Stereoselective ring-opening polymerization of D,L -lactide, initiated by aluminum isopropoxides bearing tridentate nonchiral schiff-base ligands

Article abstract of DOI:10.1002/pola.25851

Ring-opening polymerization of D,L-lactide was stereoselectively achieved using newly designed aluminum alkoxide complexes as initiators. These half-SALEN aluminum complexes bearing tridentate nonchiral Schiff-base ligands are racemates, which provide chi

Full text of DOI:10.1002/pola.25851

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Stereoselective Ring-Opening Polymerization of D,L-Lactide,
Initiated by Aluminum Isopropoxides Bearing Tridentate Nonchiral
Schiff-Base Ligands
Kouki Matsubara,1 Chihiro Terata,1 Hiromichi Sekine,1 Kenji Yamatani,1 Tatsuo Harada,1
Kiyoka Eda,1 Misato Dan,1 Yuji Koga,1 Munehisa Yasuniwa2
1Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma, Jonan-Ku, Fukuoka 814-0180, Japan
2Department of Applied Physics, Faculty of Science, Fukuoka University, Nanakuma, Jonan-Ku, Fukuoka 814-0180, Japan
Correspondence to: K. Matsubara (E-mail: kmatsuba@fukuoka-u.ac.jp)
Received 22 March 2011; accepted 10 November 2011; published online 29 November 2011
DOI: 10.1002/pola.25851
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ABSTRACT: Ring-opening polymerization of D,L-lactide was
stereoselectively achieved using newly designed aluminum
alkoxide complexes as initiators. These half-SALEN aluminum
complexes bearing tridentate nonchiral Schiff-base ligands
are racemates, which provide chirality in the aluminum
centers, efficiently afforded a stereoblock copolymer of D,L-LA.
2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
50: 957–966, 2012
KEYWORDS: aluminum
Schiff-base
complexes;
lactide;
polyesters;
polymers
ring-opening polymerization;
stereospecific
INTRODUCTION Ring-opening polymerization (ROP) of
lactide (LA), a cyclic dimer of lactic acid, is easily initiated
by metal carboxylates and alkoxides.1 The applications of
high-molecular-weight poly-LA, especially poly(L-LA) (PLLA)
from L-LA are now widely developed as biomedical materi-
als, such as drug delivery systems,2 bioresorbable sutures,
implants,3 scaffolds for tissue engineering,4 and biodegrad-
able plastics based on biosustainable resources.5 A series of
recent studies investigating the polymerization of D,L-LA in
the presence of SALEN aluminum alkoxides, where SALEN is
N,N’-bis(salicylidene)alkyldiimine, gave highly isotactic ster-
for efficient stereocontrolled ROP,7–9 whereas bidentate
ligands in the aluminum complexes, such as diphenoxides,14
thiophenoxides,15 phenoxyimines,16 and hydroxy-substituted
heteroscorpionate17 were useful in the polymerization of
L-LA but were unsuitable in stereoregulated ROP of D,L-LA to
form isotactic PDLLA. Although several metal complexes
bearing tridentate ligands have been prepared as efficient
catalysts for ROP of LA, these compounds have not been
employed for stereoselective ROP of D,L-LA.18 Recently zinc
complexes bearing chiral tridentate diaminophenoxy ligands
were found to initiate poorly stereoselective polymerization
(Pm ¼ 0.54) of D,L-LA,19 and tridentate O^N^O or O^S^O
complexes of the Group III metals also polymerize D,L-LA in
heterotactic selection.20
eoblock copolymers of PLA,6 of which the melting points
ꢁ
were as high as ꢀ200 C. This isotactic stereocontrol method
using poorly applicable D,L-LA may facilitate the use of ster-
eoblock PDLLA for similar applications to those of PLLA. In
this stereoregulated polymerization initiated by several alu-
minum,7–9 zinc,10 and titanium complexes,11 two regulating
mechanisms were proposed: (i) the enantiomorphic site con-
trol (ESC) and (ii) chain end control (CEC). In the ESC pro-
cess, the chiral ligand in the metal complex stereoselectively
recognizes the D- or L-isomer of lactide,7 whereas in the CEC
process the chiral polymer-chain end selects one of the
enantiomers, leading to the stereoregulated single-site ring-
opening reaction of LA.8 The theoretical12 and experimen-
tal13 data support the mechanisms of the stereoselective
ROP of LA.
In the previous ESC processes, the stereochemistry at the
x-end has been regulated only by the chirality in the ligands.
This is despite the fact that the coordination of a nonchiral
bridging ligand can generate an asymmetric center in zirco-
nium or titanium which gives rise to the ESC process in the
single-site stereocontrolled polymerization of propylene.21
We believed that rigid tridentate ligand having only a Cs
symmetrical plane can form a stable chiral metal center. A
tetrahedral chiral aluminum center may also easily isomerize
to form a racemate under thermal conditions. However, if
the aluminum complexes bearing such nonchiral tridentate
ligands can form rigid octahedral chiral geometry with lac-
tide coordination, the stereoselective ROP of D,L-LA could be
Most of the previously reported studies showed that the tet-
radentate ligands in the aluminum complexes were effective
Additional Supporting Information may be found in the online version of this article.
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2011 Wiley Periodicals, Inc.
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controllable. Consequently, we studied the preparation of
new aluminum complexes with asymmetric metal centers
bearing Cs symmetric tridentate Schiff-base ligands, such as
aminoalcohol- or aminophenol-derived salicylaldimine,22 and
their application to initiating the ROP of D,L-LA. These race-
mic complexes efficiently initiated the polymerization to
form isotactic stereoblock copolymers of PDLLA, and one of
the stereoblock copolymers of PDLLA was successfully gen-
erated at room temperature. Darensbourg et al. recently
showed that similar stereoselective ROP of D,L-LA initiated by
nonchiral half-SALEN complexes of aluminum was dependent
on the structures of dimerized aluminum isomers.23 How-
ever, chirality at the aluminum center was not focused and
stereocontrol at room temperature was not realized in that
report.
tate solution and dried under reduced pressure (10ꢂ2 Pa)
before use. Other reagents were used as purchased. Espe-
cially, aluminum isopropoxide (99.99þ%) was purchased
from Aldrich Chem. 2-Amino-6-methylphenol was prepared
according to the literature.24 Schiff-base ligand (rac)-2 from
(rac)-valinol was prepared according to the literature.25
Preparation of 2-(tert-Butyl)-6-
[[(2-hydroxyphenyl)imino]methyl]phenol (1a)
To a solution of 2-aminophenol (545.6 mg, 0.500 mmol) and
3-(tert-butyl)-2-hydroxy-benzaldehyde (0.856 mL, 0.500 mmol)
in methanol was added Na2SO4 (4.1 g) in a 50-mL round-bot-
tom flask equipped with a reflux condenser. The suspension
was stirred at 70 ꢁC for 20 h. After filtration to remove
Na2SO4, the solvent was concentrated and some amount of
hexane was added. Then it was cooled at ꢂ30 ꢁC, orange
crystals of 1a were separated (1.271 g, 4.72 mmol, 94% yield).
