Trinuclear and tetranuclear magnesium alkoxide clusters as highly active initiators for ring-opening polymerization of l-Lactide

Article abstract of DOI:10.1021/ic401459a

Trinuclear and tetranuclear magnesium alkoxide clusters supported by bulky phenolates with triangular or rhombic structures were readily synthesized in acceptable yields via the reaction of 2-N,N-dimethylaminoethanol/methoxyethanol, different phenols, and dibutylmagnesium. These complexes have been characterized using ¹H and ¹³C NMR, elemental analyses, and X-ray crystallography. The experimental results indicate that these clusters are efficient and excellent initiators for the ring-opening polymerizations (ROPs) of l-lactide (LA) and afford polylactides with desired molecular weights and narrow polydispersity indexes (PDIs). Complex 2 can even catalyze the ROP of 4000 equiv of l-lactide in 1 min in a controlled model. Kinetic studies indicate that the polymerization is first-order for both the trinuclear magnesium complex 3 and LA. However, for the tetranuclear magnesium complex 5, the polymerization rate is first order for 5 and second order for LA.

Full text of DOI:10.1021/ic401459a

Article
pubs.acs.org/IC
Trinuclear and Tetranuclear Magnesium Alkoxide Clusters as Highly
Active Initiators for Ring-Opening Polymerization of L‑Lactide
Yuan Gao, Zhongran Dai, Jinjin Zhang, Xinxian Ma, Ning Tang, and Jincai Wu*
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied
Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of
China
S
* Supporting Information
ABSTRACT: Trinuclear and tetranuclear magnesium alk-
oxide clusters supported by bulky phenolates with triangular or
rhombic structures were readily synthesized in acceptable
yields via the reaction of 2-N,N-dimethylaminoethanol/
methoxyethanol, different phenols, and dibutylmagnesium.
1
These complexes have been characterized using H and 13C
NMR, elemental analyses, and X-ray crystallography. The
experimental results indicate that these clusters are efficient
and excellent initiators for the ring-opening polymerizations
(ROPs) of L-lactide (LA) and afford polylactides with desired molecular weights and narrow polydispersity indexes (PDIs).
Complex 2 can even catalyze the ROP of 4000 equiv of L-lactide in 1 min in a controlled model. Kinetic studies indicate that the
polymerization is first-order for both the trinuclear magnesium complex 3 and LA. However, for the tetranuclear magnesium
complex 5, the polymerization rate is first order for 5 and second order for LA.
valuable. Magnesium alkoxides usually tend to oligomerize via
intermolecular O−Mg donor−acceptor bonds. Ashby et al.
noted that the thermodynamically preferred solution compo-
sition of alkyl magnesium alkoxides correlates with the steric
bulk of the alkoxy group and the coordinating ability of the
solvent. In coordinating solvents, such as ether and
tetrahydrofuran (THF), alkylmagnesium alkoxides can form
dimeric solvates, while applying benzene or toluene as solvent
leads to unsolvated cubic tetramers for bulky alkoxy groups
such as tert-butoxy and isopropoxy, but the less-bulky n-
propoxy group causes the formation of oligomers13 (Chart 1).
Driess et al. also reported that in a low coordinating solvent
(i.e., toluene), reactions between different ratios of bulky
alkanol and dimethyl magnesium afforded methyl alkoxide
magnesium clusters with concurrent biscubic structure in yields
of 74%−96%14 (Chart 1).
To avoid the different structures of magnesium alkoxides in
noncoordinating or coordinating solvents, we use the
bidentated 2-N,N-dimethylaminoethanol/methoxyethanol as
the alcohol and introduce a bulky phenoxy into the system to
inhibit the dissociation and aggregation reactions. Thermody-
namically stable magnesium alkoxides are anticipated in both
the coordinating and noncoordinating solvents. The exper-
imental results indicate that this method is suitable for the
synthesis of magnesium alkoxides with acceptable yields, and a
series of novel trinuclear magnesium alkoxide clusters with
triangular structures and tetranuclear magnesium alkoxide
INTRODUCTION
■
Considerable attention has been devoted to polylactic acid in
recent years, because of its wide range of applications in the
biomedical, packaging, and agricultural fields, since it is a
renewable resource and readily biodegradable.1 The conven-
tional method for synthesizing polylactide (PLA) is the ring-
opening polymerization (ROP) of lactide, which results in a
well-controlled molecular weight and low polydispersity
(PDI).2 Many complexes including aluminum,3 titanium,4
zinc,5 magnesium,6 tin,7 lanthanides,8 and other metals9 have
been reported as excellent initiators/catalysts for the controlled
ROP of lactides to afford polymers with both high and
controlled molecular weights. Among these complexes,
magnesium alkoxides have attracted considerable attention,
because of their nontoxicity and high activity.10 Undesirable
side reactions, such as backbiting and trans-esterification, can be
minimized with a sterically encumbered ligand.11 Despite these
advantages, the requirement of delicate, complicated supporting
ligands leads to a high cost for the synthesis, limiting its
practical value. Therefore, some highly active magnesium
initiator/catalyst for the ROP of lactide with simply synthesized
ligands have recently been reported to address this issue.12
Herein, a series of facilely synthesized trinuclear and
tetranuclear magnesium alkoxide clusters bearing bulky simple
monophenols as well as their performance in the ROP of
lactide are reported. The results indicate that these magnesium
alkoxides are highly active for the ROP of lactide under
controlled models.
The method for synthesizing multinuclear magnesium
Received: June 9, 2013
alkoxide clusters described herein is also interesting and
Published: December 23, 2013
© 2013 American Chemical Society
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Chart 1. Key Structures of the Magnesium Alkoxides
clusters with rhombic structures are reported in this work
(Chart 1).
Scheme 1. Synthesis of 1, 2, and 3
RESULTS AND DISCUSSION
■
Synthesis and Structures. The reaction of 2,6-di-tert-
butylphenol (HL1):DMEA (2-N,N-dimethylaminoetha-
nol):MgnBu2 in toluene at room temperature with a ratio of
1:1:1 for the initially designed product shown in Chart 2
Chart 2. Initially Designed Structure (Unsuccessful)
and sharp NMR peaks indicates that thermodynamically stable
complex 3 is the main product.
