Aluminum complexes of bidentate phenoxy-amine ligands: Synthesis, characterization and catalysis in ring-opening polymerization of cyclic esters

Article abstract of DOI:10.1016/j.jorganchem.2013.02.012

A series of aluminum complexes 1a-5a bearing bidentate phenoxy-amine ligands were synthesized and characterized by NMR spectroscopy and elemental analyzes. The variation of the ortho-substituent of phenoxy moiety and the amino group of the ancillary ligands has crucial influence on the stoichiometric structure of the obtained aluminum complexes. Reaction of 2-(N-benzyl-N-R ³-aminomethyl)-4-R²-6-R¹-phenols (R¹ = ^tBu, R² = Me, R³ = ethyl, 1; R¹ = R² = cumyl, R³ = ethyl, 2; R¹ = R² = cumyl, R³ = ^tBu, 3) with either one or half equivalent of AlMe₃ exclusively afforded monoligated complexes L ^Bu,EtAlMe₂ (1a), L^Cumyl,EtAlMe₂ (2a) and L^Cumyl,BuAlMe₂ (3a), whereas the treatment of 2-(N-benzyl-N-R³-aminomethyl)-4,6-dichlorophenols (R³ = ethyl, 4; R³ = ^tBu, 5) with 0.5 equivalent of AlMe ₃ resulted in the formation of (L^Cl,Et)₂AlMe (4a) or (L^Cl,Bu)₂AlMe(HL^Cl,Bu) (5a) respectively. All of these aluminum methyl complexes can efficiently polymerize rac-lactide to atactic polymers and actively initiate the ring-opening polymerization of ε-caprolactone.

Full text of DOI:10.1016/j.jorganchem.2013.02.012

Journal of Organometallic Chemistry 731 (2013) 23e28
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Aluminum complexes of bidentate phenoxy-amine ligands: Synthesis,
characterization and catalysis in ring-opening polymerization of cyclic esters
*
Yuan Wang, Haiyan Ma
Shanghai Key Laboratory of Functional Materials Chemistry, and Laboratory of Organometallic Chemistry, East China University of Science and Technology, 130 Meilong Road,
Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of aluminum complexes 1ae5a bearing bidentate phenoxy-amine ligands were synthesized and
characterized by NMR spectroscopy and elemental analyzes. The variation of the ortho-substituent of
phenoxy moiety and the amino group of the ancillary ligands has crucial influence on the stoichiometric
structure of the obtained aluminum complexes. Reaction of 2-(N-benzyl-N-R3-aminomethyl)-4-R2-6-R1-
phenols (R1 ¼ tBu, R2 ¼ Me, R3 ¼ ethyl, 1; R1 ¼ R2 ¼ cumyl, R3 ¼ ethyl, 2; R1 ¼ R2 ¼ cumyl, R3 ¼ tBu, 3)
with either one or half equivalent of AlMe3 exclusively afforded monoligated complexes LBu,EtAlMe2 (1a),
Received 9 October 2012
Received in revised form
31 January 2013
Accepted 12 February 2013
Keywords:
L
Cumyl,EtAlMe2 (2a) and LCumyl,BuAlMe2 (3a), whereas the treatment of 2-(N-benzyl-N-R3-aminomethyl)-
Phenoxy-amine
Aluminum complex
Ring-opening polymerization
rac-Lactide
4,6-dichlorophenols (R3 ¼ ethyl, 4; R3 ¼ tBu, 5) with 0.5 equivalent of AlMe3 resulted in the formation of
(LCl,Et)2AlMe (4a) or (LCl,Bu)2AlMe(HLCl,Bu) (5a) respectively. All of these aluminum methyl complexes can
efficiently polymerize rac-lactide to atactic polymers and actively initiate the ring-opening polymeri-
zation of -caprolactone.
3
3 -Caprolactone
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
activity [17e21]. In 2009, Nomura et al. [20] reported a series of Al
complexes containing bidentate phenoxy-imine ligands, and those
bearing fluorinated aromatic substituents were found to effectively
Polylactide (PLA), as one of the most promising synthetic polymer
derived from renewable resources, has recently received much
attention due to its biodegradable, biocompatible and permeable
properties [1e3]. It has a broad range of potential applications,
ranging from surgical implants, drug delivery systems to degradable
packaging [4e6]. The physical and mechanical properties of PLA are
intimately linked to the chain stereochemistry, and hence the ste-
reoregular polymerization of lactides has become a key focus of
attention [7]. Ring-opening polymerization (ROP) of lactides cata-
lyzed by well-defined metal complexes proves to be an effective and
versatile method to produce tactic polymers [8,9]. Among the cata-
lysts/initiators studied, aluminum alkoxide initiators, especially
those bearing tetradentate Salen- and Salan-ligands are of great in-
terest for their excellent control over the stereoselectivity in the ROP
ofrac-lactide [10e13], despiteofshowingmuchloweractivities when
compared with other metal catalyst/initiators [14e16]. Bidentate li-
gands, such as phenoxy-amines/imines, are not as rigid as the tetra-
dentate relatives and capable of providing a larger space for the
coordination/insertion of monomers when complexed with a metal
center, which may be beneficial for an improvement of the catalytic
polymerize rac-lactide at 80 ꢀC and
-CL at 50 ꢀC. Moreover, recent
3
reports by Pappalardo [21] utilized dimethyl(salicylaldiminato) li-
gands to stabilize aluminum complexes which showed activities in
the homo- and copolymerization of lactides and -caprolactone. In
3
comparisonwith phenoxy-imines, phenoxy-amine ligands may have
more diverse chemistry due to the phenyl ring and the amino moiety
aligning in different planar arrays. Such more flexible framework
allows enough capacity to form
s or p bonding and may lead to
various coordination connections to the metal center. In this paper,
we reported a series of aluminum complexes supported by bidentate
phenoxy-amine ancillary ligands and evaluated their catalytic
behavior towards rac-lactide and -CL polymerization.
3
2. Results and discussion
2.1. Synthesis of LAlMe2-type complexes 1ae3a
Initially, with the aim of preparing L2AlMe-type complexes, we
performed the reactions of 2-(N-benzyl-N-R3-aminomethyl)-4-R2-
6-R1-phenols (R1 ¼ tBu, R2 ¼ Me, R3 ¼ ethyl, 1; R1 ¼ R2 ¼ cumyl,
R3 ¼ ethyl, 2; R1 ¼ R2 ¼ cumyl, R3 ¼ tBu, 3) with AlMe3 in 2:1 ratio in
dry toluene respectively, however they were failed to afford the
target L2AlMe complexes at ambient temperature, or in refluxing
* Corresponding author. Tel./fax: þ86 21 64253519.
