Magnesium and zinc complexes supported by N, O-bidentate pyridyl functionalized alkoxy ligands: Synthesis and immortal ROP of ε-CL and l-LA

Article abstract of DOI:10.1021/om300113p

The N,O-bidentate pyridyl functionalized alkoxy ligands 2-(6-methyl-2-pyridinyl)-1,1-dimethyl-1-ethanol (L¹-H) and 2-(6-methyl-2-pyridinyl)-1,1-diphenyl-1-ethanol (L²-H) have been prepared by treatment of acetone and benzophenone with monolithiated 2,6-lutidine. Deprotonolysis of the ligands L¹-H and L²-H with 1 equiv of MgⁿBu₂ and ZnEt₂ in toluene by releasing butane and ethane, respectively, gave the corresponding dimeric metal-monoalkyl complexes [L¹MgⁿBu]₂ (1), [L²MgⁿBu]₂ (2), [L¹ZnEt]₂ (3), and [L²ZnEt]₂ (4). Complexes 1-4 were characterized by ¹H and ¹³C NMR spectroscopy analysis, and the molecular structures of 1, 3, and 4 were further confirmed by X-ray diffraction analysis. The investigation of the catalytic behavior of these complexes toward ε-caprolactone (ε-CL) and l-lactide (l-LA) polymerizations showed that the Mg-based complexes gave higher activity than those attached to zinc metal, probably owing to the greater ionic character of the magnesium metal. Remarkably, the magnesium complex 2 exhibited a striking immortal nature in the presence of primary alcohols where up to 500 PCL chains grew from each Mg active center when benzyl alcohol was employed, while, in particular, in the presence of triethanolamine, complex 2 also displayed an immortal mode for the polymerization of l-LA.

Full text of DOI:10.1021/om300113p

Article
pubs.acs.org/Organometallics
Magnesium and Zinc Complexes Supported by N,O-Bidentate Pyridyl
Functionalized Alkoxy Ligands: Synthesis and Immortal ROP of ε-CL
and L-LA
Yang Wang,†,‡ Wei Zhao,†,‡ Dongtao Liu,† Shihui Li,† Xinli Liu,† Dongmei Cui,*,† and Xuesi Chen†
†State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, China
‡Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
S
* Supporting Information
ABSTRACT: The N,O-bidentate pyridyl functionalized alkoxy ligands 2-(6-methyl-2-pyridinyl)-1,1-dimethyl-1-ethanol (L1−H)
and 2-(6-methyl-2-pyridinyl)-1,1-diphenyl-1-ethanol (L2−H) have been prepared by treatment of acetone and benzophenone
with monolithiated 2,6-lutidine. Deprotonolysis of the ligands L1−H and L2−H with 1 equiv of MgnBu2 and ZnEt2 in toluene by
releasing butane and ethane, respectively, gave the corresponding dimeric metal-monoalkyl complexes [L1MgnBu]2 (1),
1
[L2MgnBu]2 (2), [L1ZnEt]2 (3), and [L2ZnEt]2 (4). Complexes 1−4 were characterized by H and 13C NMR spectroscopy
analysis, and the molecular structures of 1, 3, and 4 were further confirmed by X-ray diffraction analysis. The investigation of the
catalytic behavior of these complexes toward ε-caprolactone (ε-CL) and L-lactide (L-LA) polymerizations showed that the Mg-
based complexes gave higher activity than those attached to zinc metal, probably owing to the greater ionic character of the
magnesium metal. Remarkably, the magnesium complex 2 exhibited a striking “immortal” nature in the presence of primary
alcohols where up to 500 PCL chains grew from each Mg active center when benzyl alcohol was employed, while, in particular, in
the presence of triethanolamine, complex 2 also displayed an immortal mode for the polymerization of L-LA.
health issues associated with the toxicity of some metal-based
residues.14 Moreover, the resulting single or linear PCL and
PLA materials suffer the problems of brittleness and high
process viscosity, which could be overcome by introducing
flexible monomer units into these polymer backbones to
prepare PCL-based and PLA-based block copolymers, or by
forming such polymers with star-shaped or dendrimer, or
hyperbranch topological architectures.15−17 However, most of
these metal-based coordination catalysts lack livingness, and the
resultant polymer chain ends have no functionality; thus, the
above target is difficult to reach.
Our interest has been concentrated on the biobenign
calcium, magnesium, and zinc catalysts18 because they
participate in the human metabolism.19 Herein, we wish to
report the synthesis, characterization, and catalysis of novel
magnesium and zinc derivatives coordinated by monovalent
N,O-bidentate pyridyl alkoxy ligands. In the presence of excess
benzyl alcohol, some complexes exhibit an interesting perform-
ance in immortal polymerization. Thus, each metal propagating
species generates more than 1 and up to 500 PCL molecules
INTRODUCTION
■
For the past decades, in view of their excellent biodegradable,
biocompatible, and permeable properties both in the environ-
ment and in vivo, polyesters, in particular polycaprolactone
(PCL), polylactide (PLA), and their copolymers, have found
wide applications as packaging materials and devices in more
sophisticated pharmaceutical and medical industries for
controlled drug release and antibody and gene delivery and
scaffolds in tissue engineering,1 as well as the promising
alternatives of synthetic petrochemical-based plastics.2
Although polyesters can be produced by condensation
polymerization or ring-opening polymerization (ROP) by
anionic, cationic, and organic compounds, the ROP via a
coordination−insertion mechanism initiated by metal-based
complexes has been commonly accepted as a more efficient
manner by providing polyesters with well-controlled molecular
weight, composition, and regularity.2d,3 To date, a huge number
of metal-based complexes, including potassium,4 lithium,5
magnesium,6 zinc,6a,b,f,g,i,7 iron,8 calcium,9 aluminum,10 stan-
nous,11 yttrium,12 and lanthanide,13 have been innovated, and
their catalytic performances have been widely investigated.