EXPERIMENTAL
t
1H NMR (CDCl3): d 1.47 (s, 9H, Bu), 5.76 (br, 1H, OH), 6.93
Techniques
(t, J ¼ 7.6 Hz, 1H, ArH), 6.97 (t, J ¼ 7.6 Hz, 1H, ArH), 7.04 (d,
J ¼ 8.0 Hz, 1H, ArH), 7.15 (d, J ¼ 7.6 Hz, 1H, ArH), 7.22 (t, J ¼
8.0 Hz, 1H, ArH), 7.30 (d, J ¼ 7.6 Hz, 1H, ArH), 7.44 (d, J ¼
7.6 Hz, 1H, ArH), 8.70 (s, 1H, N¼¼CH), 12.87 ppm (s, 1H, OH);
13C NMR (CDCl3): d 29.68 (C(CH3)3), 35.27 (C(CH3)3), 116.13
(Ph), 118.62 (Ph), 119.25 (Ph, 4ꢁ), 119.56 (Ph), 121.36 (Ph),
128.91 (Ph), 131.39 (Ph), 131.45 (Ph), 136.12 (Ph, 4ꢁ), 138.13
(Ph, 4ꢁ), 150.22 (Ph, 4ꢁ), 160.30 (Ph, 4ꢁ), 165.05 ppm
(N¼¼CH); IR (m, cmꢂ1): 3370 (OH), 1614 (CN), 1593 (CN). MS
(ESI-MS): calcd. (C7H19NO2þHþ), 270.149; found, 270.157.
1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were
recorded on a VARIAN Mercury Y plus 400 MHz spectrometer
at room temperature in CDCl3. Chemical shifts (d)
were recorded in ppm from the internal standard (CHCl3). IR
spectra were recorded in cmꢂ1 on a PERKIN ELMER Spectrum
One spectrometer equipped with a universal diamond ATR.
Size exclusion chromatography analysis of the polymers
was carried out using a JASCO HPLC system equipped with
a RI-2031Plus differential refractometer and an UV-2075
Plus UV–VIS detector (254 nm) using two Shodex KF-
804LþKF-804L columns at 40 ꢁC. THF was used as eluent
(1.0 mL minꢂ1). Calibration was carried out using standard
samples of polystyrene. The ELEM. ANAL. was carried out with
YANACO CHN Corder MT-5, AUTO-SAMPLER. Electrospray
ionization time-of-flight mass spectrometry (ESI-TOF MS) was
carried out on a JEOL JMS-T100 mass spectrometer. The sam-
ple solutions in methanol (for ligands) or isopropanol (for Al
complexes) were directly infused. Thermal analysis was car-
ried out with a differential scanning calorimeter, PERKIN
ELMER Pyris 1. A PDLLA sample sealed in an aluminum sam-
ple pan was always kept under a nitrogen atmosphere during
DSC measurement and the second scan was recorded. A
PDLLA sample for wide-angle X-ray diffraction (WAXD) study
was squeezed in a copper cylinder, and the both ends were
sealed by a thin polyimide film to prevent moisture. Mono-
chromated Cu Ka radiation (k ¼ 1.542 Å) was used as an inci-
dent X-ray beam at 50 kV and 80 mA. The X-ray beam passed
through the sample, and a WAXD pattern was obtained with a
position-sensitive proportional counter system (RIGAKU).
Preparation of 2-(tert-Butyl)-6-[[(2-hydroxy-3-
methylphenyl)imino]methyl]phenol (1b)
This was prepared as 1a from 2-amino-6-methylphenol (250.1
mg, 2.03 mmol) and 3-(tert-butyl)-2-hydroxy-benzaldehyde
1
(0.348 mL, 2.03 mmol). Orange crystals, yield: 74%. H NMR
t
(CDCl3): d 1.46 (s, 9H, Bu), 2.32 (s, 3H, 3-CH3), 5.93 (br, 1H,
OH), 6.86 (t, J ¼ 7.8 Hz, 1H, ArH), 6.92 (t, J ¼ 7.8 Hz, 1H, ArH),
6.99 (d, J ¼ 7.6 Hz, 1H, ArH), 7.08 (d, J ¼ 7.2 Hz, 1H, ArH),
7.29 (d, J ¼ 7.6 Hz, 1H, ArH), 7.43 (d, J ¼ 7.6 Hz, 1H, ArH),
8.69 (s, 1H, N¼¼CH), 12.86 ppm (br, 1H, OH); 13C NMR (CDCl3):
d 15.74 (3-CH3), 29.33 (C(CH3)3), 34.94 (C(CH3)3), 115.58
(Ph), 118.87 (Ph), 119.27 (Ph, 4ꢁ), 120.02 (Ph), 125.04 (Ph,
4ꢁ), 129.84 (Ph), 130.93 (Ph), 131.05 (Ph), 135.30 (Ph, 4ꢁ),
137.75 (Ph, 4ꢁ), 148.13 (Ph, 4ꢁ), 159.91 (Ph, 4ꢁ), 164.50 ppm
(N¼¼CH); IR (m, cmꢂ1): 3514 (OH), 1596 (C¼¼N); MS(ESI-MS):
calcd. (C8H21NO2þHþ), 284.165; found, 284.186.
Preparation of 1-[(2-Hydroxy-3-tert-
butylbenzylideneamino)methyl]cyclohexanol (3)
To a solution of 1-Aminomethyl-1-cyclohexanol hydrochloride
(400.0 mg, 2.50 mmol) and 3-(tert-butyl)-2-hydroxy-benzalde-
hyde (0.410 mL, 2.30 mmol) in methanol (16 mL) was added
K2CO3 (334 mg, 2.50 mmol) in a 50-mL round-bottom flask
equipped with a reflux condenser. The solution was refluxed
for four days. The solvent was removed under reduced
pressure and the residual solid was extracted with toluene.
Removal of toluene from the solution gave a white solid, 3, in
Materials
Toluene used for polymerization and toluene-d8 were dis-
tilled from sodium benzophenone ketyl solution under
reduced pressure just before use. Hexane used for washing
the crude mixture of the Al complexes was distilled from a
sodium benzophenone ketyl solution containing small
amount of bis[2-(2-methoxyethoxy)ethyl]ether. Pyridine,
ethyl acetate, and chloroform-d were distilled over calcium
hydride before use. Lactides were stored under nitrogen at
ꢂ35 ꢁC and recrystallized three times from dried ethyl ace-
1
86% yield (572 mg, 1.98 mmol). H NMR (CDCl3): d 1.41 (d,
J ¼ 3.4 Hz, 6H, CyH), 1.44 (s, 9H, tert-Bu), 1.50 (br, CyH),
1.53 (br, CyH), 1.63 (br, CyH), 3.56 (s, ¼¼NACH2A, 2H), 6.88
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(t, J ¼ 7.8 Hz, 1H, PhH), 7.30 (d, J ¼ 7.8 Hz, 1H, PhH), 7.42 (d,
J ¼ 7.8 Hz, 1H, PhH), 8.46 ppm (s, 1H, N¼¼CH); 13C NMR
(CDCl3): d ¼ 21.83 (CH2 in Cy), 25.78 (CH2 in Cy), 29.31
(C(CH3)3), 34.82 (C(CH3)3), 35.75 (CH2 in Cy), 70.17 (CAOH),
71.42 (CH2-N), 117.88 (Ph), 118.62 (Ph-OH, 4o), 129.54 (Ph),
129.84 (Ph), 137.42 (Ph, 4ꢁ), 160.43 (Ph, 4ꢁ), 167.63 (N¼¼CH);
IR (m, cmꢂ1): 3301 (OH), 1621 (C¼¼N); MS(ESI-MS): calcd.
