Dimethylaluminium aldiminophenolates: Synthesis, characterization and ring-opening polymerization behavior towards lactides

Article abstract of DOI:10.1039/c2dt31215h

The stoichiometric reaction of the salicylaldimine derivatives (L1-L12) with trimethylaluminium afforded the corresponding dimethylaluminium aldiminophenolates (C1-C12), which were fully characterized by NMR spectroscopy and elemental analysis. The molecu

Full text of DOI:10.1039/c2dt31215h

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PAPER
Dimethylaluminium aldiminophenolates: synthesis, characterization and
ring-opening polymerization behavior towards lactides†
Wenjuan Zhang,*a Youhong Wang,a Wen-Hua Sun,*a,b Lin Wanga and Carl Redshaw*c
Received 6th June 2012, Accepted 7th August 2012
DOI: 10.1039/c2dt31215h
The stoichiometric reaction of the salicylaldimine derivatives (L1–L12) with trimethylaluminium
afforded the corresponding dimethylaluminium aldiminophenolates (C1–C12), which were fully
characterized by NMR spectroscopy and elemental analysis. The molecular structures of the
representative complexes C1, C6, and C8 were determined by the single-crystal X-ray diffraction, which
revealed distorted tetrahedral geometry at aluminium. Activation of the dimethylaluminium
aldiminophenolates for the ring-opening polymerization required one equivalent of BnOH. On the basis
of the polymerization results for L-lactide, D-lactide or rac-lactide, higher efficiency was observed for the
ROP of D-lactide, and the nature of the ligands present significantly affected the observed catalytic
activities and the properties of the resultant polylactides.
Recent aluminium compounds bearing Schiff-base ligands
Introduction
exhibited the ROP of lactides in an effective manner,8 with good
stereo-control.9a–c To the best of our knowledge, there are a few
Polylactides (PLAs) have been intensively studied over the past
decade because of their favorable biodegradability and biocom-
patibility, and are considered as environmentally friendly
materials with high potential for use in biomedical, and pharma-
ceutical applications.1 Lactide exists as two enantiomers as
L-lactide and D-lactide, and four kinds of substrates as L-lactide,
D-lactide, meso-lactide and rac-lactide. Since L-lactic acid is the
common natural form, and L-lactide and rac-lactide are the most
commonly employed synthetic substrates, it is reasonable that
the ring opening polymerization (ROP) of L-lactide and the
stereo-selective polymerization of rac-lactide have been attractive
for polylactide (PLA) synthesis in recent years.2 To efficiently
achieve the ROP of lactides, various initiators have been
explored by employing different organometallic compounds
based on metals such as aluminium,3a,b lithium,3c calcium,3d–f
iron,3g–i tin,3j,k zinc,3l and rare earth elements.3m–p The first
example of stereo-selective polymerization using (R)-(Sal-
Binap)-AlOMe, reported by the Spassky group,4 encouraged the
development of more well-defined metal complex catalysts for
the selective polymerization of lactides,5 along with studies to
understand the mechanism of the chain-end6 and/or active sites.7
reports employing lower coordinated aluminium compounds for
the ROP of lactides.9d–h However, some bidentate dialkylalumi-
nium compounds exhibited highly efficient ROP of ε-caprolac-
tone,10 and it has been shown that dimethyl(salicylaldiminato)
aluminium could initiate the living ROP of ε-caprolactone and
co-polymerization of ε-caprolactone with L-lactide or D-lac-
tides.11 Very recently, benzhydryl-substituted anilines were used
to develop ligands for late-transition metal compounds, which
showed higher activity and better thermo-stability in ethylene
polymerization due to their large steric bulk.12 Herein, bulky
salicylaldehyde Schiff-base compounds are prepared and used to
prepare dimethylaluminium aldiminophenolates, which exhibited
enhanced catalytic activity toward the ROP of lactides. The syn-
thesis and characterization of the title aluminium compounds are
reported, along with the results of their catalytic performance in
the ROP of L-lactide, D-lactide and rac-lactide.
Results and discussion
Synthesis and characterization of methylaluminium
aldiminophenolates (C1–C12)
aKey Laboratory of Engineering Plastics and Beijing National
Laboratory for Molecular Sciences, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, China. E-mail: whsun@iccas.ac.cn;
Fax: +86 10 62618239; Tel: +86 10 62557955
The ligands (L1–L12) were prepared according to our previous
work.13 The stoichiometric reaction of L1–L12 with one equiva-
lent of AlMe3 in toluene at −20 °C gave the corresponding
dimethylaluminium aldiminophenolates (C1–C12) (Scheme 1).
bState Key Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou 730000, China
All aluminium compounds were characterized by H/13C NMR
1
cSchool of Chemistry, University of East Anglia, Norwich, NR4 7TJ,
UK. E-mail: carl.redshaw@uea.ac.uk; Fax: +44 (0)1603 592003;
Tel: +44 (0)1603 593137
†CCDC 885173–885175 for C1, C6, and C8. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2dt31215h
1
spectroscopy and by elemental analysis. Comparison of the H
NMR spectra of C1–C12 with those of the corresponding
ligands L1–L12 revealed that additional resonances appeared in
the high field region (δ −0.30 to −1.07 ppm), which are
This journal is © The Royal Society of Chemistry 2012
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Table 1 Selected bond length and bond angles
Complex
C1
C6
C8
Bond length (Å)
Al–N(imine)
Al–O1
Al–C8(Me)
Al–C9(Me)
O1–C2
1.9647(19)
1.7698(17)
1.962(3)
1.956(3)
1.324(2)
1.302(3)
1.453(2)
1.969(2)
1.7713(19)
1.967(3)
1.965(3)
1.325(3)
1.308(3)
1.460(3)
1.974(2)
1.7814(18)
1.965(3)
1.957(3)
1.330(3)
1.299(3)
1.457(3)
N1–C7(Aryl)
N1–C10
Bond angles (°)
O1–Al–C9
O1–Al–C8
C9–Al–C8
O1–Al–N1
C9–Al–N1
C8–Al–N1
111.78(11)
111.06(11)
117.63(12)
94.14(8)
110.72(10)
109.00(10)
110.23(12)
112.15(13)
114.74(14)
93.86(8)
115.04(11)
109.03(11)
112.47(11)
113.12(11)
112.94(13)
93.94(8)
111.75(12)
111.22(10)
Scheme 1 Synthesis of the dimethylaluminium compounds C1–C12.
Fig. 2 ORTEP drawing of C6 with thermal ellipsoids drawn at the
30% probability level. Hydrogen atoms are omitted for clarity.
Fig. 1 ORTEP drawing of C1 with thermal ellipsoids drawn at the
30% probability level. Hydrogen atoms are omitted for clarity.
attributed to the methyl groups attached to the aluminium
centers, meanwhile the O–H signals (single peak around 12.4 to
13.9 ppm) of the free ligands disappeared. Furthermore, the reso-
nances from −8.6 to −10.1 ppm in 13C NMR spectra of C1–12
confirmed the formation of the Al–CH3 bond.
[1.7698(17) Å] are typical, while the Al–O1 bond length is
1.7698(17) Å, indicating the typical characteristic of σ-bond.
Fig. 2 displays the molecular structure of dimethylaluminium
2-tert-butyl-6-((2,6-dibenzhydryl-4-isopropylphenylimino)-
methyl)-4-methylphenolate [Me2AlL6] (C6), which possesses a
very similar geometry to that of C1; selected bond lengths and
bond angles are collected in Table 1. As is seen in Fig. 2, the dis-
torted tetrahedral geometry around the Al center in C5 is as
found in complex C1, though slight differences are observed
within the bond lengths and bond angles. The bond lengths of
Al–C8, Al–C9, Al–N1 and Al–O1 in C6 [1.967(3), 1.965(3),
1.969(2) and 1.7713(19) Å] are much longer than those in C1
[1.962(3), 1.956(3), 1.9647(19) and 1.7698(17) Å]. At the same
time, in C6 the bond angles of C9–Al1–C8 and O1–Al1–N1
[114.74(14)°, 93.86(8)°] are much smaller than those in C1
which are 117.63(12)° and 94.14(8)°, respectively, which may
be due to the existence of increased steric hindrance around the
Al center in C1.