It is interesting that complexes 6 and 7 cannot be obtained
by changing the bidentate alcohol to 2-methoxyethanol, even
with the exact ratio of 2:4:3. The reaction mixture of 2-
methoxyethanol and MgnBu2 with HL1 or HL2 afforded
complex 4 or 5 in toluene at 60 °C, respectively, which implies
that tetrametallic complexes 4 and 5 are more thermodynami-
cally stable, possibly due to the less sterically encumbered
alcohol, as deduced from the structure analysis described in
Scheme 2.
yielded a mixture evidenced by the complicated NMR
spectrum. Although a single crystal of complex 1 was obtained
from the mixture by recrystallization, the purification of
complex 1 is difficult. While heating the reaction mixture up
to 60 °C for another 24 h, pure complex 2 can be obtained after
recrystallization with an acceptable yield of 52%, which suggests
that trimetallic complex 2 is the thermodynamically preferred
complex (Scheme 1). The exclusive formation of trimetallic
complex 2 is readily achieved by altering the reagent ratio of
HL1:DMEA:MgnBu2 to 2:4:3, resulting in a slightly high yield
of 61% based on MgnBu2. Because the NMR signals for the
dimethylaminoethanol in complex 2 are relatively broad due to
the weak coordination to magnesium, which is discussed in the
structure section below, we also attempted to synthesize
complex 3 to evaluate the validity of this method. A similar
trimetallic structure can be achieved with the 2,6-diisopropyl-
phenol (HL2) ligand in a stoichiometric ratio of 2:4:3 of
HL2:DMEA:MgnBu2 at 60 °C for 24 h, and the one set of clear
Complexes 2, 3, 4, and 5 are air-sensitive, colorless crystals
and are stable under a nitrogen atmosphere. These complexes
are soluble in THF and CH2Cl2 and can be purified via
recrystallization from THF or CH2Cl2/hexane. These com-
plexes were fully characterized by elemental analyses, as well as
1
by H and 13C NMR spectroscopy. The results are consistent
with their respective formulas.
A single crystal of complex 1 was obtained by cooling a hot
THF solution of the reaction mixture and crystallized in
monoclinic space group P21/n. An ORTEP drawing of
complex 1 is shown in Figure 1. This tetranuclear magnesium
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trinuclear magnesium clusters in centrosymmetric monoclinic
space groups of P21/c and P21/n, respectively. The ORTEP
drawings for these complexes are depicted in Figures 2 and 3.
The two similar trinuclear magnesium clusters have triangular
structures in which three Mg atoms are capped by one μ3
alkoxide above the 3Mg plane. In addition, the other three
alkoxides bridge two Mg atoms at each edge. The near right
angles of Mg3−O6−Mg1 (93.51(6)°), Mg3−O6−Mg2
(91.46(6)°), and Mg1−O6−Mg2 (98.19(6)°) for 2 and
Mg3−O6−Mg1 (95.64(3)°), Mg3−O6−Mg2 (92.23(3)°),
and Mg1−O6−Mg2 (96.30(3)°) for 3 suggest that the
trinuclear magnesium complexes can be considered cubane-
like molecules short of a corner, as shown on the right side of
Figures 2 and 3. In these two structures, the three Mg atoms are
all pentacoordinated by four O atoms and one N atom for Mg1
and Mg2 and three O atoms and two N atoms for Mg3. In
comparison, complex 2 is more crowded because of the t-butyl
groups. Therefore, the distances of Mg1−N1 (2.339(2) Å) and
Mg2−N3 (2.4542(18) Å) are longer than the related bond
distances of Mg1−N1 (2.3172(11) Å) and Mg2−N3
(2.2843(10) Å) in complex 3, which may imply complex 2 is
more active than complex 3 because of the facile substitution of
N1 or N3 by LA in the ROP initiating progress. It is notable
that the four alkoxy groups can be divided into one inner
alkoxide and three peripheral alkoxides. The inner alkoxide
bridging three Mg atoms as μ3 ligand is anticipated to be less
active for the ROP of LA than the three μ2 bridge peripheral
alkoxides. In fact, peripheral alkoxides are very active, and the
activity of the inner alkoxide is kinetically suppressed in this
work.
Scheme 2. Synthesis of 4 and 5
alkoxide complex consists of a rhombus fragment (i.e., Mg1−
Mg2−Mg1A−Mg2A), where four edges are supported by μ2
alkoxides and both triangles are capped by μ3 alkoxides lying
on different sides of the rhombus plane. The near right angles
of O3−Mg2A−Mg2 (91.53(6)°), Mg2A−O4A−Mg2
(91.13(7)°), Mg2−O4−Mg1 (95.36(7)°), Mg2A−O4−Mg1
(93.91(7)°), O3−Mg2−O4 (82.34(7)°), and O4−Mg2A−
O4A (88.87(7)°) suggest that the tetranuclear magnesium
complex can also be considered a bis-cubane-like molecule
sharing a common plane of Mg2−O4−Mg2A−O4A short of
two corners, as shown on the right side of Figure 1. Because the
sterically encumbered phenols coordinate to Mg1 and Mg1A,
the formation of the rhombic structure is more suitable in terms
of energy than the normal cubane-like structure, as shown in
Chart 1. In this structure, Mg1 is five-coordinated by O1 of
phenol, O2A, O3, O4, and N1 of dimethylaminoethoxy; Mg2 is
also pentcoordinated by O2, O3, O4, O4A, and N2 of
dimethylaminoethanol. N3 and N3A do not coordinate to Mg,
which is most likely due to the sterically crowded
dimethylamino group. In addition, the serious vibration of
the two noncoordinated dimethylamino groups may give rise to
the thermodynamic instability of complex 1.
Single crystals of complexes 4 and 5 were also obtained in
THF or CH2Cl2/hexane, and they were crystallized as
tetranuclear magnesium clusters in centrosymmetric triclinic
space group P1. ORTEP drawings of these complexes are
̅
shown in Figures 4 and 5. These two tetranuclear magnesium
alkoxide complexes are composed of a rhombus fragment of
Mg1−Mg2−Mg1A-Mg2A and the four Mg2+ ions lay in the
planes defined by the four Mg atoms with almost no deviations.
