Synthesis of high-molecular-weight poly(ε-caprolactone) catalyzed by highly active bis(amidinate) tin(ii) complexes

Article abstract of DOI:10.1039/c0dt01050b

A series of bis(amidinate) tin(ii) complexes is synthesized and shown to rapidly polymerize ε-caprolactone (ε-CL) in the presence and absence of benzyl alcohol giving high-molecular-weight poly(ε-CL) (M_n up to 160,600 Da). Ligands having electron donating groups were found to accelerate the polymerization by making the complex more nucleophilic.

Full text of DOI:10.1039/c0dt01050b

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Synthesis of high-molecular-weight poly(e-caprolactone) catalyzed by highly
active bis(amidinate) tin(II) complexes†
Khamphee Phomphrai,* Chatyapha Pongchan-o, Wipavee Thumrongpatanaraks, Preeyanuch Sangtrirutnugul,
Palangpon Kongsaeree and Manat Pohmakotr
Received 18th August 2010, Accepted 10th November 2010
DOI: 10.1039/c0dt01050b
A series of bis(amidinate) tin(II) complexes is synthesized
and shown to rapidly polymerize e-caprolactone (e-CL) in
the presence and absence of benzyl alcohol giving high-
molecular-weight poly(e-CL) (Mn up to 160,600 Da). Ligands
having electron donating groups were found to accelerate the
polymerization by making the complex more nucleophilic.
reaction of triethyl orthoformate with the corresponding anilines.
Reactions of 2 equiv of ligands 1a–1e with Sn[N(SiMe3)2]2 gave the
bis(amidinate) tin(II) complexes 2a–2e, respectively (Scheme 1).
All complexes were isolated in moderate to high yields. Although
related bis(amidinate) tin(II) complexes have been reported,9 they
have never been used as a catalyst for the polymerization of e-CL.
There has been increasing attention towards biodegradable and
biocompatible polymers during the past decade. Polyesters such as
polyglycolide, polylactide (PLA), poly(e-caprolactone) (PCL) and
their copolymers have received significant attention.1 Their prop-
erties such as tensile strength, degradation rate, and compatibility
in the body are undoubtedly determined by the stereochemistry,
composition, molecular weight, and dispersity of the polymers.
Such polyesters have been successfully synthesized using various
metal alkoxide complexes.2 While electropositive metal complexes
such as Zn, Mg, and Ca have been found to be highly active, the
less active tin(II) complex specifically tin(II) bis(2-ethylhexanoate),
Sn(Oct)2, has been used industrially.2,3 This is due to its high
solubility and thermal stability in the molten monomers, thus
allowing the melt polymerization in the absence of solvents. In
addition, several well-defined tin(II) complexes have been shown
to be active for the polymerization of cyclic esters giving high-
molecular-weight polymers with narrow polydispersity index.2,4
For organocatalysts, N-heterocyclic carbenes (NHCs) were re-
ported to be efficient catalysts for high molecular weight linear
PLA,5 and recently, for linear PCL.6 However, in the absence of
suitable initiators, Culkin et al. later reported the NHC catalysts
to generate cyclic PLA in THF.7
Scheme 1 Synthesis of bis(amidinate) tin(II) complexes.
Complex 2e was characterized crystallographically (see ESI†)
indicating a four-coordinate tin complex having a distorted square
pyramidal geometry where Sn is at the top of the pyramid and
the four nitrogen atoms are at the base similar to the related
bis(amidinate) tin(II) complexes reported earlier.9 The position of
the lone-pair electrons above Sn atom is clearly evidenced.
Complex 2e was tested for the solvent-free polymerization of
e-CL using a low e-CL:2e molar ratio of 10 : 1 at 110 ◦C for
1 min (>99% conversion by 1H NMR) in order to study the
polymer structure by mass spectrometry and NMR. MALDI-
TOF spectrum of the resulting polymer is shown in Fig. 1 (top).
