Suppressing cyclic polymerization for isoselective synthesis of high-molecular-weight linear polylactide catalyzed by sodium/potassium sulfonamidate complexes

Article abstract of DOI:10.1021/acs.macromol.6b02271

A new sodium/potassium crown ether complex system with a series of bichelating sulfonamides as ligands was developed for the ring-opening polymerization (ROP) of rac-lactide. In this system, the side reaction of cyclic polymerization can be suppressed very well because of very different ROP rates initiated by BnOH and sulfonamide anion. The synthesis of high molecular weight linear polylactide with molecular weight high up to 107 kg/mol was successful. The best isoselectivity also can reach to a high value of P_m = 0.84. The NMR analysis of the reaction mixture of rac-lactide and complex 3 together with kinetic studies suggests the mechanism of ROP in the absence of alcohol is a coordination-insertion mechanism. After addition of BnOH, the ROP rate can increase remarkably due to the cooperation interaction of alcohol and complex 3.

Full text of DOI:10.1021/acs.macromol.6b02271

Article
pubs.acs.org/Macromolecules
Suppressing Cyclic Polymerization for Isoselective Synthesis of High-
Molecular-Weight Linear Polylactide Catalyzed by Sodium/Potassium
Sulfonamidate Complexes
Changjuan Chen,†,‡ Yaqin Cui, Xiaoyang Mao, Xiaobo Pan, and Jincai Wu*
†
†
†
,†
†State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Key Laboratory of
Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, Lanzhou University, Lanzhou 730000, People’s Republic
of China
‡
College of Chemistry and Pharmaceutical Engineering, Huanghuai University, Zhumadian 463000, People’s Republic of China
*
S Supporting Information
ABSTRACT: A new sodium/potassium crown ether complex system with a series of bichelating sulfonamides as ligands was
developed for the ring-opening polymerization (ROP) of rac-lactide. In this system, the side reaction of cyclic polymerization can
be suppressed very well because of very different ROP rates initiated by BnOH and sulfonamide anion. The synthesis of high
molecular weight linear polylactide with molecular weight high up to 107 kg/mol was successful. The best isoselectivity also can
reach to a high value of P = 0.84. The NMR analysis of the reaction mixture of rac-lactide and complex 3 together with kinetic
m
studies suggests the mechanism of ROP in the absence of alcohol is a coordination−insertion mechanism. After addition of
BnOH, the ROP rate can increase remarkably due to the cooperation interaction of alcohol and complex 3.
INTRODUCTION
sodium and potassium ions are nontoxic for the human body, it
is valuable to discover some excellent sodium or potassium
catalysts for the synthesis of polymers, especially for medical
■
Coordination polymerization reactions have been applied
widely in the synthesis of many kinds of polymers due to the
good controllability on molecular weights, dispersity, stereo-
regularity, and other properties of polymers compared to the
same polymers prepared by other techniques such as free
3
,22−27
applications.
Recently, we synthesized several sodium
and potassium monophenolate complexes with an active center
embedded in a confined space constructed by crown ether and
28−32
1
−10
bulky group of phenol ligand (Chart 1).
When both
radical polymerization.
Obviously catalysts play a key role,
monomer and polymer chain approach the active center, the
confined space surrounding a metal ion can enforce the
interactions between monomer, polymer chain, and ligand; the
weaker Lewis acid of alkali metal ions will not be a
shortcoming. With this design, we succeeded in the isoselective
synthesis of biodegradable and biocompatible polylactide using
potassium/sodium phenolate crown ether complexes as
catalysts via the ring-opening polymerization (ROP) of rac-
among which many coordination metal complexes have been
widely used in many famous polymerization reactions because
their activities and stereoselectivities can be easily adjusted via
1
1−13
tuning ligands.
To our interest, alkali metal complexes are
rarely utilized to catalyze stereocontrolled polymerization
1
4−16
reactions,
which is an important aspect of polymerization
reactions because different tactic polymers will own different
4
,17−21
chemical and physical characters.
The most possible
lactide; the isoselectivity even can reach to P = 0.94 at −70
m
reason is that the monomer and polymer chain cannot
coordinate strongly on alkali metal ions in the coordination
polymerization progress because of the weak Lewis acidity of
alkali metal ions; as a result, the important interactions for a
high stereoselectivity between monomer and polymer chain or
between monomer and ligand are weak. Considering that
28
°
C.
Received: October 19, 2016
Revised: November 23, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.6b02271
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Chart 1. Alkali Metal Crown Ether Complexes
polymerization of rac-lactide initiated directly by sulfonamide
group will become slow. Importantly, the decreased ROP rate
allows us to study and understand this system in details. With
this aim in mind, complexes 1−7 with different electronic and
bulky effects were synthesized in high yields (Scheme 1).
RESULTS AND DISCUSSION
■
Structures of Complexes 1−7. As shown in Figure 1,
sodium and potassium crown ether ions are coordinated with
one nitrogen atom and one oxygen atom of sulfonamide anion
(
NO chelating model) in complexes of 1 and 4−7. But in
complex 2, potassium crown ether cation is coordinated with
two oxygen atoms of sulfonamide anion (OO chelating model).
The most possible reason is that the four bulky isopropyl
groups in complex 2 lead the NO chelating model complex to
be unstable. DFT calculations at the B3LYP 6-311+G(d) level
showed the OO chelating model isomer of complex 2 is slightly
more stable than the NO chelating model isomer, with a
decreased energy of 0.56 kcal/mol. The calculation results of
sodium complex 1 with a same ligand show that the NO
chelating isomer is more stable than OO chelating isomer with
a decreased energy of 3.54 kcal/mol because of small hindrance
of 15-crown-5 and the slightly higher Lewis acidity of sodium
Although this type of alkali metal complex seems to be a
good catalyst for the ROP of rac-lactide as other excellent
3
3−50
isoselective metal complex systems of aluminum,
5
1−60
61−75
76−94
indium,
zinc,
lanthanum,
and other com-
there are some problems which have to be
9
5−97
plexes,
overcome within these phenolate alkali metal complex systems.
One important thing is that the phenoxy group as an auxiliary
ligand of an alkali metal complex itself is also highly active to
initiate an uncontrollable ROP of lactide giving cyclic
polylactide via a coordination−insertion mechanism, which is
a serious competing side reaction to synthesize desirable linear
polymers because this reaction rate is similar to the ROP
1
14,30,32,98
ion. Varied temperature H NMR in deuterated toluene was
initiated by alcohol.
Sometimes this side reaction is not
performed in order to find a possibility of exchange between
two chelating models, but complexes 1 and 2 do not split into
two sets signals even at −60 °C possibly due to very small
energy differences between these two type isomers (Figure S1).
