Ring-opening polymerization of lactide using salen-aluminum complexes bearing Schiff-base ligands derived from cis-1,2-cyclohexanediamine

Article abstract of DOI:10.1080/00958972.2015.1121248

Three aluminum complexes supported by salen ligands derived from cis-1,2-cyclohexanediamine and salicylaldehyde derivatives were synthesized. They were characterized by ¹H, ¹³C NMR spectra, and elemental analysis. X-ray diffraction a

Full text of DOI:10.1080/00958972.2015.1121248

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Ring-opening polymerization of lactide using Salen
aluminum complexes bearing Schiff-base ligands
derived from cis-1,2-cyclohexanediamine
Yuan Cui, Dongni Li, Bo Gao, Yuqi Zhou, Li Chen, Baoguo Qiu, Yanhui Li, Qian
Duan & Ninghai Hu
To cite this article: Yuan Cui, Dongni Li, Bo Gao, Yuqi Zhou, Li Chen, Baoguo Qiu, Yanhui Li,
Qian Duan & Ninghai Hu (2015): Ring-opening polymerization of lactide using Salen aluminum
complexes bearing Schiff-base ligands derived from cis-1,2-cyclohexanediamine, Journal of
Coordination Chemistry, DOI: 10.1080/00958972.2015.1121248
To link to this article: http://dx.doi.org/10.1080/00958972.2015.1121248
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Date: 24 November 2015, At: 03:07

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2015.1121248
Ring-opening polymerization of lactide using Salen aluminum complexes
bearing Schiff-base ligands derived from cis-1,2-cyclohexanediamine
YUAN CUI†, DONGNI LI†‡, BO GAO*†§, YUQI ZHOU†, LI CHEN†, BAOGUO QIU†, YANHUI LI†§,
QIAN DUAN*† and NINGHAI HU†§
†
7
‡
§
School of Materials Science and Engineering, Changchun University of Science and Technology,
989 Weixing Road, Changchun 130022, China
China–Japan Union Hospital of Jilin University, Jilin University, 126 Xiantai Street, Changchun, 130033, China
Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy
of Sciences, 5625 Renmin Street, Changchun 130022, China
Three aluminum complexes supported by Salen ligands derived from
cis-1,2-cyclohexanediamine and salicylaldehyde derivatives were synthesized. They were
1
13
characterized by H, C NMR spectra and elemental analysis. X-ray diffraction analysis
revealed that aluminum was in distorted square pyramidal geometry in 2. These complexes were
employed as catalysts for the ring-opening polymerization of L-lactide and rac-lactide.
Complex 2 showed the highest activity among these complexes with isopropanol for the ROP of
L-lactide and 3 showed the highest stereoselectivity for the ROP of rac-lactide attaining partially
isotactic polylactide with a P of 0.75. The kinetic data of the polymerization utilizing 3 as
m
catalyst showed that the polymeric rate was first-order to the monomer and catalyst.
Keywords: Aluminum; Salen; Biodegradable; Lactide; Ring-opening polymerization
1
. Introduction
People are searching for sustainable and environment-friendly polymers, which could take place
of the more normally used petroleum-based polymers [1]. Biocompatible, biodegradable and
sustainable polylactide (PLA), whose starting materials is from corn or sugar beets, is becoming
one of the most promising polymers as a partial replacement for petroleum-based polymers [2].
PLA has been exploited for a scope of applications including disposal containers, bone fracture
fixation devices, controlled release drug carriers, sutures, scaffolds, textiles, tissue engineering,
*Corresponding authors. Emails: gaobo@ciac.ac.cn (B. Gao); duanqian88@hotmail.com (Q. Duan)
1

