Schiff base aluminum catalysts containing morpholinomethyl groups in the ring opening polymerization of rac-lactide

Article abstract of DOI:10.1007/s11426-015-5461-3

A series of Schiff base aluminum(III) complexes bearing morpholinomethyl substituents were synthesized. Comprehensive investigations on their stereoselective and kinetic features in the ring opening polymerization of lactide were carried out. The ring ope

Full text of DOI:10.1007/s11426-015-5461-3

SCIENCE CHINA
Chemistry
•
ARTICLES •
doi: 10.1007/s11426-015-5461-3
·
SPECIAL TOPIC · Progress in Synthetic Polymer Chemistry
Schiff base aluminum catalysts containing morpholinomethyl
groups in the ring opening polymerization of rac-lactide
1
2
1*
2*
2
2
Zhijie Guo , Ranlong Duan , Mingxiao Deng , Xuan Pang , Chenyang Hu & Xuesi Chen
1
Department of Chemistry, Northeast Normal University, Changchun 130024, China
2
Key Laboratory of Polymer Ecomaterials, Chinese Academy of Sciences; Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China
Received March 30, 2015; accepted May 22, 2015
A series of Schiff base aluminum(III) complexes bearing morpholinomethyl substituents were synthesized. Comprehensive
investigations on their stereoselective and kinetic features in the ring opening polymerization of lactide were carried out. The
ring opening polymerization proved to be first-order in the catalyst and the monomer. Linear relationships between the number-
average molecular weight of the polylactide and the monomer conversion were consistent with a well-controlled polymeriza-
tion. The propagation rate was strongly affected by morpholinomethyl substituents on the salicylaldehyde moiety.
polylactide, morpholinomethyl, ring opening polymerization
1
Introduction
tion process, much research has been directed toward the
development of catalyst systems [6,8,10,14–16]. Nomura et
al. [17] and Chen et al. [16] reported a series of Schiff base
aluminum complexes as N,N,O,O-tetradentate ligands, and
showed that the electronic nature of the substituents on the
salicylaldehyde moiety of the catalysts greatly affected the
catalytic performance. Our work is based on the notion that,
by appropriate choice of the substituents on the auxiliary
ligand, it is possible to improve the catalytic properties.
Carpentier synthesized a series of complexes stabilized by a
chelating bis(morpholinomethyl)phenoxy ligand, complexes
which proved to be very effective catalysts in the ROP of
Polylactide (PLA) is a biocompatible and biodegradable
polyester produced from annually renewable resources,
such as corn and sugar roots [1]. As petroleum resources
gradually depleted, PLA, with its unique features, has be-
come one of the most promising alternatives to petroleum-
based plastics [2,3]. The physical and mechanical properties
of PLA are largely determined by the stereochemistry of the
polymer chain [4–8]. Metal-catalyzed ring opening
polymerization (ROP) of lactide (LA) is a common method
for the synthesis of PLA. In recent years, catalysts with an-
cillary Schiff base ligands and combined with a discrete
metal center have been widely used in the ROP of lactide
L-LA, with a high [LA]
0
/[Initiator] ratio (up to 5000) [18].
Because of the lone pair electrons on the N atom, the mor-
pholinomethyl substituent acts as a Lewis base activating
group, which can control the nucleophilic activity of the
catalyst [19] and therefore enhance the polymerization pro-
cess. Inspired by this work, we have synthesized a series of
complexes bearing a morpholinomethyl substituent on the
organic ligand, and we have investigated the effect of this
substituent on the ROP of rac-LA (Scheme 1).
[9–13]. The central challenge for this class of catalysts is to
combine high stereoselectivity with high polymerization
rate.
