Controlled ring-opening polymerization of ε-caprolactone catalyzed by a rare earth Schiff-base complex

Article abstract of DOI:10.1016/S1872-2067(10)60096-4

Lanthanide Schiff-base complexes with the formula [3,5-tBu ₂-2-(O)C₆H₂CH=NC₆H₅] ₃Ln(THF) (Ln = Sc, Y, La, Nd, and Gd) were synthesized by the metathesis reaction of anhydrous LnCl₃ with a Schiff-base sodium salt in good yields (> 80). The complex containing a neodymium center was characterized by X-ray diffraction and the structure around the neodymium atom could be described as a pentagonal bipyramid. The ring-opening polymerization (ROP) of ε-caprolactone (CL) was successfully carried out using the lanthanide Schiff-base complexes as catalysts, and the neodymium complex leads to a controlled ROP of CL. The influence of the reaction conditions on the monomer conversion, molecular weight, and molecular weight distribution of the resultant polymers was investigated. The polymerization rate was first-order with respect to the monomer concentration. End-group analyses of the oligomer of CL showed that the polymerization underwent a coordination-insertion mechanism.

Full text of DOI:10.1016/S1872-2067(10)60096-4

CHINESE JOURNAL OF CATALYSIS
Volume 31, Issue 8, 2010
Online English edition of the Chinese language journal
RESEARCH PAPER
Cite this article as: Chin J Catal, 2010, 31: 965–971.
Controlled Ring-Opening Polymerization of ε-Caprolactone
Catalyzed by a Rare Earth Schiff-Base Complex
NI Xufeng#, ZHU Weiwei, SHEN Zhiquan*
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering,
Zhejiang University, Hangzhou 310027, Zhejiang, China
Abstract: Lanthanide Schiff-base complexes with the formula [3,5-tBu2-2-(O)C6H2CH=NC6H5]3Ln(THF) (Ln = Sc, Y, La, Nd, and Gd)
were synthesized by the metathesis reaction of anhydrous LnCl3 with a Schiff-base sodium salt in good yields (> 80%). The complex
containing a neodymium center was characterized by X-ray diffraction and the structure around the neodymium atom could be described
as a pentagonal bipyramid. The ring-opening polymerization (ROP) of ε-caprolactone (CL) was successfully carried out using the lantha-
nide Schiff-base complexes as catalysts, and the neodymium complex leads to a controlled ROP of CL. The influence of the reaction con-
ditions on the monomer conversion, molecular weight, and molecular weight distribution of the resultant polymers was investigated. The
polymerization rate was first-order with respect to the monomer concentration. End-group analyses of the oligomer of CL showed that the
polymerization underwent a coordination-insertion mechanism.
Key words: ε-caprolactone; ring-opening polymerization; rare-earth catalyst; Schiff-base; crystal structure
Aliphatic polyester poly(ε-caprolactone) (PCL) is biode-
gradable and biocompatible [1] and is attracting interest be-
cause of its numerous applications in the biomedical field. An
efficient way to synthesize PCL is by the metal complex
metal Schiff-base complexes show high catalytic activity in
various reactions [14–17]. Furthermore, the phenoxy-imine
(FI) complexes of nickel (II) and the bis(phenoxyimine) group
4 metal complexes have been used as excellent catalysts for
the polymerization of olefins [18–21]. Few rare earth Schiff
base complex catalysts have been reported for the ring-opening
polymerization of cyclic esters as well as the related lactides
[22–24]. In these catalytic reactions, Schiff-base ligands are
stable and act as ancillary ligands to stabilize the Ln-active
bond.
catalyzed
ring-opening
polymerization
(ROP)
of
ε-caprolactone (CL). Therefore, the development of simple,
convenient, and efficient metal complexes as catalysts for the
ROP of ε-caprolactone is important practically and theoreti-
cally. Catalysts for CL polymerization are traditionally based
on Sn, Al, Zn, and lanthanide compounds [2–7]. Rare earth
complexes are very attractive catalysts because of their low
toxicity, low cost, and high reactivity [8–11].
Our group has developed some rare earth complexes as
catalysts for the ROP of cyclic esters and they have worked
very well [25–31]. We synthesized rare earth complexes
bearing three Schiff-base ligands ([3,5-tBu2-2-(O)C6H2CH=N-
C6H5]3Ln(THF)) by a facile reaction between anhydrous rare
earth chlorides and a Schiff-base sodium salt. They were thus
employed as single component catalysts for the controlled ROP
of CL.
