Ring-opening polymerization of l-lactide catalyzed by calcium complexes

Article abstract of DOI:10.1039/c2dt32487c

A series of calcium complexes supported by NNO-tridentate ketiminate ligands have been synthesized and characterized. X-ray structural studies indicate that these complexes bear a mononuclear feature with hexa-coordinated calcium centers bonding to two ketiminate ligands. Polymerization of l-lactide catalyzed by these complexes proceeds rapidly and well controlled in the presence of a variety of alcohols. In addition, these complexes reveal good catalytic activity even with the addition of up to 80 equiv. of benzyl alcohol as a transfer agent. Furthermore, kinetic studies show ROP of l-lactide is a first-order dependency on [LA] and a second-order dependency on [BnOH]. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt32487c

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Ring-Opening Polymerization of L-Lactide Catalyzed by Calcium Complexes
MonꢀWei Hsiao, ChuꢀChieh Lin*
Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, R.O.C.
eꢀmail: cchlin@mail.nchu.edu.tw
Abstract
A series of calcium complexes supported by NNOꢀtridentate ketiminate ligands have been
synthesized and characterized. Xꢀray structural studies indicate that these complexes bear a
mononuclear feature with hexaꢀcoordinated calcium centers bonding to two ketiminate ligands.
Polymerization of Lꢀlactide catalyzed by these complexes proceeds rapidly and well controlled in
the presence of a variety of alcohols. In addition, these complexes reveal good immortal feature
with the addition of up to 80 equiv of benzyl alcohol as a transfer agent. Furthermore, kinetic
studies show ROP of
[BnOH].
Lꢀlactide is a firstꢀorder dependency on [LA] and secondꢀorder dependency on
Introduction
Because of their medical and environmental applications, such as tissue engineering, drug
delivery systems, and packing, biodegradable and biocompatible aliphatic polyesters have drawn
intensive attention.1 The major method to produce wellꢀdefined polyesters is ringꢀopening
polymerization (ROP) of cyclic esters via an effective initiating system. Over the past two decades,
the polymerization of cyclic esters initiated/catalyzed by various metal complexes has been
investigated.2 Since complete removal of catalyst residues from the polymer is often impossible, it
is highly preferable to use nonꢀ or low toxic metals for biomedical purpose. Among all nonꢀtoxic
metals, calcium is one of the most suitable metals for this intention.3 In order to obtain
wellꢀcontrolled polymers, a wide range of ligands have been designed to prevent side reactions
during polymerization.4 Among these various ligands systems, ketiminate ligands are easily
modified and reveal good control on polymerization.5 Recently, a series of magnesium and zinc

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alkoxides coordinated with NNOꢀtridentate ketiminate ligands have shown high activities toward
ROP of lactides.5a, 5d In extend this research we describe herein the preparation and catalytic activity
of a series of calcium complexes supported by NNOꢀtridentate ketiminate ligands.
Results and Discussion
Synthesis and characterization of calcium complexes. A series of NNOꢀtridentate ketiminate
ligands were prepared by the condensation reaction of N,Nꢀdimethylenediamine with the
corresponding diketone precursors in refluxing ethanol according to literature procedures.5d, 6
Followed by the reaction of Ca{N(SiMe3)2}2(THF)2 with 2 molar equiv of ketiminate ligands in
toluene at room temperature, calcium complexes 1ꢀ9 can be obtained in moderate to high yield
(Scheme 1). Furthermore, [L1Ca(
Lꢀlactate)]2 (1a) was obtained by the reaction of 1 with
Lꢀlactate as
1
shown in Scheme 2. All of these ligands and their calcium complexes were determined by H and
13C spectra, and elemental analysis. Fully assignment of complex 8, which is further confirmed by
the HMBC, HSQC, NOE and TOCSY NMR spectroscopic studies, is given in Table 1 (Figure
S1ꢀS3). For instance, the resonance at 2.26 ppm is assigned to H6 (Chart 1) and resonances at 6.93,
7.21, and 8.53 ppm are assigned to the carbons in the same phenyl ring based on TOCSY spectrum.
Furthermore, it clearly indicates that two set of resonances at 3.00ꢀ2.96 and 2.82ꢀ2.78 ppm are
referred to the methylene protons at H5 according to HSQC spectrum which was further verified by
HMBC and NOESYꢀ1D spectrums. It is interested to note that the methyl group on the pyrrole ring
(H13, Chart 1) shows a resonance at 1.52 ppm that is ca. 0.7 ppm upfield than the expected value.
The unexpected result is speculated that the methyl group (C13) locates above the C14ꢀC19 phenyl
ring which is further verified by the Xꢀray diffraction structural studies of complex 2.

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R
R
N
N
NH
N
N
toluene
O
2
Ca
N
N
[Ca(SiNMe2)2(THF)2]
+
N
O
O
N
N
r.t., overnight
N
N
R
1, R=H
5, R=pꢀCl
6, R=pꢀF
7, R=oꢀF
8, R=oꢀMe
9, R=oꢀCF3
2, R=pꢀPh
3, R=pꢀMe
4, R=pꢀOMe
Scheme 1. Preparation of calcium complexes 1-9.
Scheme 2. Preparation of complex 1a from the reaction of 1 with methylꢀLꢀlactate

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Table 1. 1H and 13C NMR spectroscopic data of complex 8.a
17
18
19
16
15
21
5
6
14
4
13
N
N
3 2
20
N
O
1
Ca
N
N
N
O
N
7
N
8
9
12
11
10
Position
1H (ppm)
13C (ppm)
163.3
148.2
101.7
172.4
48.5
Position
9, 11
10
1H (ppm)
7.21 (t)
6.93 (t)
1.52 (s)
ꢀ
13C (ppm)
128.6
1
ꢀ
ꢀ
ꢀ
ꢀ
2
123.5
3
13
16.0
4
5
14,19
15
140.0, 134.1
127.1
3.00ꢀ2.96 (m),
2.82ꢀ2.78 (m)
6.79 (d)
7.09ꢀ7.03 (m)
1.91 (s)
2.11 (s)
6
2.26 (br)
ꢀ
59.8
16ꢀ18
20
130.6, 128.5, 126.7
45.0
7
141.7
120.3
8, 12
8.53 (d)
21
19.2
;a Observed in tolueneꢀd8, 600 MHz, 60 oC.
Variable 1H NMR studies of complex 8. It is worth noting that 1H NMR spectrum of complex 8 at
o
20 C shows two singlet resonances at δ 1.554 and 1.548 ppm (Figure 1a) for the methyl group on
the pyrrole ring. It is believed that the existence of two conformations in solution: synꢀ and
1
antiꢀforms (Figure 2). In order to confirm this speculation, a variable temperature H NMR
experiment was performed in tolueneꢀd8 for complex 8. It was found that two resonances coalesce
o
into a single peak at 47 C (Figure 1a), and the energy barrier for this transformation is calculated
by Eyring’s equation, ꢀG‡ = 16.77 kcal molꢀ1.
In addition, the hydrogen atoms at ethylene group, Ha and Hb (Figure 3) also reveal fluxional
o
o
feature. These two hydrogen atoms show a broad resonance at 60 C, but it coalescences at ~20 C
(Figure 2b) indicating that the nitrogen atom of the N,Nꢀdimethyl amino group undergoes a

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coordination/dissociation process.
Figure 1. Variable temperature 1H NMR studies of complex 8 in tolueneꢀd8. (a) He region (b) Ha
and Hb regions.
Figure 2. Conformation change of complex 8.