Since the substituents on the ancillary organic ligands
affect the stereoselectivity and the rate of the polymeriza-
*
Corresponding authors (email: dengmx330@nenu.edu.cn; xpang@ciac.ac.cn)
©
Science China Press and Springer-Verlag Berlin Heidelberg 2015
chem.scichina.com link.springer.com

2
Guo et al. Sci China Chem September (2015) Vol.58 No.9
2
.2 Synthesis of 3-tert-butyl-2-hydroxy-5-(morpholino-
methyl)benzaldehyde
3
-tert-Butyl-2-hydroxybenzaldehyde was purchased from
Aldrich (USA). To a suspension of paraformaldehyde
(
(
880 mg, 29.4 mmol) in glacial HOAc (11 mL), morpholine
2.32 mL, 26.6 mmol) was added. The suspension was
stirred at ambient temperature for 3 h. Next, a solution of
-tert-butyl-2-hydroxybenzaldehyde (4.74 g, 26.6 mmol) in
3
absolute ethanol (45 mL) was added, and the mixture was
heated to reflux for 4 d. After cooling to ambient tempera-
ture, the mixture was brought to pH 8 with saturated
Na
2
CO
3 2 2 4
, extracted with CHCl , dried over Na SO , and
concentrated. Chromatography on SiO
2
(10% EtOAc/
hexane) afforded 3-tert-butyl-2-hydroxy-5-(morpholinome-
thyl)benzaldehyde (Compound B, Scheme 2) in 60% yield
Scheme 1 Synthesis and structures of pro-ligands 1–5 and complexes
1
a–5a.
(
4.42 g, 15.99 mmol).
2
.3 Synthesis of pro-ligands 1–5
2
Experimental
pro-Ligands 1 and 2: A solution of Compound A (0.21 mol)
in absolute ethanol (20 mL) was added dropwise to a stirred
solution of propane-1,3-diamine (0.1 mol) or 2,2-dimethyl-
propane-1,3-diamine (0.1 mol) in absolute ethanol (50 mL).
The mixture was refluxed for 12 h and cooled to ambient
temperature. After removal of the solvent under vacuum, a
yellow powder was obtained. Petroleum ether was used for
recrystallization.
All experiments were carried out under dry nitrogen at-
mosphere, using standard Schlenk techniques. Materials
used for the preparation of pro-ligands 1–5 were purchased
from Aldrich, and used without further purification. Tolu-
ene was distilled from Na-benzophenone. 2-Propanol and
2
ethyl acetate were distilled from CaH and protected by
argon atmosphere. rac-lactide (Purac) was purified by re-
crystallization from ethyl acetate and dried under vacuum at
room temperature before use. NMR spectra were recorded
on Bruker AV 300 (Germany) and 400 MHz spectrometers
in CDCl at 25 °C. Chemical shifts are given in parts per
3
million from tetramethylsilane. Gel permeation chromatog-
raphy (GPC) measurements were conducted with a Waters
pro-Ligands 3 and 4: A solution of Compound B (0.21
mol) in absolute ethanol (20 mL) was added dropwise to a
stirred solution of propane-1,3-diamine (0.1 mol) or
2
,2-dimethylpropane-1,3-diamine (0.1 mol) in absolute
ethanol (50 mL). The mixture was refluxed for 12 h and
cooled to ambient temperature. After removal of the solvent
under vacuum, flash chromatography on SiO
2 3
(5% CH OH/
5
1
15 GPC system with CHCl
mL/min, at 35 °C). The molecular weights were calibrated
3
as the eluent (flow rate:
EtOAc) afforded ligand 3 in 80% yield and pro-ligand 4 in
5
5% yield.
against polystyrene (PS) standards.
pro-Ligand 5: A solution of 3,5-di-tert-butyl-2-hydro-
xybenzaldehyde (0.21 mol) in absolute ethanol (20 mL) was
added dropwise to a stirred solution of propane-1,3-diamine
2
.1 Synthesis of 5-tert-butyl-2-hydroxy-3-(morpholino-
methyl)benzaldehyde
(0.1 mol) in absolute ethanol (50 mL). The mixture was
refluxed for 20 h and cooled to ambient temperature. Di-
5
-tert-Butyl-2-hydroxybenzaldehyde was purchased from
chloromethane and ethanol were used for crystallization.
