Ring-opening polymerization of L-lactides catalyzed by zinc-sodium/lithium heterobimetallic complexes in the presence of water

Article abstract of DOI:10.1002/ejic.201001283

Two heterobimetallic complexes, [(TBBP)₂Zn][(Na) ₂(THF)₄] (3) and [(TBBP)₂Zn][(Li) ₂(THF)₄] (4) can initiate the ring-opening polymerization of L-lactide in a controlled fashion. Complex 3 can initiate the ring-opening polymerization of unsublimed L-lactide under moist conditions even in the presence of H₂O, to yield a high conversion and with controllable molecular weight. Copyright

Full text of DOI:10.1002/ejic.201001283

SHORT COMMUNICATION
DOI: 10.1002/ejic.201001283
Ring-Opening Polymerization of -Lactides Catalyzed by Zinc–Sodium/
L
Lithium Heterobimetallic Complexes in the Presence of Water
Lei Wang,[a] Xiaobo Pan,[b] Lihui Yao,[a] Ning Tang,[a] and Jincai Wu*[a]
Keywords: Ring-opening polymerization / Heterometallic complexes / Zinc / Lactides
Two heterobimetallic complexes, [(TBBP)2Zn][(Na)2(THF)4]
unsublimed L-lactide under moist conditions even in the
(3) and [(TBBP)2Zn][(Li)2(THF)4] (4) can initiate the ring-
opening polymerization of L-lactide in a controlled fashion.
presence of H2O, to yield a high conversion and with control-
lable molecular weight.
Complex 3 can initiate the ring-opening polymerization of
Introduction
presence of water, and catalyst II can also initiate the ROP
of lactides and can almost compensate for the deactive cata-
lyst I. Two new bulky heterobimetallic aryloxides were syn-
thesized and crystallographically characterized (Scheme 2).
The catalytic activities of these complexes toward the ROP
of l-lactide (l-LA) are also presented. Preliminary results
show that complex 3 catalyzes the ROP of l-LA in the pres-
ence of water (50 equiv. with respect to the catalyst) under
a controlled model.
Poly(lactide) (PLA)[1] and their copolymers are the most
promising biodegradable and biocompatible synthetic mac-
romolecules. Because of their biodegradable, biocompatible,
and permeable properties,[2] these materials can be used in
a broad range of practical applications such as medicine,
pharmaceutics and tissue engineering.[3] Owing to the ad-
vantages of well-controlled molecular weight and low poly-
dispersity (PDI), bulky ligand supported metal-based cata-
lytic systems (e.g., Al,[4] Li,[5] Mg,[6] Zn,[7] Ca,[8] Fe,[9] Sn,[10]
Ti,[11] or Y[12]) have attracted considerable attention for the
ring-opening polymerization (ROP) of cyclic esters. It is
necessary to note that most well-defined metal initiators re-
quire rigorously anhydrous conditions for their air- and
moisture-sensitivity. Therefore, there is an exigent need to
develop a practical means for the well-controlled ROP of
lactides in laboratories and industry without access to inert-
atmosphere facilities or rigorously dried solvents and to re-
duce costs. ROP of unsublimed rac-lactide even in the pres-
ence of water or 1 equiv. benzoic acid initiated by a robust
zirconium Schiff base alkoxide,[13] a zinc Schiff base com-
plex,[14] and iron alkoxides[15] have been described by Da-
vidson and Gibson and co-workers. Nevertheless, the ROP
of lactides in the presence of water still remains relatively
unexplored, especially the mechanism of ROP of lactides in
the presence of water.
Scheme 1. Strategy for the anti-water ROP of the l-lactide system.
Scheme 2. Preparation of compounds 1–4.
