Preparation and characterization of bisphenolato magnesium derivatives: An efficient catalyst for the ring-opening polymerization of ε-caprolactone and L-lactide

Article abstract of DOI:10.1039/b912443h

A series of magnesium complexes have been prepared and characterized, in which [(EDBP-Me)Mg(μ-OBn)]₂ (4) has shown high activity toward the ring-opening polymerization (ROP) of ε-caprolactone and L-lactide. 2,2′-Ethylidene-bis(4,6-di-tert-butyl

Full text of DOI:10.1039/b912443h

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PAPER
www.rsc.org/dalton | Dalton Transactions
Preparation and characterization of bisphenolato magnesium derivatives: an
efficient catalyst for the ring-opening polymerization of e-caprolactone and
L-lactide†
Min-Yi Shen, Ya-Liu Peng, Wen-Chou Hung and Chu-Chieh Lin*
Received 24th June 2009, Accepted 14th September 2009
First published as an Advance Article on the web 6th October 2009
DOI: 10.1039/b912443h
A series of magnesium complexes have been prepared and characterized, in which
[(EDBP-Me)Mg(m-OBn)]2 (4) has shown high activity toward the ring-opening polymerization (ROP)
of e-caprolactone and L-lactide. 2,2¢-Ethylidene-bis(4,6-di-tert-butylphenol)-monomethyl ether
([EDBP-(Me)H]) is prepared by the reaction of 2,2¢-ethylidene-bis(4,6-di-tert-butylphenol) (EDBP-H2)
with dimethyl sulfate. The reaction of [EDBP-(Me)H] with nBu2Mg yields [(EDBP-Me)MgnBu]2, which
further reacts with benzyl alcohol and N,N-dimethylethylenediamine giving complexes
[(EDBP-Me)Mg(m-OBn)]2 (4) and {[EDBP(Me)]Mg(m-Me2NCH2CH2NH)}2 (5), respectively.
Experimental results indicate that the activity of complex 4 toward cyclic esters is higher than that of
[(m-EDBP)Mg]2/(BnOH).
Introduction
Currently, polyesters such as polylactide (PLA) and poly(e-
caprolactone) (PCL) are of great interest due to their wide
biomedical, pharmaceutical and environmental applications.1 The
most effective method for the preparation of PLA and PCL is
the ring-opening polymerization (ROP) of related cyclic esters,2
where metal alkyls,3 amides,4 thiolates,5 aryl oxides,6 alkoxides7
and enolates8 in the presence or absence of alcohol have been used
as initiators. Among them, single-site metal alkoxides have the
highest activity with a good control over the polymer molecular
weight.7
Chart 1 Structure of (a) bisphenol ligands and (b) EDBP-RTs-H.
bis(4,6-di-tert-butylphenol)] has been investigated. As a result, a
series of EDBP-H2 derivatives, EDBP-RTs-H (Chart 1b), have
been prepared from the reaction of EDBP-H2 with the related
thionyl chloride.13 Experimental results indicate that the activity
of the magnesium complexes can be dramatically increased by
the modification of the bisphenol system. In this research, a
different approach is used in which [EDBP-(Me)H] derived from
EDBP-H2 is synthesized. The catalytic activity of its magnesium
complex, [(EDBP-Me)Mg(m-OBn)]2 towards ROP of LA and
e-CL is investigated. The mechanism for the polymerization of
e-CL initiated by [(EDBP-Me)Mg(m-OBn)]2 will also be discussed.