E-mail address: haiyanma@ecust.edu.cn (H. Ma).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.02.012

24
Y. Wang, H. Ma / Journal of Organometallic Chemistry 731 (2013) 23e28
toluene and even with elongated reaction time. Large excess of free
ligand could be detected in the reaction residue in addition to some
unidentifiable structures. Likely, AlMe3 behaving as a deprotonat-
ing agent, readily deprotonated the first equivalent of proligand to
form “LAlMe2” species which however hardly reacted with the
second equivalent of proligand to release methane, as observed in
the reported (salicylaldiminato)2AlR system [20].
1/2 eq. AlMe3
toluene
N
O
N
O
N
Al
Me
Cl
Cl
Cl
OH
Cl
Cl
Cl
Effective preparation of aluminum complexes of proligands 1e3
was achieved by treating the proligand with one equivalent of
AlMe3 in toluene or tetrahydrofuran at room temperature for 24 h
(Scheme 1). Isolation of the crude solids followed by recrystalliza-
tion from a mixture of toluene and n-hexane afforded analytically
pure complexes 1ae3a in moderate yields, which were character-
ized by NMR spectroscopy and elemental analysis to have a stoi-
chiometric structure of LAlMe2. In the 1H NMR spectra of complexes
1ae3a, the disappearance of the OeH signal of the proligand and
the appearance of two separated AleCH3 resonances with each
integrating to three protons demonstrate the formation of expected
aluminum complexes. The four benzylic protons appear as four
separated doublets, indicating that the amino moiety of ligand is
rigidly coordinated to the aluminum center and the rotation of N-
benzyl group is hindered on the NMR time scale at room temper-
ature. The methylene protons of NCH2CH3 fragment display as two
broad resonances with each integrating to one proton, also sug-
gesting the coordination of the nitrogen atom to the metal center,
and the mentioned protons are therefore chemically inequivalent.
4a
4
Scheme 2. Synthesis of L2AlMe-type complex 4a.
similar to 4 except that a tert-butyl group was introduced to the
nitrogen atom. This modification however led to a dramatic change
in the reaction with AlMe3. The treatment of 5 with AlMe3 in 2:1
ratio in toluene at room temperature afforded complex 5a with a
stoichiometric structure of (L)2AlMe(HL) unexpectedly (Scheme 3).
Three discrete singlets corresponding to the C(CH3)3 protons at
d
1.36, 0.99, 0.95 ppm (9H, 9H, 9H) are displayed in the 1H NMR
spectrum of 5a, which shift to either downfield or upfield when
compared with the corresponding resonance of the free ligand 5.
d 8.24 ppm (OeH) inte-
grating to one proton, in line with one equivalent of ligand being
not deprotonated, but weakly coordinated to the metal center [22].
Besides, there displays a broad singlet at
The appearance of AleCH3 signal at
d
ꢁ0.69 ppm (3H) together with
all the above-mentioned features therefore indicates the formation
of a L2AlMe(HL) product without doubt. Complex 5a could be
likewise synthesized by varying the proligand 5/AlMe3 ratio to 3:1
and higher yield could be obtained.
2.2. Synthesis of L2AlMe-type complex 4a
Notably, the attempt to prepare L2AlMe-type complex using
proligand 4 by means of classical methane elimination reaction with
AlMe3 in 2:1 M ratio was successful (Scheme 2). The reaction pro-
ceeded cleanly at room temperature either in toluene or in tetrahy-
drofuran. The target complex 4a is readily soluble in THF, but
sparingly soluble in aromatic and aliphatic hydrocarbons. The
In order to exclude the weakly coordinated ligand, we tenta-
tively performed the reaction in THF which may serve as Lewis
basic reagent to inhibit the coordination of free ligand. However,
the modified reaction afforded the same product without notice-
able contamination by L2AlMe or L2AlMe$THF species. This obser-
vation indicates that the tert-butyl substituent of the amino moiety
exerts a crucial influence on the structure of resultant aluminum
complex, which might not be only a straightforward steric effect.
appearanceofa high-field signal (
d
¼ ꢁ0.88, 3H) assignabletoAleCH3
protons and one triplet peak (
d
¼ 1.11, 6H) accounting for the methyl
protons of NCH2CH3 moieties in 1:2 ratio in the 1H NMR spectrum of
4a, revealed the formation of a L2AlMe-type complex. It is suggested
that the strong electron-withdrawing effect of chloro substituents on
the phenyl ring may increase the BrØnsted acidity of the phenol
group, which is then capable of reacting with the less reactive AleMe
bond of the initially formed LAlMe2 species. In comparison with the
bulky tert-butyl and cumyl groups in proligands 1e3, the less steric
bulkiness of chloro substituents in proligand 4 may also account for
the enhanced reaction activity.
2.4. Ring-opening polymerization of rac-lactide
All of the synthesized aluminum methyl complexes were eval-
uated for the catalytic ring-opening polymerization of rac-lactide
with a prescribed equivalent ratio on the catalyst and monomer.
Representative results for different initiators are summarized in
Table 1.
In general, the experimental results clearly showed that all the
aluminum complexes 1ae5a were active initiators for the ring-
opening polymerization of rac-lactide in toluene at 90 ꢀC
(Table 1). For the series of monoligated complexes 1ae3a, with
the increase of steric bulkiness of the ortho-substituent on the
phenoxy moiety, the rate of polymerization decreased. For
example, the activity of complex 1a bearing an ortho tert-butyl
group was significantly higher than that of the cumyl analog
(complex 2a); 90% monomer conversion in 60 h vs. 94% conver-
sion in 240 h implied that the introduction of a more steric bulky
cumyl group caused a distinctly prohibitive effect to the monomer
coordination/insertion (Table 1, entry 1e3). Although both of
complexes 2a and 3a possess an ortho-cumyl group on the
phenoxide moiety, the polymerization of rac-LA using 3a as the
initiator was more efficient, which reached a monomer conversion
of 86% in 48 h, whereas a much longer polymerization time was
required for the polymerization run by 2a to reach a quantitative
monomer conversion under identical polymerization conditions
(Table 1, entry 3, 4). This might be ascribed to a steric effect of the
2.3. Synthesis of L2AlMe(HL)-type complex 5a
To explore the influence of the amino group on the structure of
resultant aluminum complex, proligand 5 was targeted, which was
R3
N
O
1eq. AlMe3
N
R3
Me
toluene
or THF
R2
Al
Me
R2
OH
R1
R1
1-3
1
2
R = Bu, R = Me, R3 = Et
t
1a
2a
R1 = R2 = CMe2Ph, R3 = Et
3a R1 = R2 = CMe2Ph, R3 = Bu
t
Scheme 1. Synthesis of LAlMe2-type complexes 1ae3a.

