Catalytic improvement of titanium complexes bearing bis(aminophenolate) in ring-opening polymerization of l-lactide and ε-caprolactone

Article abstract of DOI:10.1016/j.molcata.2014.07.003

This study synthesized and examined a series of titanium aminophenoxide complexes as catalysts for the ring-opening polymerization of l-lactide and ε-caprolactone. These Ti complexes are more active for LA than for CL. The interaction between coordinating

Full text of DOI:10.1016/j.molcata.2014.07.003

Journal of Molecular Catalysis A: Chemical 394 (2014) 97–104
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Catalytic improvement of titanium complexes bearing
bis(aminophenolate) in ring-opening polymerization of l-lactide and
␧-caprolactone
Hsiu-Wei Ou, Hsing-Yin Chen, His-Ching Tseng, Mon-Wei Hsiao, Yu-Lun Chang,
Nai-Yuan Jheng, Yi-Chun Lai, Tzung-Yu Shih, Yu-Ting Lin, Hsuan-Ying Chen∗
Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 80708, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
This study synthesized and examined a series of titanium aminophenoxide complexes as catalysts for
the ring-opening polymerization of l-lactide and ␧-caprolactone. These Ti complexes are more active for
LA than for CL. The interaction between coordinating atoms of the ligands and the central metal ions has
a considerable influence on the resulting catalyses. The complex with thiophenyl groups demonstrated
the highest catalytic activity, due to the lability of thiophenyl groups. Rapid changes between association
and dissociation can be used to tune the electronic density of Ti in order to avoid contending with the
coordination of l-lactide and ␧-caprolactone to increase activity. In addition, kinetic results indicate a
first-order dependency on [LOMeTi(OiPr)2] and a first-order dependency on [CL] and [LA] respectively.
© 2014 Elsevier B.V. All rights reserved.
Received 12 May 2014
Received in revised form 1 July 2014
Accepted 2 July 2014
Available online 11 July 2014
Keywords:
Titanium complex
␧-Caprolactone
l-Lactide
Ring-opening polymerization
1. Introduction
complexes are presented in Fig. 2. The following literatures are the
examples of the ROPs employing Ti complexes with Salen ligands
Aliphatic polyesters such as poly(␧-caprolactone) (PCL),
poly(lactide) (PLA), and their copolymers have demonstrated wide
applicability in a variety of fields [1], due to their biodegradability,
biocompatibility, and permeability. The preparation of PCL and
PLA has subsequently undergone intensive study and ring-opening
polymerization (ROP) has been the most common approach.
Many metal complexes [2] have been used as catalysts for the
ROP of cycloesters; however, materials employed in medical
applications require low cytoxicity. As a result, Ti complexes [3d]
are the commonly used catalysts in ROP, due to the low cost
of the precursor and the high oxidation state associated with
polyanionic ligands. Salen type ligands (bis(iminophenol)) (Fig. 1)
are commonly employed because the framework between the
two nitrogen atoms can be altered according to one’s objectives.
For example, linear alkyl groups produce fluxional ligands, while
cycloalkyl groups (e.g. the cyclohexyl group) produce the rigid
and chiral ligands. The phenyl group produces a rigid ligand with
higher electronic density. In addition, the substitute group on the
phenol ring can be altered according to the steric or electronic
properties. The three octahedral coordination isomers of metal
[3].
In 2006, Gibson [3a] reported that the trans-Ti complex with
Salen ligand provides greater catalytic activity than the cis form
with regard to rac-lactide polymerization. In the same year, Gibson
[3b] reported that electron-withdrawing substituents within the
Salen framework of Ti complexes can reduce the propagation rate
of lactide (LA) monomers.
Another similar ligand is bis(aminophenol) (Salan type) [4]
(Fig. 3). The framework between two nitrogens can be altered and
the functional groups (R3) associated with the nitrogen may also
be used to control the steric effect. In the following, we outline
the important work on ␧-caprolactone (CL) and LA polymeriza-
tion using Ti and Zr bis(aminophenolate) complexes. Davidson [4c]
reported that the steric bulky substituents on the phenolate ring
decreased the polymerization rate of CL and LA using Zr complexes
as the catalysts; however, the opposite is true for Ti complexes.
Kol [4a] reported that the electron donating substituent (in the
R1 and R2 positions) of ligands increases the activity of Ti com-
plexes in LA polymerization but opposite results for Zr complexes.
Kim [4d] reported a slight increase in catalytic activity from the
methyl to benzyl groups in the R3 position of Ti complexes. Few
studies have investigated the steric effect (in the R3 position of Ti
complexes) on the catalytic activity of corresponding Ti complexes
for ROP of lactide, due to difficulties involved in the synthesis of
∗
Corresponding author. Tel.: +886 7 3121101x2585; fax: +886 7 3125339.
E-mail address: hchen@kmu.edu.tw (H.-Y. Chen).
http://dx.doi.org/10.1016/j.molcata.2014.07.003
1381-1169/© 2014 Elsevier B.V. All rights reserved.

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H.-W. Ou et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 97–104
faster than that of tetradentate bis(aminophenolate)Ti complexes.
