Synthesis, characterization of sodium and potassium complexes and the application in ring-opening polymerization of L-lactide

Article abstract of DOI:10.1016/j.inoche.2010.09.021

A novel sterically bulky phenol (2,4-di-tert-butyl-6-(1-(3,5-di-tert-butyl- 2-(2-(2-methoxyethoxy)ethoxy)phenyl)ethyl)phenol)(HL) and corresponding dimeric sodium and potassium complexes [ML]₂ (1: M = Na, 2: M = K) have been prepared and struct

Full text of DOI:10.1016/j.inoche.2010.09.021

Inorganic Chemistry Communications 14 (2011) 26–30
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, characterization of sodium and potassium complexes and the application
in ring-opening polymerization of L-lactide
a
a
a
a
b
,a
,a
⁎
⁎
Longhai Chen , Lei Jia , Feixiang Cheng , Lei Wang , Chu-chieh Lin , Jincai Wu , Ning Tang
a
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry,
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
b
Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel sterically bulky phenol (2,4-di-tert-butyl-6-(1-(3,5-di-tert-butyl-2-(2-(2-methoxyethoxy)ethoxy)
phenyl)ethyl)phenol)(HL) and corresponding dimeric sodium and potassium complexes [ML]2 (1: M=Na, 2:
M=K) have been prepared and structurally characterized. Experimental results showed that complexes 1
and 2 can efficiently initiate the ring-opening polymerization of lactide in a controlled fashion, yielding
polymers with expected molecular weight and low polydispersity indexes.
Received 21 August 2010
Accepted 20 September 2010
Available online 29 September 2010
Keywords:
Sodium
© 2010 Elsevier B.V. All rights reserved.
Potassium
Catalyst
Lactide
Ring-opening polymerization
Polylactide (PLA) is one of the most important biodegradable materials
for its wide applications in biomedical and pharmaceutical fields [1], and
over the past three decades much attention has been devoted to the
development of new catalytic/initiating systems for the preparation of
polylactide (PLA). Among several catalytic systems reported previously,
the ring-opening polymerization (ROP) of lactide is the most effective
method for the synthesis of PLA [2]. Many metal complexes have been
used to initiate/catalyze ring-opening polymerization of lactides due to
the advantages of well controlled molecular weight and low polydisper-
sity index (PDI) [3]. As a result, a variety of metal complexes coordinated
with sterically bulky ligands such as β-diketiminate, salen, diol, etc., have
been developed and used as catalytic/initiating systems for the ROP of
lactides [4]. Although these complexes are excellent catalysts for the ROP
of lactides with high yields, their utilization to some extent is limited by
difficulties in removal of the catalyst from the resultant polymers as well
as the toxicity of metal cation [5]. To address this issue, many attempts
have been made to discover the nontoxic metal complexes (e.g., sodium
[6], potassium [7], magnesium [8], calcium [9], iron [10]) and highly active
metal-free [11] catalysts for the ROP of lactides. Due to the fact that sodium
and potassium cations are nontoxic, essential for life and also readily
available, sodium and potassium cations are preferentially selected as one
component of metal complex catalyst during our investigation on the
