Comparative Study of lactide polymerization with lithium, sodium, magnesium, and calcium complexes of BHT

Article abstract of DOI:10.1021/om300121c

A series of Li, Na, Mg, and Ca complexes with 2,6-di-tert-butyl-4- methylphenol (BHT) as the ligand has been synthesized, and their reactivity for the ring-opening polymerization of lactide has been studied. The Ca complex with 2,2′-ethylidenebis(4,6-di-tert-butylphenol) (EDBP) as the ligand also has been synthesized to compare with the BHT systems. All complexes, in the presence of benzyl alcohol as initiator, exhibit high activity for the ring-opening polymerization of lactide. Polymerization activities follow the order of Na ? Li > Ca ? Mg. Additionally, metal-BHT systems are more efficient than metal-EDBP systems, and this superiority was pronounced in THF solution. Four model metal-BHT complexes with coordinated dimethoxyethane (DME) have been characterized by single-crystal X-ray diffraction. These complexes exhibited a wide variation in the C_ipso-O-metal angle (159.3° to 180.0°), and DFT calculations suggested that this flexibility allows the metal to vary its electron density and thereby expedite the catalytic cycle that requires both monomer activation and substrate lability.

Full text of DOI:10.1021/om300121c

Article
pubs.acs.org/Organometallics
Comparative Study of Lactide Polymerization with Lithium, Sodium,
Magnesium, and Calcium Complexes of BHT
Hsuan-Ying Chen,†,‡ Laurent Mialon,† Khalil A. Abboud,† and Stephen A. Miller*,†
†The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, University of Florida, Gainesville,
Florida 32611-7200, United States
‡Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 80708, Taiwan, R.O.C.
S
* Supporting Information
ABSTRACT: A series of Li, Na, Mg, and Ca complexes with 2,6-di-tert-
butyl-4-methylphenol (BHT) as the ligand has been synthesized, and their
reactivity for the ring-opening polymerization of lactide has been studied.
The Ca complex with 2,2′-ethylidenebis(4,6-di-tert-butylphenol) (EDBP)
as the ligand also has been synthesized to compare with the BHT systems.
All complexes, in the presence of benzyl alcohol as initiator, exhibit high
activity for the ring-opening polymerization of lactide. Polymerization
activities follow the order of Na ≫ Li > Ca ≫ Mg. Additionally, metal−
BHT systems are more efficient than metal−EDBP systems, and this
superiority was pronounced in THF solution. Four model metal−BHT
complexes with coordinated dimethoxyethane (DME) have been
characterized by single-crystal X-ray diffraction. These complexes exhibited a wide variation in the Cipso−O−metal angle
(159.3° to 180.0°), and DFT calculations suggested that this flexibility allows the metal to vary its electron density and thereby
expedite the catalytic cycle that requires both monomer activation and substrate lability.
Scheme 1. A Multitude of Metal Complexes Have Been
Investigated for the Ring-Opening Polymerization (ROP) of
Lactide to Form Polylactic Acid (PLA)
INTRODUCTION
■
Because their synthesis and processing are both established and
inexpensive, petroleum-based plastics in commodity applica-
tions are extensively used in nearly every region of the world.
However, their functional success has coincided with serious
pollution of the ecosystem. Additional concern is warranted
because the availability of these materials is subject to
petrochemical shortages or price fluctuations. These conditions
stimulate chemists to investigate petroleum-free and biode-
gradable materials, such as polylactic acid (PLA), which is
currently one of the leaders among biobased thermoplastics.
PLA is a multiuse, biodegradable, aliphatic polyester derived
from 100% renewable natural resources, such as corn or beets.
PLA shows great promise in a wide range of commodity
applications, including thermoplastics, films, and fibers. The
biorenewability, biodegradability, and biocompatibility of PLA
have contributed to its increasing number of applications, many
of which are in the medical field.1
PLA can be made by direct condensation of lactic acid or the
ring-opening polymerization (ROP) of lactide. Because the
direct condensation route is an equilibrium reaction, the
elimination of small amounts of water during the late stages of
polymerization usually limits the ultimate molecular weight
attainable by this approach. Thus, most attention has been
concentrated on the ring-opening polymerization, and many
metal complexes (e.g., Al,2 Li,3 Na,4 Mg,5 Ca,6 Fe,7 Sn,8 Zn,9
and lanthanide10) have been used as initiators/catalysts for the
ROP of lactide (Scheme 1). Al and Sn complexes are common
ROP catalysts because of their stability toward air and moisture;
however, Al complexes often exhibit low activity and Sn is
cytotoxic. Because residual catalyst levels are typically quite low,
metals, such as Sn, are difficult to remove from the resultant
polymer. Efforts to address these activity and cytotoxicity issues
have resulted in recent lithium, sodium, magnesium, calcium,
and highly active metal-free catalystsincluding triazabicyclo-
decene (TBD)11 and N-heterocyclic carbene (NHC)11b,12
organocatalyststhat are competent for the ROP of lactide.
In addition to metal toxicity, concerns about ligand toxicity
have also been addressed. For example, the Lin research group
has developed a series of catalysts by using 2,2′-ethylidenebis-
(4,6-di-tert-butylphenol) (EDBP-H2), which has been approved
by the U.S. Food and Drug Administration as an indirect food
additive, namely, as a polymer antioxidant in food packaging.13
Complexes of this ligand with Li,3b Na,4 and Mg5d are effective
catalysts for the controlled ROP of lactide. Undesirable side
Received: February 14, 2012
Published: July 19, 2012
© 2012 American Chemical Society
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reactions, such as backbiting and transesterification, can be
minimized through the use of sterically bulky ligands, such as
EDBP.
possibility of improving polymerization control while main-
taining high activity. Additionally, a Ca EDBP complex is
prepared for direct comparison to the BHT analogue so that
the two competing ligand strategies can be directly compared
for the same metal. Importantly, butylated hydroxytoluene
(BHT) is generally regarded as safe (GRAS) by the U.S. Food
and Drug Administration; it is one of the most common food
antioxidants and is permitted for direct addition to food.14
Okuda’s group also developed aluminum alkoxides2h (Chart
1) by employing 2,2′-methylenebis(6-tert-butyl-4-methylphe-
Chart 1. Previous Aluminum Complexes with MBMP and
BHT Ligands2h
RESULTS AND DISCUSSION
■
Complex Synthesis and Crystal Structure Determi-
nation. The reactions of 2,6-di-tert-butyl-4-methylphenol
n
n
(BHT-H) with BuLi, NaH, Bu2Mg, and (((CH3)3Si)2N)2Ca
in tetrahydrofuran (THF) produced [BHTLi·THF]2 (1),
[BHTNa·THF]3 (2), BHT2Mg·THF2 (3), and BHT2Ca·THF3
(4), respectively, as shown in Scheme 2. For investigating the
polymerization mechanism, the foregoing complexes (1−4)
were dissolved in 1,2-dimethoxyethane (DME) to substitute
THF and obtain crystals of [BHTLi·DME]2 (6),
BHTNa·DME2 (7), BHT2Mg·DME (8), and BHT2Ca·DME2
(9), respectively. Additionally, EDBPCa·THF2 (5) was
synthesized by adding EDBP-H2 to a mixture of calcium iodide
and NaN(Si(CH3)3)2 (Scheme 3) in order to compare with 4.
