Ring-Opening polymerization of lactides catalyzed by natural Amino-Acid based zinc catalysts

Article abstract of DOI:10.1021/ic902271x

A series of chiral NNO-tridentate Schiff base ligands derived from natural amino acids were reacted with zinc(bis-trimethylsilylamide)₂ to provide metal complexes which have been fully characterized. One of these derivatives was further reacted

Full text of DOI:10.1021/ic902271x

2360 Inorg. Chem. 2010, 49, 2360–2371
DOI: 10.1021/ic902271x
Ring-Opening Polymerization of Lactides Catalyzed by Natural Amino-Acid
Based Zinc Catalysts
Donald J. Darensbourg* and Osit Karroonnirun
Department of Chemistry, Texas A&M University, College Station, Texas 77843
Received November 16, 2009
A series of chiral NNO-tridentate Schiff base ligands derived from natural amino acids were reacted with zinc(bis-
trimethylsilylamide)2 to provide metal complexes which have been fully characterized. One of these derivatives was
further reacted with p-fluorophenol to yield a phenoxide complex. X-ray crystallographic studies reveal the zinc Schiff
base amide complexes to be monomeric, whereas, the p-fluorophenolate complex was shown to be dimeric with
bridging phenoxide ligands. All zinc complexes were shown to be very effective catalysts for the ring-opening
polymerization (ROP) of lactides at ambient temperature, producing polymers with controlled and narrow molecular
weight distributions. These enantiomerically pure zinc complexes did not show selectivity toward either L- or D-lactide,
that is, kD(obsd)/kL(obsd) ≈ 1. However, steric substituents on the Schiff base ligands exhibited moderate to excellent
stereocontrol for the ROP of rac-lactide. Heterotactic polylactides were produced from rac-lactide with Pr values
ranging from 0.68 to 0.89, depending on the catalyst employed and the reaction temperature. The reactivities of the
various catalysts were greatly affected by substituents on the Schiff base ligands, with sterically bulky substituents
being rate enhancing.
Introduction
in medical applications, such as, biodegradable sutures, slow
release drug delivery, and tissue engineering.4-6
Presently there is considerable interest in synthesizing
useful polymeric materials wholly or in part from renewable
resources.1 For example, major efforts are underway for
industrially preparing the most widely used polymers world-
wide, polyethylene and poly(ethylene terephthalate), from
renewable resources, that is, ethanol-based ethylene2 and
ethylene glycol derived from sugar and molasses.3 Polyesters
represent another class of polymers that can serve as alter-
native materials for petrochemical-based polymers and are
derived from 100% renewable resources. That is, polylactides
are aliphatic polyesters obtained by polymerizing lactic acid
which is available from the fermentation of sugar (beet or
cane) or starch (corn, wheat, potatoes, or manioc). Of
importance, these polymers have the highly desirable proper-
ties of biocompatibility and biodegradability. As a conse-
quence, polymers such as polylactides have been widely used
The use of metal-based catalysts for the ring-opening
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chem.tamu.edu. Fax (979)845-0158.
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pubs.acs.org/IC
Published on Web 02/01/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2361
of Sn,10-14 Ge,15 Y,16 Fe,17-23 Ti/Zr,14,24-33 Mg,34-48
Al,49-73 Ca,35,74-77 Na,78 Li,79Zn,34-36,43,44,47,48,55,80-101
and In.102-104Although a large variety of metal derivatives
effectively catalyzes the ROP of lactide, it is preferable to use
biocompatible metals since polylactides are widely utilized in
food packaging and biomedical applications. Because the
lactide monomer consists of two stereocenters, it exists as
tion properties of the polylactide depend upon the tacticity of
the polymer. Isotactic
L- or
D-polylactides can easily be
obtained from enantiomerically pure
L
- or -lactide, respec-
D
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respectively. These processes are depicted in eqs 1 and 2.
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2362 Inorganic Chemistry, Vol. 49, No. 5, 2010
-leucine, and -methionine and their complexes with the
Darensbourg and Karroonnirun
L
L
Scheme 1
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L- and
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Synthesis and Characterization of Proligands and Their
Zinc Complexes. The synthesis of chiral diamines 4a-c is
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Scheme 1. Condensation reactions of these diamines
4a-d with 3,5-di-tert-butyl-2-hydroxybenzaldehyde at
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and hydrogen, respectively (Scheme 2). The reactions of
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Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2363
Scheme 2
Scheme 3
coordination mode along with one amido group. The
geometry of the zinc complexes is a distorted tetra-
hedron with zinc at the center. The complexes are shown
in Figure 1, with atomic labeling for non-hydrogen
atoms. Selected bond distances and angles are listed in
Table 1. On the other hand, if the amido group is replaced
by p-fluorophenolate to give complex 6e, the solid-state
structure is shown to be dimeric with a Zn(1)/O(2)/Zn(2)/
O(3) planar core bridged by the two oxygen atoms of
p-fluorophenol as illustrated in Figure 2. The dimeric struc-
ture shows that one zinc center adopts a distorted tetra-
hedral geometry, in comparison with the other zinc center
which possesses distorted trigonal bipyramidal geometry.
Polymerization Studies. Ring-Opening of Lactides.