development of novel effective catalysts for ROP of lactides. EDBPH2 is an
interesting ligand because it has been approved as an indirect food
additive (as an antioxidant in polymer packaging) by the U.S. Food and
Drug Administration [12], and some of its metal complexes are excellent
catalysts with good controlled features for ROP of cyclic ester [13]. Based
on these views, we have designed and synthesized a novel sterically bulky
monovalent phenol ligand derived from EDBPH2 and its related nontoxic
sodium and potassium catalysts. The catalytic activities of sodium and
potassium complexes towards ROP of lactides have been investigated, and
the positive experimental results have proved that two designed catalysts,
especially with sodium cation, are effective in the current ROP of lactides.
According to the previous studies, a novel sterically bulky phenol
and the corresponding metal complexes were prepared in an almost
quantitative yield (Scheme 1) 1 [14]. Single crystals of 1 and 2 suitable
1
(a) Data for ligand 3: (0.481 g, 89% Yield). 1H NMR (300 MHz, CDCl3): δ 7.27 (1H, d,
J=2.4Hz, ArH); 7.23 (1H, d, J=2.4Hz, ArH); 7.13 (1H, d, J=2.4Hz, ArH); 7.09 (1H, d,
J=2.4Hz, ArH); 4.68 (1H, q, J=7.2Hz, CH); 4.14–4.02 (2H, m, CH2); 4.02–3.89 (2H, m, CH2);
3.82–3.78 (2H, m, CH2); 3.68–3.63 (2H, m, CH2); 3.36 (3H, s, CH3); 1.71 (3H, d, J=6.9Hz,
CH3); 1.38 (9H, s, C(CH3)3); 1.36 (9H, s, C(CH3)3); 1.29 (9H, s, C(CH3)3); 1.24 (9H, s, C(CH3)3).
13C NMR (75 MHz, CDCl3): δ 151.13; 150.68; 146.58; 141.63; 140.95; 138.00; 135.24; 131.24;
123.34; 122.29; 121.28; 120.19; 74.95; 72.01; 71.09; 70.32; 59.03; 35.39; 35.06; 34.58; 34.35;
31.74; 31.40; 29.73; 20.83. LC–MS: m/z 558.2 [M+NH4]+. Anal. Calcd for C35H56O4: C, 77.73;
H, 10.44. Found: C, 77.69; H, 10.42. (b) Data for sodium complex 1: (0.596 g, 91% Yield). 1
H
NMR (300 MHz, CDCl3): δ 7.46(2H, br, ArH);7.23 (2H, br, ArH); 7.18 (2H, br, ArH); 6.99 (2H, s,
ArH); 4.68 (2H, br, CH); 3.96 (4H, br, CH2); 3.86 (4H, br, CH2); 3.68 (4H, br, CH2); 3.55 (4H, br,
CH2); 3.37 (6H, s, CH3);2.81 (6H, s, CH3); 1.44 (18H, s, C(CH3)3); 1.40 (18H, s, C(CH3)3); 1.36
(18H, s, C(CH3)3); 1.26 (18H, s, C(CH3)3). 13CNMR(75 MHz,CDCl3): δ152.13; 151.68; 148.48;
143.63; 141.85; 138.70; 135.44; 132.04; 123.64; 122.89; 121.48; 120.39; 76.88; 72.13; 71.14;
59.06; 35.56; 35.87; 35.00; 34.63; 34.03; 31.90; 31.86; 21.75; 14.43 Anal. calcd for
C70H110Na2O8: C, 74.69; H, 9.85. Found: C, 74.60; H, 9.83. (c) Data for complex 2: Yield 87%.
1H NMR (300 MHz, CDCl3): 1H NMR (300 MHz, CDCl3): δ7.51(2H, br, ArH);7.33 (2H, br, ArH);
7.24 (2H, br, ArH); 6.98(2H, s, ArH); 4.73 (2H, br, CH); 4.12 (4H, br, CH2); 3.95 (4H, br, CH2);
3.67 (4H, br, CH2); 3.61 (4H, br, CH2); 3.42 (6H, s, CH3);2.83 (6H, s, CH3); 1.53 (18H, s, C
(CH3)3); 1.46 (18H, s, C(CH3)3); 1.40 (18H, s, C(CH3)3); 1.28 (18H, s, C(CH3)3). 13C NMR (75
MHz, CDCl3): δ 153.13; 152.18; 149.08; 143.73; 141.65; 138.77; 135.64; 132.14; 123.84;
122.79; 121.86; 120.99; 76.96; 72.23; 71.44; 59.86; 36.06; 35.88; 35.67; 34.93; 34.53; 31.96;
32.06; 21.95; 14.83 Anal. calcd for C70H110K2O8: C, 72.62; H, 9.58. Found: C, 72.60; H, 9.51.
⁎
Corresponding author. Fax: +86 931 8912582.
E-mail addresses: wujc@lzu.edu.cn (J. Wu), chenlh2006@lzu.cn (L. Chen).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.09.021

