Ring-opening polymerization of β-butyrolactone catalyzed by efficient magnesium and zinc complexes derived from tridentate anilido-aldimine ligand

Article abstract of DOI:10.1002/pola.24334

Magnesium (Mg) and zinc (Zn) complexes incorporating tridentate anilido-aldimine ligand, (E)-2, 6-diisopropyl-N-(2-((2-(piperidin-1-yl) ethylimino)methyl)phenyl)aniline (AA^Pip-H, 1), were synthesized and structurally characterized. The reaction

Full text of DOI:10.1002/pola.24334

Ring-Opening Polymerization of b-Butyrolactone Catalyzed by Efficient
Magnesium and Zinc Complexes Derived from Tridentate Anilido-
Aldimine Ligand
YI-CHANG LIU,1 CHIA-HER LIN,1 BAO-TSAN KO,1 RONG-MING HO2
1Department of Chemistry, Chung Yuan Christian University, Chung-Li 32023, Taiwan, Republic of China
2Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, Republic of China
Received 27 March 2010; accepted 20 August 2010
DOI: 10.1002/pola.24334
Published online 5 October 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Magnesium (Mg) and zinc (Zn) complexes incorpo-
rating tridentate anilido-aldimine ligand, (E)-2, 6-diisopropyl-N-
(2-((2-(piperidin-1-yl)ethylimino)methyl)phenyl)aniline (AAPip-H,
1), were synthesized and structurally characterized. The reac-
tion of AAPip-H (1) with MgnBu2 or ZnEt2 in equivalent propor-
tions afforded the monomeric complex [(AAPip)MgnBu] (2) or
[(AAPip)ZnEt] (3), respectively. The coordination modes of
these complexes differ in the solid state: Mg complex 2 shows
onstrated in a living fashion with a narrow polydispersity
index, PDI 1.01–1.10. The number-averaged molecular
¼
weight (Mn) of the produced poly(3-hydroxybutyrate) (PHB) is
quite close to the expected Mn over diverse molar ratios of
monomer to 9-AnOH. A greater ratio of monomer to alcohol
catalyzed by Zn complex 3 served to form PHB with a large
molecular weight (Mn > 60000). An effective method to pre-
pare PHB-b-PCL and PEG-b-PHB by the ring-opening copoly-
a
four-coordinated and distorted tetrahedral geometry,
merization of b-BL catalyzed by zinc complex 3 is reported.
C
V
whereas Zn complex 3 adopts a trigonal planar geometry with
a three-coordinated Zn center. Complexes 2 and 3 are efficient
catalysts for the ring-opening polymerization of b-butyrolac-
tone (b-BL) in the presence of 9-anthracenemethanol (9-AnOH).
The polymerization of b-BL with the Zn catalyst system is dem-
2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
48: 5339–5347, 2010
KEYWORDS: anilido-aldimine; b-butyrolactone; catalyst; magne-
sium; ring-opening polymerization; zinc
thanide alkoxide,7(a) and aminoalkoxybis(phenolate) yttrium
alkoxide9(b–d) catalysts, most systems studied for the ROP of
b-BL produce PHB of low-molecular weight and their cata-
lytic activities are poor. Among these three successful initiat-
ing systems, the latter yttrium alkoxide catalyst9(b,c) demon-
strates not only the greatest activity but also the greatest
stereoselectivity (Pr ¼ 0.94) for syndioselective polymeriza-
tion of racemic b-BL. With regard to toxicity, zinc remains an
attractive candidate as a metal center with modified ancillary
ligands for ROP of cyclic esters.11
INTRODUCTION Biodegradable polymers, such as poly(e-cap-
rolactone) (PCL), poly(lactide) (PLA), and poly(hydroxyalka-
noates) (PHA) as well as their copolymers attract attention
as a promising alternative to conventional petrochemical-
based polymers.1 These polymers are also widely used in
the biomedical, pharmaceutical and environmental fields
because of their biodegradable, biocompatible, and permea-
ble properties.2 Among these biodegradable polymers, PHA
combine advantages of the film-barrier properties of polyest-
ers and the thermomechanical properties of oil-based poly-
propylene.3 A common PHA is isotactic poly(3-hydroxybuty-
rate) (PHB), which is a highly crystalline (Tm ꢀ 180 ꢁC)
aliphatic polyester produced by bacteria and other living
organisms.4 Bacteria-mediated polymerizations to synthesize
PHB commonly demand expensive production, thereby
retarding its practicality in many applications. Another con-
venient and promising synthetic route to PHB is the ring-
opening polymerization (ROP) of b-butyrolactone (b-BL)
using metal-alkoxide initiators or catalysts derived from vari-
ous metal complexes,5 such as aluminium,6 lanthanide,7
tin(II),8 and yttrium9 as well as zinc complexes.10 Except dis-
crete b-diketiminate zinc alkoxide,10(c,e) bis(guanidinate) lan-
Recently, we have investigated the synthesis and characteri-
zation of well-defined magnesium and zinc complexes sup-
ported by the NNN-tridentate anilido-aldimine (AA) ligand
(type I, Chart 1).12 These complexes have shown high per-
formance for the ROP of e-caprolactone (CL) and L-lactide
(LA) in the presence of benzyl alcohol, and catalyze the poly-
merization of e-CL and L-LA in a living fashion, giving poly-
mers with a narrow polydispersity index (PDI). On the basis
of these e-CL and L-LA polymerizations, we envisaged the
use of a tridentate anilido-aldimine Mg or Zn system to pre-
pare PHB and to investigate the catalytic behavior of these
complexes bearing such a ligand. Minor changes in the ligand
Additional Supporting Information may be found in the online version of this article. Correspondence to: B.-T. Ko (E-mail: btko@cycu.edu.tw)
C
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 5339–5347 (2010) 2010 Wiley Periodicals, Inc.
