Synthesis of biodegradable and biorenewable polylactides initiated by aluminum complexes bearing: M -xylylenediamine derivatives via the ring-opening polymerization of lactides

Article abstract of DOI:10.1039/c9nj01268k

A series of aluminum complexes bearing [ONNO]-type ligands were prepared using m-xylylenediamine and modified salicylaldehyde. These complexes were characterized by NMR spectroscopy and elemental analysis. These complexes were employed in l-lactide and ra

Full text of DOI:10.1039/c9nj01268k

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Synthesis of biodegradable and biorenewable
polylactides initiated by aluminum complexes
bearing m-xylylenediamine derivatives via the
ring-opening polymerization of lactides†
Cite this:DOI: 10.1039/c9nj01268k
Dongni Li,ab Bo Gao*acd and Qian Duan*ac
A series of aluminum complexes bearing [ONNO]-type ligands were prepared using m-xylylenediamine
and modified salicylaldehyde. These complexes were characterized by NMR spectroscopy and elemental
analysis. These complexes were employed in L-lactide and rac-lactide polymerization. Upon activation
with isopropanol, complex 3 showed the highest activity (monomer conversion 97.4%) amid these
aluminum complexes for the ring-opening polymerization of L-lactide and complex 2 showed the highest
stereoselectivity for the ring-opening polymerization of rac-lactide providing isotactic polylactide (PLA)
with a Pm of 0.70. The polymerization kinetics utilizing 2 as an initiator was researched in detail. The
kinetics of the polymerization data revealed that the rate of polymerization was first-order with respect to
the monomer and the initiator. There was a linear relationship between the L-lactide conversion and the
number-average molecular weight of PLA.
Received 11th March 2019,
Accepted 3rd April 2019
DOI: 10.1039/c9nj01268k
rsc.li/njc
textiles and packaging.2 Polylactide is usually synthesized by
ring-opening polymerization (ROP) of lactide, which is initiated
Introduction
Today, non-renewable petroleum-based materials are widely by metal complexes such as tin,3 aluminum,4 zinc,5 magnesium,6
used in our daily lives because of their low cost of synthesis iron,7 titanium,8 indium9 and rare-earth metals.10 Aluminum is
and processing. However, with the gradual depletion of fossil an effective initiator for the synthesis of PLA through ROP,
resources, the application of petroleum-based polymers has because it efficiently influences the microstructure of the polymer
seriously polluted our environment, and more and more atten- by ancillary ligands.4,11 In the past twenty years, a lot of efforts
tion to environmental protection has led to the search for were made on the ROP of rac-lactide (rac-LA) through a stereo-
sustainable and environmentally friendly polymers that can selective initiator containing an ancillary ligand to obtain PLA
replace non-renewable petroleum-based polymers.1 Starting having high stereoregularity. Many research groups have
from corn or sugar beets, biodegradable, biocompatible and attempted to clarify the relationship between the stereo-
biorenewable polylactide (PLA) is becoming one of the most regularity of poly(rac-lactide) and the aluminum complex based
promising alternatives to petroleum-based polymers.1a,b PLA on salen-type Schiff bases11,12 (Fig. 1). Among them, Spassky
has been widely used in many fields such as fracture fixation et al. found an aluminum initiator supported by a salen-type
devices, blood transfusion devices, sutures, controlled release Schiff base ligand prepared from R-(+)-1,10-dinaphthalene-2,20-
drug carriers, tissue engineering stents, disposal containers, diamine, which could highly stereo-control the polymerization
of rac-LA. The PLA’s Tm is higher than the Tm of PLLA with
optical purity.11a Coates’ team found that aluminum complexes
a School of Materials Science and Engineering, Changchun University of Science and
with Schiff bases provide enriched isotactic PLA.11b Nomura
Technology, 7989 Weixing Road, Changchun 130022, China.