etc. [1, 2]. Presence of two chiral centers in the lactide (LA) monomer results in different LA
stereoisomers, namely L-lactide (L-LA), D-lactide (D-LA) and meso-lactide (figure 1). The
stereochemistry of the polymer chains influences PLA’s physical and chemical properties [2b].
PLA is generally synthesized by the ring-opening polymerization (ROP) of lactide catalyzed by
metal complexes, such as complexes of tin [3], aluminum [4], zinc [5], magnesium [6], iron [7],
titanium [8], indium [9] and rare-earth metals [10]; organo-catalysts [11] and enzymes [12].
Aluminum catalysts are effective catalysts employed for ROP in the synthesis of PLA due to
their high effectiveness to control the stereo-regularities of the polymer [4, 13]. Efforts have been
tried to attain PLA with high stereo-regularities, derived from rac-lactide (rac-LA) and catalyzed
by stereoselective catalysts based on Schiff-base ligands [4g, 13, 14]. Some groups had
attempted to explain the relation among rac-lactide, aluminum catalysts based on Salen-type
Schiff base and stereo-regularities of PLA [13, 14] (figure 2).
Spassky [13a] discovered an aluminum catalyst (see figure 2) supported by a Salen-type
Schiff-base ligand, which was derived from R-(+)-1,1’-dinaphthalene-2,2’-diamine, could
stereo-control polymerization of rac-LA. Subsequently, Coates [13b] reported that a chiral
aluminum complex bearing a Salen-type Schiff base could afford enriched isotactic PLA. Feijen
et al. [13e] reported a chiral catalyst (R,R)-cyclohexylSalenAlOiPr [(R,R)-1] polymerized
rac-LA to get isotactic stereoblock PLA; the effect of the phenyl ring substituent on
stereoselectivity in the ROP of the rac-lactide initiated by Salen aluminum complexes was not
further studied.
Chen, Pang, Gao and Duan [14] reported a number of aluminum complexes based on
Salen ligands. These complexes were proved to be effective catalysts for the controlled ROP of
LA. Moreover, the more sterically hindered alkyl aluminum catalysts could effectively control
polymer microstructures and promote the formation of an isotactic or heterotactic bias in the
polymerization of rac-LA. Intrigued by the success of aluminum complexes based on Salen
ligands in polymerization catalysis [13, 14], we are very interested in researching the catalytic
behavior of aluminum complexes based on Salen type Schiff bases derived from salicylaldehyde
derivatives (salicylaldehyde, 3,5-dichlorosalicylaldehyde and 3,5-di-tert-butylsalicylaldehyde
from Sigma-Aldrich) and cis-1,2-cyclohexanediamine (see scheme 1). A ligand derived from
cis-1,2-cyclohexanediamine has not been reported before. In this work, we report a series of
2

Salen aluminum complexes with Schiff-base ligands derived from cis-1,2-cyclohexanediamine
and the applications as catalysts for the ROP of LA.
Scheme 1. The preparation of pro-ligands and complexes.
2
2
. Experimental
.1. General
All experiments used standard Schlenk or glovebox technology. THF and toluene were distilled
from sodium and benzophenone immediately before use. L-lactide and rac-lactide from Aldrich,
-3
crystallized from dry toluene, were purified just before use by sublimation in vacuo (10 mbar,
bath temperature: 85 °C) and distributed into glass ampoules equipped with breakseals.
1
13
Elemental analyses were performed by a Varian EL microanalyzer. H NMR and C NMR
spectra were performed on a Bruker AV 300M apparatus at 25 °C in CDCl for compounds and
3
polymers. The monomer conversions were confirmed by the integral at 1.65 ppm which was
attributed to LA monomer and 1.59 ppm which was attributed to PLA in CDCl . P values were
3
m
1
computed from different tetrad intensities measured by homonuclear decoupled H NMR
spectrum. Gel permeation chromatography (GPC) measurements were conducted with a Waters
–
1
5
15 GPC with CHCl as the eluent (flow rate: 1 mLmin , at 35 °C). The molecular weight was
3
adjusted through the PS standard. Crystallographic data were gathered on a Bruker APEX CCD
diffractometer using graphite-monochromated MoK radiation (λ = 0.71073 Å) at 187.5 K. All
α
non-hydrogen atoms were refined anisotropically. Hydrogens were introduced in calculated
3
positions with the displacement factors of the host carbons. AlEt ,
3