Single crystals of compounds C1, C6, and C8 were obtained
from their toluene/n-heptane solutions at −30 °C. The molecular
structure of the dimethylaluminium 2,4-di-tert-butyl-6-((2,6-
dibenzhydryl-4-isopropylphenylimino)methyl)phenolate (C1) is
illustrated in Fig. 1 and selected bond length and bond angles
are listed in Table 1. The Al center is coordinated with one N
atom and one O atom which adopts a similar coordination model
as observed in their titanium analogues.14 The geometry at alu-
minium can be best described as distorted tetrahedral, as evi-
denced in the bond angles for O1–Al1–N1 94.14(8), O1–Al1–
C9 111.78(11), C9–Al1–N1 110.72(10), C8–Al1–N1 109.00(10),
O1–Al1–C8 111.06(11). The Al–C bond distances in C2
[Al–C8 1.962(3) Å, Al–C9 1.956(3) Å] and the Al–O1 bond
11588 | Dalton Trans., 2012, 41, 11587–11596
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Catalytic behavior toward ring opening polymerization (ROP)
of lactides (LA)
The molecular structure of dimethylaluminium 2-tert-butyl-6-
((2,6-dibenzhydryl-4-isopropylphenylimino)methyl)-4-methyl-
phenol [Me2AlL8] (C8, Fig. 3) reveals the distorted tetrahedral
geometry around the Al center, similar to those of compounds
C1 and C6. The selected bond lengths and bond angles are
collected in Table 1, indicating slight differences in the bond
distances: Al–N(imine) (1.974(2) Å, Al–O1 1.7814(18) Å and
O1–C2 1.330(3) Å in C8 are longer than those in C1
[1.9647(19), 1.7698(17) and 1.324(2) Å] and C6 [1.969(2),
1.7713(19) and 1.325(3) Å]. The bond length of N1–C7(Aryl) in
C8 [1.299(3)] is shorter than that in C1 [1.302(3) Å] and C6
[1.308(3) Å], while the bond length of Al–C8(Me), Al–C9(Me)
and N1–C10 in C8 [1.965(3), 1.957(3), 1.457(3) Å] are similar
to those of C1 [1.962(3), 1.956(3), 1.453(2) Å] and C6
[1.967(3), 1.965(3) and 1.460(3) Å]. Meanwhile, the bond
angles of O1–Al–C9 and O1–Al–C8 [112.47(11)°, 113.12(11)°]
in C8 are much smaller than those in C1 [111.78(11)°
and 111.06(11)°] and C6 [110.23(12)° and 112.15(13)°].
However, the bond angle of C9–Al–C8 in C8 [112.94(13)°]
is much smaller that those in C1 [117.63(12)°] and C6
[114.74(14)°].
Inspired by previous use of alkylaluminium compounds as
efficient catalysts for the ring-opening polymerization (ROP) of
cyclic esters,15 the title compounds C1–C12 have been investi-
gated for the catalytic behavior toward the ROP of lactides such
as L-lactide, D-lactide and rac-lactide. Despite the use of dialkyl-
aluminium iminophenolates as initiators for the ROP of ε-CL
and LA, as well co-polymerization with ε-CL,11 the shortage of
high catalytically efficient systems and formation of low molecu-
lar polyesters can be overcome through the fine tuning of ligands
with various substituents; in our case by incorporating the bulky
benzhydryl group.
Ring opening polymerization of L-lactide by C1–C12/BnOH
system. The compound C1 was employed to optimize the reac-
tion conditions, and the results are tabulated in Table 2. It was
necessary to activate C1 for the ROP of L-lactide using BnOH.
On use of stoichiometric or more amounts of BnOH, it was
found that trace polymeric product formed within 48 h at various
reaction temperatures up to 60 °C, and good conversion yields
were achieved at higher temperature above 80 °C. This is con-
sistent with observation in literature,16 indicating the formation
of active species requires an activation energy. Over the reaction
temperature range from 80 to 120 °C, at the molar ratio of L-LA :
Al : BnOH of 250 : 1 : 1 (runs 1, 2, 4 and runs 6, 9, 11, Table 2),
the optimum condition is 100 °C, suggesting that activation
energy is essential to form the active species, which decays at
120 °C. With the reaction time prolonged at 80 or 100 °C, the
reactions resulted in a higher conversion yields (runs 1 vs. 6; 2
vs. 8, 9, Table 2) and produced polymers with higher molecular
weights. At 120 °C, however, less polymers were obtained with
lower molecular weights (runs 4, 5, 11, and 12, Table 2), confi-
rming the decomposition of the active species and degradation
of the resultant polyesters.17 When using two equivalents of
BnOH, less polymers with lower molecular weights formed
compared to results when employing a stoichiometric amount of
BnOH (runs 2 vs. 3, 4 vs. 5, 6 vs. 7, 9 vs. 10, 11 vs. 12), which
Fig. 3 ORTEP drawing of C8 with thermal ellipsoids drawn at the
30% probability level. Hydrogen atoms are omitted for clarity.
Table 2 The ROP of L-lactide by C1/BnOHa
Run
Complex
L-LA : Al : BnOH
T/°C
Time/h
Polymer (mg)/yield (%)
Mnb × 10−4
PDI
1
2
3
4
5
6
7
8
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 2
250 : 1 : 1
250 : 1 : 2
250 : 1 : 1
250 : 1 : 2
250 : 1 : 1
250 : 1 : 1
250 : 1 : 2
250 : 1 : 1
250 : 1 : 2
100 : 1 : 1
500 : 1 : 1
1000 : 1 : 1
1500 : 1 : 1
2000 : 1 : 1
80
24
24
24
24
24
48
48
36
48
48
48
48
48
48
48
48
48
143 (39.7)
324 (89.9)
192 (53.3)
154 (42.7)
132 (36.6)
284 (78.8)
258 (71.6)
338 (93.9)
360 (100.0)
303 (84.2)
80 (22.2)
1.15
1.97
0.63
0.89
n.d.
1.78
n.d.
2.27
2.82
0.68
0.81
n.d.
1.17
3.00
3.04
3.20
3.19
1.10
1.16
1.22
1.10
n.d.
1.23
n.d.
1.18
1.21
1.40
1.09
n.d.
1.15
1.29
1.35
1.49
1.63
100
100
120
120
80
80
100
100
100
120
120
100
100
100
100
100
9
10
11
12
13
14
15
16
17
Trace
144 (100.0)
675 (93.7)
1263 (87.8)
1751 (81.1)
1990 (69.1)
a Conditions: 10 μmol Al, 1.0 M L-LA toluene solution, 100 °C, 48 h. b GPC data in THF vs. polystyrene standards, using a correcting factor 0.56.19
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is consistent with excessive BnOH acting as a chain transfer
reagent in such catalytic systems.18 With one equivalent of
BnOH at 100 °C for 48 h, there was complete conversion (run 9,
Table 2), and as a consequence, additional trials were conducted
under such conditions, except where employing various ratios of
monomer L-LA (runs 13–17, Table 2).
catalytic activities in terms of their significant differences, the
conversion yields are as follows: C1 (p-iPr, 100%) > C2 (p-Cl,
95%), C4 (o-iPr, 98.1%) > C3 (o-Me, 86.4%), C6 (p-iPr,
93.1%) > C7 (p-Me, 79.2%), C9 (p-Me, 83.9%) > C8 (p-
CHPh2, 70.3%), suggesting that the catalytic activities were
enhanced by incorporating more soluble alkyl substituents. In
terms of the electronic influence of substituents, it is likely that
electron-donating substituents enhance the catalytic behavior of
On increasing the mole ratio of LA/Al from 100 to 2000 (runs
9 and 13–17, Table 2), the resultant polymers showed increased
molecular weights from 1.35 to 5.20 × 104 g mol−1 along with
slightly broader molecular weight polydispersities (1.15 to 1.63),
although the conversion rate gradually decreased. This obser-
vation is generally true that higher monomer ratio results in
polymer with higher molecular weight, but tends to occur to the
detriment of monomer conversion. It is noteworthy that the PLA
obtained has a relatively higher molecular weight at 5.20 ×
104 g mol−1, which is generally considered an attractive property
because polyesters with high molecular weight possess better
mechanical properties for widespread utilization.9 As shown in
Table 2, the PLA polymers exhibited molecular distributions in
the range 1.05–1.65, and the GPC curves revealed unimodal
characteristics consistent with a (semi-) single-site active species.
The obtained PLA polymers showed much lower molecular
weights than the calculated Mn values on the basis of the molar
ratio of monomer and Al-catalyst, which may be attributed to
chain transfer resulting from chain termination, suggesting a
somewhat higher deviation from “living polymerization” on
increasing monomer ratio. By comparison, the ROP of ε-CL by
the aluminium analogs produced polymers with higher molecu-
lar weight on increasing the concentration of ε-CL.11
The ring opening polymerization of L-LA by the aluminium
compounds C1–C12 showed conversion yields from 70.3% to
100% (Table 3), which indicated good to high catalytic activities
were achieved by these aluminium compounds. In general, the
majority of the resultant PLAs had similar molecular weights
(Mn = 2.41–2.91 × 104) and narrow distributions (PDI =
1.10–1.25) (runs 1–9, Table 3), with the exception of experimen-
tal runs 10–12 in Table 3, in which the differences typically rely
on the group situated ortho to the hydroxyl group within the
ligands used. Though there is no clear trend for the observed
t
their aluminium complexes such that C5 (R1, R2 = Bu, 90.6%)
> C12 (R1, R2 = H; 88.9%) > C10 (R1, R2 = I, 83.9%).