LIVING RING-OPENING POLYMERIZATION OF b-BUTYROLACTONE, LIU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
the Scientific Information Service Corporation chromatogra-
phy data solution 3.1 edition.
Synthesis of (E)-2,6-Diisopropyl-N-(2-((2-(Piperidin-1-
yl)Ethylimino)Methyl)Phenyl)Aniline (AAPip-H, 1)
A
mixture of 1-(2-aminoethyl)piperidine (0.39 mL, 2.7
mmol), 2-(2,6-diisopropyl-phenylamino)-benzaldehyde,
A
(0.76 g, 2.7 mmol) and anhydrous MgSO4 (2.0 g) were
stirred in reflux hexane (20 mL) for 12 h. After filtration,
volatile materials were removed under vacuum to give pale
yellow solids. Yield: 0.94 g (89%). Pale yellow crystals were
obtained from the saturated Et2O solution. 1H NMR (C6D6,
ppm): d 10.92 (s, 1H, PhNH), 8.19 (s, 1H, HC¼¼N), 7.10–7.25
(m, 3H, ArH), 6.95 (t, J ¼ 9.0 Hz, 1H, PhH), 6.60 (t, J ¼ 9.0
Hz, 1H, PhH), 6.38 (d, J ¼ 9.0 Hz, 2H, PhH), 3.58 (t, J ¼ 6.0
Hz, 2H, HC¼¼NCH2CH2N), 3.33 (m, 2H, CH(CH3)2), 2.55 (t, J ¼
6.0 Hz, 2H, HC¼¼NCH2CH2N), 2.32 (m, 4H, NCH2), 1.46 (m,
4H, CH2CH2), 1.29 (m, 2H, CH2CH2), 1.17 (d, J ¼ 6.0 Hz, 6H,
CH(CH3)2), 1.11 (d, J ¼ 6.0 Hz, 6H, CH(CH3)2). 13C NMR
(CDCl3, ppm): d 164.9, 149.3, 147.4, 135.1, 133.3, 130.9,
127.1, 123.6, 116.5, 114.8, 111.5, 60.1, 59.1, 54.8, 28.3, 25.9,
24.8, 24.2, 22.8. Anal. calcd for C26H37N3: C, 79.75; H, 9.52;
N, 10.73%. Found: C, 79.51; H, 10.17; N, 10.41%.
CHART 1
architecture might result in dramatic enhancements of poly-
merization activity and selectivity.9(c),10(c) For this reason, we
designed and synthesized a novel tridentate AA ligand with
the pendant arm of the piperidinyl group (type II, Chart 1).
Herein, we describe the synthesis, characterization, and b-BL
polymerization catalytic studies of magnesium and zinc
derivatives supported by a NNN-tridentate AA ligand of this
kind. We also report an effective method to prepare some
block copolymers of the corresponding PHB.
Synthesis of Complex [(AAPip)MgnBu] (2)
nBu2Mg (3.0 mL, 1.0 M in heptane, 3.0 mmol) was added
slowly to an ice cold solution (0 ꢁC) of AAPip-H (1) (0.98 g,
3.0 mmol) in 30.0 mL of hexane under a dry nitrogen atmos-
phere. The mixture was then stirred at ambient temperature
for 2 h, during which a yellow precipitate was formed. The
resulting precipitate was collected by filtration and then
dried in vacuo to give yellow solids. Yield: 1.20 g (85%). 1H
NMR (C6D6, ppm): d 7.66 (s, 1H, HC¼¼N), 7.25–7.32(m, 3H,
NArH), 6.98 (d, J ¼ 6.0 Hz 1H, PhH), 6.82 (t, J ¼ 9.0 Hz, 1H,
PhH), 6.31–6.41 (m, 2H, PhH), 3.45 (m, 2H, CH(CH3)2), 2.87
(t, J ¼ 6.3 Hz, 2H, HC¼¼NCH2CH2N), 2.16 (t, J ¼ 6.3 Hz, 2H,
HC¼¼NCH2CH2N), 1.54 (m, 6H, N(CH2CH2)2, CH2CH2CH3),
1.48 (d, J ¼ 7.2 Hz, 6H, CH(CH3)2), 1.36 (m, 6H, CH2CH2CH2,
MgCH2(CH2)2CH3), 1.22 (d, J ¼ 7.2 Hz, 6H, CH(CH3)2), 1.06
(t, J ¼ 13.8 Hz, 3H, CH2CH2CH3), ꢂ0.23 (t, J ¼ 8.7 Hz, 2H,
MgCH2(CH2)2CH3). 13C NMR (C6D6, ppm): d 170.6, 160.8,
147.8, 144.8, 137.7, 133.7, 125.3, 125.0, 119.8, 117.0, 112.0,
55.8, 54.3, 52.0, 33.7, 33.1, 29.4, 26.3, 26.1, 25.1, 24.4, 15.3,
8.2. Anal. calcd for C30H45N3Mg: C, 76.34; H, 9.61; N, 8.90%.
Found: C, 75.19; H, 8.50; N, 8.81%.