Complexes 115 and 316 are known and have been subjected
to previous structural determination by single-crystal X-ray
structure analysis. Here, we report X-ray structural character-
ization of complexes 2, 4, 6, 7, 8, and 9. The analysis revealed
that complex [BHTNa·THF]3 (2) consists of three symmetry-
related [BHTNa·THF] units bridged by the oxygen atoms of
nol) (MBMP) and 2,6-di-tert-butyl-4-methylphenol (BHT) as
ligands. For ε-caprolactone polymerization, the BHT complex
was more active than the MBMP complex, but it was also less
controlled, as judged by broader molecular weight distributions
and inconsistent polymerization efficiencies. The authors
postulated that initiation might occur at both Al-CH3 and Al-
OCH(CH3)2. A trade-off generally exists between greater
polymerization control with multidentate and sterically bulky
ligands (such as EDBP) and greater polymerization activity
with monodentate ligands (such as BHT).
This work targets the preparation of a series of Li, Na, Mg,
and Ca complexes bearing the BHT ligand to assess the
Scheme 2. Synthesis of BHT−Metal−THF Complexes 1−4 and BHT−Metal−DME Complexes 6−9
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Scheme 3. Synthesis of EDBPCa·THF2 (5)
phenoxide groups, forming a six-membered ring, as shown in
Figure 1. Oxygen atoms of phenoxide (O1) and THF (O2) are
Figure 2. BHT2Ca·THF3 (4) with 50% probability ellipsoids.
Figure 3. [BHTLi·DME]2 (6) with 50% probability ellipsoids.
unit with approximate mirror-image stereochemistry at sodium
(Figure 4). The apical oxygens around each pentacoordinate
Figure 1. [BHTNa·THF]3 (2) with 50% probability ellipsoids.
sp2-hybridized because the sums of the angles are 360.0° and
356.6°, respectively. Sodium atom Na1A showed a similar
trigonal-planar arrangement since the sum of the angles was
359.9°. Sodium also exhibits apparently weak bonding to the
phenoxide ipso-carbon (C1 attached to O1) with a Na−C
distance of 2.744 Å.
X-ray structural analysis revealed that complex
BHT2Ca·THF3 (4)17 is a trigonal bipyramid with two BHT
ligands (O1 and O2) and one THF ligand (O3) occupying the
equator and two THF ligands (O4 and O5) residing in axial
positions, as shown in Figure 2. The axial O4−Ca−O5 angle is
essentially linear at 177.6°, and the equatorial O1−Ca−O2
angle is unusually wide at 151.8° because of the steric bulk of
the BHT ligands, the π systems of which choose to occupy the
same plane. The Cipso−O−Ca angles are essentially linear at
174.4° and 178.0°.
X-ray single-crystal structure analysis of [BHTLi·DME]2 (6)
showed a dimeric nucleus with tricoordinate lithium centers
bridged via the BHT oxygen atoms (Figure 3). Perhaps because
of the steric bulk of the BHT ligands, the DME ligand on each
lithium atom failed to chelate, leaving ethereal functionality free
in the crystal. The BHT (O1) and bound DME (O2) oxygens
showed sp2 hybridization with angle sums of 355.5° and 359.6°,
respectively.
Figure 4. BHTNa·DME2 (7) with 50% probability ellipsoids.
sodium atom derive from different DME ligands. The
remaining DME oxygens occupy equatorial positions, along
with the BHT oxygen. The apical O−Na−O angles are 142.1°
and 141.1° for each structure, and the sums of the equatorial
angles are 359.7° and 360.0°. Interestingly, one of the Cipso
−
O−Na angles is linear at 180.0°, while the other is bent at
165.8°. This solid-state observation suggests that a phenoxide
ligand has a somewhat shallow energy potential to convert
between oxygen sp and sp2 hybridization. The high polymer-
ization activities exhibited by metal−BHT complexes may be
related to this electronic versatility at sodium, insofar as the
The molecular structure of BHTNa·DME2 (7) showed two
slightly different distorted trigonal pyramids in the asymmetric
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lability of solvent, monomers, or initiators is increased. This
phenomenon is discussed in greater detail in the polymerization
section below.
The magnesium complex BHT2Mg·DME (8) is essentially
tetrahedral at Mg with a BHT−Mg−BHT coordination angle
of 120.8° and an O−Mg−O coordination angle for chelated
DME of 78.6° (Figure 5). Note that the aryl BHT ligands of
sion) and the number-average molecular weight (Mn), and
the consistently low polydispersity indices (PDIs) of 1.11−1.23
(Figure 7), in addition to the linear relationship of (1/[lactide]
− 1/[lactide]0) versus reaction time, indicating rate =
kobs[lactide]2 (see the Supporting Information). When the
concentration of catalyst 1 was increased to 1.5 mM (Table 1,
entry 6), the PDI was substantially broader at 1.49. When
unpurified LA was employed, the PDI was also quite broad
(1.45) owing primarily to a low Mn, likely caused by impurities,
such as water. In comparison with the chelated lithium complex
3b
EDBPLi·THF3 (Table 1, entry 5 vs entry 9), compound 1
showed a somewhat higher activity, as measured by monomer
conversion. In a toluene/THF mixed-solvent system (entries 8
and 10), it appears that THF is deleterious to the measured
conversion.