There are two different mechanisms for the stereoselec-
tive ROP of rac-lactide,112 that is, a chain-end control58
and an enantiomorphic site-control mechanism.70,113 To
examine this latter mechanism we have synthesized en-
antiomerically pure zinc complexes 6a-c, and these plus
the achiral complex 6d were tested for the ROP of L- and
D-lactide under the same reaction conditions to compare
their selectivity and reactivity for this polymerization
process. All reactions were carried out in C6D6 at ambient
temperature and were monitored by H NMR spectro-
1
scopy. The experimental observations reveal all zinc
complexes to be active for the ROP of D- and L-lactide,
and the resulting polymers were obtained with the
expected molecular weights and with low polydispersity
indices. To quantitatively compare the selectivity of
each complex for the ROP of D- and L-lactide, the rates
of the reactions were investigated. The polymerization
reactions were found to be first-order in monomer as
illustrated in Figure 3 for several catalytic systems.
Table 2 summarizes the rate constants (kobsd) for these
various processes. As evident in Figure 3 and Table 2 the
ratio of kD(obsd) over kL(obsd) is close to unity for reactions
catalyzed by complexes 6a-c, indicative of a lack of
preferences of the enantiomerically pure complexes for
L- or D-lactide. Complex 6a was found to be the most
active of the zinc complexes. The ROP of L-lactide
utilizing the dimeric complex 6e began slowly for the
first 5 h, followed by a rate consistent with the more
active monomeric catalyst 6d (Figure 4). This is pre-
sumably due to dimeric disruption leading to a more
active monomeric species as previously illustrated by
Lin and co-workers.87
(112) Zhang, L.; Nederberg, F.; Messman, J. M.; Pratt, R. C.; Hedrick,
J. L.; Wade, C. G. J. Am. Chem. Soc. 2007, 129, 12610–12611.
(113) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 2002, 124, 1316–
1326.

2364 Inorganic Chemistry, Vol. 49, No. 5, 2010
Darensbourg and Karroonnirun
Figure 1. X-ray crystal structures of (a) 6a, (b) 6b, (c) 6c, and (d) 6d. Thermal ellipsoids represent the 50% probability surfaces. Hydrogen atoms are
omitted for the sake of clarity.
˚
Table 1. Selected Bond Lengths (A) and Angles (deg) for Zn Complexes 6a-d
6a
6b
6c
6d
Bond Lengths
Zn1-N1
Zn1-N2
Zn1-N3
Zn1-O1
2.017 (4)
2.198 (3)
1.920 (4)
1.933 (3)
2.030 (4)
2.193 (4)
1.921 (4)
1.930 (3)
2.022 (2)
2.211 (2)
1.913 (2)
1.9406 (18)
2.010 (5)
2.211 (6)
1.917 (5)
1.930 (4)
Bond Angles
N3-Zn1-O1
N3-Zn1-N1
O1-Zn1-N1
N3-Zn1-N2
O1-Zn1-N2
N1-Zn1-N2
115.60 (14)
136.85 (16)
91.24 (13)
110.14 (14)
121.26 (14)
78.64 (14)
119.64 (15)
133.83 (15)
92.20 (13)
114.20 (16)
112.07 (14)
77.40 (14)
116.01 (9)
138.63 (9)
91.58 (8)
108.68 (9)
120.65 (8)
78.21 (8)
122.2 (2)
132.3 (2)
90.6 (2)
112.0 (2)
110.9 (2)
81.4 (2)
Figure 2. X-ray crystal structure of complex 6e. Thermal ellipsoids
represent the 50% probability surfaces. Hydrogen atoms are omitted
˚
for the sake of clarity. Selected bond lengths (A) and angles (deg):
Zn2-N3: 2.050 (6), Zn2-N4: 2.301 (5), Zn2-O2: 2.080 (4), Zn2-O3:
2.016 (5), Zn2-O4: 1.977 (4), O4-Zn2-O3: 106.69 (17), O4-Zn2-N3:
89.61 (18), O3-Zn2-N3: 126.78 (19), O4-Zn2-O2: 95.69 (16),
O3-Zn2-O2: 80.79 (16), N3-Zn2-O2: 148.99 (18), O4-Zn2-N4:
161.08 (19), O3-Zn2-N4: 92.24 (18), N3-Zn2-N4: 78.79 (19),
O2-Zn2-N4: 87.10 (17).
The rates of ROP of L-lactide in the presence of com-
plex 6a or 6d were found to be independent of the addi-
tion of tetrahydrofuran (THF) to the benzene solvent.
Figure 5 illustrate the lack of influence of significant
concentrations of THF on kobsd at ambient temperature
for either of the catalyst systems. This is to be contrasted
with comparable calcium complexes which were shown to
be more active upon addition of the coordinating THF
solvent.76 That is, in this instance the larger calcium
center is better able to expand its coordination number,
thereby enhancing the nucleophilicity of the propagating
chain end.
As noted in Table 2 and Figure 3d, the rate of ROP of
L-lactide for complex 6d is slower than its more sterically
hindered analogues, complexes 6a and 6b. Another

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2365
Table 2. Rate Constants for the ROP of D- or L-Lactide in the Presence of Zinc
Complexesa
entry
M
kD(obsd) (h-1
)
kL(obsd) (h-1
)
kD/kL
1
2
3
4
5
6a
6b
6c
6d
6e
3.13
2.38
0.07
2.73
2.30
0.06
0.22
0.20b
1.14
1.03
1.16
a Each reaction was performed in C6D6 at ambient temperature.