However, the amounts of catalysts in the obtained polymers are
usually high, which may raise concerns regarding the potential
Received: February 12, 2012
© XXXX American Chemical Society
A
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with an adjustable molecular weight and narrow polydispersity,
and simultaneously, the PCL chain ends are capped with a
hydroxyl functionality. The hydroxyl ends facilitate incorpo-
ration of flexible monomer units, in particular, of bioactive
substituents, such as drugs or fluorescent tags, to construct
novel PCL-based block copolymers or functionalized PCL, in
one pot.20,21
RESULTS AND DISCUSSION
■
Synthesis and Structure Characterization. Treatment of
2,6-lutidine lithium salt with acetone and benzophenone
afforded the corresponding ligands 2-(6-methyl-2-pyridinyl)-
1,1-dimethyl-1-ethanol (L1−H) and 2-(6-methyl-2-pyridinyl)-
1,1-diphenyl-1-ethanol (L2−H) as a yellow oil or white powder
in 78% and 85% yields, respectively. Deprotonolysis of L1−H
and L2−H by equimolar amounts of MgnBu2 and ZnEt2 in
toluene with the releasing of butane and ethane, respectively,
gave dimeric metal-monoalkyl complexes [L1MgnBu]2 (1),
[L2MgnBu]2 (2), [L1ZnEt]2 (3), and [L2ZnEt]2 (4) in
Figure 1. ORTEP diagram of the molecular structure of complex 1.
Thermal ellipsoids are drawn at the 35% probability level. All of the
hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Mg(1)−O(1) 1.9712(14), Mg(1)−O(1A) 1.9880(14),
Mg(1)−C(8) 2.132(2), Mg(1)−N(1) 2.1860(17); O(1)−Mg(1)−
O(1A) 85.09(5), O(1)−Mg(1)−C(8) 124.14(8), O(1A)−Mg(1)−
C(8) 132.14(8), C(8)−Mg(1)−N(1) 109.19(8), O(1)−Mg(1)−N(1)
110.89(6), O(1A)−Mg(1)−N(1) 89.68(6).
1
quantitative yields (Scheme 1). H NMR spectrum analysis
Scheme 1. Preparation of Complexes 1−4
shows that the resonances of the methylene protons CH2 from
zinc complexes 3 and 4 give singlet peaks at 2.90 and 3.80 ppm,
respectively, which shift downfield slightly compared with those
in the corresponding neutral ligands (2.73 ppm for L1−H; 3.65
1
ppm for L2−H). The H NMR spectra of the magnesium
complexes 1 and 2 give different topologies from their zinc
analogues, in which the methylene groups CH2 show broad
doublet resonances at 3.16 and 2.61 ppm for complex 1 and
doublet−doublet resonances at 4.14 ppm (2JH−H = 16 Hz) and
3.49 ppm (2JH−H = 12 Hz) for complex 2.
The structures of complexes 1, 3, and 4 were defined further
by the X-ray diffraction technique, as shown in Figures 1−3. All
of these complexes adopt dimeric structures, where two anionic
lutidine functionalized alkoxy ligands bridge two metal alkyl
moieties via oxygen atoms, generating a Mg2O2 or Zn2O2
planar core in the center of each molecule. All complexes are C2
symmetric. The metal oxygen bond distances are odd, but
comparable to the reported values:22 Mg(1)−O(1A) 1.988(1)
Å versus Mg(1)−O(1) 1.971(1) Å (1); Zn(1)−O(1) 2.030(1)
Å versus Zn(1)−O(1A) 1.998(1) Å (3); Zn(1)−O(1)
2.025(5) Å versus Zn(1)−O(2) 2.009(6) Å (4). Meanwhile,
all the metal carbon bonds (Mg(1)−C(8) 2.132(2) Å, Zn(1)−
C(11) 1.984(2) Å, Zn(1)−C(21) 2.049(10) Å, Zn(2)−C(43)
1.988(10) Å) and the metal nitrogen bonds (Mg(1)−N(1)
2.186(2) Å, Zn(1)−N(1) 2.165(1) Å, Zn(1)−N(1) 2.137(7)
Å, Zn(2)−N(2) 2.171(6) Å) fall in the reasonable ranges for
those reported in the literature.22
Figure 2. ORTEP diagram of the molecular structure of complex 3.
Thermal ellipsoids are drawn at the 35% probability level. All of the
hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Zn(1)−O(1) 2.0300(12), Zn(1)−O(1A) 1.9976(12),
Zn(1)−C(11) 1.9837(19), Zn(1)−N(1) 2.1648(15), O(1)−Zn(1)−
O(1A) 83.78(5), O(1)−Zn(1)−C(11) 123.42(7), O(1A)−Zn(1)−
C(11) 130.05(7), C(11)−Zn(1)−N(1) 119.03(7), O(1A)−Zn(1)−
N(1) 100.06(5), O(1)−Zn(1)−N(1) 89.44(5).
mode, among which the magnesium complexes 1 and 2
displayed a similar high activity, independent of the ligand type,
to transfer 100 equiv of ε-CL into PCL in less than 1 min,
whereas their zinc counterparts 3 and 4 needed 4 h to reach
17% and 92% conversions, respectively (Table 1, entries 1−4).
