Ring-opening polymerization of lactides catalyzed by magnesium complexes coordinated with NNO-tridentate pyrazolonate ligands

Article abstract of DOI:10.1002/pola.26426

A series of magnesium benzylalkoxide complexes, [LⁿMg(μ-OBn)] ₂ (1-14) supported by NNO-tridentate pyrazolonate ligands with various electron withdrawing-donating subsituents have been synthesized and characterized. X-ray crystal structural studies revealed that Complexes 1-3, 5, 7, 9, and 10 are dinuclear bridging through benzylalkoxy oxygen atoms with penta-coordinated metal centers. All of these complexes acted as efficient initiators for the ring-opening polymerization of L-lactide and rac-lactide. Based on kinetic studies, the activity of these metal complexes is significantly influenced by the electronic effect of the ancillary ligands with the electron-donating substituents at the phenyl rings enhancing the polymerization rate. In addition, the "living" and "immortal" character of 6 has paved a way to synthesize as much as 40-fold polymer chains of polylactides with a very narrow polydispersity index in the presence of a small amount of initiator. Among all of magnesium complexes, Complex 6 exhibits the highest stereoselectivity toward ring-opening polymerization of rac-lactide with Pr up to 88% in THF at 0 °C.

Full text of DOI:10.1002/pola.26426

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Ring-Opening Polymerization of Lactides Catalyzed by Magnesium
Complexes Coordinated with NNO-Tridentate Pyrazolonate Ligands
Hui-Ju Chuang, Hsiao-Li Chen, Jian-Li Ye, Zn-Yun Chen, Pei-Ling Huang, Tzu-Ting Liao,
Tsung-En Tsai, Chu-Chieh Lin
Department of Chemistry and Center of Nanoscience and Nanotechnology, National Chung Hsing University,
Taichung 402, Taiwan, Republic of China
Correspondence to: C.-C. Lin (E-mail: cchlin@mail.nchu.edu.tw)
Received 7 August 2012; accepted 13 October 2012; published online 8 November 2012
DOI: 10.1002/pola.26426
ABSTRACT: A series of magnesium benzylalkoxide complexes,
[LnMg(l-OBn)]2 (1–14) supported by NNO-tridentate pyrazolo-
nate ligands with various electron withdrawing-donating sub-
situents have been synthesized and characterized. X-ray crystal
structural studies revealed that Complexes 1–3, 5, 7, 9, and 10
are dinuclear bridging through benzylalkoxy oxygen atoms
with penta-coordinated metal centers. All of these complexes
acted as efficient initiators for the ring-opening polymerization
of L-lactide and rac-lactide. Based on kinetic studies, the activ-
ity of these metal complexes is significantly influenced by the
electronic effect of the ancillary ligands with the electron-
donating substituents at the phenyl rings enhancing the poly-
merization rate. In addition, the ‘‘living’’ and ‘‘immortal’’ char-
acter of 6 has paved a way to synthesize as much as 40-fold
polymer chains of polylactides with a very narrow polydisper-
sity index in the presence of a small amount of initiator.
Among all of magnesium complexes, Complex 6 exhibits the
highest stereoselectivity toward ring-opening polymerization of
rac-lactide with Pr up to 88% in THF at 0 ꢀC.
2012 Wiley
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696–707
KEYWORDS: catalysts; kinetics (polym.); polyesters; ring-open-
ing polymerization
INTRODUCTION Because of their magnificent properties, poly-
lactide (PLA) has attracted intensive attention over the past two
decades.1 Ring-opening polymerization (ROP) of lactides cata-
lyzed by metal complexes is the most promising method to pre-
pare PLA owing to the advantages of good molecular-weight
control and low polydispersity (PDI).2 For biomedical purposes,
the most desirable features for lactide polymerization catalysts
are compounds with high activity, low toxicity and the ability to
produce high molecular weight with low polydispersity poly-
mers. Therefore, Groups 1 and 2 metals as well as zinc are espe-
cially interested because of their low cost, remarkably high activ-
ity and minimal toxicity.3 Recently, a number of homogeneous
catalysts with a single active site incorporating with a variety of
ligands have been reported to be effective catalysts/initiators for
ROP of lactides. Among them, ligands such as Schiff-base and b-
diketiminate ligands, have been intensively investigated because
they are readily used for fine-tuning of their steric and electronic
properties.4,5 In addition, pyrazolone frameworks are also easily
modified and have demonstrated a variety of coordination
modes.6 Metal pyrazolonates have been introduced in the ring
opening polymerization.7
chain, it is become more and more in demand for the well con-
trolled the microstructure of PLA.8 By far, aluminum complexes
constitute the most effective and versatile synthetic strategy
for preparing amorphous to semicrystalline PLAs. For instance,
isotactic PLA was facile produced by aluminum complexes
bearing with various ligands, such as chiral binaphthyl Schiff
base,9 homosalen ligands10 and aminophenoxide ligands.11
Metal complexes other than aluminum such as b-diiminate
(BDI)-stabilized zinc and magnesium alkoxides,12 amino-
amino-bis(phenols) supported lanthanide and yttrium com-
plexes,13 rare-earth (OSSO)-bisphenolate complexes,14 trispyra-
zolylborate calcium complexes,15 and indium trichloride16 have
also shown efficient heteroselectivities toward polymerization
of rac-lactides. In addition, pyrazolone ketiminate ligands coor-
dinated magnesium, zinc and aluminum complexes have
revealed excellent catalytic activity in ROP of lactides.17 We
report herein the effect of the substituents on the NNO-ligand
affecting the activity of magnesium complexes.
EXPERIMENTAL
General
Because of the mechanical properties of PLA rely on the ster-
eochemistry of insertion of the lactide monomer into the PLA
All manipulations were carried out under a dry nitrogen
atmosphere. Solvents were dried by refluxing at least 24 h
Additional Supporting Information may be found in the online version of this article.
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over sodium/benzophenone (hexane and toluene), phospho-
rus pentaoxide (CH2Cl2), or CaH2 (benzyl alcohol). L-lactide
(LA) was purchased from Bio Invigor Corporation and
recrystallized from a toluene solution prior to use. Deuter-
ated solvents (Aldrich) were dried over molecular sieves. 1-
phenyl-3-methyl-pyrazol-5-one, 1-p-chlorophenyl-3-methyl-
4.92 (4H, br, CH2Ph), 2.87 (4H, t, J ¼ 6.0 Hz, NCH2CH2), 2.16
(4H, t, J ¼ 6.0 Hz, NCH2CH2), 1.94 (12H, s, N(CH3)2), 1.42
(6H, s, CH3C¼¼N). 13C NMR (CDCl3, ppm): d 171.6 (CO),
162.9, 151.7, 148.7, 146.6, 140.0, 134.8, 129.0, 128.2, 128.1,
126.7, 126.5, 126.0, 125.4, 123.8, 120.3 (Ar and Py), 101.5
(C¼¼CAN), 66.4 (CH2Ph), 59.1 (NCH2CH2), 46.3 (NCH2CH2),
44.8 (N(CH3)2), 34.7 (C(CH3)3), 31.3 (C(CH3)3), 15.5 (CCH3).
Anal. Calcd. (found) for C64H76Mg2N8O4: N, 10.47 (10.35); C,
pyrazol-5-one
1-p-methoxyphenyl-3-methyl-pyrazol-5-one
and 1,3-diphenyl-pyrazol-5-one were synthesized according
to the literature method.18 All the reagents were purchased
from Acros or Aldrich and used without further purification.
1H and 13C NMR spectra were recorded on a Varian Mer-
ꢀ
71.48 (70.97); H, 7.16 (7.36)%. Mp: 280–282 C.