The ROP of L-lactide (LA) catalyzed by sodium complex 2
(1.0 mM) employing benzyl alcohol (BnOH) (10.0 mM) as an
initiator has been systematically examined in toluene (20 mL)
at room temperature, as shown in Table 2. The results indicate
that complex 2 was an efficient catalyst, reaching conversions of
>90% within 1 h at room temperature when the [LA]0/
[BnOH]0 quotient ranged from 15 to 120 (Table 2, entries 1−
9). Polymerization control is demonstrated by the linear
relationship between FWlactide([LA]0/[BnOH]0)(% conver-
sion) and Mn, and the consistently low polydispersity indices
(PDIs) of 1.09−1.34 (Figure 8), in addition to the linear
relationship of (1/[lactide] − 1/[lactide]0) versus reaction
time, indicating a rate = kobs[lactide]2 (see the Supporting
Information). When the concentration of 2 was increased to 1.5
mM (Table 2, entry 10), the Mn obtained by GPC trended
smaller compared with the Mn obtained by NMR; still, the
polydispersity index (1.30) was reasonably low. When
unpurified lactide was employed (Table 2, entry 11), the Mn
obtained by GPC was significantly smaller than the calculated
value, possibly because of impurities, such as water. Compared
to the chelated sodium complex EDBPNa(MeOH)2THF2
(Table 2, entry 8 vs entry 13),4 compound 2 exhibited
somewhat higher reactivity. In THF, the reaction rate
discrepancy between these two catalysts was much larger
(Table 2, entry 12 vs entry 14), about a factor of 30.
Figure 5. BHT2Mg·DME (8) with 50% probability ellipsoids.
this divalent metal are not coplanar, but instead adopt a
propeller-like structure. This contrasts the structure of divalent
calcium reported above (4, Figure 2), which shows aryl/aryl
coplanarity despite having a higher coordination number of 5.
The Cipso−O−Mg angles show significant deviation from
linearity at 167.2° and 160.9°.
X-ray structural analysis revealed that complex
BHT2Ca·DME2 (9) is a distorted octahedron bearing two
BHT ligands with a compressed axial angle (O1−Ca−O2) of
149.6° and two DME ligands in approximate equatorial
positions, as shown in Figure 6. One of the Cipso−O−Ca
The ROP of L-lactide (LA) catalyzed by magnesium complex
3 (3.0 mM) employing benzyl alcohol (BnOH) (10.0 mM) as
an initiator has been systematically examined in toluene (20
mL) at room temperature, as shown in Table 3. The results
indicate that compound 3 was an unusual catalyst because only
half of the BnOH functioned as an initiator (Mn calcd ≈ 0.5 Mn
NMR). It is possible that a certain fraction of the alcohol served
to stabilize the Mg complex and was thus incapacitated as an
Figure 6. BHT2Ca·DME2 (9) with 50% probability ellipsoids.
1
initiator. H NMR evidence suggests this to be the case since
addition of up to 4 equiv of BnOH to complex 3 shows no free
BnOH, but instead signals for both strongly and weakly
coordinated BnOH (see the Supporting Information).
Conversions reached >90% within 16 min at room temperature
while the [LA]0/[BnOH]0 quotient ranged from 5 to 120
(Table 3, entries 1−8). Polymerization control was demon-
strated by a linear relationship between FWlactide([LA]0/
[BnOH]0)(% conversion) and Mn obtained by GPC, and the
low polydispersity indices (PDIs) of 1.07−1.31 (Figure 9), in
addition to the linear relationship of (1/[lactide] − 1/
[lactide]0) versus reaction time, indicating a rate = kobs[lactide]2
(see the Supporting Information). When the concentration of 3
was decreased to 1.5 mM (Table 3, entry 10), the reaction
became very sluggish. When unpurified LA was employed
angles is 159.3°, and the other is 177.0°. The variation in this
angle is similar to that observed in the two unique molecules
found in the unit cell of BHTNa·DME2 (7, Figure 4).
Ring-Opening Polymerization of L-Lactide. The ROP of
L-lactide (LA) catalyzed by lithium complex 1 (1.0 mM)
employing benzyl alcohol (BnOH) (10.0 mM) as an initiator
has been systematically examined in toluene (20 mL) at room
temperature, as shown in Table 1. The results indicate that
complex 1 was an efficient catalyst, reaching conversions of
>90% within 1 h at room temperature when the [LA]0/
[BnOH]0 quotient ranged from 15 to 100 (Table 1, entries 1−
5). Polymerization control is demonstrated by the linear
relationship between FWlactide([LA]0/[BnOH]0)(% conver-
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a
Table 1. Ring-Opening Polymerization of L-Lactide by Li Complexes 1 and EDBPLi·THF
b
c
c
d
b
entry
[LA]0/[cat]0/[BnOH]0
time (min)
conv. (%)
Mn GPC
Mw/Mn
Mn calcd
Mn NMR
e
1
15/0.2/1
35/0.2/1
50/0.2/1
75/0.2/1
100/0.2/1
100/0.3/1
100/0.3/1
50/0.1/1
100/0.3/1
50/0.1/1
60
60
60
60
60
60
60
16
60
16
99
99
90
90
92
93
95
73
83
3700
9100
1.12
1.09
1.12
1.14
1.23
1.49
1.45
1.11
1.12
2100
5000
2500
5600
e
2
e
3
10 800
16 500
20 700
12 600
6500
6500
7600
e
4
9700
8500
e
5
13 300
13 400
13 700
5300
13 600
11 400
16 200
6100
f
6
g
7
h
8
8900
i
9
22 800
12 000
13 600
j
10
11
a
b
c
In 20 mL of toluene at 20 °C; quenched with ethanol in hexane. By 1H NMR analysis. GPC vs calibrated PS standards using a correction factor of
0.58.18 By FWlactide([LA]0/[BnOH]0)(% conversion). [1] = 1.0 mM. [1] = 1.5 mM. [1] = 1.5 mM; L-lactide is unpurified. [1] = 0.5 mM in 8
mL of toluene and 2 mL of THF. [EDBPLi·THF3] = 1.5 mM. [EDBPLi·THF3] = 0.5 mM in 8 mL of toluene and 2 mL of THF. Low yield
d
e
f
g
h
i
j
prevented suitable PLLA isolation.
Figure 7. Linear plot of polylactide Mn (GPC) versus FWlactide([LA]0/
[BnOH]0)(% conversion), with polydispersity indexes (Mw/Mn)
indicated by circles (from 1, Table 1, entries 1−5).