Schiff-base ligands are considered to be privileged ligands
and they are some of the most widely used non-cyclopenta-
dienyl compounds in the coordination chemistry of transition
and lanthanide metals [12]. They have also played an important
role in progressing coordination chemistry over the past two
decades [13]. Because Schiff-base ligands have a hard donor
atom framework and tunable steric and electronic effects, many
Received date: 26 March 2010.
*Corresponding author. Tel: +86-571-87953739; Fax: +86-571-87953727; E-mail: zhiquan_shen@163.com
#Corresponding author. Tel: +86-571-87953739; Fax: +86-571-87953727; E-mail: xufengni@zju.edu.cn
Foundation item: Supported by the National Natural Science Foundation of China (20774078), the Special Funds for National Basic Research Program of China
(973 Program, G2005CB623802), and the Scientific Research Foundation for Returned Scholars, Ministry of Education of China.
Copyright © 2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(10)60096-4

NI Xufeng et al. / Chinese Journal of Catalysis, 2010, 31: 965–971
1 Experimental
dissolved in 30 ml ethanol, phenylamine (6.5 ml, 0.07 mol) was
added dropwise into the solution. Then, the mixture was heated
and refluxed for 3 h. Orange crystals formed after the reaction
solution was cooled to room temperature. The fine crystalline
product was then filtered and recrystallized in ethanol (10.7 g,
49.4%). Anal. Calc. for C21H27NO: C, 81.51; H, 8.79; N, 4.53.
Found: C, 81.81; H, 8.84; N, 4.59. 1H NMR: δ = 1.33–1.48 (m,
18H, –CH3), 7.22–7.45 (m, 7H, phenyl-H), 8.63 (s, 1H,
CH=N), 13.69 (s, 1H, –OH).
1.1 General
All experiments were performed under argon with the rig-
orous exclusion of air and moisture using standard Schlenk
techniques. Toluene, n-hexane, and tetrahydrofuran (THF)
were refluxed over sodium/benzophenone and distilled just
before use. ε-Caprolactone was purchased from Acros, dried
over CaH2 for 4 d and stored over 4A molecular sieves under
argon after distillation under reduced pressure. Anhydrous rare
earth chloride was prepared from Ln2O3 and HCl in accordance
with a standard procedure [32]. Other reagents were used as
received.
1.3 Synthesis of [3,5-tBu2-2-(O)C6H2CH=NC6H5]3Nd
(THF) complex (1)
As shown in Scheme 1, NdCl3 (0.54 g, 2.15 mmol) was
suspended in 20 ml THF and stirred overnight at room tem-
perature. A THF solution (20 ml) of N-phenyl-3,5-di-tert-
butyl-salicylaldimine (2.17 g, 7.0 mmol) was added dropwise
to a THF (10 ml) suspension of NaH (0.20 g, 8.30 mmol) at
room temperature and further stirred for 4 h. After filtration,
the yellow solution was added to the NdCl3-THF suspension.
The mixture was stirred at 80 °C in an oil bath for 4 d and then
the formed precipitate was separated from the reaction mixture
by centrifugation. The solvent was vaporized under reduced
pressure and the residue was dissolved in n-hexane.
[3,5-tBu2-2-(O)C6H2CH=NC6H5]3Nd(THF) was isolated as
yellow crystals at room temperature after a few days (1.67 g,
53.8%). Anal. Calc. for C67H86N3NdO4: C, 70.49; H, 7.59; N,
3.68; Nd, 12.63. Found: C, 70.64; H, 7.72; N, 3.74; Nd,
12.78%. IR (KBr, cm–1): 2 957 (s), 2 903 (s), 2 867 (s), 1 608
(s), 1 585 (s), 1 546 (s), 1 532 (s), 1 484 (s), 1 459 (s), 1 430 (s),
1 413 (s), 1 384 (s), 1 359(s), 1 320 (s), 1 273 (s), 1 256 (s),
1 190 (s), 1 164 (s), 1 026 (s), 985 (w), 920 (w), 874 (m), 835
(s), 788 (m), 764 (m), 742 (s), 699 (w), 638 (w), 519 (s).
The other rare earth Schiff-base complexes were synthesized
by the same method described above.