Metal complexes supported by a variety of ligands such as
b-diketiminate, salen, and many others have been developed and
used as catalytic/initiating systems for ROP of lactide. In addition,
bisphenolate system is also attracting considerable attention,
because it is easily modified and functionalized. As a result, many
bisphenolato ligands (Chart 1a) have been introduced through a
link of various functional groups and their metal complexes have
been proven to be effective initiators.9 Recently, many lithium,
sodium, magnesium, zinc and aluminium bulky bisphenolato
complexes have been used as initiators/catalysts for ROP of lactide
in our laboratory.2d,10 Among these metals, magnesium is non-toxic
and essential for human life,11 therefore, magnesium complexes
are good candidates as catalysts/initiators in the preparation of
biomedical grade PLA. However, the reactivity of magnesium
complexes [LMg(THF)]2 derived from these bisphenol ligands
show only moderate activity in the ROP of LA in the presence
of additional alcohols.12 In order to enhance the activity of mag-
nesium, modification of EDBP-H2 [EDBP-H2: 2,2¢-ethylidene-
Results and discussion
Synthesis and characterization
The monovalent bisphenolato ligand, [2-(2-methoxy-3,5-di-
tert-butylphenyl)-2¢-ethylidene(4,6-di-tert-butylphenol), EDBP-
(Me)H, 1] was prepared in high yield (84%) by the reaction of
2,2¢-ethylidenebis(4,6-di-tert-butylphenol) (EDBP-H2) with one
molar equivalent of potassium carbonate in acetone followed
by the addition of a stoichiometric amount of dimethyl sulfate
(Scheme 1). Due to the replacement of one of the phenol
protons to the methyl group, two aryl rings of compound 1 are
chemically non-equivalent; this phenomenon has been verified by
Department of Chemistry, National Chung Hsing University, Taichung 402,
Taiwan, Republic of China. E-mail: cchlin@mail.nchu.edu.tw; Fax: +886-
4-22862547; Tel: +886-4-22840411
† Electronic supplementary information (ESI) available: Experimental
details. CCDC reference numbers 737433–737435. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b912443h
9906 | Dalton Trans., 2009, 9906–9913
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Scheme 1 Preparation of EDBP-(Me)H and its magnesium complexes.
1H NMR spectroscopic studies. For instance, four resonance peaks
corresponding to aromatic protons (d = 7.28, 7.19, 7.13, 7.12 ppm,
1H each) and four peaks arising from tert-butyl groups (d = 1.38,
1.36, 1.34, 1.23 ppm, 9H each) were observed.
Further reaction of compound 1 with a stoichiometric amount
n
of Bu2Mg in toluene produces {[m-EDBP(Me)]MgnBu}2 (2) in
near quantitative yield. Compound 2 is verified by spectroscopic
studies as well as elemental analysis. Attempts to grow suitable
crystals of 2 for X-ray structure determination were unsuccessful.
Like many other magnesium complexes, compound 2 is rather
thermally unstable and ligand exchange of compound 2 occurs
in a warm hexane (40 ◦C) solution yielding [(EDBP-Me)2Mg]
(3) as crystalline solids. However, compound 3 can also be
n
prepared in high yield from the reaction of Bu2Mg with two
molar equivalents of [EDBP-(Me)H]. The molecular structure of
compound 3 (Fig. 1) shows, it is a homoleptic complex with a
tetra-coordinated Mg atom supported by the oxygen atoms of the
EDBP(Me)- ligand in which there is a 2-fold axis passing through
Fig. 1 Molecular structure of 3 as 20% ellipsoids. All the hydro-
gen atoms and methyls at the t-butyl groups of the EDBP-Me-
ligand are omitted for clarity. Selected bond lengths (A) and an-
gles (deg): Mg–O(1) 1.865(2), Mg–O(2) 2.091(2), O(1)–Mg(1)–O(2)
100.40(10), O(2)–Mg–O(2A) 129.27(17), O(1)–Mg–O(1A) 110.94(14),
O(2)–Mg–O(1A) 107.77(9).
˚
˚
the Mg atom. The Mg–O(2) bond length of 2.091(2) A is longer
˚
than the Mg–O(1) distance 1.865(2) A, attributed to the neutral
coordination bonding from the methoxy group.
the Mg(1)–O(3)–Mg(1A)–O(3A) plane. The longer Mg–O(2) bond
Treatment of compound 2 with two molar equivalents of benzyl
alcohol in toluene at ambient temperature generates a magnesium
benzyl alkoxide, {[EDBP(Me)]Mg(m-OBn)}2 (4) in moderate yield
˚
length [2.080(2) A] than the Mg–O(1) bond length [1.852(2)] is as
expected because of a dative bonding feature of Mg–O(2) similar
to the results found in 3.
