pierre Brignou,c Abderrahmane Amgoune,ab
Sophie M. Guillaume,*c Jean-Franc
¸
ois Carpentierc and Didier Bourissou*ab
Received 30th June 2011, Accepted 25th July 2011
DOI: 10.1039/c1cc13915k
A dual catalytic system combining an original cationic zinc
complex and a tertiary amine is shown to promote efficiently
the polymerization of lactide under mild conditions.
lactide can indeed be promoted efficiently and in a controlled
manner upon combining an original cationic zinc complex
(a Lewis acidic metal fragment activating the monomer) and a
tertiary amine (activating the initiating/propagating alcohol).
The new discrete cationic complex [{NNO}Zn]+[B(C6F5)4)]ꢀ
(2) was prepared by treatment of the previously reported
{NNO}ZnEt complex ({NNO} = 2,4-di-tert-butyl-6-{[(20-di-
methylaminoethyl)methylamino]methyl}phenolate) (1)5 with
1 equiv. of [HNMe2Ph]+[B(C6F5)4]ꢀ in THF (Scheme 1).w
Turner’s reagent, [HNMe2Ph]+[B(C6F5)4]ꢀ, was selected for
this protonolysis, based on the non-coordinative nature of the
borate anion that should make the resulting cationic species
quite electrophilic toward the monomer. The identity of 2 was
established on the basis of 1H, 13C, 19F, 11B NMR spectroscopy,
including 2D experiments (1H–1H COSY, 1H–13C HMBC
and HMQC)w, and elemental analysis. The presence of a single
set of sharp 1H and 13C NMR resonances (THF-d8, 25 1C)
supports a monomeric structure for complex 2 in solution.
The combination of 2 with pentamethylpiperidine (PMP) as
a base was examined for the ROP of rac-lactide (LA) in the
presence of neo-pentanol (neo-PentOH) as an initiator. We
found that, under optimized conditions, complete conversion
of the monomer can be achieved within 3 h at room tempera-
ture to give PLAs in up to 98% yield with Mn values up to
14 500 g molꢀ1 and Mw/Mn values ranging between 1.2 and
1.4.6 Representative polymerization results are summarized in
Table 1. In order to probe the cooperativity of the dual
organometallic/organic catalytic system, we carried out the
ROP of LA in the presence of neo-PentOH with only 2 or
PMP (entries 1 and 2). In both cases, no polymerization
Over the last two decades, considerable achievements have
been reported on the development and application of metal
complexes and organic compounds to the ring-opening poly-
merization (ROP) of cyclic esters and carbonates.1,2 In this
area, metal-alkoxide based initiators have attracted major
interest for the preparation of biodegradable polymers, and
impressive levels of activity and control have been achieved.1
In parallel, a variety of organic compounds have been shown
to catalyze efficiently ROP, providing a valuable metal-free
approach to well-controlled polyesters and polycarbonates.2
Noticeably, metal-alkoxides typically promote ROP via a
coordination–insertion mechanism, while a variety of mecha-
nistic pathways are encountered in organo-catalyzed ROP
(i.e., nucleophilic/electrophilic activation of the monomer,
basic activation of the initiating/propagating alcohol, or
bifunctional activation of the monomer and alcohol).
The organometallic and organic approaches of ROP are
complementary and feature different advantages. Metal
complexes generally display higher activities and better
(stereo)selectivities, whereas organocatalysts exhibit higher
functional group tolerance3 and inherently allow one to work
under catalytic conditions. With the aim of combining the
respective advantages of these two strategies and of exploring
new mechanistic pathways, we recently became interested in
investigating dual organometallic/organic systems for ROP
catalysis. The association of transition metal and organic
catalysis has recently emerged as a powerful approach in
organic synthesis,4 but to the best of our knowledge, the
feasibility of such a dual catalytic approach has not been
substantiated so far in ROP. Herein we report that the ROP of
a
´
Universite de Toulouse, UPS, LHFA, 118 route de Narbonne,
31062 Toulouse, France. E-mail: dbouriss@chimie.ups-tlse.fr;
Fax: +33 5 61 55 82 04; Tel: +33 5 61 55 68 03
b CNRS, LHFA, UMR 5069, 31062 Toulouse, France
c
´
Sciences Chimiques de Rennes, UMR 6226 CNRS - Universite de
Rennes 1, Campus de Beaulieu, 35042 Rennes, France.