This could be attributed to the higher ionic character of the
central Mg2+ ion than that of the Zn2+ ion,6i,23 which facilitates
coordination of ε-CL. Therefore, in the case of Mg-based
precursors, the ligand type showed a minor effect on the
catalytic activity, whereas in contrast, for zinc complexes,
Ring-Opening Polymerization of ε-Caprolactone. The
ROP of ε-CL initiated by complex 1, 2, 3, or 4 was carried out
in THF at room temperature. All of these complexes 1−4 were
active in the absence of alcohol, albeit in a less-controlled
B
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ligands displayed more obvious influences on the activity
electronically and sterically:6a,e,24 complex 4 bearing the
sterically bulky and electron-withdrawing L2 ligand (phenyl
substituent) gave a much higher activity than complex 3
attached to the sterically less-hindered and electron-donating L1
ligand (methyl substituent). As metal alkoxide initiators that
efficiently mimic the propagating groups of the presumed active
species usually exhibit much more promising performances
than their alkyl analogues, alcohols are always chosen to
combine with equivalent metal alkyl precursors to generate in
situ the metal alkoxide initiators via alcoholysis.13b,d,25,26 Thus,
we investigated the catalytic behaviors of the binary systems
formed by complexes 1−4 in combination with isopropanol
(iPrOH), benzyl alcohol (BnOH), and triethanolamine (TEA),
respectively. When BnOH was used to activate complexes 1
and 2, the generated magnesium alkoxide initiators (Mg−OBn)
behaved similarly to their metal alkyl precursors (Mg−nBu), but
in a more controlled manner. The resultant PCL had molecular
weights close to the theoretic values with a very narrow
polydispersity. In contrast, activations of the zinc complexes 3
and 4 with BnOH did not perform smoothly to give
complicated products that did not bring about obvious
improvements in the catalytic performances than their
congeners (Table 1, entries 5−8).27 Fixing complex 2 as the
Figure 3. ORTEP diagram of the molecular structure of complex 4.
Thermal ellipsoids are drawn at the 35% probability level. All of the
hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Zn(1)−O(2) 2.035(5), Zn(1)−C(21) 2.049(10),
Zn(2)−C(43) 1.988(10), Zn(1)−O(1) 2.011(6), Zn(1)−N(1)
2.137(7), Zn(2)−N(2) 2.171(6), O(1)−Zn(1)−O(2) 85.8(2),
O(2)−Zn(1)−C(21) 126.3(3), O(2)−Zn(1)−N(1) 95.5(2),
C(21)−Zn(1)−N(1) 115.5(3).
i
precursor, switching the alcohol to PrOH and a polyol
N(CH2CH2OH)3, respectively, the generated binary systems,
unfortunately, were low active and even inert in the latter case
(Table 1, entries 9 and 11). Thus, complex 2 and BnOH
Table 1. Ring-Opening Polymerization of ε-CL Initiated by Complexes 1−4
a
b
c
d
d
entry
cat.
[CL]0/[I]0
ROH
[OH]0/[I]0
time (min)
conv. (%)
Mn,calcd × 10−4
Mn,SEC × 10−4
Mw/Mn
1
2
1
2
3
4
1
2
3
4
2
2
2
2
100
100
100
100
100
100
100
100
600
600
600
500
1000
1500
2000
500
500
500
500
500
500
1000
2000
5000
0
0
1
1
100
100
17
1.13
1.13
0.20
1.15
1.14
1.14
0.25
1.14
4.59
6.79
1.45
5.66
11.4
17.0
22.6
2.84
1.89
1.42
1.14
0.72
0.58
0.12
0.12
4.47
3.11
0.27
2.33
1.09
1.23
0.23
1.40
3.07
7.22
nd.
2.00
2.00
1.59
2.05
1.21
1.14
1.94
1.15
1.07
1.30
nd.
3
0
240
240
1
4
0
92
5
BnOH
BnOH
BnOH
BnOH
iPrOH
BnOH
TEA
1
100
100
21
6
1
1
7
1
240
240
5
8
1
99
9
1
67
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
1
2
100
7
1
5
BnOH
BnOH
BnOH
BnOH
BnOH
BnOH
BnOH
BnOH
BnOH
BnOH
BnOH
BnOH
BnOH
1
2
100
100
100
100
100
100
100
100
100
100
100
100
100
6.55
16.4
20.8
46.4
3.15
1.74
1.60
1.25
0.91
0.76
1.20
1.26
1.63
1.77
1.14
1.06
1.08
1.06
1.07
1.05
1.03
1.03
1.09
e
2
1
1
e
2
1
5
e
2
1
30
30
60
120
120
120
120
60
120
2
2
2
2
2
2
2
3
4
5
8
10
100
200
500
e
b
2
0.18
e
b
2
0.18
e
b
2
300
0.12
0.14
a
b
c
Polymerizations were performed in THF, at 25 2 °C, [CL]0 = 0.88 M. Obtained from 1H NMR analysis. Calculated by ([CL]0/[I]0) × 114.14
d
× conv. (%); with addition of alcohol, ([CL]0/[OH]0) × 114.14 × conv. (%) + MROH
. Determined by size exclusion chromatography (SEC)
against a polystyrene standard. Mn values were obtained using a correcting factor for polycaprolactone (0.56).30 [CL]0 = 2.63 M.
e
C
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established the best initiation system (Table 1, entry 10) and
was studied in detail. With the monomer-to-Mg ratio varying
from 100 to 1500, the polymerization performed smoothly to
give PCL with a molecular weight increasing from Mn = 1.14 ×
104 g/mol to Mn = 20.8 × 104 g/mol, well consistent with the
theoretic values. Note that, when the ratio was over 2000, the
polymerization was too rapid and the polymerization system
became extremely viscous, which made the monomer diffusion
difficult and aroused the deviation of the molecular weight of
the resultant PCL from the theoretic value and the broadened
PDI (Table 1, entries 6 and 12−15). Remarkably, complex 2
was so tolerant to the protic BnOH that, with an increase of
BnOH loading from 2 to 10 equiv, the polymerization went
rapidly; in addition, the molecular weight of the resultant PCL
decreased reversibly with respect to the value calculated based
on BnOH loading, while the molecular weight distribution
remained constant (Table 1, entries 16−21; Figure 4). This
1
Figure 5. H NMR spectrum of PCL-40 in CDCl3.