1
(47%). H NMR spectroscopic studies reveal the formation of a
{[EDBP(Me)]Mg(m-Me2NCH2CH2NH)}2 (5) is prepared
from the reaction of {[m-EDBP(Me)]MgBu}2 with N,N-
dimethylethylenediamine in a 1:2 ratio with a similar procedure to
that of 4. Single crystals of 5 suitable for structure determination
were obtained from a CH2Cl2 solution. The molecular structure
of compound 5 (Fig. 3) reveals a dimeric feature with 5,4,5
rings in which each of the magnesium atoms coordinates with
the nitrogen atom of the amino group, a nitrogen atom of the
-NMe2 group and the phenoxy oxygen atom. These 5,4,5 rings
form a ladder structure. There is an inversion center lying at
trace amount of 3 along with 4. Compound 3 can be removed
by crystallization of the mixture. Compound 4 is relatively more
thermally stable than complex 2. It is stable even in a warm (40 ◦C)
hexane solution for 6 h. Single crystals of 4 suitable for X-ray
structural determination were obtained from a toluene solution.
The molecular structure shows that complex 4 (Fig. 2) is dimeric
in which each Mg atom adopts a distorted tetrahedral geometry
coordinated to one phenoxy oxygen, one methoxy oxygen and two
bridged oxygen atoms of the benzyl alkoxy groups. The complex
is a C2 symmetric compound which has a 2-fold axis through
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Fig. 3 Molecular structure of 5 as 20% ellipsoids. All the hydro-
gen atoms and methyls at the t-butyl groups of the EDBP-Me- lig-
and are omitted for clarity. Selected bond lengths (A) and angles
Fig. 2 Molecular structure of 4 as 20% ellipsoids. All of the hydrogen
atoms are omitted for clarity. Selected bond lengths (A) and angles
˚
˚
(deg): Mg(1)–O(1) 1.852(2), Mg(1)–O(2) 2.080(2), Mg(1)–O(3) 1.959(2),
Mg(1)–O(3A) 1.943(2). O(1)–Mg(1)–O(2) 110.97(8), O(1)–Mg(1)–O(3)
117.59(9), O(1)–Mg(1)–O(3A) 124.40(9), O(2)–Mg(1)–O(3) 108.35(8),
O(2)–Mg(1)–O(3A) 108.08(8), O(3)–Mg(1)–O(3A) 84.27(9).
(deg): Mg(1)–O(2) 1.836(2), Mg(1)–N(1) 2.076(3), Mg(1)–N(2) 2.167(3),
Mg(1)–N(1A) 2.090(3); N(1)–Mg(1)–N(2) 85.65(13), N(1)–Mg(1)–N(1A)
90.81(12), O(2)–Mg(1)–N(2) 111.97(12), O(2)–Mg(1)–N(1A) 121.90(12),
Mg(1)–N(1)–Mg(1A) 89.19(12).
the Mg(1)–N(1)–Mg(1A)–N(1A) plane, hence the complex is an I
symmetric compound. The coordinated behavior of EDBP(Me)-
in complex 5 differs from the situation found in complex 4.
The molecular structure of 5 shows that in the presence of the
stronger coordination group (-NMe2), the methoxy group of the
EDBP(Me)- ligand swings away from the Mg atom. The result
reveals the hemilabile property of the EDBP(Me) ligand. By
combining the structure studies of compounds 5 and 4, we present
evidence for the hemilabile property of the reduced bisphenolato
(EDBP(Me)) ligand. For a specific requirement, the application
of the hemilabile property in catalyst design would be a good
consideration.14
In the cas◦e of CL, conversion up to 96% can be achieved within
1 h at 25 C. The number average molecular weight (Mn) of the
polymer increases linearly with the increase of the [M]/[4] ratio,
implying a living character of complex 4 (Table 1, entries 1–4;
Fig. 4).
The polydispersity indices (PDIs) of these polymers are quite
low ranging from 1.07 to 1.08. The molecular weight of PCL and
the chemical nature of the structure chain ends are established by
1
1H NMR studies. For example, the H NMR spectrum of PCL-
50 (the number 50 indicates the designed [M]0/[4] ratio) gave an
integral ratio close to 2:2:48 for Hb (-OCH2Ph from PCL at the
benzyl ester chain end), Hg (-CH2 from PCL at the hydroxy end)
and Hf ({-CH2OC(O)-}n), respectively (Fig. 5). The result indicates
that the polymerization is initiated by the insertion of the benzyl
alkoxy group to e-caprolactone. By comparing the molecular
weight of PCL and the [M]/[4] ratio, it can be concluded that
both benzyl alkoxy groups act as initiators in the polymerization.
ROP of L-lactide (L-LA) initiated by 4 was also performed.