Synthesis of [(L3)Mg(l-OBn)]2 (3)
1
cury-400 (400 MHz for H and 100 MHz for 13C) spectrome-
Following the procedures for 1 where L3H (1.51 g, 4 mmol)
was used as a reagent. Yield: 3.46 g (85%). 1H NMR (CDCl3,
ppm): d 8.22 (4H, d, J ¼ 7.6 Hz, ArH), 7.43–7.36 (8H, m,
ArH), 7.23 (4H, d, J ¼ 7.2 Hz, ArH), 7.17–7.10 (4H, m, ArH),
6.88 (4H, d, J ¼ 8.4 Hz, ArH), 6.79 (4H, d, J ¼ 8.4 Hz, ArH),
4.94 (4H, br, CH2Ph), 3.82 (6H, s, OCH3), 2.87 (4H, t, J ¼ 6.0
Hz, NCH2CH2), 2.17 (4H, t, J ¼ 6.0 Hz, NCH2CH2), 1.93 (12H,
s, N(CH3)2), 1.48 (6H, s, CH3C¼¼N). 13C NMR (CDCl3, ppm): d
171.2 (CO), 170.3, 163.0, 159.7, 155.7, 153.1, 148.7, 140.1,
137.4, 130.1, 128.3, 128.2, 128.1, 126.7, 126.0, 123.8,
120.3, 114.0, 108.0 (Ar and Py), 101.7 (C¼¼CAN), 66.4
(CH2Ph), 59.2 (NCH2CH2), 55.3 (OCH3), 46.3 (NCH2CH2),
44.8 (N(CH3)2), 15.5 (CCH3). Anal. Calcd. (found) for
ter with chemical shifts given in parts per million from the
internal TMS. Microanalyses were performed using a Her-
aeus CHN-O-RAPID instrument. Melting Point was performed
using a Bu¨chi 535 m. The GPC measurements were per-
formed on a Waters 1515 Isotratic HPLC Pump system
equipped with a differential Waters 2414 Refractive Index
detector using THF (HPLC grade) as an eluent. The chro-
matographic column was Water Styragel Column (HR4E) and
the calibration curve was made by primary polystyrene
standards to calculate Mn (GPC). Molecular weight of poly-
mer is greater than 200,000 were performed using a Super
Co-150 system equipped with a differential Jasco RI-2031
plus Detector. The Index detector was using THF (HPLC
grade) as an eluent. The chromatographic column was Water
C
58H64Mg2N8O6: N, 11.01 (10.89); C, 68.44 (68.34); H, 6.34
ꢀ
(6.47)%. Mp: 261–263 C.
R
StyrageV HR4E THF 7.8 ꢁ 300 mm2 Column and the calibra-
tion curve was made by polystyrene primary standards to
Synthesis of [(L4)Mg(l-OBn)]2 (4)
calculate Mn (GPC).
Following the procedures for 1d where L4H (1.67 g, 4
mmol) was used as a reagent. Yield: 3.31 g (84%). 1H NMR
(CDCl3, ppm): d 8.20 (4H, d, J ¼ 8.0 Hz, ArH), 7.64 (4H, d, J
¼8.0 Hz, ArH), 7.43–7.38 (10H, m, ArH), 7.28–7.24 (4H, m,
ArH), 7.20–7.13 (4H, m, ArH), 6.96 (4H, d, J ¼ 8.0 Hz, ArH),
4.96 (4H, br, CH2Ph), 2.79 (4H, t, J ¼ 5.6 Hz, NCH2CH2), 2.20
(4H, t, J ¼ 5.6 Hz, NCH2CH2), 1.70 (12H, s, N(CH3)2), 1.41
(6H, s, CH3C¼¼N). 13C NMR (CDCl3, ppm): d 169.7 (CO),162.8,
147.9, 146.2, 141.1, 139.7, 130.8 (CF3), 128.3, 128.1, 127.3,
126.5, 126.1, 125.7, 125.1, 124.3, 122.4, 120.3, 119.4 (Ar and
Py), 101.1 (C¼¼CAN), 66.3 (CH2Ph), 58.9 (NCH2CH2), 46.4
(NCH2CH2), 44.8 (N(CH3)2), 15.6 (CCH3). Anal. Calcd. (found)
for C58H58F6Mg2N8O4: N, 10.25 (10.26); C, 63.69 (63.59); H,
5.35 (5.73)%.
Synthesis of [(L1)Mg(l-OBn)]2 (1)
MgnBu2 (4.5 mL, 1.0 M in hexane, 4.5 mmol) was added to a
solution of benzyl alcohol (0.84 mL, 8 mmol) in toluene (20
mL). The mixture was stirred for 12 h at room temperatuꢀre
and then dried in a vacuum. The mixture was cooled to 0 C
and then a solution of ligand L1H (1.46 g, 4 mmol) in 30 mL
toluene was added slowly and stirred for 6 h. The resulting
precipitate was collected by filtration and dried under a vac-
uum to give white powder. Yield: 3.43 g (87%).
1H NMR (CDCl3, ppm): d 8.24 (4H, d, J ¼ 7.6 Hz, ArH), 7.46–
7.38 (8H, m, ArH), 7.28–7.24 (4H, m, ArH), 7.19–7.12 (8H,
m, ArH), 6.78 (4H, d, J ¼ 8.0 Hz, ArH), 4.95 (4H, br, CH2Ph),
2.87 (4H, t, J ¼ 6.0 Hz, NCH2CH2), 2.37 (6H, s, ArCH3) 2.17
(4H, t, J ¼ 6.0 Hz, NCH2CH2), 1.94 (12H, s, N(CH3)2), 1.47
(6H, s, CH3C¼¼N). 13C NMR (CDCl3, ppm): d 171.5 (CO),162.8,
148.6, 146.4, 139.9, 138.3, 134.7, 129.2, 128.2, 128.0, 126.7,
126.0, 123.8, 120.2 (Ar and Py), 101.5 (C¼¼CAN), 66.3
(CH2Ph), 58.9 (NCH2CH2), 46.3 (NCH2CH2), 44.8 (N(CH3)2),
21.3(ArCH3), 15.6 (CCH3). Anal. Calcd. (found) for
C58H64Mg2N8O4: N, 11.37 (11.31); C, 70.67 (70.46); H, 6.54
(6.64)%.
Synthesis of [(L5)Mg(l-OBn)]2 (5)
Following the procedures for 1 where L5H (1.47 g, 4 mmol)
was used as a reagent. Yield: 3.14 g (79%). 1H NMR (CDCl3,
ppm): d 8.21 (4H, d, J ¼ 7.6 Hz, ArH), 7.40 (8H, t, J ¼ 6.8
Hz, ArH), 7.25 (4H, d, J ¼7.6 Hz, ArH), 7.18–7.11 (4H, m,
ArH), 7.03 (4H, d, J ¼ 8.8 Hz, ArH), 6.73 (4H, t, J ¼ 8.4 Hz,
ArH) 4.92 (4H, br, CH2Ph), 2.83 (4H, t, J ¼ 6.0 Hz, NCH2CH2),
2.18 (4H, t, J ¼ 5.2 Hz, NCH2CH2), 1.94 (12H, s, N(CH3)2),
1.44 (6H, s, CH3C¼¼N). 13C NMR (CDCl3, ppm): d 170.3 (CO),
163.9, 162.9, 161.4, 148.2, 146.4, 139.9, 133.6, 128.8, 128.7,
128.2, 128.1, 126.5, 126.0, 124.0, 120.3 (Ar and Py), 101.5
(C¼¼CAN), 66.3 (CH2Ph), 59.1 (NCH2CH2), 46.3 (NCH2CH2),
44.8 (N(CH3)2), 15.5 (CCH3). Anal. Calcd. (found) for
Synthesis of [(L2)Mg(l-OBn)]2 (2)
Following the procedures for 1 where L2H (1.62 g, 4 mmol)
was used as a reagent. Yield: 3.55 g (83%). 1H NMR (CDCl3,
ppm): d 8.21 (4H, d, J ¼ 8.0 Hz, ArH), 7.42 (4H, d, J ¼ 7.2
Hz, ArH), 7.40–7.35 (10H, m, ArH), 7.24 (4H, d, J ¼ 5.2 Hz,
ArH), 7.19–7.10 (4H, m, ArH), 6.81 (4H, d, J ¼ 8.4 Hz, ArH),
C
56H58F2Mg2N8O4: N, 11.28 (11.08); C, 67.69 (68.15); H,
ꢀ
5.88 (5.36)%. Mp: 268–270 C.