The near right angles of Mg2−O4A−Mg2A (94.89(9)°),
Mg2−O4−Mg1 (95.04(9)°), and Mg2A−O4−Mg1
Single crystals of complexes 2 and 3 were prepared in a
solution of THF, and these complexes were crystallized as
Figure 1. ORTEP drawing of complex 1 with thermal ellipsoids at the 30% probability level. Selected bond lengths: Mg1−O1, 1.9270(17) Å; Mg1−
O2, 1.9971(19) Å; Mg1−O3, 2.0054(19) Å; Mg1−O4, 2.1755(18) Å; Mg1−N1, 2.347(2) Å; Mg2−O3, 1.961(2) Å; Mg2−O2, 1.9974(19) Å;
Mg2−O4, 2.040(2) Å; Mg2−O4A, 2.074(2) Å; Mg2−N2, 2.209(2) Å; Mg1−Mg2A, 3.1063(10) Å; Mg1−Mg2, 3.1173(10) Å; Mg2−Mg2A,
2.9379(16) Å; and Mg2−Mg1A, 3.1063(10) Å. Selected bond angles: O3−Mg1−O4, 78.02(7)°; O3−Mg2−O2, 171.75(9)°; O3−Mg2−O4,
82.34(7)°; O4−Mg2A−O4A, 88.87(7)°; O3−Mg2−N2, 94.18(8)°; O2−Mg2−N2, 80.91(8)°; O4−Mg2−N2, 117.02(8)°; O4A−Mg2−N2,
152.12(8)°; O3−Mg2A−Mg2, 91.53(6)°; Mg2A−O4A−Mg2, 91.13(7)°; Mg2−O4−Mg1, 95.36(7)°; and Mg2A−O4−Mg1, 93.91(7)°. Symmetry
code: −x, 1 − y, 1 − z.
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Figure 2. ORTEP drawing of complex 2 with thermal ellipsoids at the 30% probability level. Selected bond lengths: Mg1−Mg3, 3.0409(9) Å; Mg1−
Mg2, 3.1863(9) Å; Mg2−Mg3, 2.9924(9) Å; Mg1−O1, 1.9170(15) Å; Mg1−O4, 1.9935(16) Å; Mg1−O3, 2.0150(15) Å; Mg1−O6, 2.1057(15) Å;
Mg1−N1, 2.339(2) Å; Mg2−O2, 1.9303(15) Å; Mg2−O3, 1.9683(15) Å; Mg2−O5, 2.0148(15) Å; Mg2−O6, 2.1103(15) Å; Mg2−N3, 2.4542(18)
Å; Mg3−O5, 1.9397(15) Å; Mg3−O4, 1.9744(15) Å; Mg3−O6, 2.0686(15) Å; Mg3−N4, 2.178(2) Å; Mg3−N2, 2.235(2) Å. Selected bond angles:
Mg3−O6−Mg1, 93.51(6)°; Mg3−O6−Mg2, 91.46(6)°; Mg1−O6−Mg2, 98.19(6)°; Mg2− O3−Mg1, 106.24(7)°; Mg3−O4−Mg1, 100.06(7)°;
Mg3−O5−Mg2, 98.33(6)°; O4−Mg1−O3, 105.92(6)°; O4−Mg1−O6, 80.94(6)°; O3−Mg1−O6, 77.21(6)°; O2−Mg2−O3, 128.68(7)°; O3−
Mg2−O5, 110.88(6)°; O3−Mg2−O6, 78.10(6)°; O5−Mg3−O6, 84.01(6)°; O5−Mg3−O4, 126.38(7)°; and O5−Mg3−O6, 84.01(6)°.
Figure 3. ORTEP drawing of complex 3 with thermal ellipsoids at the 30% probability level. Selected bond lengths: Mg1−Mg3, 3.0404(5) Å; Mg2−
Mg3, 2.9842(5) Å; Mg2−Mg1, 3.0915(5) Å; Mg2−O2, 1.8938(9) Å; Mg2−O3, 1.9589(9) Å; Mg2−O5, 2.0255(9) Å; Mg2−O6, 2.0937(9) Å;
Mg2−N3, 2.2843(10) Å; Mg1−O1, 1.9198(9) Å; Mg1−O4, 1.9867(9) Å; Mg1−O3, 2.0224(9) Å; Mg1−O6, 2.0564(9) Å; Mg1−N1, 2.3172(11) Å;
Mg3−O5, 1.9364(9) Å; Mg3−O4, 1.9778(9) Å; Mg3−O6, 2.0464(9) Å; Mg3−N4, 2.1916(10) Å; Mg3−N2, 2.2178(11) Å. Selected bond angles:
Mg3−O5−Mg2, 97.72(4)°; Mg3 −O4−Mg1, 100.15(4)°; Mg3−O6−Mg1, 95.64(3)°; Mg3−O6−Mg2, 92.23(3)°; and Mg1−O6−Mg2, 96.30(3)°.
(94.60(9)°) for 4 and Mg2A−O4−Mg2 (96.77(6)°), Mg2A−
O4−Mg1 (95.09(7)°), and Mg2−O4−Mg1 (93.77(6)°) for 5,
suggest that the tetranuclear magnesium complex can also be
considered a bis-cubane-like molecule sharing a common plane
of Mg2−O4−Mg2A−O4A short of two corners, as shown in
the right side of Figures 4 and 5. Six-coordinated Mg2 and
Mg2A in complexes 4 and 5 are different from the related Mg
atoms, which are just pentacoordinated in complex 1, clearly
because of the more crowded dimethylamino group, compared
to the methoxy group. The Mg atoms tend to be octahedrally
coordinated when permitted by the surrounding environment.
Therefore, we believe that tetranuclear complexes 4 and 5 are
more thermodynamically stable than the unsynthesized
complexes 6 and 7. Because the sterically encumbered phenols
coordinate to Mg1 and Mg1A, the formation of the rhombic
structure is more suitable in terms of energy than the normal
cubic-like structure, as shown in Chart 1. It is also worth to
note that the six alkoxy groups in the two complexes can be
divided into two inner alkoxides and four peripheral alkoxides.