Experimental results show that complex 4 is a highly efficient
initiator for the polymerization of L-LA. Conversion up to 99%
can be achieved within 10 min at 0 ◦C. Linearity between the
molecular weight of PLLA and the monomer/initiator ratio is
ROP of e-caprolactone and L-lactide initiated by
{[EDBP(Me)]Mg(l-OBn)}2
The catalytic activities of complex 4 towards ROP of e-
caprolactone (CL) and L-lactide (LLA) have been systematically
studied and the experimental results are listed in Table 1. Con-
1
version of CL and L-LA is determined based on the H NMR
spectroscopic studies. The molecular weight and polydispersity
of poly(L-lactide) (PLLA) and PCL are measured by both gel-
1
permission chromatography (GPC) and H NMR spectroscopy.
Table 1 Ring-opening polymerization of e-caprolactone and L-lactide initiated by 4
entry
Monomer
[M]:[4]
Mn (Calc.)a
Mn (NMR)b
Mn (GPC)c
PDIc
t (min)
T (◦C)
Conv. (%)
1
2
3
4
5
6
7
8
9
e-CL
e-CL
e-CL
e-CL
L-LA
L-LA
L-LA
L-LA
L-LA
50:1
2800
5500
7800
11000
3700
7200
9800
14400
7200
3000
6200
8700
13000
9300
6100
1.07
1.07
1.08
1.08
1.09
1.07
1.06
1.18
1.17
60
60
60
60
10
10
10
15
25
25
25
25
0
0
0
94
95
90
96
>99
>99
90
>99
>99
100:1
150:1
200:1
50:1
100:1
150:1
200:1
50(50):1
12800
17000
23500
13800
28400
38000
50000
29900
15900
22000
d
—
0
0
16300
10(10)
1
a Calculated from the molecular weight of monomer times [M]0/2[4]0 times the conversion plus the molecular weight of BnOH. b Obtained from H
NMR analysis. c Obtained from GPC analysis which were calibrated with a polystyrene standard. The differences between theoretical calculated and
GPC analyzed values could be illustrated in the literature. The corrected Mn of PLLA would be obtained by multiplying the value obtained from GPC
by 0.58 and 0.62 for PCL.15 d Not available.
9908 | Dalton Trans., 2009, 9906–9913
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Fig. 4 Polymerization of cyclic esters initiated by 4.
Fig. 5 1H NMR spectrum of PCL-50 in CDCl3.
observed (Table 1, entries 5-8; Fig. 4). The living character can be
further verified by the stepwise addition of L-lactide (entry 9). It
is interesting to note that the molecular weight of PLLA is twice
that expected indicating that only one of the two benzyl alkoxy
groups are active in the polymerization. A similar result has been
observed in the [(EDBP-RTs)Mg(OBn)]2 system. However, the
reason for the difference between the polymerization of CL and
L-LA is still unknown. By comparing the activity of complex
4 with our earlier reported [(m-EDBP)Mg]2/BnOH system,12 the
activity of 4 is highly enhanced by the modification of the divalent
EDBP2- ligand to a monovalent EDBP-Me- ligand. This difference
is probably attributed to two situations. One cause is the higher
activity of the BnO- group than that of the BnOH, the other is the
weak coordination ability of the methoxy group to act as a switch
during polymerization, therefore, stabilizing the intermediate.
Proposed mechanism for the ROP of e-caprolactone initiated by
{[EDBP(Me)]Mg(l-OBn)}2
With the previous illustrations, the methoxy group of [EDBP-
(Me)-] leaves the Mg atom when a stronger coordination ligand
is in the environment. By considering this hemilabile property of
EDBP(Me) in complex 4, we propose the mechanism of the ROP
process of PCL as shown in Scheme 2. In the first step, the methoxy
group of EDBP(Me) dissociates from the Mg atom, and then the
oxygen atom of the carbonyl group of the cyclic ester coordinates
to the Mg atom. Secondly, the alkoxide attacks the carbon atom
of the carbonyl group via an insertion reaction, opening the ring
and forming a new alkoxide that contains the benzyl group in
the end chain. Finally, in the third step, the methoxy group
of EDBP(Me) re-coordinates to the Mg atom, stabilizing the
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Scheme 2 Proposed mechanism for the ROP of e-caprolactone.
complex. Repeating the cycle, propagation continues until all of the
monomer is consumed, producing a polyester pendant on the Mg
atom. Polyester would be obtained by terminating the reaction.