mechanism, and with a hydroxyl functionality because of the
immortal polymerization.25,28 The alcoholysis reaction of
magnesium complex 2 and excess benzyl alcohol was
1
monitored by the H NMR spectroscopy technique, which
proved the formation of magnesium alkoxide active species
Mg−OCH2Ph (br 4.48 ppm) with the releasing of butane
(1.34, 0.97 ppm) and the absence of ligand extrusion (Figure
S1, Supporting Information). This further confirmed that the
polymerization proceeded in the coordination−insertion
mechanism, not via the activated monomer mechanism.29
The hydroxyl ends provide a convenient approach to construct
novel PCL-based block copolymers or functionalized PCL, in
one pot.
Ring-Opening Polymerization of L-Lactide. The cata-
lytic performances of complexes 1−4 toward the polymer-
ization of L-lactide (L-LA) were also investigated. In the absence
of alcohols, the magnesium complexes showed much higher
activity for the ROP of L-LA at room temperature than the
corresponding zinc analogues that initiated the polymerization
at high temperature to reach quantitative yields (Table 2,
entries 1−4). Nevertheless, the resultant PLLAs from both Mg-
based and Zn-based systems had molecular weights slightly
larger than the calculated values and medium polydispersity
(PDI = 1.52−1.67). 1H NMR spectroscopy analysis was
employed to monitor the polymerization with complex 2 as the
initiator under a very low monomer-to-Mg ratio, anticipated to
obtain the end-group information of oligomeric PLA. The
result showed that nBu was found as one of the PLA chain ends
(4.22, 1.50, 1.27, 0.90 ppm), and the signals from the L2−H
ligand were not observed, suggesting that the Mg−nBu species
initiated the polymerization, not the chelating ligand (Figure
S3, Supporting Information). Addition of 1 equiv of benzyl
alcohol, BnOH, to activate complexes 1 and 2, respectively,
seemed not to make any improvement in the catalytic
performances. Increasing the BnOH loading (OH-to-Mg = 6)
Figure 4. Dependence of the molar mass Mn,SEC (PDI indicated in
parentheses) on [BnOH]0-to-[Mg]0 ratio. Conditions: [CL]0-to-
[Mg]0 ratio = 500; [CL]0 = 0.88 M; solvent, THF; Tp = 25 °C.
result indicated that an excess amount of BnOH did not
terminate the polymerization as usual but behaved as a chain
transfer agent; in particular, it aroused a living chain-transfer
polymerization. Strikingly, when the OH-to-Mg ratio varied
from 100 to 500 while the CL-to-Mg ratio varied from 1000 to
5000, the polymerization still performed in obviously high
activities so that 100% conversion could be achieved, albeit in a
prolonged time (1−5 h), to provide PCL with narrow
molecular weight distributions (PDI = 1.03−1.09) (Table 1,
entries 22−24). This result indicated that the binary system 2/
BnOH possessed an immortal nature that up to 500 PCL
chains grew from each Mg metal center. The molecular weight
of the resultant PCL and its chain ends were defined by the 1H
NMR study. For example, the 1H NMR spectrum (Figure 5) of
PCL-40 (the number 40 indicates the designed [CL]0/
[BnOH]0 value) gives a singlet resonance at 5.12 ppm assigned
to Hf from the benzyloxide chain end −OCH2Ph, a multiple
resonance around 3.60 ppm attributed to Ha from methylene
−CH2 attaching to the hydroxyl end, and a triplet centered at
4.12 ppm arising from He ({−RCH2OC(O)−}n). The integral
ratio of these resonances is 2:2:81, in precise accordance with
the polymerization degree of 40. These results revealed that the
PCL macromolecular chains are capped with benzylmethoxide
at one end, probably attributed to the coordination−insertion
i
or using alcohol, PrOH, aroused indeed a more controllable
system without backbiting or transesterification (Table 2,
entries 5−9 and 11); however, the catalytic activity dropped
obviously (Table 2, entries 10 and 12). The resulting PLA
macromolecular chain is capped with the hydroxyl group at one
end, and with the PhCH2O− group at the other end
(PhCH2O− is overlapped by methane in the polymer chain
at about 5.12 ppm, Figure 6), suggesting probably a
coordination insertion mechanism. To our delight, when TEA
was employed to combine with complex 2, the generated Mg−
D
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Table 2. Ring-Opening Polymerization of L-Lactide Catalyzed by Complexes 1−4
a
b
c
d
d
entry
cat.