The inner alkoxides bridging three Mg atoms as μ3 ligand are
anticipated to be less active for the ROP of lactide than the four
μ2 bridge peripheral alkoxides. In fact, our experimental results
indicate that the peripheral alkoxides are also very active, and
the activities of the inner alkoxides are kinetically suppressed, as
observed for complexes 2 and 3.
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Figure 4. ORTEP drawing of complex 4 with thermal ellipsoids at the 30% probability level. Selected bond lengths: Mg2−Mg2A, 3.090(2) Å; Mg2−
Mg1A, 3.1100(15) Å; Mg2−Mg1, 3.1108(15) Å; Mg1−O1, 1.893(2) Å; Mg1−O6A, 1.946(3) Å; Mg1−O2, 1.976(3) Å; Mg1−O4, 2.127(2) Å;
Mg1−O3, 2.344(3) Å; Mg1−Mg2, 3.1099(15) Å; Mg2−O2, 1.997(3) Å; Mg2−O6, 2.025(3) Å; Mg2−O4, 2.090(2) Å; Mg2−O4, A2.105(2) Å;
Mg2−O7, 2.134(3) Å; and Mg2−O5, 2.201(2) Å. Selected bond angles: Mg2−O4A−Mg2A, 94.89(9)°; Mg2−O4−Mg1, 95.04(9)°; Mg2A−O4−
Mg1, 94.60(9)°; O2−Mg2−O4, 79.45(10)°; O4−Mg2A−O4A, 85.11(9)°; O2−Mg2−O6, 172.10(11)°; O2−Mg1−O4, 79.03(10)°; and O6A−
Mg1−O4, 81.58(10)°. Symmetry code = 2 − x, −y, −z.
Figure 5. ORTEP drawing of complex 5 with thermal ellipsoids at the 30% probability level. Selected bond lengths: Mg1−Mg2, 3.0650(13) Å;
Mg1−Mg2A, 3.0803(12) Å; Mg2−Mg1A, 3.0803(12) Å; Mg2−Mg2A, 3.1052(14) Å; Mg1−O1, 1.8769(19) Å; Mg1−O6, 1.9508(17) Å; Mg1−O2,
1.9795(17) Å; Mg1−O4, 2.1101(17) Å; Mg1−O3, 2.177(2) Å; Mg2−O2A, 1.9808(17) Å; Mg2−O6, 2.0170(18) Å; Mg2−O4A, 2.0648(16) Å;
Mg2−O4, 2.0886(16) Å; Mg2−O7, 2.1462(17) Å; Mg2−O5, 2.1711(17) Å. Selected bond angles: Mg2A−O4−Mg2, 96.77(6)°; Mg2A−O4−Mg1,
95.09(7)°; Mg2−O4−Mg1, 93.77(6)°; O2A−Mg2−O6, 171.44(8)°; O2A−Mg2A−O4, 80.09(7)°; O6−Mg2A−O4, 100.24(7)°; O6−Mg2−O4,
81.71(7)°; and O4A−Mg2−O4, 83.23(6)°. Symmetry code = 1 − x, 1 − y, −z.
Ring-Opening Polymerization of L-Lactide Initiated by
Clusters 2 and 3.
active Mg-alkoxide bond in the lactide insertion reaction during
the propagation process. The number-average molecular
weights (Mn determined from GPC) of the produced polymers
are similar to the calculated molecular weights based on three
polymer chains growing per one molecular cluster in which the
less-active inner alkoxide is possibly kinetically suppressed by
the three preferred active peripheral alkoxides. The ROP
initiated by cluster 2 are living,15 which can be elucidated by the
fact that the molecular weights of the polymers increased
linearly with the ratio of [LA]0:[2]0 ranging from 100:1 to
500:1 (see entries 1−5 in Table 1, and Figure 6) and the
narrow PDIs (1.02−1.16). The performance of the living
polymerization was further confirmed by second feed experi-
ments (entry 10) in which another portion of the L-LA
monomer was added after the polymerization of the first
addition of L-LA was completed. Moreover, complex 2 can
polymerize up to 4000 equiv of L-LA at ambient temperature
within 1 min with a controlled molecular weight and narrow
PDI. Comparing to the highly active dimeric magnesium
alkoxides supported by complicated bulky ligands reported
The ring-opening polymerizations (ROP) of L-lactide (L-LA)
employing magnesium alkoxide clusters 2 and 3 as initiators
were systematically examined. Representative results are
collected in Table 1. The polymerizations of L-lactide almost
went to completion in 5 and 20 min for 2 and 3, respectively,
when a 400:1 (monomer: initiator) ratio was used (entries 4
and 12 in Table 1) with a complex concentration of 1 mM in
CH2Cl2 (10 mL) at 30 °C. Therefore, complex 2 is more active
than 3, which is most likely due to the more crowded t-butyl
groups, resulting in weak coordination between the dimethy-
lamino groups and the Mg atom and the lactide monomer
easily coordinating to magnesium in the initiating stage. The
repulsion of the bulky t-butyl groups may also lead to a highly
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a
Table 1. Ring-Opening Polymerization (ROP) of L-Lactide (L-LA) Initiated by Magnesium Clusters 2 and 3
b
c
d
entry
complex
[LA]0/[I]0
t (min)
conversion (%)
Mcal (g mol−1
)
Mobs (g mol−1
)
polydispersity index, PDI
1
2
3
4
5
6
7
8
9
2
2
2
2
2
2
2
2
2
2
2
3
2
2
2
2
2
2
100:1
200:1
5
5
92
>99
97
4500
9600
5600
10500
15600
20000
23000
55000
81000
94100
102100
23000
30000
17000
5100
1.16
1.13
1.10
1.16
1.12
1.16
1.07
1.03
1.02
1.17
1.19
1.08
1.30
1.63
1.56
1.52
1.43
300:1
5
14000
19000
24000
44000
91000
125000
136000
20000
29000
19000
4900
400:1
5
98
500:1
5
98
1000:1
2000:1
3000:1
4000:1
200(200):1
600:1
1
90
1
95
1
87
1
71
e
10
5(5)
1
>99
>99
89
f
11
12
400:1
20
8
g
13
100:1
>99
87
g
14
200:1
8
8440
10400
14000
16000
16400
37000
g
15
300:1
8
87
13000
15500
17000
g
16
400:1
8
80
g
17
500:1
8
70
g
18
1000:1
8
72
35000
1.44
a
b
c
Reactions performed in 10 mL of dichloromethane at 30 °C, [Cat.]0 = 1.0 mM. Lactide conversion as determined by 1H NMR. Calculated from
d
the molecular weight of L-LA times [LA]0/3 times monomer conversion plus MDMEA
polystyrene standard and corrected using the Mark−Houwink factor of 0.58. Polymerization of L-LA with complex 2 for 5 min followed by the
addition of another portion of L-LA. Reactions performed in 10 mL of toluene at 30 °C, [Cat.]0 = 0.17 mM. Melt polymerization at 110 °C.