and Ishii have focused on the effects of the backbone that
E-mail: gaobo@ciac.ac.cn, duanqian88@hotmail.com; Fax: +86-431-85583015;
connects the two Schiff bases and the substituent effects of the
Tel: +86-431-85583015
salicylidene moieties.13 Feijen’s group utilized a racemic and
homochiral bulky Jacobsen ligand–Al complex to obtain a
polymer with a Tm of around 185 1C.14
b Department of Blood Transfusion, China–Japan Union Hospital of Jilin University,
Jilin University, 126 Xiantai Street, Changchun, 130033, China
c Engineering Research Center of Optoelectronic Functional Materials,
Ministry of Education, Changchun 130022, China
In recent years, we have studied a number of mononuclear
and binuclear aluminum complexes based on the Schiff base
ligands. These complexes proved to be highly efficient single
point initiators in the ROP of L-LA and rac-LA. And these
d Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9nj01268k
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Scheme 2 Preparation of aluminum complexes 1–4.
1
characterized by H, 13C{1H} NMR spectroscopy and elemental
analysis. The characteristic signals, singlets at d 8.45, 8.48
and 8.36 ppm, were assigned to the protons of –NQCH– in
pro-ligands a, b and c.
Fig. 1 Initiators for stereoselective ROP of rac-lactide.
Preparation of mononuclear aluminum complexes 1, 2 and 3
As shown in Scheme 2, aluminum complexes 1–3 were prepared
in high yields by reaction of commensurable trimethyl-
aluminum with the corresponding pro-ligands under nitrogen
and were isolated as a yellow or orange solid from toluene in
83.7–92.3% yield.
Preparation of binuclear aluminum complexes 4
As shown in Scheme 2, aluminum complex 4 was prepared by
reaction of 2 eq. trimethyl-aluminum and 1 eq. pro-ligand b
under nitrogen and a yellow solid was isolated from toluene in
Scheme 1 Preparation of pro-ligands a, b and c.
aluminum complexes can modulate the microstructure of the 89.3% yield.
PLA, inducing the formation of isotactic preferences in the ROP
All aluminum complexes were characterized by NMR
spectroscopy and elemental analysis. The 1H and 13C{1H}
of rac-LA.12
However, as far as we know, few have investigated salen type NMR spectra of mononuclear aluminum complexes 1, 2 and
Schiff bases derived from m-xylylenediamine derivatives as 3 showed one ligand and a single aluminum atom in these
bridging segments (Schemes 1 and 2) and the role of their aluminum complexes, and similar resonances in the region of
metal complexes in the ROP of LA was studied. With the ꢀ0.63 to ꢀ1.01 ppm for the methyl protons of the Al–CH3
encouragement of the effective utilization of aluminum group. Meanwhile, compared with the pro-ligands’ hydroxyl
complexes containing Schiff-bases,12 we hold that the aluminum characteristic peaks of 13.36–14.36 in the low field, there
complex bearing salen type Schiff-bases containing a bulky were no corresponding signals in the mononuclear aluminum
m-xylylenediamine derivative is a potential initiator to well complexes, such as complex 2 (Fig. 2), which can be observed in
control the ROP of L-LA and rac-LA. We were curious to study the 1H NMR spectrum. Interestingly, the 1H NMR spectra of 1, 2
the catalytic properties of aluminum complexes containing and 3 revealed that the –NCH2 groups of the ligands in these
m-xylylenediamine derivatives. In this paper, the preliminary complexes were inequivalent. For example, two groups of
experimental data of Salen aluminum complexes based on resonances from –NCH2 (5.37 and 4.90 ppm) were observed
Schiff-base ligands containing m-xylylenediamine derivatives in the 1 H NMR spectrum (7 + 70 in Fig. 2). It appeared that the
were reported, which were used as initiators to obtain polylactide geometry of 2 in solution was ‘‘b’’ rather than ‘‘a’’ (Fig. S1,
by the ROP of lactides.