Interestingly, the repeating mass is assigned to [e-CL]n + Na+. The
mass of the end group is clearly missing. This is in agreement
Following the success of carbene initiators, we hypothesized
that, being three rows below carbon in the periodic table, the
stannylene8 could be an efficient initiator for the polymerization
of cyclic esters as well. Herein, we reported the synthesis of highly
active bis(amidinate) tin(II) complexes and their application in the
solvent-free polymerization of e-caprolactone (e-CL) leading to
high-molecular-weight PCL.
Amidine ligands were chosen in this work due to the ease
of ligand preparation and modification. The substituents on
the aryl groups can be modified systematically to maximize
the catalyst activity. Ligands 1a–1e were prepared from the
Center for Catalysis, Department of Chemistry and Center of Excellence
for Innovation in Chemistry, Faculty of Science, Mahidol University,
Rama 6 Road, Bangkok, 10400, Thailand. E-mail: sckpp@mahidol.ac.th;
Fax: +662-354-7151; Tel: +662-201-5146
† Electronic supplementary information (ESI) available: Synthesis and
characterization of 2a–e, polymerization procedure, ORTEP drawing of 2e,
water contact angles of PCL, and GPC traces. CCDC reference number
795658. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0dt01050b
Fig. 1 MALDI-TOF spectra of PCL synthesized without addition of
BnOH (top) and with addition of 1 equiv of BnOH (bottom).
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Dalton Trans., 2011, 40, 2157–2159 | 2157
©

Table 1 Solvent-free polymerizations of e-CL at 110 ◦
C
with the 1H NMR spectrum of the resulting polymer as shown in
Fig. 2(a) where a triplet peak of the HOCH2- end group at 3.6
ppm is not observed. These results suggest that the majority, if
not entirety, of the low-molecular-weight polymer is cyclic PCL.10
The peaks with much lower intensities observed at 1600-2200
Da (e.g. 1753.3, 1867.2, 1981.4, 2098.5) are assigned to linear
H[CL]nOH+Na+. This minor linear polymer having H2O end
group may arise during polymer precipitation and purification.
For comparison, a polymerization using e-CL:2e molar ratio
of 10 : 1 was performed under the same condition but with the
Entry Catalyst [CL]/[Sn] Time/min Conv.a (%) Mn (Dalton) PDIb
b
1
2
3
4
5
6
7
8
2ac
2bc
2cc
2dc
2ec
2a
2b
2c
500
500
500
500
500
500
500
500
500
500
1,000
5,000
10,000
3
3
3
3
3
2
2
2
2
2
5
20
60
50
>99
>99
>99
>99
>99
47
75
30
80
97
69,100
40,900
107,400
42,200
70,400
66,300
48,600
99,100
46,600
69,800
70,100
137,400
160,600
30,700
1.70
1.78
1.90
1.84
1.65
1.55
1.85
1.69
1.85
1.85
1.80
1.94
2.07
1.32
9
2d
2e
1
addition of 1 equiv of benzyl alcohol (>99% conversion by H
NMR). MALDI-TOF spectrum of the resulting polymer is shown
10
11
12
13
14
2e
>99
82
94
2e
in Fig. 1 (bottom). The repeating mass is assigned to H[e-CL]nOBn
2e
1
+ Na+. The H NMR spectra of the resulting polymer shown in
c
Sn(Oct)2 500
58
Fig. 2(b) clearly reveals a triplet peak of the HOCH2- end group at
3.6 ppm confirming the linear structure of the polymer. At higher
e-CL:2e molar ratio, the NMR signal of the end group is too
small to be observed. The polymerization mechanism accounted
for the observed linear and cyclic PCL is still under investigation. A
similar mechanism to the NHC catalyst system is possible by using
the lone-pair electrons to attack the ester group of monomer.7
However, a polymerization initiated by anionic ligand followed by
intramolecular transesterification leading to cyclic PCL cannot be
ruled out at this point.
a Conversions determined by 1H NMR spectroscopy. b Determined by
GPC, calibrated using polystyrene standards. Correction factor of 0.68
was also applied.11 c Polymerization with addition of 1 equiv of BnOH.
at 110 ◦C using e-CL:Sn molar ratio of 500 : 1 and a fixed time
at 2 min. The polymerization results were summarized in Table 1,
entries 6–10. All catalysts were also active for the polymerizations
of e-CL. The order of activity based on conversion is 2e > 2d > 2b
> 2a > 2c. The polymerization using 2e is the fastest giving 97%
conversion in only 2 min. The polymer has high Mn of 69,800 Da
and a PDI of 1.85. Other catalysts were less active producing PCL
with Mn (46,600–99,100 Da) and PDI of 1.55–1.85. For complex
2e, a plot of Mn and PDI versus % conversion is shown in Fig. 3.