(C20H24AlNO3)þHþ), 290.212; found, 290.207.
NACH), 6.74 (t, J ¼ 7.8 Hz, 1H, ArH), 7.14 (d, J ¼ 7.6 Hz, 1H,
ArH), 7.43 (d, J ¼ 7.2 Hz, 1H, ArH), 8.21 (s, 1H, N¼¼CH); 13C
NMR (CDCl3): d 20.15, 20.51, 27.86, 28.02, 29.51, 29.57,
29.71, 35.42 (4ꢁ), 61.55, 62.75, 74.35, 116.44, 119.75 (4ꢁ),
131.80, 132.28, 140.87 (4ꢁ), 164.06 (4ꢁ), 167.94. MS(ESI-
MS): calcd. ((C19H30AlNO3)þ(C16H23AlNO2)þ), 635.3580;
found, 635.3585.
Preparation of 1-[(2-Hydroxy-3-
Preparation of 2-(tert-Butyl)-6-
[[(2-hydroxyphenyl)imino]methyl]phenolate
Aluminum Complex (4a)
tert-butylbenzylideneamino)methyl]cyclohexanolate
Aluminum Complex (6)
This was prepared as 4a from aluminum isopropoxide
(0.468 mg, 2.30 mmol) and the ligand 3 (698 mg, 2.30 mmol).
To
a solution of aluminum isopropoxide (102.1 mg,
1
0.500 mmol) in dried toluene (2 mL) was added the ligand 1a
(134.7 mg, 0.500 mmol) in a 20 mL Schlenk tube under dried
nitrogen atmosphere (equipped in a Globe box (MBraun
UNI-Lab)). The solution was stirred at 80 ꢁC for 24 h, and then,
the solvent was removed in vacuo from the resulted suspen-
sion. The residual bright yellow solid was washed with
small amount of hexane and dried, yielding 4a (137.7 mg,
0.390 mmol, 78% yield). 1H NMR (CDCl3): d 1.20 (d, J ¼ 7.0 Hz,
3H, CH(CH3)2), 1.21 (d, J ¼ 6.2 Hz, 3H, CH(CH3)2), 1.32 (s, 9H,
C(CH3)3), 4.02 (m, 1H, CH(CH3)2), 6.09 (t, J ¼ 7.8 Hz, 1H, Ph),
6.44 (t, J ¼ 8.3 Hz, 1H, Ph), 6.52 (t, J ¼ 7.5 Hz, 1H, Ph), 6.90 (d,
J ¼ 7.5 Hz, 1H, Ph), 7.13 (d, J ¼ 7.8 Hz, 1H, Ph), 7.15 (d, J ¼
8.3 Hz, 1H, Ph), 7.48 (d, J ¼ 8.3 Hz, 1H, Ph), 8.00 (s, 1H, N¼¼CH).
MS(ESI-MS): calcd. ((C18H27NO2)ꢃ2þHþ), 707.3216; found,
707.3221.
White powder, yield: 73%. H NMR (toluene-d8): d 1.15-1.85
(m, 10H, CH2 in cyclohexanol), 1.27 (d, J ¼ 6.2 Hz, 3H, CH3),
t
1.36 (d, J ¼ 6.2 Hz, 3H, CH3), 1.64 (s, 9H, Bu), 2.77 (brd, J ¼
9.0 Hz, 1H, NACH2), 3.75 (brd, J ¼ 9.0 Hz, 1H, NACH2), 3.80
(sept, J ¼ 6.2 Hz, 1H, CH in alkoxide), 6.70 (t, J ¼ 7.4 Hz, 1H,
ArH), 6.92 (d, J ¼ 7.6 Hz, 1H, ArH), 7.39 (d, J ¼ 7.2 Hz, 1H,
ArH), 7.59 (s, 1H, N¼¼CH); 13C NMR (CDCl3): d 22.37
(C(CH3)2), 24.35 (CH2 in Cy), 24.41 (CH2 in Cy), 26.55 (CH2 in
Cy), 30.07 (C(CH3)3), 35.28 (C(CH3)3), 37.23 (CH2 in Cy),
38.12 (CH2 in Cy), 66.87 (C(CH3)2), 69.41 (CH2-N), 70.47
(C-OH), 116.06 (Ph), 120.66 (Ph-OH, 4ꢁ), 130.41 (Ph),
131.20 (Ph), 141.30 (Ph, 4ꢁ), 162.77 (Ph, 4ꢁ), 165.44
(N¼¼CH). MS(ESI-MS): calcd. ((C21H32AlNO3)þ(C18H25AlNO2)þ),
687.3893; found, 687.3886
General Polymerization Procedure
Preparation of 2-[[(2-Hydroxy-3-
The general procedure for the polymerization of lactides in
pyridine is as follows. To a 20-mL screw-capped glass sample
tube, lactide (216.2 mg, 1.50 mmol), aluminum complex
(0.015 mmol), and pyridine (0.2 mL) were added in a glove
box. The tube was sealed and stirred at desirable temperature
for several hours. After dilution of the adhesive product with
small amount of THF, the solution was poured into the stirring
methanol (100 mL) at room temperature to form white
precipitate as a PLA. To remove the aluminum residues, repre-
cipitation in methanol was repeated for several times.
methyl-phenyl)imino]methyl]-6-tert-butylphenolate
Aluminum Complex (4b)
This was prepared as 4a from aluminum isopropoxide
(103.3 mg, 0.5057 mmol) and the ligand 1b (143.3 mg,
0.5057 mmol). Yellow powder, yield: 84%. 1H NMR (CDCl3):
d 1.08 (d, J ¼ 6.4 Hz, 3H, CH(CH3)2), 1.19 (d, J ¼ 6.4 Hz, 3H,
CH3), 1.31 (s, 9H, C(CH3)3), 2.12 (s, 3H, CH3), 4.27 (sept, J ¼
6.4 Hz, 1H, CH(CH3)2), 6.70 (t, J ¼ 7.6 Hz, 1H, ArH), 6.86 (t,
J ¼ 7.6 Hz, 1H, ArH), 7.03 (d, J ¼ 7.6 Hz, 1H, ArH), 7.30 (d,
J ¼ 7.6 Hz, 1H, ArH), 7.34 (d, J ¼ 7.6 Hz, 1H, ArH), 8.62 (s,
1H, N¼¼CH); 13C NMR (CDCl3): d 16.23 ((CH3)2CH), 24.76
((CH3)2CH), 29.21 ((CH3)3C), 67.39 ((CH3)2CH), 111.31 (Ph),
116.39 (Ph), 117.49 (Ph), 130.22 (Ph), 160 56 ppm (N¼¼CH).