Aldrich (USA). To a suspension of paraformaldehyde
1
(
(
880 mg, 29.4 mmol) in glacial HOAc (11 mL), morpholine
2.32 mL, 26.6 mmol) was added. The suspension was
pro-Ligand 1: H NMR (300 MHz, CDCl
3
): =13.53 (s,
PhOH, 2H), 8.40 (s, NCH, 2H), 7.42 (s, PhH, 2H), 7.18 (s,
PhH, 2H), 3.76 (m, CH CH O, 8H), 3.63 (s, PhCH N, 4H),
2
2
2
stirred at ambient temperature for 3 h. Next, a solution of
-tert-butyl-2-hydroxy-benzaldehyde (4.74 g, 26.6 mmol) in
5
absolute ethanol (45 mL) was added, and the mixture was
heated to reflux for 4 d. After cooling to ambient tempera-
ture, the mixture was brought to pH 8 with saturated K CO ,
2
3
extracted with CHCl , dried over Na SO , and concentrated.
3
2
4
Chromatography on SiO2 (30% EtOAc/hexane) afforded
-tert-butyl-2-hydroxy-3-(morpholinomethyl)benzaldehyde
Compound A, Scheme 2) in 58% yield (4.28 g, 15.46
mmol).
5
(
Scheme 2 Compounds used for the preparation of pro-ligands 1–5.

Guo et al. Sci China Chem September (2015) Vol.58 No.9
3
3
.71 (m, CCH
CCH C, 2H), 1.32 (s, C(CH
CDCl ): =165.68 (NCH), 157.45, 140.64, 130.96, 126.66,
2
N, 4H), 2.55 (m, CH
2
CH
2
N, 8H), 2.09 (m,
33.83 (ArC(CH
0.27 (AlCH CH
8.66%; found: C 68.65, H 8.60, N 8.70%.
3
)
3
), 31.32 (C(CH
3
)
3
), 8.88 (AlCH
2
CH
3
),
1
3
2
3
)
3
, 18H). C NMR (100 MHz,
2
3
). Elem. anal.: calcd: C 68.70, H 8.57, N
3
1
1
24.25, 117.83 (aromatic carbons), 56.79 (CCH
2
N), 53.65
O), 31.82
).
Complex 2a: H NMR (400MHz, CDCl
3
): =8.04 (s,
(
(
CH
CH
2
CH
2
N), 56.67 (PhCH
), 33.93 (ArC(CH
2
N), 67.05 (CH
2
CH
2
NCH, 2H), 7.50 (s, PhH, 2H), 7.02 (s, PhH, 2H), 3.71 (m,
CH CH O, 8H), 3.94 (s, PhCH N, 4H), 3.53 (s, CCH N,
4H), 2.45 (s, CH CH N, 8H), 1.29 (s, C(CH , 18H), 0.94
(s, C(CH , 6H), 0.86 (m, Al-CH CH , 3H), 0.11 (m,
Al-CH , 2H). C NMR (100 MHz, CDCl ): =168.91
2
CCH
2
3
)
3
), 31.47 (C(CH
3
)
3
2
2
2
2
1
pro-Ligand 2: H NMR (300 MHz, CDCl ): =13.60 (s,
2
2
3 3
)
3
PhOH, 2H), 8.37 (s, NCH, 2H), 7.46 (s, PhH, 2H), 7.19 (s,
PhH, 2H), 3.74 (m, CH CH O, 8H), 3.65 (s, PhCH N, 4H),
.50 (s, CCH N, 4H), 2.55 (s, CH CH N, 8H), 1.33 (s,
C(CH , 18H), 1.09 (s, C(CH
CDCl ): =166.06 (NCH), 157.50, 140.67, 130.87, 126.72,
3
)
2
2
3
1
3
2
2
2
2
3
3
2
2
2
(NCH), 162.85, 137.83, 133.88, 128.16, 127.22, 117.57
(aromatic carbons), 71.33 (CCH N), 53.17 (CH CH N),
1
3
3
)
3
3 2
) , 6H). C NMR (100 MHz,
2
2
2
3
56.34 (PhCH N), 67.25 (CH CH O), 36.13 (CH CCH ),
2
2
2
3
3
2
1
24.31, 117.79 (aromatic carbons), 68.33 (CCH
2
N), 53.63
O), 36.32
), 24.25
33.95 (ArC(CH
3
)
3
), 31.33 (C(CH
3
)
3
), 26.00 (CH
CCH
2
),
(
(
(
CH
CH
CH
2
3
3
CH
CCH
CCH
2
N), 56.41 (PhCH
2
N), 67.08 (CH
), 33.95 (ArC(CH ), 31.46 (C(CH
).