In spite of this, the small energy differences may hint the
exchange between two chelating models is possible especially
when the nitrogen atom of sulfonamide anion interacts with
other substrates (for example, alcohol via a hydrogen bond).
Anyway, compared to sodium/potassium complexes with
phenoxy as mono-chelating ligand reported by us (Chart 1),
the ligands in these sulfonamidate complexes are bis-chelating
and potentially can keep this bis-chelating model when the
nitrogen atom of sulfonamide group reacts with alcohol or
attacks the carbonyl group of lactide because another oxygen
atom of sulfonamide group can replace this nitrogen atom to
coordinate to metal ion.
serious upon addition of 1 equiv of alcohol, but in some cases
excess equivalents of alcohol toward alkali metal complex is
needed to suppress this side reaction which will give rise to a
big obstacle to obtain high molecular weight polymers.
The molecular weight directly influences the mechanical and
28,30−32
sorptive properties of polylactide which sometimes are
99,100
important for its final use.
For example, high molecular
weight polylactide is necessary for application in bone plates or
101
temporary internal fixation of broken or damaged bones. To
our knowledge, no alkali metal complexes have been reported
to synthesize polylactide with molecular weight >100 kg/mol
successfully. With an attempt to know the alcohol effect in
these alkali metal complex systems and to suppress this side
reaction of cyclic polymerization further for improving the
highly isoselective alkali metal catalytic system, here we use
bichelating sulfonamide to replace monochelating phenol as
ligands (Scheme 1). The bichelating effect of sulfonamide
ligand can make the dissociation of ligand from sodium/
potassium ion be more difficult and lead to the nucleophilicity
ROP of rac-Lactide Catalyzed by Complexes 1−7. All
of complexes 1−7 are efficient catalysts for the ROP of rac-
lactide as shown in Table 1. The polymerization of rac-lactide
in the presence of alcohol catalyzed by complex 3 was
accomplished in 91% conversion within 30 min with a 100:1:1
ratio of [rac-LA] :[3] :[BnOH] in toluene (Table 1, entry 3,
102,103
of sulfonamide anion to be low;
thus, the rate of cyclic
0
0
0
Scheme 1. Synthesis of Complexes 1−7
[
Cat.] = 2.0 mM, room temperature). However, the same
0
reaction consumed 5 h in THF to reach a 31% conversion
Table 1, entry 1), and only 15% monomer can be converted to
polymer in CH Cl (Table 1, entry 2). The different reaction
(
2
2
rates can be attributed to the competitive coordination of THF
and the different solvent polarities which was also found in our
2
8,30
previous works.
Therefore, toluene is chosen as an
optimized solvent. Compared to previous phenolate sodium
and potassium crown ether complexes (Chart 1), the activity of
complex 3 is moderate for the ROP of rac-lactide at room
temperature. The reason may be the basicity of sulfonamide
anion is weaker than phenoxy because the negative charge of
nitrogen can distribute to both sulfonyl group and aromatic
group. At room temperature, the molecular weight distribution
(
Đ = 1.32, Table 1, entry 3) of polymer is somewhat broad and
the isoselectivity is not very high (Pm = 0.65). When the
temperature decreases to 0 °C, a high P value of 0.76 and a
m
B
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Figure 1. Molecular structures of 1−2 and 4−7 with probability ellipsoids at 30% (all of the hydrogen atoms are omitted for clarity).
narrow polydispersity Đ = 1.12 can be achieved (Table 1, entry
). The isoselectively can further be improved up to P = 0.84
sodium complexes 1 and 3, potassium complexes 2 and 4 are
4
slightly less active (Table 1, entries 4−7), which suggests the
activation of monomer is important in this system because
sodium ion owns stronger Lewis acidity than potassium ion. It
is interesting that sodium complexes 5−7 show very different
activities for the ROP of rac-lactide; the order of activities is 5 >
6 > 7 (Table 1, entries 8−10). Because these complexes have
similar bulky hindrances around the active center but different
electronic effect groups at a remote position, the different
activities hint that the electron-donating group can accelerate
the ROP reaction because the electron-donating group can
increase the basicity of nitrogen atom of sulfonamide anion.
The two activity tendencies of these complexes with different
metal ions and different electronic effect groups demonstrate
both activations of monomer and alcohol are important in
which the monomer is activated by metal ion and alcohol is
activated by sulfonamide anion group. Because complexes 1−7
can be considered as ion-paired complexes, the electronic effect
of a substituted group can adjust the basicity of anion ligand
m
when the temperature is −30 °C (Table 1, entry 11, Figure 2,
and Figure S2). In comparison, sodium complexes 1 and 3 with
more bulky substituted groups afford better isoselectivities than
the analogous sodium complexes 5−7 (Table 1, entries 4−10).
The isoselectivities of potassium complexes 2 and 4 are also
lower than that of sodium complexes 1 and 3, respectively,
which indicates the isoselectivity of this system is sensitive to
the surrounding of the active center constructed by crown ether
and substituted groups of sulfonamide ligand. No epimerization
was found in the polymerization of L-lactide catalyzed by
complex 3 (Figures S3 and S4). To our pleasure, the molecular
weights of polymers agree well with calculated values and
increase linearly with the ratios of [rac-LA] to [BnOH]0
0
(
Table 1, entries 4 and 12−17, and Figure S5), and the
molecular weight distributions also are narrow at 0 °C. Second-
feed experiments also confirmed the molecular weights of
polymers are under control (Table 1, entry 21). Compared to
C
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a
Table 1. rac-Lactide Polymerization Catalyzed by 1−7
b
c
d
e
entry
Cat.
[Cat.] /[rac-LA] /[BnOH]
0
t (h)
conv (%)
Mn,obsd (g/mol)
Mn,calcd (g/mol)
Đ
Pm
0
0
f
1
3
3
3
3
1
2
4
5
6
7
3
3
3
3
3
3
3
3
3
3
1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
1/100/1
1/25/1
5
5
31
15
91
94
89
41
90
90
91
20
87
97
80
80
97
94
87
90
90
97
92
3500
2800
4600
2300
1.38
1.43
1.32
1.12
1.09
1.06
1.05
1.07
1.08
1.05
1.04
1.07
1.13
1.19
1.26
1.28
1.37
1.07
1.36
1.41
1.06
0.60
0.60
0.65
0.76
0.75
0.69
0.69
0.73
0.71
0.63
0.84
0.78
0.76
0.77
0.78
0.81
0.78
0.76
0.63
0.63
0.75
g
2
h
3
0.5
1
12800
13800
11300
3100
13200
13600
12900
6000
4
5
6
7
8
9
24
56
6
11600
11200
12300
2400
13100
13100
13200
3000
5
10
56
24
0.5
2
1
1
1
1
1
1
1
1
1
1
2
2
0
1
2
3
4
5
6
7
8
9
0
1
k
12700
3500
12600
3600
1/200/1
1/300/1
1/400/1
1/500/1
24100
36300
56400
67700
107000
12200
26600
30400
23100
34700
56000
67800
125400
13100
3200
5
9
11
36
3
1/1000/1
500/1/5
1/25/0
h
h
8
1/100/0
1/50(50)/1
12
1(1)
14000
13100
13400
a
b
1
c
Conditions: reactions were performed in 5 mL of toluene, 0.01 mmol of catalyst, at 0 °C. Determined by H NMR spectroscopy. Experimental
1
04−106 d
M and Đ determined by GPC in THF against polystyrene standards and corrected using the factor 0.58.
weight of rac-LA × [LA] /[BnOH] × conversion + M
homonuclear-decoupled H NMR spectrum. In 5 mL of THF, at room temperature. In 5 mL of CH Cl , at room temperature. In 5 mL of
toluene, at room temperature. In 5 mL of toluene, at −30 °C.