ESI†), i.e. the aluminum atom in 2 exhibits a trigonal bipyr-
amidal geometry and the two N donors occupied axial and
equatorial positions. Similar configurations have also been
seen in other aluminium complexes.16
Additionally, we used the representative pro-ligand b bulky
group to react with 2 eq. trimethylaluminum to obtain the
Results and discussion
Preparation of pro-ligands a, b and c
As shown in Scheme 1, pro-ligands a, b and c were prepared expected dinuclear complex 4. The 1H NMR spectra of complex
in good yields (73.6–87.7%) by condensation reaction of 4 showed that the two central Al atoms were in an equivalent
m-xylylenediamine and modified salicylaldehydes in absolute state, and ꢀ0.94 ppm in the high field (‘8’ in Fig. 2) belonged
ethanol according to the literature.15 All pro-ligands were to the proton of the methyl group attached to the aluminum.
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Fig. 3 Representative GPC traces (Table 1, entries 2 and 4).
which meant that the polymerization was well-controlled. For
mononuclear complexes, as the substituent on the phenyl group
increased in volume, the activity of these complexes decreased
while the electron-withdrawing substituent increases the rate of
polymerization. Complex 3 showed the highest activity in the
mononuclear complexes under the same reactive conditions
(Table 1, entries 1–4). The higher electronegativity of the substitu-
Fig. 2 Stacked figure of the representative 1H NMR spectra of pro-ligand b,
complex 2 and complex 4.
Only one singlet from ꢀNCH2 (4.71 ppm) was observed in the ents on the ligand may increase the Lewis acidity of the aluminium
1H NMR spectrum (7 + 70 in Fig. 2). This chemical phenomenon centre, thereby increasing the reaction rate. A similar situation has
also occurred in the dinuclear aluminum complex as reported been reported previously.19 In contrast to mononuclear complex 2,
by Yao and Wang.17
binuclear aluminum complex 4 showed a higher activity, which
may be due to the reduction of steric hindrance, which accelerated
chain propagation during the polymerization process.
ROP of L-LA and rac-LA
Four aluminum complexes were evaluated as initiators for the
The homonuclear decoupled 1H NMR spectrum of the
ROP of L-LA or rac-LA. The polymerization was carried out in methine part20 of a typical poly(rac-LA) (Table 1, entry 13)
21
toluene or tetrahydrofuran and the representative polymeriza- (Fig. 4) was obtained. The Pm value, 0.70, indicated that
tion data are shown in Table 1. These aluminum complexes isotactic multi-block polymers were obtained. The experimental
showed moderate to high activity by co-catalysis of isopropanol results showed that the Pm selectivities increased from 0.52 to
at 70 1C (monomer conversion of 48.6–97.4%). The Mn value of 0.62 due to the increase of the substituent group on the phenyl
PLA was calculated using 1H NMR and GPC. The number ring from hydrogen atoms to tert-butyls (Table 1, entries 8 and
average molecular weight (of GPC18) of all polymers is close 9). The reaction temperature had a significant effect on the
to the theoretical molecular weight (calculated from the mono- stereoscopic regularity of the poly(rac-LA). When the tempera-
mer/Al molar ratio); the Ð values of PLA were relatively narrow ture reduced from 70 to 30 1C, the Pm increased from 0.62 to
(1.08–1.22), such as 1.08 and 1.11 (Table 1, entries 2 and 4, Fig. 3), 0.70 (Table 1, entries 9, 12 and 13). Binuclear aluminum
Table 1 Polymerization data of LA employing complexes 1–4a
c
d
d
e
Entry Complex monomer T (1C) t (h) [LA]0/[Al]0 Conv.b (%) Mn(calcd) ꢁ 10ꢀ4 MnGPC ꢁ 10ꢀ4 Mn ꢁ 10ꢀ4 Ðd
kapp (hꢀ1
)
Pm
1
2
3
4
5
6
7
8
1
2
3
4
2
2
2
1
2
3
4
2
2
L-LA
L-LA
L-LA
L-LA
L-LA
L-LA
L-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
70
70
70
70
70
70
70
70
70
70
70
50
30
12
12
12
12
20
12
10
12
12
12
12
14
20
100
100
100
100
100
75
68.7
53.3
77.5
60.3
97.4
92.1
94.7
62.0
51.7
75.8
57.9
48.6
51.3
0.99
0.77
1.12
0.87
1.40
0.99
0.68
0.89
0.74
1.09
0.83
0.70
0.74
1.74
1.33
1.95
1.50
2.40
1.68
1.19
1.55
1.29
1.92
1.43
1.22
1.30
1.01
0.77
1.13
0.87
1.39
0.97
0.69
0.90
0.75
1.11
0.83
0.71
0.75
1.20 n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
0.52
0.62
0.55
—
1.08 n.a.