Y. Wang, H. Ma / Journal of Organometallic Chemistry 731 (2013) 23e28
25
N
O
N
Me
Al
Cl
Cl
O
1/2 or 1/3 eq. AlMe3
N
toluene
or THF
Cl
Cl
Cl
OH
HO
Cl
N
Cl
5
Cl
5a
Scheme 3. Synthesis of L2AlMe(HL)-type complex 5a.
amino group of the ligand framework. It is believed that the
introduction of a steric bulky tert-butyl group to the nitrogen
atom might facilitate the dissociation of nitrogen donor from the
metal center, which would be beneficial for the coordination and
insertion of lactide monomer from both steric and electronic point
of views.
As indicated in Table 1, the bisligated complex 4a was unex-
pectedly more active than the monoligated relatives, with a
monomer conversion of 94% being observed in 48 h (Table 1, entry
5). It is suggested that the electron-withdrawing effect of chloro
substituents on the phenoxy unit of the ligand played a dominant
role, which increased the electrophilicity of the metal center and
thus enhanced the activity of the catalyst by facilitating the coor-
dination of monomer. Similar observations have been previously
reported by Chen’s [23,24] and Gibson’s groups [13].
The more sterically demanding complex 5a slowed down the
polymerization to some extent, affording a monomer conversion of
81% within 54 h under the same conditions (Table 1, entry 8). By
comparison with the polymerization results of complexes 1a and
2a, the coordination of the undeprontonate phenol molecule to the
aluminum center during the polymerization initiated by 5a is
suggested, since otherwise higher activity than that of 4a towards
the polymerization of lactide should be observed due to the same
effect of tert-butyl amino group in the ligand.
the polymerization temperature to 70 ꢀC, with only 15% of mono-
mer converted after 48 h.
Since aluminum alkoxide species are usually superior initiators
for the ROP of cyclic esters, the polymerization of rac-lactide using
4a in the presence of 2-propanol was investigated. According to the
results, it is apparent that the polymerizations of rac-LA using 4a or
4a/2-propanol system are essentially the same (Table 1, entry 5, 7).
In order to confirm the role of 2-propanol and spectroscopically
characterize the aluminum isopropoxide species, the in-situ alco-
holysis reaction of aluminum methyl complex 4a with equal
equivalent of 2-propanol was monitored by 1H NMR spectroscopy.
The complete disappearance of AleCH3 signal and the appearance
of multiplets at 0.88 ppm corresponding to eCH(CH3)2 suggested
the formation of aluminum isopropoxide species. Besides, no res-
onances attributable to free ligand 4 were displayed. Due to the
severe overlapping of many resonances, a well-defined aluminum
species could not be deduced. We tentatively assume that different
aluminum isopropoxide aggregates might be formed. A possible
explanation for no difference in activity between 4a and 4a/2-
propanol system might be due to the serious aggregation of the
in-situ formed aluminum isopropoxide species, which was not so
active as anticipant.
With regard to the stereoselectivity of these aluminum com-
plexes towards the ROP of rac-lactide, the homonuclear decoupled
1H NMR spectra of polymer samples were recorded and the Pr
values were calculated by integrating the signals of different triads
in the methine region. As shown in Table 1, the polymerizations of
rac-LA catalyzed by complexes 1ae4a all furnished nearly atactic
polymers with Pr values between 0.45 and 0.53. The minor differ-
ences in the tacticites also revealed that the substituents on the
ligand framework hardly influenced the stereocontrol ability of the
active center. The sterically bulky 5a however imposed a slight
preference for heterotactic enchainment in rac-LA polymerization
(Table 1, entry 8). Likely a chain-end control mechanism might be
account for this heterotactic preference, arose from the crowded
coordination environment around the metal center. Therefore, as
suggested previously, the coordination of the unprotonated proli-
gand during the polymerization process should be conclusive.
As depicted in Table 1, the measured number average molecular
weights of the polymer samples determined by GPC somewhat
deviate from the calculated values on the basis of monomer con-
version. In addition, the polydispersity indices are relatively broad,
ranging from 1.25 to 1.80. One of the possible explanations for this
observation is that in the polymerization process, the initiation was
slow in comparison with propagation (metal-alkyl group is known
to be poor for initiation owing to its less nucleophilic nature), and
the initiation might not be quantitative [25].
It was also found that the temperature had a significant influ-
ence on the catalytic activity. As shown in Table 1 (Entry 5, 6), the
observed activity of 4a was dramatically decreased upon lowering
Table 1
The ring-opening polymerization of rac-lactide catalyzed by complexes 1ae5a.a
c
d
d
Entry
Cat.
Temp.
(ꢀC)
Time
(h)
Conv.b
(%)
Mn,calcd
Mn
Mw/Mn
Pre
(104)
(104)
1
2
3
4
5
6
7f
8
1a
2a
2a
3a
4a
4a
4a
5a
90
90
90
90
90
70
90
90
60
48
240
48
48
48
90
4.8
94
86
94
15
94
81
1.30
0.07
1.35
1.24
1.35
0.22
1.35
1.17
2.55
e
1.32
e
0.50
e
1.75
2.76
1.60
e
1.80
1.69
1.25
e
0.45
0.51
0.53
e
48
54
1.47
1.11
1.35
1.79
0.52
0.59
a
[rac-LA]/[Al] ¼ 100:1, [rac-LA] ¼ 1.0 M, toluene.
Determined by 1H NMR spectroscopy.
Mn,calcd ¼ ([rac-LA]/[Al]) ꢂ 144.13 ꢂ conv.%.
Determined by GPC.
b
c
d
e
Pr is the probability of forming a new r-dyad, determined by homonuclear
decoupled 1H NMR spectroscopy.
f
[rac-LA]/[Al]/[iPrOH] ¼ 100:1:1.

26
Y. Wang, H. Ma / Journal of Organometallic Chemistry 731 (2013) 23e28
2.5. Ring-opening polymerization of
3
-caprolactone
NMR spectra were recorded on a Bruker Avance-400 spec-
trometer at ambient temperature. Chemical shifts for 1H and 13C
{1H} NMR spectra were referenced internally using the residual
solvent resonances and reported relative to tetramethylsilane
(TMS). Electron impact mass spectra were recorded on a G2577A.