Here, a new strategy was designed to overcome the air-
sensitive hindrance (Scheme 1). As a good initiator for the
ROP of lactides, catalyst I decomposes to catalyst II in the
Results and Discussion
Syntheses and Characterization of Zinc–Sodium/Lithium
Heterobimetallic Complexes
[a] State Key Laboratory of Applied Organic Chemistry, Key
Laboratory of Nonferrous Metal Chemistry and Resources
Utilization of Gansu Province, College of Chemistry and
Chemical Engineering, Lanzhou University,
As shown in Scheme 2, the monosodium/lithium com-
plexes 1 and 2 are obtained by the reaction of 3,3Ј,5,5Ј-
tetra-tert-butyl-2,2Ј-biphenol(TBBPH2)
nBuLi. These two complexes can be dissolved in toluene,
THF, CHCl3 and CH2Cl2. Crystals of complex 2 are ob-
tained by cooling the saturated THF solution; the molecu-
lar structure of complex 2 is depicted in Figure 1. [(TBBP)2-
Lanzhou 73000, People’s Republic of China
E-mail: wujc@lzu.edu.cn
with
sodium/
[b] Qingdao Institute of Bioenergy and Technology, Chinese
Academy of Sciences,
Qingdao 266101, People’s Republic of China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201001283.
632
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Ring-Opening Polymerization of l-Lactides
Zn][(Na)2(THF)4] (3) or [(TBBP)2Zn][(Li)2(THF)4] (4) were ration. After complex 3 is exposed to air for several hours,
1
obtained as colourless crystalline solids in 50 and 53% the H NMR spectrum in CDCl3 shows very similar peaks
yields, respectively, from the reaction of complex 1 or 2 and as those in the spectrum of the 3 under a protective nitrogen
Zn(Et)2 in a ratio of 2:1 in THF. Complex 3 and 4 are atmosphere, which also indicates the stabilization of this
soluble in THF, slightly soluble in toluene, and insoluble in complex. In order to understand the interaction between
1
n-hexane. The ORTEP drawings of the molecular structures complex 3 and water, a H NMR titration of the CDCl3
of complexes 3 and 4 are given in Figures 2 and S1. The solution of complex 3 with water was performed. The re-
two molecular structures are similar; the geometries around sults show that complex 3 hydrolyzes to complex 1 partially
the zinc atoms are distorted tetrahedral, with coordination and then to the free ligand, as evidenced by the shift in the
by four oxygen atoms from the phenoxy group with an peak at δ = 6.73 ppm for complex 3 to δ = 7.30 ppm, which
average Zn–O bond length of 1.941(4) Å for Zn1–O and can be attributed to the monosodium complex 1, and by
1.952(3) Å for Zn2–O, respectively. The sodium/lithium the appearance of a broad peak for the hydroxy group at δ
atoms are surrounded by two oxygen atoms from THF and = 5.10 ppm, which indicates the decomposition of complex
two phenyl oxygen atoms from two bulky ligands.
3 to the TBBPH2 ligand (Figure S2). The ESI mass spec-
trum, collected under moist conditions in methanol/ace-
tone, shows a series of peaks for sodium complexes, which
also confirms the hydrolysis of complex 3 (Figure S3).
Ring-Opening Polymerization of L-Lactides
The ROP of l-lactide that employs 3/4 (0.02 mmol) as
catalyst was systematically examined in toluene at 90 °C, as
shown in Table 1. The catalytic experimental results show
that complexes 3 and 4 are efficient catalysts for the ROP
of l-lactide, and the polymerization occurs to about 90%
conversion within 48 h with the ratio 100:1 to 250:1 for
[monomer]/[complex]. By taking into account the lower
solution state volume of PLA relative to polystyrene stan-
dards used for the GPC measurements, and on the basis of
Figure 1. Molecular structure of 2 with ellipsoids given at the 30%
probability level (methyl carbon atoms of the tert-butyl groups and
all of the hydrogen atoms are omitted for clarity). Selected bond
lengths [Å]: Li–O1 1.877(8), Li–O3 2.013(10), Li–O4 1.996(9), Li–
O5 1.962(10).