Ring opening polymerization of D-lactide by C1–C12/BnOH
system. Under the above optimum conditions, but using D-LA
instead of L-LA, the ring opening polymerizations were almost
quantitative, indicating a more efficient ROP process for D-LA
than for L-LA. In order to figure out the differences in their cata-
lytic behaviour, the polymerization results were recorded over
24 h and are tabulated in Table 4. In general, all obtained poly-
mers possessed very narrow distributions (PDI: 1.09–1.17),
narrower than the polymers from L-lactide, indicating less trans-
esterification in the polymerization process of D-lactide. The
different polymerization rates with L-lactide and D-lactide could
possibly be explained by the different coordination of the
monomer isomers at the active species.20
The compounds C1, C2, C6, and C7 bearing ligands derived
from 2,6-dibenzhydylanilines showed high conversation rates
from 91.1% to 93.3% (runs 1, 2, 5, and 6, Table 4). Interestingly,
compounds C3, C4, C8, and C9 (runs 3, 4, 7, and 8, Table 4)
having ligands derived from 2-dibenzhydylanilines showed even
better conversation (nearly 100%), suggesting that too much
bulkiness at the ligands can prevent the polymerization on the
aluminium species. In addition, compound C11 also exhibited
high efficiency (98.9%), producing a narrower distribution of
polymer (PDI: 1.15) for ROP of D-lactide, which is different to
the ROP of L-lactide (PDI: 1.56).
Ring opening polymerization of rac-lactide by C1–C12/BnOH
system. Given the significant differences in the ROP of either
L-LA or D-LA, the ROP of rac-LA (consisting of 50% L-LA and
50% D-LA) was conducted over 48 hours, and the results are
summarized in Table 5. In general, the obtained polymers exhib-
ited broader molecular weight distributions (PDI: 1.32–1.69)
than those polymers obtained from either L-LA or D-LA,
Table 3 The ROP of L-lactide by C1–C12a
L-
Polymer (mg)/
Table 4 The ROP of D-lactide by C1–C12a
Run Complex LA : Al : BnOH yield (%)
Mn b × 10−4 PDI
1
2
3
4
5
6
7
8
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
360 (100)
342 (95.0)
311 (86.4)
353 (98.1)
326 (90.6)
335 (93.1)
285 (79.2)
253 (70.3)
300 (83.3)
328 (91.1)
302 (83.9)
320 (88.9)
2.82
2.41
2.77
2.61
2.69
2.79
2.91
2.76
2.82
1.60
1.49
n.d
1.21
1.25
1.22
1.21
1.18
1.19
1.11
1.10
1.16
1.52
1.56
n.d
Run
Complex
Polymer (mg)/yield (%)
Mnb × 10−4
PDI
1
2
3
4
5
6
7
8
9
10
C1
C2
C3
C4
C6
C7
C8
C9
C11
C12
334 (92.8)
328 (91.1)
357 (99.2)
360 (100)
333 (92.5)
336 (93.3)
340 (94.5)
358 (99.4)
356 (98.9)
288 (80.0)
3.39
3.09
3.11
2.95
4.52
3.30
3.43
3.83
2.57
1.90
1.17
1.12
1.12
1.13
1.09
1.10
1.13
1.11
1.14
1.18
9
10
11
12
a Conditions: 10 μmol Al, 1.0 M L-LA toluene solution, 100 °C, 48 h.
b GPC data in THF vs. polystyrene standards, using a correcting factor
0.58.19
a Conditions: 10 μmol D-LA : Al : BnOH: 250 : 1 : 1, Al, 1.0 M D-LA
toluene solution, 100 °C, 24 h. b GPC data in THF vs. polystyrene
standards, using a correcting factor 0.58.19
11590 | Dalton Trans., 2012, 41, 11587–11596
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Table 5 The ROP of rac-lactide by C1–C12a
rac-
Polymer (mg)/
Run Complex LA : Al : BnOH yield (%)
Mn b × 10−4 PDI
1
2
3
4
5
6
7
8
C1
C2
C3
C4
C5
C6
C7
C8
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 1
250 : 1 : 0
250 : 1 : 1
360 (100.0)
354 (98.3)
281 (78.1)
346 (96.1)
356 (98.9)
360 (100.0)
360 (100.0)
314 (87.2)
246 (68.3)
339 (94.2)
354 (98.3)
220 (61.1)
50 (13.9)
2.96
2.32
1.68
1.82
2.62
2.34
2.37
2.86
1.49
1.64
3.25
2.20
0.42
0.47
1.38
1.32
1.43
1.46
1.35
1.33
1.35
1.48
1.38
1.69
1.67
1.65
2.31
2.20
9
C9
10
11
12
13
14
C10
C11
C12
AlMe3
AlMe3
62 (17.2)
a Conditions: 10 μmol Al, 1.0 M rac-LA toluene solution, 100 °C, 48 h.
b GPC data in THF vs. polystyrene standards, using a correcting factor
0.58.19
Fig. 4 Decoupled 1H NMR spectra of obtained poly(rac-LA).
suggesting that there was less control and more transesterification
occuring.21
It is necessary to mention the different reaction times were
used for the ROP of D-LA which was 24 h, whilst for both the
ROP of L-LA or rac-LA, it was 48 h. Compounds C1, C2, C6,
and C7 bearing ligands derived from 2,6-dibenzhydylanilines
exhibited high efficiency for ROP of rac-LA with almost 100%
conversation (runs 1, 2, 6, and 7, Table 5). For the other com-
pounds, the rate of conversation followed the order C4 > C8 >
C3 > C9 (runs 3, 4, 8, and 9, Table 5).
For comparison, blank experiments conducted using Me3Al,
both in the presence and absence of benzyl alcohol, were carried
out. The result was positive for the ring opening polymerization
of rac-lactide, but with low efficiency at 100 °C (runs 13 and 14,
Table 5). The obtained polymers showed low molecular weights
and broader polydispersity. Thus, the nature of ligands used
herein was crucial in terms of higher efficiency and better con-
trollability for the ring opening polymerization of lactides.
Moreover, the resultant PLA of rac-lactides are likely amor-
phous, indicating poor stereo-selectivity in the polymerization.
The decoupled 1H NMR spectra of poly(rac-lactide)s showed
some differences within the methane range related to the micro-
structure. Fig. 4 displays selected charts of the decoupled 1H
NMR spectra of three different samples with slight differences in
the isotacticity of the polylactides;22 the poly(rac-lactide) by
C11 showed a better stereo-selectivity.
ligands, exhibited low selectivity in the ROP of rac-lactide, illus-
trating significant differences for the ROP of the L-LA, D-LA,
and rac-LA series.
Conclusions
A series of highly sensitive dimethylaluminium aldiminopheno-
lates (C1–C12) was synthesized and fully characterized by
1H/13C NMR spectroscopy and elemental analysis. The molecu-
lar structures of representative compounds C1, C6, and C8 were
confirmed by single crystal X-ray diffraction as distorted tetra-
hedral at aluminium. These aluminium compounds exhibited
good to high activities toward the ROP of L-lactide, D-lactide,
rac-lactides in the presence of one equivalent of BnOH; the ROP
of D-lactide is most facile. This study provides a rare example of
the use of bidentate ligands at aluminium for the ROP of various
lactides. The nature of the ligands can influence the catalytic be-
havior of their aluminium compounds and the properties of the
resultant polymers. Thus, the fine tuning of substituents on
ligands remains an interesting topic for improving the catalytic
activities and better selectivity of the resultant products.
Regarding the role of benzyl alcohol, 13C NMR spectra of the
obtained polymers showed clear signals for –CH2Ph (δ: 128.9,
128.2, 125.2, 50.1), suggesting that the benzyloxy group used to
activate the aluminium species also played a part in the termin-
ation process. It was reported that the insertion of the lactide into
the aluminium–alkoxide bond resulted in lactide acyl-oxygen
cleavage.23 In the polymerization process, the ligand architec-
tures exert a significant influence on the tacticity of the growing
polymer chain, and there are two scenarios possible: if a chain
end of R stereochemistry selects D-lactide, then isotactic PLA
forms; if this chain end selects L-lactide then atactic polymers
forms.24 The current aluminium systems employing bulky
Experimental section
General considerations
All reactions were performed using standard Schlenk techniques
in an atmosphere of high-purity nitrogen or glovebox techniques.
Toluene, n-heptane and THF were dried by refluxing over
sodium and benzophenone, distilled under nitrogen and stored
over activated molecular sieves (4 Å) for 24 h in a glovebox
prior to use. C6D6 was dried over activated 4 Å molecular
sieves. CH2Cl2 and CDCl3 were dried over CaH2 for 48 h, dis-
tilled under nitrogen and stored over activated molecular sieves
(4 Å) in a glovebox prior to use. AlMe3 were purchased from
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 11587–11596 | 11591

View Article Online
Aldrich and used as received. Elemental analyses were per-
2-((2-benzhydryl-4,6-dimethylphenylimino)methyl)-6-tert-butyl-
4-methylphenol (L4) was obtained as an orange yellow solid.