EXPERIMENTAL
General
All manipulations were carried out under a dry nitrogen
atmosphere. Solvents and reagents were dried by refluxing
for at least 24 h over sodium/benzophenone (hexane, tolu-
ene, tetrahydrofuran [THF]), or over calcium hydride
(CH2Cl2). b-BL was mixed with CaH2 and filtered, followed
by the distillation under reduced pressure. Deuterated sol-
vents and ethylene glycol were dried over 4 Å molecular
sieves. 2-bromobenzaldehyde (acros), ethylene glycol (Acros),
p-toluenesulfonic acid (Acros), Pd(OAc)2 (Lacaster), sodium
tert-butoxide (Acros), 2, 6-diisopropylaniline (Acros), tri-
fluoroacetic acid (Acros), 9-anthracenemethanol (9-AnOH)
(Acros), and 1-(2-Aminoethyl)piperidine (Acros) were pur-
chased and used without further purification. MgnBu2 (1.0 M
in heptane) and diethylzinc (1.0 M in hexane) were pur-
chased from Aldrich (http://www.sigmaaldrich.com) and
used as received. 2-(2, 6-Diisopropyl-phenylamino)benzalde-
hyde (A) was prepared by according to our previous
report.13 1H NMR and 13C NMR spectra were recorded on a
Bruker Avance (300 MHz for 1H and 75 MHz for 13C) spec-
trometer with chemical shifts given in parts per million
(ppm) from the peak of internal tetramethylsilane. Micro-
analyses were performed using a Heraeus CHN-O-RAPID
instrument. GPC measurements were performed on a JASCO
PU-2080 plus system equipped with a RI-2031 detector
using THF (HPLC grade) as an eluent. The chromatographic
column was Phenomenex Phenogel 5 l 103 Å, and the cali-
bration curve used to calculate Mn(GPC) was produced from
polystyrene standards. The GPC results were calculated using
Synthesis of Complex [(AAPip)ZnEt] (3)
To an ice cold solution (0 ꢁC) of AAPip-H (1) (2.16 g, 6.6
mmol) in 30.0 mL of hexane was slowly added an Et2Zn (6.6
mL, 1.0 M in hexane, 6.6 mmol) under a dry nitrogen atmos-
phere. The mixture was then stirred for 2 h while the tem-
perature was slowly increased to ambient temperature. Vola-
tile materials were removed under vacuum, and the residue
was washed twice by the hexane solvent to give yellow sol-
ids. Yield: 2.81 g (88%).1H NMR (C6D6, ppm): d 7.78 (s, 1H,
HC¼¼N), 7.26 (m, 3H, ArH), 6.99 (d, J ¼ 7.8 Hz, 1H, PhH),
6.88 (t, J ¼ 7.2 Hz, 1H, PhH), 6.46 (d, J ¼ 7.8 Hz, 1H, PhH),
6.40 (t, J ¼ 7.5 Hz, 1H, PhH), 3.29 (m, 2H, CH(CH3)2), 3.24
(t, J ¼ 6.6 Hz, 2H, HC¼¼NCH2CH2N), 2.33 (t, J ¼ 6.6 Hz, 2H,
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ARTICLE
SCHEME 1 Synthetic Routes of
(a) ligand AAPip-H (1) and (b)
complexes (2)–(3).
HC¼¼NCH2CH2N), 2.16 (m, 4 H, NCH2), 1.37 (m, 4H, CH2CH2),
1.25–1.28 (m, 11H, CH(CH3)2, ZnCH2CH3, ACH2CH2CH2),
1.12 (d, J ¼ 6.9 Hz, CH(CH3)2), 0.55 (q, J ¼ 8.7 Hz, 2H,
ZnCH2CH3). 13C NMR (CDCl3, ppm): d 169.4, 157.1, 145.0,
143.6, 136.9, 133.3, 124.6, 123.6, 116.0, 114.0, 112.1, 59.7,
58.3, 54.6, 27.8, 25.9, 24.2, 24.1, 11.9, ꢂ1.88. Anal. calcd for
a similar approach as described above, but b-BL (0.82 mL,
10.0 mmol) in toluene (10 mL) was used. After prepolymeri-
zation for 6 h, e-CL (2.20 mL, 20.0 mmol) was then added.
The reaction mixture was stirred for another 4 h and was
quenched using the procedures described previously. Yield:
2.63 g (84%).
C
28H41N3Zn: C, 69.34; H, 8.52; N, 8.66%. Found: C, 68.79; H,
Synthesis of PEG-b-PHB Diblock Copolymer Catalyzed
by the Tridentate Anilido-Aldimine Zn Complex
8.42; N, 8.55%.