Calculated from the molecular
n
e
. Determined by analysis of all of the tetrad signals in the methine region of the
0
0
BnOH
1
f
g
h
2
2
k
1
Figure 2. Homonuclear-decoupled H NMR spectrum of PLA ([rac-LA] :[3] :[BnOH] = 100:1:1, P = 0.84, Table 1, entry 11).
0
0
0
m
more remarkably than the ability to adjust the Lewis acidity of
central metal ion; the above two activity tendencies are not in
conflict. In this system, the key step may be the cooperation of
the two activation progresses which can further be confirmed
by the following kinetic studies.
0.78 at 0 °C (Table 1, entry 17). This phenomenon can change
our prejudice of sodium complexes in the synthesis of high
molecular weight polylactide because alkali complexes usually
suffer from serious side reactions of cyclization, trans-
7
,107−111
esterification, and even epimerization.
It also indicates
What is more, complex 3 also can catalyze the immortal ROP
of 500 equiv of rac-lactide in the presence of 5 equiv of external
BnOH as a co-initiator, affording a controlled molecular weight
and a relatively narrow polydispersity (Table 1, entry 18). It is
worth to note that complex 3 even can catalyze the ROP of
this kind of sodium/potassium complex can be comparable to
that of known aluminum, rare-earth-metal, and zinc catalysis
systems. In addition, this system also can hold high
isoselectivity.
Kinetic Studies and Proposed Mechanism. In order to
get more insight into the reason for the ability of complex 3 in
the synthesis of high molecular weight polylactide, we did more
experiments to verify the ROP mechanism including NMR and
1000 equiv of rac-lactide in the presence of 1 equiv of BnOH,
affording desirable high molecular weight of 107 kg/mol,
acceptable polydispersity of Đ = 1.37, and not bad P value of
m
D
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1
Figure 3. H NMR spectrum of the mixture of rac-lactide and complex 3 in C D , 25 °C with different ratios: (a) 10:1, (b) 10:4, (c) 10:6, (d) 10:8,
6
6
and (e) 10:10.
Figure 4. MALDI-TOF spectrum (matrix: DCTB; ionization salt: CF CO Na; solvent: CHCl ) of poly(rac-LA): (a) [rac-LA] :[3] = 100:1, [3] =
3
2
3
0
0
0
2
.0 mM, 18% conversion, at room temperature; (b) [rac-LA] :[3] = 100:1, [3] = 2.0 mM, 32% conversion, at room temperature; (c) [rac-LA] :
0
0
0
0
[
3] = 25:1, [3] = 2.0 mM, 90% conversion, at room temperature (Table 1, entry 19).
0 0
kinetics experiments. In previous phenolate sodium/potassium
complex system, both rates of the ROP progress in the absence
and in the presence of alcohol are fast at a similar level.
4). The molecular weights are not expected, and the
polydispersities are high in the absence of alcohol (Table 1,
entries 19 and 20). When rac-lactide and complex 3 with
different ratios of 10:1, 10:4, 10:6, 10:8, and 10:10 were mixed
in deuterated benzene for about 15 min, the peak of methine of
lactide shifts from 3.82 to 4.21 ppm, which may indicate this
chemical shift is an equilibrium position between coordinated
14,28,32
The ROP progress in the absence of alcohol proceeds via a
coordination−insertion mechanism and usually gives cyclic
polymer, which gives rise to a difficulty of the controllability of
the ROP of lactide in the presence of alcohol. While it is
gratifying that in this sodium/potassium sulfonamidate complex
system the ROP of rac-lactide proceeds slowly in the absence of
alcohol with a 100:1 ratio of [rac-LA] :[3] in toluene, the
1
lactide and free lactide (Figure 3). The VT H NMR of the
mixture of rac-lactide and complex 3 with a ratio of 1:1 did not
1
succeed to show a split of this peak, but most H NMR signals
0
0
polymerization consumes about 12 h to reach a 97% conversion
at room temperature (Table 1, entry 20), but the same reaction
in the presence of BnOH just needs 1 h at 0 °C (Table 1, entry
of lactide and ligand shift when temperature decreased from
room temperature to −60 °C (Figure S6), which can be
attributed to the different dissociation constants of adduct of
E
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Figure 5. (a) Semilogarithmic plots of the rac-lactide versus time in toluene at 25 °C with catalyst 3 as an initiator. [3] = 1.0, 1.5, 2.0, and 2.5 mM;
0
[
=
rac-LA] = 0.1 M. (b) Plots of ln k versus ln[3] for the polymerization of rac-lactide with catalyst 3 as an initiator in toluene at 25 °C. [rac-LA]
0 obs 0 0
0.1 M.
Scheme 2. Proposed Mechanism for the ROP of rac-Lactide Catalyzed by Complex 3 in the Absence of Alcohol
complex 3 and lactide at different temperatures. Fortunately, a
careful analysis of the MALDI-TOF spectrum of a final polymer
demonstrated some polymer chains have an end group of
sulfonamide (Figure 4c, Table 1, entry 19), which can be
verified by a series of weak peaks at 463 + 72n + 23. In the
MALDI-TOF spectrum, the main series of peaks of 72n + 23
series peaks of cyclic ester also exist (Figures 4a and 4b). At
lower conversions, more linear polymer chains can be found in
the isolated products. The end groups of linear polymers are
hydroxyl and carboxylic acid, which may result from the
hydrolysis of short polymer chain with an amide end. The
component of cyclic ester seems to increase when the
conversion becomes high (Figures 4a, 4b, and 4c). Thus, the
cyclic polymer can happen in the whole ROP progress and does
not just result from overrun polymerizations. The sulfonamide
group may keep coordinating to sodium ion in the ROP
progress, which can lead the cyclic polymerization to happen
easily. The linear polymer chains can be looked as precursors of
cyclic polymers; thus, the components of cyclic polymers
increase at high conversions.