The polymer has low PDI at low conversion and higher PDI at
later stage. The Mn of the polymer also increases with increasing
conversion. Interestingly, polymerization in toluene at 70 ◦C for 25
h using e-CL:2e = 500 : 1 with and without alcohol addition gave
no polymer. Blank polymerizations (no catalyst) with and without
alcohol addition at 110 ◦C were also carried out for 2 days giving
no polymer.
Fig. 2 1H NMR spectra of PCL synthesized from e-CL:2e ratio = 10 : 1
(a) without addition of BnOH and (b) with addition of 1 equiv of BnOH.
Complexes 2a–e were used to polymerize e-CL under solvent-
free condition at 110 ◦C using e-CL:Sn:BnOH molar ratio of
500 : 1 : 1 and a fixed time at 3 min. The polymerization results
were summarized in Table 1, entries 1–5. All catalysts were highly
active for the polymerization of e-CL giving complete conversion
in only 3 min. The polymers have high Mn (42,200–107,400 Da)
and broad PDI of 1.65–1.90. The unusually high Mn and broad
PDI are indicative of slow initiation compared to the propagation.
This is also supported by the observation that the complexes 2a–
e are soluble in e- CL only at elevated temperature but not at
room temperature. In addition, the broad PDI could be a result
of the mass transportation problems due to high viscosity. For
comparison, a commercial Sn(Oct)2 was used as catalyst under
identical condition as shown in entry 14. The polymerization was
much slower compared to our catalysts giving only 58% conversion
in 50 min.
Fig. 3 Molecular weight data (PDI in parenthesis) of PCL obtained at
different conversion using e-CL:2e molar ratio of 500 : 1 at 110 ◦C.
Complexes 2a–e can be classified into two groups having
different electronic (2a–c) and steric (2a, 2d, 2e) contributions.
In term of electronic contribution, the order of reactivity is 2c
(R¢ = CF3) < 2a (R¢ = H) < 2b (R¢ = OMe) in agreement with
the increasing electron donation of the aryl groups. Based on the
polymerization mechanism for NHC catalyst system,7 the metal
in complex 2b is electron-rich and more nucleophilic and hence
more susceptible to attack e-CL. In term of steric hindrance, the
For polymerizations without the addition of alcohol, complexes
2a–e were used to polymerize e-CL under solvent-free condition
i
order of reactivity is 2a (R = H) < 2d (R = Me) < 2e (R = Pr) in
2158 | Dalton Trans., 2011, 40, 2157–2159
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The Royal Society of Chemistry 2011
©

agreement with the increasing steric hindrance of the ortho-alkyl
groups. This was surprising at first since more hindered complex
gave higher activity. However, these results are not unprecedented
because there are reports that sterically demanding substituents
enhanced the catalytic activity of the catalysts.12 In addition, the
iPr groups in 2e are more electron donating making the complex
more nucleophilic.
For complex 2e, the polymerizations using higher e-CL:Sn
molar ratios of 1000, 5000, and 10000 were performed at 110 ◦C
(Table 1, entries 11–13). Great care was taken to exclude moisture
Notes and references
1 (a) A. Arbaoui and C. Redshaw, Polym. Chem., 2010, 1, 801–826; (b) K.
E. Uhrich, S. M. Cannizzaro, R. S. Langer and K. M. Shakesheff, Chem.
Rev., 1999, 99, 3181–3198; (c) A. P. Gupta and V. Kumar, Eur. Polym.
J., 2007, 43, 4053–4074.