L. Chen et al. / Inorganic Chemistry Communications 14 (2011) 26–30
27
O
O
O
OH
OH
DMF, 110 °C, 12h
+
O
O
TsO
OH
3
O
O
O
O
M
M
O
O
O
O
1 M=Na
2 M=K
Scheme 1. Preparation of ligand and corresponding metal complexes.
for X-ray structural determination were obtained from toluene 2. The
ORTEP drawing of the molecular structures of 1 and 2 are given in
Figs. 1 and 2, respectively. The molecular structures of these
compounds show that the two complexes have dimeric character,
bridging with the oxygen atoms of the phenol group with similar
structures. The geometry around of the sodium atoms in 1 is distorted
tetrahedron, sodium interacted with two bridging phenolate oxygen
atoms, and two oxygen atoms of 2-(2-methoxy-ethoxy)-ethyl group.
Two sodium atoms in this complex are equivalent with Na–O bond
distance of Na(1)–O(3) 2.494(2) Å, Na(1)–O(4) 2.409(3) Å, Na(1)–O
(1) 2.183(2) Å, Na(1)–O(1A) 2.302(2) Å. The molecular structure of 2
is all similar to that of 1 with K–O bond distance of K(1)–O(3) 2.692
(3) Å, K(1)–O(4) 2.686(4) Å, K(1)–O(1) 2.481(3) Å, K(1)–O(1A) 2.625
(3) Å.
Complexes 1 and 2 (0.02 mmol) as catalyst are systematically
tested for the ring-opening polymerization of lactides in THF
(10 mL) at 60 °C, as shown in Table 1. Experimental results indicate
that both complex 1 and 2 are efficient in the ROP of L-lactides, and
the polymerization is completed within 36 h at 60 °C. The reaction
conversion could reach 93.4% with complex 1 as the catalyst at a
monomer-to-catalyst ratio of 100:1 (Table 1, entry 1). It is
interesting that EDBP-Na reported by Lin [6] catalyzes the ROP of
L-lactide with methanol as initiator, while complex 1 can catalyzes
the reaction directly without methanol. Actually complex 1 cannot
activate methanol to initiate the ROP of lactide, because methanol
cannot easily replace long ether group to coordinate to Na+ and be
activated to initiate the ROP reaction. For the different ROP
mechanism, the activity of complex 1 is lower comparing to
EDBP-Na. A good polymerization control is demonstrated by the
linear relationship between Mn and [LA]0/[complex]0 and the
polymers with low PDI, ranging from 1.28 to 1.36 (Fig. 3). The 1H
NMR of PLLA with [LA]0/[Complex]0 ratio of 100 show
a
characteristic methine peak (Fig. 4) at 4.36 ppm and broad
carboxyl terminal group peak at 3.74 ppm in CDCl3, indicating
that the two terminal groups of the polymer are the hydroxyl and
carboxyl terminal group respectively. LC–MS mass spectrum was
employed to investigate the components of the end group of the
polymer at the same ratio. The spectrum shows that oligomers HO
(COCHMeO)nH·M+ (M = K, Na) were obtained in the protic
reagent (Fig. 5). Furthermore, epimerization of the chiral centers
in PLLA does not occur, which confirmed by the study of
homonuclear decoupled 1H NMR in the methine region [15].
Additionally, an acceptable polymerization control also is
demonstrated by the study on the linear relationship between Mn
and [LA]0/[complex]0 with complex 2 as the catalyst at every
monomer-to-catalyst (Fig. 6) (Table 1, entry 6–9). The PDI of the
2
(a) Crystal data for sodium complex: C42H63NaO4, M=654.91, monoclinic, space
group P2(1)/c, a=16.6089(4) Å, b=14.6139(4) Å, c=17.1502(4)
Å
,
α=90.00,
β=91.7240(10), γ=90.00,
V
=4160.83(18) Å3, T=296(2) K, Z=4, Dc=1.044
g/cm3, F000 =1428, 2θmax =26.50, 24367 reflections collected, 8593 unique (R-
int =0.0808), no. of observed reflections 3732 (IN2σ(I)); R1 =0.0684, wR2 =0.1808.
(b) Crystal data for potassium complex: C42H63KO4, M=671.02, monoclinic, space
group P2(1)/c, a=16.6160(4) Å, b=15.0181(4) Å, c=17.1759(5)
β=90.3110(10), γ=90.00, V=4286.0(2) Å3, T=296(2) K, Z=4, Dc=1.038 g/cm3,
000 =1460, 2θmax =20.40, 13877 reflections collected, 4211 unique (Rint =0.0302),
no. of observed reflections 3097 (IN2σ(I)); R1 =0.0559, wR2 =0.1507.
Å , α=90.00,
Fig. 1. X-ray structure of complex 1 as 30% ellipsoids (methyl carbons of the tert-butyl
groups are omitted for clarity, hydrogen atoms omitted). Selected bond lengths (Å): Na
(1)–O(3) 2.494(2), Na(1)–O(4) 2.409(3), Na(1)–O(1) 2.183(2), Na(1)–O(1A) 2.302(2).
F