.
Obtained from GPC analysis and calibrated using a
e
f
g
■
▼
Figure 6. Relationship between Mn ( )/PDI ( ) of the polymer and
the initial molar ratios [LA]0/[2]0 for polymerization of L-LA initiated
by complex 2 in dichloromethane at room temperature (see entries 1−
5 in Table 1).
1
Figure 7. H NMR analysis of poly(L-LA) obtained from polymer-
ization of L-LA initiated by complex 2.
previously by us and the Lin group,6b,k complex 2 can show
slightly higher activity for ROP of LA in CH2Cl2 at room
temperature, even with a low concentration of [Mg]0. The H
LA into the magnesium-alkoxy bond. The ESI mass spectrum
of the final polymer, shown in Figure 8, also confirmed the
coordination insertion mechanism evidenced by the series of
peaks at 72m+89+1, which can be assigned to
(CH3)2NCH2OH + m(C3H4O2) + H+.16 Complex 2 was also
employed to initiate the melt polymerization of L-lactide at 110
°C with various [LA]0/[complex]0 ratios ranging from 100:1 to
1000:1 (see entries 13−18 in Table 1), and the molecule
weights are close to the expected values. The PDIs of the final
polymers (1.3−1.63) are slightly broad, which can be attributed
to the low diffusion of the monomer in the highly viscous melt
polymerization system as well as to some side reactions at high
temperature.
1
NMR of the PLA-50:1 produced from complex 2 at an initial
[LA]0/[complex]0 ratio of 50:1 exhibits a characteristic
quadruple methine peak (Figure 7) at 5.16 ppm, indicating
no serious epimerization of the chiral centers in the polymers,
1
which was also confirmed using homonuclear-decoupled H
NMR studies in the methine region. The peaks at 4.23 ppm
(−CH2OCH2N(CH3)2), 2.56 ppm (−CH2N(CH3)2), and 2.26
ppm (−N(CH3)2) correspond to the dimethylamino group
[OCH2CH2N(CH3)2], and the peak at 4.34 ppm was assigned
to the methine hydrogen in the polymer terminal group of
(HOCHMe−). The integral ratio of the dimethylamino group
to the methine hydrogen indicates that the polymer chain is
capped with one dimethylamino group and one hydroxyl group,
which suggests that the polymerization occurs via insertion of L-
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Figure 8. ESI-MS spectrum of poly(L-LA) prepared by ROP of L-LA.
■
▼
Figure 9. Relationship between Mn ( )/PDI ( ) of the polymer and
the initial molar ratios [LA]0/[4]0 for the polymerization of L-LA
initiated by complex 4 in dichloromethane at room temperature (see
entries 1−5 in Table 2).
Ring-Opening Polymerization of L-Lactide Initiated by
Clusters 4 and 5.
2). The final polymer with a ratio of 50:1 (monomer: complex
4) shows peaks at 4.29 ppm (OCH2−), 3.58 ppm (−CH2−
OCH3), 3.37 ppm (−OCH3), and 4.34 ppm (HOCHMe−)
indicating the presence of terminal groups of methoxyethanoxy
and hydroxyl groups, which confirms that the polymerization
proceeds via insertion of L-lactide into the magnesium-alkoxy
bond (Figure 10).17 The end groups of this obtained oligomer
were analyzed and approved by ESI-MASS too (Figure 11).
The high activity of complex 4 was also verified by ROP of
2000 equiv, which was completed in 3 min and produced a
molecular weight and narrow PDI that were close to the
expected values (entry 8 in Table 2). Melt polymerizations of L-
lactide using complex 4 as the initiator at various [LA]0/
[complex]0 ratios ranging from 200:1 to 500:1 (see entries 10−
13 in Table 2) are performedunder controlled model yielding
the expected molecule weights with slightly broad PDIs for the
final polymers (1.38−1.40), which might be due to the low rate
of diffusion of lactide in the highly viscous melt polymerization
system.
Ring-opening polymerizations (ROP) of L-LA employing
magnesium clusters 4 and 5 as initiators were also conducted.
Representative results are provided in Table 2. The polymer-
ization was nearly complete in 6 min for 4 with an initiator/
monomer ratio of 1:150 and in 60 min for complex 5 with a
conversion of 70%. Therefore, complex 4 is more active than
complex 5, which is similar to the results obtained for
complexes 2 and 3 possibly for similar reasons, and the activity
of complex 4 is similar to that of complex 2 (see entries 1 and 9
in Table 2). The number-average molecular weights of the
produced polymers are also close to the calculated molecular
weights based on four polymer chains growing per one
molecular cluster, in which the two less active inner alkoxides
are kinetically suppressed by the four very active peripheral
alkoxides. The polymerization progress is living, as shown in
Figure 9, and the polymers were obtained with desirable
molecular weights and narrow PDIs (see entries 1−6 in Table
Ring-Opening Polymerization of rac-Lactide. Polymer-
izations of rac-lactide by complexes 2−5 were also performed,
and the results are shown in Table 3. The homonuclear
a
Table 2. ROP of L-Lactide Initiated by Magnesium Clusters 4 and 5
b
c
d
entry
complex
[LA]0/[I]0
t (min)
conversion (%)
Mcal (g/mol)
Mobs (g/mol)
polydispersity index, PDI
1
2
3
4
5
6
7
8
9
4
4
4
4
4
4
4
4
5
4
4
4
4
100:1
150:1
200:1
300:1
400:1
500:1
1000:1
2000:1
150:1
200:1
300:1
400:1
500:1
6
6
97
95
>99
95
94
91
82
70
70
96
91
83
80
3500
5200
4000
5300
1.14
1.20
1.19
1.21
1.16
1.16
1.14
1.04
1.06
1.38
1.40
1.40
1.38
6
7300
8600
6
11000
14000
16400
29500
50000
3800
12000
14800
17400
29600
41000
3500
6
6
4
3
60
8
e
10
7000
8700
e
11
8
10000
12000
14500
11000
11000
13000
e
12
8
e
13
8
a
b
c
Reactions performed in 10 mL of dichloromethane at 30 °C, [Cat.] = 1.0 mM. Lactide conversion as determined by 1H NMR. Calculated from
d
the molecular weight of L-LA times [LA]/3 times monomer conversion yield plus Mmethoxyethanol
polystyrene standard and corrected using the Mark−Houwink factor of 0.58. Melt polymerization at 110 °C.