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Synthesis of [(L6)Mg(l-OBn)]2 (6)
129.01, 128.22, 128.19, 128.12, 127.01, 126.72, 126.17,
125.27, 124.23, 120.83, 114.75, 112.96 (Ar and Py), 101.05
(C¼¼CAN), 66.45 (CH2Ph), 59.72 (NCH2CH2), 55.14 (OCH3),
46.91 (NCH2CH2), 44.83 (N(CH3)2). Anal. Calcd. (found) for
Following the procedures for 1 where L6H (1.46 g, 4 mmol)
was used as a reagent. Yield: 2.76 g (70%). 1H NMR (CDCl3,
ppm): d 8.20 (4H, d, J ¼ 8.0 Hz, ArH), 7.46 (4H, d, J ¼ 6.4
Hz, ArH), 7.37 (4H, t, J ¼ 7.2 Hz, ArH), 7.25–7.10 (2H, m,
ArH), 6.76–6.66 (2H, m, ArH), 4.90(4H, br, CH2Ph), 2.83–2.75
(4H, m, NCH2CH2), 2.20 (4H, m, NCH2CH2), 2.01–1.98 (6H,
m, ArCH3), 1.91 (12H, s, N(CH3)2), 1.37 (6H, s, CH3C¼¼N). 13C
NMR (CDCl3, ppm): d 171.7, 163.0, 148.4, 140.0, 137.5,
133.9, 133.9, 130.2, 128.5, 128.2, 128.1, 126.7, 126.5, 126.2,
126.1, 123.9, 120.4, 120.3 (Ar and Py), 100.8 (C¼¼CAN), 66.3
(CH2Ph), 58.7 (NCH2CH2), 45.9 (NCH2CH2), 42.0 (N(CH3)2),
19.0,18.5 (ArCH3), 14.7 (CCH3). Anal. Calcd. (found) for
C
68H68N8O6Mg2: N, 9.81 (9.78); C, 71.52 (70.90); H, 6.00
(5.93)%. IR (KBr, cmꢂ1): 1597 (C¼¼N), 1486 (C¼¼O), 1454
ꢀ
(C¼¼C). Mp: 270–272 C.
Synthesis of [(L10)Mg(l-OBn)]2 (10)
Following the procedures for 1d where L10H (1.66 g, 4
mmol) was used as a reagent. Yield: 3.51 g (81%). 1H NMR
(CDCl3, ppm): d 8.22 (4H, d, J ¼ 8.8 Hz, ArH), 7.39–7.32 (8H,
m, ArH), 7.25–7.21 (4H, m, ArH), 7.18–7.15 (2H, m, ArH),
6.88–6.76 (8H, m, ArH), 4.94 (4H, br, CH2Ph), 3.82 (6H, s,
OCH3), 2.91 (4H, t, J ¼ 5.6 Hz, NCH2CH2), 2.21 (4H, t, J ¼ 5.6
Hz, NCH2CH2), 1.96 (12H, s, N(CH3)2), 1.47 (6H, s, CH3C¼¼N).
13C NMR (CDCl3, ppm): d 171.1 (CO), 162.9, 159.8, 149.1,
138.6, 129.8, 128.7, 128.3, 128.2, 128.1, 126.6, 126.1, 121.0,
113.9 (Ar and Py), 101.7 (C¼¼CAN), 66.4 (CH2Ph), 59.2
(NCH2CH2), 55.2 (OCH3), 46.4 (NCH2CH2), 44.8 (N(CH3)2),
15.4 (CCH3). Anal. Calcd. (found) for C58H64Cl2Mg2N8O6: N,
10.31 (10.39); C, 64.11 (64.11); H, 5.75 (6.59)%.
C
58H64Mg2N8O4: N, 11.37 (10.90); C, 70.69 (70.45); H, 6.54
ꢀ
(7.07)%. Mp: 292–294 C.
Synthesis of [(L7)Mg(l-OBn)]2 (7)
Following the procedures for 1 where L7H (1.51 g, 4 mmol)
was used as a reagent. Yield: 2.97 g (73%). 1H NMR (CDCl3,
ppm): d 8.22 (4H, d, J ¼ 7.6 Hz, ArH), 7.49 (4H, d, J ¼ 7.2
Hz, ArH), 7.37–7.32 (6H, m, ArH), 7.22 (4H, t, J ¼ 7.2 Hz,
ArH), 7.16–7.08 (4H, m, ArH), 6.97–6.89 (2H, m, ArH), 6.75–
6.73 (2H, m, ArH), 4.92–4.85 (4H, br, CH2Ph), 3.63 (6H, s,
OCH3), 2.93–2.82 (4H, m, NCH2CH2), 2.12–2.06 (2H, m,
NCH2CH2), 1.85 (12H, br, N(CH3)2), 1.46 (6H, s, CH3C¼¼N).
13C NMR (CDCl3, ppm): d 168.9, 163.1, 155.3, 148.4, 140.1,
130.0, 128.1, 128.0, 127.3, 126.9, 125.9, 123.6, 120.9, 120.2,
110.8 (Ar and Py), 101.1 (C¼¼CAN), 66.3 (CH2Ph), 58.3
(NCH2CH2), 55.3 (OCH3), 45.8 (NCH2CH2), 44.7 (N(CH3)2),
14.9 (CH3C¼¼N). Anal. Calcd. (found) for C58H64N8O6Mg2: N,
11.01 (10.41); C, 68.44 (67.99); H, 6.34 (6.22)%. Mp: 258–
Synthesis of [(L11)Mg(l-OBn)]2 (11)
Following the procedures for 1 where L11H (1.57 g, 4 mmol)
was used as a reagent. Yield: 3.64 g (87%). 1H NMR (CDCl3,
ppm): d 8.09 (4H, d, J ¼ 7.2Hz, ArH), 7.40–7.35(10H, m,
ArH), 7.13 (4H, d, J ¼ 7.6Hz, ArH), 6.93 (4H, d, J ¼ 7.2 Hz,
ArH), 6.78 (4H, d, J ¼ 7.2 Hz, ArH) 4.90 (4H, s, CH2Ph), 3.82
(6H, s, OCH3), 2.84 (4H, t, J ¼ 5.6 Hz, NCH2CH2), 2.36 (6H, s,
ArCH3), 2.15 (4H, t, J ¼ 6.4 Hz, NCH2CH2), 1.92 (12H, s,
N(CH3)2), 1.44 (6H, s, CH3C¼¼N). 13C NMR (CDCl3, ppm): d
172.4 (CO), 162.4, 156.3, 155.7, 148.1, 138.3, 133.7, 129.5,
129.2, 128.2, 128.1 126.8, 126.7, 121.8, 120.9, 113.4 (Ar and
Py), 101.3 (C¼¼CAN), 66.4 (CH2Ph), , 59.0 (NCH2CH2), 55.5
(OCH3), 46.3 (NCH2CH2), 44.8 (N(CH3)2), 21.3 (ArCH3), 15.5
(CCH3). Anal. Calcd. (found) for C60H68Mg2N8O6: N, 10.50
(10.71); C, 68.66 (68.91); H, 6.53 (6.55)%.