Yield: 83.9%. H NMR (CDCl3, 400 MHz, ppm): δ 13.15 (s,
1
formed using a PE2400II Series (Perkin-Elmer Co.). H NMR
spectra and 13C NMR spectra were recorded on a Bruker
1
1
DMX-400 (400 MHz for H, 100 MHz for 13C) instrument. All
1H, OH-H), 7.59 (s, 1H, CHvN), 7.42 (s, 1H, Ph-H), 7.22 (m,
9H, Ph-H), 7.13 (m, 5H, Ph-H), 7.02 (m, 5H, Ph-H), 6.98 (m,
2H, Ph-H), 6.93 (m, 2H, Ph-H), 5.45 (s, 1H, CHPh2-H), 5.42 (s,
1H, CHPh2-H), 2.87 (m, 1H, CH(CH3)2-H), 1.48 (s, 9H, t-Bu-
H), 1.27 (s, 9H, t-Bu-H), 1.08 (d, 6H, J = 6.79, CH(CH3)2-H).
13C NMR (CDCl3, 100 MHz, ppm): 169.5 (CHvN), 158.1,
144.5, 144.2, 143.5, 142.5, 139.6, 136.5, 133.9, 129.5, 129.4,
129.2, 129.1, 128.3, 128.1, 127.9, 126.4, 126.0, 125.9, 125.8,
117.8, 51.8, 50.6, 35.0, 34.0, 31.6, 31.4, 29.4, 23.4, 22.4. IR
(KBr, cm−1): 3661 (O-H) (w), 3022, 2961, 2028, 1621 (CHvN)
(s), 1593, 1441, 1386, 1316, 1261, 1200, 1165, 11289, 1076,
1026, 981, 863, 742, 698. Mp: 179 °C. Anal. Calcd For
C50H53NO: C 87.80, H 7.81, N 2.05. Found: C 87.91, H 7.73,
N 2.04%.
spectra were obtained in the solvent indicated at 25 °C unless
otherwise noted and chemical shifts are given in ppm and are
1
referenced to SiMe4 (d 0.00, H, 13C). The GPC measurements
were performed on a set of Water-515 HPLC pump, a Waters
2414 refractive index detector, and a combination of Styragel
HT-2, HT-3 and HT-4, the effective molar mass ranges of which
are 100–10 000, 500–30 000 and 5000–600 000, respectively.
THF was used as the eluent (flow rate: 1 mL min−1, at 40 °C).
Molecular weights and molecular weight distributions were cal-
culated using polystyrenes as standard.
Synthesis of ligand L2, L3, L4
2,4-Di-tert-butyl-6-((2,6-dibenzhydryl-4-chlorophenylimino)-
methyl)phenol (L2). The mixture of 3,5-di-tert-butyl-2-hydroxy-
benzaldehyde (2.34 g, 10.0 mmol), 2,6-dibenzhydryl-4-chloro-
benzenamine (4.60 g, 10.0 mmol) and a catalytic amount of
p-toluenesulfonic acid (0.30 g) in 100 mL ethanol was refluxed
for 12 h. The solution was then cooled to room temperature, 2,4-
di-tert-butyl-6-((2,6-dibenzhydryl-4-chlorophenylimino)methyl)-
Synthesis of compounds C1–C12
Dimethylaluminium 2,4-di-tert-butyl-6-((2,6-dibenzhydryl-4-
isopropylphenylimino)methyl)phenolate [Me2AlL1] (C1). Into a
stirred solution of 2,4-di-tert-butyl-6-((2,6-dibenzhydryl-4-iso-
propyl-phenylimino)methyl)phenol (0.684 g, 1.0 mmol) in
toluene (15.0 mL), 1.0 mL (1.0 mmol), AlMe3 solution (1.0 M
solution in toluene) was added drop-wise at −30 °C. The slurry
was allowed to warm slowly to room temperature and was stirred
for 3 h, and the solution became clear. Following concentration
to 5 mL in vacuo, 10 mL n-heptane was added, and the solution
was placed in the freezer (−30 °C) to afford C1 as a yellow
powder. Yield: 90.2%. 1H NMR (CDCl3): δ 7.38 (s, 1H,
CHvN), 7.28 (m, 5H, Ph-H), 7.21 (m, 3H, Ph-H), 7.08 (d, 4H,
J = 7.48, Ph-H), 6.97 (m, 6H, Ph-H), 6.80 (d, 4H, J = 6.90,
Ph-H), 6.71 (s, 2H, Ph-H), 5.74 (s, 2H, CHPh2-H), 2.71 (m, 1H,
CH(CH3)2-H), 1.43 (s, 9H, t-Bu-H), 1.14 (s, 9H, t-Bu-H), 1.05
(d, 6H, J = 6.84, CH(CH3)2-H), −0.48 (s, 6H, Al(CH3)2).
13C NMR (CDCl3): δ 176.2, 161.2, 143.0, 142.8, 142.6, 140.4,
139.0, 129.9, 129.7, 128.7, 128.4, 126.9, 126.5, 124.5, 119.0,
52.4, 51.9, 35.0, 29.4, 21.2, 20.4, −8.9. Anal. Calcd for
C52H58AlNO: C 84.40, H 7.90, N 1.89; Found: C 84.33, H 7.95,
N 1.95%.
1
phenol (L2) was obtained as a yellow solid in 82.8%. H NMR
(CDCl3, 400 MHz, ppm): δ: 12.95 (s, 1H, OH-H), 7.30 (s, 1H,
CHvN-H), 7.27–7.20 (m, 3H, Ph-H), 7.20–7.13 (m, 6H, Ph-H),
7.12–7.06 (m, 6H, Ph-H), 7.04–6.95 (m, 5H, Ph-H), 6.70 (s, 2H,
Ph-H), 6.32 (s, 1H, Ph-H), 6.18 (s, 1H, Ph-H), 5.47 (s, 2H,
CHPh2-H), 1.44 (s, 9H, t-Bu-H), 1.22 (s, 9H, t-Bu-H). 13C NMR
(CDCl3, 100 MHz, ppm): δ 170.6 (CHvN), 157.9, 147.1,
142.8, 141.5, 137.5, 129.7, 129.6, 128.9, 128.8, 128.5, 127.5,
127.1, 126.6, 117.4, 52.5, 35.1, 31.8, 31.5, 29.6. IR (KBr,
cm−1): 3659 (O-H) (w), 3025, 2959, 1620 (CHvN) (s), 1591,
1445, 1327, 1238, 1201, 1163, 1120, 1035, 988, 867, 743, 700.
Mp: 179 °C. Anal. Calcd For C47H46ClNO: C 83.47, H 6.86,
N 2.07. Found: C 83.54, H 6.81, N 2.09%.
2,4-Di-tert-butyl-6-((2,4-dibenzhydryl-6-methylphenylimino)-
methyl)phenol (L3). Using the same procedure as for L2, 2,4-
di-tert-butyl-6-((2,4-dibenzhydryl-6-methylphenylimino)methyl)-
phenol (L3) was obtained as an orange yellow solid. Yield:
1
90.5%. H NMR (CDCl3, 400 MHz, ppm): δ 13.12 (s, 1H, OH-
Dimethylaluminium 2,4-di-tert-butyl-6-((2,6-dibenzhydryl-4-
chlorophenylimino)methyl)phenolate [Me2AlL2] (C2). The syn-
thesis of C2 was carried out by the same procedure as that of
C1, except 2,4-di-tert-butyl-6-((2,6-dibenzhydryl-4-chlorophe-
nyl-imino)methyl)phenol was used. Yield: 84.0%. 1H NMR
(CDCl3): 7.40 (s, 1H, CHvN), 7.30 (m, 6H, Ph-H), 7.25 (m,
2H, Ph-H), 7.20 (m, 5H, Ph-H), 7.08 (m, 2H, Ph-H), 7.02 (m,
2H, Ph-H), 7.00 (m, 2H, Ph-H), 6.96 (m, 2H, Ph-H), 6.86 (s,
1H, Ph-H), 6.82 (m, 2H, Ph-H), 5.75 (s, 2H, CHPh2-H), 1.57
(s, 9H, t-Bu-H), 1.27 (s, 9H, t-Bu-H), −0.30 (s, 6H, Al(CH3)2).
13C NMR: 177.2, 162.0, 143.1, 142.5, 142.2, 141.8, 141.5,
130.6, 129.7, 128.8, 128.5, 127.0, 126.8, 122.2, 118.2, 52.6,
52.0, 35.3, 33.8, 31.3, −8.6. Anal. Calcd for C49H51AlClNO:
C 80.36, H 7.02, N, 1.91. Found: C 80.28, H 7.11, N, 1.83%.