.
Obtained from GPC analysis, calibrated using
e
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alkoxide clusters. For experimental ease, the less active
complexes (i.e., 3 and 5) were employed in the kinetics
studies. The polymerization rates were analyzed in CH2Cl2 at
1
30 °C and monitored by H NMR using a fixed monomer
concentration ([LA]0 = 0.2 M) and various concentrations of
complex 3 (0.9−1.1 mM). The plots of ln([LA]0/[LA]t), as a
function of time, exhibited a good linear relationship. This
linear relationship indicates that the polymerization proceeds
with a first-order dependence on the monomer concentration
(see Figure S1 in the Supporting Information, kobs = 0.087
min−1, [LA]0 = 0.2 M, [3]0 = 0.9 mM). The experimental
gradient of the least-squares fitted line for ln(kobs) vs ln[3]0 was
0.97(5), which is consistent with a first-order dependence on
[3]0 (Figure S2 in the Supporting Information). Therefore, the
overall rate equation is
d[LA]
dt
1
1
−
= k[LA] [3] (k = 94.42 M−1 min−1)
Figure 10. 1H NMR analysis of poly(L-LA) obtained from
polymerization of L-LA initiated by complex 4 with a [LA]0:[4]0
ratio of 50:1.
The kinetics initiated by tetranuclear complex 5 was also
studied in CH2Cl2 at 30 °C ([LA]0 = 0.2 M, [5]0 = 1.5−2.75
mM). In this system, plots of (1/[LA]t − 1/[LA]0) versus time
are linear, indicating that the polymerization proceeds with a
second-order dependence on the monomer concentration
(Figure S3 in the Supporting Information, kobs = 0.2145 M−1
min−1, [LA]0 = 0.2 M, [5]0 = 1.5 mM). The experimental
gradient of the least-squares fitted line for ln(kobs) vs ln[5]0 was
0.95(1), implying that the polymerization proceeds with a first-
order dependence on [5]0 (Figure S4 in the Supporting
Information). The rate law of
d[LA]
dt
1
−
= k[LA]2[5] (k = 137.08 M−2 min−1)
is the same as that determined in our previous dinuclear
magnesium system.5j
The results of the polymerization kinetics studies indicate a
first-order dependence on both [LA] and [3] when complex 3
is used as the initiator. When complex 5 is used as the initiator,
the polymerization rate has a second-order dependence on
[LA] and a first-order dependence on [5]. These results are
reasonable because trinuclear complex 3 is too crowded to
simultaneously activate two lactides. However, the simulta-
neous activation of two lactides is possible for the less bulky
tetranuclear complex 5. The second-order dependence on [LA]
for complex 5 can also be attributed to the symmetry of
tetranuclear complex 5 where the four Mg atoms can be divided
into two identical parts with a long distance separation related
by an inversion center. Therefore, it is possible that the two
lactides can be activated at the two parts at the same time.
Based on the NMR and mass spectra of the final polymers,
the ROP polymerization mechanism for this system is a
coordination−insertion mechanism. As Duda and Penczek et al.
Figure 11. ESI-MS spectrum of poly(L-LA) prepared by ROP of L-LA
with a [LA]0:[4]0 ratio of 50:1.
1
decoupled H NMR spectra in the methine region of the PLA
derived from complexes 2−5 reveal that only slightly
heterotactic polymers were obtained with Pr = 0.55, 0.54,
0.61, 0.56 for complexes 2−5, respectively, when CH2Cl2 was
used as the solvent.18 No improvements can be achieved by
alternating the solvent to toluene or THF. The low selectivity
may result from the insufficient bulk of the ligand. The
modification of this type of ligand is now in progress in our
laboratory.
Kinetics Studies Using 3 and 5 as Initiators. Kinetics
studies were conducted to gain insight into the polymerization
progress for these trinuclear and tetranuclear magnesium
a
Table 3. ROP of rac-Lactide Initiated by Magnesium Clusters 2, 3, 4, and 5
b
c
d
entry
complex
[rac-LA]0/[I]0
t (min)
conversion (%)
Mcal (g/mol)
Mobs (g/mol )
polydispersity index, PDI
Pr
1
2
3
4
2
3
4
5
200
200
200
200
6
30
7
95
57
96
51
9200
5600
7000
3800
8900
5000
8700
2800
1.20
1.07
1.33
1.01
0.55
0.54
0.61
0.56
70
a
b
c
Reactions performed in 10 mL of dichloromethane at 30 °C, [Cat.]0 = 1.0 mM. Lactide conversion as determined by 1H NMR. Calculated from
the molecular weight of rac-LA times [LA]0/3 times monomer conversion plus MDMEA or rac-LA times [LA]0/4 times monomer conversion yield
plus Mmethoxyethanol. Obtained from GPC analysis, calibrated by polystyrene standard and corrected using the Mark−Houwink factor of 0.58.
d
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pointed out, the alkoxides in aggregated and nonaggregated
Sn(OBu)27e can show different activities toward ROP of LA; it
is reasonable, to a certain extent, to propose that peripheral
alkoxides are more active than inner alkoxide for ROP of LA in
highly active alkoxide clusters 2−5, because of different
environmental factors. But the actual intermediates in the
polymerization progress still cannot be determined, because of
the absence of direct evidence, which will be the subject of
further, future investigations.
