Air-stable copper derivatives as efficient catalysts for controlled lactide polymerization: Facile synthesis and characterization of well-defined benzotriazole phenoxide copper complexes

Article abstract of DOI:10.1002/pola.26780

Air-stable copper catalysts supported by bis-BTP ligands (BTP = N,O-bidentate benzotriazole phenoxide) were synthesized and structurally characterized. The reactions of Cu(OAc)₂·H₂O with 2.0 molar equivalents of sterically bulky 2-(2H-benzotriazol-2-yl)-4,6-bis(1- methyl-1-phenylethyl)phenol (^CMe2PhBTP-H) and 2-(2H-benzotriazol-2- yl)-4,6-di-tert-butylphenol (^t-BuBTP-H) in refluxing ethanol solution afforded monomeric copper complexes [(^CMe2PhBTP)₂Cu] (1) and [(^t-BuBTP)₂Cu] (2), respectively. The four-coordinated copper analogue [(^TMClBTP)₂Cu] (3) resulted from treatment of 2-tert-butyl-6-(5-chloro-2H-benzotriazol-2-yl)-4-methylphenol (^TMClBTP-H) as the ligand under the same synthetic method with ligand to metal precursor ratio of 2:1, but treatment of complex 3 in acetone gave five-coordinated monomeric complex [(^TMClBTP)₂Cu(Me ₂CO)] (4). X-ray diffraction of single crystals indicates that Cu complex 4 assumes a distorted square pyramidal geometry, penta-coordinated by two BTP ligands, and one Me₂CO molecule. Catalysis for lactide (LA) polymerization of BTP-containing Cu complexes in the presence of various alcohol initiators was investigated. Complex 3 initiated by 9-anthracenemethanol catalyzes the ring-opening polymerization effectively not only in a living fashion but also in an immortal manner, yielding polymers with the predictable molecular weights and narrow molecular weight distributions. Initiations from multifunctional alcohols were able to produce PLLAs with two-arm linear and three-arm star-shaped molecular architectures. The controlled character of Cu complex 3 also enabled us to synthesize the PEG-b-PLLA copolymer. Copyright

Full text of DOI:10.1002/pola.26780

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Air-Stable Copper Derivatives as Efficient Catalysts for Controlled Lactide
Polymerization: Facile Synthesis and Characterization of Well-Defined
Benzotriazole Phenoxide Copper Complexes
Chen-Yu Li,1 Shi-Jie Hsu,1 Chin-lung Lin,1 Chen-Yen Tsai,1 Jun-Han Wang,1 Bao-Tsan Ko,1,2
Chia-Her Lin,1 Hsi-Ya Huang1
1Department of Chemistry, Chung Yuan Christian University, Chung-Li 32023, Taiwan
2Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan
Correspondence to: B.-T. Ko (E-mail: btko@dragon.nchu.edu.tw) or C.-H. Lin (E-mail: chiaher@cycu.edu.tw)
Received 2 February 2013; accepted 16 May 2013; published online
DOI: 10.1002/pola.26780
ABSTRACT: Air-stable copper catalysts supported by bis-BTP
ligands (BTP 5 N,O-bidentate benzotriazole phenoxide) were
synthesized and structurally characterized. The reactions of
Cu(OAc)2ÁH2O with 2.0 molar equivalents of sterically bulky
2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol
penta-coordinated by two BTP ligands, and one Me2CO mole-
cule. Catalysis for lactide (LA) polymerization of BTP-containing
Cu complexes in the presence of various alcohol initiators was
investigated. Complex 3 initiated by 9-anthracenemethanol cata-
lyzes the ring-opening polymerization effectively not only in a
“living” fashion but also in an “immortal” manner, yielding
polymers with the predictable molecular weights and narrow
molecular weight distributions. Initiations from multifunctional
alcohols were able to produce PLLAs with two-arm linear and
three-arm star-shaped molecular architectures. The controlled
character of Cu complex 3 also enabled us to synthesize the
(
(
CMe2PhBTP-H) and 2-(2H-benzotriazol-2-yl)-4,6-di-tert-butylphenol
t-BuBTP-H) in refluxing ethanol solution afforded monomeric
copper complexes [(CMe2PhBTP)2Cu] (1) and [(t-BuBTP)2Cu] (2),
respectively. The four-coordinated copper analogue
[(TMClBTP)2Cu] (3) resulted from treatment of 2-tert-butyl-6-(5-
chloro-2H-benzotriazol-2-yl)-4-methylphenol (TMClBTP-H) as the
ligand under the same synthetic method with ligand to metal
precursor ratio of 2:1, but treatment of complex 3 in acetone
gave five-coordinated monomeric complex [(TMClBTP)2Cu
(Me2CO)] (4). X-ray diffraction of single crystals indicates that
Cu complex 4 assumes a distorted square pyramidal geometry,
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KEYWORDS: benzotriazole phenoxide; biodegradable; catalysis;
copper; lactide; polyesters; ring-opening polymerization (ROP)
because the metal center can be stabilized by N,O-bidentate
chelation to realize the single active site catalyst. For
instance, aluminum alkoxides bearing Salen-type ligands
demonstrates not only good activities but also high stereo-
selectivity with Pm values up to 97% for stereoselective
ROP of rac-lactide.4 Although many catalytic systems have
been reported for the preparation of polyesters with con-
trolled molecular weights and narrow polydispersity indices
(PDIs), moisture- and air-tolerant metal complexes with
well-characterized structures that catalyze ROP effectively
are rare.5 For this reason, late transition metal complexes,
such as nickel(II) and copper(II) complexes seem to be pro-
spective candidates, and only a few studies of their catalytic
performance toward ROP of cyclic esters have been
reported.6 With the exception of recently reported diketimi-
nate copper isopropoxide catalyst,6h well-defined Cu com-
plexes that previously studied catalyze LA polymerization
INTRODUCTION Environmentally friendly polyesters such as
poly(E-caprolactone) (PCL) and poly(lactide) (PLA) as well
as their copolymers have received considerable attention
due to a promising alternative of conventional petrochemi-
cal based polymers.1 Such polymers also have been widely
utilized in the biomedical, pharmaceutical and environmen-
tal fields owing to their biodegradable, biocompatible, and
permeable properties.2 A convenient and useful synthetic
route to PLA and other aliphatic polyesters is the ring-
opening polymerization (ROP) of cyclic esters using metal-
alkoxide initiators or catalysts derived from various
main-group metal complexes. As a result, a variety of metal
complexes including aluminum, calcium, lithium, magne-
sium, and tin(II) as well as zinc complexes have been devel-
oped to achieve great catalytic activities in a controlled
manner.3 Particularly, well-defined metal complexes sup-
ported by phenoxyimine ligands attract increasing attention
Additional Supporting Information may be found in the online version of this article.