Elemental analysis was performed on a Flash EA1112 in-
strument and the rare earth metal content was measured using
the EDTA-titration method. 1H NMR spectra were obtained on
a Bruker Avance DMX500 spectrometer in CDCl3 at 25 °C
with TMS as an internal reference. The molecular weight (Mn)
and molecular weight distributions (MWD) of the formed
polymers were measured by gel permeation chromatography
(GPC) at 40 °C using THF as eluent at a flow rate of 1.0 ml/min
against standard polystyrenes on a Waters 1515 apparatus with
three PL Mixed-C columns. FT-IR spectra of the samples were
measured as KBr pellets using a Bruker Vector 22 FT-IR
spectrometer. X-ray diffraction (XRD) was performed as de-
scribed below: suitable single crystals of a complex were
sealed in thin-walled glass capillaries, and data collection was
performed at 20 °C on a Bruker SMART diffractometer with
graphite-monochromated Mo Kα radiation (λ = 0.071 073 nm).
The SMART program package was used to determine the
unit-cell parameters. An absorption correction was applied
using SADABS. The crystal structures of the neodymium
containing complex was solved by a direct method using the
SHELXS-97 program and expanded by the Fourier technique.
All the non-hydrogen atoms were refined anisotropically. Hy-
drogen atoms were all generated geometrically (C–H bond
lengths fixed at 0.095 nm) with assigned appropriate isotropic
thermal parameters.
1.4 Polymerization of CL
All polymerizations were carried out in 20 ml ampoules
using the Schlenk technique under argon. The catalyst was
dissolved in toluene at a concentration of 2.0 mol/L. As a
typical example, 0.55 ml (5.0 mmol) CL and the desired
amount of solvent and catalyst solution were injected into a
flame-dried ampoule sequentially at a specific temperature and
1.2 Synthesis of N-phenyl-3,5-di-tert-butyl-
salicylaldimine
3,5-Di-tert-butyl-salicylaldehyde (16.5 g, 0.07 mol) was
N
N
N
CHO
NH2
NaH/THF
Ln
THF
Nd
THF
EtOH
OH
OH
O
O
+
LnCl3/THF
3
3
Reflux
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
Complex 1
Ln = Sc, Y, La, Nd, Gd
Scheme 1. Synthesis and structure of rare earth Schiff-base complexes.

NI Xufeng et al. / Chinese Journal of Catalysis, 2010, 31: 965–971
polymerized over a predefined time. The polymerization was
in situ by the reaction of N-phenyl-3,5-di-tert-butyl-salicy-
laldimine with NaH in THF, giving complexes in isolated
yields of up to 80%. All these complexes have good solubility
in THF, toluene, and even in hexane.
terminated by adding ethanol containing 5% HCl and the so-
lution was poured into excess ethanol to precipitate the prod-
uct, filtered and dried in vacuum at room temperature for 24 h
to constant weight.
Among the above-mentioned complexes, only the neo-
dymium complex (complex 1) provided a suitable single
crystal for X-ray structural determination. Complex 1 was also
characterized by elemental analysis and FT-IR. In the IR
spectrum of complex 1, stretching vibration bands due to imine
groups at 1 608 cm−1 were observed. The paramagnetic prop-
erty of complex 1 did not allow a resolvable NMR spectrum to
be obtained. A definitive solid-state structure of complex 1 was
provided by a single crystal X-ray determination. The ORTEP
diagram of complex 1 is shown in Fig. 1, and the corresponding
crystallographic data are given in Table 1. The selected bond
lengths and angles are also listed in Table 1. The neodymium
ion in complex 1 is ligated by three salicyladiminato ligands
and a THF molecule. The geometry around the seven-coor-
dinate neodymium metal center can be best described as a
distorted pentagonal bipyramidal geometry (Fig. 2) with two
oxygen atoms (O2, O3) from the Schiff-base ligands occupying
the axial positions while two oxygen atoms (O4, O1) and three
nitrogen atoms (N1, N2, and N3) form a plane. The average
Nd-N(Schiff-base) distance in complex 1 is 0.269 6 nm and the
average Nd–O(Ar) bond length is 0.223 6 nm.
1.5 Oligomer for the end-group analysis
The oligomerization of CL was carried out with the Nd
containing complex 1 in toluene at 30 °C with [CL]/[Nd] = 20
and a monomer concentration [CL] = 2.0 mol/L. The reaction
was terminated with isopropanol containing 5% HCl after 5
min. The oligomer was precipitated from ethanol and purified
by redissolving in THF, followed by precipitation in ethanol.
After filtration, the product was dried in vacuum for 24 h.