C44H82Mg3N4O6: C 63.21, H 9.89, N 6.70. Found: C 63.25, H
9.80, N 6.74. H NMR (600 MHz, CDCl3): δ (ppm) 7.18 (s,
1
Ar−H, 4H), 6.42 (m, Ar−H, 2H), 4.08−3.81 (m, −CH2O,
8H), 2.68 (s, N(CH3)2, 3H), 2.59 (s, N(CH3)2, 6H), 2.47−2.33
(m, N(CH3)2, 15H), 2.24−2.07 (m, −CH2N(CH3)2, 8H), 1.60
(s, C(CH3)3, 36H); 13C NMR (150 MHz, CDCl3): δ (ppm)
167.32 (Ar), 165.27 (Ar), 137.43 (Ar), 136.86 (Ar), 128.88
(Ar), 128.07 (Ar), 125.15 (Ar), 124.76 (Ar), 124.20 (Ar),
137.43 (Ar), 123.71 (Ar), 109.98 (Ar), 65.38 (−CH2OCH2N-
(CH3 )2 ), 64.84 (−CH2 OCH2 N(CH3 )2 ), 63.63
(−CH2OCH2N(CH3)2), 58.66 (−CH2N(CH3)2), 58.08
(−CH2N(CH3)2), 57.19 (−CH2N(CH3)2), 47.11 (N(CH3)2),
46.49 (N(CH3)2), 45.80 (N(CH3)2), 43.69 (N(CH3)2), 35.07
(−C(CH3)3), 30.52 ((CH3)3), 30.11 ((CH3)3).
CONCLUSION
■
Five magnesium alkoxide clusters supported by two bulky
phenols were synthesized, in which the triangular structures are
thermodynamically stable for magnesium alkoxide clusters with
2-N,N-dimethylaminoethanol as the alcohol, and the rhombic
structures are thermodynamically stable for magnesium
alkoxide clusters with methoxyethanol as the alcohol. The
ring-opening polymerization (ROP) results show that the four
thermodynamically stable complexes are highly active yielding
desirable molecular weights with narrow PDIs. In addition,
complex 2 was able to initiate the polymerization of 4000 equiv
of lactide (LA) in 1 min. Because of the different steric
environments in the trinuclear magnesium and tetranuclear
magnesium clusters, the kinetics studies indicated that the
polymerization was first-order in both the trinuclear magnesium
complex 3 and LA. However, for tetranuclear magnesium
complex 5, the polymerization rate was first-order in 5 and
second order in LA.
Synthesis of Complex 3. A mixture of MgnBu2 (3.8 mL,
3.8 mmol), DMEA (0.5 mL, 5.0 mmol), and 2,6-diisopropyl-
phenol (0.445 g, 2.5 mmol) in toluene was stirred at 0 °C for 1
h. Next, the solution was heated to 60 °C and stirred for 24 h. A
white haze gradually formed. The solvent then was removed
under vacuum, and recrystallization with hexane afforded a
white solid. Yield: 0.39 g (40%). Anal. Calcd. for
C40H74Mg3N4O6: C 61.60, H 9.56, N 7.18. Found: C 62.0, H
1
9.62, N 7.06. H NMR (600 MHz, CDCl3): δ (ppm) 6.98 (d,
Ar−H, 2H, J = 7.2 Hz), 6.93 (d, Ar−H, 2H, J = 7.2 Hz), 6.46
(dd, Ar−H, 1H, J = 7.2 Hz), 6.42 (dd, Ar−H, 1H, J = 7.2 Hz),
3.96−3.85 (m, −CH2O, 8H), 3.59−3.52 (m, −CH2N, 6H),
3.21−3.17 (m, −CH(CH3)2, 1H), 3.07−3.02 (m, −CH(CH3)2,
1H), 2.73−2.69 (m, −CH(CH3)2, 2H), 2.54−2.52 (m,
N(CH3)2, 6H), 2.46 (s, N(CH3)2, 3H), 2.36−2.24 (m,
N(CH3)2, 15H), 2.06 (t, −CH2N, 2H), 1.27−1.56 (m,
−CH(CH3)2, 24H); 13C NMR (150 MHz, CDCl3): δ (ppm)
160.92 (Ar), 160.79 (Ar), 135.68 (Ar), 122.13 (Ar), 110.86
(Ar), 110.67 (Ar), 64.85 (−CH2OCH2N(CH3)2), 64.29
(−CH2OCH2N(CH3)2), 64.01 (−CH2OCH2N(CH3)2), 62.61
(−CH2OCH2N(CH3)2), 59.04 (−CH2N(CH3)2), 58.44
(−CH2N(CH3)2), 58.00 (−CH2N(CH3)2), 56.79 (−CH2N-
(CH3)2), 47.00 (N(CH3)2), 46.31 (N(CH3)2), 46.16(N-
(CH3)2), 45.68 (N(CH3)2), 45.22 (N(CH3)2), 4.52 (N(CH3)2),
43.69 (N(CH3)2), 42.96 (N(CH3)2), 27.02 (−CH(CH3)2),
25.54 (−CH(CH3)2), 24.35 (−(CH3)2), 23.76 (−(CH3)2),
22.69 (−(CH3)2).