˚
Phenogel 5 m 103 A and the calibration curve used to calculate Mn
(GPC) was produced from polystyrene standards. The GPC results
were calculated using the SISC chromatography data solution 1.0
edition.
Conclusions
Preparation of 2,4-di-tert-butyl-6-(1-(3,5-di-tert-butyl-2-
Three novel magnesium complexes of a monovalent bisphenolato
ligand have been synthesized and fully characterized by spectro-
scopic methods as well as XRD structural determination. The
result of the catalytic ROP reaction indicates that complex 4
is a better complex than [(EDBP)Mg(THF)]2 for this purpose.
Combining the characterization of complexes 4 and 5 reveals
the hemilabile property of the ligand EDBP(Me). This property
enables the complex to act as a switch for controlled ligand
coordination and stabilizes the intermediate.
methoxyphenyl)ethyl)phenol [EDBP-(Me)H] (1)
A mixture of EDBP-H2 (0.88 g, 2 mmol) and potassium carbonate
(0.28 g, 2 mmol) in acetone (40 mL) was stirred at room
temperature for 0.5 h. Dimethyl sulfate (0.38 g, 3 mmol) was then
added and the resulting mixture was stirred for another 48 h. The
mixture was filtered and the filtrate was dried in vacuo giving white
powder. The white powder was redissolved in hot hexane (50 mL)
and dried over MgSO4. The hexane solution was then concentrated
to ca. 30 mL and was cooled to -20 ◦C. White crystalline solid was
obtained after 1 day at -20 ◦C. Yield: 0.76 g (84%). Anal. Cacld
for C31H48O2: C, 82.24; H, 10.69%. Found: C, 82.00; H, 9.92%.1H
NMR (CDCl3, ppm): d 7.28, 7.19, 7.13, 7.12 (s, 4H, Ph-H); 4.52
(q, 1H, HCCH3 J = 7.2 Hz); 3.96 (s, 3H, OCH3); 1.76 (d, 3H,
Experimental
General conditions
HCCH3, J = 7.2 Hz); 1.38, 1.34, 1.36 1.23 (s, 36H, C(CH3)3). 13
C
All manipulations were carried out under a dry nitrogen atmo-
sphere and all glassware was flame-dried under vacuum before
use. Solvents were dried by refluxing for at least 24 h over
sodium/benzophenone (hexane, toluene and tetrahydrofuran) or
phosphorus pentoxide or anhydrous magnesium sulfate (benzyl
alcohol) and freshly distilled prior to use. Deuterated solvents
NMR (CDCl3, ppm): d 152.7, 150.5, 146.9, 141.6, 141.4, 137.9,
135.5, 131.4, 123.0, 122.3, 119.9, 114.8 (Ph); 63.7 (OCH3); 35.3,
35.0, 34.6, 34.4 (C(CH3)3); 31.7, 31.4, 30.3, 29.7 (C(CH3)3); 29.9
(Ph2CCH3); 20.3 (Ph2CCH3). Mass spectrum (EI, m/z): 452.6
(M+). Mp: 169–171 ◦C. IR (KBr, cm-1): 3510(s), 2963(s), 2868(s),
1767(m), 1599(m), 1456(s), 1427(s), 1392(m), 1361(s), 1321(m),
1289(s), 1224(s), 1154(m), 1133(m), 1108(m), 1065(m), 988(s),
932(m), 904(m), 881(m), 811(m), 799(br), 769(br).
˚
and e-caprolactone were dried over 4 A molecular sieves. L-
lactide is purified from the recrystallization of the toluene solution.
MgnBu2 (1.0 M in heptane), N,N-dimethylethylenediamine, 2,2¢-
ethylidenebis(4,6-di-tert-butyl phenol) (EDBP-H2), potassium
carbonate, dimethyl sulfate and acetic acid were purchased and
used without further purification. 1H and 13C NMR spectra were
Synthesis of [l-(EDBP-Me)MgBu]2 (2)
1
recorded on a Varian Mercury-400 (400 MHz for H, 100 MHz
MgnBu2 (2.20 mL, 1.0 M in heptane, 2.20 mmol) was added
◦
for 13C) spectrometer with chemical shifts given in ppm referenced
to TMS as internal standard. Infrared spectra were obtained
from a Bruker Equinox 55 spectrometer. Microanalyses were
performed using a Heraeus CHN-OS-RAPID instrument. The
electron impact mass spectrum (EIMS) was obtained using a
JEOL JMS-SX/SX 102A mass spectrometer. GPC measurements
were performed on a Hitachi L-7100 system equipped with a
differential Bischoff 8120 RI detector using THF (HPLC grade)
as an eluent. The chromatographic column was Phenomenex
slowly to an ice cold (0 C) toluene solution (15 mL) of EDBP-
(Me)H (0.905 g, 2.0 mmol). The mixture was stirred for 8 h
while the temperature was increased to room temperature slowly.