Polymerization of b-BL Catalyzed by Tridentate
Anilido-Aldimine Mg or Zn Complex
A typical polymerization procedure was exemplified by the
synthesis of PEG45-b-PHB49 (the number 45 and 49 indicate
the degree of polymerization of PEG and PHB). The living
A typical polymerization procedure was exemplified by the
synthesis of PHB-100 (the number 100 indicates the
designed [b-BL]0/[9-AnOH]0). To a rapidly stirring solution
of [(AAPip)ZnEt] (3) (0.097 g, 0.2 mmol) and 9-anthracene-
methanol (9-AnOH, 0.042 g, 0.2 mmol) in toluene (5 mL)
was added b-butyrolactone (1.63 mL, 20.0 mmol). The reac-
tion mixture was then stirred at 55 ꢁC for 4 h. The conver-
polymerization of b-BL was synthesized by
a similar
approach as described above, but poly(ethylene glycol)
methyl ether (Mw ¼ 2000, PDI ¼ 1.05) (0.4 g, 0.2 mmol) in
toluene (10.0 mL) was used as the initiator. The reaction
mixture was stirred at 55 ꢁC for 12 h and was then
quenched using the procedures described previously. Yield:
1.03 g (86%).
1
sion yield (99%) of PHB-100 was analyzed by H NMR spec-
troscopic studies. After the reaction was quenched by the
addition of an excess water (1.0 mL), the polymer was pre-
cipitated into hexane (100 mL). The final polymer was then
dissolved in CH2Cl2 (5.0 mL) and purified on precipitation
again in hexane (60 mL), collected, and dried under vacuum.
Yield: 1.51 g (88%).
X-Ray Crystallographic Studies
Suitable crystals of complex 2 and 3 were sealed in thin-
walled glass capillaries under nitrogen atmosphere and were
mounted on Brucker SMART APEX2 diffractometer. Intensity
data were collected in 1350 frames with increasing w (width
of 0.5 per frame). The absorption correction was based on
the symmetry-equivalent reflections using SADABS program.
The space group determination was based on a check of the
Laue symmetry and systematic absence, and was confirmed
using the structure solution. The structures were solved with
direct methods using a SHELXTL package. All non-H atoms
were located from successive Fourier maps, and hydrogen
Synthesis of PHB-b-PCL Diblock Copolymer Catalyzed by
the Tridentate Anilido-Aldimine Zn Complex
A typical polymerization procedure was exemplified by the
synthesis of PHB(50)-b-PCL(100) (the number 50 or 100
indicates the designed [b-BL]0/[9-AnOH]0 or [CL]0/[9-
AnOH]0). The living prepolymer PHB-50 was synthesized by
LIVING RING-OPENING POLYMERIZATION OF b-BUTYROLACTONE, LIU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Scheme 1. To develop a useful approach for the synthesis of
bi-, tri- or multi-dentate AA, we prepared the ligand precur-
sor, 2-(2, 6-diisopropyl-phenylamino)benzaldehyde (A), in
three steps as shown in Scheme 1(a).13 The novel AA ligand,
(E)-2, 6-diisopropyl-N-(2-((2-(piperidin-1-yl)ethylimino)me-
thyl)phenyl)aniline (AAPip-H, 1) was prepared in 89% yield
on condensation of 1-(2-aminoethyl)piperidine with com-
pound A in one molar equivalent under refluxing hexane so-
lution in the presence of anhydrous MgSO4. Although this
strategy required four steps to achieve the final AA ligand,
the method is simple in that products are readily purified
with only recrystallization from a hexane or Et2O solution
without chromatographic technique. Compound 1 was iso-
1
lated as a pale yellow solid and characterized by H and 13C
FIGURE 1 Molecular structure of compound 1 with probability
1
˚
NMR spectra as well as microanalysis. The H NMR spectrum
ellipsoids drawn at 20% level. Selected bond lengths/A and
of the ligand exhibited signals at 8.19 ppm for the aldimine
proton of HC¼¼N, and the corresponding 13C NMR signal was
around 164.9 ppm, indicating the formation of the desired
tridentate AA ligand. The N-H signal at the down field of
10.92 ppm indicates a hydrogen bond between N-H and
N(imine), which was further verified by X-ray structural
determination. Single crystals suitable for X-ray determina-
tion of compound 1 were obtained from a saturated Et2O so-
lution. The molecular structure of 1 (Fig. 1) exhibits an
intramolecular hydrogen bond of NAH…N between the amine
and imine groups. The distance of N…H (2.082 Å) is substan-
tially shorter than the Van der Waals distance of 2.75 Å for
the N and H atom. The six-membered ring (N(1), C(1), C(2),
C(19), N(2), H(1a)) formed from the NAH…N hydrogen-bond
is almost coplanar with mean deviation of 0.082 Å.
angles/deg: N(1)-C(1) 1.374(3), N(1)-C(7) 1.423(3), N(2)-C(19)
1.271(3), N(2)-C(20) 1.463(3), N(2)….H(1a) 2.082, N(1)-H(1a)….N(2)
130.0.
atoms were refined using a riding model. Anisotropic ther-
mal parameters were used for all non-H atoms, and fixed
isotropic parameters were used for H-atoms. Crystallographic
data of complexes 1–3 are summarized in Supporting Infor-
mation Table S1.