ꢀ
260 C.
Synthesis of [(L8)Mg(l-OBn)]2 (8)
Following the procedures for 1d where L8H (1.79 g, 4
mmol) was used as a reagent. Yield: 3.96 g (83%). 1H NMR
(CDCl3, ppm): d 8.35 (4H, d, J ¼ 11.2 Hz, ArH), 7.56 (4H, d, J
¼10 Hz, ArH), 7.44–6.85 (26H, m, ArH), 6.65 (4H, d, J ¼
10.4 Hz, ArH), 5.05 (4H, d, J ¼ 13.6 Hz, CH2Ph), 3.05 (4H, t, J
¼ 6.8 Hz, NCH2CH2), 2.17 (4H, t, J ¼ 7.2 Hz, NCH2CH2), 1.95
(12H, s, N(CH3)2), 1.14 (18H, s, C(CH3)3). 13C NMR (CDCl3,
Synthesis of [(L12)Mg(l-OBn)]2 (12)
Following the procedures for 1 where L12H (1.74 g, 4 mmol)
was used as a reagent. Yield: 3.71 7.36 (4H, d, J ¼ 8.4 Hz,
ArH), 7.24 (4H, d, J ¼ 7.6 Hz, ArH), 6.92 (2H, d, J ¼ 9.2 Hz
ArH), 6.82 (4H, d, J ¼ 7.6 Hz, ArH), 4.88 (4H, s, CH2Ph), 3.82
(6H, s, OCH3,), 2.86 (4H, t, J ¼ 5.2 Hz, NCH2CH2), 2.18 (4H, t,
J ¼ 6.0 Hz, NCH2CH2), 1.92 (12H, s, N(CH3)2), 1.38 (6H, s,
CH3C¼¼N), 1.33 (18H, s, C(CH3)3). 13C NMR (CDCl3, ppm): d
171.6 (CO), 162.4, 156.2, 148.1, 146.6, 134.8, 133.6, 128.0,
128.9, 128.2, 126.7 126.4, 126.0, 125.3, 121.8, 113.4 (Ar and
Py), 101.1 (C¼¼CAN), 66.3 (CH2Ph), 59.0 (NCH2CH2),
55.5(OCH3), 46.2 (NCH2CH2), 44.8 (N(CH3)2), 34.6 (C(CH3)3),
31.4 (C(CH3)3), 15.5 (CCH3). Anal. Calcd. (found) for
ppm):
d 171.34 (CO), 163.17, 152.48, 151.31, 140.13,
135.17, 132.72, 128.99, 128.36, 128.23, 126.92, 126.97,
126.79, 126.38, 126.11, 124.39, 124.22, 120.86 (Ar and Py),
101.01 (C¼¼CAN), 66.54 (CH2Ph), 59.66 (NCH2CH2), 46.93
(NCH2CH2), 44.86 (N(CH3)2), 34.41 (C(CH3)3), 31.08
(C(CH3)3). Anal. Calcd. (found) for C74H80N8O4Zn2: N, 9.38
(9.82); C, 75.08 (75.08); H, 6.27 (6.27)%. IR (KBr, cmꢂ1):
1596 (C¼¼N), 1485 (C¼¼O), 1456 (C¼¼C). Mp: 260–262 C.
Synthesis of [(L9)Mg(l-OBn)]2 (9)
ꢀ
Following the procedures for 1d where L9H (1.76 g, 4
mmol) was used as a reagent. Yield: 3.75 g (82%). 1H NMR
(CDCl3, ppm): d 8.35 (4H, d, J ¼ 8.4 Hz, ArH), 7.55–6.92
(26H, m, ArH), 6.64 (4H, d, J ¼ 8.0 Hz, ArH), 6.40 (4H, d, J ¼
8.4 Hz, ArH), 5.02 (4H, br, CH2Ph), 3.62 (6H, s, OCH3), 3.06
(4H, t, J ¼ 5.6 Hz, NCH2CH2), 2.17 (4H, t, J ¼ 5.6 Hz,
NCH2CH2), 1.95 (12H, s, N(CH3)2). 13C NMR (CDCl3, ppm): d
170.81 (CO), 163.16, 159.59, 152.37, 140.10, 135.22, 130.18,
C66H80Mg2N8O6: N, 9.92 (10.12); C, 70.15 (70.23); H, 7.14
(7.32)%.
Synthesis of [(L13)Mg(l-OBn)]2 (13)
Following the procedures for 1 where L13H (1.63 g, 4 mmol)
was used as a reagent. Yield: 3.32 g (77%).1H NMR (CDCl3,
ppm): d 8.10 (4H, d, J ¼ 8.4 Hz, ArH), 7.41 (4H, d, J ¼ 7.2
Hz, ArH), 7.24 (4H, d, J ¼ 7.2 Hz, ArH), 7.16 (2H, d, J ¼ 6.8
698
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1
Hz, ArH), 6.93 (4H, d, J ¼ 8.4 Hz, ArH), 6.88 (4H, d, J ¼ 7.6
Hz, ArH), 6.81 (4H, d, J ¼ 8.0 Hz, ArH) 4.90 (4H, s, CH2Ph),
3.82 (6H, s, OCH3), 2.88 (4H, t, J ¼ 5.2 Hz, NCH2CH2), 2.18
(4H, t, J ¼ 5.2Hz, NCH2CH2), 1.93 (12H, s, N(CH3)2), 1.46
(6H, s, CH3C¼¼N). 13C NMR (CDCl3, ppm): d 171.2 (CO),
162.4, 159.6, 156.2, 148.2, 133.6, 130.0, 128.2, 126.6, 126.0,
121.7, 113.9 113.4, (Ar and Py), 101.5 (C¼¼CAN), 66.5
(CH2Ph), 59.1 (NCH2CH2), 55.5, 55.2 (OCH3), 46.3 (NCH2CH2),
44.8 (N(CH3)2), 15.5 (CCH3). Anal. Calcd. (found) for
(Pr ¼ 0.88) were analyzed by H NMR spectroscopic studies.
THF (10.0 mL) was added to a mixture of Complex 6 (0.049
g, 0.05 mmol) and rac-lactide (0.72 g, 5.0 mmol) at 0 C. Af-
ꢀ
ter the solution was stirred for 96 h, the reaction was then
quenched by the addition of deionized water (0.1 mL), and
the polymer was precipitated on pouring the mixture into
n-hexane (100.0\mL) to give white crystalline solids. The
white solid was dissolved in CH2Cl2 (10.0 mL) and then
n-hexane (100.0 mL) was added for re-crystallization and
was then evaporated to dryness under vacuum to yield a
white crystalline solid. Yield: 0.51 g (70%).
C60H68Mg2N8O6: N, 10.40 (9.98); C, 66.86 (66.21); H, 6.36
(6.32)%.