Monomer and catalyst concentrations were held constant at 0.34 M
and 0.0069 M respectively. The kobsd values were determined by the slope
of the plots of ln([LA]0/[LA]t) vs time. b Linear portion of plot which
occurs after 5 h.
Figure 4. ln([LA]0/[LA]t) vs time plot for the ROP of L-lactide by the
dimeric complex 6e at ambient temperature.
Figure 5. Independence of the rate constant for the ROP of L-lactide on
the THF concentration in C6D6 at ambient temperature. (a) Complex 6a
as catalyst, (b) complex 6d as catalyst.
Figure 3. ln([LA]0/[LA]t) vs time plots for the ROP of D-lactide (blue
solid circles) or L-lactide (red solid triangles) catalyzed by various zinc
complexes at ambient temperature. (a) complex 6a, (b) complex 6b, (c)
complex 6c, and (d) complex 6d.
differentiating feature noted for the ROP of L-lactide in
the presence of complex 6d is seen in the 1H NMR spectra
in C6D6 (Figure 6) during the catalytic reaction. That is, a
methine proton resonance at 4.15 ppm is observed which
corresponds to a meso-lactide, indicating that epimeriza-
tion of the lactide monomer occurs during the poly-
merization process. This is further seen in the ROP of
L-lactide catalyzed by complex 6d where the anticipated
isotactically pure polylactide exhibits small defects in
microstructure as shown in Figure 7a. With the other
closely related, but sterically more encumbered catalysts,
complexes 6a-c, this isomerization process is not ob-
Figure 6. 1H NMR spectrum in C6D6 during the ROP of L-lactide in the
presence of complex 6d at ambient temperature. Methine proton of meso-
lactide observed at 4.15 ppm.
The resulting polylactides were isolated and purified by
precipitation from CH2Cl2 with 5% HCl in methanol
followed by drying in vacuo. The molecular weights
and polydispersity indices of the purified polymers were
determined by gel permeation chromatography (dual RI
and light scattering detectors) in THF using polystyrene
as a standard. These experimental findings indicate that
the polymerization process is well-controlled (Table 3).
The living character of the process can be noted by the low
polydispersites over the range of 1.05-1.07, and the linear
relationship between Mn and the monomer/initiator ratio
as depicted in Figure 8.
1
served as illustrated by the H NMR spectrum of the
polylactide in Figure 7b. In the case of complex 6c, the
sulfur atom from the amine backbone possibly coordi-
nates to the zinc center following dissociation of the
dimethylamine arm resulting in the slowest rate of cata-
lysis for all the complexes (vide infra).
The ROP of rac-lactide catalyzed by complexes 6a-d
was performed in chloroform at ambient temperature.

2366 Inorganic Chemistry, Vol. 49, No. 5, 2010
Darensbourg and Karroonnirun
Figure 8. Linear relationship observed between Mn and monomer/
initiator ratio of polylactide produced from rac-lactide catalyzed by
complex 6a at ambient temperature in CHCl3.
the ligand’s architecture is expected to play a major role in
stereoselectivity.18,115,116 Complexes 6a-d were evalu-
ated for the ROP of rac-lactide in CHCl3 at various
temperatures, and the afforded polymers’ tacticities were
assigned using the methine proton regions with homo-
nuclear decoupling as described by Hillmyer and co-
workers.117
Pr values were calculated from the ratio of the (area of
isi and sis)/(total area in methine proton region) from the
decoupled 1H NMR spectra shown in Figure 9. As
illustrated in Figures 9 and Table 4, complex 6c produced
the highest degree of heterotacticity in the polylactide
produced from rac-lactide with a Pr = 0.83 at ambient
temperature which increases to 0.89 at -30 °C. By way of
contrast, the least sterically hindered zinc derivative,
complex 6d, provided polylactide with the lowest degree
of heterotacticity (Pr = 0.68 at ambient temperature). In
this latter instance, decreasing the polymerization tem-
perature to -30 °C provided a Pr value of 0.87. Since the
chiral center in complexes 6a-c did not exhibit any
selectivity in the ROP of L- or D-lactide, we suggest that
the stereoselectivity in the polymerization of rac-lactide
occurs via a chain-end mechanism.52 Because the steric
bulk of the ligands in complexes 6a and 6b is rather
remote relative to the metal center, this rate effect may
be due to a more facile dissociation of the dimethylamine
arm during the polymerization process involving these
catalysts. Indeed, Mehrkhodavandi and co-workers have
shown in related diamino-phenol zinc complexes that the
dimethylamine arm of the ligand is hemilabile.101 To
examine this possibility, we carried out an experiment
where 6 equiv of 2-methylcyclohexanone were added to
Figure 7. 1H NMR spectra of polylactide in CDCl3 prepared from L-
lactide. (a) In the presence of complex 6d, with 1f2 illustrating expanded
methine region before and after homonuclear decoupling. (b) In the
presence of complex 6a, with 3f4 illustrating expanded methine region
before and after homonuclear decoupling.