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Crystal structure studies. Single crystals suitable for Xꢀray studies of 2 were obtained from slowly
cooling CH2Cl2/hexane solution and the molecular structure of complex 2 is shown in Figure 3. The
solid state structure of complex 2 bears a mononuclear feature with a hexaꢀcoordinated calcium
center bonding to four N and two O atoms of two NNOꢀtridentate ketiminate ligands. The geometry
around the calcium center is a distorted trigonal antiꢀprismatic with the calcium atom is sitting on a
C2ꢀsymmetry. Ca, O(1), C(1), C(2), C(4) and N(3) atoms are almost coplanar with the standard
deviation of 0.0587 À. The dihedral angle between plane CaꢀO(1)ꢀC(1)ꢀC(2)ꢀC(4)ꢀN(3) and
CaꢀO(1a)ꢀC(1a)ꢀC(2a)ꢀC(4a)ꢀN(3a) is ca. 66.5o. The bond distance of CaꢀO(1) (2.2460(18)
Å) is
slightly shorter than that found in the Schiff base calcium complex (the average CaꢀO bond distance
.
is 2.364 Å for dinuclear species, [(DAIP)2Ca]2 THF)3a, but is somewhat longer than that in
[(tBuDAIP)CaN(SiMe3)2].THF (2.1914(16) Å)3b. Furthermore, the distance between calcium and
the imine nitrogen atom, CaꢀN(3) distance is 2.466(2) Å which is shorter than that in
.
[(DAIP)2Ca]2 THF (2.752(5)) and similar to that in [(tBuDAIP)CaN(SiMe3)2].THF (2.474(2)).
.
However, the distance between CaꢀN(4) (2.581(2) Å) shows close to those in [(DAIP)2Ca]2 THF
(2.550(5) Å
) and in [(tBuDAIP)CaN(SiMe3)2].THF (2.600(2) Å).
Single crystals of 1a suitable for Xꢀray diffraction were obtained from toluene solution at ꢀ18
oC and the molecular structure is shown in Figure 4. The solid state structure of complex 1a reveals
a dinuclear feature bridging through the alkoxy oxygen atom of Lꢀlactate with a six coordinated
calcium center. The structure of 1a suggests that in the presence of ROH and lactide, complex 1 a
ligand will dissociate during addition of alcohol, forming a metal alkoxide and another alcohol
group and carbonyl group of
Scheme 3).
Lꢀlactate will coordinate to the calcium center (intermediate C,

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Figure 3. ORTEP drawing of complex 2 with thermal ellipsoids drawn at the 50% probability level.
Hydrogen atoms were omitted for clarity. Selected bond lengths [Å] and bond angles (o): CaꢀO(1)
2.2460(18), CaꢀO(1A) 2.2460(18), CaꢀN(3) 2.466(2), CaꢀN(3A) 2.466(2), CaꢀN(4) 2.581(2), CaꢀN(4A)
2.581(2), O(1A)ꢀCaꢀO(1) 90.07(9), O(1A)ꢀCaꢀN(3A) 78.67(6), O(1)ꢀCaꢀN(3A) 127.33(7), O(1A)ꢀCaꢀN(3)
127.33(7), O(1)ꢀCaꢀN(3) 78.67(6), N(3A)ꢀCaꢀN(3) 146.27(10), O(1A)ꢀCaꢀN(4) 92.25(7), O(1)ꢀCaꢀN(4)
144.56(6), N(3A)ꢀCaꢀN(4) 87.68(7), N(3)ꢀCaꢀN(4) 71.93(7), O(1A)ꢀCaꢀN(4A) 144.56(6), O(1)ꢀCaꢀN(4A)
92.25(7), N(3A)ꢀCaꢀN(4A) 71.93(7), N(3)ꢀCaꢀN(4A) 87.68(7), N(4)ꢀCaꢀN(4A) 105.65(10).

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Figure 4. ORTEP drawing of complex 1a with thermal ellipsoids drawn at the 50% probability level.
Hydrogen atoms were omitted for clarity. Selected bond lengths [Å] and bond angles (o): CaꢀO(1) 2.240(4),
CaꢀO(2) 2.267(4), CaꢀO(2A) 2.306(3), CaꢀO(3) 2.428(4), CaꢀN(3) 2.506(4), CaꢀN(4) 2.538(5), O(1)ꢀCaꢀO(2)
104.05(15), O(1)ꢀCaꢀO(2A) 110.41(13), O(2)ꢀCaꢀO(2A) 78.16(14), O(1)ꢀCaꢀO(3) 92.27(15), O(2)ꢀCaꢀO(3)
146.75(14), O(2A)ꢀCaꢀO(3) 68.97(14), O(1)ꢀCaꢀN(3) 78.61(13), O(2)ꢀCaꢀN(3) 105.68(14), O(2A)ꢀCaꢀN(3)
169.34(15), O(3)ꢀCaꢀN(3) 105.84(15), O(1)ꢀCaꢀN(4), 148.60(15), O(2)ꢀCaꢀN(4) 92.92(17), O(2A)ꢀCaꢀN(4)
98.71(15), O(3)ꢀCaꢀN(4) 87.29(16), N(3)ꢀCaꢀN(4) 71.37(15).
Ring-opening polymerization of
complexes 1 has been systematically studied as shown in Table 2. Experimental results reveal that
complex 1 has very poor activity toward ROP of ꢀlactide in the absence of alcohol (Table 2, entry
L-lactide. Ringꢀopening polymerization of Lꢀlactide catalyzed by
L
1), but it shows good catalytic activity in the presence of benzyl alcohol as an initiator. The
conversion can reach over 90% within 2 hours at 30 oC in the [M]0/[I]0 ratio ranging from 25 to 250
(entry 2ꢀ7). With the linear relationship between Mn versus [M]0/[I]0 and low polydispersity index
(PDI) indicate good control manner and “living” character for the polymerization process. However,