Besides the molecular ion, only the most characteristic peaks are
reported as m/z (percent of base peak). Elemental analyzes were
performed on an EA-1106 instrument. Spectroscopic analyzes of
polymers were performed in CDCl3. Gel permeation chromatog-
raphy (GPC) analyzes were carried out on a Waters 1515 Breeze
instrument in THF at 25 ꢀC, at a flow rate of 1 mL/min. Calibration
standards were commercially available narrowly distributed linear
polystyrene samples that cover a broad range of molar masses
(103 < M < 2 ꢂ 106 g/mol).
Moreover, complexes 1ae5a were shown to be active for the
ring-opening polymerization of -CL (Table 2). The observed trend
concerning the effects of the aromatic substituents and the amino
group on the catalytic activities in the ROP of -CL was similar to
those in the ROP of rac-lactide. Complexes 1a, 3a and 4a featured
similar activity for the ROP of -CL, achieving approximately full
3
3
3
monomer conversion after 6 h at 50 ꢀC (Table 2, entry 1, 3, 4). A
slower reaction rate was observed by employing 5a as the initiator.
The polymerization required relatively long time (36 h) to reach
84% conversion under identical conditions. Among these com-
plexes, complex 2a exhibited the lowest activity, and an extended
polymerization time was necessary to achieve high monomer
conversion (90%, after 84 h).
The GPC analysis of purified PCL showed relatively broad mo-
lecular weight distributions with PDI in the range of 1.28e1.68,
suggesting that the polymerization hardly proceeded in a well-
controlled manner. Being different from the results of rac-lactide
polymerization, the measured number average molecular weights
4.2. Synthesis of proligands
4.2.1. Synthesis of 2-[(N-benzyl-N-ethyl)aminomethyl]-6-tert-butyl-
4-methylphenol (1)
To a stirred solution of 2-tert-butyl-4-methylphenol (3.29 g,
20.0 mmol) and N-ethylbenzylamine (2.70 g, 20.0 mmol) in ethanol
(20 mL) was added paraformaldehyde (0.90 g, 30.0 mmol) at room
temperature. The mixture was refluxed overnight and then cooled
down to room temperature. All the volatiles were removed under
reduced pressure to give pale yellow oil. Colorless oil was isolated
by silica gel chromatography (elution with petroleum ether/ethyl
acetate ¼ 50:1). Yield: 4.60 g (73.9%). 1H NMR (CDCl3, 400 MHz):
of produced poly( -caprolactone)s agree with the theoretical
3
values, except for the case by employing complex 1a.
3. Conclusions
In summary, we reported five new aluminum methyl complexes
bearing bidentate phenoxy-amine ligands, which could be effi-
ciently prepared by straightforward methane elimination from the
corresponding proligand and trimethylaluminum under mild con-
ditions. All of these complexes proved to be effective catalysts to-
wards the ring-opening polymerization of rac-lactide and
d
11.14 (br, 1H, OH), 7.30e7.20 (m, 5H, ArH), 6.95 (s, 1H, ArH), 6.65 (s,
1H, ArH), 3.70 (s, 2H, ArCH2), 3.54 (s, 2H, PhCH2), 2.51 (q, 2H,
3J ¼ 7.2 Hz, CH2CH3), 2.20 (s, 3H, ArCH3),1.39 (s, 9H, C(CH3)3),1.06 (t,
3H, 3J ¼ 7.2 Hz, CH2CH3). 13C{H} NMR (CDCl3, 100 MHz):
d 154.3,
137.3, 136.2, 129.5, 128.4, 127.4, 127.3, 127.0, 126.5, 122.2 (all AreC),
57.4 (ArCH2), 57.2 (PhCH2), 46.2 (NCH2CH3), 34.6 (C(CH3)3), 29.5
(C(CH3)3), 20.8 (ArCH3), 10.8 (NCH2CH3). Anal. Calcd. for C21H29NO:
C, 80.98; H, 9.38; N, 4.50. Found: C, 81.21; H, 9.86; N, 4.29%.
3 -caprolactone, and furnished predominantly atactic polylactides in
the case of ROP of rac-lactide. While the monoligated complexes
especially 2a with cumyl substituents on the ligand framework
initiated relatively slow polymerizations, much faster polymeriza-
tions were found for the bisligated catalyst 4a with chloro sub-
stituents on the phenoxy moiety of the ligand.
4.2.2. Synthesis of 2-[(N-benzyl-N-ethyl)aminomethyl]-4,6-dicumylp-
henol (2)
To a stirred solution of 2,4-dicumylphenol (6.96 g, 20.0 mmol,
95%) and N-ethylbenzylamine (2.70 g, 20.0 mmol) in ethanol
(20 mL) was added paraformaldehyde (0.90 g, 30.0 mmol) at room
temperature. The mixture was refluxed overnight and then cooled
down to room temperature. A pale yellow solution was obtained.
After concentration and being kept at ꢁ20 ꢀC, colorless crystals
were obtained. Yield: 6.40 g (67.0%). 1H NMR (CDCl3, 400 MHz):
4. Experimental section
4.1. General considerations
All manipulations of air and/or moisture-sensitive compounds
were carried out under an atmosphere of dry argon using standard
Schlenk techniques or in a glove box. Toluene, n-hexane and
tetrahydrofuran were freshly distilled from sodium benzophenone
ketyl prior to use. rac-Lactide (Aldrich) was recrystallized with dry
toluene and then sublimated twice under vacuum at 80 ꢀC.
d
10.84 (br, 1H, OH), 7.33e7.13 (m, 14H, ArH), 6.90 (d, 2H, 3J ¼ 6.8 Hz,
ArH), 6.77 (s, 1H, ArH), 3.66 (s, 2H, ArCH2), 3.39 (s, 2H, PhCH2), 2.40
(q, 2H, 3J ¼ 7.2 Hz, CH2CH3), 1.71 (s, 6H, C(CH3)2Ph), 1.70 (s, 6H,
C(CH3)2Ph), 0.97 (t, 3H, 3J ¼ 7.2 Hz, CH2CH3). 13C{H} NMR (CDCl3,
3 -Caprolactone was distilled over CaH2 under reduced pressure. N-
100 MHz): d 153.7, 151.6, 151.4, 139.9, 137.0, 135.2,129.7, 128.3, 127.9,
ethylbenzylamine and N-tert-butylbenzylamine were synthesized
according to literature procedures [26,27]. All other chemicals were
commercially available and used after appropriate purification.