E-mail: sophie.guillaume@univ-rennes1.fr; Fax: +33 2 2323 6939;
Tel: +33 2 2323 5880
Scheme 1 Synthesis of cationic zinc complex [{NNO}Zn]+
[B(C6F5)4)]ꢀ (2).
-
w Electronic supplementary information (ESI) available: Experimental
procedures and analytical data. See DOI: 10.1039/c1cc13915k
c
9828 Chem. Commun., 2011, 47, 9828–9830
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Table 1 Dual ROP catalyzed by the combination of 2 and amines in the presence of neo-PentOH as an initiatora
Conv.b Mn,theo
Solvent Temp./1C Time/h (%)
/
Mn,NMR
/
Mn,SEC
g molꢀ1
/
c
d
d
Entry [LA]0 : [neo-PentOH]0 : [2]0 : [amine]0 Amine
g molꢀ1 g molꢀ1
Mw/Mn
1
2
3
4
5
6
7
8
100 : 1 : 1 : 0
100 : 1 : 0 : 1
—
Tol
Tol
Tol
Tol
THF
DCM
DCM
50
50
50
50
50
25
25
25
25
25
25
25
25
25
25
24
24
3
3
24
3
0
0
—
—
—
—
—
—
—
—
PMP
PMP
PMP
PMP
PMP
Et3N
100 : 1 : 0.5 : 0.5
100 : 1 : 0.2 : 0.2
100 : 1 : 0.5 : 0.5
100 : 1 : 0.5 : 0.5
100 : 1 : 0.5 : 0.5
100 : 1 : 0.5 : 0.5
100 : 1 : 0.5 : 0.5
50 : 1 : 0.2 : 0.2
70 : 1 : 0.2 : 0.5
50 : 1 : 0.5 : 0.5
100 : 2 : 0.5 : 0.5
100 : 4 : 0.5 : 0.5
100 : 6 : 0.5 : 0.5
98
95
94
92
90
56
3
96
98
97
87
95
90
14 200
13 768
13 600
13 336
12 904
8152
—
7000
9965
7072
6496
3688
2488
14 500
14 500
12 500
13 900
13 900
8000
—
7300
10 200
6300
6100
4400
3100
11 700
12 600
8700
12 700
13 100
—
1.34
1.32
1.70
1.34
1.33
—
8
Collidine DCM
PhNMe2 DCM
168
144
3
2.1
2.5
5
9
10
11
12
13
14
15
PMP
PMP
PMP
PMP
PMP
PMP
DCM
DCM
DCM
DCM
DCM
DCM
7100
10 300
6800
5700
4300
3000
1.22
1.21
1.22
1.17
1.30
1.30
5
5.5
a
b
c
neo-PentOH = neo-pentanol, PMP = 1,2,2,6,6-pentamethylpiperidine. Monomer conversion was determined by 1H NMR. Calculated from
[LA]0/[neo-PentOH]0 ꢁ LA conversion ꢁ MLA + Mneo-PentOH, with MLA = 144 g molꢀ1 and Mneo-PentOH = 88 g molꢀ1
.
Determined by SEC
d
using a viscosimeter or a RI detector vs. PS standards.
occurred after 24 h at 50 1C, indicating that the combination
of the two components (i.e., 2 and PMP) is essential.7–9
By analogy with the mode of action of bifunctional
organocatalysts,2,10 we postulate that the electrophilic zinc
cation activates the monomer by coordination to the carbonyl
moiety while the amine activates the initiating/propagating
alcohol through hydrogen bonding (Scheme 2). Noteworthily,
Tolman et al.5 have shown that zinc-alkoxides derived from 1
are active towards ROP of lactide, and the in situ formation of
such species might be considered as an alternative pathway.