1.15 n.a.
1.11 n.a.
1.09 0.1703
1.08 0.2280
1.08 0.3388
1.22 n.a.
1.10 n.a.
1.19 n.a.
1.09 n.a.
1.10 n.a.
1.08 n.a.
50
100
100
100
100
100
100
9
10
11
12
13
0.67
0.70
a
The polymerization reactions were carried out in toluene solution except that some reactions were carried out in THF at 30 1C, [LA]0 = 0.5 mol Lꢀ1
,
[isopropanol]/[Al] = 1.0. Measured by 1H NMR. Calculated from the molecular weight of LA ꢁ [LA]0/[Al]0 ꢁ conversion + Mwisopropanol
.
Obtained
b
c
d
from GPC analysis and calibrated against the polystyrene standard. The true value of number-average molecular weights could be calculated
18 e
2
according to formula Mn = 0.58MnGPC
.
Pm is the probability of meso linkages, according to [mmm] = Pm + (1 ꢀ Pm)Pm/2, [mmr] = [rmm] =
(1 ꢀ Pm)Pm/2, [rmr] = (1 ꢀ Pm)2/2, [mrm] = [(1 ꢀ Pm)2 + Pm (1 ꢀ Pm)]/2.12c
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Fig. 4 Homonuclear decoupled 1H NMR spectrum of the methine part of
poly(rac-LA) by complex 2 at 25 1C, Pm = 0.70, Table 1, entry 13, 500 MHz,
CDCl3.
Fig. 6 The relationship between Mn or Ð of the polymer and L-LA con-
version employing complex 2/isopropanol, [LA]0/[Al]0 = 100, at 70 1C in
toluene.
complex 4 in the ROP of rac-LA with isopropanol could control
well the molecular weight but not the selectivity in the poly-
merization (Table 1, entry 11; Fig. S2, ESI†).
The kinetics of ROP of L-LA under some conditions such as
monomer/initiator ratios were researched in toluene employing 2.
The molecular weight (Mn) of the polymers increased linearly
accompanied by the increase of the monomer conversion and
the Ð values of these polymers were narrow (1.05–1.09), this
illustrated the living characteristic of the catalytic systems (Fig. 6).
In order to calculate the order in the initiator, kapp was plotted
versus the concentration of 2 (Fig. 5 and 7). Under each condition,
the first-order kinetics in a monomer was researched (Fig. 5). So the
ROP of LA by 2 speculatively complied with the formula:
ꢀd[LA]/dt = kapp[LA]
(1)
where kapp = kp[Al]x and kp is the propagation speed constant. As
shown in Fig. 6, kapp is proportional to the concentration of 2,
which means that the LA polymerization initiated by 2 is also a
first-order reaction for the initiator. So the ROP of LA by 2
followed the entire kinetic equation:
Fig. 7 kapp versus the [Al]0 of 2/the isopropanol initiator for the LA
polymerization at 70 1C in toluene ([LA]0 = 0.5 mol Lꢀ1; kp = kapp/[Al]0;
L-LA, [Al]0 = 0.005 mol Lꢀ1, kapp = 0.1703 hꢀ1; L-LA, [Al]0 = 0.0067 mol Lꢀ1
kapp = 0.2280 hꢀ1; L-LA, [Al]0 = 0.010 mol Lꢀ1, kapp = 0.3388 hꢀ1).