127.7, 127.2, 126.7, 125.8, 125.6, 125.4, 124.7, 121.6 (all AreC), 57.6
(ArCH2), 56.6 (PhCH2), 46.1 (NCH2CH3), 42.5 (C(CH3)2Ph), 42.0
(C(CH3)2Ph), 31.1 (C(CH3)2Ph), 29.6 (C(CH3)2Ph), 10.6 (NCH2CH3).
Anal. Calcd. for C34H39NO: C, 85.49; H, 8.23; N, 2.93. Found: C,
85.34; H, 8.48; N, 2.78%.
Table 2
The ring-opening polymerization of 3
-CL catalyzed by complexes 1ae5a.a
Conv.b (%)
Mn,calcdc (104)
Mn (104)
Mw/Mn
d
d
Entry
Cat.
Time (h)
4.2.3. Synthesis of 2-[(N-benzyl-N-tert-butyl)aminomethyl]-4,6-dicu-
mylphenol (3)
1
2
3
4
5
1a
2a
3a
4a
5a
6
84
6
6
36
98
90
98
100
84
1.12
1.03
1.12
1.14
0.96
8.02
3.70
1.06
1.17
0.86
1.64
1.60
1.68
1.32
1.28
This compound was prepared by a similar procedure to that
described for 2. By using 2,4-dicumylphenol (6.96 g, 20.0 mmol,
95%), N-tert-butylbenzylamine (3.26 g, 20.0 mmol), and para-
formaldehyde (0.90 g, 30.0 mmol), colorless crystal was obtained
from ethanol. Yield: 5.86 g (57.9%). 1H NMR (CDCl3, 400 MHz):
a
[
3
-CL]/[Al] ¼ 100:1, [
3
-CL] ¼ 1.0 M, toluene, 50 ꢀC.
b
c
Determined by 1H NMR spectroscopy.
d
7.30e7.06 (m, 14H, ArH), 7.01e6.99 (m, 2H, ArH), 6.65 (d, 1H,
Mn,calcd ¼ ([
3
-CL]/[Al]) ꢂ 114.14 ꢂ conv.%.
Determined by GPC.
4J ¼ 2.0 Hz, ArH), 3.76 (s, 2H, ArCH2), 3.62 (s, 2H, PhCH2), 1.63 (s, 6H,
d

Y. Wang, H. Ma / Journal of Organometallic Chemistry 731 (2013) 23e28
27
C(CH3)2Ph), 1.61 (s, 6H, C(CH3)2Ph), 1.08 (s, 9H, C(CH3)3). 13C{H}
NMR (CDCl3, 100 MHz): 154.1, 151.5, 151.3, 140.6, 139.7, 134.78,
4.3.2. Synthesis of 2-[(N-benzyl-N-ethyl)aminomethyl]-4,6-
dicumylphenolate aluminum dimethyl (2a)
d
128.9, 128.2, 127.9, 127.7, 126.8, 126.5, 125.9, 125.4, 125.3, 124.7,
123.1 (all AreC), 56.8 (ArCH2), 54.4 (PhCH2), 54.3 (NC(CH3)3), 42.5
(C(CH3)2Ph), 42.2 (C(CH3)2Ph), 31.2 (C(CH3)2Ph), 29.6 (C(CH3)2Ph),
27.03 (NC(CH3)3). Anal. Calcd. for C36H43NO: C, 85.50; H, 8.57; N,
2.77. Found: C, 85.70; H, 8.48; N, 2.63%.
This compound was prepared by a similar procedure to that
described for 1a. By using AlMe3 (1.0 mL, 2.0 mmol, 2.0 M in
toluene) and proligand 2 (0.955 g, 2.00 mmol), a white powder was
obtained after recrystallization from a mixture of toluene and n-
hexane. Yield: 0.74 g (69%). 1H NMR (CDCl3, 400 MHz):
d 7.40 (d, 1H,
4J ¼ 2.0 Hz, ArH), 7.36e7.29 (m, 5H, ArH), 7.23e7.14 (m, 7H, ArH),
7.08e7.01 (m, 3H, ArH), 6.56 (d, 1H, 4J ¼ 2.0 Hz, ArH), 3.83 (d, 1H,
2J ¼ 14 Hz, ArCH2), 3.78 (d, 1H, 2J ¼ 14 Hz, ArCH2), 3.46 (d, 1H,
2J ¼ 14 Hz, ArCH2), 3.17 (d, 1H, 2J ¼ 14 Hz, ArCH2), 2.61 (dq, 1H,
2J ¼ 14 Hz, 3J ¼ 7.2 Hz, NCH2CH3), 2.48 (dq, 1H, 2J ¼ 14 Hz,
3J ¼ 7.2 Hz, NCH2CH3), 1.75 (s, 3H, C(CH3)2Ph), 1.73 (s, 3H,
C(CH3)2Ph), 1.71 (s, 3H, C(CH3)2Ph), 1.63 (s, 3H, C(CH3)2Ph), 1.30 (t,
3H, 3J ¼ 7.2 Hz, CH2CH3), ꢁ1.05 (s, 3H, AlCH3), ꢁ1.16 (s, 3H, AlCH3).
4.2.4. Synthesis of 2-[(N-benzyl-N-ethyl)aminomethyl]-4,6-dichloro-
phenol (4)
This compound was prepared by a similar procedure to that
described for 1. By using 2,4-dichlorophenol (3.24 g, 20.0 mmol), N-
ethylbenzylamine (2.70 g, 20.0 mmol), and paraformaldehyde
(0.90 g, 30.0 mmol), a colorless oil was isolated by Silica gel chro-
matography (elution with petroleum ether/ethyl acetate ¼ 20:1).
Yield: 3.94 g (63.5%). 1H NMR (CDCl3, 400 MHz):
d
7.37e7.15 (m, 6H,
13C{H} NMR (CDCl3, 100 MHz):
d 156.0, 152.3, 151.6, 137.8, 137.4,
ArH), 6.83 (d, 1H, 4J ¼ 2.4 Hz, ArH), 3.70 (s, 2H, ArCH2), 3.61 (s, 2H,
131.9, 129.7, 128.8, 128.3, 128.0, 127.6, 127.4, 126.8, 125.5, 125.4,
125.3, 124.4, 119.1 (all AreC), 57.9 (ArCH2), 50.0 (PhCH2), 46.6
(NCH2CH3), 42.4 (C(CH3)2Ph), 42.1 (C(CH3)2Ph), 31.2 (C(CH3)2Ph),
31.1 (C(CH3)2Ph), 30.5 (C(CH3)2Ph, 27.9 (C(CH3)2Ph), 10.3
(NCH2CH3), ꢁ10.2 (AlCH3), ꢁ10.9 (AlCH3). Anal. Calcd. for
C36H44AlNO: C, 81.01; H, 8.31; N, 2.62. Found: C, 81.08; H, 8.49; N,
2.56%.