RESULTS AND DISCUSSION
Synthesis and Solid-State Structural Determination
The synthetic routes to new tridentate ligand (AAPip-H, 1)
and its Mg and Zn complexes (2 and 3) are shown in
FIGURE 2 ORTEP drawing of complex 2 with probability ellip-
soids drawn at 30% level; hydrogen atoms are omitted for
FIGURE 3 ORTEP drawing of complex 3 with probability ellip-
˚
clarity. Selected bond lengths/A and angles/deg: Mg-N(1)
soids drawn at 30% level; hydrogen atoms are omitted for
˚
2.0610(19), Mg-N(2) 2.060(2), Mg-N(3) 2.349(2), Mg-C(27)
2.134(3), N(2)-Mg-N(1) 89.21(8), N(2)-Mg-N(3) 75.23(8), N(1)-Mg-
N(3) 133.53(8), N(2)-Mg-C(27) 127.70(10), N(1)-Mg-C(27)
120.09(9), C(27)-Mg-N(3) 103.44(9).
clarity. Selected bond lengths/A and angles/deg: Zn-N(1)
1.9710(15), Zn-N(2) 1.9966(16), Zn-C(27) 1.969(2), N(1)-Zn-C(27)
125.80(8), N(1)-Zn-N(2) 93.87(7), C(27)-Zn-N(2) 136.71(9),
Zn….N(3) 2.833(2).
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ARTICLE
TABLE 1 Ring-Opening Polymerization of b-Butyrolactone (b-BL) Catalyzed by Complexes 2 and 3 in the Presence of 9-AnOH
[b-BL]0/[Cat.]0
/
Entry Catalyst
[9-AnOH]0
Conv. (%)a
Temp. (ꢁC)
Time (h)
Mn (calcd.)b
Mn (obsd.)c
Mn (NMR)a
PDIc
Prd
1e
2e
3f
4g
5h
6h
7h
8h
9h
10h
11j
12h
13k
14k
a
2
2
2
2
2
3
3
3
3
3
3
3
3
3
100/1/1
100/2/1
100/1/1
100/1/1
100/1/1
100/1/1
100/1/1
25/1/1
44
30
30
30
30
30
30
55
55
55
55
55
55
55
55
6
6
4,000
3,800
2,900
6,500
6,800
8,100
8,700
2,300
4,500
6,600
46,700
2,300
10,100l
15,800l
6,500
5,600
5,000
7,900
8,600
10,600
10,200
2,700
5,600
7,900
64,300
2,600
16,800
27,500
6,900
8,200
3,700
7,700
8,800
9,000
9,300
2,300
4,700
6,700
58,600
2,300
12,800
19,000
1.28
1.76
1.37
1.30
1.28
1.01
1.01
1.10
1.03
1.02
1.06
1.10
1.08
1.12
0.68
0.69
0.60
0.67
0.64
0.47
0.45
42
31
6
73
6
76
16
16
4
92
99
i
97
4
–
i
50/1/1
99
4
–
i
75/1/1
99
4
–
i
600/5/1
100/1/4
50/1/1;50/1/1
50/1/1;100/1/1
90
8
–
i
96
4
–
i
>99; 99
>99; 99
6; 4
6; 4
g
–
i
–
Obtained from 1H NMR determination.
[Cat.]0 ¼ 0.02 M, 5 mL of toluene was used as the solvent.
[Cat.]0 ¼ 0.04 M, 5 mL of toluene.
Not available.
b
h
i
Calculated from the molecular weight of b-butyrolactone times [b-
BL]0/[9-AnOH]0 times conversion yield plus the molecular weight of 9-
AnOH.
j
[Cat.]0 ¼ 0.05 M, 10 mL of toluene.
c
k
Obtained from GPC analysis and calibrated by polystyrene standard.
[3]0 ¼ 0.02 M, prepolymerization of b-BL in toluene (10ml) for 6h fol-
d
Pr is the probability of racemic linkages between b-BL units and is
lowed by the addition of CL for another 4h.
determined from the methylene region by 13C{1H} NMR spectroscopy.
Calculated from the molecular weight of b-BL times [b-BL]0/[9-AnOH]0
plus the molecular weight of CL times [CL]0/[9-AnOH]0 times conversion
yield plus the molecular weight of 9-AnOH.
l
e
[Cat.]0 ¼ 0.01 M, 10 mL of toluene, 9-AnOH was used as the alcohol.
f
10 mL of THF was used as the solvent.