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CHART 1 Several types of benzotriazole phenoxide (BTP) derivatives.
to produce PLA with poor controlled character and their
catalytic activities are low. For instance, Cu(II) complexes6b
of N,O-bidentate Schiff base ligands were demonstrated to
be active catalysts for bulk ROP of L-lactide with moderate
molecular weights and acceptable molecular weight distri-
butions (Mn 5 3000–16000; PDI 5 1.08–1.58), but poly-
merizations with only maximum conversion yields of 70–
80% and uncontrollable behaviors were observed.
butylphenol (t-BuBTP-H), 2-tert-butyl-6-(5-chloro-2H-benzo-
triazol-2-yl)-4-methylphenol (TMClBTP-H), poly(ethylene gly-
col)
methyl
ether
(PEG-OH),
tert-butyl
bis(2-
hydroxyethyl)carbamate (BOC-2ROH), triethanolamine (TEA-
3ROH) and 9-anthracenemethanol (9-AnOH) were purchased
from Aldrich and used without further purification. 2,2’-(hexa-
decylimino)diethanol (NC16H33-2ROH) was purchased, recrys-
tallized by hexane. 1H NMR and 13C NMR spectra were
recorded on Bruker Aveance (300 and 400 MHz) spectrome-
ter with chemical shifts given in parts per million from the
peak of internal TMS. Microanalyses were performed using
a Heraeus CHN-O-RAPID instrument. Gel permeation chro-
matography (GPC) measurements were performed on a
Jasco PU-2080 plus system equipped with a RI-2031 detec-
tor using THF (HPLC grade) as an eluent. The chromato-
graphic column was Phenomenex Phenogel 5l 103 Å and
the calibration curve used to calculate Mn(GPC) was pro-
duced from polystyrene standards. The GPC results were
calculated using the Scientific Information Service Corpora-
tion (SISC) chromatography data solution 3.1 edition. UV/
Vis absorption spectra were taken on an Evolution 201 UV-
Visible spectrophotometer.
Inspired by the successful catalytic systems of metal phenox-
yimine complexes in the ROP, our current focus has been to
design and to develop a new family of phenolate ligands con-
taining nitrogen donors, benzotriazole phenoxide (BTP)
derivatives (Chart 1). These unmodified-BTP ligands (type I,
Chart 1) are commercially available, whereas amino-BTP
ligands (type II-III, Chart 1) can be obtained with ease of
modification via one-pot Mannich condensation.7 The synthe-
sis and catalysis of well-characterized BTP-containing Al,5b,8
Zn9 and Mg10 complexes were reported recently, and the lat-
ter magnesium complexes incorporated by amine-BiBTP
ligands have demonstrated to be efficient and bifunctional
catalysts towards ROP of E-caprolactone and cycloaddition of
CO2 with cyclohexene oxide.10b To extend studies for metal
benzotriazole phenoxide complexes and to investigate differ-
ences of polymerization activity and controlled character
with different metal systems, a direct reaction between com-
mercially available BTP-H ligands and Cu(OAc)2ÁH2O as the
metal precursor was approached. We envisaged the use of
copper catalytic system to prepare PLAs and to explore the
catalytic behavior of these Cu complexes bearing such
ligands. In this article, we present the synthesis, structure
and ROP catalysis of air-stable copper derivatives based on
sterically bulky BTP ligands.
Synthesis and Characterization of Copper Complexes 1–4
Synthesis of Complex [(CMe2PhBTP)2Cu] (1)
To a mixture of 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-
phenylethyl)-phenol (1.78 g, 4.0 mmol), Cu(OAc)2ÁH2O (0.39
g, 2.0 mmol) were dissolved in ethanol (50 mL). The solution
was heated under reflux for 24 h and then cooled to room
temperature. After the reaction was completed, the dark
brown precipitate occurred during the reaction was collected
by filtration and then dried in vacuo to give dark brown sol-
ids. Yield: 1.55 g (81%). Anal. calc. for C60H56CuN6O2: N,
8.78; C, 75.33; H, 5.90%; Found: N, 9.01; C, 75.33; H, 6.24%.
EXPERIMENTAL
Synthesis of Complex [(t-BuBTP)2Cu] (2)
General Considerations
To a mixture of 2-(2H-benzotriazol-2-yl)-4,6-di-tert-butylphe-
nol (1.30 g, 4.0 mmol), Cu(OAc)2ÁH2O (0.39 g, 2.0 mmol)
were dissolved in ethanol (50 mL). The solution was heated
under reflux for 24 h and then cooled to room temperature.
After the reaction was completed, the dark green precipitate
occurred during the reaction was collected by filtration and
then dried in vacuo to give dark green solids. Yield: 0.71 g
(50%). Anal. calc. for C40H48CuN6O2: N, 11.86; C, 67.82; H,
6.83%; Found: N, 11.86; C, 67.81; H, 6.82%.
Solvents and reagents were dried by refluxing for at least 24
h over sodium/benzophenone [hexane, toluene, tetrahydrofu-
ran (THF)], or over phosphorus pentoxide (CH2Cl2). Deuter-
ated solvents were dried over
4 Å molecular sieves.