Figure 8. Linear plot of polylactide Mn (GPC) versus FWlactide([LA]0/
[BnOH]0)(% conversion), with polydispersity indexes (Mw/Mn)
indicated by circles (from 2, Table 2, entries 1−9).
differential polymerization activity was amplified by an order of
magnitude.
The ROP of L-lactide (LA) catalyzed by calcium complexes 4
and 5 (0.5 mM) employing benzyl alcohol (BnOH) (10.0 mM)
as an initiator has been systematically examined in toluene (20
mL) at room temperature, as catalogued in Table 4. The results
(entry 9), the Mn obtained by GPC became much smaller than
expected. In comparison with the chelating magnesium
complex (EDBPMg·THF)2,5d compound 3 showed a much
higher bulk polymerization activity: about 150 times (Table 3,
entry 8 vs entry 12). In THF (entry 11 vs entry 13), the
a
Table 2. Ring-Opening Polymerization of L-Lactide by Na Complexes 2 and EDBPNa(MeOH)2(THF)2
b
c
c
d
b
entry
[LA]0/[cat]0/[BnOH]0
time (min)
conv. (%)
Mn GPC
Mw/Mn
Mn calcd
Mn NMR
e
1
15/0.2/1
20/0.2/1
25/0.2/1
30/0.2/1
40/0.2/1
50/0.2/1
60/0.2/1
100/0.2/1
120/0.2/1
100/0.3/1
100/0.3/1
100/0.5/1
100/0.5/0
100/0.5/0
3
3
90
91
89
91
85
87
99
99
96
99
99
68
98
53
3600
4500
1.09
1.13
1.13
1.16
1.34
1.24
1.34
1.18
1.24
1.30
1.53
1.39
1.11
1.13
1900
2600
1900
2900
e
2
e
3
3
5900
3200
3600
e
4
3
7300
4000
3900
e
5
3
9000
4900
5500
e
6
3
11 200
15 100
23 900
29 600
18 800
8900
6300
6200
e
7
8
8600
9200
e
8
8
14 300
16 600
14 300
14 300
9800
14 700
17 100
16 200
17 800
10 900
15 300
7700
e
9
8
f
10
6
g
11
6
h
12
10
8
11 300
23 100
5500
i
13
14 100
7600
j
14
240
a
b
c
In 20 mL of toluene at 20 °C; quenched with ethanol in hexane. By 1H NMR analysis. GPC vs calibrated PS standards using a correction factor of
0.58.17 By FWlactide([LA]0/[BnOH]0)(% conversion). [2] = 1.0 mM. [2] = 1.5 mM. [2] = 1.5 mM; L-lactide is unpurified. [2] = 2.5 mM in 20
mL of THF. [EDBPNa(MeOH)2(THF)2] = 2.5 mM; from ref 4. [EDBPNa(MeOH)2(THF)2] = 2.5 mM in 20 mL of THF; from ref 4.
d
e
f
g
h
i
j
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a
Table 3. Ring-Opening Polymerization of L-Lactide by Magnesium Complexes 3 and (EDBPMg·THF)2
b
c
c
d
b
entry
[LA]0/[cat]0/[BnOH]0
time (min)
conv. (%)
Mn GPC
Mw/Mn
Mn calcd
Mn NMR
e
1
5/0.6/1
16
16
99
90
92
93
99
99
92
95
96
15
95
79
2700
7000
1.07
1.28
1.27
1.21
1.22
1.12
1.16
1.31
1.35
1.31
1.30
1.37
1.10
700
1900
1680
4200
e
2
15/0.6/1
35/0.6/1
55/0.6/1
65/0.6/1
75/0.6/1
85/0.6/1
120/0.6/1
100/0.6/1
100/0.3/1
50/0.6/1
100/1/1
50/0.6/1
e
3
16
17 900
28 600
32 400
37 800
40 700
58 800
31 700
7500
4600
10 500
e
4
16
7400
15 300
e
k
5
16
9300
e
k
k
k
6
16
10 700
11 300
16 400
13 800
2100
e
7
16
e
8
20
f
9
40
18 800
4700
g
10
60
h
11
2.5
35 h
33 h
20 100
14 000
6800
10 500
11 000
4400
i
12
11 400
j
13
53
8200
3800
a
b
c
In 20 mL of toluene at 20 °C; quenched with ethanol in hexane. By 1H NMR analysis. GPC vs calibrated PS standards using a correction factor of
0.58.17 By FWlactide([LA]0/[BnOH]0)(% conversion). [3] = 3.0 mM. [3] = 3.0 mM; L-lactide is unpurified. [3] = 1.5 mM. [3] = 5.0 mM in 8
mL of toluene and 2 mL of THF. [EDBPMg·THF] = 2.5 mM. [EDBPMg·THF] = 2.5 mM in 8 mL of toluene and 2 mL of THF. Signals for the
d
e
f
g
h
i
j
k
benzyl alkoxy end group are too small to be integrated.
the linear relationship of (1/[lactide] − 1/[lactide]0) versus
reaction time (indicating rate = kobs[lactide]2; see the
Supporting Information) and the linear relationship between
FWlactide([LA]0/[BnOH]0)(% conversion) and Mn obtained by
GPC (Figure 10), this latter relationship for complex 5 deviates
from linearity at higher molecular weights (Figure 11).
Moreover, the polydispersity index range was lower for 4
(1.08−1.17) than for 5 (1.04−1.32). When the concentration
of 4 was increased from 0.5 to 1.5 mM (Table 4, entry 6), a
higher molar reactivity and a narrow PDI (1.07) were found.
Although this catalyst will ring-open polymerize unpurified
lactide, the Mn value drops somewhat below the expected value.
Under comparable conditions, the BHT complex 4 exhibited a
somewhat higher polymerization activity than chelating
Figure 9. Linear plot of polylactide Mn (GPC) versus FWlactide([LA]0/
complex 5 (EDBPCa·THF2): about 2 times greater (Table 4,
[BnOH]0)(% conversion), with polydispersity indexes (Mw/Mn)
entry 5 vs entry 12). In THF, the bulk catalytic activity of 4 was
indicated by circles (from 3, Table 3, entries 1−8).
about 3 times that of 5 (Table 4, entry 8 vs entry 14).