+
can be assigned to n(C H O ) + Na (Figure 4c and Table 1,
3
4
2
entry 19), which can prove the polymer mostly is cyclic
polylactide. The cyclic structure of a final polymer can further
1
be confirmed by the H NMR spectrum in which no obvious
polymer chain end can be found (Figure S7).
Because overrun polymerization processes can easily lead to
significant intramolecular transesterifications to give cyclic
polymers, we stopped the ROP reaction with a 100:1 ratio of
[
LA] :[3] at 18% and 32% conversions, respectively, the
Kinetic studies of polymerization of rac-lactide in the absence
0
0
MALDI-TOF spectra of isolated polymers demonstrate that
of alcohol were performed to establish the reaction order with
F
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1
Figure 6. H NMR spectra in benzene-d for (a) [rac-LA] :[3] = 10:1, [3] = 1.0 mM, 15 min, at room temperature; (b) [rac-LA] :[3] :[BnOH] =
6
0
0
0
0
0
0
1
0:1:1, [3] = 1.0 mM, 15 min, at room temperature; and (c) the mole ratio of (rac-LA:3:BnOH) = 10:1:1, [3] = 10 mM, 15 min, at room
0
0
temperature.
Scheme 3. Proposed Mechanism for the ROP of rac-Lactide Catalyzed by Complex 3 in the Presence of Alcohol
−1
respect to lactide and complex 3. The results indicated that the
reaction rate has first-order dependence on both the
concentration of rac-lactide and complex 3 (Figure 5). At
values of kobs are 0.0020, 0.0028, 0.0042, and 0.0054 min ,
respectively. The overall rate equation is shown in eq 1, and the
is 2.03 M− min at 25 °C.
1
−1
reaction constant of k
1
various concentrations of complex 3 ([3] = 1.0, 1.5, 2.0, and
0
1
1
v = −d[LA]/dt = k [LA] [3]
(1)
2
.5 mM) and the concentration of [rac-LA] fixed at 0.1 M, the
1
1
0
G
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Thus, a coordination−insertion mechanism is reasonable for
the ROP of lactide in the absence of alcohol. As proposed in
Scheme 2, monomer first coordinates to sodium ion as
1
evidenced by the H NMR spectrum (Figure 3 and Figure
S6). The sulfonamide end-group found in the MALDI-TOF
spectrum suggests sulfonamide anion itself can attack the
carbonyl group of lactide to initiate the ROP progress directly;
consequently, the lactide inserts into the sodium nitrogen bond
in complex 3; after that, a new polymer alkoxy was formed
which can propagate to extend the polylactide chain. The
sulfonamide anion can be regenerated by the backbiting
reaction of alkoxy group of polymer chain to give a cyclic
polylactide, and complex 3 is also regenerated. The peaks with a
difference of molar mass of ∼72 Da in MALDI-TOF MS
indicate an intramolecular attack by the metal−alkoxy group to
any ester positions of the propagation chain also can happen. In
the whole progress, the sulfonamide group may keep
coordinating on sodium ion which leads the cyclic polymer-
ization to become easy.
Figure 7. MALDI-TOF spectrum of poly(rac-LA) prepared by the
ROP of rac-LA (Table 1, entry 12).
As shown in Figure 6 and Figure S8, when rac-lactide,
BnOH, and complex 3 were mixed in deuterated benzene with
a ratio of 10:1:1 for about 15 min (Figure 6b), 93% monomer
was converted to polymer, which confirmed further the
polymerization of rac-lactide in the presence of BnOH is faster
than that in the absence of alcohol (Figure 6a). In Figure 6c,
the concentration of complex 3 was increased from 1.0 to 10
mM, and 10 equiv of rac-LA was added into the NMR tube to
keep the mole ratio of rac-LA:3:BnOH at 10:1:1. In this
situation, rac-LA cannot completely dissolve at this high
concentration in deuterated benzene; the presence of large
amounts of unconsumed lactide in the system allows us more
easily to detect details of this reaction. The new complicated
multiple signals from 4.04 to 4.30 ppm can be ascribed to the
methine proton close to the hydroxyl group of alcohol, which is
similar to the chemical shift of the methene protons of BnOH
charge of +1, which can be assigned to m(C H O ) + BnOH +
6
8
4
+
H O . A series of weak peaks with a difference in molecular
3
mass of ∼72 Da suggest some transesterification reaction
happens during this polymerization process. But we believe the
transesterification reaction is not serious because the molecular
weights of polymers are desirable and polydispersities are
narrow.
Kinetic studies of polymerization of rac-lactide in the
presence of alcohol were performed to establish the reaction
order with respect to lactide, complex 3, and BnOH. The
results indicated that the reaction rate exhibits first-order
dependence on all components of rac-lactide, complex 3, and
BnOH (Figure 8). At various concentrations of complex 3
([3]
[rac-LA]
of kobs1 are 0.0206, 0.0334, 0.0413, and 0.0518 min
respectively. The overall rate equation is shown in eq 2, and
= 1.0, 1.5, 2.0, and 2.5 mM) and both concentrations of
0
and [BnOH] fixed at 0.1 M and 1.0 mM, the values
0
0
−
1
(
4.29 ppm) and the methine proton of ethyl lactate (4.04 ppm)
(Figures S9 and S10). Because partial alcohol molecules may
4
−2
−1
coordinate to complex 3 or be hydrogen-bonded to complex 3,
and some may be free in solution, the proton signals of different
alcohol seem to be so complicated. The reaction of complex 3
the reaction constant of k
2
is 2.11 × 10 M min at 25 °C.
1
1
1
v = −d[LA]/dt = k [LA] [3] [BnOH]
(2)
2
2
1
and BnOH was conducted in a H NMR tube with deuterated
Based on all above information, the mechanism as shown in
Scheme 3 seems to be suitable for the ROP of lactide in the
presence of BnOH. Lactide can be activated after coordinating
benzene as a solvent (Figure S9); the methene proton of
BnOH downshifts from 4.29 to 4.38 ppm. This shift value
proves that there are some interactions between BnOH and
complex 3. We think the interactions include a hydrogen bond
interaction between alcohol and sulfonamide group and the
coordination of an oxygen atom of BnOH to a sodium ion. We
believe the hydrogen bond interaction is important in the key
step of catalytic cycle (Scheme 3); otherwise, the ROP rate
cannot increase so dramatically relative to the ROP in the
absence of alcohol, and both metal effect and the electronic
effect of substituted group on ligand for the ROP of lactide
cannot be so remarkable (Table 1, entries 5 vs 6, 4 vs 7, and 8
+
+
to K or Na ; at the same time BnOH can be activated by the
sulfonamide anion via a hydrogen bond, and then the activated
BnOH can attack the carbonyl group of lactide to initiate the
ROP reaction. The analysis of Lewis acidity of metal ions and
electronic effect of substituted group for the ROP progress
informs us both activations of monomer and BnOH are
important in the key step of catalytic progress, which also can
be proved by the kinetic equation. The cooperation of the two
activation reactions leads the ROP reaction to proceed
smoothly, and the hydrogen bond interaction between alcohol
and sulfonamide anion should be an important factor for
increasing the ROP reaction rate because it can activate alcohol
(A, Scheme 3) and even can help to break the carbon−oxygen
bond of lactide (C, Scheme 3).