3
,5-di-tert-butylsalicylaldehyde, cis-1,2-cyclohexanediamine, isopropanol, p-toluenesulfonic
acid, 3,5-dichlorosalicylaldehyde and salicylaldehyde were from Sigma-Aldrich.
2
.2. Synthesis of pro-ligands
General procedure: a mixture of cis-1,2-cyclohexanediamine (5.0 mmol), modified
salicylaldehyde (10.0 mmol) and p-toluenesulfonic acid (0.20 mmol) in toluene (150 mL)
were refluxed for 4-18 h. After solvent evaporation at reduced pressure, the crude product
was purified by flash chromatography on silica gel using petroleum ether/acetic ether (V
1 2
/V
=
12/1) as the eluent. Then the product as yellow powder was acquired in 71.2–90.3% yields.
Pro-ligand L3 was prepared according to the reference [15].
1
2
.2.1. Pro-ligand L1. H NMR (300 MHz, CDCl ) δ 13.33 (bs, 2H, OH), 8.13 (s, 2H, N=CH),
3
7
.20 (t, J = 6.2 Hz, 2H, ArH), 7.09 (d, J = 6.0 Hz, 2H, ArH), 6.80 (d, J = 6.1 Hz, 2H, ArH), 6.72
1
3
(t, J = 6.0 Hz, 2H, ArH), 3.24–3.20 (m, 2H, N-CH-), 1.97–1.34 (m, 8H, (CH ) ) ppm. C NMR
2
4
(
75 MHz, CDCl
3
) δ 166.24 (2C, N=CH), 158.01, 134.55, 129.94, 126.29, 123.47, 116.38 (12C,
ArC), 68.96 (2C, N-CH-), 30.02, 27.39 ppm (4C, (CH ) ). Anal. Calcd for C H N O (%): C,
2
4
20 22
2
2
7
4.51; H, 6.88; N, 8.69. Found: C, 74.49; H, 6.84; N, 8.65. MS, (EI) m/z: calcd. for C H N O
20 22 2 2
[M]+ 322.2. Found: 322.1.
1
2
.2.2. Pro-ligand L2. H NMR (300 MHz, CDCl
3
) δ 13.74 (bs, 2H, OH), 8.25 (s, 2H, N=CH),
7
.29 (s, 2H, ArH), 7.12 (s, 2H, ArH), 3.41–3.15 (m, 2H, N-CH-), 2.06–1.24 (m, 8H, (CH ) ) ppm.
2
4
1
3
C NMR (75 MHz, CDCl ) δ 169.57 (2C, N=CH), 157.43, 137.97, 131.34, 127.68, 125.82,
3
1
18.87 (12C, ArC), 69.23 (2C, N-CH-), 31.54 (2C, (CH
2
)
2
2 2
), 26.44 ppm (2C, (CH ) ). Anal.
Calcd for C H Cl N O (%): C, 52.20; H, 3.94; N, 6.09. Found: 52.24; H, 3.96; N, 6.11. MS,
2
0
18
4
2
2
4 2 2
(EI) m/z: calcd. for C20H18Cl N O [M]+ 458.0. Found: 458.1.
2
.3. Synthesis of Salen aluminum complexes
General procedure: pro-ligand (2.0 mmol) dissolved in 20 mL of toluene was added to a stirred
solution of AlEt (2.0 mmol) in 10 mL of toluene. The reaction mixture was stirred at 40 °C
3
overnight and then cooled to ambient temperature. The solvent was evaporated at reduced
pressure to leave a powder. The product was purified by washing with dry hexane (1.0 mL×2).
4