[LA]0/[I]0
ROH
[OH]0/[I]0
time (min)
conv. (%)
Mn,calcd × 10−4
Mn,SEC × 10−4
Mw/Mn
1
2
1
2
100
100
100
100
100
100
100
100
600
600
600
600
600
600
900
900
0
0
5
5
100
100
100
89
1.45
1.45
1.45
1.28
1.09
0.59
1.45
0.33
5.28
0.35
8.66
1.03
25.9
4.34
13.0
1.45
2.35
2.02
2.08
1.27
1.23
0.46
1.44
0.53
4.60
0.32
8.62
1.00
18.9
4.31
13.7
1.92
1.52
1.53
1.63
1.67
1.32
1.10
1.48
1.11
1.28
1.05
1.64
1.06
1.95
1.35
1.09
1.14
e
3
3
0
1020
1020
15
25
5
e
4
4
0
5
1
1
2
2
BnOH
BnOH
BnOH
BnOH
iPrOH
iPrOH
BnOH
BnOH
TEA
1
75
6
2
81
7
1
100
44
8
2
5
f
9
2
1
5
61
f
10
11
12
13
14
15
16
2
6
60
5
24
f
2
1
100
71
f
2
6
60
5
f
2
1
100
100
100
f
2
TEA
6
60
5
f
2
TEA
3
f
2
TEA
27
60
100
a
b
c
In THF, at 25 2 °C, [LA]0 = 0.83 M. Determined by 1H NMR spectroscopy. Mn,calcd = ([LA]0/[I]0) × 144.13 × conv. (%); with addition of
d
alcohol, Mn,calcd = ([LA]0/[OH]0) × 144.13 × conv. (%) + MROH. Determined by size exclusion chromatography against a polystyrene standard. Mn
values were obtained using a correcting factor for polylactide (0.58).30 70 2 °C. [LA]0 = 2.08 M.
e
f
CONCLUSION
■
We have demonstrated that a new series of N,O-bidentate
pyridyl functionalized alkoxyl ligated magnesium and zinc alkyl
complexes have been synthesized selectively via a protonolysis
reaction between the pyridyl alcohols and the corresponding
dibutylmagnesium and diethylzinc. All of these complexes are
well defined by NMR spectroscopy and X-ray diffraction
analyses, adopting dimeric structures. The magnesium alkyl
complexes show much higher activities toward both ε-CL and
L-LA polymerizations than the corresponding zinc analogues,
which might be attributed to the higher ionic properties of
Mg2+ than that of Zn2+. The bulkiness and the electron-
withdrawing substituent lead to increasing of the catalytic
activity to the attached zinc precursors but show no influence
on that of Mg-based precursors. Strikingly, the binary system
composed of magnesium complex 2 and benzyl alcohol, BnOH,
displays excellent catalytic performances, which initiates the
polymerization of ε-CL under a wide range of OH-to-Mg
values (1−500) and high ε-CL loadings up to 5000, suggesting
an immortal polymerization characteristic where up to 500 PCL
molecules grew from each Mg active center, apparently. In
contrast, the combination of complex 2 and triethanolamine,
TEA, establishes an efficient catalytic system for the immortal
polymerization of L-LA to provide PLLA with a predicted
molecular weight and narrow molecular weight distribution.
Both polymerizations are performed in a coordination−
insertion mechanism, and the resultant PCL and PLLA chains
are automatically end-capped by hydroxyl groups, which
facilitate the preparation of PCL- and PLLA-based block
functionalized copolymers in one pot.
1
Figure 6. H NMR spectrum of PLLA-20 in CDCl3.
OCH2CH2N− active species initiated the polymerization in a
livingness mode with high activity, which was dramatically
different from its behavior toward ε-CL polymerization. Such a
distinguished performance remained when both OH-to-Mg and
LLA-to-Mg values were high (Table 2, entries 13−15),
suggesting an immortal polymerization mode, and each
magnesium active species grew up to 27 PLLA polymer chains
1
(Table 2, entry 16). The H NMR spectrum of the oligomeric
polymer showed clearly the resonances from three HOCH-
(CH3)−CO end groups and a triethanolamine core for each
macromolecule (Figure S4, Supporting Information). The
completely opposite behavior of the binary systems 2/BnOH
and 2/TEA toward the polymerizations of CL and LA is under
investigation.
EXPERIMENTAL SECTION
■
General Methods. All operations were carried out under an
atmosphere of argon using standard Schlenk techniques. Solvents were
reagent grade, dried by standard methods,31 and distilled under
E
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Article
nitrogen prior to use. Toluene, tetrahydrofuran, and n-hexane were
dried over 4 Å molecular sieves for a week and distilled before use.
C6D6 was purchased from Cambridge Isotopes, dried over Na, and
stored in the glovebox. 2,6-Lutidine was purchased from Fluka and
used after being carefully dried. 2-(6-Methyl-2-pyridinyl)-1,1-dimethyl-
1-ethanol (L1−H) and 2-(6-methyl-2-pyridinyl)-1,1-diphenyl-1-etha-
nol (L2−H) were prepared as in the literature.32 MgnBu2 and ZnEt2
were purchased from Sigma-Aldrich. All reactions were carried out
under a dry and oxygen-free argon atmosphere by using Schlenk
techniques or under a nitrogen atmosphere in an MBRAUN glovebox.
ε-CL was dried over calcium hydride and distilled under nitrogen prior
to its use. L-Lactide was recrystallized with dry toluene and then
sublimed three times under vacuum at 80 °C. Benzyl alcohol,
isopropanol, and triethanolamine were dried over calcium hydride
prior to distillation. All other chemicals were commercially available
and used after appropriate purification. Glassware and flasks used in
the polymerization were dried in an oven at 115 °C overnight and
exposed to a vacuum−argon cycle three times.
120.99 (1C, m-Py−C), 78.3 (1C, Py−C−CH2−), 46.78 (1C, Py−
CH2−), 24.12 (1C, Py−CH3) ppm. Elemental analysis calcd (%) for
C20H19NO: C, 83.01; H, 6.62; N, 4.84. Found: C, 82.90; H, 6.55; N,
4.50.
(L1MgnBu)2 (1). In an Mbraun glovebox, in a 25 mL flask, a toluene
(10 mL) solution of L1−H (165 mg, 1 mmol) was mixed with a
toluene solution (5 mL) of MgnBu2 (1 mL, 1 M, 1 mmol). The
reaction mixture was stirred at room temperature for 4 h and
concentrated to approximately 2 mL, then cooled to −35 °C, to afford
crystalline solids of 1 (211 mg, yield = 81.8%). Single crystals suitable
for X-ray diffraction were obtained from a toluene−hexane solution.