2 Results and Discussion
2.1 Synthesis and characterization of the lanthanide
Schiff-base complexes
The lanthanide Schiff-base complexes were synthesized by a
salt metathesis reaction between LnCl3 and Schiff-base sodium
salts in a non-aqueous solution. Anhydrous LnCl3 reacted with
three equivalents of the sodium salts of the Schiff-bases formed
Table 1 Crystallographic data and selected bond lengths and angles for the Nd containing complex 1
Value Parameter
C67H86N3NdO4 R(Nd1–O2)/nm
Parameter
Value
Empirical formula
Formula weight (g/mol)
Temperature (oC)
Wavelength (nm)
Crystal system
Space group
a/nm
0.2215(5)
0.2226(5)
0.2547(2)
0.2691(6)
0.2697(6)
0.2699(6)
166.57(17)
93.15(18)
90.10(18)
90.94(14)
94.06(13)
144.08(15)
89.55(18)
103.79(18)
69.80(16)
74.56(15)
97.48(18)
69.98(17)
83.01(17)
131.73(15)
152.28(17)
68.62(17)
101.63(18)
144.54(18)
69.04(16)
136.58(17)
70.27(17)
1141.63
23(1)
R(Nd1–O3)/nm
R(Nd1–O4)/nm
0.071073
triclinic
P-1 (2)
R(Nd1–N1)/nm
R(Nd1–N2)/nm
R(Nd1–N3)/nm
1.14235(6)
1.63503(8)
1.71564(9)
79.5588(16)
85.2927(13)
79.9808(16)
3.0991(3)
2
∠O2–Nd1–O3/(°)
∠O2–Nd1–O1/(°)
∠O3–Nd1–O1/(°)
∠O2–Nd1–O4/(°)
∠O3–Nd1–O4/(°)
∠O1–Nd1–O4/(°)
∠O2–Nd1–N1/(°)
∠O3–Nd1–N1/(°)
∠O1–Nd1–N1/(o)
∠O4–Nd1–N1/(°)
∠O2–Nd1–N3/(°)
∠O3–Nd1–N3/(°)
∠O1–Nd1–N3/(°)
∠O4–Nd1–N3/(°)
∠N1–Nd1–N3/(°)
∠O2–Nd1–N2/(°)
∠O3–Nd1–N2/(°)
∠O1–Nd1–N2/(°)
∠O4–Nd1–N2/(°)
∠N1–Nd1–N2/(°)
∠N3–Nd1–N2/(°)
b/nm
c/nm
α/(°)
β/(°)
γ/(°)
V/nm3
Z
Dcalc/(g/cm3)
μ/mm−1
1.22333
0.885
F(000)
1202
Crystal size (mm)
2θ range (°)
Reflections collected
Independent reflections
Reflections with I ≥ 2.0 σ (I)
Parameters refine
Goodness-of-fit on F2
R
0.19×0.18×0.08
51.0
25672
11384
5690
695
1.053
0.0768
WR
0.1435
R(Nd1–O1)/nm
0.2268(4)

NI Xufeng et al. / Chinese Journal of Catalysis, 2010, 31: 965–971
of CL under certain conditions and the scandium complex
showed almost no catalytic activity for the ROP of CL. The
catalytic activity sequence for the different rare earth elements
is Y > Nd > La > Gd > Sc. The ROP of CL was also carried out
with the neodymium complex as a catalyst at 10 and 30 °C. A
higher reaction temperature favors the ROP of CL. Only a
small amount of PCL was isolated at 10 °C.
We also found that the neodymium complex acts as an active
single-component catalyst for the ROP of CL and gives a
polymer with a high molecular weight and a reasonably narrow
MWD. The polymerization process of the CL catalyzed by the
neodymium complex was investigated in detail and the results
are shown in Figs. 3–5.
When the CL/Nd molar ratio increases, the molecular weight
of PCL increases linearly while the MWD broadens slightly
from 1.33 to 1.52, as shown in Fig. 3. The kinetics of the po-
lymerization reaction at 50 °C in toluene with a CL/Nd molar
ratio of 200 was determined. The conversion increases as the
polymerization time increases and a first-order kinetics plot of
the polymerization is shown in Fig. 4. Figure 5 indicates that
the number-average molecular weight Mn of PCL increases
proportionally to the monomer conversion while the MWD of
Fig. 1. ORTEP view of complex 1 with atom labels and showing 25%
probability displacement ellipsoids.
60000
50000
1
2
Fig. 2. Conformation of the neodymium complex.