1
2
.3.1. Complex 1. H NMR (300 MHz, CDCl ) δ 8.22 (s, 1H, N=CH), 7.97 (s, 1H, N=CH), 7.25
3
(m, 2H, ArH), 7.07 (d, J = 8.9 Hz, 2H, ArH), 6.88 (m, 2H, ArH), 6.64 (m, 2H, ArH), 3.65–3.51
(
m, 1H, N-CH-), 2.76–2.61 (m, 1H, N-CH-), 2.22–1.44 (m, 8H, (CH ) ), 0.79 (t, J = 8.2 Hz, 3H,
2
4
13
AlCH
2
CH
3
2 3 3
), –0.17 ppm (q, J = 8.2 Hz, 2H, AlCH CH ). C NMR (75 MHz, CDCl ): δ 167.25
(1C, N=CH), 164.33 (1C, N=CH), 156.41, 134.17, 132.23, 130.04, 128.75, 127.57, 125.00,
1
2
23.24, 122.13, 120.78, 117.54, 113.82 (12C, ArC), 67.66, 62.09 (2C, N-CH-), 29.79, 27.10,
4.23, 23.17 (4C, (CH ), 9.55 (1C, AlCH CH ), –0.47 ppm (1C, AlCH CH ). Anal. Calcd for
2
)
4
2
3
2
3
C H AlN O (%): 70.20; H, 6.69; N, 7.44. Found: 70.23; H, 6.71; N, 7.47.
22
25
2
2
1
2
.3.2. Complex 2. H NMR (300 MHz, CDCl ) δ 8.27 (s, 1H, N=CH), 7.75 (s, 1H, N=CH), 7.37
3
(s, 1H, ArH), 7.30 (s, 1H, ArH), 7.13 (s, 1H, ArH), 7.02 (s, 1H, ArH), 3.69–3.56 (m, 1H, N-CH-),
2
–
1
1
.80–2.67 (m, 1H, N-CH-), 2.27–1.19 (m, 8H, (CH
2
)
4
), 0.82 (t, J = 8.1 Hz, 3H, AlCH
2
CH
3
),
1
3
0.15 ppm (q, J = 8.1 Hz, 2H, AlCH CH ). C NMR (75 MHz, CDCl ): δ 169.22 (1C, N=CH),
2
3
3
66.74 (1C, N=CH), 158.22, 137.53, 134.28, 132.39, 129.98, 129.03, 126.94, 125.07, 124.28,
22.49, 119.00, 115.37 (12C, ArC), 68.92, 64.11 (2C, N-CH-), 30.22, 29.07, 26.19, 24.02 (4C,
(
CH ) ), 9.90 (1C, AlCH CH ), –0.18 ppm (1C, AlCH CH ). Anal. Calcd for C H AlCl N O
2
4
2
3
2
3
22 21
4
2
2
(%): C, 51.39; H, 4.12; N, 5.45. Found: C, 51.36; H, 4.10; N, 5.42. Crystals of 2 suitable for an
X-ray structure determination were grown from toluene solution. CCDC: 908768.
1
2
.3.3. Complex 3. H NMR (300 MHz, CDCl ) δ 8.15 (s, 1H, N=CH), 8.01 (s, 1H, N=CH), 7.24
3
(m, ArH, 1H), 7.18 (m, ArH, 1H), 7.10 (m, ArH, 1H), 6.89, (m, ArH, 1H), 3.55–2.91 (m, 2H,
N-CH-), 1.92–1.41 (m, 8H, (CH ) ), 1.32 (s, 9H, C(CH ) ), 1.25 (s, 9H, C(CH ) ), 1.19 ppm
2
3
3 3
3 3
(
s, 9H, C(CH ) ), 1.08 (s, 9H, C(CH ) ), 0.72 (t, J = 8.3 Hz, 3H, AlCH CH ), –0.29 ppm (q, J =
3
3
3 3
2
3
1
3
8
.3 Hz, 2H, AlCH CH ). C NMR (75 MHz, CDCl ) δ 159.72 (1C, N=CH), 156.55 (1C,
2
3
3
N=CH), 149.33, 140.30, 130.50, 129.12, 126.03, 125.92, 123.24, 122.41, 120.03, 118.79,
10.45, 108.76 (12C, ArC), 60.17 (1C, N-CH-), 58.94 (1C, N-CH-), 35.54 (1C, C(CH ), 34.78
1C, C(CH ) ), 33.73 (1C, C(CH ) ), 32.28 (1C, C(CH ) ), 28.10, 27.81, 26.10, 24.52 (4C,
1
3 3
)
(
(
3
3
3 3
3 3
CH ) ), 25.55, (3C, C(CH ) ), 24.43, (3C, C(CH ) ), 22.77, (3C, C(CH ) ), 21.05 ppm (3C,
2
4
3 3
3 3
3 3
C(CH ) ), 7.89 (1C, AlCH CH ), –2.24 ppm (1C, AlCH CH ). Anal. Calcd for C H AlN O
3
3
2
3
2
3
38 57
2
2
(%): C, 75.96; H, 9.56; N, 4.66. Found: C, 75.94; H, 9.53; N, 4.62.
5