MS(ESI-MS): calcd. (C21H27AlNO3þHþ), 368.1801; found,
368.1806.
RESULTS AND DISCUSSION
Preparation of Aluminum Alkoxides Bearing
Tridentate Schiff-Base Ligands
We planned to design both chiral and achiral bridging
ligands in the asymmetric aluminum complexes, because it
was important to compare these complexes with and without
chirality in the ligand in the stereoregulation system. These
Schiff-base ligands were obtained in excellent yields by the
reaction of 2-hydroxy-3-tert-butyl-benzaldehyde with an
equivalent of either 2-aminophenol and 6-methyl-2-amino-
phenol, (rac)-valinol, or 1-(2-amino-ethyl)-cyclohexanol.25
Besides valinol, these aminophenols and aminoalcohols do
not have chirality. All the ligands reacted with aluminum iso-
propoxide (Al(OiPr)3) efficiently upon heating at 80 ꢁC to
form complexes 4a, 4b, (rac)-5 and 6 in high yields after
washing the resulting solid with hexane [Scheme 1 (1), (2),
and (3)]. These complexes were fully characterized on the
basis of 1H and 13C NMR spectra; however, X-ray structural
Preparation of 2-[(1-Hydroxy-3-methylbutan-2-
ylimino)methyl]-6-tert-butylphenolate Aluminum
Complex ((rac)-5)
This was prepared as 4a from aluminum isopropoxide
(1.040 g, 5.090 mmol) in dried toluene (10 mL) and a ligand,
(rac)-2-((1-hydroxy-3-methylbutan-2-ylimino)methyl)-6-tert-
butylphenol, (1.341 g, 5.090 mmol). White powder, yield:
90%. 1H NMR (CDCl3): d 0.90 (d, J ¼ 6.0 Hz, 3H, CH3), 0.93
(d, J ¼ 6.0 Hz, 3H, CH3), 0.95 (d, J ¼ 6.8 Hz, 3H, CH3), 1.07
t
(d, J ¼ 6.8 Hz, 3H, CH3), 1.50 (s, 9H, Bu), 2.50 (oct, J ¼ 6.8
Hz, 1H, CH in valinol), 3.01 (dd, J ¼ 5.2, 10.4 Hz, 1H,
OACH2), 3.80 (sept, J ¼ 6.0 Hz, 1H, CH in alkoxide), 4.02
(dd, J ¼ 5.2, 9.6 Hz, 1H, OACH2), 4.41 (d, J ¼ 9.2 Hz, 1H,
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C21H28AlNO3 (368.1801 m/z, as the theoretical mass). As a
typical example, the 1H NMR spectrum for 4b (Fig. 1)
showed that a set of signals due to the aryl group of 4b
appeared from d 6.70 to 7.34, and this set of signals is simi-
lar to those for the ligand 1b which range between d 6.86
and 7.43. The integrated ratios between the signals due to
the imine proton (d 8.62) and the methine proton (d 4.27)
in the aluminum isopropyl group were identical, indicating
that two of the three isopropoxy groups in Al(OiPr)3 were
removed (Scheme 2). Solvent signals could not be found in
the spectra other than a negligible amount of hexane. In the
absence of non-coordinating solvent, the compound could be
stabilized by the formation of a dimeric structure that forms
AlꢄꢄꢄO bonds, as reported previously.18,23 However, the
dimeric isomers of the cis and trans forms could not be
1
detected.23 The H NMR spectrum for (rac)-5 gave only one
set of the signals assigned as one of the stereoisomers. It is
of interest that no signals due to the other stereoisomers of
(rac)-5 and/or its dimer, where the chirality combination
of chiral carbon derived from D,L-valinol and aluminum is
different, were detected in the 1H NMR spectrum at room
temperature, suggesting that chirality at the D,L-valinol unit
can regulate the chirality of the aluminum center and/or the
structure of the dimer.
SCHEME 1 Preparation of Schiff-base aluminum complexes of
4a-b, (rac)-5 and 6 in toluene at 80 ꢁC for 24 h.
Although, in the 13C NMR spectrum, the CC or CD in ligand 3
gave rise to two signals at d 35.75 or 21.83, whereas for
complex 6 independent four signals due to CC, CC’, CD, and
CD’ appeared at d 37.23, 38.12, 24.35, and 24.41, respectively
analysis of these compounds was not possible, because we
could not obtain the crystals with enough size to analyze.
Although 4a was barely soluble in common organic solvents
at room temperature, the other complexes were character-
ized using 1H and 13C NMR, and ESI-MS spectroscopy. The
compound 4a was considered to not only form dimers but
also oligomerize or polymerize via a strong AlꢄꢄꢄO network.
In the ESI-MS spectra of 4a in isopropanol, the mass signal
at 707.3221 m/z was detected, assigned as a protonated
dimer, C40H51N2O6Al2, of which the theoretical mass is
707.3216 m/z, whereas no signals due to a monomeric form
were detected (see the Supporting Information). The similar
mass signals showing only its dimeric structure was
observed in the spectrum of (rac)-5 and 6. The result
supported the dimeric structure of 4a, (rac)-5 and 6 in solu-
tion. It is of interest that the mass signal at 368.1806 m/z
was detected for only 4b in the ESI-MS spectrum in
isopropanol, showing a protonated monomeric form of 4b,
1
(Fig. 2). Additionally, two H resonances due to diasterotopic
methylene protons between the imine nitrogen and a cyclo-
hexylene group in the half-SALEN ligand of 6 appeared at d
2.77 and 3.75; however, the corresponding protons in 3
were equivalent, observed at
d 3.56. The preliminary
experiment of the various-temperature 1H NMR (400 MHz)
spectrum for 6 showed that these signals coalesced around
70 ꢁC into one broad signal. The activation energy of the
isomerization equilibrium was about 66 kcal molꢂ1, and was
estimated from the coalescence temperature and the differ-
ence of these signals at 22 ꢁC. Measuremeꢁnt of the NMR
spectra of 6 at temperatures greater than 70 C was not pos-
sible due to limitation of the instrument, and the averaged
signal of the diastereotopic protons was not observed. The
result suggests that equilibrium between the stereoisomers
FIGURE 1 1H NMR spectra for 4b in CDCl3. Signals marked as ꢃ arise from the solvents.