H), 7.42 (s, 1H, CHvN), 7.25–7.22 (d, 4H, J = 7.47, Ph-H),
7.19–7.12 (m, 9H, Ph-H), 7.04 (d, 4H, J = 7.24, Ph-H), 6.95 (d,
4H, J = 6.51, Ph-H), 6.85 (s, 1H, Ph-H), 6.63 (s, 1H, Ph-H),
6.57 (s, 1H, Ph-H), 5.50 (s, 1H, CHPh2-H), 5.40 (s, 1H, CHPh2-
H), 2.11 (s, 3H, CH3-H), 1.48 (s, 9H, t-Bu-H), 1.28 (s, 9H, t-Bu-
H). 13C NMR (CDCl3, 100 MHz, ppm): 169.0 (CHvN), 158.2,
144.7, 144.1, 143.6, 142.5, 140.2, 136.8, 135.0, 129.6, 129.5,
129.4, 129.3, 128.5, 128.2, 128.1, 126.6, 126.2, 126.1, 125.9,
117.7, 58.3, 35.2, 34.1, 31.5, 29.5, 18.8, 18.4. IR (KBr, cm−1):
3660 (O-H) (w), 3020, 2963, 2027, 1620 (CHvN) (s), 1592,
1441, 1386, 1318, 1263, 1202, 1165, 1129, 1076, 1028, 981,
862, 742, 696. Mp: 178 °C. Anal. Calcd For C48H49NO: C
87.90, H 7.53, N 2.14. Found: C 87.99, H 7.50, N 2.19%.
2,4-Di-tert-butyl-6-((2,4-dibenzhydryl-6-isopropylphenylimino)-
methyl)phenol (L4). Using the same procedure as for L2,
Dimethylaluminium 2,4-di-tert-butyl-6-((2,4-dibenzhydryl-6-
methylphenylimino)methyl)phenolate [Me2AlL3] (C3). The
11592 | Dalton Trans., 2012, 41, 11587–11596
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synthesis of C3 was carried out by the same procedure as that
of C1, except 2,4-di-tert-butyl-6-((2,4-dibenzhydryl-6-methyl-
phenylimino)methyl)phenol was used. Yield: 83.8%. H NMR
C49H52AlNO: C 84.32, H 7.51, N 2.01. Found: C 84.38, H 7.45,
N 2.13%.
1
Dimethylaluminium 2-tert-butyl-6-((2,6-dibenzhydryl-4-methyl-
phenylimino)methyl)-4-methylphenol [Me2AlL7] (C7). The syn-
thesis of C7 was carried out by the same procedure as that of
C1, except 2-tert-butyl-6-((2,6-dibenzhydryl-4-methylphenyl-
imino)methyl)-4-methylphenol was used. Yield: 85.9%. 1H
NMR (CDCl3): 7.29 (s, 1H, CHvN), 7.27 (m, 5H, Ph-H), 7.22
(s, 1H, Ph-H), 7.08 (m, 6H, Ph-H), 7.00 (m, 6H, Ph-H), 6.85 (m,
4H, Ph-H), 6.67 (s, 2H, Ph-H), 5.73 (s, 2H, CHPh2-H), 2.19 (s,
3H, CH3-H), 2.03 (s, 3H, CH3-H), 1.42 (s, 9H, t-Bu-H), −0.50
(s, 6H, Al(CH3)2-H). 13C NMR: 177.2, 161.6, 147.1, 143.0,
142.9, 142.4, 139.7, 139.1, 130.0, 129.9, 128.4, 128.3, 127.2,
126.5, 118.3, 52.1, 33.8, 31.4, 29.5, 24.0, −8.6. Anal. Calcd for
C47H48AlNO: C 84.27, H 7.22, N 2.09. Found: C 84.35, H 7.17,
N 2.14%.
(CDCl3): 7.36 (s, 1H, CHvN), 7.29 (m, 5H, Ph-H), 7.23 (m,
3H, Ph-H), 7.07 (m, 4H, Ph-H), 7.00 (m, 6H, Ph-H), 6.82 (m,
4H, Ph-H), 6.68 (m, 2H, Ph-H), 5.74 (s, 1H, CHPh2-H), 5. 52 (s,
1H, CHPh2-H), 2.53 (s, 3H, CH3-H), 1.43 (s, 9H, t-Bu-H),
1.16 (s, 9H, t-Bu-H), −0.47 (s, 6H, Al(CH3)2). 13C NMR: 176.5,
161.4, 146.8, 144.2, 142.7, 140.8, 133.0, 129.7, 129.5, 129.4,
129.3, 129.1, 128.5, 128.3, 128.1, 126.6, 126.3, 126.0, 125.4,
122.4, 117.8, 56.3, 52.9, 35.2, 31.6, 29.6, 21.6, 17.9, −8.8.
Anal. Calcd for C50H54AlNO: C 84.35, H 7.65, N 1.97. Found:
C 84.41, H 7.72, N 1.89%.
Dimethylaluminium 2,4-di-tert-butyl-6-((2,4-dibenzhydryl-6-
isopropylphenylimino)methyl)phenolate [Me2AlL4] (C4). The
synthesis of C4 was carried out by the same procedure as that of
C1, except 2,4-di-tert-butyl-6-((2,4-dibenzhydryl-6-isopropyl-
Dimethylaluminium 2-tert-butyl-6-((2,4-dibenzhydryl-6-methyl-
phenylimino)methyl)-4-methylphenol [Me2AlL8] (C8). The syn-
thesis of C8 was carried out by the same procedure as that of
C1, except 2-tert-butyl-6-((2,4-dibenzhydryl-6-methylphenyl-
imino)methyl)-4-methylphenol was used. Yield: 87.1%. 1H
NMR (CDCl3): 7.27 (s, 1H, CHvN), 7.25 (m, 3H, Ph-H), 7.19
(m, 1H, Ph-H), 7.15 (m, 4H, Ph-H), 7.13 (m, 4H, Ph-H), 7.04 (t,
4H, J = 7.72, Ph-H), 6.94 (m, 4H, Ph-H), 6.88 (d, 2H, J = 7.35,
Ph-H), 6.60 (s, 1H, Ph-H), 6.03 (s, 1H, Ph-H), 5.72 (s, 1H,
CHPh2-H), 5.41 (s, 1H, CHPh2-H), 2.16 (s, 3H, CH3-H), 2.10
(s, 3H, CH3-H), 1.42 (s, 9H, t-Bu-H), −0.54 (s, 3H, Al(CH3)2-
H), −0.77 (s, 3H, Al(CH3)2-H). 13C NMR: 175.6, 162.3, 143.8,
142.8, 142.7, 142.5, 141.0, 138.6, 132.1, 130.1, 129.8, 129.5,
129.4, 129.3, 129.2, 128.7, 128.4, 128.2, 126.5, 125.5, 118.9,
56.5, 51.9, 35.1, 29.5, 20.5, 18.8, −8.5, −9.3. Anal. Calcd for
C47H48AlNO C 84.27, H 7.22, N 2.09. Found: C 84.36, H 7.14,
N 2.01%.
1
phenylimino)methyl)phenol was used. Yield: 85.7%. H NMR
(CDCl3): 7.27 (s, 1H, CHvN), 7.25 (m, 3H, Ph-H), 7.19 (m,
1H, Ph-H), 7.13 (m, 4H, Ph-H), 7.10 (m, 4H, Ph-H), 7.04 (m,
4H, Ph-H), 6.98 (m, 4H, Ph-H), 6.78 (m, 3H, Ph-H), 6.03 (s,
1H, Ph-H), 5.72 (s, 1H, CHPh2-H), 5.45 (s, 1H, CHPh2-H), 2.82
(m, 1H, CH(CH3)2-H), 1.43 (s, 9H, t-Bu-H), 1.16 (s, 9H, t-Bu-
H), 1.09 (d, 6H, J = 6.92, CH(CH3)2-H), −0.50 (s, 6H,
Al(CH3)2). 13C NMR: 176.9, 161.7, 147.1, 145.0, 143.0, 142.9,
142.5, 139.9, 139.0, 138.5, 130.0, 129.2, 128.9, 128.4, 128.3,
127.5, 126.8, 126.0, 125.5, 124.2, 118.7, 56.6, 54.3, 34.1, 31.4,
29.5, 28.1, 22.9, 22.6, −8.8. Anal. Calcd for C52H58AlNO: C
84.35, H 7.65, N 1.97. Found: C 84.28, H 7.58, N 2.03%.
Dimethylaluminium 2,4-di-tert-butyl-6-((2,6-diisopropyl-
phenylimino)methyl)phenol [Me2AlL5] (C5). The synthesis of
C5 was carried out by the same procedure as that of C1, except
2,4-di-tert-butyl-6-((2,6-diisopropylphenylimino)methyl)phenol
was used. Yield: 89.5%. 1H NMR (CDCl3): 8.06 (s, 1H,
CHvN), 7.50 (t, 1H, J = 7.56, Ar-H), 7.31 (d, 1H, J = 7.58,
Ar-H), 7.02 (m, 3H, Ar-H), 3.10 (m, 2H, CH(CH3)2-H), 1.39 (s,
9H, t-Bu-H), 1.10 (s, 9H, t-Bu-H),1.27 (d, 12H, J = 6.76,
CH(CH3)2-H), −0.80 (s, 6H, Al(CH3)2-H). 13C NMR: 173.1,
165.1, 142.5, 138.1, 135.2, 129.2, 128.4, 124.4, 122.3, 118.8,
118.0, 39.8, 31.7, 31.2, 28.5, 28.0, 23.4, −9.8. Anal. Calcd for
C29H44AlNO: C 77.46, H 9.86, N 3.12. Found: C 77.36, H 9.79,
N, 3.17%.