In addition, the pendent functional group can influence the catalytic
activity of Ti complexes because the pendant atoms of coordinated
ligands provide the metal with electrons through coordination,
which weakens the metal-alkoxide bond [6]. Herein, we designed
a series of Salan ligands and their Ti complexes. According to the
result of CL and l-LA polymerization using these Ti complexes as the
catalysts, the best pendent functional group for increasing the cat-
alytic activity can be found out and it investigated the underlying
catalytic mechanism.
N
N
N
=
N
N
N
N
N
N
R2
OH HO
R2
H2
C
n
N
R1
R1
Fig. 1. Salen type ligands.
X
O
X
2. Results and discussion
N
N
N
N
N
N
M
M
M
O
O
X
X
X
X
O
2.1. Synthesis and characterization of Ti complexes
O
O
All ligands were prepared through the condensation reactions of
arylaldehydes and ethylenediamine to form diimines. The diimines
were reduced using NaBH4 and reacted with 2,4-di-tert-butyl-6-
(chloromethyl)phenol to form hexadentate bis(aminophenol). All
ligands reacted with a stoichiometric quantity of titanium iso-
propoxide in THF to produce a moderate yield of Ti compounds
(Fig. 5). The formula and structure were confirmed by 1H and
13C NMR spectra, elemental analysis, and X-ray crystal analysis.
The X-ray structure of LFTi(OiPr)2 (Fig. 6) illustrates the octahe-
dral geometry of the Ti complex, with the cis position between
two isopropoxides and the trans position between two phenolate
groups. The axial angle of O(2)-Ti-O(1) is 168.80(7) and the equa-
torial angles between N(2)-Ti-N(1), O(4)-Ti-N(2), O(4)-Ti-O(3),
and O(3)-Ti-N(1) are 73.84(6), 95.09(7), 107.73(7), and 89.87(7)◦,
respectively. The distances between the Ti atom and O(1), N(1),
O(2), O(3), O(4) and N(2) are 1.8853(15), 2.3856(18), 1.8915(15),
β-cis
α-cis
trans
Fig. 2. Octahedral coordination isomers of metal complexes with Salen ligands.
bis(aminophenol) using bulky or coordinated groups in the R3 posi-
tion. In addition, the common coordination number of Ti complexes
is six and the atoms in the R3 position did not easily coordinate to
Ti for the Ti complexes with bis(aminophenolate) and two alkoxide
ligands. A survey of Ti complexes bearing Salen or Salan ligands for
the cycloester polymerization showed that their catalytic activity
is low because the coordination spheres of Ti atoms bearing Salen
or Salan ligands are congested and the compound structure have
to transform into less-six-coordination states to accept monomer
or form the unlikely seven coordination states with monomer
donating.
Neves [5] attempted to synthesize Zn complex using hex-
adentate bis(aminophenol). The synthesis of this Salan ligand is
presented in Fig. 4. Inspired by this ligand, we planned to synthesize
a series of bis(aminophenol) ligands with two extra pendent coor-
dinating groups and their Ti complexes. If the size of Ti is too small
to form a seven or eight coordination complex, the extra coordi-
nated groups in the R3 positions are useless and may even compete
against the coordination of monomers, thereby decreasing the
polymerization rate. If two extra pendent coordinating groups are
labile and can help the Ti complexes to transform into less six coor-
dination states, the polymerization rate of bis(aminophenolate)Ti
complexes with two extra pendent coordinating groups will be
˚
1.8178(16), 1.8334(16), and 2.3910(18) A, respectively, confirming
the distortion of the structure from an ideal octahedral topol-
ogy. In addition, the angles of C(47)-O(4)-Ti and C(50)-O(3)-Ti are
134.11(14) and 138.66(15)◦, which exceeds 109.5◦. This means that
there are ␲ bonds between Ti and oxygen of isopropoxides and
shorter bond distances than the bonds between Ti and oxygen of
phenolate groups. Compared with LBnTi(OiPr)2 [4d], the bonds of
Ti Oi−Pr are longer in LFTi(OiPr)2 than LBnTi(OiPr)2 (1.818(4) and
1.800(5) A) and the angles of C Oi−Pr Ti are smaller in LFTi(OiPr)2
˚
than LBnTi(OiPr)2 (138.3(4) and 148.9(5)◦). There is no interaction
between Ti and F (no coordination and resonance effect) but the
bonding between Ti and Oi−Pr is changed.
R3
R3
N
N
N
=
N
N
N
N
N
N
R2
R2
OH HO
H2
C
n
N
R1
R1
Fig. 3. Bis(aminophenol) ligands (Salan type).
tBu
2
tBu
But
But
OH
HO
tBu
tBu
O
N
H2N
N
N
NH HN
NaBH4
OH Cl
+
N
N
N
N
2
N
N
N
N
NH2
Fig. 4. Preparation of hexadentate bis(aminophenol) ligand.

H.-W. Ou et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 97–104
99
H2N
O
N
N
N
H H
N
NaBH4
+
R
R
R
R
2
R
NH2
*
*
*
tBu
*
*
R =
O
F
S
O
2
tBu
OHCl
LFu-H
LF-H
LBn-H LOMe-H
LTh-H
R
O
tBu
tBu
OH
HO
R
tBu
tBu
HO
N
R
Ti(OiPr)4
R
OiPr
N
N
N
Ti
O
N
N
iPrO
OH
R
R
Fig. 5. Synthesis of bis(aminophenol) ligands and their Ti complexes.