EXPERIMENTAL SECTION
■
All syntheses and manipulations of air- and moisture-sensitive
materials were performed under a dry nitrogen atmosphere,
using standard Schlenk techniques or a glovebox. Hexane,
toluene, and tetrahydrofuran (THF) were distilled over sodium
benzophenone. CH2Cl2 was distilled from P2O5. 2-N,N-
Dimethylaminoethanol and methoxyethanol were dried with
CaH2 for 24 h at 60 °C and then distilled under a nitrogen
atmosphere. L-LA was purchased from Daigang BIO Engineer
Limited Co. of China and recrystallized from toluene. MgnBu2
(1 mol/L in hexane) and various phenols were purchased from
Acros Company. 1H NMR and 13C NMR spectra were
recorded on a Varian Mercury Plus 600 MHz or a 300 MHz
Synthesis of Complex 4. A mixture of MgnBu2 (5.1 mL,
5.1 mmol), 2-methoxyethanol (0.6 mL, 7.5 mmol), and 2, 6-di-
tert-butylphenol (0.575 g, 2.5 mmol) in toluene was stirred at 0
°C for 1 h. Next, the solution was heated to 60 °C and stirred
for 24 h. A yellow haze gradually formed. The solvent then was
removed under vacuum, and recrystallization with hexane
afforded a white solid. Yield: 0.626 g (66%). Anal. Calcd for
1
spectrometer. H NMR chemical shifts are reported in ppm
versus the residual protons in deuterated solvents as follows: δ
7.26 CDCl3, 13C NMR chemical shifts are reported in ppm
versus residual 13C in the solvent: δ 77.0. The mass
spectroscopic data were obtained using a Thermo Scientific
Orbitrap Elite MS (LTQ Orbitrap Elit).
1
C46H84Mg4O14: C 57.65, H 8.83. Found: C 57.75, H 8.78. H
Synthesis of Complex 1. A mixture of MgnBu2 (5.1 mL,
5.1 mmol), DMEA (0.5 mL, 5.0 mmol), and 2,6-di-tert-
butylphenol (1.15 g, 5 mmol) in toluene was stirred at 0 °C for
1 h. The solution was stirred at room temperature for 24 h. A
pale yellow haze gradually formed. The solvent then was
removed under vacuum, and recrystallization with hexane
afforded a solid white mixture.
NMR (300 MHz, CDCl3): δ (ppm) 7.12 (d, Ar−H, 4H, J = 7.2
Hz), 6.36 (dd, Ar−H, 2H, J = 7.2 Hz), 3.9−3.78 (m, OCH2−,
12H), 3.72−3.64 (m, −CH2−OCH3, 12H), 3.4 (s, −CH2−
OCH3, 18H), 1.48 (s, C(CH3)3, 18H), 1.45 (s, C(CH3)3,
18H); 13C NMR (75 MHz, CDCl3): δ (ppm) 137.54 (Ar),
125.3 (Ar), 124.27 (Ar), 110.5 (Ar), 75.00 (−CH2O), 75.68
(−CH2O), 60.30 (−CH2−OCH3), 58.96 (−CH2−OCH3),
57.99 (−OCH3), 35.31 (−C(CH3)3), 30.78 (−C(CH3)3).
Synthesis of Complex 5. A mixture of MgnBu2 (5.1 mL,
5.1 mmol), 2-methoxyethanol (0.6 mL, 7.5 mmol), and 2,6-
diisopropylphenol (0.445 g, 2.5 mmol) in toluene was stirred at
0 °C for 1 h. Next, the solution was heated to 60 °C and stirred
for 24 h. A pale haze gradually formed. The solvent then was
removed under vacuum, and recrystallization with hexane
Synthesis of Complex 2. A mixture of MgnBu2 (3.8 mL,
3.8 mmol), DMEA (0.5 mL, 5.0 mmol), and 2,6-di-tert-
butylphenol (0.575 g, 2.5 mmol) in toluene was stirred at 0 °C
for 1 h. Next, the solution was heated to 60 °C and stirred for
24 h. A dark yellow haze gradually formed. Then, the solvent
was removed under vacuum, and recrystallization with hexane
afforded white solid. Yield: 0.61 g (61%). Anal. Calcd for
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afforded a white solid. Yield: 0.445 g (51%). Anal. Calcd for
C42H76Mg4O14: C 55.91, H 8.49. Found: C 56.06, H 8.52. H
Research Funds of the Central Universities of China (No.
lzujbky-2012-67), and the Project for National Basic Science
Personnel Training Fund (No. J1103307) are gratefully
acknowledged.
1
NMR (300 MHz, CDCl3): δ (ppm) 6.97 (d, Ar−H, 4H, J = 7.2
Hz), 6.48 (dd, Ar−H, 2H, J = 7.2 Hz), 3.81−3.72 (m, OCH2−,
12H), 3.67−3.60 (m, −CH(CH3)2, 4H), 3.58−3.52 (m,
−CH2−OCH3, 12H), 3.48 (s, −OCH3, 18H), 1.21 (d,
CH(CH3)2, 24H, J = 6 Hz); 13C NMR (75 MHz, CDCl3): δ
(ppm) 159.46 (Ar), 134.84 (Ar), 121.14 (Ar), 110.2 (Ar),
75.57 (−CH2O), 75.23 (−CH2O), 74.17 (−CH2O), 59.33
(−CH2−OCH3), 57.95 (−CH2−OCH3), 57.42 (−OCH3),
24.97 (−CH(CH3)2), 22.89 (−CH(CH3)2).
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J. Macromolecules 2002, 35, 7538.
Crystallographic Studies. The data were collected at low
temperature (100−200 K) on SuperNova (Dual) X-ray
diffraction diffractometer with graphite-monochromated Mo
Kα radiation (λ = 0.71073 Å). The structures were solved by
direct methods of Siemens SHELXTL PLUS program.19 Non-
hydrogen atoms were refined with anisotropic displacement
parameters during the final cycles. All H atoms were placed by
geometrical considerations and were added to the structure
factor calculation.
ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files giving crystallographic data for 1, 2, 3, 4, and 5
(CCDC Reference Nos. 942413, 942414, 942415, 942416, and
942417, respectively) are available. A PDF file showing the
relationship between ln([LA]0/[LA]t) versus time for complex
3, ln kobs versus ln[3]0 for the polymerization of L-lactide with
complex 3, ln([LA]0/[LA]t) versus time for complex 5, and ln
kobs versus ln[5]0 for the polymerization of L-lactide with
complex 5 is also provided. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Tel.: 86 931 8912580. Fax: 86 931 8912582. E-mail: wujc@
lzu.edu.cn.
■
́
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Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science
Foundation of China (Nos. 21071069, 21171078, and
21271092), State Education Ministry and Fundamental
725
dx.doi.org/10.1021/ic401459a | Inorg. Chem. 2014, 53, 716−726

Inorganic Chemistry
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