2
CH
2
2
10.00 (AlCH CH
3
), 1.54 (AlCH CH
2
3
). Elem. anal.: calcd: C
3
)
3
3
)
3
69.41, H 8.81, N 8.30%; found: C 69.60, H 8.92, N 8.41%.
Complex 3a: H NMR (400 MHz, CDCl ): =8.16 (s,
NCH, 2H), 7.41 (s, PhH, 2H), 7.01 (s, PhH, 2H), 3.73 (m,
2
1
2
3
1
pro-Ligand 3: H NMR (300 MHz, CDCl ): =13.94 (s,
3
PhOH, 2H), 8.40 (s, NCH, 2H), 7.25 (s, PhH, 2H), 7.09 (s,
PhH, 2H), 3.73 (m CH CH O, 8H), 3.44 (s, PhCH N, 4H),
.72 (m, CCH N, 4H), 2.45 (s, CH CH N, 8H), 2.14 (m,
CCH C, 2H), 1.32 (s, C(CH
CDCl ): =166.09 (NCH), 159.57, 137.23, 130.58, 130.08,
CH
4H), 2.45 (m, CH
(s, C(CH ) , 18H), 0.87 (m, Al-CH CH , 3H), 0.09 (m,
2
CH
2
O, 8H), 3.41 (s, PhCH
2
N, 4H), 3.63 (m, CCH
2
N,
CH N, 8H), 2.25 (m, CCH
2
2
C, 2H), 1.43
2
2
2
2
3
2
2
2
3
3
2
3
1
3
13
)
, 18H). C NMR (100 MHz,
Al-CH , 2H). C NMR (400 MHz, CDCl ): =163.38
2
3
3
2
3
3
(NCH), 161.70, 140.97, 133.00, 128.23, 128.03, 118.38
1
26.60, 118.30 (aromatic carbons), 56.82 (CCH
2
N), 53.52
O), 31.82
).
(aromatic carbons), 55.32 (CCH N), 53.54 (CH CH N),
62.79 (PhCH N), 67.02 (CH CH O), 30.46 (CH CCH ),
2 2 2 2 2
35.04 (ArC(CH ) ), 29.24 (C(CH ) ), 8.90 (AlCH CH ),
3 3 3 3 2 3
2 3
0.27 (AlCH CH ). Elem. anal.: calcd: C 68.70, H 8.57, N
2
2
2
(CH
CH
2
CH
2
N), 63.08 (PhCH
), 34.80 (ArC(CH
2
N), 67.04 (CH
2
CH
2
(
2
CCH
2
3
)
3
), 29.40 (C(CH
3
)
3
1
pro-Ligand 4: H NMR (300 MHz, CDCl ): =13.82 (s,
3
PhOH, 2H), 8.37 (s, NCH, 2H), 7.29 (s, PhH, 2H), 7.18 (s,
PhH, 2H), 3.74 (m, CH CH O, 8H), 3.45 (s, PhCH N, 4H),
.53 (s, CCH N, 4H), 2.45 (s, CH CH N, 8H), 1.32 (s,
C(CH , 18H), 1.08 (s, C(CH
CDCl ): =166.61 (NCH), 159.59, 137.25, 130.60, 130.05,
8.66%; found: C 68.60, H 8.60, N 8.65%.
Complex 4a: H NMR (400 MHz, CDCl ): =8.04 (s,
NCH, 2H), 7.41 (s, PhH, 2H), 7.02 (s, PhH, 2H), 3.73 (m,
1
2
2
2
3
3
2
2
2
1
3
3
)
3
3 2
) , 6H). C NMR (100 MHz,
CH CH O, 8H), 3.41 (s, PhCH N, 4H), 3.53 (s, CCH N,
2
2
2
2
4H), 2.45 (s, CH
2
CH
2
N, 8H), 1.43 (s, C(CH
3
)
3
, 18H), 0.94
3
1
26.58, 118.32 (aromatic carbons), 68.31 (CCH
2
N), 53.50
O), 36.30
3 3
) ), 24.27
(s, C(CH ) , 6H), 0.88 (m, Al-CH CH , 3H), 0.06 (m,
3
2
2
3
13
(
(
(
CH
CH
CH
2
3
3
CH
CCH
CCH
2
N), 62.83 (PhCH
2
N), 67.04 (CH
), 34.81 (ArC(CH ), 29.41 (C(CH
).