Compounds 2 and 3 were synthesized via an alkane elimina-
tion reaction in hexane. The tetra-coordinated monomeric
complex [(AAPip)MgnBu] (2) was synthesized in 85% yield
from the reaction of 1 with MgnBu2 (one equiv.), followed by
recrystallization from toluene solution. However, treatment
of ligand 1 with ZnEt2 in stoichiometric proportion gave the
three-coordinated monomeric complex [(AAPip)ZnEt] (3) in
88% yield as shown in Scheme 1(b). Attempts to obtain the
magnesium or zinc alkoxide complexes modified by the tri-
dentate AA ligand were unsuccessful. Complexes 2 and 3
were obtained as yellow crystalline solids and were fully
characterized by spectroscopic studies as well as X-ray struc-
tural characterizations. The formation of the expected com-
plexes was demonstrated by the disappearance of the N–H
NMR signal of AAPip-H and the appearance of the resonance
for alkyl protons of MgnBu or ZnEt in the high-field region
(ꢂ0.23 or 0.55 ppm).
3 appear in Figures 2 and 3, respectively. Complex 2 exhibits
a monomeric structure with a four-coordinated Mg center,
containing one six- and one five-membered chelating ring.
Single crystals of complexes 2 and 3 suitable for X-ray struc-
tural determination were obtained on slowly cooling a satu-
rated toluene (2) or hexane (3) solution at 0 ꢁC. Oak Ridge
Thermal Ellipsoid Plot (ORTEP) drawings displaying selected
bond lengths and angles of the molecular structure of 2 and
FIGURE 4 Polymerization of b-BL catalyzed by 3 in toluene at
55 ꢁC. The relationship between Mn(n)/(PDI(h) of polymer and
the initial molar ratio [b-BL]0/[9-AnOH]0 is shown.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 1H NMR spectra of PHB-25 (Table 1, entry 8) in CDCl3.
The bond angles around Mg in 2, from 75.23(8)ꢁ to
133.53(8)ꢁ, exhibit a distorted tetrahedral geometry. The
Mg–N(1) (amido) distance (2.0610(19) Å) almost equals the
Mg–N(2) (imine) distance (2.060(2) Å) and is significantly
shorter than the pendant arm bond length of Mg–N(3)
(2.349(2) Å). These bond distances of Mg complex 2 in this
work resemble those found in a Mg complex with tridentate
AA ligands.12 The six-membered ring consisting of a Mg cen-
ter chelated with amido and imine nitrogen atoms of the
ligand is almost planar with the metal atom lying out of the
plane by 0.0172 Å for 2. For comparison with Mg complex 2,
the zinc center of complex 3 adopts a trigonal planar geome-
try with the metal center coordinated by amido and imine
nitrogen atoms of the tridentate AAPip ligand, forming a six-
membered chelating ring. This ring is also almost coplanar
with mean deviation of 0.0397 Å. The pendant nitrogen
atom does not coordinate to the zinc center and interacts
only weakly; the distance between Zn and N(3) is 2.833(2)
Å, somewhat shorter than the sum of Van der Waals radii of
2.94 Å for the Zn and N atoms. The Zn–N (amido) distance
(1.9710(15) Å for 3) is shorter than the Zn–N (imine) dis-
tance (1.9966(16) Å for 3), which are similar to those found
in Zn complexes with bidentate AA ligands.14 The torsion
angles between the six-membered chelating ring and the aro-
matic ring attached to the amido nitrogen atom are 78.8ꢁ for
2 and 98.2ꢁ for 3.
Ring-opening Polymerization of b-Butyrolactone
Based on lactone polymerizations catalyzed with tridentate
AA magnesium and zinc derivatives,12 alkyl complexes 2 and
3 were expected to behave as catalysts toward the ROP of b-
butyrolactone (b-BL) in the presence of alcohol. In this
FIGURE 6 1H NMR spectra of PHB(50)-b-PCL(50) (Table 1, entry 13) in CDCl3.
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ARTICLE
eral trials on running polymerization with THF as solvent
and increasing concentration with 2 as the catalyst (Table 1,
entries 1–5). On this basis of optimal polymerization condi-
tions (Table 1, entry 5), the conversion yield attained ꢀ76%
in 16 h; the PDI of the produced poly(b-BL) is narrow,
<1.30, but the number-average molecular weight (Mn deter-
mined from 1H NMR) of the obtained polymer is slightly
higher than the molecular weight calculated from the molar
ratio of monomer to 9-AnOH. In comparison with complex 2,
b-BL polymerization catalyzed by complex 3 shows good ac-
tivity and excellent ‘‘controlled’’ fashion (Table 1, entry 6 vs
5) under the optimal conditions of complex 2. The polymer-
ization is completed within several hours at 55 ꢁC (Table 1,
entry 7). Consequently, polymerizations of b-BL employing
catalyst 3 under the best catalytic conditions were investi-
gated systematically (Table 1, entries 7–10). The conversion
yields achieved >97% in toluene within 4 h at 55 ꢁC with
the ratio of [b-BL]0/[9-AnOH]0 from 25 to 100. The living
character of the polymerization was demonstrated by a lin-
ear relationship between the number-average molecular
weight (Mn) and the ratio of monomer to initiator ([b-BL]0/
[9-AnOH]0) as shown in Figure 4; the obtained PHB had a
narrow PDI, ranging from 1.01 to 1.10. The 1H NMR spec-
trum of PHB-25 (number 25 indicates the designed [b-BL]0/
[9-AnOH]0 ratio) (Fig. 5) indicates that the polymer chain is
capped with one 9-anthracenemethyl ester and one hydroxyl
chain end. This result suggests that the (AAPip)ZnEt/9-AnOH
system might involve in situ formation of metal 9-anthrace-
nemethoxide active species, and that polymerization occurs
through an insertion of the 9-anthracenemethyl alkoxy group
into the b-BL.15 To realize the capability of producing PHB
with the higher molecular weight in a controlled manner, a
greater ratio of monomer to 9-AnOH catalyzed by Zn com-
FIGURE 7 GPC profiles of copolymerization of PHB-b-PCL cata-
lyzed by complex 3: (peak A) after prepolymerization of b-BL
(50 equiv to 9-AnOH, Mn ¼ 5600, PDI ¼ 1.03); (peak B) after
block copolymerization of PHB-b-PCL ([b-BL]0/[9-AnOH]0/[e-CL]0
¼ 50/1/50, Mn ¼ 16800, PDI ¼ 1.08).