L-Lactide and rac-lactide were recrystallized from a toluene
solution and sublimation twice prior to use. Cu(OAc)2ÁH2O,
2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phe-
nol
(
CMe2PhBTP-H),
2-(2H-benzotriazol-2-yl)-4,6-di-tert-
2
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Synthesis of Complex [(TMClBTP)2Cu] (3)
4.19 (br, -OCH2CH2N-), 3.40 (br, -OCH2CH2N-), 2.71 (br, -CH3
CHOH(C@O)-), 1.75 (m, -NCH2CH2CH2-), 1.56 (d, -CH3CHO
(C5O)-), 1.41 (s, -CH3-).
To a mixture of 2-tert-butyl-6-(5-chloro-2H- benzotriazol-2-
yl)-4-methylphenol (1.26 g, 4.0 mmol), Cu(OAc)2ÁH2O (0.39
g, 2.0 mmol) were dissolved in ethanol (50 mL). The solution
was heated under reflux for 24 h and then cooled to room
temperature. After the reaction was completed, the dark
green precipitate occurred during the reaction was collected
by filtration and then dried in vacuo to give dark green sol-
ids. Yield: 0.83 g (60%). Anal. calc. for C34H34Cl2CuN6O2: N,
12.12; C, 58.92; H, 4.94%; Found: N, 11.64; C, 59.20; H,
5.09%.
Polymerization with triethanolamine (TEA-3ROH) initiator in
toluene (10 mL) were stirred at 110ꢀC for 48 h. The conver-
sion yield (95%) of PLLA-150 (the number 150 indicates the
[L-LA]0/[TEA-3ROH]0 ratio) was analyzed by 1H-NMR spec-
troscopic analysis. 1H-NMR (300 MHz, ppm, CDCl3): d 5.14
(q, -CHO(C@O)-), 4.33 (br, -CH3CHOH(C@O)-), 4.12 (t, -OCH2
CH2N-), 2.77 (t, -OCH2CH2N-), 2.69 (br, -CH3CHOH(C@O)-),
1.56 (d, -CH3CHO(C@O)-).
Synthesis of Complex [(TMClBTP)2Cu(Me2CO)] (4)
To a solution of [(TMClBTP)2Cu] (3) (0.69 g, 1.00 mmol) was
dissolved in acetone (30 mL). The solution was stirred at
25ꢀC for 16 h and was removed the volatile components in
vacuo to give pale green solids. Black crystals of 0.07 g
(Yield: 10%) were obtained from the saturated acetone solu-
tion. Anal. calc. for C37H40Cl2CuN6O2: N, 11.19; C, 59.16; H,
5.37%; Found: N, 11.18; C, 59.36; H, 5.45%.
Synthesis of PEG-b-PLLA Diblock Copolymer Catalyzed by
the Complex 3
A typical polymerization procedure was exemplified by the
synthesis of PEG(17)-b-PLLA(100) (the number 17 and 100
indicate the degree of polymerization of ethylene oxide and
[L-LA]0/[PEG-OH]0). The living polymerization of L-LA was
synthesized by a similar approach as described above, but
poly(ethylene glycol) methyl ether (Mw 5 750, 0.075 g, 0.1
mmol) in toluene (10.0 mL) was used as the initiator. The
reaction mixture was stirred at 110ꢀC for 6 h and was then
quenched using the procedures described previously. Yield:
1.13 g (85%).
General Procedure for Ring-Opening Polymerization
Polymerization of L-Lactide Catalyzed by Complex 3 in the
Presence of 9-anthracenemethanol (9-AnOH)
A typical polymerization procedure was exemplified by the
synthesis of PLLA-100 (the number 100 indicates the
[L-LA]0/[9-AnOH]0 ratio). Polymerizations were carried out
X-Ray Crystallographic Studies
Suitable crystals of 1 and 2 were sealed in thin-walled glass
capillaries under a nitrogen atmosphere and mounted on
Bruker APEX2 diffractometer to collect diffraction data at
296K. Suitable crystals of complex 4 were mounted onto
glass fiber using perfluoropolyether oil and cooled rapidly in
a stream of cold nitrogen gas to collect diffraction data at
100 K using Bruker APEX2 diffractometer. Intensity data
were collected in 1350 frames with increasing w (width of
0.5ꢀ per frame). The absorption correction was based on the
symmetry-equivalent reflections using SADABS program.11
The space group determination was based on a check of the
Laue symmetry and systematic absence, and was confirmed
by the structure solution. The structures were solved with
direct methods using a SHELXTL package.11 All non-H atoms
were located from successive Fourier maps, and hydrogen
atoms were treated as a riding model on their parent C
atoms. Anisotropic thermal parameters were used for all
non-H atoms, and fixed isotropic parameters were used for
H-atoms. Drawing of the molecules was done using Oak
Ridge Thermal Ellipdoid Plots (ORTEP).12 Crystallographic
data of complexes 1, 2, and 4 are summarized in Supporting
Information Table S1.
under
a
dry nitrogen atmosphere. To
(3) (0.034 g, 0.05
a
solution of
mmol), 9-
[(TMClBTP)2Cu]
anthracenemethanol (9-AnOH, 0.020 g, 0.1 mmol) and L-lac-
tide (1.44 g, 10.0 mmol) in toluene (10 mL) were stirred at
ꢀ
110 C for 6 h. The conversion yield (93%) of PLLA-100 was
analyzed by 1H-NMR spectroscopic analysis. After the reac-
tion was quenched by the addition of excess water (0.5 mL),
the polymer was precipitated into hexane (100 mL). The
final polymer was then dissolved in THF (20 mL) and puri-
fied upon precipitation again in MeOH (150 mL), collected
and dried under vacuum. Yield: 1.20 g (90 %).