Mn
40000
30000
20000
10000
0
3
4
5
6
7
8
MWD
2.2 Polymerization of CL as catalyzed by the lanthanide
Schiff-base complexes
The polymerization of CL was initially performed with
various rare earth Schiff-base complexes (Ln(Salen)3) (Ln =
Sc, Y, La, Nd, and Gd) as shown in Table 2. We found that the
yttrium complex had the highest catalytic activity for the ROP
0
50
100
150
200
250
CL/Nd molar ratio
Table 2 ROP of CL catalyzed by Ln(Salen)3
Fig. 3. Relationship between the Mn of PCL and the concentration of
a
complex 1. Polymerization conditions: [CL] = 2.0 mol/L in toluene, 50
°C.
Element n(CL)/n(Ln) T/°C t/min Conversion (%) Mn
MWD
1.31
1.30
1.34
1.42
1.38
1.14
1.18
1.47
1.51
—
Y
100
200
50
50
50
50
50
50
50
50
50
50
50
50
30
30
10
30
30
30
30
30
30
30
30
30
60
60
60
60
60
100
74.5
19200
33400
15800
18300
26100
09600
11400
17700
18900
—
Y
Nd
Nd
Nd
La
La
Gd
Gd
Sc
100
100
2.5
2.0
1.5
1.0
0.5
0.0
100
200
50
51.5
100
55.8
100
50
90.0
43.3
—
100
50
Sc
100
100
200
100
—
—
—
Nd
Nd
Nd
88.2
41.7
—
10400
13800
—
1.52
1.66
—
0
20
40
60
80
Reaction time (min)
Fig. 4. Plot of ln([M]0/[M]) versus reaction time for the polymerization
of CL. Polymerization conditions: CL/Nd molar ratio = 200, and the rest
is the same as in Fig. 3.
General polymerization conditions: [CL] = 2.0 mol/L in toluene.
aMeasured by GPC against standard PSt samples.

NI Xufeng et al. / Chinese Journal of Catalysis, 2010, 31: 965–971
N
O
O
a: acyl-oxygen bond cleavage
N
i-PrOH
CL
CH3
O HC CH3
H O
Nd O
+
+
NdCl3
HCl
O
OH
n
t-Bu
t-Bu
t-Bu
t-Bu
O
N
Nd
a
+
O
O
3
b
t-Bu
t-Bu
Nd
O
CH3
H3C HC O
O
N
i-PrOH
N
CL
O
+
NdCl3
+
O H
n
HCl
O
b: alkyl-oxygen bond cleavage
OH
t-Bu
t-Bu
t-Bu
t-Bu
Scheme 2. Initiation and termination reaction modes in the ROP of CL
To study the polymerization mechanism, a sample of low
molecular weight PCL quenched with isopropanol was pre-
1
1
2
3
4
5
6
7
8
pared and subjected to H NMR analysis, as shown in Fig. 6.
40000
One end of the PCL chain is the esterified isopropyl group
–COOCH(CH3)2 according to the signals at δ = 4.98 (multiplet,
Hf) for the CH group and δ = 1.23 (doublet, Hg) for the CH3
groups. Furthermore, the other end of the chain is –CH2OH
according to the methylene proton signal at δ = 3.63 (triplet,
He). These signals prove that the ring of CL opened by
acyl-oxygen bond cleavage (Scheme 2 a). On the contrary, no
30000
20000
10000
0
Mn
MWD
1
isopropyl ether end group signal is detected in the H NMR
spectrum, indicating that the ring of CL does not open by a
break of the alkyl-oxygen bond (Scheme 2 b). This type of ring
opening is similar to that previously reported [33].
0
20
40
60
80
100
Conversion (%)
Fig. 5. Plots of Mn versus monomer conversion. Polymerization condi-
tions are the same as in Fig. 4.
3 Conclusions
c
c
e
d
b
O
CH
H
H
H2
C
H
H
3
C2 C2
C
C2 C2
C
Lanthanide Schiff-base complexes [3,5-tBu2-2-(O)C6H2-
CH=NC6H5]3Ln(THF) were prepared by the reaction of anhy-
drous LnCl3 with the sodium salts of Schiff-bases in THF with
a 3:1 molar ratio and in good yield. The neodymium contain-
ing complex was structurally characterized and was used as a
single-component catalyst for the controlled ring-opening
polymerization of ε-caprolactone giving PCL with a high mo-
lecular weight and a narrow molecular weight distribution.
O
C
H
CH
3
HO
C
2
C
2
C
O
H
2
H
H
H
2
H
2
O
g
n
f
c
c
a
b
d
a
d
b
c
e
g
f
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