1
conversions from H NMR analysis, the observed polymer
molecular weights are consistent with the propagation of
only one PLA chain from each complex. Further, a good
polymerization control is demonstrated by the linear rela-
tionship between the number-average molecular weight
(Mn) and the monomer-to complex ratio ([M]0/[complex]0)
and polymers with PDIs ranging from 1.26 to 1.46 (Fig-
ure 3). Furthermore, epimerization of the chiral centres in
PLLA does not occur as observed by the homonuclear de-
1
coupled H NMR studies in the methine region.[16] End-
group analysis of the polymer reveals the existence of a ter-
minal hydroxy group, evidenced by the peak at 4.36 ppm in
Table 1. Ring-opening polymerization of l-LA with complexes 3
and 4.[a]
Entry Catalyst [M]0/[complex]0 Conv.
[%]
Mn
Mn
PDI
(obsd.)[b] (calcd.)[c]
Figure 2. Molecular structure of 3 with ellipsoids given at the 30%
probability level (methyl carbon atoms of the tert-butyl groups and
all of the hydrogen atoms are omitted for clarity). Selected bond
lengths [Å]: Zn2–O5 1.947(3), Zn2–O5(A) 1.947(3), Zn2–O6
1.957(3), Zn2–O6(A) 1.957(3), Na1–O6 2.225(4), Na1–O6(A)
2.225(4), Na1–O7 2.257(5), Na1–O7(A) 2.257(5), Na2–O5 2.219(4),
Na2–O5(A) 2.219(4), Na2–O8 2.256(6), Na2–O8(A) 2.256(6).
1
2
3
4
5
6
7
8
3
3
3
3
4
4
4
4
125
150
200
250
100
150
175
200
90.6
89.5
91.0
91.6
89.0
91.3
87.0
95.0
18800
19300
25500
29000
12800
22200
23900
27400
16400
19300
26200
33000
12300
19700
22200
27360
1.36
1.40
1.42
1.45
1.40
1.46
1.40
1.26
After the crystals were exposed to air for several hours,
the colour of complex 4 changed from colourless to yellow,
which indicates the lability of complex 4. The crystals of
complex 3 are stable in air under moist conditions and can
[a] Conditions: All manipulations were carried out under a dry
nitrogen atmosphere, 0.02 mmol of complex, 10 mL of toluene,
90 °C, reaction time 48 h. [b] Obtained from GPC analysis times
0.58[18] and calibrated by polystyrene standard. [c] Mn (calcd.) =
be exposed to air for a few hours without apparent deterio- ([M]0/[complex]0) ϫ 144.13 ϫ conv.
Eur. J. Inorg. Chem. 2011, 632–636
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633

L. Wang, X. Pan, L. Yao, N. Tang, J. Wu
SHORT COMMUNICATION
1H NMR spectrum, which indicates that the polymer is lin- Table 2. Ring-opening polymerization of l-LA with complexes 3
and 4 under moist conditions.[a]
ear chain. The existence of a terminal hydroxy group as the
end group on isolated polylactide samples and no epimeri-
zation possibly suggests that lactide polymerization is initi-
ated by deprotonation of the monomer and propagation oc-
curs through a coordination–insertion mechanism.[3c,17]
Entry Catalyst [M]0/[complex]0 Conv.
[%]
Mn
Mn
PDI
(obsd.)[b] (calcd.)[c]
1
2
3
4
5
6
3[d]
3[d]
3[d]
3[d]
3[e]
4[f]
100
125
150
175
100
100
87.0
90.0
92.0
93.7
85.0
25.4
17300
21700
24700
27300
13000
n.d.
12500
19400
23200
26900
12200
n.d.
1.47
1.49
1.50
1.36
1.33
n.d.
[a] Conditions: 0.02 mmol of complex, 10 mL of toluene, 90 °C,
reaction time 48 h. [b] Obtained from GPC analysis times 0.58[18]
and calibrated by polystyrene standard. [c] Mn (calcd.) = ([M]0/
[complex]0) ϫ 144.13 ϫ conv. [d] The lactide was not recrystallized,
the polymerization reactions were exposed to air. [e] 18 μL of H2O
added. [f] 3.6 μL of H2O added.