2 (a) O. Dechy-Cabaret, B. Martin-Vaca and D. Bourissou, Chem. Rev.,
2004, 104, 6147–6176; (b) Z. Zhong, P. J. Dijkstra and J. Feijen, J.
Biomater. Sci., Polym. Ed., 2004, 15, 929–946; (c) M. H. Chisholm,
Inorg. Chim. Acta, 2009, 362, 4284–4290; (d) K. Nakano, N. Kosaka,
T. Hiyama and K. Nozaki, Dalton Trans., 2003, 4039–4050; (e) B. J.
O’Keefe, M. A. Hillymer and W. B. Tolman, J. Chem. Soc., Dalton
Trans., 2001, 2215–2224; (f) M. J. Stanford and A. P. Dove, Chem.
Soc. Rev., 2010, 39, 486–494; (g) C. A. Wheaton, P. G. Hayes and B.
J. Ireland, Dalton Trans., 2009, 4832–4846; (h) J. Wu, T.-L. Yu, C.-T.
Chen and C.-C. Lin, Coord. Chem. Rev., 2006, 250, 602–626; (i) R.
H. Platel, L. M. Hodgson and C. K. Williams, Polym. Rev., 2008, 48,
11–63.
or impurities from the reaction. In entry 13, PCL with Mn
=
160,600 Da was obtained in 1 h. For 1000 : 1 and 5000 : 1 molar
ratios, the polymerizations finished in 5 and 20 min having the
Mn¢s of 70,100 and 137,400 Da, respectively.
The difference between surfaces of PCL synthesized with and
without addition of BnOH was also observed by water contact
angle analysis.13 Polymers having similar molecular weights based
on GPC were chosen. The average water contact angle of PCL
(Table 1, entry 10, synthesized without addition of alcohol) is
107.4 2.5◦ while that of entry 5 (with addition of BnOH) is 92.8
0.6◦.
3 (a) P. Dege´e, P. Dubois, R. Jeroˆme, S. Jacobsen and H.-G. Fritz,
Macromol. Symp., 1999, 144, 289; (b) H. R. Kricheldorf, J. Polym.
Sci., Part A: Polym. Chem., 2004, 42, 4723–4742.
4 (a) N. Nimitsiriwat, V. C. Gibson, E. L. Marshall and M. R. J. Elsegood,
Dalton Trans., 2009, 3710–3715; (b) N. Nimitsiriwat, V. C. Gibson, E.
L. Marshall and M. R. J. Elsegood, Inorg. Chem., 2008, 47, 5417–5424;
(c) N. Nimitsiriwat, V. C. Gibson, E. L. Marshall, A. J. P. White, S. H.
Dale and M. R. J. Elsegood, Dalton Trans., 2007, 4464–4471; (d) K. B.
Aubrecht, M. A. Hillmyer and W. B. Tolman, Macromolecules, 2002,
35, 644–650.
In conclusion, a series of bis(amidinate) tin(II) complexes were
successfully synthesized and characterized. The major advantage
is the simplicity of complex preparation and a large number of
possible ligand library to fine tune both reactivity and selectivity
of the catalysts. The bis(amidinate) tin(II) complexes are active
for the polymerization of e-CL in the pressence and absence of
alcohol giving high-molecular-weight PCL. Although the low-
molecular-weight PCL synthesized using a low e-CL:Sn molar
ratio of 10 : 1 in the absence of alcohol was shown to be cyclic
based on mass spectrometry and NMR, the exact topology
(linear or cyclic) of high-molecular-weight PCL synthesized using
higher e-CL:Sn molar ratio is still uncertain due to the lack of
access to required instruments (e.g. viscometer, light scattering
detector) used to prove the cyclic structure.7,14 Nonetheless, the
bis(amidinate) tin(II) complexes are highly active for the solvent-
free polymerization of e-CL in the presence and absence of BnOH
giving high-molecular-weight PCL. This catalyst system is much
more active than the commercial Sn(Oct)2 catalyst. Amidinate
ligands having electron-donating group were found to accelerate
the polymerization by making the complex more nucleophilic.