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the MALDIꢀTOF experiments reveal odd number of lactic acid molecular unit indicating that
transesterification probably occurs during polymerization. (Fig S3). 1H NMR spectrum of PLLAꢀ50
reveals that the polymer chain end capped with a benzyl ester and a hydroxyl group with the
integration ratio close to 5:1 between He and Hc (Figure 5). Based on these results, it is believed that
the polymerization proceeds with the “coordinationꢀinsertion” mechanism reported in literatures.2d, 7
For environmental and economic purpose, it is very important to use as small amount of
calcium complexes as possible. In order to examine the “immortal” character of these catalysts,
polymerization of
Lꢀlactide in the presence of excess benzyl alcohol was also performed.
Experimental results show that complex 1 demonstrates good control manner with low
polydispersity index in the addition of up to 80 equiv of benzyl alcohol as a chain transfer agent
(entry 7) and up to 90% conversion within 40 min. However, the conversion of polymerization
cannot over 80% even in the prolong of reaction time when [M]0/[Ca]0 ratio of 160 is used (entry
8).
Recently, block copolymers containing biodegradable copolymers and polyurethanes have
drawn attention for their potential utility as biomedical materials because they may offer good
elasticity and unique properties.8 Therefore, an ethylene glycol was introduced to act as an initiator
for furnishing the “diolizedꢀPLA” (Figure 6) which can further react with diisocyanates to produce
PUꢀPLAꢀPU, an ABBAꢀtype block copolymer. Experimental results demonstrate that
polymerization of
[M]0/[Ca]0/[ROH] ratio up to 520/1/40 (entry 9). Furthermore, it is worth noting that polymerization
of “unpurified” ꢀlactide at room temperature by complex 1 is also well controlled (entry 10).
Lꢀlactide initiated by ethylene glycol is also well controlled with
L

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Table 2. RingꢀOpening Polymerization of L
ꢀLactide Catalyzed by Complex 1a
Time
(min)
Conv.
(%)b
Entry
[LA]0/[1]0/[ROH]0
Mn (calcd.)c
Mn (obsd.)d
Mn (NMR)b PDI
1
2
3
4
5
6
7
8e
9e
10f
100/1/0
25/1/1
120
120
120
120
120
120
120
40
7
91
92
92
96
95
95
90
80
97
98
ꢀ
ꢀ
ꢀ
ꢀ
3400
6800
10100
13900
17300
34300
1400
1300
1900
1900
4900 (2800)
9700 (5600)
14800 (8600)
20200 (11700)
24400 (14200)
53200 (30900)
1500 (900)
3500 1.16
6300 1.15
9500 1.15
50/1/1
75/1/1
100/1/1
125/1/1
250/1/1
800/1/80
1600/1/160
520/1/40
104/1/8
13100 1.19
16600 1.19
33200 1.22
1300 1.13
1400 1.11
1400 1.21
1600 1.13
30
1700 (1000)
2800 (1600)
4000 (2300)
20
11g
20
a The polymerizations were performed on CH2Cl2, 30 oC, [1]0 = 5.0 mM, ROH = benzyl alcohol. b Obtained
c
d
from 1H NMR analysis. Calculated Mn = Mw(LA) x [LA]0/[ROH]0 x conversion + Mw(ROH). Obtained
from GPC analysis and calibrated by polystyrene standard. Values in parentheses are the values obtained
e
f
g
from GPC times 0.58.8 [1]0 = 1.25 mM. ROH = ethylene glycol. ROH = ethylene glycol, using
unpurified ꢀlactide.
L

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Figure 5. 1H NMR spectrum of PLLAOBnꢀ50 (50 indicates [LA]/[BnOH] = 50).

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Figure 6. 1H NMR spectrum of PLLAꢀEGꢀPLLAꢀ13 (13 indicates [LA]/[ROH] = 13).

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Figure 7. 1H NMR spectrum of PLLAꢀ50 obtained from polymerization of
(50 indicates [LA]/2[1a] = 50).
Lꢀlactide initiated by 1a
Comparison of the activities of complexes 1-9. All of calcium complexes show good catalytic
activity in the presence of benzyl alcohol expect complex 9 (Table 3). Experimental results show
that the substituents (R) at 4ꢀposition of the phenyl group of the ketiminate ligand do not
significantly affect the reactivity of the complexes. For instance, complexes with electron donating
group (R = pꢀOMe, pꢀMe) has the similar reactivity as the complexes with e electron withdrawing
group (R = pꢀF, pꢀCl) (Table 3). This result shows electron effect does not drastically affect the
catalytic activity for
Lꢀlactide polymerization. However, the catalytic activity of the complex
increases significantly by adding steric hindrance group on the ortho position of the phenyl segment
(R = oꢀMe) (Table 3, entry 8). Nevertheless, exchanging the methyl group to a small fluoride
substituent at the same position (R = oꢀF), the catalytic activity decrease drastically (Table 3, entry

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7). It is believed that the steric bulky ligands will enhance the dissociation rate of ligand and
therefore increase the polymerization rate. Furthermore, changing the substituent from oꢀMe group
to a sterically bulkier oꢀCF3 group (complex 9) on the ortho position, the activity decrease
drastically indicating that sterically too bulky ligands may hinder the coordination of monomer to
calcium center and therefore retarding the reaction rate. Similar result has been observed in the
magnesium ketiminate system5a.
In order to understand the really active species during polymerization, 1a has been prepared
from the reaction of 1 and methylꢀLꢀlactate. The activity of 1a toward ROP of Lꢀlactide is also
performed (entry 10, Table 3). Experimental results show that the conversion can reach to 99%
within 2 h at room temperature and the molecular weight of PLLA is as expected. The conversion
o
experimental result at 0 C and the observed rate constant (kobs) indicate that the activity of 1a is
similar to that of 1/BnOH.
Table 3. RingꢀOpening Polymerization of
L
ꢀLactide Initiated by Complexes 1-9a
Conv.
(%)b (calcd.)c
Mn
Mn
kobs
Conv. (%)b, e
Entry Cat.
Mn (obsd.)d
PDI
( L molꢀ1 minꢀ1)b,f
(NMR)b
6300
6600
6700
6200
6200
6300
6400
6400
ꢀi
1
2
3
4
5
6
7
8
9
92
91
94
94
97
96
95
91
10
6800
6700
6900
6900
7000
7000
7000
6700
800
9700 (5600)
11200 (6500)
11000 (6400)
8800 (5100)
11500 (6600)
10000 (5800)
9100 (5300)
9800 (5700)
ꢀi
1.15
1.12
1.14
1.14
1.10
1.09
1.08
1.08
ꢀi
1
2
3
4
5
6
7
8
9
58
61
61
58
49
53
61
80
3
0.0374
0.0321
0.0667
0.0478
0.0393
0.0545
0.0169
0.1242
ꢀi