2
.4. General procedure for lactide polymerization
The substrate and solvent were prepared in sealed glass ampoules using standard Schlenk
techniques. In a representative polymerization reaction, aluminum complex (0.50 mmol) and
isopropanol (0.50 mmol) in toluene (80 mL) were placed in a dry ampoule with a magnetic bar.
The ampoule was immersed in an oil bath at 70 °C. The solution was stirred for about
3
0 minutes, when the catalyst was activated completely by isopropanol and subsequently the
required quantities of lactide were added. After a certain reaction time, the polymer was isolated
by precipitating with cold methanol or low temperature centrifuge. The polymers were collected
and dried in vacuo at 30 °C for 24 h.
3
3
. Results and discussion
.1. Complexes preparation and characterization
As shown in scheme 1, pro-ligands L1-L3 were prepared easily in good yields (71.2 – 90.3%) by
condensation between modified salicylaldehyde and cis-1,2-cyclohexan-diamine in toluene.
Complexes 1-3 were synthesized by the reaction of L1-L3 with AlEt in toluene, specifically
3
synthetic procedure shown in scheme 1. All three complexes were sensitive to air and moisture.
1
13
3
Complexes 1-3 were characterized by H and C NMR in CDCl at room temperature and
1
13
elemental analysis. The H and C NMR spectra of 1-3 showed one Salen and one ethyl
coordinated to aluminum in these complexes. For example, the resonances (δ 8.31, 7.95 and
1
0
.82, –0.15 ppm) in the H NMR spectrum of 2 were attributed to the imine protons (N=CH) in
Salen ligand, the primary protons (AlCH CH ) and the secondary protons (AlCH CH ) in ethyl
2
3
2
3
1
of 2. H NMR spectra of 1-3 revealed that the phenyl portions of the ligands in these complexes
were inequivalent. For instance, two sets of resonances from imine protons (δ 8.31 and
1
7
.95 ppm) were observed in the H NMR spectrum, suggesting two possible isomers of 2 in
solution (see figure S1), one where the Et was on the same side as the methine H atoms and one
where the Et group on the opposite side. The geometry of 2 in the solid state was confirmed via
X-ray diffraction analysis. The molecular structure is depicted in figure 3. Selected bond
distances and angles are listed in table 1, the crystal data and structural refinement details for
2
·toluene are listed table S1. X-ray structural analysis revealed that 2 was mononuclear and Al
was coordinated by two N, two O and one C. In five-coordinate systems, the actual geometry of
6