Figure 3. Polymerization of l-LA catalyzed by 3 in toluene at
90 °C. The relationship between Mn (᭝) [PDI (᭹)] of the polymer
and the initial mol ratio [M]0/[complex]0 is shown.
By comparing with other zinc alkoxides, these two com-
plexes show a low activity for the ROP of l-LA. However,
it is interesting that complex 3 was found to be an active
catalyst for ROP of LA under moist conditions without the
protection of nitrogen. To systematically examine its sensi-
tivity to water, unsublimed l-LA was used as a monomer
and the weighing of the catalyst and the ROP reaction were
performed in air. The results are summarized in Table 2.
As expected, the higher conversion (more than 85%) was
achieved at various monomer-to complex ratios. Further,
a linear relationship between Mn and [M]0/[complex]0 was
obtained (Table 2, Entries 1–4, Figure 4). Surprisingly,
when an additional amount (18 μL) of water (50 equiv. with
respect to complex 3) was introduced into the ROP reaction
with complex 3 as the catalyst, a high conversion and a
controllable molecular weight were also obtained (Table 2,
Entry 5). Catalyst 4 can also catalyze the polymerization of
l-lactide with the addition of 3.6 μL water, but with low
conversion (Table 2, Entry 6). Complex 1 was also system-
atically examined as a catalyst for the ROP of LA; the re-
sults are summarized in Table 3. The results show that com-
plex 1 catalyzes the ROP reaction; experimental average
molecular number weights based on the propagation of one
PLA chain from two molecules of complex 1 were slightly
Figure 4. Polymerization of unsublimed l-LA catalyzed by 3 in tol-
uene at 90 °C. The relationship between Mn (᭝) [PDI (᭹)] of the
polymer and the initial mol ratio [M]0/[complex]0 is shown.
Table 3. Ring-opening polymerization of l-LA with complex 1.[a]
Entry Catalyst [M]0/[complex]0 Conv.
[%]
Mn
Mn
PDI
(obsd.)[b] (calcd.)[c]
1
2
3
4
1
1
1
1
150
200
225
250
92.3
90.0
92.0
93.7
38600
34800
48400
50600
20000
26000
30000
34000
1.33
1.40
1.44
1.43
[a] Conditions: 0.02 mmol of complex, 10 mL of toluene, 80 °C,
reaction time 24 h. [b] Obtained from GPC analysis times 0.58[18]
and calibrated by polystyrene standard. [c] Mn (calcd.) = ([M]0/
[complex]0) ϫ 144.13 ϫ conv.
Conclusions
Two new heterometallic aryloxides were synthesized as
higher than expected. Therefore, complex 3 can catalyze the catalysts for the ring-opening polymerization of l-lactide.
ROP of LA in the presence of water; this outstanding fea- Compounds 3 and 4 have been demonstrated to be efficient
ture possibly arises because the hydrolysis product complex catalysts for the “controlled” ROP of l-lactide. Complex 3
1 is also an efficient catalyst for the ROP of LA. The strat- can even catalyze the ROP of unsublimed l-lactide in the
egy mentioned in Scheme 1 was therefore achieved. Com- presence of water, with a high conversion and a controllable
plex 1 can almost compensate for the decomposed complex molecular weight. A new strategy for the ROP of lactide
3.
under moist conditions was presented.
634
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Ring-Opening Polymerization of l-Lactides
in hexane) was then slowly added to the solution. After stirring for
4 h, the solvent was removed under vacuum to afford a white solid.
The white solid was dissolved in toluene (7 mL). The toluene solu-
tion was then filtered through Celite. After the filtrate was left over-
night at room temperature, colourless crystals were obtained. Yield:
Experimental Section
General Considerations: All manipulations were carried out under
a dry nitrogen atmosphere. Solvents, l-lactide and benzyl alcohol
were purified before use. nBuLi (1.6 m in hexane) and Zn(Et)2
(1.0 m in hexane) were purchased from Aldrich and used as re-
ceived. 1H NMR and 13C NMR spectra were recorded on a Varian
Mercury-300 (300 MHz, 200 MHz) spectrometer with chemical
shifts given in parts per million from the peak of internal TMS.