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0.0608h
10g
99
7200
14600(8500)
7500
1.43
1a
48
o
b
a Rxn Cond. [LA]0/[Cat.]0/[BnOH]0=50/1/1, CH2Cl2, 30 C, [Cat.]0 = 5.0 mM, 120 min. Obtained from 1H
c
NMR analysis. Calculated Mn = Mw(LA) x [LA]0/[ROH]0 x conversion + Mw(ROH). d Obtained from GPC
analysis and calibrated by polystyrene standard. Values in parentheses are the values obtained from GPC
e
times 0.589. 0 oC, 90 min. f Rxn Cond. [Cat.]0 = 0.0025M, [LA]0 = 0.375M, [BnOH]0 = 0.01 M, CDCl3, 20
g
h
oC. Rxn Cond. [LA]0/[1a]0=100/1, CH2Cl2, 30 oC, [1a]0 = 5.0 mM, 120 min. Rxn Cond. [1a]0 = 0.0025M,
[LA]0 = 0.375M, CDCl3, 20 oC. i Not detected.
Kinetic studies of ring-opening polymerization of L-lactide. The kinetic studies of complex 1
were performed in CDCl3 at room temperature. Conversion of Lꢀlactide with different concentration
1
of BnOH was monitored by H NMR with time. The plots of ln([LA]0/[LA]t) versus time were
linear indicating the polymerization rate is a first order dependency on monomer (Figure 8a)
. Thus,
the rate law of polymerization can be written as
ꢀ
d[LA]/dt = kobs[LA], where kobs = kapp[BnOH]x.
The reaction order of BnOH can be obtained by the plotting of lnkobs versus ln[BnOH]. The slop
reveals that the polymerization is a second order dependency on BnOH and the rate constant kapp
=
4.95 molꢀ2 L2 sꢀ1 based on the intercept of fitted line of Figure 8b. The kinetic study of complex 1a was
also performed in CDCl3 at room temperature and the polymerization was found to be first order to the
monomer with kobs = 0.0608 L molꢀ1 minꢀ1 (Figure 9) which is somewhat higher than that of complex 1 in
the presence of BnOH. This result is agree with our expectation that induction time is required for
polymerization of lactide initiated by 1/BnOH.

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(a)
(b)
Figure 8. (a) ln([LA]0/[LA]t) versus time plot for Lꢀlactide polymerizations catalyzed by complex 1
in CDCl3 at room temperature with different concentration of BnOH as an initiator. (b) Linear plot
of lnkobs versus ln[BnOH] with slope = 1.97 in [LA]0 = 0.375 M and [1]0 = 2.5 mM.
Figure 9. ln([LA]0/[LA]t) versus time plot for Lꢀlactide polymerizations initiated by complex 1a in
CDCl3 at room temperature.
Proposed mechanism for polymerization of
L-lactide. Reaction of complex 1 with BnOH
indicates that dissociation of LꢀH within 10 min with [L2Ca]/[LꢀH] ratio of 5:1 (Fig. S2). The
[L2Ca]/[LꢀH] ratio decreases with time and reach an equilibrium at ca. [L2Ca]/[LꢀH] = 2/1 after 8h.

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It suggested that some ligand will dissociative during polymerization. In addition, reaction of
ꢀlactate yields the modeling compound, [L1Ca(
Lꢀlactate)]2 (Fig 4). Based
complex 1 with methylꢀ
L
on the these information and kinetic studies of complex 1, it is believed that the polymerization
proceeds with the dissociation of a ligand in the presence of excess ROH giving a dinuclear
intermediate A bridging through the oxygen atom of alkoxide group (Scheme 3). Followed by the
coordination of lactide to intermediate A forming intermediate B and by the insertion of CaꢀOBn to
Lꢀlactide, intermediate C, which can verification by the modeling compound 1a, is obtained.
Scheme 3. Proposed mechanism for the ROP of
L
ꢀlactide catalyzed by L2Ca
O
N
O
O
R'
R'
R'
O
N
N
N
N
O
N
O
O
R
O
R
O
N
O
O
2
Ca
N
Ca
ROH
Ca
ROH
N
N
ROH
N
ROH
4 ROH
-2LH
O
O
O
N
N
N
N
N
O
O
O
N
N
N
O
R
Ca
Ca
N
N
R
R'
Coordination
N
N
N
R'
R'
B
A
Insertion
RO
O
RO
RO
O
O
N
N
O
R'
O
R'
O
R'
O
N
O
N
ROH
Ca
O
N
N
OH
R
ROH
Ca
O
N
O
N
ROH
Ca
O
ROH
O
N
O
N
O
N
N
N
N
O
N
O
N
O
O
R
Ca
N
O
Ca
N
O
Ca
Alcohol exchange
R
N
N
N
N
N
N
R'
R'
R'
E
D
C
O
O
O
O
RO
O
N
O
R'
O
O
H+
N
RO
O
OH
R
O
H
ROH
Ca
Ca
O
N
n
O
O
N
N
O
N
O
R
O
O
N
N
O
R'
O
Conclusions
A series of calcium complexes supported by NNOꢀtridentate ketiminate ligands with great reactivity

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toward ROP of
Lꢀlactide have been prepared and fully characterized. Polymerization of Lꢀlactide
catalyzed by these complexes in the presence of BnOH/EG as transfer agents, proceeds rapidly with
up to 80 equiv of the alcohols. Moreover, these complexes are also extraordinarily active toward the
ROP of Lꢀlactide without any purification.
Experimental Section
General methods and materials. All manipulations were carried out under a dry nitrogen
atmosphere. Solvents were dried by refluxing at least 24 h over sodium/benzophenone (hexane,
toluene, tetrahydrofuran (THF)), phosphorus pentaoxide (CH2Cl2), or calcium hydride (benzyl
alcohol). LꢀLactide was purchased from PURAC and recrystallized from toluene twice and dried
under vacuum overnight prior to use. Deuterated solvents (Aldrich, Merck) were dried over
molecular sieves. Calcium iodide and Sodium bis(trimethylsilyl)amide were purchased from
Aldrich
and
stored
in
a
glove
box.
3ꢀmethylꢀ1ꢀphenylꢀ1Hꢀpyrazolꢀ5(4H)ꢀone,
N,Nꢀdimethylethaneꢀ1,2ꢀdiamine and other regents were purchased from Acros or Aldrich and used
without further purification. Ca{N(SiMe3)2}2(THF)2 was prepared according to the literature
procedures.9
Measurement. 1H and 13C NMR spectra were recorded on a Varian Unity Inovaꢀ600 (600 MHz for
13
1
1H and 150 MHz for C) or a Varian Mercuryꢀ400 (400 MHz for H, 100 MHz for 13C, 376 MHz
for 19F) spectrometer with chemical shifts given in ppm from the internal TMS, toluene,
triflourotoluene, or center line of CHCl3. The GPC measurements were performed on a Jasco
PUꢀ2080 Plus HPLC Pump system equipped with a Jasco RIꢀ2031 Plus refractive index detector
o
using THF (HPLC grade) as an eluent (flow rate 1.0 ml/min, at 40 C). The chromatographic
column was Jordi 15022 Column (250 mm) and the calibration curve was made by polystyrene
standards to calculate Mn(GPC). Matrixꢀassisted laser desorption ionizationꢀtimeꢀof flight mass