However, 1H NMR monitoring of a 1 : 1 : 1 mixture of 2/PMP/
neo-PentOH showed no sign of reaction under the polymer-
ization conditions, ruling out this hypothesis.
somewhat inhibits the electrophilic activation of the monomer.
In addition, a broad range of reactivities was observed
depending on the basicity of the amine [PMP (pKa = 11.2) E
NEt3 (pKa = 10.7) 4 collidine (pKa = 7.5) 4 PhNMe2
(pKa = 5.1)]. For instance, while a rapid polymerization is
observed with PMP (complete conversion of 100 equiv. within
3 h, entry 5), hardly any polymerization occurs with PhNMe2
(entry 9). Interestingly, with PMP, the catalyst loading can be
reduced (from 1 to 0.2 mol% relative to the monomer) without
noticeably slowing down the ROP (entry 4), and a faster
polymerization rate is observed when the PMP/2 ratio is
increased from 1 to 2.5 (entries 10 and 11), in line with the
activation of the initiating/propagating alcohol by the amine
through hydrogen bonding.
In accordance with the electrophilic activation of LA upon
coordination to 2, we observed a solvent effect on the activity
of the dual catalyst 2/PMP/neo-PentOH. Indeed, polymeriza-
tions proceed equally in toluene and dichloromethane (DCM),
in terms of activity (95% and 97% conversion in 3 h, respectively)
and polymerization control, but a much lower polymerization
rate is observed in THF (94% yield in 24 h) with a lower
degree of polymerization control (entries 4–6). THF is likely
competing with LA for coordination to 2, and thereby
Having established the cooperativity of the zinc cation and
amine, the polymerization behavior of the dual catalytic
system was explored in more detail. First, end-group analysis
1
by H NMR spectroscopy revealed quantitative initiation by
neo-PentOH. In addition, a linear relationship was observed
between the initial monomer-to-alcohol ratio (varying from
20 to 150) and the number-average molar mass values
(as determined by NMR and SEC analyses), indicating the
controlled character of polymerization (Fig. 1). This was
further confirmed by a second-feed experiment resulting
in a polymer chain extension. A PLA sample with Mn
=
8100 g molꢀ1, Mw/Mn = 1.29 was first prepared by complete
ROP of 50 equiv. of LA with neo-PentOH. The polymeriza-
tion was then restarted by subsequent addition of 100 equiv. of
LA to give PLA with Mn = 21 700 g molꢀ1, Mw/Mn = 1.27.w
Increasing the monomer-to-initiator feed ratio to 200 enabled
to attain higher molar mass, but at the expense of polymeriza-
tion control (Mn = 28 000 g molꢀ1 and Mw/Mn = 1.80).
According to NMR analysis (homonuclear decoupled
1H NMR spectrum of the methine region), isotactic PLLA is
Scheme 2 Bifunctional activation of the monomer (by the Lewis
acidic zinc complex 2) and initiating/propagating alcohol (by the
amine) proposed to account for the dual ROP of lactide.
c
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Chem. Commun., 2011, 47, 9828–9830 9829

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2010, 39, 165; (f) M. J. Stanford and A. P. Dove, Chem. Soc. Rev.,
2010, 39, 486; (g) J.-F. Carpentier, Macromol. Rapid Commun.,
2010, 31, 1696.
2 For reviews on the organocatalyzed ROP of lactones, see:
(a) N. E. Kamber, W. Jeong, R. M. Waymouth, R. C. Pratt,
B. G. Lohmeijer and J. L. Hedrick, Chem. Rev., 2007, 107, 5813;
(b) D. Bourissou, S. Moebs-Sanchez and B. Martin-Vaca,
C. R. Chim., 2007, 10, 775; (c) M. K. Kiesewetter, E. J. Shin,
J. L. Hedrick and R. M. Waymouth, Macromolecules, 2010,
43, 2093.