Dimethylaluminium 2-((2-benzhydryl-4,6-dimethylphenylimino)-
methyl)-6-tert-butyl-4-methylphenol [Me2AlL9] (C9). The syn-
thesis of C9 was carried out by the same procedure as that of
C1, except 2-((2-benzhydryl-4,6-dimethylphenylimino)methyl)-
1
6-tert-butyl-4-methylphenol was used. Yield: 92.1%. H NMR
(CDCl3): 7.30 (s, 1H, CHvN), 7.24 (m, 2H, Ph-H), 7.19 (m,
3H, Ph-H), 7.14 (m, 2H, Ph-H), 7.09 (m, 2H, Ph-H), 7.01 (s,
1H, Ph-H), 6.93 (m, 2H, Ph-H), 6.82 (s, 1H, Ph-H), 6.61 (s, 1H,
Ph-H), 5.75 (s, 1H, CHPh2-H), 2.23 (s, 3H, CH3-H), 2.15 (s,
3H, CH3-H), 2.12 (s, 3H, CH3-H), 1.42 (s, 9H, t-Bu-H), −0.53
(s, 3H, Al(CH3)2-H), −0.78 (s, 3H, Al(CH3)2-H). 13C NMR:
175.6, 162.3, 143.0, 140.9, 138.4, 136.6, 132.6, 132.0, 130.1,
129.9, 129.2, 128.7, 128.4, 126.6, 125.5, 125.2, 118.9, 51.9,
35.1, 29.5, 21.3, 20.5, 18.6, −8.5, −9.4. Anal. Calcd for
C35H40AlNO: C 81.20, H 7.79, N, 2.71. Found: C 81.29, H
7.71, N, 2.77%.
Dimethylaluminium 2-tert-butyl-6-((2,6-dibenzhydryl-4-isopro-
pylphenylimino)methyl)-4-ethylphenol [Me2AlL6] (C6). The
synthesis of C6 was carried out by the same procedure as that of
C1, except 2-tert-butyl-6-((2,6-dibenzhydryl-4-isopropyl-phenyl-
imino)methyl)-4-methylphenol was used. Yield: 81.6%. 1H
NMR (CDCl3): 7.29 (s, 1H, CHvN), 7.27 (m, 3H, Ph-H), 7.21
(m, 2H, Ph-H), 7.14 (m, 1H, Ph-H), 7.07 (m, 4H, Ph-H), 7.01
(m, 6H, Ph-H), 6.82 (d, 4H, J = 6.58, Ph-H), 6.72 (s, 2H, Ph-H),
5.79 (s, 1H, Ph-H), 5.74 (s, 2H, CHPh2-H), 5.16 (s, 1H, Ar-H),
2.71 (m, 1H, CH(CH3)2-H), 2.02 (s, 3H, CH3-H), 1.42 (s, 9H,
t-Bu-H), 1.05 (d, J = 6.88, 6H, CH(CH3)2), −0.5 (s, 6H,
Al(CH3)2-H). 13C NMR: 176.6, 161.8, 147.1, 143.1, 142.9,
142.5, 140.3, 138.9, 129.8, 128.4, 128.3, 127.3, 126.5, 124.5,
118.9, 52.0, 35.0, 33.8, 29.4, 24.0, 20.4, −8.9. Anal. Calcd for
Dimethylaluminium 2-benzhydryl-6-((2,6-diethylphenylimino)-
methyl)-4-methylphenol [Me2AlL10] (C10). The synthesis of
C10 was carried out by the same procedure as that of C1, except
2-benzhydryl-6-((2,6-diethylphenylimino)methyl)-4-methyl-
1
phenol was used. Yield: 93.2%. H NMR (CDCl3): 8.00 (s, 1H,
CHvN), 7.29 (m, 4H, Ph-H), 7.25 (s, 1H, Ph-H), 7.22 (m, 1H,
Ph-H), 7.19 (m, 3H, Ph-H), 7.18 (m, 4H, Ph-H), 6.99 (s, 1H,
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Ph-H), 6.89 (s, 1H, Ph-H), 6.04 (s, 1H, CHPh2-H), 2.54 (m, 4H,
(CH2CH3)2-H), 2.18 (s, 3H, Ar-CH3-H), 1.14 (t, 6H, J = 7.48,
(CH2CH3)2-H), −1.07 (s, 6H, Al(CH3)2-H). 13C NMR: 173.3,
161.0, 143.7, 139.2, 137.5, 137.1, 132.6, 129.6, 129.2, 128.4,
128.3, 126.8, 126.2, 126.1, 118.2, 49.7, 24.4, 20.6, 15.4, −10.1.
Anal. Calcd for C33H36AlNO: C 80.95, H 7.41, N 2.86. Found:
C 80.84, H 7.47, N 2.89%.
138.1, 135.2, 129.2, 128.4, 124.4, 122.3, 118.8, 118.0, 28.4,
26.1, 22.7, −9.8. Anal. Calcd for C21H28AlNO: C 74.75, H
8.36, N, 4.15. Found: C, 67.70; H, 5.67; N, 8.29%.
Ring-opening polymerization (ROP) of L-lactide, D-lactide,
rac-lactide
Typical polymerization procedures in the presence of one equi-
valent of benzyl alcohol (Table 2, run 9) are as follows. A
toluene solution of C1 (0.010 mmol, 1.0 mL toluene) and BnOH
(0.010 mmol) were added into a Schlenk tube in the glovebox at
room temperature. The solution was stirred for 2 min, and then
L-lactide (5.0 mmol) along with 1.5 mL toluene was added to
the solution. The reaction mixture was then placed into an oil
bath preheated at 100 °C, and the solution was stirred for the pre-
scribed time (48 h). The polymerization mixture was then
quenched by addition of an excess of glacial acetic acid
(0.2 mL) into the solution, and the resultant solution was then
poured into methanol (200 mL). The resultant polymer was then
collected on filter paper and was dried in vacuo.
Dimethylaluminium 2-((2,6-diisopropylphenylimino)methyl)-
4,6-diiodophenol [Me2AlL11] (C11). The synthesis of C11 was
carried out by the same procedure as that of C1, except 2-((2,6-
diisopropylphenylimino)methyl)-4,6-diiodophenol was used.
1
Yield: 87.8%. H NMR (CDCl3): 8.16 (s, 1H, CHvN), 7.84(s,
1H, Ar-H), 7.38 (s, 1H, Ar-H), 7.15 (m, 3H, Ar-H), 2.88 (m,
2H, CH(CH3)2-H), 1.07 (d, 12H, J = 8.40, CH(CH3)2-H), −0.86
(s, 6H, Al(CH3)2-H). 13C NMR: 171.5, 162.7, 153.3, 143.0,
142.0, 141.3, 129.1, 128.7, 128.3, 124.4, 119.7, 28.3, 26.2, 22.7,
−9.7. Anal. Calcd for C21H26AlI2NO: C 42.81, H 4.45, N, 2.38.
Found: C 42.72, H 4.53, N, 2.31%.