1H NMR (400 MHz, C6D6, 25 °C): δ = 7.03 (t, 2H, JH−H = 8 Hz, p-
Py−H), 6.54 (d, 2H, JH−H = 8 Hz, m-Py−H), 6.52(d, 2H, JH−H = 12
Hz, m-Py−H), 3.17 (d, 2H, JH−H = 12 Hz, Py−CH2−), 2.63 (s, 6H,
Py−CH3), 2.61 (d, 2H, JH−H = 16 Hz, Py−CH2−), 1.95 (m, 4H,
−CH2CH2CH2CH3), 1.63 (m, 4H, −CH2CH2CH2CH3), 1.54 (s, 6 H,
−CH2C(OH)(CH3)2), 1.33 (s, 6H, −CH2C(OH)(CH3)2), 1.18 (t,
6H, −CH2(CH2)2CH3), 0.02 (t, 4H, −CH2(CH2)2CH3) ppm (Figure
S5, Supporting Information). 13C NMR (100 MHz, C6D6, 25 °C): δ =
160.04 (2C, o-Py−C−CH2−), 157.40 (2C, o-Py−C−CH3), 138.92
(2C, p-Py−C), 123.66 (2C, m-Py−C), 122.27 (2C, m-Py−C), 69.95 (2
C, Py−C−CH2−), 52.09 (2C, Py−CH2−), 35.62 (4C, Py−CH2(OH)-
C−(CH3)2), 33.95 (2C, −CH2CH2CH2CH3), 32.49 (2C,
−CH2CH2CH2CH3), 31.49 (2C, −CH2CH2CH2CH3), 24.55 (2C,
Measurements. Organometallic samples for NMR spectroscopic
measurement were prepared in an Mbraun glovebox by use of NMR
tubes sealed by paraffin film. 1H and 13C NMR spectra were recorded
1
on a Bruker AV300 and a Bruker AV400 (FT, 300 MHz for H; 75
1
MHz for 13C or 400 MHz for H; 100 MHz for 13C) spectrometer.
1
1
NMR assignments were confirmed by the H−1H COSY and H-13C
HMQC experiments when necessary. The number-average molar mass
(Mn) and polydispersity index (PDI) of the polymer were measured by
means of size exclusion chromatography on a TOSOH HLC-8220
SEC (column: Super HZM-H × 3) at 40 °C using THF as eluent (the
flowing rate is 0.35 mL/min) against polystyrene standards.
Py−CH3), 15.18 (2C, −CH2(CH2)2CH3), 9.82 (2C,
−
CH2(CH2)2CH3) ppm. Elemental analysis calcd (%) for
C28H46Mg2N2O2: C, 68.45; H, 9.44; N, 5.70. Found: C, 68.39; H,
9.33; N, 5.79.
(L2MgnBu)2 (2). In an Mbraun glovebox, a toluene (10 mL) solution
of L2−H (289 mg, 1 mmol) and a toluene solution (5 mL) of MgnBu2
(1 mL, 1 M, 1 mmol) were added to a 25 mL flask. The reaction
mixture was stirred at room temperature for 2 h and concentrated to
approximately 2 mL, then cooled to −35 °C, to afford crystalline solids
Synthesis of Proligands and Complexes. 2-(6-Methyl-2-
pyridinyl)-1,1-dimethyl-1-ethanol (L1−H). The compound was
prepared using the modified method reported by Koning et al.32
2,6-Lutidine (10.0 g, 93.3 mmol) was dissolved in 200 mL of Et2O and
cooled to −40 °C, to which n-butyllithium (1.6 M in hexane, 59.4 mL,
95.0 mmol) was added under stirring. The mixture was warmed to 25
°C and reacted for 1 h. Acetone (7.00 mL, 95.0 mmol) in 25 mL of
Et2O was then added into the system, and the reaction was kept
overnight. Adding 150 mL of water to the system and extracting the
aqueous layer with dichloromethane twice gave an organic layer, which
was dried over MgSO4, filtered, and concentrated in vacuo to yield a
1
of 2 (288 mg, yield 78.0%). H NMR (400 MHz, C6D6, 25 °C): δ =
7.89 (d, 4H, JH−H = 8 Hz, o-Ar−H), 7.46 (t, 4H, JH−H = 8 Hz, m-Ar−
H), 7.39 (d, 4H, JH−H = 8 Hz, o-Ar−H), 7.31 (t, 2H, JH−H = 8 Hz, p-
Ar−H), 7.25 (t, 4H, JH−H = 8 Hz, m-Ar−H), 7.12 (t, 2H, JH−H = 8 Hz,
p-Ar−H), 6.78 (t, 2H, JH−H = 8 Hz, p-Py−H), 6.35 (d, 2H, JH−H = 8
Hz, m-Py−H), 6.20 (d, 2H, JH−H = 8 Hz, m-Py−H), 4.15 (d, 2H, JH−H
= 16 Hz, Py−CH2−), 3.50 (d, 2H, JH−H = 12 Hz, Py−CH2−), 1.88 (s,
6H, Py−CH3), 1.70 (m, 4H, −CH2CH2CH2CH3), 1.56 (m, 4H,
−CH2CH2CH2CH3), 1.15 (t, 6H, JH−H = 8 Hz, −CH2(CH2)2CH3),
−0.21 (t, 4H, JH−H = 8 Hz, −CH2(CH2)2CH3) ppm (Figure S6,
Supporting Information). 13C NMR (100 MHz, C6D6, 25 °C): δ =
160.99 (2C, o-Py−C−CH2), 154.94 (2C, o-Py−C−CH3), 137.96 (2C,
p-Py−C), 135.79 (4C, Ar−C), 128.74, 128.50, 128.26, 127.84 (4C, p-
Ar−C, 8C, m-Ar−C), 125.35 (8C, o-Ar−C), 122.75 (2C, m-Py−C),
121.81 (2C, m-Py−C), 79.43 (2C, Py−CH2−C), 50.49 (2C, Py−
CH2 −), 33.28 (2C, −CH2 CH2 CH2 CH3 ), 32.39 (2C,
−CH2CH2CH2CH3), 24.38 (2C, Py−CH3), 15.20 (2C, −
CH2(CH2)2CH3), 14.40 (2C, −CH2(CH2)2CH3) ppm. Elemental
analysis calcd (%) for C48H54Mg2N2O2: C, 77.95; H, 7.36; N, 3.79.