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spectrometry (MALDIꢀTOF MS) analyses were carried out with a Bruker Autoflex III TOF/TOF
equipped with a micro channel plate (MCP) detector. The sample was dissolved in THF, and the
matrix was 2ꢀ(4’ꢀhydroxybenzeneazo) benzoic acid (HABA). Ions formed by a pulsed UV nitrogen
laser beam with 3 ns bandwidth (nitrogen laser λ was 337 nm) were accelerated through 20 kV, and
the detection voltage was set at 1.7 kV.
General procedures for the preparation of LH are according to the method reported
previously.6
Synthesis of L2H. A mixture of 2ꢀdimethylaminoethylamine (1.09 mL, 10.0 mmol) and
3ꢀmethylꢀ1ꢀphenylꢀ4ꢀ[(4ꢀphenylphenyl)carbonyl]ꢀ4,5ꢀdihydroꢀ1Hꢀpyrazolꢀ5ꢀone (3.54 g, 10.0
mmol) were refluxed in ethanol (40 mL) for 24 h. Volatile materials were removed under vacuum
producing yellow powder. The powder was then redissolved in hot ethanol (30 mL) and cooled to
ꢀ18 oC. Yellow crystalline solid was obtained after 24 h. Yield: 3.73 g (88 %). 1H NMR (CDCl3, 400
MHz): δ 11.27 (1H, br, NH), 8.02 (2H, d, J = 8.0 Hz, ArH), 7.87 (2H, d, J = 8.0 Hz, ArH), 7.66 (2H,
d, J = 8.0 Hz, ArH), 7.50ꢀ7.11 (8H, m, ArH), 3.26 (2H, q, J = 6.0 Hz, NHCH2), 2.49 (2H, t, J = 6.4
Hz, NHCH2CH2), 2.21 (6H, s, N(CH3)2), 1.53 (3H, s, CH3C=N) ppm. 13C NMR (CDCl3, 100 MHz):
δ 165.7 (C=O), 165.3 (C=CꢀNH), 147.6 (N=CꢀCH3), 143.1, 139.5, 139.2, 130.3, 128.9, 128.6, 128.1,
127.9, 127.5, 127.0, 124.0, 119.1 (Ar), 99.7 (C=CꢀNH), 58.7 (NHCH2CH2), 45.4 (N(CH3)2), 42.4
(NHCH2), 15.5 (N=CꢀCH3) ppm. Anal. Calcd. (found) for C27H28N4O: N, 13.20 (13.09); C, 76.39
(75.66); H, 6.65 (6.73) %. Mass spectrum (EI, m/e): 424.4 (M+, 21.73 %).
Synthesis
of
L5H.
Use
the
same
procedures
for
L2H,
except
4ꢀ(4ꢀchlorobenzoyl)ꢀ3ꢀmethylꢀ1ꢀphenylꢀ1Hꢀpyrazolꢀ5(4H)ꢀone (3.13 g, 10.0 mmol) was used as the
reactant. Yield: 3.26 g (85 %). 1H NMR (CDCl3, 400 MHz): δ 11.24 (1H, br, NH), 7.99 (2H, d, J =
8.0 Hz, ArH), 7.56ꢀ7.53 (2H, m, ArH), 7.38 (2H, t, J = 8.0 Hz, ArH), 7.32ꢀ7.29 (2H, m, ArH), 7.14
(1H, t, J = 6.0 Hz, ArH), 3.19 (2H, d, J = 6.7 Hz, NHCH2), 2.47 (2H, d, J = 6.0 Hz, NHCH2CH2),

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2.20 (6H, s, N(CH3)2), 1.49 (3H, s, CH3C=N) ppm. 13C NMR (CDCl3, 100 MHZ)：δ 165.5 (C=O),
164.0 (C=CꢀNH), 147.2 (N=CꢀCH3), 139.1, 136.4, 129.8, 129.2, 128.9, 128.5, 124.0, 119.0 (Ar),
99.6 (C=CꢀNH), 58.5 (NHCH2CH2), 45.4 (N(CH3)2), 42.3 (NHCH2), 15.5 (N=CꢀCH3) ppm. Anal.
Calcd. (found) for C21H23ClN4O: N, 14.63 (14.39); C, 65.87 (65.41); H, 6.05 (6.56) %. Mass
spectrum (EI, m/e): 382.2 (M+, 3.43 %).
Synthesis
of
L7H.
Use
the
same
procedures
for
L2H,
except
4ꢀ(2ꢀfluorobenzoyl)ꢀ3ꢀmethylꢀ1ꢀphenylꢀ1Hꢀpyrazolꢀ5(4H)ꢀone (2.96 g, 10.0 mmol) was used as the
reactant. Yield: 2.74 g (74.8 %). 1H NMR (CDCl3, 400 MHZ): δ 11.28 (1H, br, NH), 7.99 (2H, d, J
= 7.6 Hz, ArH), 7.58ꢀ7.54 (1H, m, ArH), 7.40ꢀ7.25 (5H, m, ArH), 7.13 (1H, d, J = 7.6 Hz, ArH),
3.28 (2H, br, NHCH2), 2.54 (2H, br, NHCH2CH2), 2.25 (6H, s, N(CH3)2), 1.52 (3H, s, CH3C=N)
ppm. 13C NMR (CDCl3, 100 MHz): δ 165.6 (C=O), 159.8, 159.4, 157.3 (CꢀF), 147.1 (N=CꢀCH3),
139.0, 132.7, 132.6, 129.6, 128.5, 124.8, 124.7, 119.4, 119.2, 116.4, 116.2 (Ar), 100.0 (C=CꢀNH),
58.2 (NHCH2CH2), 45.3 (N(CH3)2), 42.4 (NHCH2), 14.8 (N=CꢀCH3) ppm, Anal. Calcd. (found) for
C21H23FN4O: N, 15.29 (15.41); C, 68.83 (67.90); H, 6.33 (5.99) %. Mass spectrum (EI, m/e): 366.2
(M+, 3.19 %).
Synthesis
of
L8H.
Use
the
same
procedures
for
L2H,
except
3ꢀmethylꢀ4ꢀ(2ꢀmethylbenzoyl)ꢀ1ꢀphenylꢀ1Hꢀpyrazolꢀ5(4H)ꢀone (2.92 g, 10.0 mmol) was used as
the reactant. Yield: 1.92 g (52.9 %). 1H NMR (CDCl3, 400 MHZ): δ 11.24 (1H, br, NH), 8.02 (2H, d,
J = 7.6 Hz, ArH), 7.46ꢀ7.33 (5H, m, ArH), 7.21 (1H, d, J = 8.0 Hz, ArH), 7.15 (1H, t, J = 6.4 Hz,
ArH), 3.17 (2H, q, J = 6.0 Hz, NHCH2), 2.48 (2H, t, J = 6.4 Hz, NCH2CH2) ,2.28 (3H, s , PhCH3),
2.21 (6H, s, N(CH3)2), 1.40 (3H, s, CH3C=N) ppm. 13C NMR (CDCl3, 100 MHz): δ 165.7 (C=O),
165.1 (C=CꢀNH), 147.6 (N=CꢀCH3), 139.1, 134.9, 131.1, 130.6, 130.2, 128.5, 127.1, 126.4, 124.0,
119.1 (Ar), 99.1 (C=CꢀNH), 58.4 (NHCH2CH2), 45.3 (N(CH3)2), 42.0 (NHCH2), 19.0 (PhCH3),
14.6 (N=CꢀCH3) ppm. Anal. Calcd. (found) for C22H26N4O: N, 15.46 (15.11); C, 72.90 (70.94); H,