Polymerization of L-Lactide Catalyzed by Complex 3 in the
Presence of Multifunctional Alcohol
Polymerization is similar to the procedure described for L-
lactide polymerization with 9-anthracenemethanol. Polymer-
ization with 2,2’-(hexadecylimino)bisethanol (NC16H33-2ROH)
initiator in toluene (10 mL) were stirred at 110 ꢀC for 6 h.
The conversion yield (91%) of PLLA-100 (the number 100
indicates the [L-LA]0/[NC16H33-2ROH]0 ratio) was analyzed
by 1H-NMR spectroscopic analysis. 1H-NMR (CDCl3, ppm): d
5.14 (q, -CHO(C@O)-), 4.30 (br, -CH3CHOH(C@O)-), 4.11 (br,
-OCH2CH2N-), 2.69 (br, -OCH2CH2N-), 2.43 (br, -CH3CHOH
(C@O)-), 1.75 (m, -NCH2CH2CH2-), 1.56 (d, -CH3CHO(C@O)-),
1.20 (s, -CH2-), 0.82 (t, -CH3-).
RESULTS AND DISCUSSION
Syntheses and Crystal Structure Determination
Polymerization with tert-butyl bis(2-hydroxyethyl)carbamate
(BOC-2ROH) initiator in toluene (10 mL) were stirred at
110ꢀC for 6 h. The conversion yield (93%) of PLLA-100 (the
number 100 indicates the [L-LA]0/[BOC-2ROH]0 ratio) was
analyzed by 1H-NMR spectroscopic analysis. 1H-NMR (CDCl3,
ppm): d 5.14 (q, -CHO(C@O)-), 4.30 (br, -CH3CHOH(C@O)-),
The synthesis of copper complexes (1-4) containing sterically
bulky benzotriazole phenoxide derivatives
(
CMe2PhBTP–,
t-BuBTP–, and TMClBTP–) was shown in Scheme 1. Treatment
of Cu(OAc)2ÁH2O with 2.0 mol equiv. of 2-(2H-benzotriazol-2-
yl)-4,6-bis(1-methyl-1-phenylethyl)phenol (CMe2PhBTP-H) in
refluxing ethanol (EtOH) solution gave tetra-coordinated
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SCHEME 1 Synthetic route of copper complexes 1–4.
copper complex [(CMe2PhBTP)2Cu] (1) in high yield. Simi-
larly, the monomeric copper analogue ([(t-BuBTP)2Cu] (2);
[(TMClBTP)2Cu] (3)) could also be synthesized by employing
2-(2H-benzotriazol-2-yl)-4,6-di-tert-butylphenol
or 2-tert-butyl-6-(5-chloro-2H-benzotriazol-2-yl)-4-methyl-
phenol (TMClBTP-H) as the ligand according to the same
synthetic route with metal precursor to ligand ratio of 1:2.
Attempts to synthesize the mono-BTP Cu complexes have
proved unsuccessful. Interestingly, complex 3 acetone adduct
[(TMClBTP)2Cu(Me2CO)] (4) was obtained via recrystalliza-
tion from acetone (Me2CO) solution. It was further confirmed
by X-ray structural analysis. No similar product was isolable
(
t-BuBTP-H)
FIGURE 1 ORTEP drawing of complex 1 with thermal ellipsoids drawn at the 30% level. Hydrogen atoms are omitted for clarity.
˚
Selected bond lengths (A) and angles (deg): Cu(1)-O(1) 1.875(3), Cu(1)-O(2) 1.876(3), Cu(1)-N(1) 1.973(3), Cu(1)-N(4) 1.966(3), O(1)-
Cu(1)-O(2) 167.95(12), O(1)-Cu(1)-N(1) 89.43(13), O(1)-Cu(1)-N(4) 92.85(12), O(2)-Cu(1)-N(1) 92.35(12), O(2)-Cu(1)-N(4) 89.94(12), N(1)-
Cu(1)-N(4) 158.07(14).
4
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FIGURE 2 ORTEP drawing of complex 2 with thermal ellipsoids
drawn at the 30% level. Hydrogen atoms are omitted for clarity.
˚
Selected bond lengths (A) and angles (deg): Cu(1)-O(1)
1.8719(19), Cu(1)-O(2) 1.8750(18), Cu(1)-N(1) 1.976(2), Cu(1)-N(4)
1.9568(19), O(1)-Cu(1)-O(2) 165.36(9), O(1)-Cu(1)-N(1) 90.29(9),
O(1)-Cu(1)-N(4) 91.80(8), O(2)-Cu(1)-N(1) 95.08(8), O(2)-Cu(1)-
N(4) 90.25(8), N(1)-Cu(1)-N(4) 150.30(9).
FIGURE 3 ORTEP drawing of complex 4 with thermal ellipsoids
drawn at the 50% level. Hydrogen atoms are omitted for clarity.
˚
Selected bond lengths (A) and angles (deg): Cu(1)-O(1)
while complex 1 or 2 was recrystallized from Me2CO. We
believed that the effective electron-withdrawing ability of the
-Cl moiety enhances the Lewis acidity of the copper(II) cen-
ter and therefore leads to existence of weakly bonding
between copper and oxygen atom from Me2CO in 4. All cop-
per complexes were isolated as air-stable crystalline solids
1.912(2), Cu(1)-O(2) 1.918(2), Cu(1)-N(1) 1.976(2), Cu(1)-N(4)
1.987(2), Cu(1)-O(1S) 2.452(3), O(1)-Cu(1)-O(2) 163.92(9), O(1)-
Cu(1)-N(1) 87.82(10), O(1)-Cu(1)-N(4) 92.38(9), O(2)-Cu(1)-N(1)
93.11(9), O(2)-Cu(1)-N(4) 87.83(9), N(1)-Cu(1)-N(4) 175.91(11),
O(1)-Cu(1)-O(1S) 98.47(8), O(2)-Cu(1)-O(1S) 97.61(8), N(1)-Cu(1)-
O(1S) 87.66(10), N(4)-Cu(1)-O(1S) 88.28(10).