Synthesis of [(L14)Mg(l-OBn)]2 (14)
Melting Point Depression Measurement
Following the procedures for 1 where L14H (1.59 g, 4 mmol)
was used as a reagent. Yield: 3.58 g (85%).1H NMR (CDCl3,
400ppm): d 8.08 (4H, d, J ¼ 8.8 Hz, ArH), 7.41 (4H, d, J ¼
7.2 Hz, ArH), 7.23 (4H, d, J ¼ 8.0 Hz, ArH), 7.18 (2H, d, J ¼
7.2 Hz, ArH), 7.07 (4H, d, J ¼ 8.4 Hz, ArH), 6.94 (4H, d, J ¼
9.2 Hz, ArH), 6.84 (4H, d, J ¼ 5.6 Hz, ArH), 4.91 (4H, s,
CH2Ph), 3.82 (6H, s, OCH3), 2.84 (4H, t, J ¼ 6.0 Hz,
NCH2CH2), 2.19 (4H, t, J ¼ 5.6Hz, NCH2CH2), 1.95 (12H, s,
N(CH3)2), 1.43 (6H, s, CH3C¼¼N). 13C NMR (CDCl3, ppm): d
170.3 (CO), 168.8, 162.3, 161.4, 156.3, 147.8, 146.4, 133.6,
133.5, 128.7, 128.0, 127.8 126.5, 121.1, 115.9, 113.4 (Ar and
Py), 101.3 (C¼¼CAN), 59.0 (NCH2CH2), 55.4 (OCH3), 46.3
(NCH2CH2), 44.8 (N(CH3)2), 15.5 (CCH3). Anal. Calcd. (found)
for C58H62F2Mg2N8O6: N, 10.63 (10.64); C, 66.11 (65.73); H,
5.93 (6.77)%.
[L5Mg(l-OBn)]2 (5) (0.099 g) was dissolved in a hot naph-
thalene (3.42 g). The freezing point of the mixture was
measured by slowly cooling the mixture, and the tempera-
ture was recorded once every 30 s using a Beckman ther-
mometer, which was corrected prior to use. This procedure
was repeated three times. [L5Mg(l-OBn)]2 (5) (0.116 g) in a
hot benzene (18.73 g) was measured in a similar procedure.
[L12Mg(l-OBn)]2 (12) (0.094 g) was dissolved in a hot naph-
thalene (3.53 g).
Pulse Gradient Spin Echo (PGSE) NMR Experiment
The 1H PGSE experiments were performed in CDCl3 on a
VARIAN UNITY INOVS 600 (600 MHz) instrument. The DOSY
experiments were carried out using the pulse sequence
‘‘BPPSTE.’’ Durations of gradient pulse (d) and diffusion time
(D) were 2 and 50 ms, respectively. The Diffusion gradient
level (gzlvll) are 6325, 8944, 10,954, 12,649, 14,142, 15,492,
16,733, 17,889, 18,974, and 20,000. The DAC_to_G has been
calibrated at 25 ꢀC and has a value of 0.0019400 gauss/cm
DAC. The diffusion coefficient D can be extracted by Stej-
skal–Tanner attenuation plot and the hydrodynamic radius of
complex can be calculated from the Stokes–Einstein
equation.19
Typical Polymerization Procedures of L-Lactide
A typical polymerization procedure was exemplified by the
synthesis of Entry 2 (Table 2) using complex [L1Mg(l-OBn)]2
(1) as an initiator at 30 ꢀC. The conversion yield (97%) of
1
PLLA was analyzed by H NMR spectroscopic studies. L-Lac-
tide (0.72 g, 5.0 mmol) was added to an ice cold solution of
complex [L1Mg(l-OBn)]2 (1) 0.049 g, 0.05 mmol in CH2Cl2
(10.0 mL). After the solution was stirred at 30 ꢀC for 100
min, the reaction was then quenched by the addition of
deionized water (0.1 mL), and n-hexane (60.0 mL) was then
added to the above mixture to give white solid. The white
solid was dissolved in CH2Cl2 (10.0 mL) and then n-hexane
(100.0 mL) was added for re-crystallization and was then
evaporated to dryness under vacuum to yield a white crys-
talline solid. Yield: 0.55 g (77%).
Kinetic Studies of the Polymerization of Lactide
A typical polymerization procedure was exemplified using
Complex 5 as an initiator. L-Lactide (0.072 g, 0.50 mmol) in
CDCl3 (1 mL) was added to Complex 5 (0.0049 g, 0.005
mmol) in a NMR tube under liquid nitrogen temperature.
1
The conversion of LA was monitored on the basis of H NMR
ꢀ
spectroscopic studies at 20 C.
Synthesis of L-Lactide in the Presence of Benzyl Alcohol
A typical polymerization was exemplified by the synthesis of
PLA in the presence of 38 molar equiv of BnOH. L-Lactide
(28.83 g, 200 mmol) and BnOH (1.99 mL, 19 mmol) were
dissolved in 5 mL of CH2Cl2 at 30 ꢀC, and then the mixture
was added to a rapidly stirring solution of Complex 6 (0.049
g, 0.5 mmol)ꢀ in CH2Cl2 (5 mL). The reaction mixture was
stirred at 30 C for 30 min. After the reaction was quenched
by adding the deionized water (0.1 mL), the polymer was
precipitated into n-hexane. Yield: 25.9 g (90%).
X-Ray Crystallographic Studies
Suitable crystal of 1–3, 5, 7, 9, and 10 were covered with
R
perfluoropolyether vacuum oil (Aldirch, FOMBLINVY) hold
on a glass fiber and mounted on CryoLoop (HAMPTON
RESEARCH) and cooled rapidly in a stream of cold nitrogen
gas using an Oxford diffraction Limited GEMINT R and S.
The space group determination was based on a check of the
Laue symmetry and systematic absences, and was confirmed
using the structure solution. The structure was solved by
direct methods using a SHELXTL package.20 All non-H atoms
were located from successive Fourier maps, and hydrogen
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.
Typical Polymerization Procedures of rac-Lactide
A typical polymerization procedure was exemplified by
the synthesis of Entry 6 (Table 4) using Complex 6 as an ini-
tiator. The conversion yield (94%) and Pr value of PLA
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SCHEME 1 Preparation of NNO-ketiminate ligands and their magnesium Complexes 1–15.
determined on the basis of 1H and 13C NMR spectroscopic
studies, as well as elemental analysis.
RESULTS AND DISCUSSION
Syntheses and Structural Studies
To study the substituent effects of the ligands affecting the
activity of metal complexes, various functional groups such
as alkyl, phenyl, alkoxy and halo groups incorporated to py-
razolone based ketiminate ligands were synthesized as illus-
trated in Scheme 1: (1) Pyrazolone precursors I–IV were
prepared by refluxing the corresponding b-ketoesters and
hydrazine compounds in ethanol;18 (2) Reaction of the py-
razolone compounds with substituted benzoyl chloride in
the presence of Ca(OH)2, affords the diketone intermediate;
and (3) ketoimines, L1H–L14H, were obtained as yellow solid
in reasonable yields (80–88%) by the reaction of N,N-dime-
thylethylenediamine with the corresponding diketone inter-
mediate. (4) Further reaction of L1H–L14H, with Mg(OBn)2,
heteroleptic magnesium Complexes 1–14 were obtained in
high yield (Scheme 1). The formation of magnesium Com-
plexes 1–14 can be monitored by 1H NMR spectroscopic
studies. For instance, the resonance frequency for ꢂNH pro-
ton at 11.56 ppm disappears and peaks corresponding to
CH2CH2N(CH3)2, CH2N(CH3)2, and N(CH3)2 at 3.21, 2.45, and
2.19 ppm, respectively, for the free ligands L10H shift to
upfield 2.91, 2.21, and 1.96 ppm for 10. Furthermore, the
methylene hydrogens of the bridging benzyl alkoxides were
revealed at 4.95 ppm. All of these metal complexes were
Single crystals of magnesium Complexes 1–3, 5, 7, 9, and 10
suitable for X-ray diffraction were obtained by recrystalliza-
tion from a mixture of dichloromethane–hexane solution.
The crystal structure of Complex 1 (Fig. 1) reveals a dinu-
clear feature with penta-coordinated magnesium centers
bonding to O,N,N atoms of NNO-ketiminate ligand and two
bridging benzyloxy oxygen atoms. The bond distance
between magnesium and the bridging oxygen atoms are Mg-
O(2) 1.9736(9) Å and Mg-O(2A) 1.9915(9) Å, respectively.