The volatile materials were removed under vacuum yielding white
solid. The residue was then dissolved in toluene (10 mL) and
R
filtered through Celiteꢀ. The filtrate was concentrated to ca.
5 mL. Colorless crystals were obtained at -20 ◦C after 24 h.
Yield: 0.981 g (92%). Anal. Calcd for C70H112Mg2O4: C, 78.85;
1
H, 10.59%. Found: C, 77.35; H, 9.64%. H NMR (C6D6, ppm):
9910 | Dalton Trans., 2009, 9906–9913
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R
d 7.95, 7.79, 7.51, 7.41 (s, 8H, Ph-H); 5.25 (q, 2H, HCCH3 J =
6.8 Hz); 3.63 (s, 6H, OCH3); 1.61 (d, 6H, HCCH3, J = 6.8 Hz);
1.72, 1.44, 1.36, 1.35 (s, 72H, C(CH3)3); 0.63–1.38 (m, 14H,
CH2CH2CH3); -0.34 (m, 4H, MgCH2). 13C NMR (C6D6, ppm):
d 154.1, 149.9, 149.1, 141.0, 140.9, 139.7, 139.5, 133.9, 126.5,
124.4, 123.4, 122.4 (Ph); 66.28 (OCH3); 36.5, 35.7, 34.9, 34.6
(C(CH3)3); 33.3, 32.5, 32.0, 31.6 (C(CH3)3); 31.3 (CCH3); 30.2
(CCH3); 26.3 (MgCH2CH2CH2CH3); 14.6 (MgCH2CH2CH2);
13.9 (MgCH2CH2); 1.4 (MgCH2). Mp: 183-185 ◦C.
dissolved in CH2Cl2 (10 mL) and filtrated through Celiteꢀ. The
resulting solution was concentrated to ca. 5 mL and kept at
room temperature. After 5 days, colorless crystals were obtained.
Yield: 0.521 g (46%). Anal. Calcd for C31H48O2: C, 82.24%; H,
1
10.69%. Found: C, 82.00%; H, 9.92%. H NMR (CDCl3, ppm):
d 7.44–6.86 (m, 8H, Ph-H); 4.71 (q, 2H, HCCH3 J = 6.4 Hz);
3.96 (s, 6H, OCH3); 2.64 (br, 4H, CH2CH2NMe2); 2.32 (br, 4H,
CH2CH2NMe2); 2.15 (br, 12H, CH2CH2N(CH3)2); 1.76 (d, 6H,
CCH3, J = 6.4 Hz); 1.38, 1.33, 1.30, 1.13 (s, 72H, C(CH3)3); 0.88
(br, 2H, NH).
Synthesis of [EDBP-Me]2Mg (3)
Polymerization of e-caprolactone initiated by 4
Method A. [m-EDBP(Me)]MgBu}2 (0.958 g, 1.8 mmol) was
dissolved in hot (40 ◦C) hexane (50 mL) and was filtered
A typical polymerization procedure was exemplified by the
synthesis of PCL-50 (the number 50 indicates the designed
R
through Celiteꢀ. The resulting solution was then kept in room
◦
temperature. Colorless thin crystals were obtained after 45 days.
Yield: 0.684 g (82%). Method B. Mg(nBu)2 (1.1 mL, 1.0 M in
heptane, 1.1 mmol) was added slowly to an ice cold toluene (15 mL)
solution of [EDBP-(Me)H] (1) (0.905 g, 2.0 mmol). The mixture
was stirred for 1 h while the temperature was increased slowly to
room temperature and the mixture was stirred for another 7 h. The
volatile materials were removed under vacuum giving a colorless
solid. The residue was recrystallized from a mixed hexane (50 mL)
and diethyl ether (2 mL) solution. Yield: 0.751 g (90%). Anal.