Table 3. Polylactides Produced from the ROP of rac-lactide in Chloroform at
Ambient Temperature
Mn
conversion
(%)a
c
entry complex M/I
theoreticalb GPC 0.58Mn,GPC PDI
1
2
3
4
5
6
7
8
9
6a
6a
6a
6a
6a
6b
6c
6d
6e
50
97
6 990
8 630
5 000
1.08
1.05
1.08
1.05
1.07
1.31
1.31
1.13
1.07
300 96
700 96
1000 96
2000 96
41 509
96 855
138 782
279 612
7 062
96 191 55 790
181 730 105 400
285 108 165 360
530 141 307 480
50
50
50
50
98
95
98
97
8 161
8 757
8 171
8 056
4 730
5 080
4 740
4 672
6 846
7 062
6 990
a Obtained from 1H NMR spectroscopy. For entries 1 and 6 the
reaction times were 2 h, all other reactions were carried out for 24 h.
b Theoretical Mn = (M/I) ꢁ (% conversion) ꢁ (mol. wt. of lactide). c Mn
1
complex 6d in C6D6. As seen in Figure 10, H NMR
114
values corrected by the equation: Mn = 0.58Mn,GPC
.
spectroscopy reveals a significant portion of the complex
showed the dimethylamine arm free with presumably
concomitant binding of the ketone to the metal center
via the oxygen donor. This binding would closely mimic
the coordination of the lactide monomer to the zinc
center. This reaction pathway would also account for
the greatly reduced activity and highest heterotacticity
exhibited by complex 6c which contains the methylthio
group. Nevertheless, inconsistent with an amine dissocia-
tion mechanism and lactide ROP reactivity is the obser-
vation that complex 6d in the presence of excess THF
exhibited a similar amine lability (Figure 11). However, as
As previously mentioned the physical and degradation
properties of polylactides are intimately dependent on the
tacticity of the polymer. Herein we have examined the
tacticity of the polymers resulting from the ROP of rac-
lactide as catalyzed by the series of zinc derivatives, since
(114) The Mn values for polylactide were corrected from the Mn values
determined by GPC vs polystyrene standards, according to the equation Mn
= 0.58Mn,GPC, as previously reported in the literature. (a) Barak, I.; Dubois,
ꢀ ^
ꢀ
P.; Jerome, R.; Teyssie, Ph J. Polym. Sci., Part A: Polym. Chem. 1993, 31,
305. (b) Baran, J.; Duda, A.; Kowalski, A.; Szymanski, R.; Penczek, S.
Macromol. Rapid Commun. 1997, 18, 325–333.
(115) MacDonald, R. T.; McCarthy, S. P.; Gross, R. A. Macromolecules
1996, 29, 7356–7361.
(116) Stevels, W. M.; Ankone, M. J. K.; Dijkstra, P. J.; Feijen, J.
Macromolecules 1996, 29, 6132–6138.
(117) Zell, M. T.; Padden, B. E.; Paterick, A. J.; Thakur, K. A. M.; Kean,
R. T.; Hillmyer, M. A.; Munson, E. J. Macromolecules 2002, 35, 7700–7707.

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2367
Figure 9. Homonucleardecoupled1H NMR (CDCl3, 500MH3) spectra ofthe methineregion ofpolylactideproducedfromrac-lactidewithcatalyst(a) 6a,
(b) 6b, (c) 6c, and (d) 6d. (A) Polymerization temperature ambient. (B) Polymerization temperature -30 °C.
multiple times with a planimeter.118 In general, we have
observed this method provides lower Pr values than
those routinely determined by programs provided
for deconvolutions by NMR software providers. For
example, complex 6c gave a Pr value of 0.89 at -30 °C
for the polylactide produced from rac-lactide, whereas
the FIT deconvolution program provided by Varian
Instruments yielded a value of 0.98. Hence, we conclude
that Pr values determined by this latter technique
overestimate the heterotacticity of the thus formed
polylactide.
The dependence of the thermal properties on the tacti-
city of the polylactides synthesized can be readily gleamed
from the DSC measurements listed in Table 5. As is
readily seen in Table 5, the isotactically pure polylactide
afforded from the ROP of L-lactide by complex 6a is
highly crystalline and possesses a Tg value of 60 °C, with
Tc and Tm values of 90 and 178 °C, respectively. On the
other hand, the heterotactically enriched polymers pro-
vided by complexes 6a and 6c from the ROP of rac-lactide
exhibited Tg parameters which increased with increasing
Pr values. That is, complex 6a which gave a polymer with
Table 4. Pr Values of Poly(rac-lactide) at Different Temperaturea,b
complex
Pr at -30 °C
Pr at 0 °C
Pr at 22 °C
6a
6b
6c
6d
88
85
89
87
81
80
86
79
76
76
83
68
a Each reaction was performed in CHCl3 at different temperatures.
b Pr values were calculated from the ratio of the (area of isi and sis)/(total
area in methine proton region).
noted in Figure 5 the addition of THF did not inhibit the
ROP of L-lactide.