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7.23 (6.91) %. Mass spectrum (EI, m/e): 362.2 (M+, 3.92 %).
Synthesis
of
L9H.
Use
the
same
procedures
for
L2H,
except
3ꢀmethylꢀ1ꢀphenylꢀ4ꢀ(2ꢀ(trifluoromethyl)benzoyl)ꢀ1Hꢀpyrazolꢀ5(4H)ꢀone (3.46 g, 10.0 mmol) was
used as the reactant. Yield: 3.33 g (80 %). 1H NMR (CDCl3, 400 MHZ): δ 7.89ꢀ7.87 (2H, m, ArH),
7.70 (1H, d, J = 7.2 Hz, ArH), 7.53ꢀ7.48 (2H, m, ArH), 7.36 (1H, d, J = 7.2 Hz, ArH), 7.37ꢀ7.27
(2H, m, ArH), 7.07 (1H, t, J = 7.6 Hz, ArH), 2.56 (2H, t, J = 6.4 Hz, NHCH2), 2.48 (2H, t, J = 5.6
Hz, NCH2CH2) ,2.05 (6H, s, N(CH3)2), 1.51 (3H, s, CH3C=N) ppm. 13C NMR (CDCl3, 100 MHz): δ
187.3 (C=O), 164.7 (C=CꢀNH), 149.3 (N=CꢀCH3), 141.5, 139.4, 131.9, 128.6, 128.5, 128.1, 126.5,
126.3 (PhꢀCF3), 126.2, 125.4, 124.4, 122.7, 120.7 (Ar), 104.9 (C=CꢀNH), 55.9 (NHCH2CH2), 44.6
(N(CH3)2), 36.3 (NHCH2), 15.9 (N=CꢀCH3) ppm. 19F NMR (CDCl3, 376 MHz) : δ ꢀ59.94 ppm.
Mass spectrum (EI, m/e): 416.2 (M+, 0.22 %).
General procedures for the preparation of calcium complexes 1-9. A mixture of
Ca{N(SiMe3)2}2(THF)2 (0.51 g, 1.0 mmol) and the corresponding NNOꢀtridentate liagnd (2.0 mmol)
was stirred in toluene (20 ml) at room temperature for 16 h. During which pale yellow powder
precipitated and the mixture was filtered. The resulting solid was dried under vacuum.
Complex 1. Yield: 0.45 g (61.5 %). 1H NMR (CDCl3, 400 MHZ): δ 8.10 (2H, d, J = 8.0 Hz, ArH),
7.50ꢀ7.42 (3H, m, ArH), 7.34 (2H, t, J = 8.0 Hz, ArH), 7.19 (2H, d, J = 8.0 Hz, ArH), 7.09 (1H, t, J
= 8.0 Hz, ArH), 3.17 (2H, t, J = 6.0 Hz, CꢀNCH2), 2.60 (2H, br, CꢀNCH2CH2), 2.27 (6H, s,
N(CH3)2), 1.35 (3H, s, CH3C=N) ppm. 13C NMR(CDCl3, 100 MHz): δ 171.8 (C=O), 161.9
(C=CꢀN), 149.1 (N=CꢀCH3), 140.2, 139.2, 128.9, 128.3, 128.2, 126.6, 123.5, 119.8 (Ar), 101.8
(C=CꢀN), 59.5 (CꢀNCH2CH2), 48.5 (N(CH3)2), 45.2 (CꢀNCH2), 16.1 (N=CꢀCH3) ppm. Anal. Calcd.
:
(found) for C42H46CaN8O2 N, 15.25 (14.22); C, 68.64 (70.21); H, 6.31 (6.66) %.
Complex 2. Yield: 0.85 g (95.8 %). 1H NMR: δ 8.13 (2H, d, J = 8.0 Hz, ArH), 7.73 (2H, d, J = 8.0