3 (a) O. Thillaye du Boullay, C. Bonduelle, B. Martin-Vaca and
D. Bourissou, Chem. Commun., 2008, 3105; (b) O. Thillaye du
Boullay, N. Saffon, J. P. Diehl, B. Martin-Vaca and D. Bourissou,
Biomacromolecules, 2010, 11, 1921; (c) R. J. Pounder and
A. P. Dove, Biomacromolecules, 2010, 11, 1930; (d) F. Suriano,
O. Coulembier, J. L. Hedrick and P. Dubois, Polym. Chem., 2011,
2, 528.
Fig. 1 Plot of Mn (as measured by SEC) vs. [LA]0/[neo-PentOH]0
ratio for the dual ROP of LA with the 2/PMP/neo-PentOH
(0.5 : 0.5 : 1) system at 25 1C (with Mw/Mn values, left). SEC traces
from the successive feed experiments (right).
4 (a) C. Zhong and X. Shi, Eur. J. Org. Chem., 2010, 2999;
(b) Z. Shao and H. Zhang, Chem. Soc. Rev., 2009, 38, 2745.
5 C. K. Williams, L. E. Breyfogle, S. K. Choi, W. Nam, V. G. Young
Jr., M. A. Hillmyer and W. B. Tolman, J. Am. Chem. Soc., 2003,
125, 11350.
6 The polymerization of lactide is slightly slower and less controlled
with the dual catalytic system 2/PMP than with the related zinc-
alkoxide5.
obtained by ROP of L-lactide, showing that the basic amine
does not induce epimerization of the monomer and polymer
chain. On the other hand, the PLA samples derived from
rac-lactide were found to be essentially atactic, indicating the
absence of significant chain-end control. Polymerizations
carried out in the presence of an excess of alcohol led to PLAs
with molar mass Mn in good agreement with the LA/neo-
PentOH ratio and with narrow Mw/Mn values (entries 13-15).
Thus, increasing the amount of alcohol with respect to the
catalytic system (2/amine) allows the simultaneous growth of
several chains per metal center without affecting either the
control or the rate of polymerization. This demonstrates the
possibility to carry out lactide ROP with catalytic amounts of
the metal. In comparison, the ROP of lactide through a
coordination–insertion mechanism offers this opportunity
only if the alcohol behaves as a reversible chain transfer agent
(i.e., if exchange between the active metal-alkoxide and
dormant alcohol species proceeds faster than propagation),
as previously observed for ‘‘immortal’’ ROP processes.11
In summary, a dual catalytic system combining an original
cationic zinc complex with a tertiary amine is shown to
promote efficiently and in a controlled manner the ROP of
lactide under mild conditions. The two-component catalyst
activates cooperatively the monomer and initiating/propagating
alcohol. This illustrates for the first time the possibility to
combine the organic and organometallic approaches in ring-
opening polymerization catalysis. Such a dual approach
bridges the gap between the coordination/insertion and
activated-monomer pathways. Future work will seek to
generalize this approach to other monomers (lactones, cyclic
carbonates. . .) and to explore other combinations of Lewis
acidic metal complexes and organic bases.
7 There are few examples of cationic zinc compounds that have been
used in the ROP of lactide and cyclic esters and all of these refer to
pure cationic processes; see: (a) M. D. Hannant, M. Schormann
and M. Bochmann, J. Chem. Soc., Dalton Trans., 2002, 4071;
(b) Y. Sarazin and M. Bochmann, Organometallics, 2004, 23, 3296;
(c) B. Lian, C. M. Thomas, O. L. Casagrande, C. W. Lehmann,
T. Roisnel and J.-F. Carpentier, Inorg. Chem., 2007, 46, 328;
(d) C. A. Wheaton, B. J. Ireland and P. G. Hayes, Organometallics,
2009, 28, 1282; (e) C. A. Wheaton and P. G. Hayes, Chem.