Crystal-related data indicated no coordination between Ti and
fluorine; however, the 1H NMR spectra of these complexes revealed
interesting information. In Fig. 7, protons (a) (blue) are the four H
atoms in the ethylenediamine group, protons (b) (green) are the
four H atoms of methenes in the 2,4-di-tert-butyl-6-methylphenol
group, and protons (c) (red) are the four H atoms of methenes in the
pendent group (R). When R is a phenyl group (Ti–Bn), protons (c)
are the singlet peak, which means that the benzyl group can freely
rotate. A different situation may also occur, in which the protons
(c) are the doublet of doublets peak when R is a 2-methylphenyl,
2-fluorophenyl, furan-2-yl, or thiophen-2-yl group. This splitting
pattern is predictable regardless of the timescale of fluxionality of
complex and therefore cannot be used as a diagnostic of coordina-
tion of the R group to the metal.
2.2. Polymerization of ␧-caprolactone and l-lactide
Polymerizations of l-LA and CL using Ti complexes as an
initiator in toluene were investigated at 120 ◦C (Table 1). As
shown in Table 1, different Ti complexes demonstrated different
degrees of catalytic rate (in l-LA and CL polymerization) using
different ligands. As illustrated by entries 1, 3, 5, 7, and 9 of
Table 1 for CL polymerization ([CL]/[Cat.] = 10, benzene-d6 1 mL
in a sealed NMR tube), the catalytic rate was in the following
order: LThTi(OiPr)2 > LBnTi(OiPr)2 > LFTi(OiPr)2 > LOMeTi(OiPr)2 >
LFuTi(OiPr)2. All the CL polymerization associated with these com-
plexes was completed within 80 h using [CL] = 5 M in 5 mL toluene
at 120 ◦C (entries 2, 4, 6, 8, and 10 of Table 1). The Mn(GPC) of these
polymers appeared slightly larger than Mn(cal) and Mn(NMR) and
their polydispersity index (PDI) was narrow, with the exception
of LBnTi(OiPr)2 due to reduced transesterification resulting from a
reduction in the protection of ligands.
As shown in entries 1, 3, 5, 7, and 9 of Table 2 for l-LA polymer-
ization ([LA]/[Cat.] = 10, benzene-d6 1 mL at 120 ◦C in a sealed NMR
tube), the activity of LThTi(OiPr)2 still exceeded that of LBnTi(OiPr)2
and others had lower catalytic rate than LBnTi(OiPr)2. The l-LA
polymerization using LThTi(OiPr)2 as the catalyst was finished in
20 h with [LA] = 5 M in 2 mL toluene at 100 ◦C (entries 2 of Table 2);
however, other Ti complexes showed lower catalytic rate. The range
of PDI of these polymers was 1.14–1.26. LThTi(OiPr)2 provided the
best catalytic rate for both l-LA and CL polymerization, revealing
the ability of sulfur atoms to increase the polymerization rate of Ti
complexes. According to literature reports [4c,7], Ti complexes are
in general more active for CL than for LA, but our results showed
the opposite phenomenon. Few papers [8] reported that their Ti
Fig. 6. Molecular structure of complex LF Ti(OiPr)2 as 20% probability ellipsoids
(all of the hydrogen atoms were omitted for clarity. The deposition numbers of
crystallographic data: CCDC 949467).
complexes showed higher activity for the polymerization of LA than
for the polymerization of CL without explanations.
Although LThTi(OiPr)2 presented the best catalytic activity, it
did not necessarily have the highest polymerization rate. These Ti
complexes have an octahedral geometric framework, which should
require transformation to real active intermediates with a lower
coordination number in order to accept monomers coordination;
the induction period may be the time required for transformation.
The pendent groups of the ligands may decrease the transforma-
tion time or increase the polymerization rate of their intermediate.
Therefore, the polymerization of CL and LA by these Ti com-
plexes was monitored by 1H NMR to obtain the kobs and the
induction period of these catalysts (Table 3, Figures S1–S4, and
Tables S1–S4). In addition, two d-solvents (C6D6 and CDCl3) were
used to investigate the solvent effect with different polarity. In
Table 2, LThTi(OiPr)2 had a higher kobs value than LBnTi(OiPr)2
for CL and LA polymerization in benzene-d6 and CDCl3; however,

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H.-W. Ou et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 97–104
Table 1
Polymerization of ␧-caprolactone using each of the Ti complexes as an initiator.
b
a
c
Entry
LTi(OiPr)2
LTh
Time (h)
Conv.a
Mn(Cal)
Mn(NMR)
Mn(GPC)
PDIc
1d
2e
7.75
40
30.5
40
51.25
40
56.5
40
90
>99
93
>99
91
62
86
54
93
2900
2900
1900
1600
2300
1900
800
2200
2900
1900
1800
1.02
1.26
1.20
3d
4e
LBn
LF
5d
6e
7d
8e
LOMe
LFu
1400
1.25
9d
10e
75.5
40
f
f
f
f
28
–
–
–
–
a
Obtained from 1H NMR analysis.
b
c
d
e
f
Calculated from the molecular weight of monomer × [monomer]0/2[Cat]0 × conversion yield + Mw(PriOH).