1
vs 10). The H NMR spectrum of final polymer obtained with
complex 3 as a catalyst in the presence of alcohol proved that
the polymer is the type of HO-[PLA]-OBn, capped with a
benzyl ester group on one end and a hydroxyl group on the
other with an integral ratio close to 5:1 between H (aromatic
Now we can find the ROP progress to give linear polylactide
in the presence of alcohol should be faster than the cyclic
polymerization progress when the concentration of BnOH is
not very small. For example, when the concentration of BnOH
f
proton of benzyl end group) and H (neighbor methine of
c
hydroxyl end group), which indicates the polymers are linear
and BnOH acts as a real initiator (Figure S11). The MALDI-
TOF spectrum (Figure 7 and Table 1, entry 12) also confirmed
this further by a series of peaks at 144m + 108 + 19 with a
as applied in all entries of Table 1 is 2 mM, v is about 20 times
2
faster than v . In addition, upon addition of BnOH in this
1
H
DOI: 10.1021/acs.macromol.6b02271
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Macromolecules
Article
Figure 8. (a) Semilogarithmic plots of the rac-lactide versus time in toluene at 25 °C with catalyst 3 and BnOH as a co-initiator. [3] = 1.0, 1.5, 2.0,
0
and 2.5 mM, [rac-LA] = 0.1 M, [BnOH] = 1.0 mM. (b) Plots of ln k
versus ln[3] for the polymerization of rac-lactide with catalyst 3 and
0
0
obs1
0
BnOH as a co-initiator in toluene at 25 °C. [rac-LA] = 0.1 M, [BnOH] = 1.0 mM. (c) Semilogarithmic plots of the rac-lactide versus time in
0
0
toluene at 25 °C with catalyst 3 and BnOH as a co- initiator. [BnOH] = 1.0, 1.5, 2.0, and 2.5 mM, [rac-LA] = 0.1 M, [3] = 1.0 mM. (d) Plots of ln
0
0
0
kobs2 versus ln[BnOH] for the polymerization of rac-lactide with catalyst 3 and BnOH as an initiator in toluene at 25 °C. [rac-LA] = 0.1 M, [3] =
0
0
0
1.0 mM.
system, the hydrogen bond interaction between of alcohol and
complex 3 possibly can further reduce the ability of sulfonamide
anion to initiate the cyclic polymerization directly. What is
more, because the cyclic polymerization initiated by sulfona-
mide anion directly is slow, the little amount of very small cyclic
polymer also can be consumed to be polymerized into a linear
polymer with alcohol as an initiator before enlarging the size of
cyclic polymer. Therefore, the alcohol plays a very important
role in the whole polymerization progress for the successful
synthesis of high molecular weight linear polylactide with a
desirable molecular weight.
complex 3 confirmed the hydrogen interaction between alcohol
and complex 3 can accelerate the ROP rate. Kinetic studies also
confirmed these hypotheses
EXPERIMENTAL SECTION
■
Materials and Methods. All manipulations with air- and
moisture-sensitive materials were carried out under a dry argon
atmosphere using standard Schlenk techniques or in a glovebox.
Toluene, THF, and n-hexane were distilled from sodium benzophe-
none ketyl before use. CH Cl was dried by refluxing from P O .
2
2
2
5
Chloroform-d, benzene-d , DMSO-d toluene-d , and other reagents
6
6,
8
were carefully dried and stored in a glovebox. The monomer rac-
lactide (rac-LA; Daigang BIO Engineer Ltd. of China) was
recrystallized from dry toluene and then sublimed twice prior to use.
CONCLUSION
■
In summary, we developed a new sodium/potassium crown
ether complex system to improve the catalytic performance of
sodium/potassium complexes in the ROP of rac-lactide. In this
system, bichelating sulfonamide can effectively suppress the
cyclic polymerization progress via a coordination−insertion
mechanism which leads the synthesis of high molecular weight
linear polylactide to become a reality for the sodium/potassium
complex system. Because the active centers of these sodium/
potassium sulfonamidate complexes have especial sandwich
structure constructed by the plane of the crown and the plane
of substituted groups of the sulfonamide anion, the
BnOH was dried over CaH under argon prior to distillation. All other
2
chemicals were commercially available and used after appropriate
purification.
Methods. NMR spectra were recorded on Varian Mercury Plus
1
13
3
00 MHz and JNM-ECS 400 MHz spectrometer. The H and
C
NMR spectra were referenced internally using the residual solvent
resonances and reported relative to TMS. The elemental analysis data
were measured using an Elemental Vario EL series CHN analyzer with
the samples under a nitrogen atmosphere. The molecular weights (Mn
and M ) and the molecular mass distributions (M /M ) of the
w
w
n
polymer samples were determined by gel permeation chromatography
−1
(
GPC) using THF as the eluent (flow rate: 1 mL min at 25 °C) and
isoselectivity also can reach to a high value of P = 0.84. The
narrow polystyrene standards as reference samples. The M values of
m
n
NMR analysis of the reaction mixture of rac-lactide, BnOH, and
PLAs were corrected with a Mark−Houwink factor of 0.58 to account
I
DOI: 10.1021/acs.macromol.6b02271
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
for the difference in hydrodynamic volume of polystyrene and
polylactide. The MALDI-TOF mass spectroscopic data were obtained
using DCTB as the matrix in a Bruker Daltonics Inc. BIFLEX III
MALDI-TOF mass spectrometer.