Calcd for C62H94MgO4: C, 80.27; H, 10.21%. Found: C, 80.38; H,
9.99%. 1H NMR (CDCl3, ppm): d 7.59–6.81 (m, 8H, Ph-H); 4.90
(q, 2H, HCCH3 J = 7.2 Hz); 4.48 (s, 6H, OCH3); 1.65 (d, 6H,
[CL]0/[4]0) at 25 C (Table 2, entry 1). The conversion (94%) of
PCL-50 was analyzed by 1H NMR spectroscopy. e-Caprolactone
(0.263 mL, 2.5 mmol) was added to a rapidly stirred solution of
{[EDBP(Me)]Mg(m-OBn)}2 (4) (583 mg, 0.05 mmol) in CH2Cl2
◦
(20 mL). The mixture was stirred at 25 C for 1 h during which
the viscosity of the reaction mixture increases. The reaction was
then quenched by the addition of an aqueous acetic acid solution
(0.35 N, 10 mL), and PCL was precipitated out as a white solid on
pouring the mixture into n-hexane (100 mL). The resulting solid
was filtered and washed twice with hexane and then dried under
vacuum. Yield: 0.249 g (92%).
HCCH3, J = 7.2 Hz); 1.59, 1.55, 1.31, 1.25 (s, 72H, C(CH3)3). 13
C
Polymerization of L-lactide initiated by 4
NMR (CDCl3, ppm): d 159.1, 149.5, 149.3, 141.0, 140.9, 136.1,
134.5, 131.4, 126.2, 124.4, 121.1, 119.4 (Ph); 65.65 (OCH3); 36.2,
34.8, 34.0, 33.9 (C(CH3)3); 32.9, 31.9, 31.7, 31.4 (C(CH3)3); 29.4
(Ph2CCH3); 22.7 (Ph2CCH3). Mp: 256–258 ◦C.
A typical polymerization procedure was exemplified by the syn-
thesis of PLA-50 (the number 50 indicates the designed [LA]0/[4]0)
at 0 ◦C (Table 2, entry 5). The conversion (>99%) of PLA-50 was
analyzed by 1H NMR spectroscopy. To a rapidly stirred solution
of {[EDBP(Me)]Mg(m-OBn)}2 (4) (583 mg, 0.05 mmol) in CH2Cl2
(20 mL) was added a mixture of L-lactide (0.36 g, 2.5 mmol) and
benzyl alcohol (1.0 mL, 0.10 mmol). After 10 minutes stirring, the
increase of viscosity was observed in the reaction mixture. The
reaction was quenched by the addition of an aqueous acetic acid
solution (0.35 N, 10 mL), and the polymer was precipitated out as
white solid on pouring the mixture into n-hexane (100 mL). Yield:
0.257 g (71%).
Synthesis of {[EDBP(Me)]Mg(l-OBn)}2 (4)
Benzyl alcohol (0.21 mL, 2.0 mmol) was added slowly to an ice
cold solution (0 ◦C) of {[m-EDBP(Me)]MgBu}2 (1.06 g, 1.0 mmol)
in toluene (15 mL). After the ice bath was removed, the mixture
was stirred at room temperature for 6 h and was then evaporated
to dryness under vacuum. The residue was dissolved in warm
◦
R
toluene (40 C, 10 mL) and filtered through Celiteꢀ. Colorless
crystals were obtained from concentrated solution (5 mL) at room
temperature after three days. Yield: 0.543 g (46.6%). Anal. Calcd
for C76H108Mg2O6: C, 78.27; H, 9.33%. Found: C, 77.86; H, 8.74%.
1H NMR (C6D6, ppm): d 7.62–6.97 (m, 18H, Ph-H); 5.12, 5.06 (dd,
4H, OCH2Ph, J = 14 Hz); 3.74 (q, 2H, HCCH3 J = 7.2 Hz); 3.63
(s, 6H, OCH3); 1.63 (d, 6H, HCCH3, J = 7.2 Hz); 1.82, 1.71, 1.37,
0.97 (s, 72H, C(CH3)3). 13C NMR (C6D6, ppm): d 159.3, 150.4,
149.2, 144.0, 141.0, 139.7, 135.9, 135.24, 132.8, 129.0, 127.1, 126.1,
126.0, 124.0, 121.5, 120.0 (Ph); 66.6 (OCH2Ph); 63.6 (OCH3); 35.9,
35.8, 35.7, 34.5 (C(CH3)3); 32.8, 32.6, 31.0, 30.8 (C(CH3)3); 30.6
(Ph2CCH3); 22.9 (Ph2CCH3). Mp: 202–204 ◦C (dec.).