The bond lengths between Mg and NNO-ligands are Mg-O(1)
1.9865(9) Å, Mg-N(3) 2.1769(11) Å and Mg-N(4) 2.2004(11)
Å, respectively. These data are comparable to those found in
magnesium ketiminate complexes.17(a) The solid state struc-
ture of Complexes 2, 3, 5, 7, 9, and 10 (Supporting Informa-
tion Figures S1–S6) are all similar to that of 1 and the geom-
etry around magnesium are distorted trigonal bipyramidal
with s values ranging from 0.56 to 0.73 (Table 1).21 The
crystallographic data and selected bond lengths and angles
are summarized in Table 1.
Ring-Opening Polymerization of L-Lactide
Ring-opening polymerization of LA initiated by these magne-
sium complexes [LnMg(l-OBn)]2 (1–14) were investigated in
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tion of the first addition ([LA]0/[I]0 ¼ 50) had gone to com-
pletion (Table 2, Entry 5). It is worth noting that these com-
plexes also reveal ‘‘immortal’’ character. For instance, ROP of
L-Lactide initiated by Complex 6 in the presence of benzyl
alcohol as a chain transfer reagent proceeds smoothly with
good molecular weigh controlled. Experiment results demon-
strated that as much as 38 equiv of BnOH can be added
resulting PLA with expected molecular weight and narrow
PDI (Table 3) implying the chain transfer reaction is much
faster than propagation of polymer.
Ring-Opening Polymerization of rac-Lactide
Ring-opening polymerization of rac-Lactide by these magne-
sium complexes proceeds rapidly and well controlled with
PDI ranging from 1.07 to 1.14 as shown in Table 4. These
complexes illustrate moderate to highly heterotactic selectiv-
ity in THF with Pr in the range of 0.77 to 0.87. Among them,
[L6Mg(l-OBn)]2 (6) exhibits the highest stereoselectivity
with Pr ¼ 0.88 at 0 ꢀC in THF. However, atactic PLA was
obtained when the polymerization was performed in either
CH2Cl2 (Pr ¼ 0.45) or toluene (Pr ¼ 0.58) (Fig. 3). Similar
results have been reported previously in which solvent plays
an important role in influencing the activity and stereocon-
trollity of catalysts.13(a),17(a)
FIGURE 1 The molecular structure of [(L1)Mg(l-OBn)]2 (1) with
probability ellipsoids drawn at the 40% probability level.
Hydrogen atoms were omitted for clarity.
toluene and the results are listed in Table 2. Experimental
results reveal that all of these complexes display good activ-
ities for the polymerization of LA and the reaction can reach
to completion within 100 min at 30 ꢀC except Complexes 8
and 9 (Table 2, Entries 12 and 13). The polymerization is
well controlled with good molecular weight control and nar-
row distributions ranging from 1.07 to 1.26. The ‘‘living’’
character were implied by the linear relationship between
the Mn and [LA]0/[I]0 as shown in Figure 2. The ‘‘living’’
character was further confirmed from the polymerization
resumption experiment, in which another portion of LA
monomer ([LA]0/[I]0 ¼ 50) was added after the polymeriza-
Kinetic Studies of Polymerization of L-Lactide
In order to realize the influence of magnesium complexes
[LnM(l-OBn)]2 in the mechanism of polymerization, the poly-
merization of LA initiated by the magnesium compounds 1–
4, 6, 8, and 10–14 were systematically investigated. Conver-
ꢀ
sion of LA with time at various complexes in CDCl3 at 20 C
was monitored by 1H NMR spectroscopy ([LA]0 ¼ 0.5 M;
˚
TABLE 1 Comparison of the Selected Bond Lengths (A), Bond Angles (8), and s Values for 1–3, 5, 7, 9, and 10
1
2
3
5
7
9
10
˚
Bond Lengths (A)
Mg-O(1)
1.9865(9)
1.9936(9)
1.9615(9)
2.1769(11)
2.2004(11)
1.9713(17)
2.0054(17)
1.9719(17)
2.148(2)
1.9903(10)
1.9964(9)
1.9633(17)
1.9974(17)
1.9711(16)
2.148(2)
1.9717(16)
1.9892(16)
1.9615(17)
2.160(2)
1.9655(18)
1.9840(19)
2.0341(17)
2.141(2)
1.9731(13)
1.9737(13)
1.9985(12)
2.1601(14)
2.2369(15)
Mg-O(2)
Mg-O(2A)
1.9864(10)
2.1489(11)
2.2426(11)
Mg-N(3)
Mg-N(4)
2.243(2)
2.2228(19)
2.223(2)
2.247(2)
Bond Angles (ꢀ)
O(1)-Mg-O(2)
O(2)-Mg-O(2A)
O(2)-Mg-N(3)
O(2)-Mg-N(4)
O(1)-Mg-N(4)
O(1)-Mg-N(3)
O(1)-Mg-O(2A)
O(2A)-Mg-N(3)
O(2A)-Mg-N(4)
N(3)-Mg-N(4)
s Value
94.03(4)
82.50(4)
172.40(4)
96.81(4)
135.33(4)
86.10(4)
119.72(4)
104.05(4)
104.63(4)
77.94(4)
95.19(7)
82.59(7)
172.53(8)
94.87(7)
130.78(8)
87.14(7)
115.80(7)
102.80(7)
113.24(8)
78.31(7)
95.61(4)
81.77(4)
172.99(5)
95.15(4)
135.62(5)
86.73(4)
116.79(4)
103.10(4)
107.32(4)
78.65(4)
95.48(7)
82.86(7)
173.57(7)
94.81(7)
129.61(7)
87.14(8)
116.64(7)
101.24(7)
113.54(7)
79.04(7)
93.44(7)
83.82(7)
173.22(8)
97.28(7)
139.00(7)
87.59(7)
114.72(7)
101.84(8)
105.74(7)
77.70(7)
116.28(8)
84.30(8)
105.64(8)
108.30(8)
135.38(8)
87.26(7)
92.65(7)
168.97(8)
93.57(7)
78.90(8)
116.72(5)
82.28(5)
102.67(6)
109.07(6)
133.98(6)
86.74(5)
95.77(5)
172.78(6)
94.53(5)
78.94(5)
0.62
0.70
0.62
0.73
0.57
0.56
0.65
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TABLE 2 Ring-Opening Polymerization of L-Lactide Initiated by Magnesium Complexes 1–14
Entry
[I]0
[M]0/[I]0
Time (min)
Conv. (%)a
Mn (calcd.)b
Mn (obsd.)c
Mn (NMR)d
PDIe
1
1
50/1
100
100
100
100
100(100)
60
96
97
97
98
93
96
95
96
97
96
96
95
97
98
95
97
94
91
3,600
7,100
10,600
14,000
6,800
7,000
6,900
7,000
7,100
7,000
7,000
6,900
7,100
7,200
6,900
7,100
6,800
6,600
3,000
6,800
11,000
14,000
5,800
6,900
7,000
6,800
6,500
8,100
5,800
8,100
7,500
7,100
6,000
6,600
7,400
6,800
3,600
6,800
11,400
14,500
6,700
7,200
7,100
7,500
7,100
7,800
7,900
7,700
8,300
6,500
7,100
6,900
7,000
6,200
1.18
1.12
1.10
1.13
1.09
1.10
1.09
1.10
1.09
1.09
1.26
1.06
1.09
1.09
1.18
1.18
1.10
1.11
2
1
100/1
150/1
200/1
50(50)/1
100/1
100/1
100/1
100/1
100/1
100/1
100/1
100/1
100/1
100/1
100/1
100/1
100/1
3
1
4
1
5
1
6
2
7
3
30
8
4
100
100
35
9
5
10
11
12
13
14
15
16
17
18
6
7
60
8
210
240
100
60
9
10
11
12
13
14
40
50
60
c
Reaction Conditions: 30 ꢀC, CH2Cl2 (10 mL), [I]0 ¼ 5.0 mM; [8]0–[9]0
¼
Obtained from GPC analysis times 0.58.22
Obtained from 1H NMR analysis.