28
L. Chen et al. / Inorganic Chemistry Communications 14 (2011) 26–30
Fig. 3. Polymerization of L-LA catalyzed by 1 in THF at 60 °C. The relationship between
Mn (■) ((PDI (●)) of the polymer and the initial mole ratio [LA]0/[complex]0 is
shown.
Fig. 2. X-ray structure of complex 2 as 30% ellipsoids (methyl carbons of the tert-
butyl groups are omitted for clarity, hydrogen atoms omitted). Selected bond
lengths (Å): K(1)–O(3) 2.692(3), K(1)–O(4) 2.686(4), K(1)–O(1) 2.481(3),
K(1)–O(1A) 2.625(3).
the two complexes can efficiently catalyze ring-opening polymeriza-
tion of lactides in good controlled manner with slight isotactic
selectivity. Compared with catalyst 2, it was interestingly found that
sodium complex 1 can catalyze ROP of L-lactides in THF at 60 °C giving
a narrower molecular weight distribution and higher conversion. At
the present stage, the ROP of rac-lactide by designed complexes
proceeded with the low selectivity, and a full understanding of the
factors influencing the molecular design of single-site catalyst
precursors for stereoselective ROP of lactides remains to be further
explored in detail.
polylactides obtained are narrow, ranging from 1.24 to 1.41.
Experimental results show sodium complex 1 is more active than
potassium complex 2, because Na+ is the more strong Lewis acid of
than K+ and the lactide is easier to coordinate to Na+ to be activated.
Although the two catalysts have similarity dimeric structures to
EDBP-/K reported by Pan et al., catalyst 1 and 2 are more active
which may attribute to the more bulky long ether group surrounding
the Na+ and K+ [6,7].
Complexes 1 and 2 were also examined in the polymerization of
rac-lactides, as shown in Table 1 (entry 5, 10 ). The homonuclear
decoupled 1H NMR spectrum at the methine region of the PLA derived
from 1 and 2 are isotactic predominance with Pm=0.64 for 1 and
Pm=0.59 for 2, respectively [16]. The low selectivity may be owing to
the insufficient bulk of the ligand, and the modification of this type of
ligand is actively ongoing in our laboratory.
Acknowledgments
We thank the financial supports from the National Natural Science
Foundation of China (Nos. 21071069, 20601011), Science Foundation
of Gansu Province of China (0803RJZA103), Scientific Research
Foundation for the Returned Overseas Chinese Scholars and the
Fundamental Research Funds of the Central Universities of China,
State Education Ministry (lzujbky-2009-26).
In conclusion, two nontoxic sodium and potassium bulky
phenolate complexes have been synthesized and characterized, and
Table 1
Ring-opening polymerization of lactide using complex 1 and 2a.
Entry
Complex
[LA]0/[Complex]0
Time (h)
Mn (obsd)b
Mn (calcd)c
PDI
Conversion (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
2
2
2
2
2
100:1
125:1
150:1
200:1
100:1d
100:1
125:1
150:1
200:1
100:1d
36
36
36
36
36
36
36
36
36
36
19,600 (11,400)
27,000 (15,700)
31,700 (18,400)
40,800 (23,700)
26,800 (15,500)
18,300 (10,600)
25,100 (14,600)
23,300 (13,500)
30,400 (17,600)
26,500 (15,400)
13,600
16,100
19,600
26,700
13,200
12,100
15,600
16,100
20,300
13,100
1.28
1.32
1.35
1.36
1.35
1.24
1.38
1.40
1.41
1.32
93.4
88.7
90.1
92.2
90.8
83.3
86.2
74.1
70.2
90.3
10
a.
Conditions: 0.02 mmol of complex, 10 mL of THF at 60 °C.
Obtained from GPC analysis, and calibrated by polystyrene standard. The true value of Mn could be calculated according to formula Mnobsd =0.58MnGPC
Calculated from the molecular weight of L-lactide times [LA]0/[Complex]0 times conversion yield plus the molecular weight of H2O: Mncalcd =(M/I)×conv×144+18.
b.
c.
.
d.
At 60 °C, 10 mL of THF and rac-lactide were used.

L. Chen et al. / Inorganic Chemistry Communications 14 (2011) 26–30
29
Fig. 4. 1H NMR PLA-100 (from Table 1, entry 1) in CDCl3.
Fig. 5. LC–MS mass spectrum of PLA-100 (from Table 1, entry 1).

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L. Chen et al. / Inorganic Chemistry Communications 14 (2011) 26–30
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Appendix A. Supplementary material
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graphic data for thispaper. These data can be obtained free of charge
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