Synthesis, characterization of hetero-bimetallic complex and application in the polymerization of ε-caprolactone

Article abstract of DOI:10.1016/j.inoche.2011.03.003

The reaction of 2,2′-ethylidenebis(4,6-di-tert-butylphenol)(EDBP- H₂) with sodium, Al(Me)₃ and BnOH with a molar of ratio of 1:1:1:1 in THF gives hetero-bimetallic complex of [(EDBP)Al(CH ₃)(μ₂-OBn)Na(THF)₃](1) in 85% yield. In order to investigate the different anion effects on the ROP of ε-caprolactone, complexes [(EDBP)₂Al][(Li)(THF)₂] (2) and [(EDBP)₂Al][(Na)(THF)₂] (3) were prepared in an analogous manner to that of 1 in 63% and 71% yields respectively. Experimental results show that hetero-bimetallic complex 1 is an efficient catalyst for the ring-opening polymerization of ε-caprolactone.

Full text of DOI:10.1016/j.inoche.2011.03.003

Inorganic Chemistry Communications 14 (2011) 763–766
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, characterization of hetero-bimetallic complex and application in the
polymerization of ε-caprolactone
b
b
b
b
b,⁎⁎
Xiaobo Pan a,b, , Ai Liu , Lihui Yao , Lei Wang , Jinfeng Zhang , Jincai Wu
,
⁎
Xuebo Zhao a, Chu-Chieh Lin c
Qingdao Institute of Bioenergy and Bioprocess Technology, CAS, PR China
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, ROC
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of 2,2′-ethylidenebis(4,6-di-tert-butylphenol)(EDBP-H2) with sodium, Al(Me)3 and BnOH with a
molar of ratio of 1:1:1:1 in THF gives hetero-bimetallic complex of [(EDBP)Al(CH3)(μ2-OBn)Na(THF)3](1) in
85% yield. In order to investigate the different anion effects on the ROP of ε-caprolactone, complexes
[(EDBP)2Al][(Li)(THF)2] (2) and [(EDBP)2Al][(Na)(THF)2] (3) were prepared in an analogous manner to that
of 1 in 63% and 71% yields respectively. Experimental results show that hetero-bimetallic complex 1 is an
efficient catalyst for the ring-opening polymerization of ε-caprolactone.
Received 1 December 2010
Accepted 2 March 2011
Available online 13 March 2011
Keywords:
Hetero-bimetallic complex
ε-Caprolactone
© 2011 Elsevier B.V. All rights reserved.
Catalyst
Ring-opening polymerization
Biodegradability, biocompatibility, and permeable properties of
aliphatic polyesters, such as poly(ε-caprolactone) (PCL) [1], poly
(lactide) (PLA) [2] and poly(trimethylenecarbonate)(PTMC) [3]
and their copolymers show their potential applications in the
medical field as biodegradable surgical sutures or as a delivery
medium for controlled release of drugs [4]. Therefore, there has
been increasing interest in the development of efficient catalytic
systems for the preparation of PCL, PLA and PTMC. The major
polymerization method used to synthesize these polymers has been
the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL).
An important task for developing new catalytic systems is to make
the catalyst more compatible with the purpose of biomedical
application. Though several effective initiators that initiate ROP of
CL have been reported [5], the cytotoxicity and difficulties in
removal of the catalyst from the resulting polymer have limited
their utilization.
Recently we reported some metal aryloxides supported with
2,2′-ethylidenebis(4,6-di-tert-butylphenol)(EDBPH2), such as
EDBP-Li [6], EDBP-Na [7], EDBP-K [8], EDBP-Al [9], are excellent
catalysts with good controlled features for ROP of cyclic ester
through the activation of alcohol. And EDBP-Al [9a] molecular even
can catalyze the synthesis of 400-fold polylactone chains in the ROP
of ε-caprolactone. EDBP-H2 is an attractive ligand because it has
been approved as an indirect food additive (as an antioxidant in
polymer packaging) by the U.S. Food and Drug Administration [10].
Moreover, sodium and aluminum are nontoxic and are essential for
life. Thus, aluminum and sodium complexes supported with EDBPH2
ligand have attracted considerable interest in terms of biomedical
purposes. All of these encourage us to design new EDBP nontoxic
metal complexes as catalysts for ROP of ε-CL. Herein we report one
new hetero-bimetallic complex to catalyze ring-opening polymer-
ization of ε-CL and results show it is an efficient catalyst.
The reaction of EDBP-H2 with sodium, Al(Me)3 and BnOH with a
molar of ratio of 1:1:1:1 in THF gives complex of [(EDBP)Al(CH3)
(μ2-OBn)Na(THF)3] (1) (Scheme 1) in 85% yield. 1H NMR spectra
and microanalyses of complex 1 are consistent with our expecta-
tions. This is further verified by the crystal structure studies of
complex 1. In order to investigate the different cation effects on the
ROP of lactone, complexes [(EDBP)2Al][(Li)(THF)2] (2) and
[(EDBP)2Al][(Na)(THF)2] (3) were prepared in an analogous
manner to that of 1 in 63% and 71% yield respectively. Further
reaction of compound 3 with excess BnOH in toluene produces 4 in
78% yields.
Fine colorless crystals were obtained by crystallization from
THF/hexane (5 mL:15 mL). The ORTEP drawing of 1 is given in
Fig. 1. The [(EDBP)Al(CH3)(μ2-OBn)Na(THF)3] (1) complex is
crystallized in triclinic space group P-1. Two oxygen atoms of the
EDBP ligand are nonequivalent. One oxygen atom bridges a sodium
atom and an aluminum atom, and the other one is coordinated to
the aluminum atom. The geometry around Al1 is distorted
tetrahedral, with coordination by two oxygen atoms of the phenoxy
⁎
Correspondence to: Qingdao Institute of Bioenergy and Bioprocess Technology,
CAS, PR China.
⁎⁎ Corresponding author.
E-mail addresses: panxb@qibebt.ac.cn (X. Pan), wujc@lzu.edu.cn (J. Wu).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.03.003