To summarize the observed ROP activity, the bulk
polymerization activity of the metal−BHT complexes followed
the trend of Na (2, Table 2, entry 10) ≫ Li (1, Table 1, entry
6) > Ca (4, Table 4, entry 6) ≫ Mg (3, Table 3, entry 10).
demonstrated that complex 4 is generally more efficient than 5
for catalyzing the ROP of LA with BnOH as the initiator. While
good polymerization control for complex 4 is demonstrated by
a
Table 4. Ring-Opening Polymerization of L-Lactide by Ca Complexes 4 and 5 (EDBPCa·THF2)
b
c
c
d
b
entry
[LA]0/[cat]0/[BnOH]0
time (min)
conv. (%)
Mn GPC
Mw/Mn
Mn calcd
Mn NMR
e
1
25/0.1/1
50/0.1/1
75/0.1/1
100/0.1/1
100/0.1/1
100/0.3/1
100/0.1/1
100/0.1/1
25/0.1/1
50/0.1/1
75/0.1/1
100/0.1/1
125/0.1/1
100/0.1/1
2
2
98
98
98
84
96
83
92
83
55
71
91
93
92
84
6300
12 400
16 100
20 000
23 600
22 800
18 000
16 400
2600
1.08
1.15
1.17
1.08
1.12
1.07
1.19
1.31
1.05
1.04
1.13
1.25
1.32
1.49
3500
7000
3,600
7000
e
2
e
3
2
10 600
12 100
13 800
11 900
13 200
11 900
2000
10 900
12 400
14 400
14 100
16 300
13 900
1800
e
4
1.5
2
e
5
f
6
1
g
7
1.5
22
4
h
8
i
9
i
10
4
7700
5100
5200
i
11
4
15 700
17 200
20 600
18 900
9800
9300
i
12
4
13 400
16 600
12 100
14 300
16 700
12 600
i
13
4
j
14
65
a
b
c
In 20 mL of toluene at 20 °C; quenched with ethanol in hexane. By 1H NMR analysis. GPC vs calibrated PS standards using a correction factor of
0.58.17 By FWlactide([LA]0/[BnOH]0)(% conversion). [4] = 0.5 mM. [4] = 1.5 mM. [4] = 0.5 mM; L-lactide is unpurified. [4] = 0.5 mM in 20
mL of THF. [5] = 0.5 mM. [5] = 0.5 mM in 20 mL of THF.
d
e
f
g
h
i
j
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Organometallics
Article
also related to the greater structural freedom in the former.
Crystal structure analysis of monomeric metal−BHT com-
plexes 4, 7, 8, and 9 reveals linear and bent Cipso−O−metal
angles, which vary considerably: 4, 174.4°, 178.0°; 7, 180.0°,
165.8°; 8, 167.2°, 160.9°; 9, 159.3°, 177.0°. In the case of 9
(BHT2Ca·DME2), the variation is in the same metal complex,
but in the case of 7 (BHTNa·DME2), the two different angles
are found in separate molecules that are “bond angle isomers,”
two otherwise identical molecules in the unit cell. This bond
angle variation in the solid state suggests that there is a
somewhat shallow energy potential change for moving the
metal out of linearity with the ipso-carbon and its oxygen atom.
This energy potential (energy destabilization) was modeled
(DFT B3LYP 6-31G*)19 with sodium phenoxide to produce
the potential energy relationship in Figure 12. As the Cipso−O−
Figure 10. Linear plot of polylactide Mn (GPC) versus
FWlactide([LA]0/[BnOH]0)(% conversion), with polydispersity indexes
(Mw/Mn) indicated by circles (from 4, Table 4, entries 1−5).
Figure 12. Computed energy destabilization and sodium electrostatic
charge as a function of the Cipso−O−Na bond angle for sodium
phenoxide (DFT B3LYP 6-31G*).
Figure 11. Linear plot of polylactide Mn (GPC) versus
FWlactide([LA]0/[BnOH]0)(% conversion), with polydispersity indexes
(Mw/Mn) indicated by circles (from 5, Table 4, entries 9−13).
Na angle decreased from 180°, the potential energy steadily
increased by 2.0 kcal/mol en route to 120°. Indeed, this is a
rather small energy penalty and is likely much smaller than the
combined electronic penalty and ring strain penalty that exists
for a similar deviation in a chelating EDBP−Na complex. Now
note the effect of the angle deviation on the electrostatic charge
of sodium. In the sodium phenoxide model complex, as the
angle progresses from linear (180°) to bent (120°), the
electrostatic charge on sodium decreases from +0.913 to
+0.871. This corresponds to a reduction in the electrophilicity
at sodium and would suggest an increased lability in sodium−
ligand interactions as the Cipso−O−Na angle is increasingly
bent. This available mechanism for stabilizing catalytic
intermediates and achieving lability should increase ROP
catalytic rates for metal−BHT complexes. However, more
rigid metal−EDBP complexes are expected to maintain their
metal’s high electrophilicity, which suggests a lower degree of
lability, which comports with their diminished catalytic activity.
Figure 13 generalizes the possible effects that the Cipso−O−
Na angle deviation can have on the key catalytic intermediates
during the ROP of lactide. A linear geometry is preferred to
bent in order to maximize the electrophilicity of the sodium
metal and activate the coordinated monomer toward
nucleophilic attack. After ring-opening, displacement of the
coordinated carbonyl oxygen by the lactide monomer is
facilitated by a geometry change to bent, for which the metal
is less electrophilic and lability is enhanced. Thus, the
geometrical freedom of the Cipso−O−Na angle in metal−
BHT complexes allows for a smoother modulation of the
Generally, the metal−BHT complexes were more efficient than
chelating metal−EDBP complexes, and the activity differential
was amplified when THF was employed as solvent. There are
both steric and electronic explanations that can account for this
activity difference.
Steric Effects on Bulk Polymerization Activity. Metal−
BHT complexes maintain conformational freedom about the
metal−oxygen bond, whereas metal−EDBP complexes do not
enjoy this rotational freedom because of chelation. Therefore,
one would expect the metal−BHT complexes to be sterically
more accommodating toward the coordination of the growing
polymer chain and lactide monomer, which is activated toward
carbonyl attack upon association with the Lewis acidic metal. In
fact, the lactide monomer is presumed to chelate itself,
employing both oxygen atoms from one ester group for
coordination. All of the monomeric metal−BHT−DME crystal
structures reported herein (7, 8, and 9) readily accommodate
the DME (dimethoxymethane) ligand and coordinate both of
the oxygen atoms. Sodium complex 7 (BHTNa·DME2) and
calcium complex 9 (BHT2Ca·DME2) each accommodate two
DME ligands, whereas the magnesium complex 8
(BHT2Mg·DME) only accommodates one. Fittingly, the
magnesium complex 8 is the most sluggish in the metal−
BHT series investigated (Na, Li, Ca, and Mg).