,
ꢀd[LA]/dt = kp[Al][LA]
(2)
For studying the initiation mechanism, end-group analysis
of the oligomers of L-LA, which were prepared by the ROP of the
L-LA at a low monomer to initiator ratio ([LA]t : [2]t = 20 : 1)
(Fig. 8), was performed by means of 1H NMR. It has been demon-
strated that the integral ratio of the two peaks at d 1.24 ppm which
Fig. 5 Kinetics of the ROP of L-LA by 2 with isopropanol at 70 1C in
toluene with [LA]0 = 0.5 mol Lꢀ1; kp = kapp/[Al]0. ’: L-LA, [LA]0/[Al]0 = 100,
kapp = 0.1703 hꢀ1; m: L-LA, [LA]0/[Al]0 = 75, kapp = 0.2280 hꢀ1;K: L-LA,
Fig. 8 1H NMR spectrum of a polymer sample obtained from the complex
2 system with [LA]t : [2]t = 20 : 1.
[LA]0/[Al]0 = 50, kapp = 0.3388 hꢀ1
.
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was assigned to the methyl protons from the isopropoxycarbonyl (m, 4H, ArH), 7.05 (d, J = 2.4 Hz, 2H, ArH), 4.78 (s, 4H, NCH2),
end-group and at d 4.34 ppm which was assigned to the methine 1.43 (s, 18H, C(CH3)3). 13C{1H} NMR (125 MHz, chloroform-d,
proton bonded to the hydroxyl end-group was close to 6 : 1. This 25 1C): d 166.76 (s, 2C, NQCH), 158.02 (ArC), 140.03 (ArC),
revealed that the aggregated chains were end-capped with an 138.78 (ArC), 136.67 (ArC), 128.92 (ArC), 127.39 (ArC), 127.01
isopropyl ester and a hydroxyl group,22 that is to say, the alkyl (ArC), 126.86 (ArC), 126.01 (ArC), 117.84 (ArC), 63.18 (2C, NCH2),
aluminum complex had become isopropoxy aluminum species at 35.01 (2C, C(CH3)3), 34.11 (2C, C(CH3)3), 29.40 (6C, C(CH3)3),
the start of the polymerization, so the actual initiator was the 29.14 (6C, C(CH3)3). Anal. calcd for C38H52N2O2 (%): C, 80.24; H,
isopropoxy aluminum species. The ROP may involve a coordination 9.21; N, 4.92. Found: C, 80.27; H, 9.24; N, 4.90. HRMS (m/z): calcd
insertion mechanism (Fig. S3, ESI†).13d,23
for C38H52N2O2: 568.40; found: 568.40 [M + H]+.
Pro-ligand c. The synthesis of pro-ligand c was similar to that
of pro-ligand a, using m-xylylenediamine (0.136 g, 1.00 mmol) and
3,5-di-3,5-dichlorosalicylaldehyde (0.382 g, 2.00 mmol). The product
Experimental
General considerations
1
was isolated as an orange solid. Yield: 0.421 g, 87.7%. H NMR
(500 MHz, chloroform-d, 25 1C) d 14.36 (bs, 2H, OH), 8.36 (s, 2H,
NQCH), 7.42 (t, J = 2.1 Hz, 2H, ArH), 7.37 (m, 1H, ArH), 7.27
(m, 1H, ArH), 7.25 (s, 1H, ArH), 7.22 (s, 1H, ArH), 7.19 (t, J =
2.1 Hz, 2H, ArH), 4.84 (s, 4H, NCH2). 13C{1H} NMR (125 MHz,
chloroform-d, 25 1C): d 169.85 (s, 2C, NQCH), 162.11 (ArC),
139.20 (ArC), 133.52 (ArC), 132.33 (ArC), 129.77 (ArC), 128.02
(ArC), 127.34 (ArC), 119.32 (ArC), 119.56 (ArC), 118.32 (ArC),
63.28 (2C, NCH2). Anal. calcd for C22H16Cl4N2O2 (%): C, 54.80;
H, 3.34; N, 5.81. Found: C, 54.77; H, 3.37; N, 5.83. HRMS (m/z):
calcd for C22H16Cl4N2O2: 480.00; found: 480.00 [M + H]+.