Microanalyses were performed by using a Heraeus CHN-O-RA-
PID instrument. The GPC measurements were performed on a Hit-
achi L-700 system equipped with a differential Bischoff 8120 RI
detector by using THF (HPLC grade) as an eluent running at
1 mL/min. Molecular weights and molecular weight distributions
were calculated using polystyrene as standard.
1
0.63 g (53%). H NMR (200 MHz, CDCl3, 25 °C): δ = 7.11, 6.72
(s, 8 H, Ph), 3.57 (s, 16 H, -OCH2CH2-), 1.67 (s, 16 H, -OCH2CH2-
), 1.21, 1.18 [s, 72 H, C(CH3)3] ppm. 13C NMR (200 MHz, CDCl3,
25 °C): δ = 158.88, 136.13, 135.74, 134.07, 128.22, 122.00 (Ph),
68.08 (-OCH2CH2-), 34.85, 33.98, 32.10, 31.50 (tBu), 25.30
(-OCH2CH2-) ppm. C72H108Li2O8Zn (1180.92): calcd. C 73.23, H
9.22; found C 73.53, H 9.16.
Typical Polymerization Procedures: A typical polymerization pro-
cedure is exemplified by the synthesis of PLLA-150 (the number
150 indicates the designed [M]0/[complex]0) at 90 °C (Table 1, Entry
2). The polymerization conversion was analyzed by 1H NMR spec-
troscopy. l-Lactide (0.432 g, 3.0 mmol) was added to a solution of
complex 3 (0.027 g, 0.02 mmol) in toluene (10 mL). After the solu-
tion was stirred at 90 °C for 48 h, the reaction was quenched by
the addition of an aqueous acetic acid solution (0.35 n, 10 mL).
Hexane (40 mL) was then added to the above mixture to give a
white crystalline solid. The resulting solid was washed with water
twice and then dried under vacuum.
(TBBP)Na(THF)3 (1): A solution of 3,3Ј,5,5Ј-tetra-tert-butyl-2,2Ј-
biphenol (TBBPH2) (0.820 g, 2.0 mmol) and sodium (0.050 g,
2.2 mmol) was stirred in THF (20 mL) overnight at 50 °C under a
N2 atmosphere. The solvent was removed under vacuum to afford
a white solid. Yield: 0.79 g (61%). 1H NMR (200 MHz, CDCl3,
25 °C): δ = 7.30, 7.12 (s, 4 H, Ph), 3.69 (s, 12 H, -OCH2CH2-), 1.83
(s, 12 H, -OCH2CH2-), 1.34, 1.30 [s, 36 H, C(CH3)3] ppm. 13C
NMR (200 MHz, CDCl3, 25 °C): δ = 136.64, 129.03, 128.22,
126.05, 125.29, 123.04 (Ph), 67.96 (-OCH2CH2-), 34.79, 34.16,
31.78, 29.88 (tBu), 25.52 (-OCH2CH2-) ppm. C40H65NaO5
(645.91): calcd. C 74.03, H 10.10; found C 74.86, H 9.63.
X-Ray Crystallographic Studies: A summary of the crystallographic
data is given in Table S1. Suitable crystals of 2, 3 and 4 were sealed
in thin-walled glass capillaries under a nitrogen atmosphere and
were mounted on a Bruker AXS SMART 1000 diffractometer. In-
tensity data were collected in 1350 frames with increasing w (width
of 0.3° per frame). The absorption correction was based on the
symmetry-equivalent reflections by using the program SADABS.