2
CH
2
Al-CH , 2H). C NMR (100 MHz, CDCl ): =163.40
2
3
2
3
)
3
(
(
6
3
9
6
NCH), 162.34, 142.45, 132.77, 128.44, 127.22, 117.30
aromatic carbons), 71.45 (CCH N), 53.50 (CH CH N),
O), 36.21 (CH
), 25.58 (CH
3
2
2
2
2.30 (PhCH
5.04 (ArC(CH
.88 (AlCH CH
2
N), 67.12 (CH
), 29.26 (C(CH
), 1.54 (AlCH CH
2
CH
2
3
CCH
CCH
2
),
),
3
)
3
3
)
3
3
2
2
.4 Synthesis of complexes 1a–5a
2
3
2
3
). Elem. anal.: calcd: C
In a glovebox filled with nitrogen, pro-ligands 1–5 (0.1
mmol) were dissolved in toluene (10 mL) in a phial that was
pre-dried for 2 d in a drying oven at 120 °C. Al(Et) (0.1
3
mmol) dissolved in toluene was added to the phial, and the
reaction was stirred for 12 h at 80 °C in an oil bath. Upon
completion of the reaction, the phial was cooled to ambient
9.41, H 8.81, N 8.30%; found: C 69.60, H 8.62, N 8.42%.
Complex 5a: The characterization of Complex 5a has
been previously reported elsewhere [17].
.5 Polymerization of rac-LA
2
temperature and toluene was removed under vacuum.
In a glovebox filled with nitrogen, rac-LA (7 mmol) was
added to a phial that was pre-dried for 2 d in a drying oven
at 120 °C. A stoichiometric amount of complexes 1a–5a
and 2-propanol as initiator (I) were added to the phial. Next,
the phial was put into an oil bath at 70–150 °C. At a certain
reaction time, approximately 1 mL of the reaction mixture
was withdrawn with a 2 mL syringe. PLA was precipitated
with cold ethanol. After centrifugation and removal of the
solvent, the polymer was dried in a vacuum drying oven at
45 °C for 48 h.
1
Complex 1a: H NMR (400 MHz, CDCl
3
): =8.19 (s,
NCH, 2H), 7.63 (s, PhH, 2H), 7.06 (s, PhH, 2H), 3.73 (m,
CH CH O, 8H), 3.63 (s, PhCH N, 4H), 3.52 (m, CCH N,
H), 2.53 (m, CH CH N, 8H), 2.30 (m, CCH C, 2H), 1.32
s, C(CH ) , 18H), 0.86 (m, Al-CH CH , 3H), 0.08 (m,
2
2
2
2
4
2
2
2
(
3
3
2
3
13
Al-CH , 2H). C NMR (400 MHz, CDCl ): =168.68
2
3
(
(
5
NCH), 161.45, 139.49, 130.06, 128.49, 128.03, 117.38
aromatic carbons), 59.62 (CCH N), 53.30 (CH CH N),
5.98 (PhCH N), 67.22 (CH CH O), 30.46 (CH CCH ),
2
2
2
2
2
2
2
2

4
Guo et al. Sci China Chem September (2015) Vol.58 No.9
3
Results and discussion
3
.1 Ring-opening polymerization of rac-LA in solution
According to the empirical formula:
m
n

d[LA]/dt=k [LA] [Cat]
p
the reaction order with respect to LA concentration can be
determined from the monomer conversion by fixing the
concentration of the catalyst and the polymerization tem-

Ea/RT
perature (k
kinetics in the monomer is observed. The apparent
polymerization constant (kapp=k [Cat]) can be determined
p
=Ae
) [20]. As shown in Figure 1, first-order
p
Figure 1 First-order kinetic plots for the ROP of rac-LA by applying 4a
and 2-propanol as catalyst and initiator, respectively, in toluene. [LA] =0.5
0
mol/L, T=70 °C.
from the slope of the figure [21]. To further probe the reac-
tion order with respect to the catalyst, kapp was plotted vs.
the catalyst concentration (Figure 2). The linear correlation
between kapp and [Al] revealed a first-order kinetics in cata-
lyst as well.
n
The number-average molecular weight (M ) of PLA us-
ing complexes 1a–4a showed a linear correlation with the
monomer conversion, along with low polydispersities, indi-
cating a well-controlled polymerization (Figure 3 and Table
1
n
). The correlation between M and the monomer conver-
sion using Complex 5a as catalyst has been previously re-
ported elsewhere [17].