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X. Pan et al. / Inorganic Chemistry Communications 14 (2011) 763–766
Scheme 1. Preparation of compounds 1–4.
group, one bridging oxygen atom of the benzyl alkoxide group and
one carbon atom of methyl with the bond lengths of Al1-O1 1.767
(2), Al1-O2 1.722(3), Al1-O3 1.759(3) and Al1-C30 1.948(5) Å,
respectively. The geometry around Na1 is distorted square pyramid,
with coordination by one bridging oxygen atom of O1 from ligand,
and one bridging oxygen atom of O3 from the benzyl alkoxide group
and three oxygen atoms O4, O5, O6 from three THF molecules, and
with the bond lengths of Na1-O1 2.515(3), Na1-O3 2.331(3), Na1-O4
2.379(4), Na1-O5 2.307(4) and Na1-O6 2.339(4) Å respectively.
Single crystals of 2 and 4 suitable for X-ray structural de-
termination were obtained from hexane solution. The ORTEP
drawing of the molecular structure of 2 and 4 are given in Figs. 2
and 3. In the two structures, the geometry around Al is distorted
tetrahedral and the aluminum atom is firmly fixed by four oxy-
gen atoms of two EDBP ligands with average Al–O bond lengths of
1.735(1) and 1.736(8) Å for complex 2 and 4 respectively. Two
oxygen atoms of the each EDBP ligand are nonequivalent. One just
coordinates to Al1 and the other one bridges aluminum and
Fig. 2. Molecular structure of 2 as 20% ellipsoids (methyl carbons of the tert-butyl
groups and all of the hydrogen atoms are omitted for clarity). Selected bond lengths
(Å): Al1-O1 1.682(8), Al1-O2 1.767(8), Al1-O3 1.718(9), Al1-O4 1.770(8), Li1-O2 2.07
(3), Li1-O4 1.94(3), Li1-O5 2.13(3), Li1-O6 1.93(2), C1-O1 1.334(12), C13-O2 1.409
(13), C31-O3 1.372(13) and C43-O4 1.352(13).
Fig. 1. Molecular structure of 1 as 30% ellipsoids (methyl carbons of the tert-butyl
groups and all of the hydrogen atoms are omitted for clarity). Selected bond lengths
(Å): Al1-O1 1.767(2), Al1-O2 1.722(3), Al1-O3 1.759(3), Na1-O1 2.515(3), Na1-O3
2.331(3), Na1-O4 2.379(4), Na1-O5 2.307(4) and Na1-O6 2.339(4).