The space group determination was based on a check of the Laue
symmetry, and systematic absence and was confirmed by using the
structure solution. The structures were solved by direct methods by
using the SHELXTL package. All non-H atoms were located from
successive Fourier maps, and hydrogen atoms were refined by using
a riding model. Anisotropic thermal parameters were used for all
non-H atoms, and fixed isotropic parameters were used for H
atoms. CCDC-777036, -777037 and -777038 (for complexes 1–3,
respectively) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
(TBBP)Li(THF)3 (2): A solution of 3,3Ј,5,5Ј-tetra-tert-butyl-2,2Ј-
biphenol (0.820 g, 2.0 mmol) and nBuLi (1.4 mL, 2.2 mmol, 1.6 m
in pentane) was stirred in THF (20 mL) at 0 °C under a N2 atmo-
sphere for 3 h. The solvent was removed under vacuum to afford a
white solid. The white solid was dissolved in THF (5 mL). The
THF solution was then filtered through Celite. After the filtrate
was left for several hours at room temperature, colourless crystals
were obtained. Yield: 0.66 g (52%). 1H NMR (200 MHz, CDCl3,
25 °C): δ = 7.31, 7.10 (s, 4 H, Ph), 3.65 (s, 12 H, -OCH2CH2-), 1.82
(s, 12 H, -OCH2CH2-), 1.41, 1.31 [s, 36 H, C(CH3)3] ppm. 13C
NMR (200 MHz, CDCl3, 25 °C): δ = 136.45, 128.93, 128.04,
126.17, 125.09, 122.85 (Ph), 67.96 (-OCH2CH2-), 34.96, 34.20,
31.72, 29.90 (tBu), 25.45 (-OCH2CH2-) ppm. C40H65LiO5 (629.86):
calcd. C 75.91, H 10.35; found C 76.70, H 9.85.
[(TBBP)2Zn][(Na)2(THF)4] (3): A solution of 3,3Ј,5,5Ј-tetra-tert-
butyl-2,2Ј-biphenol (0.820 g, 2.0 mmol) and sodium (0.050 g,
2.2 mmol) was stirred in THF (20 mL) overnight at 50 °C under a
N2 atmosphere. Zn(Et)2 (1.2 mL, 1.2 mmol, 1.0 m in hexane) was
then slowly added to the solution. After stirring for 6 h, the solvent
was removed under vacuum to afford a white solid. The white solid
was dissolved in THF (5 mL), and the solution was then filtered
through Celite. After the filtrate was left for several hours at room
temperature, colourless crystals were obtained. Yield: 0.60 g (50%).
1H NMR (200 MHz, CDCl3, 25 °C): δ = 136.64, 129.03, 128.22,
126.05, 125.29, 123.04(Ph), 67.96 (-OCH2CH2-), 34.79, 34.16,
31.78, 29.88 (tBu), 25.52 (-OCH2CH2-).: δ = 7.09, 6.73 (s, 8 H, Ph),
3.57 (s, 16 H, -OCH2CH2-), 1.72 (s, 16 H, -OCH2CH2-), 1.25, 1.18
[s, 72 H, C(CH3)3] ppm. 13C NMR (200 MHz, CDCl3, 25 °C ): δ
= 160.81, 135.30, 135.15, 134.69, 128.23, 121.58 (Ph), 67.91
(-OCH2CH2-), 34.66, 33.98, 31.90, 31.59 (tBu), 25.31 (-OCH2CH2-)
ppm. C72H108Na2O8Zn (1213.01): calcd. C 71.29, H8.97; found C,
71.60; H, 8.86.
Supporting Information (see footnote on the first page of this arti-
cle): Crystallographic and structure refinement data, results from
1
the H NMR titration of the CDCl3 solution, electrospray-ioniza-
1
tion mass spectra, and H NMR spectra are presented.
Acknowledgments
We thank the National Natural Science Foundation of China
(No.21071069), the Scientific Research Foundation for the Re-
turned Overseas Chinese Scholars, the State Education Ministry
and the Fundamental Research Funds for the Central Universities
of China for financial support.
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Eur. J. Inorg. Chem. 2011, 632–636
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L. Wang, X. Pan, L. Yao, N. Tang, J. Wu
SHORT COMMUNICATION
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Published Online: January 18, 2011
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