Electronic Effects on Polymerization Activity. An
electronic rationale for the greater activity of metal−BHT
complexes over metal−EDBP complexes is more subtle, but is
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exhibited high activity for the ring-opening polymerization of
lactide. Generally, metal−BHT catalysts were more efficient
than metal−EDBP systems, and their superiority was rather
evident in THF solution. Bulk polymerization activities
generally follow the order of Na ≫ Li > Ca ≫ Mg. Four
metal−BHT complexes (6, 7, 8, and 9) were prepared with
coordinated dimethoxyethane (DME), which was designed to
mimic the coordinated polymer chain during catalysis. These
complexes exhibited a wide variation in the Cipso−O−metal
angle (159.3−180.0°), suggesting a shallow potential energy
well for bond angle deviation, which was supported by DFT
(B3LYP 6-31G*) calculations on sodium phenoxide that
indicated a modest 2.0 kcal/mol penalty to bend from linear
to 120°. Calculations also indicated that the angle deviation
modulates the electrostatic charge on the metal, and this
fluxionalitynot readily possible with rigid, chelating EDBP
ligandsallows the metal to alternate between a stronger Lewis
acid when Cipso−O−metal is linear (to activate monomer) and
a weaker Lewis acid when Cipso−O−metal is bent (for increased
lability)thereby facilitating catalysis and explaining the
superior activities versus EDBP complexes.
EXPERIMENTAL SECTION
■
General. All manipulations were carried out under a dry nitrogen
atmosphere. Solvents, benzyl alcohol, and deuterated solvents were
purified before use. L-Lactide was purchased from PURASORB and
was sublimed prior to use. BHT (2,6-di-tert-butyl-4-methylphenol),
sodium hydride, n-butyllithium in hexanes (Acros), EDBP (2,2′-
ethylidenebis(4,6-di-tert-butylphenol)), di-n-butylmagnesium in hep-
tane (Aldrich), calcium iodide, and sodium bis(trimethylsily)amide
Figure 13. Proposed catalytic cycle wherein Cipso−O−Na angle
modulations allow for greater monomer activation (linear) and greater
substrate lability at sodium (bent).
electron density and a flatter catalytic potential energy surface
that results in greater catalytic efficiency.
(Alfa Aesar) were used as received. H and 13C NMR spectra were
1
recorded on a Varian Mercury-300 (300 MHz for 1H and 75 MHz for
13C) spectrometer with chemical shifts given in parts per million from
the internal TMS or center line of CHCl3. Complexes EDBP-
Li·THF33b and [EDBPMg·THF]25d were prepared following literature
procedures.
Comparative Polymer Characterization. Four polymer
samples prepared under comparable conditions with four
different catalysts (1, 2, 3, and 4) were subjected to
characterization by GPC, DSC, and NMR, obtaining the
weight-average molecular weight (Mw), number-average mo-
lecular weight (Mn), polydispersity index (PDI), glass transition
temperature (Tg), melting temperature (Tm), and isotacticity
index ([m]). The characterization results are summarized in
Table 5, and additional details are found in the Supporting
Information. The differences among the polymer properties
were not vast, although it should be noted that Li complex 1
yielded the polymer with the lowest molecular weight, broadest
PDI, and lowest melting temperature under these conditions.
Gel permeation chromatography (GPC) was performed using an
internal differential refractive index detector (DRI), internal differ-
ential viscosity detector (DP), and a Precision 2 angle light-scattering
detector (LS). The light-scattering signal was collected at a 15° angle,
and the three in-line detectors were operated in series in the order of
LS-DRI-DP. The chromatography was performed at 45 °C using two
tandem columns (10 μm PD, 7.8 mm ID, 300 mm total length) with
HPLC grade tetrahydrofuran as the mobile phase at a flow rate of 1.0
mL/min. Injections were made at a 0.05−0.07% w/v sample
concentration using a 322.5 μL injection volume. In the case of
universal calibration, retention times were calibrated versus narrow-
range molecular weight polystyrene standards. All standards were
selected to produce Mp or Mw values well beyond the analyte’s
expected range. The Precision LS was calibrated using a narrow-range
polystyrene standard with Mw = 65 500 g/mol.
CONCLUSIONS
■
Eight BHT (2,6-di-tert-butyl-4-methylphenol) complexes (1, 2,
3, 4, 6, 7, 8, and 9) and one EDBP (2,2′-ethylidenebis(4,6-di-
tert-butylphenol)) complex (5) involving the metals Li, Na, Mg,
and Ca were prepared and characterized. The molecular
structure of complexes 2, 4, 6, 7, 8, and 9 were further
confirmed by single-crystal X-ray diffraction techniques. All
complexes, in the presence of benzyl alcohol as initiator,
Differential scanning calorimetry (DSC) was performed using a
DSC equipped with a controlled cooling accessory at a heating rate of
5 °C/min unless otherwise specified. All samples were prepared in
hermetically sealed pans (5−10 mg/sample) and were analyzed using
Table 5. Polymer Characterization
a
a
a
b
b
c
entry
reference
catalyst
Mw
Mn
PDI
Tg (°C)
Tm (°C)
isotacticity [m]
1
2
3
4
Table 1, entry 5
Table 2, entry 8
Table 3, entry 4
Table 4, entry 5
1
2
3
4
25 600
28 200
34 500
26 600
20 700
23 900
28 600
23 600
1.24
1.18
1.21
1.13
50.6
56.1
50.9
53.0
152.7
168.0
166.1
154.9
99.84
99.87
99.88
99.84
a
b
c
GPC vs calibrated PS standards using a correction factor of 0.58.17 By differential scanning calorimetry (DSC). The second scan is reported. By
1H NMR analysis.
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Organometallics
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an empty pan as a reference and empty cells for a subtracted baseline.
The samples were scanned for multiple cycles to remove
recrystallization and thermal history differences between the samples,
and the results reported are from the third scan in the cycle. Although
all samples were thermally scanned from 0 to 200 °C, in some cases,
the DSC plots (see the Supporting Information) were scaled
differently for better visualization.