context, ROP of b-BL employing 2 or 3 as a catalyst in the
presence of 9-anthracenemethanol (9-AnOH) under dry N2
was systematically studied as shown in Table 1. Polymeriza-
tions were generally performed in toluene or THF with a
prescribed equivalent ratio of the catalyst, monomers and 9-
AnOH and a prescribed duration. After the reaction was
quenched by addition of water, the polymer was precipitated
into hexane. Mg complex 2 was first used to investigate the
optimal catalytic conditions; we examined the effects of the
ratio of the catalyst to 9-AnOH, the solvent and the concen-
tration for b-BL polymerization. Optimized conditions
included a catalyst concentration of 0.04 M with the same
molar equiv. of 9-AnOH in toluene (5 mL, 30 ꢁC) after sev-
plex 3 was examined. High molecular weight PHB (Mn
¼
64300) was achieved ꢁon polymerizing b-BL with [b-BL]0/[9-
AnOH]0 ¼ 600 at 55 C for 8 h (Table 1, entry 11). The PDI
of PHB with a large Mn remains narrow (ꢀ1.06), and the
molecular weight is linearly proportional to the [b-BL]/[9-
AnOH] ratio. Complex 3 catalyzes the ROP of b-BL in not
TABLE 2 Ring-Opening Polymerization of b-BL Catalyzed by Complex 3 in the Presence of PEG-OHa
[b-BL]0/[3]0/
Entry PEG-OH Mn(NMR)b
Mn (obsd.)c
PDIc
[PEG-OH]0
Mn (calcd.)d
Mn (obsd.)c
Mn (NMR)b
PDIc
PEGnPHBm
e
1
2
3
4
a
PEG17-OH (750)
PEG17-OH (750)
PEG45-OH (2000)
PEG45-OH (2000)
500
500
1.34
1.34
1.05
1.05
50:1:1
100:1:1
50:1:1
5,000
9,300
6,300
10,500
d
7,100
10,800
7,700
5,200
9,500
1.03
1.02
1.06
1.03
PEG17PHB52
PEG17PHB102
PEG45PHB49
PEG45PHB97
3,200
3,200
6,200
100:1:1
12,700
10,300
[3]0 ¼ 0.02 M, 10 mL of toluene under 55 ꢁC, 12 h, conversion yield
Calculated from [M(b-BL)x([b-BL]0/[PEG-OH]0)x(conversion yield)]þ
ꢀ99%.
M(PEG).
b
e
Obtained from 1H NMR analysis.
n or m is degree of polymerization for PEG or PHB and calculated
c
Obtained from GPC analysis and calibrated by polystyrene standard.
from 1H NMR determination.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 8 1H NMR spectra of PEG17-b-PHB52 (Table 2, entry 1) in CDCl3.
only a living fashion but also an immortal manner. Experi-
mental results show that 9-AnOH can be added four fold,
resulting in a narrow PDI polymer with Mn only 1/4 that
with the addition of [b-BL]0/[3]0/[9-AnOH]0 ¼ 100/1/1
(Table 1, entry 12 vs 7). The immortal character of complex
3 paves a way to synthesize PHB with a narrow PDI in the
presence of catalyst in a small proportion and eventually to
decrease the metal residues in the final polymer. Despite
zinc complex 3 exhibiting good catalytic activity and a well
controlled character, poor stereocontrol was observed by
using 3/9-AnOH as the initiating system in ROP of rac-b-BL
([b-BL]0/[9-AnOH]0 100) with Pr value equal to
¼
0.45.8(a),9(b),16 In comparison, initiating system [(AAPip
)
MgnBu]/9-AnOH showed somewhat improved stereocontrol,
yielding a predominantly syndiotactic PHB with Pr ¼ 0.67
(Supporting Information Fig. S1).
To further demonstrate the versatility of complex 3 for the
ROP of b-BL and e-caprolactone (e-CL), we prepared their co-
polymer, PHB-b-PCL, using polymerization with sequential
addition (Table 1, entries 13–14).17 For instance, PHB-b-PCL
block polymer (Mn ¼ 16800, Mw/Mn ¼ 1.08, entry 13) was
synthesized with the sequential ROP of b-BL ([b-BL]0/[9-
AnOH]0 ¼ 50) and e-CL monomer ([e-CL]0/[9-AnOH]0 ¼ 50)
in the presence of 3, as verified by the 1H NMR spectrum
and GPC profile as shown in Figures 6 and 7, respectively.