A point worthwhile noting regarding the measure-
ment of Pr values from 1H NMR peak areas is that the
relative peak areas presented in Table 4 were measured
(118) From expanded versions of the 1H NMR spectra illustrated in
Figure 9, and assuming the sis and isi peaks are symmetrical, the areas of these
peaks vs that of the total methine proton region were measured using a
planimeter. This mechanical integrating instrument used to measure the area
under a curve by moving a point attached to an arm of the device around the
perimeter of the defined curve can be easily demonstrated to accurately assess
areas as indicated by concomitant area measurements of known dimensions.

2368 Inorganic Chemistry, Vol. 49, No. 5, 2010
Darensbourg and Karroonnirun
Figure 10. 1H NMR spectrum (C6D6, rt) of 6d before addition of 2-methylcyclohexanone (bottom) and 15 min after addition of 6 equiv of
2-methylcyclohexanone (top).
Figure 11. 1H NMR spectrum (C6D6, rt) of 6d before addition of THF (bottom) and 15 min after addition of 10 equiv of THF (top).
a Pr value of 0.76 has a Tg of 39 °C, and complex 6c which
provided a polymer with a Pr of 0.89 has a Tg of 50 °C.
systems as depicted by a linear relationship between Mn and
% conversion, as well as, a low polydispersity index. Experi-
mental results revealed that enantiomeric pure complexes did
not preferably polymerize one enantiomer over the other as
is evident from the ratios of kD(obsd)/kL(obsd) being close to 1 for
all catalysts investigated. However, the substituent of the
chiral tridentate Schiff base ligands played a major role in
producing heterotactic polylactide from rac-lactide, and it
Summary Remarks
Herein we have reported a new series of well-characterized
chiral tridentate Schiff base ligands and their zinc complexes.
The complexes are shown to be active toward the ROP of
lactide. The polymerization processes appear to be living

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2369
Table 5. Physical and Thermal Properties of Polylactides
complex
M/I
lactide
tacticity
isotactic
hetero(Pr = 0.76)
hetero(Pr = 0.89)
Mn
decomposition temp. (°C)
Tga (°C)
Tca (°C)
Tma (°C)
6a
6a
6c
50
50
50
L-lactide
rac-lactide
rac-lactide
N/Ab
8630
8760
300
300
300
60
39
50
90
178
a Determined from second heating run. b Molecular weight was not measured because of polymer’s insolubility in THF due to its high crystallinity.
Table 6. Crystallographic Data for Complexes 6a-6d
6a
6b
6c
6d
6e
empirical formula
fw
C
32H55N3OSi2Zn
C58H114N6O2Si4Zn2
1170.65
110(2) K
monoclinic
P2(1)
10.616(3)
18.225(4)
17.992(4)
90
91.995(13)
90
3478.7(14)
1.118
C33H67N3OSSi2Zn
675.51
110(2) K
monoclinic
P2(1)
15.315(7)
10.089(5)
26.190(13)
90
104.946(6)
90
C55H108N6O2Si4Zn2
1128.57
110(2) K
monoclinic
P2(1)/c
13.530(5)
23.298(5)
10.488(5)
90.000(5)
99.190(5)
90.000(5)
3264(2)
1.148
2
0.848
C50H70F2N4O4Zn2
959.84
619.34
150(2) K
orthorhombic
P(2)1(2)1(2)1
8.719(4)
16.535(8)
24.901(12)
90
90
90
3590(3)
1.146
temperature (K)
crystal system
space group
110(2) K
monoclinic
P2(1)/n
12.128(12)
21.45(2)
20.25(2)
90
105.529(12)
90
5074(9)
1.257
˚
a(A)
˚
b(A)
˚
c(A)
R(deg)
β(deg)
γ(deg)
˚
V(A)
3910(3)
1.148
4
0.770
Dc(Mg/m3)
Z
4
0.777
2
1.805
27837
4
0.997
abs coeff (mm-1
reflections collected
independent reflections
)
32055 48125
7063 [R(int) = 0.1231] 9520 [R(int) = 0.0627] 17705 [R(int) = 0.0242] 4681 [R(int) = 0.1202] 11309 [R(int) = 0.0242]
44430
22528
data/restrains/parameters 7063/4/373
9520/4/694
1.077
R1 = 0.0402
Rw = 0.0963
R1 = 0.0515
Rw = 0.1030
17705/44/818
1.032
R1 = 0.0347
Rw = 0.0899
R1 = 0.0408
Rw = 0.0927
4681/2/337
0.965
R1 = 0.0756
Rw = 0.1935
R1 = 0.1165
Rw = 0.2405
11309/0/575
1.036
R1 = 0.0932
Rw = 0.2505
R1 = 0.1435
Rw = 0.3180
GOF on F2
final R indices
[I > 2σ(Ι)]
1.014
R1 = 0.0539
Rw = 0.1190
R1 = 0.0779
Rw = 0.1299
final R indices
(all data)
was shown that complex 6c produced the highest degree of
heterotactic polylactide with Pr values of 0.83 and 0.89 at
ambient and -30 °C, respectively. As might be anticipated,
the Tg of the heterotactic polylactides increased significantly
as the degree of heterotacticity increased.
used without further purification. Analytical elemental analysis
was provided by Canadian Microanalytical Services Ltd.