Obtained from GPC analysis and calibration based on the polystyrene standard. Mn(GPC) is the value obtained from GPC times 0.56.
Reaction condition: benzene-d6 (1 mL) in sealed NMR tube, [CL] = 1.0 M, 120 ◦C, [CL]:[Cat] = 10:1.
Reaction condition: toluene (5 mL) in Schlenk flask, [CL] = 5.0 M, 100 ◦C, [CL]:[Cat] = 50:1.
Not available.
Table 2
Polymerization of l-lactide using each of the Ti complexes as an initiator.
b
a
c
Entry
LTi(OiPr)2
LTh
Time (h)
Conv.a
Mn(Cal)
Mn(NMR)
Mn(GPC)
PDIc
1d
2e
10.5
20
15
20
26
20
38.75
20
51
20
91
>99
f
7300
2700
3100
1800
3900
–
5400
2400
3000
2400
2200
1.26
1.16
1.14
1.22
1.21
3d
4e
LBn
LF
95
37
91
43
91
24
91
53
f
–
5d
6e
f
–
7d
8e
LOMe
LFu
f
–
9d
10e
f
–
a
Obtained from 1H NMR analysis.
b
c
d
e
f
Calculated from the molecular weight of monomer × [monomer]0/2[Cat]0 × conversion yield + Mw(PriOH).
Obtained from GPC analysis and calibration based on the polystyrene standard. Mn(GPC) is the value obtained from GPC times 0.58.
Reaction condition: benzene-d6 (1 mL) in sealed NMR tube, [LA] = 1.0 M, 120 ◦C, [LA]:[Cat] = 10:1.
Reaction condition: toluene (2 mL) in Schlenk flask, [LA] = 5 M, 100 ◦C, [LA]:[Cat] = 100:1.
Not available.
LThTi(OiPr)2 had a longer induction period than LBnTi(OiPr)2 for
LA polymerization. This indicates that thiophenyl groups increase
the polymerization rate because the S is a soft atom and the
weaker bonds between Ti and S allow thiophenyl groups to dis-
sociate easily, thereby avoiding the competition with monomers.
The lability of thiophenyl groups still provides Ti electrons through
coordination, which decreases the bond between Ti and alkoxide
and increases the polymerization rate. However, this does not help
to decrease transformation time. The transformation time involved
in LA polymerization was shorter than that of CL, indicating that
LA is helpful in the transformation of these Ti complexes into the
active intermediates. In addition, Ti complexes in CDCl3 resulted
Fig. 7. 1H NMR spectra of Ti complexes. (For interpretation of the references to colour in this figure citation, the reader is referred to the web version of this article.).

H.-W. Ou et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 97–104
101
Table 3
Kinetic study of polymerization of ␧-caprolactone and l-lactide using each of the Ti complexes as an initiator in a sealed NMR tube in C6D6 and CDCl3, respectively.
Catalyst
C6D6
CDCl3
LTi(OiPr)2
CL
LA
CL
LA
Entry
kobs
Ipa
3.5
6.5
19.5
13.5
38.5
kobs
Ipa
3.5
2.75
7.5
kobs
Ipa
kobs
Ipa
LTh
LBn
LF
0.551
0.082
0.075
0.057
0.065
0.274
0.242
0.128
0.080
0.068
0.215
0.048
0.064
0.093
0.053
9
0.242
0.098
0.055
0.054
0.048
3.75
2
5
4
7
14
30
22
38
LOMe
LFu
8
15.75
a
Ip = Induction period (h).
in lower kobs and longer induction periods, compared with C6D6.
All the Ti complexes with hexadentate ligands in CDCl3 showed
greater activity than LBnTi(OiPr)2 in CL polymerization (Table 3)
and it means the pendent groups of ligands are beneficial in polar
CDCl3 and increase the polymerization rate (kobs).
form a seven-coordinate intermediate, which subsequently trans-
formed into a six-coordinate active species. During the process of
polymerization, the N atom of Salan ligand easily bonded with Ti
to form a six-coordinate structure, which decreased activity. The
thiophen-2-yl methyl group prevented the coordination of the N
atom and was easily replaced by the monomer due to its lability.