In order to investigate the influence of the morpho-
linomethyl substituent on the polymerization rate, a series
of experiments applying complexes 1a–5a were performed.
The results are plotted and summarized in Figure 4 and in
Tables 1 and 2.
Figure 2 kapp vs. the concentration of 4a, using 2-propanol initiator, for
rac-LA polymerization.
Figure 3 Plot of M
n
(■) and polydispersity (▲) of PLA vs. rac-LA conversion, using complexes 1a–4a and 2-propanol as catalysts and initiator, respec-
tively, with [M] /[I]=100. (a) Complex 1a/2-propanol; (b) Complex 2a/2-propanol; (c) Complex 3a/2-propanol; (d) Complex 4a/2-propanol.
0

Guo et al. Sci China Chem September (2015) Vol.58 No.9
5
a)
Table 1 Polymerization of rac-LA in toluene using complexes 1a–5a, and 2-propanol as initiator
b)
3 c)
3 d)
3 e)
e)
f)
Entry
Complex
1a
t (min)
105
[M]
0
/[I]
Conv. (%)
M
n (calcd)×10
13.0
M
n (NMR)×10
13.9
M
n (GPC)×10
12.6
11.8
PDI
P
m
1
2
3
4
5
6
7
8
100
100
100
80
90
89
88
93
95
92
82
88
1.12
1.19
1.13
1.08
1.05
1.15
1.03
1.14
0.74
0.76
0.86
0.87
0.87
0.85
0.85
0.91
2a
120
12.8
13.5
3a
960
12.6
14.8
11.4
4a
1100
1150
1850
2100
720
10.7
11.5
9.7
4a
100
140
200
100
13.7
15.7
14.3
18.6
24.0
10.7
4a
20.2
23.6
4a
24.5
26.4
5a
12.7
13.8
1
a) The polymerizations were carried out in toluene; [LA]
0
=0.5 mol/L, T=70 °C; b) determined by H NMR; c) Mn (calcd)=144 g/mol×[M]
M (iPrOH); d) calculated from H NMR; e) obtained from GPC analysis; f) P is the probability of the meso-enchainment of the polymer chain.
w m
0
/[I]×conversion
1
+
Table 2 Kinetic results of rac-LA polymerization using complexes 1a–5a

3
1
Entry
Complex
1a
T (°C)
70
[M]
0
/[cat]
k
app (×10 min )
20.12
17.94
2.27
k
p
(L/(mol min))
4.024
1
2
3
4
5
6
7
8
100
100
100
80
2a
70
3.588
3a
70
0.454
4a
70
2.41
0.386
4a
70
100
140
200
100
1.93
0.386
4a
70
1.37
0.383
4a
70
0.96
0.384
5a
70
2.95
0.590
The morpholinomethyl substituent in the ortho position
3.2 Stereoselectivity investigation
had a positive effect on the polymerization rate, as shown in
In order to investigate the influence of the morpholinome-
thyl substituent on the stereoselectivity of the polymer, a
series of experiments were performed at different tempera-
tures, in the range 70–150 °C. For safety, the reaction phial
was tightly sealed with a rubber stopper and copper wire.