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SCHEME 2 The proposed isomerization process of 6 (tetrahedral) between the stereoisomers. Ha and Hb are the diastereotopic protons.
as shown in Scheme 2 may exist. Another isomerization
process between dimer and monomer is likely to be possible.
Observation of sharp signals due to the other protons
and carbons of 6 in the NMR spectra at high temperatures
supported the former isomerization process.
lactide C¼¼O may be obstructed by the intermolecular AlꢄꢄꢄO
interactions.18 In particular, if 4a forms an oligomer or poly-
mer this compound would probably not function efficiently
as an initiator. In contrast, the 6-methyl substituent may
electronically and sterically weaken the AlꢄꢄꢄO interactions as
indicated from the result of the ESI-MS spectrum, therefore
forcing the separation of the inactive dimer into active
monomers with lactide coordination in solution. The poly-
merization of L-LA with (rac)-5 proceeded more slowly than
that of D,L-LA, probably because half of the racemic complex
was not active toward the L-isomer of LA (entry 7). Isotactic-
ity, Pm, of the PLA derived from L-LA was 1.00, indicating
that any epimerization process of L-LA to form meso-LA did
not occur in the polymerization process.23
ROP of Lactides
Polymerization of D,L-LA was conducted using a solution
method in toluene under nitrogen atmosphere in which
D,L-LA and the aluminum complex were mixed at a 100:1
ratio. After polymerization, the resulting solution was dis-
solved in a small amount of THF and poured into methanol
to form a PLA precipitate. To remove the aluminum residuals
and pyridine in the polymers, dissolution in THF and precipi-
tation from methanol were repeated several times. Table 1
shows the results of the ROP of lactides initiated by com-
plexes 4a, 4b, and (rac)-5 under several conditions. Interest-
ingly, using 4b and (rac)-5 in toluene at 70 ꢁC gave the
highest yields of PLA after 96 h (Table 1, entries 2 and 6,
respectively), whereas polymerization with 4a afforded PLA
in only a 21% yield (entry 1). In the previously reported sys-
tems initiated by aluminum SALEN complexes, polymeriza-
tion was completed in 12ꢂ24 h.7–9 This longer reaction time
appears to be due to the use of the tridentate ligands in
these aluminum complexes, which easily form dimers
through interactions between aluminum and phenoxide
oxygen (Scheme 3). Such dimers may have negligible activity
toward the ROP of LA, because the initial coordination of the
As an alternative method, we found that using small amount
of pyridine as a solvent dramatically enhanced the activity of
4b and (rac)-5 in the ROP of LA, which was converted into
PLA in 3ꢂ12 h at 70 ꢁC (Table 1, entries 3 and 8, respec-
tively). The reason of this faster conversion is not clear; we
considered some possibilities, such as that the pyridine coor-
dination led to the facile formation of the monomeric com-
plex (Scheme 3) and/or regulated the aluminum geometry
into five-coordinated square pyramidal or trigonal bipyrami-
dal geometry,26 that lactide can coordinate smoothly to form
the ꢁoctahedral geometry. Longer reaction times > 6 h at
70 C with 4b gave PDLLA quantitatively; however, the poly-
dispersity was worse with values of ꢀ1.8ꢂ2.0. The
FIGURE 2 13C NMR spectra for 3 and 6 in CDCl3. Because of the existence of the aluminum chirality, the signals assigned as each
of the four diastereotopic carbon atoms of CC, CC’, CD, or CD’ in 6 were separated independently.
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TABLE 1 Ring-Opening Polymerization of Lactide Initiated by 4a, 4b, (rac)-5 and 6a
ꢁ
Mn/kg molꢂ1b
Theor/kg molꢂ1c
Mw/Mn
Pm
b
d
Entry
Complex
Solvent
Monomer
Temp/ C
Time /h
Yield/%
1
2
3
4
5
6
7
8
9
a
4a
Toluene
Toluene
Pyridine
Pyridine
Pyridine
Toluene
Toluene
Pyridine
Pyridine
D,L-LA
D,L-LA
D,L-LA
D,L-LA
D,L-LA
D,L-LA
L-LA
70
70
96
96
3
21
100
86
9.7
18.9
13.7
11.8
20.0
16.4
10.7
17.2
3.2
3.0
14.4
12.4
10.6
10.9
11.5
8.1
1.09
1.14
1.45
1.38
1.27
1.14
1.13
1.14
1.19
0.76
0.80
0.79
0.87
0.88
0.86
1.00
0.87
0.92
4b
4b
70
4b
50
6
64
4b
25
72
96
96
12
24
76
(rac)-5
(rac)-5
(rac)-5
6
70
80
70
56
D,L-LA
D,L-LA
70
100
28
14.4
4.0
100
Conditions: [monomer]/[Al] ¼ 100/1, [monomer] ¼ 1.73 mol Lꢂ1
.
Pm (meso content) of PLA was calculated from the integrated ratios of
homonuclear decoupled signals at d5.16-5.22 due to tetrads, mmm,
d
b
Determined by size-exclusion chromatography (SEC) in THF, cali-
brated with polystyrene standard (the real Mn of PLA should be reduced
mrm, rmm, mmr, rrm, and mrr of the methine proton.
(0.58 times) from that of PSt).
c
Calculated on the basis of monomer/initiator ratio and monomer
conversion.
polymerization of D,L-LA with 4b could be conducted at
room temperature for 72 h with the formation of PDLLA in
high yield (entry 5). This polymer had a narrower polydis-
persity, 1.27, than the polymers produced by polymerization
Thermal Properties and X-Ray Diffraction Study of
the Stereoblock Copolymer
A DSC curve for poly(D,L-LA) (Mn ¼ 15 kg molꢂ1, Mw/Mn
¼
1.1, Pm ¼ 0.87), which was made with (rac)-5 (1 mol %) at
ꢁ
at 50 and 70 C, 1.38 and 1.45. ROP of LA at room tempera-
70 ꢁC in pyridine, was recordedꢂ1after prior heating to
ture has been shown in Lewis acidic aluminum complexes.27
On the otherꢁ hand, narrow-polydispersity PDLLA was
obtained at 70 C by the reaction with (rac)-5, in which the
Lewis acidity of the aluminum center is lower than that in
4b. The molecular weight of the PDLLA was determined to
be 10ꢂ20 kg molꢂ1 by SEC. The polymerization of D,L-LA
with 6 as an initiator did not proceed at 70 ꢁC but yielded
PDLLA at 100 ꢁC in pyridine (28% after 24 h, Mn ¼ 3.2 kg
molꢂ1, Mw/Mn ¼ 1.19) (entry 9). The low activity of 6 may
be attributed to the bulky cyclohexylene unit in the Schiff-
base ligand which prevents the coordination of LA or the
nucleophilic attack of the isopropoxide to the carbonyl moi-
ety in LA.