All air-sensitive operations were performed in a glovebox or a
Schlenk line. 1H NMR and 13C{1H} NMR were performed using
a Bruker AV 500M apparatus at 25 1C in CDCl3 for compounds
and macromolecules. The CDCl3 used to obtain the NMR
spectra of aluminium complexes was dried over CaH2 before
being used. The monomer conversions were confirmed and
the Pm values were calculated according to the literature.1–3
Gel permeation chromatography (GPC) measurements were
conducted using a Waters 515 GPC with CHCl3 as the mobile
phase (flow rate: 1 mL minꢀ1, at 35 1C). The molecular weight
was calibrated using the PS standard. Elemental analysis was
performed using a Varian EL microanalyzer. The L-LA and
rac-LA were purchased from Aldrich and were recrystallized
three times in dry EtOAc under N2. AlMe3, salicylaldehyde,
m-xylylenediamine, 3,5-dichlorosalicylaldehyde, isopropanol,
and 3,5-di-tert-butylsalicylaldehyde were obtained from Aldrich
and applied without further purification.
Synthesis of complexes
Complex 1. A mixture of a (0.344 g, 1.00 mmol) and AlMe3
(1.00 M in toluene, 1.00 mL, 1.00 mmol) in 15 mL toluene was
stirred for 10 h at 25 1C under an argon atmosphere. And it was
concentrated to 2 mL to afford a yellow powder, from which the
mother liquor was decanted, and the product was washed with
about 0.5 mL of hexane and dried under vacuum. The product
1
was isolated as a yellow solid. Yield: 0.497 g, 83.7%. H NMR
Synthesis of pro-ligands
(500 MHz, chloroform-d, 25 1C): d 8.32 (bs, 2H, NQCH), 7.91
Pro-ligand a. A mixture of m-xylylenediamine (0.136 g, (m, 2H, ArH), 7.81 (s, 1H, ArH), 7.37 (s, 2H, ArH), 7.26 (m, 2H,
1.00 mmol), salicylaldehyde (0.244 g, 2.00 mmol) and a catalytic ArH), 7.17 (dd, J = 12.6, 7.2 Hz, 1H, ArH), 6.93 (m, 2H, ArH), 6.54
quantity of formic acid in absolute ethanol (50 mL) was (m, 2H, ArH), 5.11 (s, 2H, NCH2), 4.83 (s, 2H, NCH2), ꢀ1.01
refluxed for 10 h. After solvent evaporation at reduced pressure, (s, 3H, AlCH3). 13C{1H} NMR (125 MHz, chloroform-d, 25 1C)
the crude product was purified by flash chromatography on d 171.70 (1C, NQCH), 160.84 (1C, NQCH), 146.47 (ArC), 142.50
silica gel with petroleum ether/acetic ether (V1/V2 = 12/1) and (ArC), 139.68 (ArC), 135.94 (ArC), 134.79(ArC), 134.75 (ArC),
1% NEt3 as eluents, affording 0.258 g of a yellow solid of the 129.22 (ArC), 128.46 (ArC), 117.39 (ArC), 67.21 (1C, NCH2),
product in 75.2% isolated yield. 1H NMR (500 MHz, chloro- 55.87 (1C, NCH2), ꢀ9.42 (1C, AlCH3). Anal. calcd for
form-d, 25 1C): d 13.36 (bs, 2H, OH), 8.45 (s, 2H, NQCH), 7.39–
C23H21AlN2O2 (%): C, 71.86; H, 5.51; N, 7.29. Found: C, 71.82;
7.22 (m, 8H, ArH), 6.97 (d, J = 8.3 Hz, 2H, ArH), 6.89 (t, J = H, 5.53; N, 7.25.