2.3. Kinetic study of the polymerization of CL and LA catalyzed
using LOMeTi(OiPr)2
3.1. Experimental
Kinetic studies were performed with respect to the ratio of
[M]0/[LOMeTi(OiPr)2] ([CL] = 2.0 M in toluene 5 mL and [LA] = 1.25 M
in CDCl3 1 mL) at 100 ◦C shown in Tables S5, S6, and Figures S5, S6,
S7, and S8. Preliminary results indicate a first-order dependency
on [CL] and [LA] respectively (Figures S5 and S7). By plotting ln kobs
vs. ln [LOMeTi(OiPr)2], both LOMeTi(OiPr)2 orders of 1, kapp values
of 1.82 and 0.40 were discovered for CL and LA (Figures S6 and S8),
respectively. The polymerization of CL and LA using LOMeTi(OiPr)2
at 100 ◦C demonstrated the following rate law:
Standard Schlenk techniques and a N2-filled glovebox were
used all over the isolation and treatment of all the com-
pounds. Solvents, ␧-caprolactone, l-lactide, and deuterated
solvents were purified prior to use. 2,4-Di-tert-butylphenol,
sodium borohydride, formaldehyde, 37 wt% sol. in water, triethyl-
amine, thionyl chloride, ethylenediamine anhydrous, benzalde-
hyde, 2-methoxybenzaldehyde, 2-fluorobenzaldehyde, thiophene-
2-carbaldehyde, furan-2-carbaldehyde, titanium (IV) isoprop-
oxide, sodium hydride, deuterated chloroform, l-lactide, and
␧-caprolactone were purchased from Acros. Benzyl alcohol was
purchased from Alfa. 1H and 13C NMR spectra were recorded on
a Varian Gemini 2000-200 (200 MHz for 1H and 50 MHz for 13C)
spectrometer with chemical shifts given in ppm from the internal
TMS or center line of CDCl3. Microanalyses were performed using a
Heraeus CHN-O-RAPID instrument. GPC measurements were per-
formed on a Jasco PU-2080 PLUS HPLC pump system equipped
with a differential Jasco RI-2031 PLUS refractive index detector
usi◦ng THF (HPLC grade) as an eluent (flow rate 1.0 mL/min, at
−d[CL]/dt = 1.82 × [CL]1[LOMeTi(OiPr)2]1
−d[LA]/dt = 0.40 × [LA]1[LOMeTi(OiPr)2]1
2.4. Mechanistic studies of polymerization
According to the kinetic characteristics, one monomer is con-
sumed in every polymerization cycle and the Ti complexes retain
the mononuclear form because the order of the monomer and Ti
complex is 1. Because the Ti complex is in a hexa-coordinated form,
it is incapable of bonding with a monomer. This may be explained
by the Ia mechanism, in which coordination of the monomer weak-
ens the N atom of the Salan ligand and N atom dissociates to form
a six-coordinate intermediate A (Fig. 8). After the initiation of the
monomer (by isopropoxide) to form the six-coordinate structure
B or C, structure C is more active than D if the pendent group R is
labile because R is easily replaced by the monomer, which increases
reactivity. This is the reason that the thiophen-2-yl methyl group
presented the highest catalytic activity. In addition, CL polymeriza-
tion presented a longer induction period than LA polymerization,
which indicates that LA more efficiently coordinates with Ti than
with CL, thereby promoting the formation of structure A.
˚
40 C). The chromatographic column was JORDI Gel DVB 103 A, and
the calibration curve was made by primary polystyrene standards
to calculate Mn(GPC). 2,4-Di-tert-butyl-6-(chloromethyl)phenol
[9], LBn–H2 [4d], and LBn2Ti(OiPr)2 [4d] were prepared by acid-
catalyzed condensation following literature procedures.
3.2. Synthesis of N,Nꢀ-bis(2-methoxybenzyl)-N,Nꢀ-bis[(3,5-di-
tert-butyl-2-hydroxyphenyl)methylene]-1,2-diaminoethane
(LOMe–H2)
A
mixture of ethylenediamine (6.01 g, 100 mmol) and 2-
methoxybenzaldehyde (27.2 g, 200 mmol) was refluxed for 1 day
in ethanol (150 mL). The reaction solution was cooled down in
ice bath and sodium borohydride (7.57 g, 200 mmol) was trans-
ferred to the solution slowly. After 1 h, the solution was refluxed
again for 1 day. Volatile materials were removed under vacuum
to yield yellow oil. The oil was dissolved in CH2Cl2 (200 mL) and
the solution was washed with water (2/200 ml) and the solvent
removed at reduced pressure to give the white powders. The white
powders were set with 2,4-di-tert-butyl-6-(chloromethyl)phenol
(53.55 g, 210 mmol) and NEt3 (28 mL, 200 mmol) in ethanol 400 mL
and refluxed for 1 month. Volatile materials were removed under
vacuum to yield yellow oil. The oil was dissolved in CH2Cl2 (200 mL)
and the solution was washed with water (2/200 ml) and sev-
eral drops of HCl. The yellow oil was obtained when CH2Cl2 was
3. Conclusions
A series of Salan ligands was synthesized and associated tita-
nium complexes in catalyzing the polymerization of CL and LA were
studied. The polymerization rate of CL and LA was altered according
to pendent group. Among these Ti complexes, only thiophen-2-yl
methyl group enhanced the polymerization rate of CL and LA but
was not helpful for decreasing the induction period. The underly-
ing mechanism may be the coordination of Ti by the monomer to

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H.-W. Ou et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 97–104
R
O
N
R
O
Ti
O
O
Ti
O
O
iPrO
O
N
N
N
R
OiPr
R
iPrO
OiPr
O
A
O
*
*
*
S
O
R =
R = Bn
tBu
tBu
OH
R
HO
R
tBu
tBu
R
F
*
N
O
Ti
N
N
N
O
Ti
O
N
R
R
N
O
O
R
OiPr
OiPr
O
O
O
O
O
B
C
HO
N
R
O
O
low activity
high activity if R = Th
O
O
N
OH
R
R
O
N
O
O
N
R
OiPr
O
Ti
O
O
O
D
+ H+
O
2n-2
O
O
O
n
2
H
O
Fig. 8. Possible mechanisms underlying of the polymerization by Salan Ti complex.
removed and ethanol (250 mL) was added to dissolve the oil. The
white powder was obtained and filtered after 10 day at −20 ◦C.