X-ray Diffraction Method. Data were collected at 173 K on a
Siemens SMART PLATFORM equipped with a CCD area detector
and a graphite monochromator utilizing Mo Kα radiation (λ = 0.71073
Å). Cell parameters were refined using up to 8192 reflections. A full
sphere of data (1850 frames) was collected using the ω-scan method
(0.3° frame width). The first 50 frames were remeasured at the end of
data collection to monitor instrument and crystal stability (maximum
correction on I was <1%). Absorption corrections by integration were
applied based on measured indexed crystal faces.
Synthesis of [BHTLi·DME]2 (6). In the glovebox, approximately
0.6 g of 1 was dissolved in approximately 4 mL of dimethoxyethane
(DME). The solution was heated at 80 °C for 10 min and cooled to
room temperature. Crystals suitable for an X-ray diffraction study were
obtained by storing the solution in a −35 °C freezer within the
glovebox for a week. XRD data confirm the structure of
[BHTLi·DME]2 as a dimeric complex with loss of all THF from 1.
1H NMR (C6D6): δ 1.75 (s, 18H, Ar(C(CH3)3), 2.46 (s, 3H, ArCH3),
2.68 (s, 4H, OCH2CH2O), 2.70 (s, 6H, OCH3), 7.28 (s, 2H, Ar-H).
13C NMR (C6D6): δ 21.9 (ArCH3), 31.4 (ArC(CH3)3), 35.5
(ArC(CH3)3), 59.6 (OCH3), 71.0 (OCH2CH2O), 116.9, 125.9,
136.7, 167.8 (aromatic C).
Synthesis of BHTNa·DME2 (7). In the glovebox, approximately
0.6 g of 2 was dissolved in about 4 mL of DME. The solution was
heated at 80 °C for 10 min and cooled to room temperature. Crystals
suitable for an X-ray diffraction study were obtained by storing the
solution in a −35 °C freezer within the glovebox for 3 days. XRD data
confirm the structure of BHTNa·DME2 as a monomeric complex with
loss of all THF from 2. 1H NMR (C6D6): δ 1.68 (s, 18H, ArC(CH3)3),
2.39 (s, 3H, ArCH3), 3.00 (s, 12H, OCH3), 3.18 (m, 8H,
OCH2CH2O), 7.27 (s, 2H, Ar-H). 13C NMR (C6D6): δ 21.7
(ArCH3), 31.8 (ArC(CH3)3), 35.5 (ArC(CH3)3), 59.1 (OCH3), 71.9
(OCH2CH2O), 121.3, 126.0, 137.5, 164.3 (aromatic C).
Synthesis of BHT2Mg·DME (8). In the glovebox, approximately
0.6 g of 3 was dissolved in about 4 mL of DME. The solution was
heated at 80 °C for 10 min and cooled to room temperature. Crystals
suitable for an X-ray diffraction study were obtained by storing the
solution in a −35 °C freezer within the glovebox for 1 day. XRD data
confirm the structure of BHT2Mg·DME as a monomeric complex with
loss of all THF from 3. 1H NMR (C6D6): δ 1.55 (s, 36H, ArC(CH3)3),
2.41 (s, 6H, ArCH3), 3.08 (s, 6H, OCH3), 3.10 (br, 4H, OCH2CH2O),
7.22 (s, 4H, Ar-H). 13C NMR (C6D6): δ 21.7 (ArCH3), 31.6
(ArC(CH3)3), 35.1 (ArC(CH3)3), 58.7 (OCH3), 71.5 (OCH2CH2O),
121.4, 125.7, 137.0, 160.5 (aromatic C).
Synthesis of [BHTLi·THF]2 (1). This was prepared following
literature procedures.14
Synthesis of [BHTNa·THF]3 (2). A mixture of 2,6-di-tert-butyl-4-
methylphenol (8.8 g, 40 mmol) and NaH (1.00 g, 42 mmol) was
stirred in tetrahydrofuran (30 mL) at room temperature for 24 h.
Residual NaH was removed by filtration, and the solvent was removed
under vacuum. The obtained white powder was washed with heptane
(3 × 15 mL) and dried in vacuo. Yield: 9.21 g (73.3%). Crystals
suitable for an X-ray diffraction study were obtained by storing a THF
solution in a −35 °C freezer within the glovebox for 3 days. XRD data
1
confirm the structure of [BHTNa·THF]3 as a trimeric complex. H
NMR (C6D6): δ 1.20 (m, 4H, OCH2CH2), 1.67 (s, 18H, ArC(CH3)3),
2.37 (s, 3H, ArCH3), 3.16 (m, 4H, OCH2), 7.19 (s, 2H, Ar-H). 13C
NMR (C6D6): δ 21.6 (ArCH3), 25.5 (OCH2CH2), 31.3 (ArC(CH3)3),
35.4 (ArC(CH3)3), 69.9 (OCH2CH2), 118.9, 125.8, 137.4, 166.5
(aromatic C).
Synthesis of BHT2Mg·THF2 (3). This was prepared following
literature procedures.15
Synthesis of BHT2Ca·DME2 (9). In the glovebox, approximately
0.6 g of 4 was dissolved in about 5 mL of DME. The solution was
heated at 80 °C for 10 min and cooled to room temperature. Crystals
suitable for an X-ray diffraction study were obtained by storing the
solution in a −35 °C freezer within the glovebox for 1 day. XRD data
confirm the structure of BHT2Ca·DME2 as a monomeric complex with
loss of all THF from 4. 1H NMR (CDCl3): δ 1.45 (s, 36H,
ArC(CH3)3), 2.23 (s, 6H, ArCH3), 3.55 (s, 12H, OCH3), 3.72 (s, 8H,
OCH2CH2O), 6.92 (s, 4H, Ar-H). 13C NMR (C6D6): δ 21.1 (ArCH3),
30.3 (ArC(CH3)3), 34.5 (ArC(CH3)3), 60.1 (OCH3), 71.4
(OCH2CH2O), 119.8, 125.2, 136.2, 161.8 (aromatic C).
Synthesis of BHT2Ca·THF3 (4). Two methods were employed.