X. Pan et al. / Inorganic Chemistry Communications 14 (2011) 763–766
765
Fig. 4. Polymerization of ε-CL catalyzed by 1 in toluene at 120 °C. The relationship
between Mn(■) (PDI (●)) of the polymer and the initial mole ratio [M]0/[complex]0
is shown.
Fig. 3. Molecular structure of 4 as 20% ellipsoids (methyl carbons of the tert-butyl
groups and all of the hydrogen atoms are omitted for clarity, except for the hydroxy of
benzyl alcohol). Selected bond lengths (Å): Al1-O1 1.707(8), Al1-O2 1.725(8), Al1-O3
1.811(9), Al1-O4 1.701(8), Na1-O1 2.309(10), Na1-O3 2.459(10), Na1-O5 2.271(15),
Na1-O6 2.37(2), C1-O13 1.402(9), C1-O2 1.367(8), C31-O3 1.376(8) and C43-O4
1.396(9).
polymerization with the catalysts published. [11] Analysis of PCL-
47 (Table 1, entry 1) produced from 1 at an initial [M]0/[I]0 ratio of
47 by 1H NMR (Fig. 5) shows a characteristic benzyl peak Hb at
5.11 ppm and hydroxy end group peak Hg at 3.65 ppm in CDCl3,
indicating polymers are linear capped with two end groups of the
benzyl alkoxy and hydroxy group respectively.
In fact the complexes 2, 3, 4 [12] also were applied to the ROP of ε-
Caprolactone, the experimental results show they are not active
catalysts which can be attributed to the low basic complexes and
cannot activate BnOH to initiate the ROP reaction.
lithium/sodium. The structure of 2 is somewhat different from that
of 4, in complex 2 two THF molecules coordinates to Li1, and in
complex 4 two benzyl alcohol molecules coordinate to Na1 instead
of THF.
Ring-opening polymerization of ε-caprolactone (ε-CL) employ-
ing 1 as an initiator is systematically examined under a dry nitrogen
atmosphere. Conversion of CL is determined on the basis of 1H NMR
spectroscopic studies. The molecular weight and polydispersity of
PCL are measured by gel permeation chromatography (GPC). A
typical polymerization experiment exemplified for PCL-94 ([M]0/
[I]0 =94) is described as follows. To a rapidly stirring solution of 1 in
toluene was added ε-CL (0.20 mL, 1.85 mmol). After the reaction
was quenched by the addition of excess 0.35 N acetic acid aqueous
solutions, the polymer was precipitated into n-hexane. Polymeriza-
tions of ε-CL under same reaction conditions (entries 1–4) have
been systematically examined as shown in Table 1. It was found that
the PDIs of polyesters initiated by 1 range from 1.40 to 1.47, and a
linear relationship between the number average molecular weight
(Mn) and the monomer-to-initiator ratio ([M]0/[I]0) exists as shown
in Fig. 4 (Table 1, entries 1–4). While in these experiments, the
experimental value of Mn (Mn(obsd)) obtained from the GPC
analysis is generally lower than the theoretical Mn value (calculated
on the assumption that each BnOH initiates the polymerization)
owe to the occurrence of the back-biting reaction. The cyclic
polymerization side reactions were also found for the lactide
In conclusion, a new hetero-bimetallic complex has been syn-
thesized and characterized. The hetero-bimetallic complex 1 is an
efficient catalyst for ring-opening polymerization of lactone.
Acknowledgments
We thank the financial supports from the National Natural
Science Foundation of China (No. 21071069 and 20601011), Science
Foundation of Gansu Province of China (0803RJZA103), Scientific
Research Foundation for the Returned Overseas Chinese Scholars
and the Fundamental Research Funds of the Central Universities of
China, State Education Ministry (lzujbky-2009-26).
Appendix A. Supplementary material
Supplementary data to this article can be found online at
doi:10.1016/j.inoche.2011.03.003.
Table 1
Ring-opening polymerization of ε-caprolactone catalyzed by 1a.
Entry
[CL]0/[Complex]0
Time (h)
PDI
Mn (obsd)b
Mn (calcd)c
Conversion (%)
1
2
3
4
47
94
141
188
16
16
16
16
1.42
1.47
1.43
1.40
9170 (5300)
9887 (6500)
11,368 (7600)
16,600 (9600)
5400
10,300
15,200
19,600
98
95
94
91
a
Conditions: 0.02 mmol of complex, 10 mL of toluene, 120 °C reflux.
Obtained from GPC analysis, and calibrated by polystyrene standard (corrected values listed in brackets).
Calculated from the molecular weight of ε-CL times [CL]0/[Complex]0 times conversion yield plus the molecular weight of BnOH.
b
c

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X. Pan et al. / Inorganic Chemistry Communications 14 (2011) 763–766
Fig. 5. 1H NMR of PCL-47 (from Table 1, entry 1) in CDCl3.
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