Yield: 49.38 g (67%). 1H NMR (CDCl3, 200 MHz): ı 10.76 (2H, s,
ArOH), 7.20–6.77 (12H, m, ArH), 3.73 (6H, s, BnOCH3), 3.64 (4H,
s, NCH2 BnOCH3), 3.56 (4H, s, NCH2Ar), 2.67 (4H, s, NCH2CH2N),
1.36 (18H, s, ArC(CH3)3), 1.24 (18H, s, ArC(CH3)3). 13C NMR
(CDCl3, 50 MHz): ı 158.00, 154.18, 140.12, 135.31, 131.37, 128.84,
125.11, 123.41, 122.60, 121.42, 120.33, 110.31 (Ar), 58.73 (NCH2
BnOCH3), 54.97 (BnOCH3), 53.29 (NCH2Ar), 50.20 (NCH2CH2N),
34.79 (ArC(CH3)3), 34.06 (ArC(CH3)3), 31.67 (ArC(CH3)3),29.53
(ArC(CH3)3). Elemental Analysis (C48H68N2O4) Found: N, 3.48%; C,
78.44%; H, 9.22%. Anal. Calcd: N, 3.80%; C, 78.22%; H, 9.30%. ESI-
MS(+) m/z calcd = 737.06. Found: 737.34.
analysis (C46H62F2N2O2) found: N, 3.31%; C, 77.29%; H, 8.69%. Anal.
Calcd: N, 3.93%; C, 77.49%; H, 8.76%. ESI-MS(+) m/z calcd = 712.99.
Found: 713.43.
3.4. Synthesis of N,Nꢀ-bis(furan-2-ylmethyl)-N,Nꢀ-bis[(3,5-di-
tert-butyl-2-hydroxyphenyl)methylene]-1,2-diaminoethane
(LFu–H2)
Using a method is similar to that for LOMe–H2. Yield: 44.01 g
(67%). 1H NMR (CDCl3, 200 MHz): ı 10.30 (2H, s, ArOH), 7.34 ∼ 7.21
(4H, m, ArH), 6.82 (2H, d, J = 1.0 Hz, FuranH), 6.28 (2H, dd, J = 1.6,
1.0 Hz, FuranH), 6.08 (2H, d, J = 1.6 Hz, FuranH), 3.72 (4H, s,
NCH2Furan), 3.67 (4H, s, NCH2Ar), 2.72 (4H, s, NCH2CH2N), 1.40
(18H, s, ArC(CH3)3), 1.28 (18H, s, ArC(CH3)3). 13C NMR (CDCl3,
50 MHz): ı 154.14, 150.21, 142.45, 140.60, 135.68, 123.79, 123.00,
120.90, 110.21, 109.84 (Ar), 58.66 (NCH2Furan), 50.04 (NCH2Ar),
48.48 (NCH2CH2N), 34.84 (ArC(CH3)3), 34.12 (ArC(CH3)3), 31.69
(ArC(CH3)3), 29.57 (ArC(CH3)3). Elemental analysis (C42H60N2O4)
found: N, 4.35%; C, 76.74%; H, 9.44%. Anal. Calcd: N, 4.26%; C, 76.79%;
H, 9.21%. ESI-MS(+) m/z calcd = 656.94. Found: 657.23.
3.3. Synthesis of N,Nꢀ-bis(2-fluorobenzyl)-N,Nꢀ-bis[(3,5-di-tert-
butyl-2-hydroxyphenyl)methylene]-1,2-diaminoethane
(LF–H2)
Using a method is similar to that for LOMe–H2. Yield: 59.17 g
(83%). 1H NMR (CDCl3, 200 MHz): ı 10.27 (2H, s, ArOH), 7.22–6.80
(12H, m, ArH), 3.67 (4H, s, NCH2BnF), 3.59 (4H, s, NCH2Ar),
2.71 (4H, s, NCH2CH2N), 1.39 (18H, s, ArC(CH3)3), 1.26 (18H, s,
ArC(CH3)3). 13C NMR (CDCl3, 50 MHz): ı 162.64 (d, JCF = 205.2 Hz),
160.19, 153.82, 140.65, 135.64, 131.94, 131.90, 129.46, 129.38,
124.18, 124.15, 123.68, 123.01, 120.94, 115.55, 115.32 (Ar), 58.93
(NCH2BnF), 50.85 (NCH2Ar), 50.25 (NCH2CH2N), 34.83 (ArC(CH3)3),
34.11 (ArC(CH3)3), 31.67 (ArC(CH3)3), 29.55 (ArC(CH3)3). Elemental
3.5. N,Nꢀ-bis(thiophen-2-ylmethyl)-N,Nꢀ-bis[(3,5-di-tert-butyl-2-
hydroxyphenyl)methylene]-1,2-diaminoethane
(LTh–H2)
Using a method is similar to that for LOMe–H2. Yield: 48.92 g
(71%). 1H NMR (CDCl3, 200 MHz): ı 10.19 (2H, s, ArOH), 7.22–6.90

H.-W. Ou et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 97–104
103
(6H, m, ArH, ThioH), 6.83 (2H, d, J = 1 Hz, ThioH), 6.77 (2H, d,
J = 1.8 Hz, ThioH), 3.80 (4H, s, NCH2Thio), 3.73 (4H, s, NCH2Ar),
2.75 (4H, s, NCH2CH2N), 1.42 (18H, s, ArC(CH3)3), 1.28 (18H,
(ArC(CH3)3), 30.43 (ArC(CH3)3), 26.62 and 26.19 (OCH(CH3)2). Anal.