TABLE 1 Ring-Opening Polymerization of Lactide (LA) Catalyzed by Complexes 1-3 in the Presence of 9-AnOH
Conv. (%)c
Mn (calcd.)d
Mn (obsd.)e
Mn (NMR)f
PDIg
Pm
h
Entry
Cat.
[LA]0/[Cat.]0/[9-AnOH]0
t (h)
i
1a
2a
3a
4a
5a
6a
7a
8a
9a
10b
1
2
3
3
3
3
3
3
3
3
200/1/2
200/1/2
200/1/2
50/1/2
6
6
6
6
6
6
8
6
6
6
10
92
93
95
93
93
94
93
92
94
1,600
13,500
13,600
3,600
2,500(1,500)
18,700(10,800)
21,800(12,600)
5,300(3,100)
1,440
13,700
14,400
3,600
1.10
1.75
1.13
1.19
1.18
1.11
1.19
1.09
1.12
1.36
–
i
–
i
–
i
–
i
100/1/2
300/1/2
400/1/2
400/1/8
400/1/16
200/1/2
6,900
9,800(5,700)
6,500
–
i
20,300
27,300
69,00
39,800(23,000)
48,000(27,800)
14,400(8,300)
6,300(3,600)
22,000
29,700
7,200
–
i
–
i
–
i
3,500
3,600
–
13,700
23,900(13,800)
14,500
0.50
a
f
[Cat.]0 5 0.005M, 10 mL toluene, 110 ꢀC.
rac-LA as the monomer, 0.05 mmol complexes, 10 mL toluene, 110 ꢀC.
Obtained from 1H-NMR determination.
Obtained from 1H-NMR analysis.
Obtained from GPC determination.
b
c
g
h
Pm is the probability of meso linkages between monomer units and is
d
Calculated from the molecular weight of lactide times [L-LA]0/[9-
determined from the methine region of the homonuclear decoupled 1H-
NMR spectrum.
AnOH]0 times conversion yield plus the molecular weight of 9-AnOH.
e
i
Obtained from GPC analysis and calibrated by polystyrene standard.
Not determination.
Values in parentheses are the values obtained from GPC times 0.58.20
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butyl (2C4H9) substituent at the 4- and 6-position of the
phenoxy group. Complex 1 displays the monomeric and
homoleptic features with a four-coordinated copper center
incorporated by two N,O-bidentate BTP ligands, forming two
six-membered chelating rings in a twisted conformation. The
bond angles around Cu center in 1 are classified into two
sets: one in a range 89.43(13)-92.85(12)ꢀ and the other in a
range 158.07(14)-167.95(12)ꢀ, which indicate a distorted
square planar geometry. The average bond lengths between
the Cu atom and O(phenoxy) and N(benzotriazole) are,
respectively, 1.876(3) and 1.968(3) Å for 1, which are all
comparable with those observed for the monomeric copper
complexes bearing the O,N-chelating triazole-phenolate deriv-
ative.13 In comparison, the Cu-containing bond distances and
angles for 2 are similar to those found for complex 1 as
shown in Supporting Information Table S2. The solid-state
FIGURE 4 Polymerization of L-LA catalyzed by 3 in toluene at
ꢀ
structure of
4 reveals a mononuclear species, penta-
w
110 C. The relationship between Mn(᭿)/PDI( ) of polymer and
coordinated by two O atoms and two N atoms of the biden-
tate BTP ligands and one O atom of Me2CO molecule (Fig.
3). The Cu center of complex 4 adopts a distorted square
pyramidal geometry (s 5 0.20)14 with aryloxy oxygen atoms
O(1), O(2), and benzotriazole nitrogen atoms N(1), N(4)
occupying the square base, and the Cu(1) atom is about
0.0790 Å above the O(1)N(1)O(2)N(4) mean plane. The dis-
tances between the Cu atom and atoms O(1), O(2), N(1), and
N(4) are 1.912(2), 1.918(2), 1.976(2), and 1.987(2) Å for 4,
which are slightly longer than the bond distances observed
for complexes 1 and 2 as compared in Supporting Informa-
tion Table S2. It is worth to note that the apical oxygen atom
O(1S) from Me2CO is coordinated to copper center with a
Cu(1)-O(1S) bond distance of 2.452(2) Å, which is ꢁ0.54 Å
longer than the other Cu-O distances of 1.912(2)-1.918(2) Å,
the mole ratio ([L-LA]0-[L-LA])/[9-AnOH]0 is shown.
and were characterized on the basis of microanalyses and X-
ray single crystal determinations.
Single crystals of complexes 1 and 2 suitable for X-ray struc-
tural analysis were obtained from their saturated Et2O solu-
tions. Oak Ridge Thermal Ellipsoid Plot (ORTEP) drawings
displaying selected bond lengths and angles of the molecular
structures of 1 and 2 are displayed in Figures 1 and 2,
respectively. Both complexes crystallize in monoclinic space
group P21/c with two molecules of [(BTP)2Cu] in the asym-
metric unit. The molecular structures of complexes 1 and 2
are isostructural, except either a cumyl (2CMe2Ph) or a tert-
FIGURE 5 1H-NMR spectrum of PLLA-25 (Table 1, entry 4) in CDCl3.
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the [L-LA]0/[9-AnOH]0 ratio) reveals that the PLLA chain is
capped by one 9-anthracenemethyl ester and one hydroxyl
chain end with the integration ratio close to 9:2:1:1 between
Ha, Hb, Hd, and He.16 Based on the observed results from spec-
troscopic studies of PLLA, it implies that the [(TMClBTP)2Cu]/9-
AnOH system might undergo an “activated-monomer” path,
whereas L-LA is activated by 3 and 9-AnOH behaves as a
nucleophile to proceed the insertion of an anthracenemethoxide
group into the L-LA.17 It is worthy of note that the structural
investigation of complex 4 with an weakly bonding between
copper and oxygen atom of Me2CO suggested the behavior of
“monomer activation” during the ROP of L-LA.