above for HL , 4-nitrobenzenesulfonyl chloride (2.22 g, 10 mmol) was
2
1
converted to give HL as yellow powder (0.38 g, 10% yield). H NMR
5
(400 MHz, chloroform-d, 25 °C): δ 8.31 (d, J = 8.0 Hz, Ar−H, 2H),
7.98 (d, J = 8.0 Hz, Ar−H, 2H), 7.28−7.33 (m, Ar−H, 2H), 7.20 (d, J
= 8.0 Hz, Ar−H, 2H), 7.13 (d, J = 8.0 Hz, Ar−H, 2H), 7.00−7.04 (m,
Ar−H, 2H), 5.88 (d, J = 8.0 Hz, CH, 1H), 5.03 (d, J = 8.0 Hz, NH,
Synthesis of N-(2,6-Diisopropylphenyl)-2,4,6-triisopropyl-
benzenesulfonamide (HL ). A solution containing 2,4,6-
1
1H). 13C NMR (100 MHz, DMSO-d , 25 °C): δ 151.03, 149.81,
triisopropylbenzenesulfonyl chloride (3.02 g, 10 mmol) in pyridine
6
(
20 mL) was added dropwise to a pyridine solution (20 mL) of 2,6-
129.33, 127.82, 124.52, 123.41, 120.61, 116.35, 48.40. Anal. Calcd (%)
for C H N O S: C 59.68, H 3.69, N 7.33. Found: C 59.53, H 3.56, N
diisopropylaniline (1.77 g, 10 mmol) at 118 °C. Then the reaction
mixture was stirred under reflux for 2 h and cooled to room
temperature. Pyridine was then removed under reduced pressure. The
residue was diluted with water and extracted with ethyl acetate (3 × 30
mL). The combined organic extracts were washed with brine, dried
19
14
2
5
7.21.
Synthesis of Complex 1. To a solution of N-(2,6-diisopropyl-
phenyl)-2,4,6-triisopropylbenzenesulfonamide (HL ) (0.443 g, 1.0
1
mmol) and 15-crown-5 ether (0.220 g, 1.0 mmol) in toluene (20 mL)
was slowly added NaN(SiMe ) (0.5 mL of 2.00 M solution in THF,
over MgSO , and concentrated under reduced pressure. The residue
4
3 2
was recrystallized from ethyl acetate and petroleum ether to give HL1
as white solid (1.56 g, 35% yield). H NMR (400 MHz, chloroform-d,
1.0 mmol) at 0 °C under a nitrogen atmosphere. During this progress,
the colorless solution changed to faint yellow. After stirring 6 h at
room temperature, the white precipitate formed was separated by
filtration. The solid residue was washed with 20 mL of hexane and
dried in vacuo to give complex 1 as a white powder (0.523 g, 76%).
Colorless crystals of 1 suitable for X-ray diffraction studies were
1
2
5 °C): δ 7.23 (t, J = 8.0 Hz, Ar−H, 1H), 7.11 (s, Ar−H, 2H), 7.10 (d,
J = 8.0 Hz, Ar−H, 2H), 6.09 (s, NH, 1H), 3.71 (m, CH, 2H), 3.28 (m,
CH, 2H), 2.87 (m, CH, 1H), 1.22 (d, J = 8.0 Hz, CH , 6H), 1.11 (d, J
=
3
13
8.0 Hz, CH , 12H), 0.97 (d, J = 8.0 Hz, CH , 12H). C NMR (100
3
3
MHz, chloroform-d, 25 °C): δ 152.87, 150.26, 148.43, 134.28, 129.32,
28.89, 124.23, 123.92, 34.34, 30.95, 28.49, 24.79, 23.82, 23.75. Anal.
Calcd (%) for C H NO S: C 73.09, H 9.31, N 3.16. Found: C 72.96,
obtained from a mixture of toluene and n-hexane at room temperature.
1
1
H NMR (400 MHz, benzene-d , 25 °C): δ 7.27 (s, Ar−H, 1H), 7.26
6
(s, Ar−H, 1H), 7.21 (br, Ar−H, 2H), 7.11 (t, J = 8.0 Hz, Ar−H, 1H),
4.70 (m, CH, 2H), 4.30 (m, CH, 2H), 3.20 (s, crown ether−H, 20H),
27
41
2
H 9.28, N 3.09.
Synthesis of 2,4,6-Triisopropyl-N-(9H-xanthen-9-yl)-
2.77 (m, CH, 1H), 1.40 (d, J = 8.0 Hz, CH , 12H), 1.28 (d, J = 8.0 Hz,
3
13
benzenesulfonamide (HL ). A solution containing 2,4,6-
CH , 12H), 1.20 (d, J = 8.0 Hz, CH , 6H). C NMR (100 MHz,
2
3
3
triisopropylbenzenesulfonyl chloride (3.02 g, 10 mmol) in pyridine
benzene-d , 25 °C): δ 148.74, 148.29, 145.97, 143.32, 142.99, 123.43,
6
(
20 mL) was added dropwise to a pyridine solution (20 mL) of 9H-
122.84, 121.94, 69.07, 34.52, 30.61, 28.12, 25.51, 24.97, 24.16. Anal.
Calcd (%) for C H NNaO S: C 64.79, H 8.82, N 2.04. Found: C
112
xanthen-9-amine (1.97 g, 10 mmol) at 118 °C. Then the reaction
mixture was stirred under reflux for 2 h and cooled to room
temperature. Pyridine was then removed under reduced pressure. The
residue was diluted with water and extracted with ethyl acetate (3 × 30
mL). The combined organic extracts were washed with brine, dried
37
60
7
64.58, H 8.69, N 1.98.
Synthesis of Complex 2. To a solution of N-(2,6-diisopropyl-
phenyl)-2,4,6-triisopropylbenzenesulfonamide (HL ) (0.443 g, 1.0
1
mmol) and 18-crown-6 ether (0.264 g, 1.0 mmol) in toluene (20 mL)
was slowly added KN(SiMe ) (1.0 mL of 1.00 M solution in THF, 1.0
over MgSO , and concentrated under reduced pressure. The residue
4
3 2
was recrystallized from ethyl acetate and petroleum ether to give HL2
as white solid (1.23 g, 27% yield). H NMR (300 MHz, chloroform-d,
mmol) at 0 °C under a nitrogen atmosphere. During this progress, the
colorless solution changed to faint yellow. After stirring 6 h at room
temperature, the white precipitate formed was separated by filtration.
The solid residue was washed with 20 mL of hexane and dried in
vacuo to give complex 2 as a white powder (0.642 g, 86%). Colorless
crystals of 2 suitable for X-ray diffraction studies were obtained from a
1
2
5 °C): δ 7.22−7.28 (m, Ar−H, 2H), 7.17 (s, Ar−H, 2H), 7.03−7.10
(
1
m, Ar−H, 4H), 6.90−6.95 (m, Ar−H, 2H), 5.86 (d, J = 9.0 Hz, CH,
H), 4.86 (d, J = 9.0 Hz, NH, 1H), 4.12 (m, CH, 2H), 2.95 (m, CH,
H), 1.30 (d, J = 6.0 Hz, CH , 6H), 1.17 (d, J = 6.0 Hz, CH , 12H).