Calcd (found) for (C48H72N2O4S2Ti) Found: N, 3.13%; C, 67.60%; H,
8.29%. Anal. Calcd: N, 3.28%; C, 67.58%; H, 8.51%. Mp: 244 ◦C.
1
s, ArC(CH3)3). 13C{ H} NMR (CDCl3, 50 MHz): ı 153.92, 140.75,
3.9. Synthesis of LFuTi(OiPr)2
138.07, 135.78, 127.77, 126.77, 125.69, 123.85, 123.11, 120.75
(Ar), 58.55 (NCH2Thio), 51.02 (NCH2Ar), 49.85 (NCH2CH2N),
34.86 (ArC(CH3)3), 34.13 (ArC(CH3)3), 31.67 (ArC(CH3)3), 29.53
(ArC(CH3)3). Elemental analysis (C42H60N2O2S2) found: N, 3.62%;
C, 73.22%; H, 8.98%. Anal. Calcd: N, 4.07%; C, 73.21%; H, 8.87%. ESI-
MS(+) m/z calcd = 689.07. Found: 689.29.
Using
a
method similar to that for LOMeTi(OiPr)2. Yield:
5.17 g (63%). 1H NMR (CDCl3, 200 MHz): ı 7.40 (2H, dd, J = 1.6,
0.8 Hz, ArH), 7.20 (2H, d, J = 2.4 Hz, ArH), 6.72 (2H, d, J = 2 Hz,
FuranH), 6.34 (2H, dd, J = 3.2, 1.8 Hz, FuranH), 6.13 (2H, d, J = 2.8 Hz,
FuranH), 4.86 (2H, m, J = 6 Hz, OCH(CH3)2), 4.21 (2H, d, J = 15.4 Hz,
NCH2Furan), 4.31 (2H, d, J = 13.6 Hz, NCH2Ar), 3.42 (2H, d, J = 13.6 Hz,
NCH2Ar), 2.89 (2H, d, J = 10.2 Hz, NCH2CH2N), 2.65 (2H, d, J = 10.2 Hz,
NCH2CH2N), 1.51 (18H, s, ArC(CH3)3), 1.27 (18H, s, ArC(CH3)3),
1.14 (12H, dd, J = 59.2, 6 Hz, OCH(CH3)2). 13C NMR (CDCl3, 50 MHz):
ı 159.47, 149.63, 143.00, 138.67, 135.52, 124.33, 123.24, 123.17,
112.14, 110.24 (Ar,Furan), 77.60 (OCH(CH3)2), 60.27 (NCH2 Furan),
51.23 (NCH2Ar), 47.00 (NCH2CH2N), 35.09 (ArC(CH3)3), 34.11
(ArC(CH3)3), 31.79 (ArC(CH3)3), 30.36 (ArC(CH3)3), 26.64 and 26.19
(OCH(CH3)2). Anal. Calcd (found) for (C48H72N2O6Ti) Found: N,
3.59%; C, 70.44%; H, 9.31%. Anal. Calcd: N, 3.41%; C, 70.22%; H, 8.84%.
Mp: 230 ◦C.
3.6. Synthesis of LOMeTi(OiPr)2
A mixture of LOMe–H2 (7.37 g, 10 mmol) and Ti(OiPr)4 (2.84 g,
10 mmol) in THF (50 mL) was stirred for 24 h. Volatile materi-
als were removed under vacuum to give yellow powder and
then it was washed with hexane (30 mL) and a yellow powder
was obtained. Yield: 6.57 g (73%). 1H NMR (CDCl3, 400 MHz): ı
7.32–6.64 (H, m, ArH), 4.97 (2H, m, J = 6 Hz, OCH(CH3)2), 4.27 (2H,
d, J = 20.5, 14.6 Hz, NCH2PhOCH3), 4.20 (2H, d, J = 20.5, 14.6 Hz,
NCH2PhOCH3), 4.41 (2H, d, J = 25.6, 13 Hz, NCH2Ar), 3.31 (2H, d,
J = 25.6, 13 Hz, NCH2Ar), 3.43(6H, s, NCH2PhOCH3), 3.05 (2H, d,
J = 19.6 Hz, NCH2CH2N), 2.55 (2H, d, J = 19.6 Hz, NCH2CH2N), 1.49
(18H, s, ArC(CH3)3), 1.23 (18H, s, ArC(CH3)3), 1.23 (12H, dd, J = 54.4,
6 Hz, OCH(CH3)2). 13C NMR (CDCl3, 0 MHz): ı 159.46, 159.39,
138.15, 135.55, 134.82, 129.31, 124.20, 122.98, 122.30, 120.07,
111.44 (Ar), 77.17 (OCH(CH3)2), 59.90 (NCH2 BnOCH3), 55.36
(BnOCH3), 52.92 (NCH2Ar), 45.93 (NCH2CH2N), 35.07 (ArC(CH3)3),
34.09 (ArC(CH3)3), 31.83 (ArC(CH3)3), 30.36 (ArC(CH3)3), 26.74 and
26.28 (OCH(CH3)2). Elemental analysis (C54H80N2O6Ti) Found: N,
2.90%; C, 71.32%; H, 9.08%. Anal. Calcd: N, 3.11%; C, 71.98%; H, 8.95%.