250 C at the same rate of 5 K min and slow cooling at a
ꢁ
rate of 2 K minꢂ1 [Fig. 4 (a)]. The melting temperature was
186 ꢁC, which was distinctly higher than that of a PLLA
sample with the similar molecular weight crystallized under
a normal crystallization condition,28 also indicating the for-
mation of the expected stereoblock copolymer from D,L-LA.
Glass transition point was observed around 52 ꢁC, which
agreed well with that in the previous reports for PLLA.29
The broad melting peak, starting from about 137 ꢁC, indi-
cates an inclusion of small size and/or imperfect crystals in
the sample. The formation of the stereocomplex in the
The polydispersities of the obtained PLAs were close to 1.0,
indicating a controlled/living polymerization system. First-
order kinetic plots for lactide polymerization in pyridine at
70 ꢁC [(rac)-5] or 50 ꢁC (4b) and the correlation between
monomer conversion and molecular weight or polydispersity
from SEC are shown in Figureꢁ3(a,b). An induction period
was observed in the ROP at 50 C initiated by 4b, indicating
existence of some activation process involving the formation
of the monomeric active species or the initial reaction with
LA molecules. The polymer weight increased linearly accom-
panied by monomer conversion of (rac)-5, thereby showing
the living nature of the polymerization; however, the poly-
mer weight decreased with 4b when monomer conversion
was high. The decrease in the polymer weight and the
increase in the polydispersity with the conversion also
indicated inter- and/or intramolecular transesterification
of the polymer chain at the low monomer concentration
due to higher Lewis acidity of the aluminum complex, 4b,
than that of 5.
SCHEME 3 Formation of the active monomeric form which
could be generated in the presence of pyridine.
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FIGURE 3 (a) Time versus ln[M0]/[M] plots and (b) conversion versus Mn and Mw/Mn plots with [M0]/[I] ¼ 100. (^: (rac)-5-initiated
ROP of D,L-LA at 70 ꢁC, *: 4b-initiated ROP at 50 ꢁC).
PDLLA was finally confirmed by the powder X-ray analysis.
As shown in Figure 4(b), three peaks, A, B, and C, at 12.11,
20.58, and 23.67ꢁ appeared in the WAXD pattern. Appear-
ance of three intense peaks in the diffraction angle 2y range
of 9–30ꢁ has been reported for powder patterns of stereo-
complex which is composed of PLLA and PDLA.30 The
diffraction pattern, peaks A and B, also agreed with the
reported pattern of the stereoblock copolymer, in the 2y
range of 11–22ꢁ.31 Broadness of the peaks suggests forma-
tion of the small size crystals in this sample. These facts
strongly support the presence of the stereocomplex in the
PDLLA structure.
Stereoselectivity
Investigation of the stereoselectivity in PDLLA initiated by
4a, 4b, (rac)-5, and 6 gave significant information. Table 1
lists the Pm values of the PLA initiated by these complexes
under several conditions and Figure 6 shows the partial
FIGURE 5 Homonuclear-decoupled 1H NMR spectra (400 MHz,
CDCl3, 25 ꢁC, methine region) of PLA generated from (a) (rac)-5
with L-LA (70 ꢁC), (b) 4a with D,L-LA (70 ꢁC), (c) 4b with D,L-LA
(50 ꢁC), (d) (rac)-5 with D,L-LA (70 ꢁC), and (e) 6 with D,L-LA (100
ꢁC). The Pm values of the polymers are shown in the spectra.
FIGURE 4 (a) DSC curve for PDLLA (Mn ¼ 17 kg/mol, Mw/Mn
¼
1.1) by (rac)-5. The temperature was increased at a rate of
5 K minꢂ1. The second scan was recorded. (b) WAXD pattern of
PDLLA with polyimide films. Background peaks of the polyimide
did not affect the pattern.
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tivity was still lower than previous data acquired for SALEN
aluminum complexes.7–9 Such distortion in stereoselectivity
of 4b may be due to two factors: a rather disturbed stereo-
regulation system attributed to the less-bulky tridentate
ligands than bulky tetradentate SALEN ligands; and thermal
interconversion between chiral isomers at the aluminum
center during the polymerization, which is specifically possi-
ble in the aluminum complexes bearing nonchiral tridentate
ligands. Concerning about the former, the ratios of the tet-
rads in the homonuclear decoupled NMR spectra indicate
that the intermolecular alkoxide exchange frequently
occurred in these polymerizations, because the less-bulky
tridentate ligands may allow to form dimer as previously
reported.32 Additionally, the high Lewis acidity of 4b, which
can polymerize LA at room temperature, may cleave the
polymer chain with an alkoxide anion at the low monomer
concentration to form polymers with low molecular weight
[Fig. 3 (b)]. On the other hand, the chiral interconversion at
the aluminum may occur in 6 rather frequently at high poly-
merization temperatures, as indicated in the variable-tem-
perature NMR spectra. However, we could not determine
whether such chiral interconversion in these complexes with
nonchiral ligands also takes place under polymerization con-
ditions or not. Hence, we examined ROP of (L)-LA in the
presence of 4b, (rac)-5, and 6, (monomer/initiator ratio is
30/1) under the suitable conditions for the initiators.
We found that the molecular weights of PLLA were around
4.6–7.5 kg molꢂ1, which agreed with those calculated from
monomer conversion/initiator ratios as shown in Table 2.
However, the polydispersities were larger than those
observed in ROP of D,L-LA, especially for PLLA initiated by
(rac)-5 and 6 (Table 2, entries 1 and 5). Moreover, the SEC
chromatograms of PLLA initiated by 4b at room temperature
showed bimodal distribution, where a shoulder peak was
observed at about 17 min in addition to the dominant signal
at 16 min in contrast to the monomodal SEC distribution of
PDLLA (Mn ¼ 6.6 kg molꢂ1, Mw/Mn ¼ 1.47, Pm ¼ 0.92) initi-
ated by 4b under the same conditions (Fig. 6). That suggests
existence of two different active and less active aluminum
initiators in the reaction mixture, which could be stereoiso-
mers of the aluminum complex, one of them is a suitable iso-
mer to polymerize L-LA and the other is not. This indicates
the possibility of the ESC process proceeding in stereocon-
trolled ROP of D,L-LA using the aluminum complex 4b
FIGURE 6 SEC chromatograms for PLLA (Mn ¼ 6.5 kg molꢂ1
,
Mw/Mn ¼ 1.36) and PDLLA (Mn ¼ 6.6 kg molꢂ1, Mw/Mn ¼ 1.47)
mediated by 4b at room temperature for 72 h (monomer/initia-
tor ratio is 30/1). The bimodal distribution of PLLA shows that
two different initiators, performing different activities toward
L-LA, may exist in the reaction mixture.