the complex can be described by a structural index parameter τ such that τ = (β–α)/60°, where β
and α are the two largest angles (β>α). Thus, the geometric parameter τ is applicable to
five-coordinate structures as an index of the degree of trigonality within the structural continuum
between trigonal bipyramidal (τ = 1) and square pyramidal (τ = 0) [16]. In 2·toluene (see
table 1), the Al1–N1 bond length, 2.0407(16) (Å), was the longest among these bond lengths;
O1–Al1–N2 and O2–Al1–N1 angles were 149.55(7) and 145.97(7)°, respectively. The τ value
was 0.06, which suggested the aluminum was in distorted square pyramidal geometry.
3
.2. Ring-opening polymerization of L-LA and rac-LA
All three complexes (1-3) were studied as catalysts with isopropanol for the ROP of L-LA and
rac-LA; the results are collected in tables 2 and 3. Molecular weight of the PLA polymers was
1
measured by GPC. H NMR and GPC were applied to calculate the number-averaged molecular
weights of PLA. These aluminum complexes with isopropanol showed low to high activities
(
26.4 – 97.9%) at 70 °C. The number-averaged molecular weights (M were calculated according
n
to the formula M
the monomer-to-catalyst molar ratio) with narrow molecular weight distributions (PDI: 1.08 –
.27). The polymerization velocities of rac-LA applying 1-3 were slower than for L-LA
n
= 0.58Mn(GPC) [17]) of PLA were close to theoretical (Mn(theory) calculated from
1
polymerization velocities with 1-3 at the same polymerization conditions (e.g. table 3, entry 1
and table 2, entry 1).
The data of L-LA conversions versus polymerization time are shown in figure 4. The
activities of these complexes reduced with the increase of substituent’s bulk on the phenyl parts,
while electron-withdrawing substituents increased polymerization rate. The results are similar to
those in the literature [18]. Complex 2 displayed the highest activity (95.7% monomer
conversion table 2, entry 2) at the same polymerization conditions among the three complexes
(table 2, entries 1, 2 and 3). Moreover, the size of Salen ligands affected the PDI of the PLA,
decreasing from 1.25 to 1.09 with increase of the bulk of the substituents on phenyl rings from H
t
to Bu (table 2, entries 1 and 3). The increase in bulk and its effect of decreasing the PDI has
been observed previously [13].
7

3
.3. Kinetics studies
The kinetics of the ROP of L-LA by 3 was investigated in toluene at 70 °C in various monomer
1
to catalyst mole ratios, and the monomer conversion was monitored by H NMR as a function of
the polymerization time. The curves of conversions versus time are plotted in figure 4.
First-order kinetics in monomer was observed according to equation (1):
–
d[LA]/dt = kapp [LA] (1)
where kapp was the apparent polymerization rate constant. The molecular weight of the polymers
propagated linearly depending on the monomer transformation rate. The PDI of these polymers
were relatively narrow (1.09 – 1.13), illustrating that the catalytic system possessed living feature
(figure 5). In order to deduce the order of catalyst, kapp was plotted versus the concentration of 3.
As shown in figure 6, kapp increased linearly with the 3 concentration, manifesting that the order
in catalyst was first-order. Hence, polymerization of L-LA using 3 according to the kinetic
equation of equation (2):
–
d[LA]/dt = k [LA][Al] (2)
p
–
1 –1
where k was the polymerization rate constant, k = k /[Al]. A k value of 19.12 Lmol h is
p
p
app
p
determined for the LA polymerization catalyzed with 3 in toluene at 70 °C. This value is lower
i-
–1 –1
than that of LA polymerization with Al(O Pr) (k : 36.0 Lmol h ) [4m] under similar
3
p
conditions, which is maybe because of the rather bulky nature of the Salen ligand in 3.
3
.4. Stereoselective polymerization studies
1
The homonuclear decoupled H NMR spectrum of the methine region [19] of the poly(rac-LA)
was also researched (see figure 7; table 3, entries 1-6). The P [20] value, 0.75, demonstrated
m
that these polymer chains were partially isotactic. The results showed that the P selectivities
m
t
increased from 0.57 to 0.70 with increase of the bulk of the substituents (from H to Bu) on
ligands at 70 °C (see table 3, entries 1 and 3). For 3, the P value increased from 0.70 to 0.75
m
when the reaction temperature reduced from 70 to 40 °C (table 3, entries 3 and 6). In comparison
with the aluminum catalysts (e.g. (R,R)-1 [13e], see figure S2, P : 0.92-0.93) supported by Salen
m
ligand containing trans-1,2-diaminohexane backbone, these aluminum complexes with
8