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Hz, ArH), 7.67 (2H, d, J = 8.0 Hz, ArH), 7.49 (2H, t, J = 8.0 Hz, ArH), 7.41ꢀ7.34 (3H, m, ArH),
7.27 (2H, d, J = 8.0 Hz, ArH), 7.10 (1H, t, J = 8.0 Hz, ArH), 3.25 (2H, t, J = 6 Hz, CꢀNCH2), 2.63
(2H, br, CꢀNCH2CH2), 2.30 (6H, s, N(CH3)2), 1.44 (3H, s, CH3C=N) ppm. 13C NMR(CDCl3, 100
MHz): δ 171.6 (C=O), 162.0 (C=CꢀN), 149.0 (N=CꢀCH3), 141.0, 140.2, 138.1, 128.9, 128.3, 127.6,
127.5, 127.2, 127.0, 123.5, 119.8 (Ar), 101.9 (C=CꢀN), 59.6 (CꢀNCH2CH2), 48.6 (N(CH3)2), 45.2
:
(CꢀNCH2), 16.3 (N=CꢀCH3) ppm. Anal. Calcd. (found) for C54H54CaN8O2 N, 12.68 (12.35); C,
73.11 (72.90); H, 6.14 (6.88) %.
Complex 3. Yield: 0.50 g (65.3 %). 1H NMR (CDCl3, 400 MHZ): δ 8.10 (2H, d, J = 8.0 Hz, ArH),
7.33 (2H, t, J = 8.0 Hz, ArH), 7.27 (2H, d, J = 8.0 Hz, ArH), 7.09ꢀ7.04 (3H, m, ArH), 3.17 (2H, t, J
= 6.0 Hz, CꢀNCH2), 2.57 (2H, br, CꢀNCH2CH2), 2.42 (3H, s, PhꢀCH3), 2.24 (6H, s, N(CH3)2), 1.37
(3H, s, CH3C=N) ppm. 13C NMR(CDCl3, 100 MHz): δ 172.0 (C=O), 161.9 (C=CꢀN), 149.2
(N=CꢀCH3), 140.2, 137.9, 136.3, 129.5, 128.3, 126.6, 123.4, 119.8 (Ar), 102.0 (C=CꢀN), 59.5
(CꢀNCH2CH2), 48.5 (N(CH3)2), 45.2 (CꢀNCH2), 21.3 (PhꢀCH3), 16.2 (N=CꢀCH3) ppm. Attempts to
get appropriate results of elemental analysis failed, probably due to moisture sensitivity.
Complex 4. Yield: 0.63 g (79.5 %). 1HꢀNMR: δ 8.11 (2H, d, J = 8.0 Hz, ArH), 7.34 (2H, t, J = 8.0
Hz, ArH), 7.10ꢀ6.98 (5H, m, ArH), 3.87 (3H, s, OCH3), 3.19 (2H, t, J = 4 Hz, CꢀNCH2), 2.58 (2H,
br, CꢀNCH2CH2), 2.25 (6H, s, N(CH3)2), 1.41 (3H, s, CH3C=N) ppm. 13C NMR(CDCl3, 100 MHz):
δ 171.7 (C=O), 161.9 (C=CꢀN), 159.4 (CꢀOꢀCH3), 149.2 (N=CꢀCH3), 140.3, 131.6, 128.3, 125.3,
123.4, 119.8 (Ar), 102.2 (C=CꢀN), 59.6 (CꢀNCH2CH2), 55.3 (PhꢀOꢀCH3), 48.5 (N(CH3)2), 45.2
:
(CꢀNCH2), 16.3 (N=CꢀCH3) ppm. Anal. Calcd. (found) for C44H50CaN8O4 N, 14.09 (13.84); C,
66.47 (67.06); H, 6.34 (6.44) %.
Complex 5. Yield: 0.61 g (76.5 %). 1HꢀNMR: δ 8.06 (2H, d, J = 8.0 Hz, ArH), 7.47 (2H, t, J = 8.0
Hz, ArH), 7.34 (3H, t, J = 8.0 Hz, ArH), 7.14ꢀ7.10 (3H, m, ArH), 3.13 (2H, t, J = 6.0 Hz, CꢀNCH2),
2.60 (2H, br, CꢀNCH2CH2), 2.27 (6H, s, N(CH3)2), 1.40 (3H, s, CH3C=N) ppm. 13C NMR(CDCl3,

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100 MHz): δ 170.6 (C=O), 161.9 (C=CꢀN), 148.7 (N=CꢀCH3), 140.1, 134.3, 129.2, 128.4, 125.3,
123.7, 119.9 (Ar), 101.7 (C=CꢀN), 59.5 (CꢀNCH2CH2), 48.6 (N(CH3)2), 45.2 (CꢀNCH2), 16.4
:
(N=CꢀCH3) ppm. Anal. Calcd. (found) for C42H44CaCl2N8O2 N, 13.94 (13.30); C, 62.76 (62.24); H,
5.52 (5.31) %.
Complex 6. Yield: 0.51 g (66.4 %). 1H NMR (CDCl3, 400 MHZ): δ 8.08 (2H, d, J = 8.4 Hz, ArH),
7.34 (2H, t, J = 8.0 Hz, ArH), 7.18ꢀ7.16 (4H, m, ArH), 7.09 (1H, t, J = 7.6 Hz, ArH), 3.15 (2H, t, J
= 5.2 Hz, CꢀNCH2), 2.61 (2H, br, CꢀNCH2CH2), 2.28 (6H, s, N(CH3)2), 1.38 (3H, s, CH3C=N) ppm.
13C NMR(CDCl3, 100 MHz): δ 172.0 (C=O), 161.9 (C=CꢀN), 149.2 (N=CꢀCH3), 140.2, 137.9,
136.3, 129.6, 128.3, 126.6, 125.3, 123.4, 119.7 (Ar), 102.0 (C=CꢀN), 59.5 (CꢀNCH2CH2), 48.5
:
(N(CH3)2), 45.2, 16.2 (N=CꢀCH3) ppm. Anal. Calcd. (found) for C42H44CaF2N8O2 N, 14.53 (14.92);
C, 65.43 (65.93); H, 5.75 (5.57) %.
1
Complex 7. Yield: 0.48 g (62.3 %). H NMR (CDCl3, 400 MHZ): δ 8.14ꢀ8.09 (2H, m, ArH),
7.46ꢀ7.40 (1H, m, ArH), 7.35 (2H, t, J = 8.0 Hz, ArH), 7.30ꢀ7.24 (2H, m, ArH), 7.19ꢀ7.06 (2H, m,
ArH), 3.26ꢀ3.07 (2H, m, CꢀNCH2), 2.74ꢀ2.39 (2H, m, CꢀNCH2CH2), 2.26ꢀ2.22 (6H, m, N(CH3)2),
1.44 (3H, s, CH3C=N) ppm. 13C NMR(CDCl3, 100 MHz): δ 166.0, 165.8 (C=O), 162.2, 162.1
(C=CꢀN), 159.3, 159.2, 156.82, 156.76 (CꢀF), 148.5, 148.4 (N=CꢀCH3), 140.13, 140.10, 130.6,
130.5, 129.14, 129.09, 126.5, 126.3, 125.1, 125.0 ,123.6, 119.8, 119.7, 115.8, 115.6 (Ar), 101.8
(C=CꢀN), 59.5, 59.2 (CꢀNCH2CH2), 48.4 (N(CH3)2), 45.1 (CꢀNCH2), 15.6, 15.5 (N=CꢀCH3) ppm.
:
Anal. Calcd. (found) for C42H44CaF2N8O2 N, 14.53 (14.08); C, 65.43 (64.69); H, 5.75 (5.80) %.
1
Complex 8. Yield: 0.29 g (38 %). H NMR (CDCl3, 400 MHZ): δ 8.10 (2H, d, J = 8.4 Hz, ArH),
7.34ꢀ7.28 (5H, m, ArH), 7.08 (2H, t, J = 7.2 Hz, ArH), 3.20ꢀ2.96 (2H, m, CꢀNCH2), 2.64 (2H, br,
CꢀNCH2CH2), 2.29 (6H, s, N(CH3)2), 2.26 (2H, s, CH3C=N) , 2.24 (1H, s, CH3C=N), 1.33 (3H, s,
ArꢀCH3) ppm. 13C NMR(CDCl3, 100 MHz): δ 171.8 (C=O), 162.1 (C=CꢀN), 148.9 (N=CꢀCH3),
140.2, 139.0, 133.6, 130.3, 129.0, 128.3, 128.2, 126.5, 125.3, 119.7 (Ar), 101.1 (C=CꢀN), 59.3