1
Measurements. H NMR spectra were recorded on Unityþ
300 MHz and VXR 300 MHz superconducting NMR spectro-
meters. Molecular weight determinations were carried out with
Viscotek Modular GPC apparatus equipped with ViscoGEL
I-series columns (H þ L) and Model 270 dual detector composed
of refractive index and light scattering detectors. TGA measure-
ments were performed with TGA 1000 Thermogravimetric
Analyzer by Instrument Specialists Incorporated. The samples
were scanned from room temperature to the desired temperature
under argon atmosphere with a heating rate of 5 °C/min. DSC
measurements were performed with a Polymer DSC by Mettler
Toledo. The samples were scanned from -100 to 200 °C under
nitrogen atmosphere. The glass transition temperature (Tg), the
crystallization temperature (Tc), and the melting temperature
(Tm) of polylactides were determined from the second heating at
a heating rate of 5 °C/min. X-ray crystallography was done on a
Bruker Smart 1000 diffractometer equipped with a CCD detec-
tor in a nitrogen cold stream maintained at 110 K. Crystal data
and details of the data collection for complexes 6a-6e are
provided in Table 6.
Experimental Section
Methods and Materials. All manipulations were carried out
using a double manifold Schlenk vacuum line under an argon
atmosphere or an argon filled glovebox unless otherwise stated.
Toluene and THF were freshly distilled from sodium/benzo-
phenone before use. Methanol and dichloromethane were pur-
ified by an MBraun Manual Solvent Purification System packed
with Alcoa F200 activated alumina desiccant. Pentane was
freshly distilled from CaH2. Deuterated chloroform and deut-
erated benzene from Cambridge Isotope Laboratories Inc. were
stored in the glovebox and used as received. L- and D-lactide were
gifts from PURAC America Inc., and rac-lactide was purchased
from Aldrich. These lactides were recrystallized from toluene,
dried under vacuum at 40 °C overnight, and stored in the
glovebox. Sodium bis(trimethylsilyl)amide and p-fluophenol
were purchased from Alfa Aesar, zinc chloride anhydrous was
purchased from Strem Chemicals, and all were stored in the
glovebox. N-Boc-L-phenylalanine, N-Boc-L-methionine, and
N-Boc-L-leucine were purchased from Chem-Impex interna-
tional and used as received. N,N-dimethylethylendiamine was
purchased from Acros and used as received. 3,5-Di-tert-butyl-2-
hydroxybenzaldehyde and zinc(bis-trimethylsilyl amide)2 were
prepared according to published procedure.75,76,119,120 All other
compounds and reagents were obtained from Aldrich and were
General Procedure for Synthesis of Chiral Diamines 4a-c. The
chiral diamines 4a-c were prepared according to the reported
literature111 with some modifications. To N-Boc-L-phenyl-
alanine (75 mmol) in CH2Cl2 (200 mL) was added DCC (83
mmol) followed by HOBt (83 mmol) at ambient temperature.
The dimethylamine (83 mmol, 40% aq solution) was added after
2 h. The reaction mixture was stirred overnight, and the solvent
was removed under reduced pressure to obtain a white precipi-
tate. The white precipitate was removed by filtration, and the
filtrate was washed with 10% citric acid solution followed by
washing with a saturated NaHCO3 solution. The organic layer
was separated, dried over NaSO4, and concentrated to dryness
under reduced pressure to afford the crude amides 2a-c which
were used in the next step without further purification.
(119) Darensbourg, D. J.; Holtcamp, M. W.; Struck, G. E.; Zimmer, M.
S.; Niezgoda, S. A.; Rainey, P.; Robertson, J. B.; Draper, J. D.; Reibenspies,
J. H. J. Am. Chem. Soc. 1999, 121, 107–116.
(120) Hansen, T. V.; Skattebol, L. Tetrahedron Lett. 2005, 46, 3829–3830.
e

2370 Inorganic Chemistry, Vol. 49, No. 5, 2010
Darensbourg and Karroonnirun
The N-Boc-protected amines were then deprotected with
TFA. To a solution of N-Boc-protected amines 2a-c (55 mmol)
in anhydrous CH2Cl2 (15 mL), TFA (165 mmol) was added and
stirred for overnight at room temperature. Excess TFA and
solvent were removed under reduced pressure. The resulting
yellowish oil was neutralized with sat. NaHCO3, extracted with
CH2Cl2 (10 ꢁ 30 mL), dried over Na2SO4, and evaporated to
dryness. The crude amides 3a-c were used for the next step
without purification.
The amides 3a-c (45 mmol) were dissolved in THF (30 mL)
and cannulated into a suspension of LiAlH4 (180 mmol) in THF
(60 mL) cooled in an ice bath. The reaction mixture was heated
to reflux overnight. The mixture was placed in an ice bath, and
EtOAc (100 mL) was slowly added to the mixture, followed by
saturated Na2SO4 (100 mL). The resulting white solid was
washed with EtOAc (3ꢁ 50 mL). The combined organic layers
were dried over Na2SO4, and the solvent was removed under
reduced pressure to afford slightly yellow oils 4a-c. The crude
products 4a-c were purified by vacuum distillation in a short-
path apparatus.