Dimethylaluminium 2-((2,6-diisopropylphenylimino)methyl)-
phenol [Me2AlL11] (C12). The synthesis of C12 was carried out
by the same procedure as that of C1, except 2-((2,6-diisopropyl-
Crystal structure determinations
1
phenylimino)methyl)phenol was used. Yield: 84.9%. H NMR
(CDCl3): 8.06 (s, 1H, CHvN), 7.50 (t, 1H, J = 7.56, Ar-H),
7.31 (d, 1H, J = 7.58, Ar-H), 7.23 (d, 1H, J = 7.37, Ar-H), 7.16
(t, 2H, J = 6.50, Ar-H), 6.99 (d, 1H, J = 8.48, Ar-H), 6.82
(t, 1H, J = 7.42, Ar-H), 3.07 (m, 2H, CH(CH3)2-H), 1.27 (d, 6H,
J = 6.76, CH(CH3)2-H), 1.07 (d, 6H, J = 6.80, CH(CH3)2-H),
−0.78 (s, 6H, Al(CH3)2-H). 13C NMR: 173.1, 165.1, 142.5,
Single crystals of C1, C6, and C8 suitable for single crystal
X-ray diffraction were obtained from chilled toluene/n-heptane
solution. With graphite-monochromated Mo Kα radiation (λ =
0.71073 Å), cell parameters were obtained by global refinement
of the positions of all collected reflections. Intensities were
corrected for Lorentz and polarization effects and empirical
Table 6 Crystal data and refinement details for C1, C6, and C8
C1
C6
C8
Empirical formula
Formula weight
Crystal color
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C52H58AlNO
739.97
Yellow
C49H52AlNO
697.90
Yellow
173(2)
0.71073
Monoclinic
P21/c
12.114(2)
C47H48AlNO
669.84
Yellow
173(2)
173(2)
0.71073
Triclinic
ˉ
P1
10.403(2)
12.914(3)
0.71073
Triclinic
ˉ
P1
10.054(2)
14.116(3)
b (Å)
15.129(3)
c (Å)
α (°)
17.170(3)
102.93(3)
23.294(5)
90
15.935(3)
91.37(3)
β (°)
90.60(3)
94.86(3)
2239.0(8)
2
1.098
0.082
796
99.69(3)
90
4208.3(15)
4
1.102
0.083
1496
98.60(3)
90.67(3)
2235.3(8)
2
0.995
0.076
716
γ (°)
Volume (Å3)
Z
D
calc (Mg m−3
)
μ (mm−1
F(000)
)
Crystal size (mm)
θ range (°)
Limiting indices
0.30 × 0.26 × 0.22
1.22–27.48
−13 ≤ h ≤ 13, −16 ≤ k ≤ 16,
−22 ≤ l ≤ 22
20 071
10 186
0.0330
0.28 × 0.22 × 0.20
1.61–27.46
−15 ≤ h ≤ 11, −18 ≤ k ≤ 19,
−30 ≤ l ≤ 30
19 583
9525
0.0378
0.28 × 0.25 × 0.22
1.29–27.46
−13 ≤ h ≤ 12, −18 ≤ k ≤ 18,
−20 ≤ l ≤ 20
29 159
10 182
0.0528
No. of rflns collected
No. of unique rflns
Rint
Completeness to θ (%)
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
Largest diff peak hole (e Å−3
99.1%
1.185
99.0%
1.236
99.5%
1.114
R1 = 0.0759, wR2 = 0.1869
R1 = 0.0899, wR2 = 0.2029
0.415, −0.397
R1 = 0.0844, wR2 = 0.1978
R1 = 0.0976, wR2 = 0.2118
0.567, −0.467
R1 = 0.0783, wR2 = 0.2098
R1 = 0.0948, wR2 = 0.2214
0.292, −0.317
)
11594 | Dalton Trans., 2012, 41, 11587–11596
This journal is © The Royal Society of Chemistry 2012

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Chem. J. Chin. Univ., 2006, 27, 352–355; (h) N. Nomura, R. Ishii,
Y. Yamamoto and T. Kondo, Chem.–Eur. J., 2007, 13, 4433–4451.
7 (a) T. M. Ovitt and G. W. Coates, J. Polym. Sci., Part A: Polym. Chem.,
2000, 38, 4686–4692; (b) T. M. Ovitt and G. W. Coates, J. Am. Chem.
Soc., 2002, 124, 1316–1326; (c) Z. Zhong, P. J. Dijkstra and J. Feijen,
Angew. Chem., Int. Ed., 2002, 41, 4510–4513; (d) Z. Zhong, P. J. Dijkstra
and J. Feijen, J. Am. Chem. Soc., 2003, 125, 11291–11298.
8 (a) P. Hormnirum, E. L. Marshall, V. C. Gibson, R. I. Pugh and
A. J. P. White, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 15343–15348;
(b) H. Du, X. Pang, H. Yu, X. Zhuang, X. Chen, D. Cui, X. Wang and
X. Jing, Macromolecules, 2007, 40, 1904–1913.
9 (a) N. Nomura, R. Ishii, M. Akakura and K. J. Aoi, J. Am. Chem. Soc.,
2002, 124, 5938–5939; (b) R. Ishii, N. Nomura and T. Kondo, Polym. J.,
2004, 36, 261–264; (c) Z. Tang, X. Chen, X. Pang, Y. Yang, X. Zhang
and X. Jing, Biomacromolecules, 2004, 5, 965–970; (d) A. Otero,
A. Lara-Sánchez, J. Fernández-Baeza, C. Alonso-Moreno, J. Castro-
Osma, I. Márquez-Segovia, L. F. Sánchez-Barba, A. M. Rodríguez and
J. C. Garcia-Martinez, Organometallics, 2011, 30, 1507–1522;
(e) W.-A. Ma and Z.-X. Wang, Organometallics, 2011, 30, 4364–4373;
(f) M. Bouyahyi, T. Roisnel and J.-F. Carpentier, Organometallics, 2012,
31, 1458–1466; (g) B. Lian, C. M. Thomas, O. L. C. Jr., C. W. Lehmann,
T. Roisnel and J.-F. Carpentier, Inorg. Chem., 2007, 46, 328–340;
(h) S. Gong and H. Ma, Dalton Trans., 2008, 3345–3357.
10 (a) W. Yao, Y. Mu, A. Gao, W. Gao and L. Ye, Dalton Trans., 2008,
3199–3206; (b) D. Chakraborty and E. Y. X. Chen, Organometallics,
2002, 21, 1438–1442; (c) Z. Chai, C. Zhang and Z. X. Wang, Organo-
metallics, 2008, 27, 1626–1633; (d) M. Shen, W. Zhang, K. Nomura and
W.-H. Sun, Dalton Trans., 2009, 9000–9009; (e) M. Shen, W. Huang,
W. Zhang, X. Hao, W.-H. Sun and C. Redshaw, Dalton Trans., 2010, 39,
9912–9922; (f) Z. Liu, W. Gao, J. Zhang, D. Cui, Q. Wu and Y. Mu,
Organometallics, 2010, 29, 5783–5790; (g) C. Zhang and Z.-X. Wang,
Appl. Organomet. Chem., 2009, 23, 9–18.
absorption. The structures were solved by direct methods and
refined by full-matrix least squares on F2. All hydrogen atoms
were placed in calculated positions. Structure solution and refine-
ment were performed by using the SHELXL-97 package.25
Details of the X-ray structure determinations and refinements are
provided in Table 6.
Acknowledgements
This work was supported by National Natural Science Foun-
dation of China (No. 20904059). CR wishes to thank the
EPSRC for an overseas travel grant (EP/H031855/1).
References
1 (a) Y. Ikada and H. Tsuji, Macromol. Rapid Commun., 2000, 21,
117–132; (b) G. Winzenburg, C. Schmidt, S. Fuchs and T. Kissel, Adv.
Drug Delivery Rev., 2004, 56, 1453–1466; (c) R. Auras, B. Harte and
S. Selke, Macromol. Biosci., 2004, 4, 835–864; (d) M. Vert, S. Li,
G. Spenlehauer and P. Guerin, J. Mater. Sci.: Mater. Med., 1992, 3, 432;
(e) E. Chiellini and R. Solaro, Adv. Mater., 1996, 8, 305–313;
(f) G. Swift, Acc. Chem. Res., 1993, 26, 105–110; (g) H. R. Kricheldorf,
I. Kreiser-Saunders, C. Juergens and D. Wolter, Macromol. Symp., 1996,
103, 85–102.
2 (a) G. J. Van Hummel, S. Harkema, F. E. Kohn and J. Feijen, Acta Crys-
tallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1982, 38, 1679;
(b) M. H. Chisholm, N. W. Eilerts, J. C. Huffman, S. S. Iyer, M. Pacold
and K. Phomphrai, J. Am. Chem. Soc., 2000, 122, 11845; (c) R. Auras,
L.-T. Lim and S. E. M. Selke, Poly(lactic acid): Synthesis, Structures,
Properties, Processing, and Applications, Wiley, 2010.
11 (a) D. Pappalardo, L. Annunziata and C. Pellecchia, Macromolecules,
2009, 42, 6056–6062; (b) N. Nomura, T. Aoyama, R. Ishii and T. Kondo,
Macromolecules, 2005, 38, 5363–5366; (c) J. Liu, N. Iwasa and
K. Nomura, Dalton Trans., 2008, 3978–3988.
12 (a) J. Yu, H. Liu, W. Zhang, X. Hao and W.-H. Sun, Chem. Commun.,
2011, 47, 3257–3259; (b) J. Yu, W. Huang, L. Wang, C. Redshaw and
W.-H. Sun, Dalton Trans, 2011, 40, 10209–10214; (c) W. Zhao, J. Yu,
S. Song, W. Yang, H. Liu, X. Hao, C. Redshaw and W.-H. Sun, Polymer,
2012, 53, 130–137; (d) X. Cao, F. He, W. Zhao, Z. Cai, X. Hao,
T. Shiono, C. Redshaw and W.-H. Sun, Polymer, 2012, 53, 1870–1880.
13 (a) H. Yang, W.-H. Sun, Z. Li and L. Wang, Synth. Commun., 2002, 32,
2395–2402; (b) Y. Wang, W. Zhang, W. Huang, L. Wang, C. Redshaw
and W.-H. Sun, Polymer, 2011, 52, 3732–3737.