One method followed literature procedures16 and the following. A
mixture of calcium iodide (2.93 g, 10 mmol) and sodium
bis(trimethylsilyl)amide (3.66 g, 20 mmol) was stirred in tetrahy-
drofuran (30 mL) at 50 °C for 24 h. 2,6-Di-tert-butyl-4-methylphenol
(4.4 g, 20 mmol) was transferred into the solution and stirred for a day
at room temperature. Residual precursor and NaI were removed by
filtration, and the solvent was removed under vacuum. The obtained
white powder was washed with heptane (3 × 15 mL) and dried in
vacuo. Yield: 5.20 g (75.0%). Approximately 0.6 g of 4 was dissolved in
about 5 mL of THF in the glovebox. The solution was heated at 50 °C
for 10 min and cooled to room temperature. Crystals suitable for an X-
ray diffraction study were obtained by storing the solution in a −35 °C
freezer within the glovebox for 1 day. XRD data confirm the structure
Typical Procedure for the Polymerization of L-Lactide. A
typical polymerization procedure is provided here that corresponds to
Table 1, entry 5. A solution of 1 (0.006 g, 0.02 mmol) with BnOH
(0.010 g, 0.1 mmol) in toluene (20 mL) was stirred at room
temperature (20 °C) for 5 min, and then L-lactide (1.440 g, 10 mmol)
was transferred into the solution. After 60 min, an aliquot of the
reaction was taken and the solvent was removed. The obtained residue
1
of BHT2Ca·THF3 as a monomeric complex. H NMR (CDCl3): δ
1.44 (s, 18H, ArC(CH3)3), 1.87 (m, 4H, OCH2CH2), 2.22 (s, 3H,
ArCH3), 3.93 (m, 4H, OCH2), 6.91 (s, 2H, Ar-H). 13C NMR
(CDCl3): δ 21.2 (ArCH3), 25.3 (OCH2CH2), 30.8 (ArC(CH3)3), 35.0
(ArC(CH3)3), 69.2 (OCH2CH2), 119.0, 125.4, 137.0, 162.7 (aromatic
C).
1
was analyzed by H NMR to determine the percent conversion. The
volatile components were removed from the bulk reaction under
reduced pressure. The obtained solids were dissolved in tetrahy-
drofuran (10 mL). The solution was then quenched by the addition of
ethanol (10 mL), and the polymer was precipitated upon pouring into
n-hexane (80 mL) to give a white crystalline powder. Isolated yield:
Synthesis of EDBPCa·THF2 (5). A mixture of calcium iodide (2.93
g, 10 mmol) and sodium bis(trimethylsilyl)amide (3.66 g, 20 mmol)
was stirred in tetrahydrofuran (30 mL) at 50 °C for 24 h. 2,6-Di-tert-
butyl-4-methylphenol (4.38 g, 10 mmol) was transferred into the
solution and stirred for a day at room temperature. Residual precursor
and NaI were removed by filtration, and the solvent was removed
under vacuum. The obtained white powder was washed with heptane
1
0.10 g (46%). Characterization of the poly(L-lactide) using H NMR
spectroscopy indicates highly isotactic PLA with [m] > 99% (see the
Supporting Information for details).
1
(3 × 15 mL) and dried in vacuo. Yield: 4.51 g (72.7%). H NMR
ASSOCIATED CONTENT
* Supporting Information
■
(CDCl3): δ 1.24 (s, 18 H, C(CH3)3), 1.41 (s, 18H, C(CH3)3), 1.44 (s,
3H, CHCH3), 1.70 (br, 8H, OCH2CH2), 3.49 (br, 8H, OCH2CH2),
4.82 (q, 1H, CHCH3, J = 6.9 Hz), 7.02 (s, 2H, Ar-H), 7.24 (s, 2H, Ar-
H). 13C NMR (CDCl3): δ 23.8 (CH(CH3)), 25.3 (OCH2CH2), 30.4
(C(CH3)3), 31.4 (CH(CH3)), 31.7 (C(CH3)3), 34.0 (C(CH3)3), 35.1
(C(CH3)3), 68.2 (OCH2CH2), 120.3, 122.5, 135.1, 135.2, 158.4, 160.9
(aromatic C).
S
Catalyst characterization data (including NMR and X-ray
crystallography), polymer characterization data (NMR, GPC,
and DSC), and details of the DFT, kinetics, and alcohol
coordination studies. This material is available free of charge via
the Internet at http://pubs.acs.org.
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Article
(d) Chen, M.-Z.; Sun, H.-M.; Li, W.-F.; Wang, Z.-G.; Shen, Q.; Zhang,
Y. J. Organomet. Chem. 2006, 691, 2489−2494.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: miller@chem.ufl.edu.
(8) (a) Kricheldorf, H. R.; Sumbbl, M. V.; Saunders, I. K.
Macromolecules 1991, 24, 1944−1949. (b) Aubrecht, K. B.; Hillmyer,
M. A.; Tolman, W. B. Macromolecules 2002, 35, 644−650. (c) Amsden,
B.; Wang, S.; Wyss, U. Biomacromolecules 2004, 5, 1399−1404.
(d) Nimitsiriwat, N.; Marshall, E. L.; Gibson, V. C.; Elsegood, M. R. J.;
Dale, S. H. J. Am. Chem. Soc. 2004, 126, 13598−13599.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(9) (a) Chen, H.-Y.; Tang, H.-Y.; Lin, C.-C. Macromolecules 2006, 39,
3745−3752. (b) Huang, B.-H.; Lin, C.-N.; Hsueh, M.-L.; Yu, T.-L.;
Athar, T.; Lin, C.-C. Polymer 2006, 47, 6662−6629. (c) Williams, C.
K.; Breyfogle, L. E.; Choi, S. K.; Nam, W.; Young, V. G., Jr.; Hillmyer,
M. A.; Tolman, W. B. J. Am. Chem. Soc. 2003, 125, 11350−11359.
(d) Chamberlain, B. M.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J.
Am. Chem. Soc. 2001, 123, 3229−3238. (e) Chisholm, M. H.; Gallucci,
J. C.; Zhen, H. Inorg. Chem. 2001, 40, 5051−5054.
■
This research was supported by the donors of the American
Chemical Society Petroleum Research Fund (no. 47951-AC7)
and the National Science Foundation (CHE-0848236). K.A.A.
wishes to acknowledge the National Science Foundation
(CHE-0821346) and the University of Florida for funding
the purchase of X-ray diffraction equipment.
(10) (a) Ma, H.; Spaniol, T. P.; Okuda, J. Dalton Trans. 2003, 4770−
4780. (b) Yu, C.; Zhang, L.; Tu, K.; Shen, Z. Polym. Bull. 2004, 52,
329−337.
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dx.doi.org/10.1021/om300121c | Organometallics 2012, 31, 5252−5261