1
3
3
1
3
1
C NMR (100 MHz, chloroform-d, 25 °C): δ 153.00, 151.44, 149.73,
34.98, 129.98, 129.48, 123.97, 123.63, 120.64, 116.76, 49.24, 34.36,
9.92, 24.80, 23.89. Anal. Calcd (%) for C H NO S: C 72.54, H
mixture of toluene and n-hexane at room temperature. H NMR (400
1
2
7
MHz, benzene-d , 25 °C): δ 7.34 (s, Ar−H, 1H), 7.32 (s, Ar−H, 1H),
6
7.23 (br, Ar−H, 2H), 7.16 (t, J = 8.0 Hz, Ar−H, 1H), 4.99 (m, CH,
2H), 4.47 (m, CH, 2H), 3.14 (s, crown ether−H, 24H), 2.76 (m, CH,
28
33
3
.17, N 3.02. Found: C 72.46, H 7.04, N 3.09.
Synthesis of 4-Methoxy-N-(9H-xanthen-9-yl)-
1H), 1.47 (d, J = 8.0 Hz, CH , 12H), 1.36 (d, J = 8.0 Hz, CH , 12H),
3
3
13
benzenesulfonamide (HL ). According to the procedure described
1.19 (d, J = 8.0 Hz, CH , 6H). C NMR (100 MHz, benzene-d , 25
3 6
3
above for HL , 4-methoxybenzenesulfonyl chloride (2.06 g, 10 mmol)
°C): δ 148.60, 147.70, 145.69, 144.58, 143.93, 123.15, 122.53, 121.20,
70.04, 34.52, 30.33, 28.20, 25.57, 24.84, 24.27. Anal. Calcd (%) for
2
1
was converted to give HL as white powder (1.10 g, 30% yield). H
3
NMR (400 MHz, chloroform-d, 25 °C): δ 7.86 (d, J = 8.0 Hz, Ar−H,
C H64KNO S: C 62.78, H 8.65, N 1.88. Found: C 62.70, H 8.52, N
39 8
2
H), 7.29 (d, J = 8.0 Hz, Ar−H, 2H), 7.19 (d, J = 8.0 Hz, Ar−H, 2H),
.09 (d, J = 8.0 Hz, Ar−H, 2H), 7.00−7.03 (m, Ar−H, 4H), 5.77 (d, J
1.75.
Synthesis of Complex 3. According to the procedure described
above for 1, HL (0.463 g, 1.0 mmol) was converted to give complex 3
as white powder (0.603 g, 85% yield). H NMR (400 MHz, benzene-
d , 25 °C): δ 7.93 (d, J = 8.0 Hz, Ar−H, 2H), 7.40 (s, Ar−H, 2H), 7.05
7
=
8.0 Hz, CH, 1H), 4.87 (d, J = 8.0 Hz, NH, 1H), 3.91 (s, OCH , 3H).
C NMR (100 MHz, chloroform-d, 25 °C): δ 163.13, 151.39, 133.33,
2
3
1
3
1
129.42, 123.75, 120.56, 116.87, 114.47, 55.83, 49.24. Anal. Calcd (%)
6
(
d, J = 8.0 Hz, Ar−H, 2H), 6.91−6.98 (m, Ar−H, 4H), 5.96 (s, CH,
for C H NO S: C 65.38, H 4.66, N 3.81. Found: C 65.19, H 4.54, N
20
17
4
1
1
H), 5.24 (m, CH, 2H), 3.07 (s, crown ether−H, 20H), 2.88 (m, CH,
3
.67.
H), 1.49 (d, J = 8.0 Hz, CH , 12H), 1.30 (d, J = 8.0 Hz, CH , 6H).
S y n t h e s i s o f 4 - M e t h y l - N - ( 9 H - x a n t h e n - 9 - y l ) -
3
3
13
benzenesulfonamide (HL ). According to the procedure described
C NMR (100 MHz, benzene-d
6
, 25 °C): δ 152.12, 149.61, 148.82,
4
above for HL , 4-methylbenzenesulfonyl chloride (1.90 g, 10 mmol)
142.87, 132.51, 129.73, 126.86, 123.17, 122.89, 115.43, 68.83, 48.73,
34.57, 30.26, 26.04, 24.29. Anal. Calcd (%) for C H NNaO S: C
2
1
was converted to give HL as white powder (1.08 g, 31% yield). H
4
38 52
8
NMR (400 MHz, chloroform-d, 25 °C): δ 7.79 (d, J = 8.0 Hz, Ar−H,
64.66, H 7.43, N 1.98. Found: C 64.53, H 7.32, N 1.88.
Synthesis of Complex 4. According to the procedure described
2
H), 7.33 (d, J = 8.0 Hz, Ar−H, 2H), 7.24−7.28 (m, Ar−H, 2H), 7.14
(
d, J = 8.0 Hz, Ar−H, 2H), 7.07 (d, J = 8.0 Hz, Ar−H, 2H), 6.96−7.00
above for 2, HL (0.463 g, 1.0 mmol) was converted to give complex 4
2
(
m, Ar−H, 2H), 5.76 (d, J = 8.0 Hz, CH, 1H), 4.90 (d, J = 8.0 Hz, NH,
as white powder (0.618 g, 81% yield). Colorless crystals of 4 suitable
13
1
1
4
H), 2.47 (s, CH , 3H). C NMR (100 MHz, chloroform-d, 25 °C): δ
for X-ray diffraction studies were obtained from a toluene solution at
3
1
51.37, 143.73, 138.71, 129.95, 129.59, 127.26, 123.71, 120.51, 116.85,
9.28, 21.73. Anal. Calcd for C H NO S: C 68.36, H 4.88, N 3.99.
room temperature. H NMR (400 MHz, benzene-d
, 25 °C): δ 8.04
6
(d, J = 8.0 Hz, Ar−H, 2H), 7.41 (s, Ar−H, 2H), 7.06 (d, J = 8.0 Hz,
Ar−H, 2H), 6.92−6.99 (m, Ar−H, 4H), 6.00 (s, CH, 1H), 5.38 (m,
CH, 2H), 3.09 (s, crown ether−H, 24H), 2.88 (m, CH, 1H), 1.53 (d, J
20
17
3
Found: C 68.26, H 4.69, N 3.87.
S y n t h e s i s o f 4 - N i t r o - N - ( 9 H - x a n t h e n - 9 - y l ) -
13
benzenesulfonamide (HL ). According to the procedure described
= 8.0 Hz, CH , 12H), 1.30 (d, J = 8.0 Hz, CH , 6H). C NMR (100
5
3
3
J
DOI: 10.1021/acs.macromol.6b02271
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
MHz, benzene-d , 25 °C): δ 152.45, 149.38, 148.41, 143.64, 132.25,
6
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.6b02271.
■
1
2
1
30.54, 126.66, 122.96, 122.82, 115.48, 69.98, 49.40, 34.60, 30.08,
6.10, 24.33. Anal. Calcd (%) for C H KNO S: C 62.72, H 7.37, N
.83. Found: C 62.64, H 7.19, N 1.76.