PhCH2), 2.55 (q, 2H, 3J ¼ 7.2 Hz, CH2CH3), 1.11 (t, 3H, 3J ¼ 7.2 Hz,
CH2CH3). 13C{H} NMR (CDCl3, 100 MHz):
d 152.7, 135.8, 129.3, 128.6,
128.4, 127.8, 126.6, 124.1, 123.1, 121.3 (all AreC), 57.4 (ArCH2), 56.2
(PhCH2), 46.5 (NCH2CH3), 10.8 (NCH2CH3). Anal. Calcd. for
C16H17Cl2NO: C, 61.95; H, 5.52; N, 4.52. Found: C, 62.00; H, 5.43; N,
4.31%.
4.2.5. Synthesis of 2-[(N-benzyl-N-tert-butyl)aminomethyl]-4,6-
chloro-phenol (5)
4.3.3. Synthesis of 2-[(N-benzyl-N-tert-butyl)aminomethyl]-4,6-dicu-
mylphenolate aluminum dimethyl (3a)
This compound was prepared by similar procedure to that
described for 2. By using 2,4-dichlorophenol (3.24 g, 20.0 mmol,
95%), N-tert-butylbenzylamine (3.26 g, 20.0 mmol), and para-
formaldehyde (0.90 g, 30.0 mmol), colorless solid was obtained
after two sequential recrystallization with ethyl acetate. Yield:
This compound was prepared by a similar procedure to that
described for 1a. By using AlMe3 (1.0 mL, 2.0 mmol, 2.0 M in
toluene) and proligand 3 (1.010 g, 2.000 mmol), a white powder
was obtained after recrystallization from a mixture of toluene and
n-hexane. Yield: 0.66 g (59%). 1H NMR (CDCl3, 400 MHz):
d 7.40e
3.19 g (47.1%). 1H NMR (CDCl3, 400 MHz):
d
7.25e7.13 (m, 5H, ArH),
7.29 (m, 6H, ArH), 7.24e7.10 (m, 10H, ArH), 6.71 (s, 1H, ArH), 4.35
(d, 1H, 2J ¼ 14.4 Hz, ArCH2), 4.00 (s, 2H, ArCH2), 3.88 (d, 1H,
2J ¼ 14.4 Hz, ArCH2), 1.73 (s, 3H, C(CH3)2Ph), 1.72 (s, 3H, C(CH3)2Ph),
1.66 (s, 3H, C(CH3)2Ph), 1.64 (s, 3H, C(CH3)2Ph), 1.14 (s, 9H,
C(CH3)3), ꢁ0.93 (s, 3H, AlCH3), ꢁ1.02 (s, 3H, AlCH3). 13C{H} NMR
7.03 (d, 1H, 4J ¼ 2.4 Hz, ArH), 6.68 (d, 1H, 4J ¼ 2.4 Hz, ArH), 3.91 (s,
2H, ArCH2), 3.74 (s, 2H, PhCH2), 1.27 (s, 9H, C(CH3)3). 13C{H} NMR
(CDCl3, 100 MHz):
d 152.7, 137.3, 129.5, 128.2, 127.6, 127.5, 126.0,
125.1, 122.8, 121.2 (all AreC), 57.1 (ArCH2), 55.1 (PhCH2), 52.6
(NC(CH3)3), 26.6 (NC(CH3)3). Anal. Calcd. for C18H21Cl2NO: C, 63.91;
H, 6.26; N, 4.14. Found: C, 63.86; H, 6.32; N, 4.01%.
(CDCl3, 100 MHz):
d 155.1, 152.1, 151.8, 138.4, 137.9, 135.0, 131.4,
128.5, 128.1, 128.0, 127.2, 126.8, 125.6, 125.3, 124.6, 124.3, 120.5 (all
AreC), 61.9 (ArCH2), 51.7(PhCH2), 51.5 (C(CH3)3), 42.5 (C(CH3)2Ph),
42.0 (C(CH3)2Ph), 31.3 (C(CH3)2Ph) 31.2 (C(CH3)2Ph), 29.2
(C(CH3)2Ph), 28.9 (C(CH3)2Ph), 27.9 (C(CH3)3), ꢁ5.22 (AlCH3), ꢁ8.18
(AlCH3). Anal. Calcd. for C38H48AlNO: C, 81.24; H, 8.61; N, 2.49.
Found: C, 81.13; H, 8.81; N, 2.11%.