1
homonuclear-decoupled H NMR spectra for PLLA [Fig. 5(a)]
ꢁ
and PDLꢁLA made with (c) 4bꢁat 50 C, (b) 4a and (d) (rac)-
5 at 70 C, and (e) 6 at 100 C. The distributions of the sig-
nals around 5 ppm due to tetrads of the methine proton in
PDLLA were similar to that of the isotactic stereoblock
copolymer:7d integrated ratios of three signals due to the
rmm, mmr, mrm tetrads were almost equal, whereas the
integrated ratio due to the rmr tetrad was small. The isotac-
ticity of PDLLA, Pm, made from 4b decreased depending on
the decrease of the polymerization temperature from 70 to
25 ꢁC (Table 1, entries 3–5). The best performance was
observed when 6 was used as the initiator; the Pm value is
0.92 (entry 9). These results suggest that the aluminum
system bearing the tridentate ligand can also provide stereo-
regulated polymerization of D,L-LA. Pyridine rather than
toluene did not affect the isotacticity of PDLLA, concluded
from the similar Pm values of PDLLA polymerized in pyridine
and toluene at the same temperature were similar.
The stereoregulation in PDLLA was tightly controlled with
4b especially at low temperatures. However, the stereoselec-
TABLE 2 Polymerization of L-LA initiated by 4b, (rac)-5, and 6a
Entry
Monomer
Al complex
Temp/ ꢁC
Time/h
Yield/%
Mn/kg molꢂ1b
Theor/kg molꢂ1c
Mw/Mn
1
2
3
4
5
a
L-LA
L-LA
L-LA
L-LA
L-LA
(rac)-5
4b
70
25
20
70
12
7
80
78
92
90
65
5.9 (3.4)
6.5 (3.8)
7.0 (4.1)
7.5 (4.3)
4.6 (2.7)
3.5
3.4
4.0
3.9
2.8
1.26
1.36
1.35
1.37
1.28
4b
50
4b
70
6
110
20
Conditions: [monomer]/[Al] ¼ 30/1, [monomer] ¼ 1.73 mol Lꢂ1
.
Calculated on the basis of monomer/initiator ratio and monomer
conversion.
c
b
Determined by size-exclusion chromatography (SEC) in THF, cali-
brated with polystyrene standard (the real Mn of PLA in brackets was
reduced (0.58 times) from that of PSt).
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P.; Baker, G. L.; Smith III, M. R. J. Am. Chem. Soc. 2000, 122,
1552–1553; (c) Ovitt, T. M.; Coates, G. W. J. Polym. Sci. Part A:
Polym. Chem. 2000, 38, 4686–4692; (d) Ovitt, T. M.; Coates, G.
W. J. Am. Chem. Soc. 2001, 124, 1316–1326; (e) Zhong, Z.; Dijk-
stra, P. J.; Feijen, J. Angew. Chem. Int. Ed. 2002, 41,
4510–4513; (f) Zhong, Z.; Dijkstra, P. J.; Feijen, J. J. Am. Chem.
Soc. 2003, 125, 11291–11298; (g) Hormnirun, P.; Marshall, E. L.;
Gibson, V. C.; White, A. J. P.; Williams, D. J. J. Am. Chem.
Soc. 2004, 126, 2688–2689; (h) Chisholm, M. H.; Gallucci, J. C.;
Quisenberry, K. T.; Zhou, Z. Inorg. Chem. 2008, 47, 2613–2624;
(i) Du, H.; Velders, A. H.; Dijkstra, P. J.; Sun, J.; Zhong, Z.;
Chen, X.; Feijen, J. Chem. Eur. J. 2009, 15, 9836–9845.
bearing nonchiral ligand. However, the real active structures
of the aluminum complexes in the reaction mixture are still
not clear. Additionally, undefined microstructures in the NMR
spectra for PDLLA initiated by 5 and 6 were detected, and
further studies examining the structures of the polymers is
required to clarify in more detail the polymerization
mechanism.
CONCLUSIONS
In summary, we have synthesized a series of aluminum alk-
oxides bearing nonchiral or chiral tridentate Schiff-bases,
half-SALEN ligands and achieved controlled stereoselective
polymerization of D,L-LA using these aluminum complexes.
In particular, the Pm value of the PDLLA initiated by 6 was
>0.9, whereas 4b initiated the ROP of D,L-LA efficiently even
at room temperature in a pyridine solution with high stereo-
selectivity. The behavior of these aluminum complexes in
solution, including inversion of the chirality at the aluminum
center, was investigated. The detailed identification of PDLLA
showed the isotactic stereoblock copolymers generated in
these processes. These results indicate the significance of
chiral aluminum centers in stereocontrolled polymerization.
Further studies on detailed identification of the polymers
and new initiator performing higher stereoselectivity, based
on the concept of a racemic metal complex with chirality at
the metal center, are currently in progress.
8 (a) Nomura, N.; Ishii, R.; Yamamoto, Y.; Kondo, T. Chem.
Eur. J. 2007, 13, 4433–4451; (b) Nomura, N.; Ishii, R.; Akakura,
M.; Aoi, K. J. Am. Chem. Soc. 2002, 124, 5938–5939.
9 (a) Tang, Z.; Yang, Y.; Pang, X.; Hu, J.; Chen, X.; Hu, N.; Jing,
X. J. Appl. Polym. Sci. 2005, 98, 102–108; (b) Pang, X.; Du, H.;
Chen, X.; Zhuang, X.; Cui, D.; Jing, X. J. Polym. Sci. Part A:
Polym. Chem. 2005, 43, 6605–6612; (c) Tang, Z.; Pang, X.; Sun,
J.; Du, H.; Chen, X.; Wang, X.; Jing, X. J. Polym. Sci. Part A:
Polym. Chem. 2006, 44, 4932–4938; (d) Bouyahyi, M.; Grunova,
E.; Marquet, N.; Kirillov, E.; Thomas, C. M.; Roisnel, T.; Car-
pentier, J.-F. Organometallics 2008, 27, 5815–5825; (e) Du, H.;
Velders, A. H.; Dijkstra, P. J.; Zhong, Z. Macromolecules 2009,
42, 1058–1066; (f) Buffet, J.-C.; Okuda, J. Chem. Commun.
2011, 47, 4796–4798.
10 (a) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T.
M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001,
123, 3229–3238; (b) Jensen, T. R.; Breyfogle, L. E.; Hillmyer, M.
A.; Tolman, W. B. Chem. Commun. 2004, 2504–2505.
The authors thank to Masahiko Hayashi in Kobe University,
Japan for giving useful advises concerning the Schiff-base
ligands and complexes. This study was financially supported
in part by a grant for the scientific group research in Fukuoka
University (No. 075006).
11 Russell, S. K.; Gamble, C. L.; Gibbins, K. J.; Juhl, K. C. S.;
Mitchell, W. S., III; Tumas, A. J.; Hofmeister, G. E. Macromole-
cules 2005, 38, 10336–10340.
12 Marshall, E. L.; Gibson, V. C.; Rzepa, H. S. J. Am. Chem.
Soc. 2005, 127, 6048–6051.
13 Chisholm, M. H.; Patmore, N. J.; Zhou, Z. Chem. Commun.
2005, 127–129.