Obtained from GPC.
d
e
0.625 mM.
a
Obtained from the 1H NMR analysis.
b
Calculated from the molecular weight of LA ꢁ [M]0/2[I]0 ꢁ conversion
þ Mw(BnOH).
[C]0 ¼ 5 mM). However, the semi logarithmic plots of (1/
[LA] ꢂ 1/[LA]0) versus time (min) are more fitted that using
these magnesium complexes for polymerization of LA in this
work, indicating that kinetics of polymerization favor to sec-
ond-order reaction to monomer concentration rather than
first-order reaction (see Supporting Information Figure S7).
Previously, the kinetics studies for the polymerization of lac-
tide by the similar magnesium Complex 15 (Scheme 1) were
reported to be a first order in terms of [LA].17(a) However,
after carefully reexamined the previously (Supporting infor-
mation Figure S8) and present data, we believed that the
polymerization follows a second-order reaction of [LA].
polymerization rate can be written as ꢂd[LA]/dt
¼
k[LA]2[5]n ¼ kobs[LA]2, where kobs ¼ kp[5]n, and kp is the
propagation constant. The linear relationship between
ln(kobs) versus ln[5] [Fig. 4(b)] reveals the order in the
In Table 5, their rate constant (kobs) were summarized and
the activity order of these complexes were consistent with
the conversion efficiency of LA. Therefore, the Complex 5
was used for extended research. Conversion of LA ([LA]0
¼
1
0.5 M) with time was monitored by H NMR for various con-
centrations of 5 ([5]0 ¼ 3, 4, 5, 6, and 7 mM respectively) at
20 ꢀC until monomer consumption was complete. In each
case, plots of (1/[LA] ꢂ 1/[LA]0) versus time (min) are lin-
ear, indicating polymerization proceeds with a second-order
dependence on monomer concentration [Fig. 4(a)]. Thus, the
FIGURE 2 Plot of Mn(GPC) versus [LA]0/[1]0 and the values in
parentheses are PDIs values obtained from GPC analysis.
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TABLE 3 ‘‘Immortal’’ Ring-Opening Polymerization of L-Lactide Initiated by Complex 6 in the Presence of Alcohol
Entry
[M]0/[6]0/[BnOH]0
Time (min)
Conv. (%)a
Mn (calcd.)b
Mn (obsd.)c
Mn (NMR)d
PDIe
1
2
3
4
5
6
7f
50/1/0
35
20
20
20
20
4
96
98
97
99
99
97
96
3,600
3,600
3,600
3,700
3,700
1,500
1,500
3,800
4,000
3,900
3,600
3,700
1,400
1,100
3,900
4,000
3,900
3,800
3,700
1,800
1,500
1.08
1.06
1.05
1.05
1.05
1.08
1.09
100/1/2
150/1/4
200/1/6
250/1/8
200/1/18
400/1/38
30
c
d
e
f
Reaction conditions: 30 ꢀC, CH2Cl2 (10 mL), [I]0 ¼ 5.0 mM.
Obtained from GPC analysis times 0.58.22
Obtained from 1H NMR analysis.
Obtained from GPC.
a
Obtained from the 1H NMR analysis.
b
Calculated from the molecular weight of LA ꢁ [M]0/2[I]0 ꢁ conversion
þ Mw(BnOH).
Reaction was performed at 30 ꢀC.
TABLE 4 Ring-Opening Polymerization of rac-Lactide Initiated by [LnMg(l-OBn)]2
Entry
[I]0
Solvent
Time (h)
Conv. (%)a
Mn (calcd.)b
Mn (obsd.)c
Mn (NMR)d
PDIe
Prf
1
2
THF
12
12
6
78
92
83
86
89
90
93
93
85
94
72
97
5,600
6,700
6,100
6,300
6,500
6,600
6,800
6,800
6,100
6,900
5,300
7,100
e
5,000
7,700
6,000
5,300
5,500
8,200
9,100
9,400
8,500
5,000
6,500
10,900
6,000
9,400
7,500
7,400
6,600
6,200
12,500
13,800
6,900
7,200
4,800
7,500
1.09
1.11
1.10
1.07
1.10
1.11
1.14
1.13
1.11
1.07
1.07
1.09
0.87
0.86
0.85
0.84
0.81
0.77
0.81
0.79
0.80
0.88
0.45
0.58
2
3
THF
3
4
THF
4
5
THF
12
8
5
6
THF
6
11
12
13
14
6
THF
24
24
24
24
96
24
24
7
THF
8
THF
9
THF
10g
11
12
THF
11
11
Toluene
CH2Cl2
Reaction conditions: 30 ꢀC, [I]0 ¼ 5.0 mM, [M]0 ¼ 5.0 M.
Obtained from GPC.
a
f
Obtained from the 1H NMR analysis.
Pr is the probability of racemic linkages between monomer units and
b
Calculated from the molecular weight of LA ꢁ [M]0/2[I]0 ꢁ conversion
þ Mw(BnOH).
is determined from the methine region of the homonuclear decoupled
1H NMR spectrum: [mmm] ¼ (2(1 ꢂ Pr)2 þ Pr(1 ꢂ Pr))/2; [mrm] ¼ (Pr2
þ
c
Obtained from GPC analysis times 0.58.22
Obtained from 1H NMR analysis.
Pr(1 ꢂ Pr))/2; [mmr] ¼ [rmm] ¼ (Pr(1 ꢂ Pr))/2; [rmr] ¼ Pr2/2.23
d
g
Reaction was performed at 0 ꢀC.
FIGURE 3 Homonuclear decouple 1H NMR spectrum of the methane region of PLA prepared from rac-LA with 11: (a) in THF (Pr ¼
0.77); (b) in CH2Cl2 (Pr ¼ 0.58); and (c) in toluene (Pr ¼ 0.45).
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TABLE 5 Conversion and Rate Constants for the ROP of L-Lactide Initiated by [LnMg(l-OBn)]2
Entry
Complex
Conv. (%)
kobs (minꢂ1 a
)
1
1
(R1 ¼ H, R2 ¼ CH3, R3 ¼ CH3, R4 ¼ H)
(R1 ¼ H, R2 ¼ CH3, R3 ¼ tBu, R4 ¼ H)
(R1 ¼ H, R2 ¼ CH3, R3 ¼ OCH3, R4 ¼ H)
(R1 ¼ H, R2 ¼ CH3, R3 ¼ CF3, R4 ¼ H)
(R1 ¼ H, R2 ¼ CH3, R3 ¼ F, R4 ¼ H)
(R1 ¼ H, R2 ¼ CH3, R3 ¼ H, R4 ¼ CH3)
(R1 ¼ H, R2 ¼ Ph, R3 ¼ tBu, R4 ¼ H)
(R1 ¼ Cl, R2 ¼ CH3, R3 ¼ OCH3, R4 ¼ H)
(R1 ¼ OMe, R2 ¼ CH3, R3 ¼ tBu, R4 ¼ H)
(R1 ¼ OMe, R2 ¼ CH3, R3 ¼ OCH3, R4 ¼ H)
(R1 ¼ OMe, R2 ¼ CH3, R3 ¼ F, R4 ¼ H)
(R1 ¼ H, R2 ¼ CH3, R3 ¼ H, R4 ¼ H)
b
73
1.29
1.57
1.12
0.39
0.35
1.61
0.24
0.37
1.74
1.47
1.09
0.72
2
2
77
3
3
63
4
4
5 (68b)
6 (69b)
80
5 (68b)
8 (72b)
87
5
5
6
6
7
8
8
10
12
13
14
15
9
10
11
12c
76
59
49
Monomer and catalyst concentrations were 0.5 and 0.005 M, respec-
tively. Conversions were obtained from 1H NMR analysis. The reaction
times were 20 min at 30 ꢀC.