4.3. Synthesis of aluminum complexes
4.3.1. Synthesis of 2-[(N-benzyl-N-ethyl)aminomethyl]-6-tert-butyl-
4-methylphenolate aluminum dimethyl (1a)
AlMe3 (1.0 mL, 2.0 mmol, 2.0 M in toluene) was added dropwise
to a solution of proligand 1 (0.622 g, 2.00 mmol) in toluene (20 mL)
at room temperature. Instantaneous evolution of methane was
observed. The obtained reaction solution was stirred for 24 h at
ambient temperature. After removal of all the volatiles under vac-
uum, the resultant white foam-like solid was dissolved with n-
hexane and filtrated to remove trace amount of impurities. The
filtrate was concentrated and kept at ꢁ20 ꢀC overnight, a white
powder was obtained. Yield: 0.50 g (61%). 1H NMR (CDCl3,
4.3.4. Synthesis of 2-[(N-benzyl-N-ethyl)aminomethyl]-4,6-dichloro-
phenolate aluminum methyl (4a)
AlMe3 (1.0 mL, 2.0 mmol, 2.0 M in toluene) was added dropwise
to a solution of proligand 4 (1.24 g, 4.00 mmol) in toluene (40 mL) at
room temperature. Instantaneous evolution of methane was
observed. The reaction solution was stirred for 24 h at ambient
temperature. After removal of all the volatiles under vacuum, the
resultant pale yellow powder was dissolved with tetrahydrofuran
and filtrated to remove trace amount of impurities. The filtrate was
concentrated and a minimum amount of n-hexane was added to a
slight turbid state. The solution was kept at 0 ꢀC overnight, a white
powder was obtained was obtained. Yield: 0.99 g (75%). 1H NMR
400 MHz): d 7.42e7.33 (m, 3H, ArH), 7.30e7.24 (m, 2H, ArH), 7.06 (s,
1H, ArH), 6.62 (s, 1H, ArH), 4.05 (d, 1H, 2J ¼ 14.0 Hz, ArCH2), 3.87 (d,
1H, 2J ¼ 14.0 Hz, ArCH2), 3.80 (d, 1H, 2J ¼ 14.0 Hz, ArCH2), 3.55 (d,
1H, 2J ¼ 14.0 Hz, ArCH2), 2.77 (dq, 1H, 2J ¼ 13.6 Hz, 3J ¼ 7.2 Hz,
NCH2CH3), 2.66 (dq, 1H, 2J ¼ 13.6 Hz, 3J ¼ 7.2 Hz, NCH2CH3), 2.27 (s,
3H, ArCH3), 1.40 (s, 9H, C(CH3)3), 1.37 (t, 3H, 3J ¼ 7.2 Hz,
CH2CH3), ꢁ0.61 (s, 3H, AlCH3), ꢁ0.84 (s, 3H, AlCH3). 13C{H} NMR
(CDCl3, 400 MHz):
d 7.38e7.20 (m, 12H, ArH), 6.82 (d, 2H,
4J ¼ 1.8 Hz, ArH), 4.43 (d, 2H, 2J ¼ 12.8 Hz, ArCH2), 4.03 (d, 4H,
2J ¼ 13.6 Hz, ArCH2), 3.63 (d, 2H, 2J ¼ 12.8 Hz, ArCH2), 2.86e2.77 (m,
4H, NCH2CH3), 1.11 (t, 6H, 3J ¼ 7.0 Hz, NCH2CH3), ꢁ0.88 (s, 3H,
(CDCl3, 100 MHz):
d 156.8, 138.5, 131.9, 130.2, 129.0, 128.5, 128.4,
127.9, 124.7, 119.6 (all AreC), 57.9 (ArCH2), 51.7 (PhCH2), 45.8
(NCH2CH3), 34.7 (C(CH3)3), 29.4 (C(CH3)3), 20.8 (ArCH3), 9.6
(NCH2CH3), ꢁ10.6 (AlCH3), ꢁ10.7 (AlCH3). Anal. Calcd. for
C23H34AlNO: C, 75.17; H, 9.33; N, 3.81. Found: C, 74.67; H, 9.54; N,
3.73%. MS-EI m/z (%): 367.3 (1.74, Mþ), 352.2 (100, [M ꢁ Me]þ).
AlCH3). 13C{H} NMR (CDCl3, 100 MHz):
d 153.9, 133.2, 131.9, 128.9,
128.2, 128.0, 125.1, 124.1, 121.3 (all AreC), 55.7 (ArCH2), 54.1
(PhCH2), 46.3 (NCH2CH3), 10.0 (NCH2CH3), ꢁ10.6 (AlCH3). Anal.
Calcd. for C33H35AlCl4N2O2: C, 60.01; H, 5.34; N, 4.24. Found: C,
59.92; H, 5.46; N, 3.93%.

28
Y. Wang, H. Ma / Journal of Organometallic Chemistry 731 (2013) 23e28
4.3.5. Synthesis of 2-[(N-benzyl-N-tert-butyl)aminomethyl]-4,6-
chloro-phenolate aluminum methyl {2-[(N-benzyl-N-tert-butyl-
aminomethyl)methyl]-4,6-chloro-phenol} adduct (5a)
petroleum ether. After removal of the volatiles, the residue was
subjected to 1H NMR analysis. Monomer conversion was deter-
mined by observing the methane resonance integration of mono-
mer vs. polymer in the 1H NMR (CDCl3, 400 MHz) spectrum. The
purification of the polymer in each case was managed by dissolving
the crude samples in CH2Cl2 and precipitating the polymer solution
with methanol. The obtained polymers were further dried in a
vacuum oven at 50 ꢀC for 24 h. The polymer samples were sub-
jected to GPC analysis.
AlMe3 (0.5 mL, 1.0 mmol, 2.0 M in toluene) was added dropwise
to a solution of proligand 5 (1.01 g, 3.00 mmol) in toluene (20 mL) at
room temperature. Instantaneous evolution of methane was
observed. The reaction solution was stirred for 24 h at ambient
temperature. After removal of all the volatiles under vacuum, the
resultant pale yellow foam-like solid was dissolved with toluene
and filtrated to remove trace amount of impurities. The filtrate was
concentrated and a minimum amount of n-hexane was added to a
slight turbid state. The solution was kept at 0 ꢀC overnight, a white
powder was obtained. Yield: 0.56 g (54%). 1H NMR (CDCl3,
Acknowledgments
8.24 (br, 1H, OH), 7.43 (d, 1H, 4J ¼ 2.8 Hz, ArH), 7.35 (d,
Financial supports from the National Natural Science Founda-
tion of China (NNSFC, 20604009 and 21074032) and the Funda-
mental Research Funds for the Central Universities (WK0914042,
WD1113011) are gratefully acknowledged.
400 MHz):
d
1H, 4J ¼ 2.8 Hz, ArH), 7.31e7.22 (m, 7H, ArH, overlapped with CDCl3
signal), 7.20 (d, 1H, 4J ¼ 2.5 Hz, ArH), 7.18e7.14 (m, 4H, ArH), 7.10e
7.07 (m, 4H, ArH), 6.97 (d, 1H, 4J ¼ 2.8 Hz, ArH), 6.91 (d, 1H,
4J ¼ 2.8 Hz, ArH), 6.90 (d, 1H, 4J ¼ 2 Hz, ArH), 5.07 (d, 1H, 2J ¼ 12 Hz,
ArCH2), 4.34 (d, 1H, 2J ¼ 14 Hz, ArCH2), 3.98 (d, 1H, 2J ¼ 14 Hz,
ArCH2), 3.67e3.49 (m, 9H, ArCH2), 1.36 (s, 9H, C(CH3)3), 0.99 (s, 9H,
C(CH3)3), 0.95 (s, 9H, C(CH3)3), ꢁ0.69 (s, 3H, AlCH3). 13C{H} NMR
References
[1] R.E. Drumright, P.R. Gruber, D.E. Henton, Adv. Mater. 12 (2000) 1841e1846.