Further attempts to characterize the polymers and to understand
the insight mechanism of the polymerization are in progress.
We acknowledge financial support from Commission on Higher
Education, The Thailand Research Fund, and Mahidol Uni-
versity. Financial support from the Center of Excellence for
Innovation in Chemistry (PERCH-CIC), Commission on Higher
Education, Ministry of Education is gratefully acknowledged.
This work is also supported by Center for Catalysis and Faculty of
Science, Mahidol University. We thank Pailin Srisuratsiri for help
with X-ray crystallography.
5 (a) G. W. Nyce, T. Glauser, E. F. Connor, A. Mo¨ck, R. M. Waymouth
and J. L. Hedrick, J. Am. Chem. Soc., 2003, 125, 3046–3056; (b) T.
R. Jensen, L. E. Breyfogle, M. A. Hillmyer and W. B. Tolman, Chem.
Commun., 2004, 2504–2505; (c) A. P. Dove, H. Li, R. C. Pratt, B. G. G.
Lohmeijer, D. A. Culkin, R. M. Waymouth and J. L. Hedrick, Chem.
Commun., 2006, 2881–2883.
6 N. E. Kamber, W. Jeong, S. Gonzalez, J. L. Hedrick and R. M.
Waymouth, Macromolecules, 2009, 42, 1634–1639.
7 D. A. Culkin, W. Jeong, S. Csihony, E. D. Gomez, N. P. Balsara, J.
L. Hedrick and R. M. Waymouth, Angew. Chem., Int. Ed., 2007, 46,
2627.
8 T. Gans-Eichler, D. Gudat and M. Nieger, Angew. Chem., Int. Ed.,
2002, 41, 1888–1891.
9 (a) N. Nimitsiriwat, V. C. Gibson, E. L. Marshall, A. J. P. White, S.
H. Dale and M. R. J. Elsegood, Dalton Trans., 2007, 4464–4471; (b) Y.
Zhou and D. S. Richeson, J. Am. Chem. Soc., 1996, 118, 10850–10852;
(c) P. B. Hitchcock, M. F. Lappert and M. Layh, J. Chem. Soc., Dalton
Trans., 1998, 3113–3117; (d) S. R. Foley, Y. Zhou, G. P. A. Yap and D.
S. Richeson, Inorg. Chem., 2000, 39, 924–929; (e) K. B. Aubrecht, M.
A. Hillmyer and W. B. Tolman, Macromolecules, 2002, 35, 644–650;
(f) F. Antolini, P. B. Hitchcock, A. V. Khvostov and M. F. Lappert,
Can. J. Chem., 2006, 84, 269–276.
10 (a) J. N. Hoskins and S. M. Grayson, Macromolecules, 2009, 42, 6406–
6413; (b) H. E. Dyer, S. Huijser, A. D. Schwarz, C. Wang, R. Duchateau
and P. Mountford, Dalton Trans., 2008, 32–35; (c) K. C. Hultzsch, T.
P. Spaniol and J. Okuda, Organometallics, 1997, 16, 4845–4856; (d) C.
E. Willans, M. A. Sinenkov, G. K. Fukin, K. Sheridan, J. M. Lynam,
A. A. Trifonov and F. M. Kerton, Dalton Trans., 2008, 3592.
11 H. R. Kricheldorf, G. Behnken, G. Schwarz and J. Kopf, Macro-
molecules, 2008, 41, 4102–4107.
12 M. H. Chisholm, N. W. Eilerts, J. C. Huffman, S. S. Iyer, M. Pacold
and K. Phomphrai, J. Am. Chem. Soc., 2000, 122, 11845–11854.
13 P. G. de Gennes, Rev. Mod. Phys., 1985, 57, 827–863.
14 (a) C. W. Bielawski, D. Benitez and R. H. Grubbs, Science, 2002,
297, 2041–2044; (b) D. Cho, K. Masuoka, K. Koguchi, T. Asari, D.
Kawaguchi, A. Takano and Y. Matsushita, Polym. J., 2005, 37, 506–
511.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 2157–2159 | 2159
©