Found: C, 77.55; H, 7.20; N, 3.89.
1
yellow oil of L1−H (12.5 g, 75.7 mmol, 81%). H NMR (300 MHz,
CDCl3, 25 °C): δ = 7.52 (t, 1H, JH−H = 9 Hz, p-Py−H), 7.03 (d, 1H,
JH−H = 9 Hz, m-Py−H), 6.92 (d, 1H, JH−H = 9 Hz, m-Py−H), 6.29 (s,
1H, −OH), 2.86 (s, 2H, Py−CH2−), 2.52 (s, 3H, Py−CH3), 1.21 (s,
6H, Py−CH2−C−(CH3)2) ppm. 13C NMR (100 MHz, CDCl3, 25
°C): δ = 158.89 (1C, o-Py−C), 156.76 (1C, o-Py−C), 136.65 (1C, p-
Py−C), 120.90 (1C, m-Py−C), 120.60 (1C, m-Py−C), 70.19 (1C, Py−
C−CH2−), 48.07(1C, Py−CH2−), 29.20 (2C, Py−CH2C(OH)-
(CH3)2), 23.94 (1C, Py−CH3) ppm. Elemental analysis calcd (%)
for C10H15NO: C, 72.69; H, 9.15; N, 8.48. Found: C, 72.20; H, 9.00;
N, 8.60.
2-(6-Methyl-2-pyridinyl)-1,1-diphenyl-1-ethanol (L2−H). Follow-
ing the similar procedure for the preparation of L1−H, compound L2−
H was synthesized by using 2,6-lutidine (10.0 g, 93.3 mmol), n-
butyllithium (1.6 M in hexane, 59.4 mL, 95.0 mmol), and
benzophenone (17.3 g, 95.0 mmol). The mixture was allowed to
reach ambient temperature overnight and was acidified to pH = 1 with
2 N HCl. After stirring for 1 h, the mixture was neutralized with 2 N
NaOH. The aqueous layer was extracted with ethyl acetate twice.
Combining of the organic layers, drying over MgSO4, concentration in
vacuo, and recrystallization from methanol yielded white solids (20.0 g,
69.1 mmol, 74%) with mp 124−125 °C. 1H NMR (300 MHz, CDCl3,
25 °C): δ = 8.33 (s, 1H, −OH), 7. 89(s, 2H, m-Ar−H), 7.78 (s, 2H, m-
Ar−H), 7.24 (m, 2H, o-Ar−H), 7.19 (m, 2H, o-Ar−H), 7.06 (t, 2H,
JH−H = 6 Hz, p-Ar−H), 6.87 (t, 1H, JH−H = 6 Hz, p-Py−H), 6.50 (d,
1H, JH−H = 6 Hz, m-Py−H), 6.32 (d, 1H, JH−H = 9 Hz, m-Py−H), 3.62
(s, 2H, Py−CH2−), 2.06 (s, 3H, Py−CH3) ppm. 13C NMR (75 MHz,
CDCl3, 25 °C): δ = 158.48 (1C, o-Py−C), 156.82(1C, o-Py−C),
147.43 (2C, Ar−C), 137.04 (1C, p-Py−C), 127.82 (4C, m-Ar−C),
126.32 (2C, p-Ar−C), 126.16 (4C, o-Ar−C), 121.51 (1C, m-Py−C),
(L1ZnEt)2 (3). In a Mbraun glovebox, to a 25 mL flask, were added
L1−H (248 mg, 1.5 mmol, 10 mL toluene) and ZnEt2 (1.54 mL, 1.5
mmol, 5 mL toluene). The reaction mixture was stirred at room
temperature for 4 h. Removal of volatiles gave white solids that were
washed with hexane to afford crystalline 3 (345 mg, yield 89.0%).
Single crystals suitable for X-ray diffraction were obtained from a
toluene−tetrahydrofuran−hexane solution cooled at −35 °C. 1H
NMR (300 MHz, C6D6, 25 °C): δ = 7.01 (t, 2H, JH−H = 9 Hz, p-Py−
H), 6.56 (d, 4H, m-Py−H), 2.90 (s, 4H, Py−CH2), 2.78 (s, 6H, Py−
CH3), 1.60 (t, 6H, JH−H = 6 Hz, −CH2CH3), 1.51 (s, 12H, Py−CH2−
C(CH3)2), 0.52 (q, 4H, JH−H = 9 Hz, −CH2CH3) ppm (Figure S7,
Supporting Information). 13C NMR (150 MHz, C6D6, 25 °C): δ =
158.59 (2C, o-Py−C−CH2), 156.31 (2C, o-Py−C−CH3), 136.38 (2C,
p-Py−C−CH2), 121.61 (2C, m-Py−C), 120.35 (2C, m-Py−C), 69.64
(2C, Py−CH2−C), 51.30 (2C, Py−CH2), 31.54 (4C, Py−CH2−C−
(CH3)2), 23.33 (2C, −CH2CH3), 12.59 (2C, Py−CH3), 0.67 (2C,
F
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Article
−CH2CH3) ppm. Elemental analysis calcd (%) for C24H38N2O2Zn2: C,
68.45; H, 9.44; N, 5.70. Found: C, 68.40; H, 9.25; N, 5.77.
precipitated into ethanol; it was then dried under vacuum to yield a
white solid of the polymer (0.20 g, yielding 87%).