3 (a) Ph. Dubois, C. Jacobs, R. Jerome and Ph. Teyssie, Macromolecules,
1991, 24, 2266–2270; (b) A. Kowalski, A. Duda and S. Penczek, Macro-
molecules, 1998, 31, 2114–2122; (c) H. R. Kricheldorf and C. Boettcher,
Makromol. Chem., 1993, 194, 1665–1669; (d) Z. Zhong, P. J. Dijkstra,
C. Birg, M. Westerhausen and J. Feijen, Macromolecules, 2001, 34,
3863–3868; (e) Z. Zhong, S. Schneiderbauer, P. J. Dijkstra,
M. Westerhausen and J. Feijen, J. Polym. Environ., 2001, 9, 31–38;
(f) Z. Zhong, M. J. K. Ankone, P. J. Dijkstra, C. Birg, M. Westerhausen
and J. Feijen, Polym. Bull., 2001, 46, 51–57; (g) B. J. O’Keefe,
S. M. Monnier, M. A. Hillmyer and W. B. Tolman, J. Am. Chem. Soc.,
2001, 123, 339–340; (h) B. J. O’Keefe, L. E. Breyfogle, M. A. Hillmyer
and W. B. Tolman, J. Am. Chem. Soc., 2002, 124, 4384–4393;
(i) X. Wang, K. Liao, D. Quan and Q. Wu, Macromolecules, 2005, 38,
4611–4617; ( j) A. Kowalski, J. Libiszowski, A. Duda and S. Penczek,
Macromolecules, 2000, 33, 1964–1971; (k) A. Duda, S. Penczek and
A. Kowalski, Macromol. Symp., 2000, 153, 41–53; (l) G. Schwach,
J. Coudane, R. Engel and M. Vert, Polym. Int., 1998, 46, 177–182;
(m) W. M. Stevels, M. J. K. Ankone, P. J. Dijkstra and J. Feijen, Macro-
molecules, 1996, 29, 3332–3333; (n) W. M. Stevels, M. J. K. Ankone,
P. J. Dijkstra and J. Feijen, Macromolecules, 1996, 29, 6132–6138;
(o) V. Simic, N. Spassky and L. G. Hubert-Pfalzgraf, Macromolecules,
1997, 30, 7338–7340; (p) L. Zhang, Z. Sheng, C. Yu and L. Fan, J. Mol.
Catal. A: Chem., 2004, 214, 199–202.
14 W. Zhang, Y. Wang, C. Redshaw, X. Hao and W.-H. Sun, J. Organomet.
Chem., 2012, 715, 119–128.
15 (a) M. H. Chisholm, D. Navarro-Llobet and W. J. Simonsick Jr., Macro-
molecules, 2001, 34, 8851; (b) M. H. Chisholm, C.-C. Lin, J. C. Gallucci
and B.-T. Ko, J. Chem. Soc., Dalton Trans., 2003, 3, 406–412;
(c) C.-H. Huang, F.-C. Wang, B.-T. Ko, T.-L. Yu and C.-C. Lin, Macro-
molecules, 2001, 34, 356; (d) A. Kowalski, A. Duda and S. Penczek,
Macromolecules, 1998, 31, 2114–2122; (e) W.-Y. Huang, S.-J. Chuang,
N.-T. Chunag, C.-S. A. Datta, S.-J. Chen, C.-H. Hu, J.-H. Huang,
T.-Y. Lee and C.-H. Lin, Dalton Trans., 2011, 40, 7423;
(f) K. Matsubara, C. Terata, H. Sekine, K. Yamatani, T. Harada, K. Eda,
M. Dan, Y. Koga and M. Yasuniwa, J. Polym. Sci., Polym. Chem., 2012,
50, 957–966; (g) W. Li, W. Wu, Y. Wang, Y. Yao, Y. Zhang and Q. Shen,
Dalton Trans., 2011, 40, 11378; (h) C.-Y. Li, C.-Y. Tsai, C.-H. Lin and
B.-T. Ko, Dalton Trans., 2011, 40, 1880; (i) W.-A. Ma and Z.-X. Wang,
Organometallics, 2011, 30, 4364–4373; ( j) E. L. Whitelaw, G. Loraine,
M. F. Mahon and M. D. Jones, Dalton Trans., 2011, 40, 11469–11473;
(k) M. Bouyahyi, T. Roisnel and J. F. Carpentier, Organometallics, 2012,
31, 1458–1466.
16 (a) Ph. Dubois, C. Jacobs, R. Jerome and Ph. Teyssie, Macromolecules,
1991, 24, 2266–2270; (b) C. Jacobs, Ph. Dubois, R. Jerome and
Ph. Teyssie, Macromolecules, 1991, 24, 3027–3034.
17 N. Ajellal, J.-F. Carpentier, C. Guillaume, S. M. Guillaume, M. Helou,
V. Poirier, Y. Sarazin and A. Trifonov, Dalton Trans., 2010, 39, 8363.
18 R. E. Drumright and P. R. Gruber, Adv. Mater., 2000, 12, 1841.
19 M. Save, M. Schappacher and A. Soum, Macromol. Chem. Phys., 2002,
203, 889–899.
4 N. Spassky, M. Wisniewski, C. Pluta and A. LeBorgne, Macromol.
Chem. Phys., 1996, 197, 2627–2637.
5 (a) O. Dechy-Cabaret, B. Martin-Vaca and D. Bourissou, Chem. Rev.,
2004, 104, 6147–6176; (b) B. J. O’Keefe, M. A. Hillmyer and
W. B. Tolman, J. Chem. Soc., Dalton Trans., 2001, 2215–2224;
(c) J. C. Wu, T.-L. Yu, C.-T. Chen and C.-C. Lin, Coord. Chem. Rev.,
2006, 250, 602–626.
6 (a) A. Bhaw-Luximon, D. Jhurry and N. Spassky, Polym. Bull., 2000, 44,
31–38; (b) D. Jhurry, A. Bhaw-Luximon and N. Spassky, Macromol.
Symp., 2001, 175, 67–79; (c) N. Nomura, R. Ishii, M. Akakura and
K. J. Aoi, J. Am. Chem. Soc., 2002, 124, 5938–5939; (d) R. Ishii,
N. Nomura and T. Kondo, Polym. J., 2004, 36, 261–264; (e) Z. Tang,
X. Chen, X. Pang, Y. Yang, X. Zhang and X. Jing, Biomacromolecules,
2004, 5, 965–970; (f) Z. Tang, X. Chen, Y. Yang, X. Pang, J. Sun,
X. Zhang and X. Jing, J. Polym. Sci., Part A: Polym. Chem., 2004, 42,
5974–5982; (g) Y. Yang, Z. Tang, X. Pang, H. Du, X. Chen and X. Jing,
20 (a) C. Girard and H. B. Kagan, Angew. Chem., Int. Ed., 1998, 37, 2922–
2959; (b) Z. Zhong, P. J. Dijkstra and J. Feijen, J. Am. Chem. Soc., 2003,
125, 11291–11298.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 11587–11596 | 11595

View Article Online
21 M. Bero, P. Dobrzynski and J. Kasperczyk, J. Polym. Sci., Part A: Polym.
Chem., 1999, 37, 4038–4042.
S. Penczek, Macromolecules, 1998, 31, 2114–2122; (d) A. Kowalski,
A. Duda and S. Penczek, Macromolecules, 2000, 33, 689–695.
24 (a) B. M. Chamberlain, M. Cheng, D. R. Moore, T. M. Ovitt,
E. B. Lobkovsky and G. W. Coates, J. Am. Chem. Soc., 2001, 123, 3229–
3238; (b) K. Yamada, T. Nakano and Y. Okamoto, Macromolecules,
1998, 31, 7598–7605; (c) T. Kitayama, T. Hirano and K. Hatada, Tetra-
hedron, 1997, 53, 15263–15279; (d) S. Nakahama, M. Kobayashi,
T. Ishizone and A. Hirao, J. Macromol. Sci., Pure Appl. Chem., 1997, 34,
1845–1855; (e) E. Fossum and K. Matyjaszewski, Macromolecules,
1995, 28, 1618–1625.
22 (a) H. R. Kricheldorf and C. Boettcher, Makromol. Chem., 1993, 194,
1653–1664; (b) H. R. Kricheldorf, C. Boettcher and K. U. Tonnes,
Polymer, 1992, 33, 2817–2824; (c) J. E. Kasperczyk, Macromolecules,
1995, 28, 3937–3939; (d) J. Coudane, C. UstarizPeyret, G. Schwach and
M. Vert, J. Polym. Sci., Part A: Polym. Chem., 1997, 35, 1651–1658;
(e) M. T. Zell, B. E. Padden, A. J. Paterick, K. A. M. Thakur, R. T. Kean,
M. A. Hillmyer and E. J. Munson, Macromolecules, 2002, 35, 7700–
7707.
23 (a) H. R. Kricheldorf, S. R. Lee and S. Bush, Macromolecules, 1996, 29,
1375–1381; (b) P. Dubois, C. Jacobs, R. Jérôme and P. Tessié,
Macromolecules, 1991, 24, 2266–2270; (c) A. Kowalski, A. Duda and
25 G. M. Sheldrick, SHELXTL-97, Program for the Refinement of
Crystal Structures, University of Göttingen, Göttingen, Germany,
1997.
11596 | Dalton Trans., 2012, 41, 11587–11596
This journal is © The Royal Society of Chemistry 2012