REFERENCES AND NOTES
14 Chisholm, M. H.; Lin, C.-C.; Gallucci, J. C.; Ko, B.-T. Dalton
Trans 2003, 406–412.
1 (a) O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. J. Chem. Soc.
Dalton Trans. 2001, 2215–2224; (b) Dechy-Cabaret, O.; Martin-
Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147–6176; (c) Wu,
J.; Yu, T.-L.; Chen, C.-T.; Lin, C.-C. Coord. Chem. Rev. 2006, 250,
602–626; (d) Amgoune, A.; Thomas, C. M.; Carpentier, J.-F. Pure
Appl. Chem. 2007, 79, 2013–2030.
15 Huang, C.-H.; Wang, F.-C.; Ko, B.-T.; Yu, T.-L.; Lin, C.-C.
Macromolecules 2001, 34, 356–361.
16 (a) Iwasa, N.; Fujiki, M.; Nomura K. J. Mol. Cat. A: Chem.
2008, 292, 67–75; (b) Pappalardo, D.; Annunziata, L.; Pellecchia
C. Macromolecules 2009, 42, 6056–6062.
2 (a) Uhrich, K. E.; Cannizzaro, S. M.; Langer, R. S.; Shakesheff,
K. M. Chem. Rev. 1999, 99, 3181–3198; (b) Liu, S.; Maheshwari,
R.; Kiick, K. L. Macromolecules 2009, 42, 3–13.
17 Silvestri, A.; Grisi, F.; Milione, S. J. Polym. Sci. Part A:
Polym. Chem. 2010, 48, 3632–3639.
18 (a) Doherty, S.; Errington, R. J.; Housley, N.; Clegg, W.
Organometallics 2004, 23, 2382–2388; (b) Tsai, Y.-H.; Lin, C.-H.;
Lin, C.-C.; Ko, B.-T. J. Polym. Sci. Part A: Polym. Chem. 2009,
47, 4927–4936; (c) Piedra-Arroni, E.; Brignou, P.; Amgoune, A.;
Guillaume, S. M; Carpentier, J.-F.; Bourissou, D. Chem. Com-
mun. 2011, 47, 9828–9830; (d) Mazzeo, M.; Lamberti, M.;
D’Auria, I.; Milione, S.; Peters, J. C.; Pellecchia, C. J. Polym.
Sci. Part A: Polym. Chem. 2010, 48, 1374–1382.
3 Rokkanen, P.; Bostman, O.; Hirvensalo, E. In Biodegradable
Implants in Fracture Fixation; Leung, K. S.; Hung, K. L.; Leung,
P. C., Eds.; World Scientific: Singapore, 1994; Chapter 4,
pp 189–192.
4 (a) Ikada, Y. In Tissue Engineering for Therapeutic Use 1, Y.
Ikada, Y. Yamaoka, Eds.; American Chemical Society, Washing-
ton DC, 1998, pp 1–14; (b) Ikada, Y. In Wound Close Biomateri-
als and Devices; Chu, C. C.; von Franhofer, L. A.; Greisler, H. P.,
Eds.; CRC: Boca Raton, 1992, pp 317–346; (c) Melchels, F. P.
W.; Feijen, J.; Grijpma, D. W. Biomaterials 2009, 30, 3801–3809.
19 (a) Labourdette, G.; Lee, D. J.; Patrick, B. O.; Ezhova, M. B.
Organometallics 2009, 28, 1309–1319; (b) Darensbourg, D. J.;
Karroonnirun, O. Inorg. Chem. 2010, 49, 2360–2371.
5 (a) Drumright, R. E.; Gruber, P. R.; Henton, D. E. Adv. Mater.
2000, 12, 1841–1846; (b) Chiellini, E.; Solaro, R. Adv. Mater.
1996, 8, 305–313.
20 Grunova, E.; Kirillov, E.; Roisnel, T.; Carpentier, J.-F. Dalton
Trans. 2010, 39, 6739–6752.
21 Coates, G. W. Chem. Rev. 2000, 100, 1223–1252.
6 (a) Thomas, C. M. Chem. Soc. Rev. 2010, 39, 165–173; (b)
Stanford, M. J.; Dove, A. P. Chem. Soc. Rev. 2010, 39, 486–494.
22 The chiral versions of tridentate Schiff-base ligands are
known to be useful in stereoselective oxidation and addition
reactions, for example, see. (a) Oguni, N.; Miyagi, Y.; Itoh, K.
7 (a) Spassky, N.; Wisniewski, M.; Pluta, C.; Le Borgne, A.
Macromol. Chem. Phys. 1996, 197, 2627–2637; (b) Radano, C.
WWW.MATERIALSVIEWS.COM
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965

JOURNAL OF
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ꢀ
Tetrahedron Lett. 1998, 39, 9023–9026; (b) Flores-Lopez, L. Z.;
Parra-Hake, M.; Somanathan, R.; Walsh, P. J. Organometallics
2000, 19, 2153–2160.
28 Yasuniwa, M.; Iura, K.; Dan, Y. Polymer 2007, 48, 5398–5407.
29 Tsuji, H. In Biopolymers, Polyesters III: Applications and
Commercial Products. Biopolymers, vol.4.; Doi, Y.; Steinbu-
chel, A., Eds.; Wiley-VCH: Weinheim, 2002, Chapter 5, pp 129–
178.
23 Darensbourg, D. J.; Karroonnirun, O. Organometallics 2010,
29, 5627–5634.
24 Dey, A.; Kirchner, M. T.; Vangala, V. R.; Desiraju, G. R.; Mondal,
R.; Howard, J. A. K. J. Am. Chem. Soc. 2005, 127, 10545–10559.
30 (a) Brizzolara, D.; Cantow, H.-J.; Diederichs, K.; Keller, E.;
Domb, A. J. Macromolecules 1996, 29, 191–197; (b) Rahman,
N.; Kawai, T.; Matsuba, G.; Nishida, K.; Kanaya, T.; Watanabe,
H.; Okamoto, H.; Kato, M.; Usuki, A.; Matsuda, M.; Nakajima,
K.; Honma, N. Macromolecules 2009, 42, 4739–4745, and refer-
ences cited therein.
25 Many examples of five-coordinated aluminum compounds
are known in SALEN aluminum complexes, for example, Mitra,
A.; DePue, L. J.; Parkin, S.; Atwood, D. A. J. Am. Chem. Soc.
128, 1147–1153.
26 Hayashi, M.; Miyamoto, Y.; Inoue, T.; Oguni, N. J. Org.
Chem. 1993, 58, 1515–1522.
31 Li, L.; Zhong, Z.; de Jeu, W. H.; Dijkstra, P. J.; Feijen, J. Mac-
romolecules 2004, 37, 8641–8646.
27 Bhaw-Luximon1, A.; Jhurry1, D.; Spassky N. Polym. Bull.
2000, 44, 31–38.
32 Lewinski, J.; Horeglad, P.; Wo´ jcik, K.; Justyniak, I. Organo-
metallics 2005, 24, 4588–4593.
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