Commun., 2010, 46, 8404; (f) C. A. Wheaton and P. G. Hayes,
Dalton Trans., 2010, 3861.
8 Immortal ROP of lactide mediated by cationic zinc complexes/
alcohol binary systems proceeds in a controlled fashion but at
relatively higher temperatures (100 1C); see: Y. Sarazin, V. Poirier,
T. Roisnel and J.-F. Carpentier, Eur. J. Inorg. Chem., 2010, 3423.
9 For other cationic metal complexes evaluated in ROP, see:
(a) D. Robert, M. Kondracka and J. Okuda, Dalton Trans.,
2008, 2667; (b) H. E. Dyer, S. Huijser, A. D. Schwarz, C. Wang,
R. Duchateau and P. Mountford, Dalton Trans., 2008, 32;
(c) M. Haddad, M. Laghzaoui, R. Welter and S. Dagorne, Organo-
metallics, 2009, 28, 4584; (d) L. Clark, M. G. Cushion, H. E.
Dyer, A. D. Schwarz, R. Duchateau and P. Mountford, Chem.
Commun., 2010, 46, 273; (e) B. J. Ireland, C. A. Wheaton and
P. G. Hayes, Organometallics, 2010, 29, 1079; (f) M. G. Cushion
and P. Mountford, Chem. Commun., 2011, 47, 2276; (g) Y. Sarazin,
B. Liu, T. Roisnel, L. Maron and J.-F. Carpentier, J. Am. Chem.
Soc., 2011, 133, 9069; (h) A. Otero, A. Lara-Sanchez, J. Fernandez-
Baeza, C. Alonso-Moreno, J. A. Castro-Osma, I. Marquez-
Segovia, L. F. Sanchez-Barba, A. M. Rodrıguez and J. C.
´
Garcia-Martinez, Organometallics, 2011, 30, 1507.
10 (a) A. P. Dove, R. C. Pratt, B. G. G. Lohmeijer, R. M. Waymouth
and J. L. Hedrick, J. Am. Chem. Soc., 2005, 127, 13798;
(b) R. C. Pratt, B. G. G. Lohmeijer, D. A. Long, P. N. P.
Lundberg, A. P. Dove, H. Li, C. G. Wade, R. M. Waymouth
and J. L. Hedrick, Macromolecules, 2006, 39, 7863; (c) S. Koeller,
J. Kadota, F. Peruch, A. Deffieux, N. Pinaud, I. Pianet, S. Massip,
J. M. Leger, J. P. Desvergne and B. Bibal, Chem.–Eur. J., 2010,
16, 4196; (d) C. Thomas, F. Peruch, A. Deffieux, A. Milet,
J.-P. Desvergne and B. Bibal, Adv. Synth. Catal., 2011,
353, 1049; (e) D. J. Coady, K. Fukushima, H. W. Horn,
J. E. Rice and J. L. Hedrick, Chem. Commun., 2011, 47, 3105.
11 N. Ajellal, J.-F. Carpentier, C. Guillaume, S. M. Guillaume,
M. Helou, V. Poirier, Y. Sarazin and A. Trifonov, Dalton Trans.,
2010, 8363 and references cited therein.
Notes and references
1 For reviews on the ROP of lactones with metal-alkoxide initiators,
see: (a) B. J. O’Keefe, M. A. Hillmyer and W. B. Tolman, J. Chem.
Soc., Dalton Trans., 2001, 2215; (b) O. Dechy-Cabaret, B. Martin-
Vaca and D. Bourissou, Chem. Rev., 2004, 104, 6147; (c) J. Wu,
T. L. Yu, C. T. Chen and C. C. Lin, Coord. Chem. Rev., 2006,
250, 602; (d) R. H. Platel, L. M. Hodgson and C. K. Williams,
Polym. Rev., 2008, 48, 11; (e) C. M. Thomas, Chem. Soc. Rev.,
c
9830 Chem. Commun., 2011, 47, 9828–9830
This journal is The Royal Society of Chemistry 2011