The reaction times were 50 min at 20 ꢀC.
See Ref. 17(a).
c
a
Reaction was performed in CDCl3 at 20 ꢀC. The kobs values were deter-
mined by the slope of the plots of (1/[LA] ꢂ 1/[LA]0) versus time.
initiator (slope) is 1.75. The noninteger order of 5 could be
attributed to the impurities or aggregation of metal initiators
on growing polymer chains which referred to the previous
literatures. The y-intercept of the regression line 8.38 equals
ln kp, and thus the polymerization rate constant, kp, is 4350
(Cl) (Entries 11, 4, and 9). The reason for the activity differ-
ence is probably due to that the electron-withdrawing group
causes the decreasing of electron density around the metal
and decreasing the bond strength between magnesium and
BnOꢂ group. Therefore, the metal center is less favorable for
the coordination and insertion of LA monomers resulting in a
tardy propagation rate for ring-opening polymerization. Inter-
estingly, increasing the steric hindrance on the R4 position
enhances the reactivity of the magnesium complex with the
order: 6 > 1 (Entries 7 and 2) in which the methyl group is
in the ortho and para position, respectively (Entries 2 and 7).
However, Complex 8 reveals the lowest activity among all of
these complexes probably due to the steric bulky substituents
on the pyrolone inhibiting the coordination of monomer to
metal center and therefore retarding the reaction rate.
Mꢂ2 minꢂ1 or 72.5 Mꢂ2
s
ꢂ1. Therefore, the overall rate
equation is ꢂd[LA]/dt ¼ k[LA]2[5]1.75
.
Furthermore, experiment results reveal that the conversion of
LA is consistent with the rate constant and the activity of
these magnesium complexes is enhanced by the electron-
donating ability on the R3 position of the ligand with the reac-
tivity in the order: 2 > 1 > 3 > 15 > 4 ꢃ 5 (Table 5, Entries
1–6). Similarly, the activity of magnesium complexes is also
enhanced by the electron-donating ability on the R1 position
of the ligand with in the order of 13 (OMe) > 3 (H) > 10
FIGURE 4 (a) Linear plots of (1/[LA] ꢂ 1/[LA]0) versus time demonstrating the second-order dependence on monomer concentra-
tion ([LA]0 ¼ 0.25 M; [5]0 ¼ 3, 4, 5, 6, 7 mM in d-chloroform at 20 ꢀC). (b) Linear plot of ln(kobs) versus ln[5] for the polymerization
of LA with 5 as an initiator. (CDCl3, 20 ꢀC, [LA]0 ¼ 0.25 M) (ln kobs ¼ 8.37 þ 1.75[5], R2 ¼ 0.97).
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TABLE 6 Comparison of Radius and Volume Derived from PGSE Experiment and Those Calculated from X-Ray Crystallographic
Data
PGSEa
X-Ray Crystallographic Datab
Diffusion
coef. (ꢁ10ꢂ10 m2 sꢂ1
Hydrodynamic
3
3
˚
˚
˚
˚
Complexes
)
Radius (A)
Volume (A )
Radius (A)
Volume (A )
Configuration
Bc [L1ZnEt]
6.27–8.21
5.21–5.85
5.42–5.57
5.31–5.87
6.3–4.83
7.6–6.8
7.3–5.6
7.5–6.8
1,062–473
5.1
6.8
7.3
6.7
569
Mononuclear
Dinuclear
Dinuclear
Dinuclear
[L1Mg(l-OBn)]2(1)
[L2Mg(l-OBn)]2(2)
[L5Mg(l-OBn)]2 (5)
a
1,847–1,311
1,643–1,517
1,750–1,296
1,324
1,629
1,269
c
Measured by PGSE NMR experiments in CDCl3 at 25 ꢀC.
See Ref. 24.
b
The packing index (percentage of filled space in the unit cell) was
calculated from the X-ray crystal structure.
Solution Behavior
In eq 1, the DTf is the melting point depressed (ꢀC), i (van’t
Hoff factor) is the number of particles measured/number of
particles added, and Kf (cryoscopic constant) is dependent
on the properties of the solvent, and Cm is the molality of
the solute (m). The i values obtained are 1.007 and 1.115,
respectively for Complexes 5 and 12 when using the naph-
thalene (Kf is 6.9 ꢀC/m) as the solvent. In addition, the i
value of Complex 5 is 1.002 in the benzene agent (Kf is 5.12
ꢀC/m). The results indicated nearly no dissociation of Com-
plexes 5 and 12 at both low temperature (freezing point of
benzene is 5.51 ꢀC) and high temperature (freezing point of
naphthalene is 80.29 ꢀC). Therefore, the dimeric species are
major prꢀoduct in solution at the temperature of polymeriza-
tion (30 C).
The crystal structural studies of [LnMg(l-OBn)]2 indicate
that these magnesium complexes are dinuclear in the solid
state. However, it is still unclear whether the polymerization
proceeds with a dinuclear active species or a mononuclear
species with an equilibrium between dinuclear and mono-
meric species in solution. In order to understand the behav-
ior of these magnesium complexes in solution, PGSE-NMR
experiment studies for 1, 2, and 5 were performed (Table 6).
By comparison the experiment data with that of the mono-
nuclear complex, [L1ZnEt] B, it is believed that the magne-
sium benzylalkoxide complexes [LnMg(l-OBn)]2 remain
dinuclear in solution.
The dinuclear behavior of [LM(l-OBn)]2 in solution was fur-
ther confirmed by the freezing point depression results of
[(L5)Mg(l-OBn)]2 (5) and [(L12)Mg(l-OBn)]2 (12). The disso-
ciation percentage of the dimeric species can be calculated
by the following equation:
Proposed Mechanism
To understand the polymerization process, 1H NMR studies
on the PLLA-50, prepared from a [M]0/[1] ratio of 50, were
carried out as shown in Figure 5. Based on the 1H NMR
spectrum of PLLA-50, the polymer chain is believed to be
capped with a benzyl ester group on one end and a hydroxyl
dTf ¼ i ꢁ Kf ꢁ Cm
(1)
FIGURE 5 1H NMR spectrum of PLLA-50 (50 indicates [LA]0/[1]0 ¼ 50).
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SCHEME 2 Proposed mechanism for polymerization of LA initiated by [L5Mg(l-OBn)]2 (5).
group on the other end with the integration ratio close to
5:1 between Ha and Hd, suggesting that the polymerization
procedures agree with the ‘‘coordination-insertion’’ process
found in other metal alkoxides.25
data, X-ray structural determination, freezing point depression
studies and PGSE-NMR analysis, we suggest that the polymer-
ization intermediate a initiated by the magnesium complexes
maybe exist as dimmer form.
Based on the solid state structure, PGSE-NMR, melting point
depression and kinetic studies, we conclude that the dinu-
clear species [LnMg(l-OBn)]2 is probably the only product
in solution at the temperature of polymerization (30 ꢀC).
Therefore, the mechanism for the polymerization initiated
by magnesium Complex 5 was proposed as shown in
Scheme 2 in which the hemilabile character of the tertiary
amine group of the ketiminate ligand dissociates given four-
coordinated metal center followed by the coordination of
lactide.
ACKNOWLEDGMENTS
The financial support from the National Science Council of the
Republic of China (NSC-98-2113-M-005-001-MY3) is gratefully
appreciated.
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