FIGURE 6 Multifunctional alcohol initiators used for the poly-
merization of L-LA.
indicating the weakly bonding between copper atom and
oxygen atom of acetone. Similarly, a long Cu-O bond distance
bonded by oxygen atom of O-donor solvent in the distorted
square-pyramidal coordination was also reported in tetrahy-
drofuran(THF)-containing Cu(II) complex with the Cu-O(api-
cal THF) distance of 2.476(5) Å.15
Not only the “living” character of complex 3 for L-LA polymer-
ization has enabled, but also the “immortal” manner has pro-
vided an effective way by using only small amounts of
catalyst to synthesize polyesters.18 Consequently, ROP of L-LA
catalyzed by copper complex 3 using excess equiv. ratios of
9-AnOH (up to 16 equiv.) as the chain transfer agent (entries
8-9) were examined. For instance, extra 9-AnOH can be added
16-fold amount to [L-LA]0/[Cu]0 5 400, giving a narrow PDI
polymer with Mn that is similar to molecular weight from the
addition of [L-LA]0/[3]0/[9-AnOH]0 5 50/1/2 (Table 1, entry
9 vs. 4). The catalytic performance of copper catalyst 3 is
slightly better than that of our previously reported
benzotriazole-phenolate alumoxane catalysts, in which a pro-
longed period of time (48 h) is required to reach a high con-
version yield under the same conditions.5b However, Cu
complex 3 shows the lower catalytic activity than magnesium
complex bearing the corresponding BTP ligands at the cata-
lytic condition of 80ꢀC for 5 h.10a In order to understand the
microstructure of PLA catalyzed by BTP-modified Cu com-
plexes, polymerization of rac-LA and the stereoselectivity cata-
lyzed by Cu complex 3 were explored. It was found that the
catalytic activity is not affected while rac-LA is used as the
monomer and a virtually atactic polymer (Pm 5 0.50) is
obtained (Table 1, entry 10, Supporting Information Fig. S1).
Ring-Opening Polymerization of Lactide
On the basis of our previous studies on controlled lactone poly-
merizations catalyzed by BTP-containing metal complexes
(metal 5 Al, Mg, and Zn),5b,8,9,10a the benzotriazole phenoxide
Cu derivatives have the potential to act as catalysts for the ROP
of lactide (LA) in the presence of alcohol. ROP of L-LA cata-
lyzed by Cu complexes 1-3 in the presence of 9-
anthracenemethanol (9-AnOH) was first performed to evaluate
their catalytic performances, and representative results are
listed in Table 1. Optimized conditions were found to be the Cu
catalyst concentration of 0.005 M with 9-AnOH (two molar
equiv.) in toluene (10 mL) at 110ꢀC. Experimental results
revealed that Cu complex 3 is an active catalyst to give poly(L-
LA)s with narrow molecular weight distributions (PDI < 1.20)
among these copper complexes. Accordingly, different
monomer-to-initiator ratios ([L-LA]0/[9-AnOH]0 5 25-200) cat-
alyzed by 3 were systematically studied to probe “living” fash-
ion under optimum conditions. It could be seen from Table 1,
entries 3-7 that L-LA conversion was attained 92% within 8 h
and the actual molecular weight Mn (Mn(GPC) value corrected
by a factor of 0.58) of the yielded polymers is close to the
molecular weight calculated from the molar ratio of L-LA to 9-
AnOH. Figure 4 illustrates a linear relationship between
observed Mn and ([L-LA]0-[L-LA])/[9-AnOH]0, and the PDIs of
poly(L-LA)s catalyzed by 3 range from 1.11 to 1.19, indicating
a “living” character of the polymerization. End group analysis
To demonstrate further the versatility of Cu complex 3 for
the L-LA polymerization, several multifunctional alcohol ini-
tiators such as 2, 2’-(hexadecylimino)diethanol (NC16H33
-
2ROH), tert-butyl bis(2-hydroxyethyl)carbamate (BOC-
2ROH), and triethanolamine (TEA-3ROH) were employed to
initiate polymerization of L-LA (Fig. 6). ROP of L-LA initiated
1
from the H-NMR spectrum (Fig. 5) of PLLA-25 (“25” indicates
TABLE 2 Results of the ROP of L-LA with Various Multifunctional Initiators at 110 ꢀC in Toluene Using Cu Complex 3
Entry
ROH
[L-LA]0/[3]0/[ROH]0
t (h)
Conv.(%)a
Mn (calcd.)b
Mn (obsd.)c
Mn (NMR)d
PDIe
1
2
3
4
NC16H33-2ROH
BOC-2ROH
TEA-3ROH
PEG17-OH
400/1/2
400/1/2
900/1/2
200/1/2
6
6
91
93
95
92
13,400
13,600
20,700
14,000
26,900 (15,000)
25,000 (14,500)
46,800 (27,100)
23,800 (13,300)
14,700
14,400
25,000
14,100
1.16
1.15
1.23
1.13
48
6
a
d
Obtained from 1H-NMR determination.
Obtained from 1H-NMR analysis.
Obtained from GPC analysis.
b
e
Calculated from the molecular weight of lactide times [L-LA]0/[3]0
times conversion yield plus the molecular weight of ROH.
c
Obtained from GPC analysis and calibrated by polystyrene standard.
Values in parentheses are the values obtained from GPC times 0.58.