7.5 Hz, 2H, ArH), 4.81 (s, 4H, NCH2). 13C{1H} NMR (125 MHz,
Complex 2. Complex 2 as a yellow solid was obtained in a
chloroform-d, 25 1C) d 165.77 (s, 2C, NQCH), 161.07 (ArC), similar way to 1 by stirring pro-ligand b (0.568 g, 1.00 mmol)
138.67 (ArC), 132.41 (ArC), 131.49 (ArC), 129.08 (ArC), 127.17 and AlMe3 (1.00 mmol) for 12 h at 70 1C. Yield: 0.549 g, 90.2%.
(ArC), 126.82 (ArC), 118.81 (ArC), 118.67 (ArC), 117.04 (ArC), 1H NMR (500 MHz, chloroform-d, 25 1C) d 7.93 (bs, 2H, NQCH),
63.16 (2C, NCH2). Anal. calcd for C22H20N2O2 (%): C, 76.72; H, 7.40 (d, J = 10.8 Hz, 2H, ArH), 7.28 (m, 3H, ArH), 7.16 (dd, J =
5.85; N, 8.13. Found: C, 76.77; H, 5.82; N, 8.16. HRMS (m/z): 12.9, 7.4 Hz, 1H, ArH), 6.86 (bs, 2H, ArH), 5.37 (s, 2H, NCH2),
calcd for C22H20N2O2: 344.15; found: 344.20 [M + H]+.
4.90 (s, 2H, NCH2), 1.42 (d, J = 34.7 Hz, 18H, C(CH3)3), 1.15 (s,
Pro-ligand b. The synthesis of pro-ligand b was similar to 18H, C(CH3)3), ꢀ0.63 (s, 3H, AlCH3). 13C{1H} NMR (125 MHz,
that of pro-ligand a, using m-xylylenediamine (0.136 g, 1.00 mmol) chloroform-d) d 172.04 (s, 1C, NQCH), 161.58 (s, 1C, NQCH),
and 3,5-di-tert-butylsalicylaldehyde (0.468 g, 2.00 mmol). The 140.37 (ArC), 138.91 (ArC), 135.93 (ArC), 132.03 (ArC), 129.95
product was isolated as a yellow solid. Yield: 0.418 g, 73.6%. (ArC), 129.80 (ArC), 129.41 (ArC), 128.80 (ArC), 118.09 (ArC),
1H NMR (500 MHz, chloroform-d, 25 1C) d 13.64 (bs, 2H, OH), 58.81 (1C, NCH2), 58.67 (1C, NCH2), 35.26 (2C, C(CH3)3), 34.05
8.48 (s, 2H, NQCH), 7.31 (d, J = 2.4 Hz, 2H, ArH), 7.21–7.14 (2C, C(CH3)3), 31.32 (6C, C(CH3)3), 29.25 (6C, C(CH3)3), ꢀ9.85
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(s, 1C, AlCH3). Anal. calcd for C39H53AlN2O2 (%): C, 76.94; H, Acknowledgements
8.77; N, 4.60. Found: C, 76.96; H, 8.79; N, 4.63.
This work was supported by the National Natural Science
Foundation of China (No. 21204082); the Jilin Province Science
and Technology Development Program (No. 20180101189JC
and 20190303069SF); the Youth Scientific Research Fund of
the Science and Technology Department, Jilin Province, China
(No. 20160520132JH); Foundation of Department of Education
of Jilin Province (No. 2016363 and JJKH20170606KJ); the China
Postdoctoral Science Foundation (No. 2016M601356); and the
Youth Science Foundation of Changchun University of Science
and Technology (No. XQNJJ201506). The project was supported
by the Open Research Fund of Key Laboratory of Polymer Eco-
materials, Changchun Institute of Applied Chemistry, Chinese
Academy of Sciences.