(S)-N0,N0-dimethyl-3-phenylpropane-1,2-diamine 4a. Follow-
ing the general procedure for synthesis of chiral diamines, the
title compound 4a was purified by vacuum distillation (0.5-0.7
mmHg) in a short-path apparatus at 121 °C; 4.36 g of yellowish
liquid was collected (61% yield). [R]D=þ12.64 (c=1.1, CHCl3);
1H NMR (300 MHz, CDCl3) δ = 1.20-1.80 (br s, 2H),
2.13-2.19 (m, 1H), 2.23 (s, 6H), 2.27-2.31 (m, 1H), 2.47 (dd,
J = 13.46, 8.8 Hz, 1H), 2.74 (dd, J = 13.25, 4.5 Hz, 1H),
3.09-3.18 (m, 1H), 7.19-7.25 (m, 3H), 7.26-7.32 (m, 2H);
13C NMR (300 MHz, CDCl3) δ = 49.25, 52.05, 53.35, 68.46,
116.28, 118.08, 118.78, 126.78. Anal. Calcd for C11H18N2: C,
74.11; H, 10.18; N, 15.71. Found: C, 71.98; H, 10.26; N, 14.56.
HRMS (ESI), m/z, 179.1591 [MþHþ], calcd for C11H18N2,
179.15.
(S)-N0,N0,4-trimethylpentane-1,2-diamine 4b. Following the
general procedure for synthesis of chiral diamines, the title
compound 4b was purified by vacuum distillation (0.5-0.7
mmHg) in a short-path apparatus at 110 °C; 3.75 g of product
was collected (59% yield). [R]D20=þ27.77 (c = 1.8, CHCl3); 1H
NMR (300 MHz, CDCl3) δ = 0.89 (d, J = 7.0 Hz, 3H), 0.91 (d,
J = 6.9 Hz, 3H), 1.11-1.24 (m, 2H), 1.69-1.80 (m, 1H), 2.06
(dd, J = 12.0, 3.8 Hz, 1H), 2.12-2.19 (m, 1H), 2.15 (br s, 2H),
2.22 (s, 6H), 2.90-2.98 (m, 1H); 13 C NMR (300 MHz, CDCl3) δ
= 22.00, 23.52, 24.60, 45.09, 45.81, 45.99, 67.43. Anal. Calcd for
C8H20N2: C, 66.61; H, 13.97; N, 19.42. Found: C, 61.97; H,
13.34; N, 16.53. HRMS (ESI), m/z, 144.1523 [MþHþ], calcd for
C8H20N2, 144.15.
added to 4a (0.958 g, 5.38 mmol). The solution mixture was heated
to reflux overnight and dried over Na2SO4 followed by filtration.
The volatile component was removed in vacuo to obtain 5a in 95%
yield. [R]D20 = -154.54 (c = 1.1, CHCl3); 1H NMR (300 MHz,
CDCl3) δ = 1.37 (s, 9H, C(CH3)3), 1.55 (s, 9H), 2.35 (s, 6H),
2.55-2.68 (m, 2H), 2.95 (dd, J = 13.39, 8.33 Hz, 1H), 3.13 (dd,
J = 13.39, 4.16 Hz, 1H), 3.54-3.63 (m, 1H), 7.03 (d, 7.03 (d,
J = 2.87 Hz, 1H), 7.23-7.32 (m, 5H, PhH), 7.44 (d, J = 2.65
Hz,1H), 8.13 (s, 1H), 13.89 (s, 1H, OH); 13C NMR (300 MHz,
CDCl3) δ = 29.58, 31.62, 34.21, 35.13, 41.14, 46.34, 65.14, 69.86,
117.90, 126.10, 126.28, 126.83, 128.41, 129.74, 136.54, 138.89,
139.83, 158.25, 165.63. Anal. Calcd for C26H38N2O: C, 79.14; H,
9.71; N, 7.10. Found: C, 78.56; H, 9.69; N, 6.93. HRMS (ESI), m/z,
395.3184 [MþHþ], calcd for C26H38N2O, 395.31.
(S,E)-2,4-di-tert-butyl-6-((1-(dimethylamino)-4-methylpentan-2-
ylimino)methyl)phenol (L2-H) 5b. Following the general procedure
for synthesis of tridentate Schiff base ligands, 3,5-di-tert-butyl-2-
hydroxybenzaldehyde (1.30 g, 5.54 mmol) in MeOH (30 mL) was
added to 4a (0.799 g, 5.54 mmol). The solution mixture was heated
to reflux overnight and dried over Na2SO4 followed by filtration.
The volatile component was removed in vacuo to obtain 5b in 96%
yield. [R]D21 = -18.72 (c = 1.17, CHCl3); 1H NMR (300 MHz,
CDCl3) δ = 0.87 (dd, J = 10.37, 5.88 Hz, 6H), 1.32 (s, 9H), 1.46 (s,
9H), 1.43-1.46 (m, 1H), 1.55-1.65 (m, 1H), 2.25 (s, 6H),
2.42-2.46 (m, 2H), 3.34-3.43 (m, 1H), 7.11 (d, J = 2.60 Hz,
1H), 7.38 (d, J = 2.60 Hz, 1H), 8.34 (s, 1H), 13.89 (s, 1H, OH); 13
C
NMR δ = (300 MHz, CDCl3) δ = 21.46, 23.89, 24.44, 29.59,
33.66, 34.26, 35.13, 43.36, 46.40, 65.14, 117.96, 126.06, 126.79,
136.65, 139.90, 158.37, 165.02. Anal. Calcd for C23H40N2O: C,
76.61; H, 11.18; N, 7.77. Found: C, 76.19; H, 11.05; N, 7.36.