Synthesis of Diblock Copolymer PEG-b-PHB
Copolymers containing aliphatic polyesters and poly(ethylene
glycol) methyl ether (PEG) are of great interest for use as
biodegradable scaffolds because of their ultimate degrada-
tion to natural metabolites.18 Block copolymers of PEG-b-
PHB were accordingly prepared with the ROP of b-BL in the
presence of PEG-OH catalyzed by Zn complex 3; the results,
summarized in Table 2, indicate that catalytic activities and
the controlled character are not drastically affected when
PEG-OH was used as the initiator. The molecular weights of
these block copolymers all increase nearly linearly with the
ratio of monomer to macroinitiator and have very narrow
PDIs (Table 2, entry 1–4). In the 1H NMR spectrum (Fig. 8)
of PEG17-b-PHB52 (Table 2, entry 1), a sharp singlet of meth-
ylene protons (3.72 ppm) for the PEG polymer, and a broad
signal of methine protons (5.22 ppm) for the atactic PHB
polymer show the block nature of the PEG-b-PHB copolymer.
In the GPC profile (Fig. 9), peak A corresponds to PEG-OH
(Mn ¼ 500, PDI ¼ 1.34) and peak B to PEG17-b-PHB52 after
FIGURE 9 GPC profiles of copolymerization of PEG-b-PHB:
(peak A) the prepolymer (Mn ¼ 500, PDI ¼ 1.34); (peak B) after
block copolymerizationof of PEG-b-PHB (12 h, Mn ¼ 7100, PDI
¼ 1.03).
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7 (a) Ajellal, N.; Lyubov, D. M.; Sinenkov, M. A.; Fukin, G. K.;
Cherkasov, A. V.; Thomas, C. M.; Carpentier, J.-F.; Trifonov, A.
A. Chem Eur J 2008, 14, 5440–5448; (b) Grunova, E.; Kirillov, E.;
Roisnel, T.; Carpentier, J.-F. Organometallics 2008, 27,
5691–5698.
polymerization with b-BL at 50 equiv. A unimodal peak with
increased molecular weight (Mn ¼ 7100, PDI ¼ 1.03) was
observed (peak B), confirming the formation of the block
copolymer.
CONCLUSION
8 (a) Kemnitzer, J. E.; McCarthy, S. P.; Gross, R. A. Macromole-
cules 1993, 26, 1221–1229; (b) Hori, Y.; Suzuki, M.; Yamaguchi,
A.; Nishishita, T. Macromolecules 1993, 26, 5533–5534; (c)
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A novel tridentate AA ligand and two Mg and Zn complexes
containing the tridentate AA ligand were synthesized and
fully characterized. Both complexes 2 and 3 are efficient cat-
alysts for the ROP of b-BL in the presence of 9-anthracene-
methanol. Zn complex 3 was demonstrated to catalyze effi-
ciently the ROP of b-BL in not only a living fashion but also
an immortal manner, yielding polymers with the expected
molecular weights and very narrow PDIs. The catalytic activ-
ity of Zn complex 3 is greater than that of Mg complex 2 in
polymerization. On the basis of catalytic and structural stud-
ies of complexes 2 and 3, the three-coordinated geometry of
Zn complex 3 causes a more greatly electron-deficient nature
and the higher Lewis acidity at the Zn2þ center. The harder
Zn2þ metal center of complex 3 hence provides a superior
combination of monomer activation and alkoxide nucleophi-
licity to result the improved performance for b-BL polymer-
ization. The living character of Zn complex 3 also enabled us
to prepare PHB-b-PCL and PEG-b-PHB copolymers.
9 (a) Leborgne, A.; Pluta, C.; Spassky, N. Macromol Rapid
Commun 1994, 15, 955–960; (b) Amgoune, A.; Thomas, C. M.;
Ilinca, S.; Roisnel, T.; Carpentier, J.-F. Angew Chem Int Ed
2006, 45, 2782–2784; (c) Ajellal, N.; Bouyahyi, M.; Amgoune,
A.; Thomas, C. M.; Bondon, A.; Pillin, I.; Grohens, Y.; Carpent-
ier, J.-F. Macromolecules 2009, 42, 987–993; (d) Kramer, J.
W.; Treitler, D. S.; Dunn, E. W.; Castro, P. M.; Roisnel, T.;
Thomas, C. M.; Coates, G. W. J Am Chem Soc 2009, 131,
16042–16044.
Financial supports from the National Science Council, Taiwan
(NSC97-2113-M-033-005-MY2) and from the CYCU Distinctive
Research Area project (CYCU-98-CR-CH) are greatly appreci-
ated. The authors also thank Chu-Chieh Lin and Ya-Sen Sun for
their valuable discussions.
10 (a) Leborgne, A.; Spassky, N. Polymer 1989, 30, 2312–2319;
(b) Tanahashi, N.; Doi, Y. Macromolecules 1991, 24,
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2009, 9010–9019; (e) Guillaume, C.; Carpentier, J.-F.; Guillaume,
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