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FIGURE 7 1H-NMR (400 MHz, CDCl3) spectra of trihydroxylfunctionalized star-shaped PLLA (Table 2, entry 3).
from alcohols NC16H33-2ROH and BOC-2ROH gives two-arm
linear PLLA while initiation from alcohol TEA-3ROH pro-
duces three-arm star-shaped PLLA (Scheme 2). Table 2 sum-
marizes the conditions and results of Cu-catalyzed
polymerization of L-LA in the presence of varied alcohol ini-
tiators. Relative to 9-AnOH as the initiator, the catalytic effi-
ciency is little affected while NC16H33-2ROH or BOC-2ROH
is altered as the initiator (Table 2, entries 1-2). For instance,
the synthesized PLLA with two-arm linear architecture from
BOC-2ROH exhibited the desired molecular weight (Mn
5
14,500 g/mol) and low PDI (<1.20). However, initiating
activities are drastically changed with alcohol TEA-3ROH as
the initiator. As can be seen from Table 2, [(TMClBTP)2Cu]/
TEA-3ROH catalytic system also shows effective activity in a
controlled fashion, but with duration of polymerization up to
48 h to attain conversion ꢁ95%. To understand the chemical
nature of three-armed polymer architecture initiated by TEA-
1
3ROH, H-NMR studies were conducted as shown in Figure 7.
1
The H-NMR spectrum of the resultant PLLA clearly displayed
resonances from a triethanolamine-core and three HOCH(Me)CO
chain ends for each macromolecule with the integration ratio
ꢁ2:1 between Hc and Hb, which was in agreement with a three-
arm star-shaped PLA catalyzed by the Salan-ligated yttrium alkyl
complex.19
Encouraged by controlled catalysis of lactide polymerization
initiated from multifunctional alcohol initiators, we also
examined the capability of producing PEG-PLLA block copol-
ymer with the controlled molecular weight and a narrow
PDI. Block copolymer of PEG-b-PLLA was therefore synthe-
sized by the ROP of L-LA in the presence of PEG-OH cata-
lyzed by Cu complex 3, and the results are shown in Table 2,
entry 4. As expected, the catalytic activity and the controlled
manner remained when PEG-OH is employed as the macroi-
nitiator. The block nature of the PEG-b-PLLA copolymer was
1
1
confirmed by its H-NMR spectrum and GPC profile. H-NMR
spectrum (Fig. 8) of PEG-b-PLLA indicates that a sharp sin-
glet of methylene protons (Hc, 3.60 ppm) for the PEG seg-
ment and a quartet signal of methine protons (Ha, 5.14 ppm)
for the PLLA segment, suggesting the binary composition of
the PEG-b-PCL copolymer. The GPC profiles are shown in Fig-
SCHEME 2 Synthesis of two-arm linear and three-arm star-
shaped PLLAs by L-LA polymerizations catalyzed by Cu com-
plex 3 with various multifunctional alcohol initiators.
ure 9; peak A is relative to macroinitiator PEG-OH (Mn
5
500 g/mol, PDI 5 1.34) and peak B is to PEG-b-PLLA after
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FIGURE 8 1H-NMR spectrum of PEG-b-PLLA (Table 2, entry 4) in CDCl3.
copolymerization with L-LA at 100 equiv. An increased
molecular weight and its narrow distribution (Mn 5 23800
g/mol, PDI 5 1.13) were observed (peak B), verifying the
formation of the block copolymer.
[(TMClBTP)2Cu(Me2CO)] (4), the copper center is extra coordi-
nated by one acetone molecule, suggesting the Cu center in
four-coordinated complex [(TMClBTP)2Cu] (3) can behave
effectively as one active site to activate monomers during
ring-opening polymerization (ROP). Experimental results indi-
cate that 3/9-anthracenemethanol catalytic system was able to
catalyze ROP of L-lactide (L-LA) with “living” and “immortal”
characters, giving polymers with the expected molecular
weights and narrow molecular weight distributions. Initiations
from multifunctional alcohols were able to produce polymers
with diverse molecular architectures, such as two-arm linear
and three-arm star-shaped PLLAs. The controlled fashion of
complex 3 also enabled preparation of the PEG-b-PLLA copoly-
CONCLUSIONS
Four new copper(II) complexes based on N,O-bidentate benzo-
triazole phenoxide ligands have been synthesized and charac-
terized by ELEM. ANAL. as well as X-ray single crystal structural
determination. On the basis of the crystal structure of
1
mer, which were characterized by H-NMR and GPC.
ACKNOWLEDGMENT
The authors gratefully acknowledge the financial support from
the National Science Council, Taiwan (NSC101-2113-M-005-
020-MY3 to B.-T. Ko, NSC101-2113-M-033-007-MY3 to C.-H.
Lin, and NSC100-2632-M-033-001-MY3 to H.-Y. Huang). We
also thank Professor Chu-Chieh Lin for valuable discussions.
REFERENCES AND NOTES
1 R. E. Drumright, P. R. Gruber, D. E. Henton, Adv. Mater. 2000,
12, 1841–1846.
2 (a) R. Gref, Y. Minamitake, M. T. Peracchia, V. Trubetskov, V.
Torchilin, R. Langer, Science 1994, 263, 1600–1603; (b) T. Fuji-
sato, Y. Ikada, Macromol. Symp. 1996, 103, 73–83; (c) B.
Jeong, Y. H. Bae, D. S. Lee, S. W. Kim, Nature (London) 1997,
388, 860–862.
FIGURE 9 GPC profiles of copolymerization of PEG-b-PLLA: (peak
A) the prepolymer (Mn 5 500 g/mol, PDI 5 1.34); (peak B) the block
copolymer of PEG-b-PLLA (Mn 5 23800 g/mol, PDI 5 1.13).
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3 (a) B. J. O’Keefe, M. A. Hillmyer, W. B. Tolman, J. Chem.
Soc. Dalton Trans 2001, 2215–2224; (b) A. C. Albertsson, I. K.
Varma, Biomacromolecules 2003, 4, 1466–1486; (c) O. Dechy-
Cabaret, B. Martin-Vaca, D. Bourissou, Chem. Rev. 2004, 104,
6147–6176; (d) J. Wu, T.-L. Yu, C.-T. Chen, C.-C. Lin, Coord.
Chem. Rev. 2006, 250, 602–626; (e) R. H. Platel, L. M. Hodgson,
C. K. Williams, Polym. Rev. 2008, 48, 11–63; (f) C. Wheaton, P.
Hayes, B. Ireland, Dalton Trans. 2009, 4832–4846.