cis-1,2-diaminohexane backbone showed lower stereoselectivities (P : 0.75) for the ROP of
m
rac-LA.
3
.5. Mechanism of lactide polymerization
For investigating the mechanism of initiation, end group analysis of the oligomer prepared by the
ROP of the L-LA at small monomer to catalyst mole ratio ([LA] : [2] = 15 : 1) was measured
0
0
1
by H NMR spectrum (figure 8). It showed an apparent triplet of two overlapping doublets at
δ 1.24 ppm (“a” in figure 8) and a multiplet at δ 4.99 ppm (“e”) with an integral ratio close to 6:1.
These peaks were attributed to the methyl protons of the isopropoxycarbonyl and the methine
proton, respectively. An eventual 1:1 ratio between multiplets at 4.99 ppm (“e”) and 4.34 ppm
(
“d”) confirm the other end group. This clearly revealed that the polymer was capped with one
isopropyl ester group and one hydroxyl group [14b, 14c, 21], and the ROP proceeded through a
so-called coordination insertion mechanism [22].
4
. Conclusion
A number of Salen aluminum Schiff-base complexes were synthesized. These complexes were
applied as catalysts in lactide polymerization. Kinetic data showed polymerizations were living
with narrow molar mass distributions. Electron-withdrawing substituents raise the
polymerization rate. Microstructural analysis of the polymers catalyzed by these complexes
discovered that the Salen ligands have ability to affect the tacticity of the polymer.
Supplementary data
CCDC 908768 contains the supplementary crystallographic data for 2; the SHELXTL-97
crystallographic software package was used for refinements [23]. This data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (+44)
1
223-336-033; or E-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 21204082);
China Postdoctoral Science Foundation (No. 2014M561268); Jilin Science & Technology
9

Department, Science and Technology Development Project (No. 20140204017GX); Science and
Technology Bureau of Changchun City (No. 2013060); Training Programs of Innovation and
Entrepreneurship for Undergraduates (No. 2015X094).
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[20]
P
(
m
is the probability of meso linkages: [mmm] = P
m
+ (1 − P
m
m
)P
m
m
/2, [mmr] = [rmm] =
2
2
1 − P )P /2, [rmr] = (1 − P ) /2, and [mrm] = [(1 − P ) + P (1 − P )]/2. See:
m
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14

Figure 1. Stereoisomers of lactides.
Figure 2. Salen aluminum catalysts for the ROP of lactide.
15

Figure 3. Perspective view of 2·toluene with thermal ellipsoids drawn at 30% probability level.
Hydrogens were omitted for clarity.
Figure 4. Kinetics of the ROP of L-LA by 3 at 70 °C in toluene (see table 2, entries 4, 5 and 6).
–
1
–1
[
4
0
LA] = 0.5 mol L , [isopropanol] /[Al] = 1; ■: [Al] = 0.005 mol L , [LA] /[Al] = 100, k
=
0
0
0
0
0
0
app
-2
–1
–1
-2 –1
.155 × 10 h ; ▲: [Al] = 0.010 mol L , [LA] /[Al] = 50, k = 8.298 × 10 h ; □: [Al] =
0
0
0
app
0
–1 -2 –1
.0067 mol L , [LA] /[Al] = 75, k = 5.550 × 10 h , where k was the apparent
0
0
app
app
polymerization rate constant.
16