Complex 3. Complex 3 as an orange solid was acquired in a
similar way to 1 using pro-ligand c (0.480 g, 1.00 mmol) and
1
AlMe3 (1.00 mmol). Yield: 0.482 g, 92.3%. H NMR (500 MHz,
chloroform-d, 25 1C): d 8.40 (bs, 2H, NQCH), 7.90–7.80 (m, 2H,
ArH), 7.62 (d, J = 10.6 Hz, 2H, ArH), 7.40 (m, 1H, ArH), 7.29
(d, J = 10.4 Hz, 1H, ArH), 6.95 (s, 2H, ArH), 5.42 (s, 2H, NCH2),
4.97 (s, 2H, NCH2), ꢀ0.89 (s, 3H, AlCH3). 13C{1H} NMR
(125 MHz, chloroform-d, 25 1C) d 173.01 (1C, NQCH), 162.35
(1C, NQCH), 147.20 (ArC), 144.45 (ArC), 140.23 (ArC), 137.00
(ArC), 135.66 (ArC), 136.03 (ArC), 130.64 (ArC), 129.71(ArC),
118.23 (ArC), 67.87 (1C, NCH2), 56.38 (1C, NCH2), ꢀ9.34 (1C,
AlCH3). Anal. calcd for C23H17AlCl4N2O2 (%): C, 52.90; H, 3.28;
N, 5.36. Found: C, 52.93; H, 3.34; N, 5.31.
Complex 4. Complex 4 as a yellow solid was acquired in a
similar way to 1 using 1 eq. pro-ligand b (0.568 g, 1.00 mmol)
and 2 eq. AlMe3 (2.00 mmol). Yield: 0.608 g, 89.3%. H NMR
Notes and references
1
(500 MHz, chloroform-d, 25 1C) d 8.16 (s, 2H, NQCH), 7.51
(d, J = 2.6 Hz, 2H, ArH), 7.43 (m, 1H, ArH), 7.29 (dd, J = 7.7, 1.7
Hz, 1H, ArH), 7.25 (s, 1H, ArH), 7.21 (s, 1H, ArH), 7.00 (d, J =
2.6 Hz, 2H, ArH), 4.71 (s, 4H, NCH2), 1.40 (s, 18H, C(CH3)3), 1.27
(s, 18H, C(CH3)3), ꢀ0.94 (s, 12H, Al(CH3)2). 13C{1H} NMR
(125 MHz, chloroform-d, 25 1C): d 168.92 (s, 2C, NQCH),
138.93 (ArC), 138.06 (ArC), 137.90 (ArC), 129.59 (ArC), 129.07
(ArC), 128.36 (ArC), 128.26 (ArC), 125.33 (ArC), 119.15 (ArC), 60.22
(4C, NCH2), 35.01 (2C, C(CH3)3), 33.94 (2C, C(CH3)3), 31.43
(6C, C(CH3)3), 29.61 (6C, C(CH3)3), ꢀ10.20 (4C, Al(CH3)2). Anal.
calcd for C37H31AlCl2N2O (%): C, 71.96; H, 5.06; N, 4.54. Found:
C, 72.01; H, 5.10; N, 4.61.
Polymerization of lactide in solution. In a representative
polymerization reaction, the aluminum complex (50 mmol) was
loaded in a flame-dried vessel containing a magnetic bar. The
ampulla was immersed in an oil bath at 70 1C. The solution was
stirred for about 10 minutes, when the initiator was activated
completely by the co-initiator and subsequently the required
quantity of lactides in toluene (100 mL) was added. After a
certain reaction time, the polymer was isolated by precipitating
with cold methanol or using a refrigerated centrifuge. The solid
was collected and dried under vacuum at 35 1C for 40 h.
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Conflicts of interest
There are no conflicts to declare.
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