Because the catalysts studied herein were achiral, the ROP
reactions of rac-LA were assumed to be controlled by a
chain-end mechanism [13,17]. According to this mechanism,
the stereogenic center of the last inserted monomer on the
Figure 4. The kapp and k
rized in Table 2. The k values of complexes 1a and 2a with
morpholinomethyl substituents in the ortho position are
.024 and 3.588 L/(mol min), respectively, almost ten times
higher than those of complexes 3a (0.454 L/(mol min)) and
a (0.386 L/(mol min)) bearing bulky tert-butyl groups in
the ortho position. In general, single site catalysts are repre-
sented by the formula L
MR, where M is the metal center,
surrounded by the ancillary ligand L . The steric and elec-
p
of complexes 1a–5a are summa-
p
4
4
n
n
tronic properties of the ligand affect the bonding between
the ligand and the metal center, that is, the substituents on
the ligand have a significant influence on the polymeriza-
tion activity and on the stereoselectivity. As illustrated in
Figure 4, the k
p
values of complexes 3a and 4a are slightly
lower than those of Complex 5a, because these complexes
differ only in the para substituent. Hence, we speculate that
because of its electron-donating ability, the morpholinome-
thyl substituent at the para position is less efficient in im-
proving the polymerization rate, compared with the
tert-butyl. The morpholinomethyl moiety can increase the
rate of ROP of LA when present at the ortho position,
whereas it does not affect remarkably the reaction rate when
at the para position. This finding can be presumably at-
tributed to the synergistic effect of steric hindrance and
electronic impact of the morpholinomethyl group at the
ortho position; nevertheless, a more complicated rationale
cannot be excluded.
Figure 4 First-order kinetic plots for the ROP of rac-LA using complex-
es 1a–5a and 2-propanol as catalysts and initiator, respectively, in toluene,
with [LA] =0.5 mol/L, [M]/[I]=100:1, and T=70 °C. (1a) Complex 1a/
0
2-propanol; (2a) Complex 2a/2-propanol; (3a) Complex 3a/2-propanol; (4a)
Complex 4a/2-propanol; (5a) Complex 5a/2-propanol.

6
Guo et al. Sci China Chem September (2015) Vol.58 No.9
polymer chain determines which enantiomer of rac-LA
2
could independently activate the Et Zn. Thus, the morpho-
monomers is enchained. Thus, two outcomes were obtained:
linomethyl group in the ortho position can act as a nucleo-
phile and can stabilize the metal center. However, taking
into account bond lengths and bond angles, presumably the
electron-donating ability of the morpholinomethyl substitu-
ent in the para position played a role in stabilizing the metal
center of the catalyst as the temperature increases.
(1) isotactic PLA, formed when the stereogenic centers fa-
vor a meso-enchainment (kR/RR>>kR/SS or kS/SS>>kS/RR); and (2)
heterotactic PLA, formed when the stereogenic centers fa-
vor a racemic enchainment (kR/SS>>kR/RR or kS/RR>>kS/SS) [22].
In an attempt to increase the selectivity (P
plexes were designed with two different amine bridges. The
of complexes having 2,2-dimethylpropane-1,3-diamine
m
), the four com-
P
m
4
Conclusions
as amine bridge is 0.03–0.05 higher compared with that of
complexes using propane-1,3-diamine.
1
The homonuclear decoupled H NMR spectrum of the
A series of Schiff base aluminum(III) complexes bearing a
morpholinomethyl substituent have been synthesized. The
morpholinomethyl substituent in the ortho and para posi-
tions of the phenolate rings exhibit a dramatic influence on
the ROP of rac-LA. The reaction is first-order with respect
to the monomer and the catalyst. The morpholinomethyl
substituent is shown to be a less efficient group than the
bulky tert-butyl substituent in the stereoselective polymeri-
zation of rac-LA. Nevertheless, the morpholinomethyl sub-
stituent shows a positive effect on the rate of ROP of
rac-LA.
PLA’s methine region using Complex 3a is illustrated in
Figure 5. The stereo microstructures of the PLA chain can
be deduced from the spectrum. The stereoselectivity data of
rac-LA using complexes 3a and 5a are plotted in Figure 6.
An increase in temperature led to a decrease in stereoselec-
tivity. At 70 °C, Complex 5a had the highest stereoselectiv-
ity of 0.91, which gradually decreased to 0.67 when the
temperature increased to 150 °C. The same trend was ob-
served with Complex 3a. However, the decrease in selectiv-
ity was less severe with Complex 3a than with Complex 5a.
According to a previous report [19], in the addition of di-
ethylzinc to aldehydes, an apical coordination site on the
salen metal center of the catalyst could act as a Lewis acid
site to activate the aldehyde, whereas the tethered base
This work was supported by the National Natural Science Foundation of
China (51173183, 51233004, 51390484, 51321062, 51473029).
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