[2] M.J. Stanford, A.P. Dove, Chem. Soc. Rev. 39 (2010) 486e494.
[3] C.A. Wheaton, P.G. Hayes, B.J. Ireland, Dalton Trans. 38 (2009) 4832e4846.
[4] K.E. Uhrich, S.M. Cannizzaro, R.S. Langer, K.M. Shakesheff, Chem. Rev. 99
(1999) 3181e3198.
[5] R.H. Platel, L.M. Hodgson, C.K. Williams, Polym. Rev. 48 (2008) 11e63.
[6] G. Schmack, B. Tändler, R. Vogel, R. Beyreuther, S. Jacobsen, H.-G. Fritz, J. Appl.
Polym. Sci. 73 (1999) 2785e2797.
[7] J.R. Sarasua, R.E. Prud’homme, M. Wisniewski, A. Le Borgne, N. Spassky,
Macromolecules 31 (1998) 3895e3905.
[8] B.J. O’Keefe, M.A. Hillmyer, W.B. Tolman, J. Chem. Soc. Dalton Trans. (2001)
2215e2224.
(CDCl3, 100 MHz):
d 154.8, 151.3, 151.2, 142.4, 142.3, 136.6, 135.9,
131.4, 129.9, 129.7, 129.2, 128.8, 128.6, 128.5, 128.0, 127.9, 127.8,
127.7, 127.6, 126.1, 126.0, 125.7, 125.6, 123.2, 123.0, 122.4, 121.5, 121.1,
120.8 (all AreC), 66.8 (ArCH2), 55.5 (ArCH2), 55.4 (ArCH2), 55.2
(ArCH2), 55.1 (ArCH2), 54.8 (ArCH2), 54.6 (NC(CH3)3), 49.0
(NC(CH3)3), 48.9 (NC(CH3)3), 27.2 (NC(CH3)3), 27.1 (NC(CH3)3), 25.8
(NC(CH3)3), ꢁ11.4 (AlCH3). Anal. Calcd. for C55H64AlCl6N3O3: C,
62.63; H, 6.12; N, 3.98. Found: C, 62.66; H, 6.05; N, 3.84%.
[9] A. Stopper, J. Okuda, M. Kol, Macromolecules 45 (2012) 698e704.
[10] E.L. Whitelaw, G. Loraine, M.F. Mahon, M.D. Jones, Dalton Trans. 40 (2011)
11469e11473.
4.4. Ring-opening polymerization procedures
[11] Z.Y. Zhong, P.J. Dijkstra, J. Feijen, J. Am. Chem. Soc. 125 (2003) 11291e11298.
[12] N. Nomura, R. Ishii, M. Akakura, K. Aoi, J. Am. Chem. Soc. 124 (2002)
5938e5939.
[13] P. Hormnirun, E.L. Marshall, V.C. Gibson, A.J.P. White, D.J. Williams, J. Am.
Chem. Soc. 126 (2004) 2688e2689.
[14] L. Wang, H. Ma, Macromolecules 43 (2010) 6535e6537.
[15] X. Liu, X. Shang, T. Tang, N. Hu, F. Pei, D. Cui, X. Chen, X. Jing, Organometallics
26 (2007) 2747e2757.
[16] M. Bouyahyi, N. Ajellal, E. Kirillov, C.M. Thomas, J.-F. Carpentier, Chem. Eur. J.
17 (2011) 1872e1883.
[17] D.J. Darensbourg, O. Karroonnirun, S.J. Wilson, Inorg. Chem. 50 (2011)
6775e6787.
[18] N. Iwasa, S. Katao, J. Liu, M. Fujiki, Y. Furukawa, K. Nomura, Organometallics
28 (2009) 2179e2187.
[19] S. Dagorne, F.L. Bideau, R. Welter, S. Bellemin-Laponnaz, A. Maisse-François,
Chem. Eur. J. 13 (2007) 3202e3217.
[20] N. Nomura, T. Aoyama, R. Ishii, T. Kondo, Macromolecules 38 (2005)
5363e5366.
4.4.1. General procedure of rac-lactide polymerization
To a solution of rac-lactide (288 mg, 2.00 mmol) in toluene
(1.5 mL), the solution of aluminum complex (0.02 mmol) in toluene
(0.5 mL) was added. The total volume was 2.0 mL. The mixture was
then immerged into an oil bath of 90 ꢀC and stirred for desired time
interval. The polymerization was quenched by addition of wet pe-
troleum ether. After removal of the volatiles, the residue was sub-
jected to 1H NMR analysis. Monomer conversion was determined
by observing the methine resonance integration of monomer vs.
polymer in the 1H NMR (CDCl3, 400 MHz) spectrum. The purifica-
tion of the polymer in each case was managed by dissolving the
crude samples in CH2Cl2 and precipitating the polymer solution
with methanol. The obtained polymers were further dried in a
vacuum oven at 50 ꢀC for 24 h. The polymer samples were sub-
jected to GPC analysis and homonuclear decoupled NMR
measurement.
[21] D. Pappalardo, L. Annunziata, C. Pellecchia, Macromolecules 42 (2009)
6056e6062.
[22] W. Braune, H. Ma, T.P. Spaniol, J. Okuda, Organometallics 24 (2005)
1953e1958.
[23] X. Pang, H. Du, X. Chen, X. Wang, X. Jing, Chem. Eur. J. 14 (2008) 3126e3136.
[24] H. Du, A.H. Velders, P.J. Dijkstra, J. Sun, Z. Zhong, X. Chen, J. Feijen, Chem. Eur. J.
15 (2009) 9836e9845.
4.4.2. General procedure of
3
-caprolactone polymerization
To a solution of -caprolactone (1.0 mL, 2.0 mmol) in toluene, the
3
[25] M.R.T. Breteler, Z. Zhong, P.J. Dijkstra, A.R.A. Palmans, J. Peeters, J. Feijen,
J. Polym. Sci. Part. A Polym. Chem. 45 (2007) 429e436.
[26] O. Phanstiel, Q.X. Wang, D.H. Powell, M.P. Ospina, B.A. Leeson, J. Org. Chem. 64
(1999) 803e806.
solution of aluminum complex (0.02 mmol) in toluene (1.0 mL) was
added. The total volume was 2.0 mL. The mixture was then
immerged into an oil bath of 50 ꢀC and stirred for desired time
interval. The polymerization was quenched by addition of wet
[27] W. Cheung, S. Zheng, W. Yu, G. Zhou, C. Che, Org. Lett. 14 (2003) 2535e2538.