Figure 5. Plots of PLA’s M and PDI as a function of monomer conversion using 3 with
n
isopropanol, [LA] /[3] /[isopropanol] = 100:1:1 in toluene at 70 °C.
0
0
0
Figure 6. k versus the concentration of 3 for the L-LA polymerization in toluene at 70 °C
app
–
1
–1 –1
(
[LA] = 0.5 mol L , k = 4.16 L mol h ).
0
p
17

1
Figure 7. Homonuclear decoupled H NMR spectrum of the methine part of poly(rac-LA) using
3
at 40 °C, P
m 3
= 0.75, in CDCl (table 3, entry 6).
1
Figure 8. H NMR spectrum of poly(L-LA) oligomer prepared by 2 with isopropanol when
[
LA] : [2] = 15 : 1.
0
0
18

Table 1. Selected bond lengths (Å) and angles (deg) for 2·toluene.
Al1–N1
Al1–O1
Al1–C21
2.0407(16)
1.8296(13)
1.963(2)
Al1–N2
Al1–O2
2.0277(16)
1.8162(13)
O2–Al1–N2
O1–Al1–C21
O2–Al1–C21
C21–Al1–N2
O1–Al1–N1
88.15(6)
104.05(8)
107.82(8)
105.84(8)
88.02(6)
O1–Al1–O2
O2–Al1–N1
O1–Al1–N2
88.25(6)
145.97(7)
149.55(7)
C21–Al1–N1 105.89(8)
N2–Al1–N1 78.37(6)
19

[
a]
Table 2. Representational polymerization data of L-LA with 1-3.
[
b]
[c]
[d]
[e]
[d]
Entry Complex [LA]
0
/[Al]
0
T h
Conv. %
M
n(theory)
M
n(GPC)
M
n
PDI
1
2
3
4
5
6
1
2
3
3
3
3
100
100
100
100
50
48
48
48
95
34
54
87.2
95.7
82.8
97.9
93.2
95.1
12600
13800
11900
14100
6700
22200
24400
20400
25000
11200
17500
12900
14100
11800
14500
6500
1.25
1.20
1.09
1.13
1.10
1.08
75
10300
10200
[
a]
-1
The polymerization reactions proceeded in toluene solution at 70 °C, [LA] = 0.5 mol L ,
0
[
b]
1
[c]
[
0 0
isopropanol] /[Al] = 1. Measured by H NMR. Calculated from the molecular weight of
[
d]
LA × [LA] /[Al] × conversion. from GPC analysis and calibrated against polystyrene
standard. The actual value of number-averaged molecular weights have to be calibrated by
0
0
[
e]
n
M = 0.58Mn(GPC) [17].
20

[
a]
Table 3. Representational polymerization data of rac-LA with 1-3.
[
b]
[c]
[d]
[e]
[d]
[f]
m
Entry Complex T h
Conv. %
M
n(theory)
M
n(GPC)
M
n(actual)
PDI
P
1
2
3
4
5
6
1
2
3
3
3
3
48
43.6
60.7
26.4
38.3
85.9
22.6
6300
8700
3800
5500
12400
3300
10400
16000
6800
6000
9300
3900
5300
12500
3100
1.22
1.27
1.15
1.18
1.20
1.14
0.57
0.59
0.70
0.67
0.69
0.75
48
48
95
9200
192
148
21600
5400
[
a]
[
a]
The polymerization reactions were carried out in toluene solution at 70 °C except that a
-1
reaction, entry 6, proceeded in THF at 40 °C; [LA]
[
LA × [LA] /[Al] × conversion. Obtained from GPC analysis and calibrated against
0
[c]
= 0.5 mol L , [LA]
0
0
/[Al] = 100,
[
b]
1
isopropanol] /[Al] = 1. Measured by H NMR. Calculated from the molecular weight of
0
0
[
d]
0
0
[
e]
polystyrene standard. The actual value of M
n
has to be calibrated by M
n
= 0.58Mn(GPC) [17].
[
f]
1
Homonuclear decoupled H NMR spectrum of the methine part of poly(rac-LA).
Graphical abstract
21