Synthesis of Complex 5. According to the procedure described
*
S
40
56
9
above for 1, HL (0.367 g, 1.0 mmol) was converted to give complex 5
3
as white powder (0.512 g, 84% yield). Colorless crystals of 5 suitable
1H NMR and 13C NMR spectra of compounds HL −
1
for X-ray diffraction studies were obtained from a deuterated benzene
1
HL and complexs 1−7, the polymerization studies,
5
solution at room temperature. H NMR (400 MHz, benzene-d , 25
6
computational details (PDF)
°
C): δ 8.30 (d, J = 8.0 Hz, Ar−H, 2H), 7.85 (d, J = 8.0 Hz, Ar−H,
Crystallographic data for complex 1 (CIF)
Crystallographic data for complex 2 (CIF)
Crystallographic data for complex 4 (CIF)
Crystallographic data for complex 5 (CIF)
Crystallographic data for complex 6 (CIF)
Crystallographic data for complex 7 (CIF)
2
H), 6.95−6.99 (m, Ar−H, 2H), 6.85−6.91 (m, Ar−H, 4H), 5.90 (s,
13
CH, 1H), 3.32 (s, OCH , 3H), 3.11 (s, crown ether−H, 20H).
C
3
NMR (100 MHz, benzene-d , 50 °C): δ 160.59, 151.84, 132.26,
6
1
29.28, 129.04, 127.04, 115.48, 113.53, 69.28, 54.92, 49.46. Anal.
Calcd (%) for C H NNaO S: C 59.10, H 5.95, N 2.30. Found: C
30
36
9
5
9.06, H 5.83, N 2.24.
Synthesis of Complex 6. According to the procedure described
above for 1, HL (0.351 g, 1.0 mmol) was converted to give complex 6
4
as white powder (0.470 g, 79% yield). Colorless crystals of 6 suitable
AUTHOR INFORMATION
Corresponding Author
E-mail wujc@lzu.edu.cn (J.W.).
■
*
for X-ray diffraction studies were obtained from a deuterated benzene
1
solution at room temperature. H NMR (400 MHz, benzene-d , 25
6
°
C): δ 8.33 (d, J = 8.0 Hz, Ar−H, 2H), 7.91 (d, J = 8.0 Hz, Ar−H,
2
H), 7.10 (d, J = 8.0 Hz, Ar−H, 2H), 7.04 (d, J = 8.0 Hz, Ar−H, 2H),
.97 (t, J = 8.0 Hz, Ar−H, 2H), 6.90 (t, J = 8.0 Hz, Ar−H, 2H), 5.96
ORCID
6
Jincai Wu: 0000-0002-8233-2863
Notes
The authors declare no competing financial interest.
13
(
s, CH, 1H), 3.05 (s, crown ether−H, 20H), 2.13 (s, CH , 3H). C
3
NMR (100 MHz, benzene-d , 50 °C): δ 151.58,148.27, 138.15,
6
1
2
32.15, 129.41, 128.85, 127.57, 126.91, 122.66, 115.26, 68.86, 49.17,
1.31. Anal. Calcd (%) for C H NNaO S: C 60.69, H 6.11, N 2.36.
30
36
8
Found: C 60.33, H 6.18, N 2.56.
Synthesis of Complex 7. According to the procedure described
ACKNOWLEDGMENTS
Financial support from the National Natural Science
Foundation of China (No. 21271092, 21401161, and
■
above for 1, HL (0.382 g, 1.0 mmol) was converted to give complex 7
5
as white powder (0.521 g, 83% yield). Yellow crystals of 7 suitable for
2
1671087), the Science Foundation of Gansu Province of
X-ray diffraction studies were obtained from a toulene solution at
1
China (1308RJ2A121), the project for the National Basic
Science Personnel Training Fund (J1103307), and the ScGrid
of the Supercomputing Center of the Chinese Academy of
Sciences are gratefully acknowledged. We also appreciated the
help of Bingbing Wu at the School of Chemistry and Materials
Science, University of Chinese science and technology for the
experiments.
room temperature. H NMR (400 MHz, benzene-d , 25 °C): δ 8.24
6
(
d, J = 8.0 Hz, Ar−H, 2H), 8.05 (d, J = 8.0 Hz, Ar−H, 2H), 7.65 (d, J
8.0 Hz, Ar−H, 2H), 7.03 (d, J = 8.0 Hz, Ar−H, 2H), 6.95 (t, J = 8.0
Hz, Ar−H, 2H), 6.81 (t, J = 8.0 Hz, Ar−H, 2H), 5.84 (s, CH, 1H),
=
1
3
2
°
6
.96 (s, crown ether−H, 20H). C NMR (100 MHz, benzene-d , 50
6
C): δ 151.58,148.05, 131.84, 128.46, 127.27, 123.60, 122.80, 115.52,
8.77, 49.43. Anal. Calcd (%) for C H N NaO S: C 55.76, H 5.33,
29
33
2
10
N 4.48. Found: C 55.62, H 5.19, N 4.39.
General Polymerization Procedure. A typical polymerization
REFERENCES
procedure is illustrated by the synthesis of PLA-([rac-LA] /[cat.] /
■
0
0
[
BnOH] = 100:1:1; Table 1, entry 4). A solution of complex 3 (0.007
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2
2
polymer sample was further dried in a vacuum oven at 50 °C for 8 h to
1
a constant weight for GPC and H NMR analyses.
General Procedure for Kinetic Studies. The polymerizations
were carried out 25 °C in a glovebox. Complex 3 (0.007 g, 0.010
mmol) and BnOH (100 μL, 0.10 M in toluene) were added a solution
of rac-lactide (0.144 g, 1.0 mmol) in toluene (10 mL). At appropriate
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benzoic acid (1.0 μL, 0.10 M in toluene). The solvent was removed in
1
vacuo, and the percent conversion was determined by H NMR in
CDCl3.
Crystal Structure Determination. X-ray diffraction measure-
ments were performed on a SuperNova (Dual) X-ray diffraction
diffractometer with a graphite-monochromated Cu/Mo Kα radiation
source (λ = 1.54184/0.71073 Å). The structures were solved by direct
1
13
methods using the Siemens SHELXTL PLUS program.
Non-
hydrogen atoms were refined with anisotropic displacement
parameters during the final cycles. All hydrogen atoms were placed
by geometrical considerations and were added to the structure-factor
calculations. The crystal parameters and results of the structure
refinements are summarized in Table S1.
K
DOI: 10.1021/acs.macromol.6b02271
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DOI: 10.1021/acs.macromol.6b02271
Macromolecules XXXX, XXX, XXX−XXX