Mp:258 ◦C.
3.10. General procedures for the polymerization
A typical polymerization procedure was exemplified by the syn-
thesis of entry 7 (Table 1) using complex LOMeTi(OiPr)2 as a catalyst.
The polymerization conversion was analyzed by 1H NMR spectro-
scopic studies. Toluene (5.0 mL) was added to a mixture of complex
LOMeTi(OiPr)2 (0.1 mmol) and ␧-caprolactone (1.14 g, 10 mmol) at
120 ◦C. After the solution was stirred for 80 h, the reaction was then
quenched by adding to a drop of ethanol, and the polymer was pre-
cipitated pouring into n-hexane (30.0 mL) to give white solids. The
white solid was dissolved in CH2Cl2 (5.0 mL) and then n-hexane
(70.0 mL) was added to give white crystalline solid. Yield: 0.88 g
(77%).
3.7. Synthesis of LFTi(OiPr)2
Using a method is similar to that for LOMeTi(OiPr)2. Yield:
5.96 g (68%). 1H NMR (CDCl3, 200 MHz): ı 7.32–6.63 (12H, m,
ArH), 4.94 (2H, m, J = 6 Hz, OCH(CH3)2), 4.37 (2H, d, J = 15.6 Hz,
NCH2BnF), 4.29 (2H, d, J = 15.6 Hz, NCH2BnF), 4.38 (2H, d, J = 13 Hz,
NCH2Ar), 3.25 (2H, d, J = 13 Hz, NCH2Ar), 2.99 (2H, d, J = 20 Hz,
NCH2CH2N), 2.29 (2H, d, J = 20 Hz, NCH2CH2N), 1.50 (18H, s,
ArC(CH3)3), 1.24 (18H, s, ArC(CH3)3), 1.22 (12H, dd, J = 58, 6 Hz,
OCH(CH3)2). 13C NMR (CDCl3, 50 MHz): ı 162.28 (d, JCF = 245.8 Hz),
161.05, 159.33, 138.69, 135.52, 134.88, 134.83, 130.16, 130.07,
124.28, 123.71, 123.67, 123.30, 123.27, 120.35, 120.20, 115.93,
115.69 (Ar), 77.60 (OCH(CH3)2), 59.92 (NCH2 BnF), 52.49 (NCH2Ar),
46.14 (NCH2CH2N), 35.10 (ArC(CH3)3), 34.12 (ArC(CH3)3), 31.78
(ArC(CH3)3), 30.38 (ArC(CH3)3), 26.72 and 26.24 (OCH(CH3)2). Ele-
mental analysis (C52H74 F2N2O4Ti) found: N, 3.19%; C, 71.38%; H,
8.74%. Anal. Calcd: N, 3.19%; C, 71.21%; H, 8.50%. Mp: 266 ◦C.
Acknowledgments
Financial support from the Ministry of Science and Technology
(Grant NSC 101-2113-M-037-009) and the Fundamental Research
Funds from Kaohsiung Medical University (KMU-Q100011). We
thank Center for Research Resources and Development at Kao-
hsiung Medical University for the instrumentation and equipment
support.
Appendix A. Supplementary data
Supplementary material related to this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.molcata.
2014.07.003.
3.8. Synthesis of LThTi(OiPr)2
Using a method similar to that for LOMeTi(OiPr)2. Yield: 6.05 g
(71%). 1H NMR (CDCl3, 200 MHz): ı 7.30 (2H, dd, J = 5.2, 1.2 Hz,
ThioH), 7.22 (2H, d, J = 2.4 Hz, ArH), 7.00 (2H, d, J = 5.4 Hz, d,
J = 3.4 Hz, ThioH), 6.76 (2H, d, J = 3.4 Hz, d, J = 1 Hz, ThioH), 6.74
(2H, d, J = 2.4 Hz, ArH), 4.90 (2H, m, OCH(CH3)2), 4.48 (2H, d,
J = 15.6 Hz, NCH2Thio), 4.44 (2H, d, J = 15.6 Hz, NCH2Thio), 4.20 (2H,
d, J = 13.2 Hz, NCH2Ar), 3.56 (2H, d, J = 13.2 Hz, NCH2Ar), 2.76 (2H,
d, J = 10 Hz, NCH2CH2N), 2.53 (2H, d, J = 10 Hz, NCH2CH2N), 1.51
(18H, s, ArC(CH3)3), 1.27 (18H, s, ArC(CH3)3), 1.15 (12H, dd, J = 58,
6 Hz, OCH(CH3)2). 13C NMR (CDCl3, 50 MHz): ı 159.88, 138.91,
135.59, 134.80, 130.41, 126.91, 126.48, 124.35, 123.49, 123.47
(Ar,Thio), 77.86 (OCH(CH3)2), 59.59 (NCH2 Thio), 53.58 (NCH2Ar),
47.20 (NCH2CH2N), 35.13 (ArC(CH3)3), 34.14 (ArC(CH3)3), 31.82
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