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(CꢀNCH2CH2), 48.1 (N(CH3)2), 43.2 (CꢀNCH2), 19.2 (PhꢀCH3), 15.5 (N=CꢀCH3) ppm. Anal. Calcd.
:
(found) for C44H50CaN8O2 N, 14.69 (14.66); C, 69.26 (68.90); H, 6.61 (6.42) %.
Complex 9. Yield: 0.26 g (30 %). 1H NMR (CDCl3, 400 MHZ): δ 7.87 (2H, br, ArH), 7.70 (1H, br,
ArH), 7.49 (2H, br, ArH), 7.24 (3H, br, ArH), 7.06 (1H, br, ArH), 2.76 (2H, br, CꢀNCH2), 2.36 (2H,
br, CꢀNCH2CH2), 2.26 (6H, s, N(CH3)2), 1.48 (3H, s, CH3C=N) ppm. 13C NMR(CDCl3, 100 MHz):
δ 187.7 (C=O), 164.8 (C=CꢀN), 149.0 (N=CꢀCH3), 139.8, 139.1, 131.7, 129.0, 128.8, 128.6, 128.5,
128.2, 126.7, 126.4, 126.2, 126.2, 125.3, 124.5, 122.5, 120.3 (Ar), 106.4 (C=CꢀN), 60.8
(CꢀNCH2CH2), 45.1 (N(CH3)2), 37.9 (CꢀNCH2), 15.6 (N=CꢀCH3) ppm. 19F NMR(CDCl3, 376 MHz):
δ ꢀ60.06 ppm. Anal. Calcd. (found) for C44H44CaF6N8O2: N, 12.87 (13.94); C, 60.68 (62.05); 5.09
(5.71) %.
Synthesis of complex 1a. MethylꢀLꢀlactate (0.10 ml, 1.0 mmol) was added to the suspension of 1
(0.735 g, 1.0 mmol) in toluene, during which, the mixture changed to yellow solution and then
yellow suspension. After stirred at room temperature overnight, pale brown powder was obtained
1
by filtration. Yield: 0.29 g (30 %). H NMR (CDCl3, 400 MHZ): δ 8.04 (2H, d, J = 7.6 Hz, ArH),
7.48ꢀ7.16 (7H, m, ArH), 7.02 (1H, t, J = 7.6 Hz, ArH), 4.62 (1H, q, J = 6.0 Hz, CH for lactate), 3.73
(3H, s, OCH3 for lactate), 3.17ꢀ3.16 (2H, m, CꢀNCH2), 2.49ꢀ2.42 (2H, m, CꢀNCH2CH2), 2.17 (6H, s,
N(CH3)2), 1.35ꢀ1.34 (6H, m, CH3C=N and CH3 for lactate) ppm. 13C NMR(CDCl3, 100 MHz): δ
171.8 (C=O), 170.9 (C=O), 162.3 (C=CꢀN), 149.1 (N=CꢀCH3), 140.5, 139.5, 128.9, 128.7, 128.3,
126.9, 123.0, 119.8 (Ar), 101.4 (C=CꢀN), 58.8 (CꢀNCH2CH2), 52.6, 48.6 (N(CH3)2), 45.5, 45.0,
21.4 (CH3 for lactate), 15.4 (N=CꢀCH3) ppm. Due to difficulty of removing complex 1 from 1a,
attempts to get suitable results of elemental analysis failed.
Typical general procedures for polymerization of L-lactide. A typical polymerization procedure
was exemplified by the synthesis of PLLAꢀ50 (the number 50 indicates the designed
[LA]0/[BnOH]0) catalyzed by 1 at room temperature. The conversion of polymerization was

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1
analyzed by H NMR spectroscopic studies. Complex 1 (0.0367g, 0.05 mmol) was added to a
solution of ꢀlactide (0.36 g, 2.5 mmol) and benzyl alcohol (5 ꢁl, 0.05 mmol) in CH2Cl2 (10 ml).
L
The mixture was stirred at room temperature for 2 hours and was then quenched by the addition of
deionized water (2 mL). The polymer was precipitated by pouring the above mixture into nꢀhexane
(40 mL) to give white crystalline solids. Yield: 0.32 g (85%).
X-ray crystallographic studies. Single crystals of complex 2 were obtained from slowly cooling
CH2Cl2/hexane solution. Suitable crystals were immersion with FOMBLIN®Y under nitrogen
atmosphere and mount on an Oxford Gemini S diffractometer. Intensity data were collected in 1350
frames with increasing ω (width of 0.3o per frame). The absorption correction was based on the
symmetry equivalent reflections using SADABS program10. The space group determination was
based on a check of the Laue symmetry and systematic absences, and was confirmed using the
structure solution. The structure was solved by direct methods using a SHELXTL package11. All
nonꢀH atoms were located from successive Fourier maps and hydrogen atoms were refined using a
riding model. Anisotropic thermal parameters were used for all nonꢀH atoms, and fixed isotropic
parameters were used for H atoms.
Acknowledgements. Financial support from the National Science Council of the Republic of China
is gratefully appreciated. (NSCꢀ98ꢀ2113ꢀMꢀ005ꢀ001ꢀMY3)
Supporting Information. PGSE studies of complex 2, 1H NMR studies of the eaction of complex
2 with BnOH, MALDIꢀTOF Mass spectrometry of PLLAꢀ25, and crystallographic studies of 2
(CCDC 864111) and 1a (CCDC 906296) are available in the supporting information.
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Graphic Abstract
Ring-Opening Polymerization of L-Lactide Catalyzed by Calcium
Complexes
Mon-Wei Hsiao, Chu-Chieh Lin*
Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan,
R.O.C.
e-mail: cchlin@mail.nchu.edu.tw
Polymerization of
L-lactide catalyzed by calcium complexes proceeds rapidly
and well controlled in the presence of a variety of alcohols.