9 (a) J.-Y. Li, C.-Y. Li, W.-J. Tai, C.-H. Lin, B.-T. Ko, Inorg. Chem.
Commun. 2011, 14, 1140–1144; (b) Y.-E. Tai, C.-Y. Li, C.-H. Lin,
Y.-C. Liu, B.-T. Ko, Y.-S. Sun, J. Polym. Sci. Part A: Polym. Chem.
2011, 49, 4027–4036; (c) C.-Y. Sung, C.-Y. Li, J.-K. Su, T.-Y. Chen,
C.-H. Lin, B.-T. Ko, Dalton Trans. 2012, 953–961.
10 (a) C.-Y. Li, J.-K. Su, C.-J. Yu, Y.-E. Tai, C.-H. Lin, B.-T. Ko,
Inorg. Chem. Commun. 2012, 20, 60–65; (b) C.-Y. Li, C.-R. Wu,
Y.-C. Liu, B.-T. Ko, Chem. Commun. 2012, 48, 9628–9630.
4 H.-L. Chen, S. Dutta, P.-Y. Huang, C.-C. Lin, Organometallics
2012, 31, 2016–2025.
11 G. M. Sheldrick, Acta Cryst. 2008, A64, 112–122.
12 M. N. Burnett, C. K. Johnson, ORTEPIII, Report ORNL-6895,
Oak Ridge National Laboratory, Oak Ridge, TN, 1996.
5 (a) M. D. Jones, M. G. Davidson, C. G. Keir, L. M. Hughes, M.
F. Mahon, D. C Apperley, Eur. J. Inorg. Chem. 2009, 635–642;
(b) C.-Y. Li, C.-Y. Tsai, C.-H. Lin, B.-T. Ko, Dalton Trans. 2011,
1880–1887; (c) L. Wang, X. Pan, L. Yao, N. Tang, J. Wu, Eur. J.
Inorg. Chem. 2011, 632–636.
13 K. Butsch, A. Klein, S. Nitsche, K. Stirnat, J. R. Hawkett, E. J.
L. McInnes, M. Bauer, Dalton Trans. 2012, 41, 11464–11475.
14 A. W. Addison, T. N. Rao, J. Reedijk, J. V. Rijn, G. C. Ver-
schoor, J. Chem. Soc. Dalton Trans. 1984, 1349–1356.
6 (a) J. Sun, W. Shi, D. Chen, C. Liang, J. Appl. Polym. Sci.
2002, 86, 3312–3315; (b) A. John, V. Katiyar, K. Pang, M. M.
Shaikh, H. Nanavati, P. Ghosh, Polyhedron 2007, 26, 4033–
4044; (c) S. Bhunora, J. Mugo, A. Bhaw-Luximon, S. Mapolie,
J. V. Wykc, J. Darkwa, E. Nordlander, Appl. Organometal.
Chem. 2011, 25, 133–145; (d) L.-L. Chen, L.-Q. Ding, C. Zeng, Y.
15 S. Muhammad, M. Muhammad, H. Madeleine, A. Javeed, A.
Kibriya, Acta Cryst. 2008, E64, m1139.
16 J. Wu, Y.-Z. Chen, W.-C. Hung, C.-C. Lin, Organometallics
2008, 27, 4970–4978.
17 (a) C. M. Silvernail, L. J. Yao, L. M. R. Hill, M. A. Hillmyer,
W. B. Tolman, Inorg. Chem. 2007, 46, 6565–6574; (b) C.-A.
Huang, C.-L. Ho, C.-T. Chen, Dalton Trans. 2008, 3502–3510; (c)
M.-T. Chen, P.-J. Chang, C.-A. Huang, K.-F. Peng, C.-T. Chen,
Dalton Trans. 2009, 9068–9074.
€
Long, X.-Q. Lu, J.-R. Song, D.-D. Fana, W.-J. Jin, Appl. Organo-
metal Chem. 2011, 25, 310–316; (e) L. Ding, Wenjuan J.; Z.
€
Chu, L. Chen, X.-Q. Lu, G. Yuan, J. Song, D. Fan, F. Bao, Inorg.
Chem. Commun. 2011, 14, 1274–1278; (f) G. Xiao, B. Yan, R.
€
Ma, W. J. Jin, X.-Q. Lu, L. Q. Ding, C. Zeng, L. L. Chen, F. Bao,
18 V. Poirier, T. Roisnel, J.-F. Carpentier, Y. Sarazin, Dalton
Trans. 2009, 9820–9827.
Polym. Chem. 2011, 2, 659–1; (g) R. R. Gowda, D. Chakraborty,
J. Molec. Catal A: Chem. 2011, 349, 86–93; (h) T. J. J. White-
horne, F. Schaper, Chem. Commun. 2012, 48, 10334–10336.
19 W. Zhao, D. Cui, X. Liu, X. Chen, Macromolecules 2010, 43,
6678–6684.
7 (a) J.-Y. Li, Y.-C. Liu, C.-H. Lin, B.-T. Ko, Acta Cryst. 2009, E65,
o2475; (b) M.-J. Chen, B.-H. Wu, C.-Y. Li, C.-H. Lin, B.-T. Ko,
Acta Cryst. 2012, E68, o2944.
20 The Mn (GPC) value is multiplied by a factor of 0.58, giving
the actual Mn of the polylactide, see: Baran, J.; Duda, A.;
Kowalski, A.; Szymanski, R.; Penczek, S. Macromol Rapid Com-
mun 1997, 18, 325–333; Biela, T.; Duda, A.; Penczek, S. Macro-
mol Symp 2002, 183, 1–10.
8 W.-J. Tai, C.-Y. Li, P.-H. Lin, J.-Y. Li, M.-J. Chen, B.-T. Ko,
App. Organometal. Chem. 2012, 26, 518–527.
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