(L2ZnEt)2 (4). In a Mbraun glovebox, L2−H (434 mg, 1.5 mmol, 10
mL of toluene) and ZnEt2 (1.54 mL, 1.5 mmol, 5 mL of toluene) were
added to a 25 mL flask. The reaction mixture was stirred at room
temperature for 2 h. Removal of volatiles gave white solids that were
washed with hexane to afford crystalline 4 (517 mg, yield 90%). Single
crystals of 4 suitable for X-ray diffraction were obtained from a
toluene−tetrahydrofuran−hexane solution cooled at −35 °C. 1H
NMR (400 MHz, C6D6, 25 °C): δ = 7.68 (br, 8H, o-Ar−H), 7.34 (br,
8H, m-Ar−H), 7.21 (br, 4H, p-Ar−H), 6.87 (t, 2H, JH−H = 8 Hz, o-
Py−H), 6.44 (d, 2H, JH−H = 8 Hz, m-Py−H), 6.39 (d, 2H, JH−H = 8
Hz, m-Py−H), 3.80 (br, 4H, Py−CH2−), 2.00 (s, 6H, Py−CH3), 1.30
(t, 6H, JH−H = 8 Hz, −CH2CH3), 0.15 (t, 4H, −CH2CH3) ppm
(Figure S8, Supporting Information). 13C NMR (100 MHz, C6D6, 25
°C): δ = 158.85 (2C, o-Py−C−CH2), 157.54 (2C, o-Py−C−CH3),
151.01 (4C, Ar−C), 137.29 (2C, p-Py−C), 127.95, 127.70 (8C, o-Ar−
C, 8C, m-Ar−C), 126.08 (4C, p-Ar−C), 123.81 (2C, m-Py−C), 121.40
(2C, m-Py−C), 79.78 (2C, Py−CH2−C), 50.81 (2C, Py−CH2), 23.07
(2C, −CH2CH3), 13.19 (2C, Py−CH3), 2.42 (2C, −CH2CH3) ppm.
Elemental analysis calcd (%) for C44H46N2O2Zn2: C, 69.02; H, 6.06;
N, 3.66. Found: C, 68.95; H, 5.98; N, 3.76.
Synthesis of Benzyl Ester End-Capped PLLA. A typical
polymerization procedure was exemplified by the synthesis of PLLA-
20 (the number 20 indicates the designed [M]0/[LA]0). To a rapidly
stirring solution of 2 (0.0370 g, 0.1 mmol) and L-lactide (0.29 g, 2.0
mmol) in THF (5 mL) was added a BnOH/toluene mixture solution
(0.1 mmol, 1 mL). The reaction mixture was stirred at room
temperature for 1 h. The acidic ethanol (0.5 mL of a 1.0 M HCl
solution in EtOH) was added to the system to terminate the reaction.
The white precipitate was resolved in dichloromethane and then
precipitated into ethanol; it was then dried under vacuum to yield a
white solid of the polymer (0.24 g, yielding 82%).
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files for 1, 3 and 4; 1H NMR spectra of complexes 1−4; 1H
NMR spectra of complexes 2 and 3 in the presence of alcohol;
and 1H NMR spectra of PLA oligomers. This material is
available free of charge via the Internet at http://pubs.acs.org.
X-ray Crystallographic Studies. Suitable crystals of complex 1, 3,
or 433 were sealed in thin-walled glass capillaries. Data collection was
performed at 20°C on a Bruker SMART diffractometer with graphite-
monochromated Mo Kα radiation (l = 0.71073 Å). The SMART
program package was used to determine the unit cell parameters. The
absorption correction was applied using SADABS.34 The structures
were solved by direct methods and refined on F2 by full-matrix least-
squares techniques with anisotropic thermal parameters for non-
hydrogen atoms. Hydrogen atoms were placed at calculated positions
and were included in the structure calculation without further
refinement of the parameters. All calculations were carried out using
the SHELXS-97 program.35 Molecular structures were generated using
the ORTEP program.36
Polymerization of ε-CL Catalyzed by Complex 2. A typical
polymerization procedure was exemplified by the synthesis of PCL-
100 (the number 100 indicates the designed [CL]0/[BnOH]0). To a
rapidly stirring THF solution (5 mL) of complex 2 (0.0163 g, 0.044
mmol) and benzyl alcohol (BnOH, 0.0048 g, 0.044 mmol) was added
ε-CL (0.5 g, 4.4 mmol). The reaction mixture was stirred at 25 °C for
1 min. The yield (100%) of PCL-100 was analyzed by 1H NMR
spectroscopic studies. The acidic ethanol (0.5 mL of a 1.0 M HCl
solution in EtOH) was added to the system to terminate the reaction.
The resultant white precipitate was purified by redissolving the
polymer in THF (10 mL), being precipitated in enthanol (60 mL).
Finally, the white polymer solid was dried under vacuum to a constant
weight (0.50 g, yielding 100%).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dmcui@ciac.jl.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Project Nos. 20904051, 51021003) for financial support, and
Prof. D. Cui also wants to thank the State Key Lab of Rare-
earth Sources Utilization for financial support.
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singlet at 0.90 ppm is attributed to CH3CH3, an alcoholysis product,
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These data suggest that benzyl alcohol is consumed completely;
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