X-ray crystallographic studies
Suitable crystals of 3, 4 and 5 were sealed in thin-walled glass
capillaries under a nitrogen atmosphere and mounted on a
Bruker AXS SMART 1000 diffractometer. Intensity data were
collected in 1350 frames with increasing w (width of 0.3◦ per
frame). The absorption correction was based on the symmetry-
equivalent reflections using the SADABS program. The space
group determination was based on a check of the Laue symmetry
and systematic absences and confirmed by using the structure
solution. The structure was solved by direct methods or Patterson
methods using an SHELXTL package. All non-H atoms were
located from successive Fourier maps, and hydrogen atoms were
refined using a riding model. Anisotropic thermal parameters
were used for all non-H atoms, and fixed isotropic parameters
were used for H atoms. One solvated ether molecule, one solvated
toluene molecule and two CH2Cl2 molecules were observed to co-
recrystallize with complexes 3, 4 and 5, respectively. Due to the
Synthesis of {[EDBP(Me)]Mg[l-(CH3)2NCH2CH2NH]}2 (5)
N,N-Dimethylethylenediamine (0.22 mL, 2.0 mmol) was added
slowly to {[m-EDBP(Me)]MgBu}2 (1.06 g, 1.0 mmol) in toluene
(20 mL) at 0 ◦C. The ice bath was removed and the solution
was warmed to room temperature and stirred for 8 h. All volatile
materials were removed under vacuum, and the residue was
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Table 2 Crystallographic data of complexes 3–5a
3·Et2O
4·2C7H8
5·2CH2Cl2
Empirical formula
FW
Temp (K)
C66 H104 Mg O5
1001.8
293
C90 H124 Mg2 O6
1350.51
293
C72 H120 Cl4 Mg2 N4 O4
1296.4
293
Cryst. syst.
Space group
Monoclinic
C2/c
26.224(4)
14.652(2)
18.448(3)
90
105.519(3)
90
6829.5(19)
4
0.974
0.067
2208
19352
6754 [R(int) = 0.1032]
6754/5/313
0.0786
0.2071
1.022
Monoclinic
C2/c
18.354(3)
14.909(3)
30.879(5)
90
100.342(4)
90
8313(2)
4
1.079
0.079
2944
21853
8057 [R(int) = 0.0683]
8057/1/430
0.0755
0.2081
1.152
Triclinic
P-1
˚
a/A
10.566(2)
10.567(2)
18.362(4)
84.166(5)
80.160(4)
77.437(5)
1967.3(8)
1
1.094
0.211
704
10833
7631 [R(int) = 0.0272]
7631/1/389
0.0766
0.2167
1.050
˚
b/A
˚
c/A
a/deg
b/deg
g /deg
3
˚
Volume/A
Z
Density (calcd)/Mg/m3
Abs. coeff./mm-1
F(000)
No. of reflns collected
No. of indep reflns
No. of data/restraints/params
R1b
wR2c
GoFd
-3
˚
Min., max. residual density e A
0.377 and -0.310
0.338 and -0.272
0.394 and -0.564
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
a With disordered solvent contribution. b R1 = | (|Fo| - |Fc|)/ |Foꢀ. c R2 = { [w(Fo - Fc2)2]/ [W(Fo2)2]}1/2, w = 0.10. d GoF = [ w(Fo
-
2
2
Fc2)2/(Nreflns - Npapams)]1/2
.
disorder of these solvent molecules, their bond distances and bond
angles are not reliable. Crystallograhic data of complexes 3–5 are
summarized in Table 2.
6 M. H. Chisholm, J. C. Gallucci and H. S. Zhen, Inorg. Chem., 2001, 40,
5051.
7 Many alkoxides have been reported to initiate ROP of lactide, and been
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Acknowledgements
Financial support from the National Science Council (NSC95-
2113-M-005-016-MY3 and the Ministry of Economic Affairs
(97-EC-17-A-S1-111) of Taiwan, Republic of China is gratefully
appreciated.
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