HRMS (ESI), m/z, 361.3292 [MþHþ], calcd for C23H40N2O,
361.31.
(S,E)-2,4-di-tert-butyl-6-((1-(dimethylamino)-4-(methylthio)-
butan-2-ylimino)methyl)phenol (L3-H) 5c. Following the general
procedure for synthesis of tridentate Schiff base ligands, 3,5-di-
tert-butyl-2-hydroxybenzaldehyde (1.03 g, 4.41 mmol) in
MeOH (30 mL) was added to 4a (0.72 g, 4.41 mmol). The
solution mixture was heated to reflux overnight and dried
over Na2SO4 followed by filtration. The volatile component
21
was removed in vacuo to obtain 5c in 88% yield. [R]D
=
-62.80 (c = 1.21, CHCl3); 1H NMR (300 MHz, CDCl3) δ =
1.33 (s, 9H), 1.46 (s, 9H), 1.84-2.08 (m, 2H), 2.10 (s, 3H), 2.26
(s, 6H), 2.38-2.65 (m, 4H), 3.44-3.56 (m, 1H), 7.12 (d, J = 2.63
Hz, 1H), 7.39 (d, J = 2.63 Hz, 1H), 8.39(s, 1H), 13.70 (s, 1H,
OH); 13C NMR (300 MHz, CDCl3) δ = 15.57, 29.73, 31.11,
33.27, 34.26, 35.24, 46.45, 65.54, 66.52, 177.87, 126.32, 127.11,
136.75, 140.10, 158.39, 166.26. Anal. Calcd for C22H38N2OS: C,
69.79; H, 10.12; N, 7.40; S, 8.47 Found: C, 69.88; H, 10.16; N,
7.17; S, 8.27. HRMS (ESI), m/z, 379.2673 [MþHþ], calcd for
C22H38N2OS, 379.27.
(S)-N0,N0-dimethyl-4-(methylthio)butane-1,2-diamine 4c. Follow-
ing the general procedure for synthesis of chiral diamines, the title
compound 4c was purified by vacuum distillation (1.2 mmHg) in a
short-path apparatus at 125 °C; 4.22 g of product was collected
(40% yield). [R]D21 = þ12.22 (c = 1.14, CHCl3); 1H NMR (300
MHz, CDCl3) δ = 1.29-1.43 (m, 2H), 1.76-1.86 (m, 2H), 1.88 (s,
3H), 2.00(s, 6H), 2.30-2.46 (m, 2H), 2.71-2.81(m, 2H);13CNMR
(300 MHz, CDCl3) δ = 15.39, 30.74, 35.04, 45.49, 47.33, 66.68.
Anal. Calcd for C7H18N2S: C, 51,80; H, 11.18; N, 17.26; S, 19.76
Found: C, 51.51; H, 10.97; N, 15.11; S, 17.09. HRMS (ESI), m/z,
163.1309 [MþHþ], calcd for C7H18N2S, 163.13.
(E)-2,4-di-tert-butyl-6-((2-(dimethylamino)ethylimino)methyl)-
phenol (L4-H) 5d121. Following the general procedure for synth-
esis of tridentate Schiff base ligands, 3,5-di-tert-butyl-2-hydroxy-
benzaldehyde(2.00g, 8.53 mmol) inMeOH(30mL) was addedto
4d (N0,N0-dimethylethane-1,2-diamine) (0.75 g, 8.53 mmol). The
solution mixture was heated to reflux overnight and dried over
Na2SO4 followed by filtration. The volatile component was
1
removed in vacuo to obtain 5d in 92% yield. H NMR (300
MHz, CDCl3) δ = 1.32 (s, 9H), 1.65 (s, 9H), 2.02 (s, 6H), 2.28 (t,
J = 7.03, 2H), 3.31 (t, J = 7.03, 2H), 6.98 (d, J = 2.52, 1H), 7.56
(d, J = 2.52, 1H), 7.84 (s, 1H), 14.26 (s, 1H, OH); 13C NMR (300
MHz, CDCl3) δ = 29.8, 31.7, 34.3, 35.4, 45.7, 57.8, 60.1, 118.7,
126.3, 126.7, 137.1, 140.0, 158.9, 166.8. Anal. Calcd for
C19H32N2O: C, 74.95; H, 10.59; N, 9.20. Found: C, 74.94; H,
10.61; N, 9.02. HRMS (ESI), m/z, 305.2620 [MþHþ], calcd for
C19H32N2O, 305.25.
General Procedure for Synthesis of Tridentate Schiff Base
Ligands. 3,5-Di-tert-butyl-2-hydroxybenzaldehyde75,76,120 (1.0
equiv) in MeOH (30 mL) was added to 4a-d, (1.0 equiv). The
solution mixture was heated to reflux overnight and dried over
Na2SO4 followed by filtration. The volatile component was
removed in vacuo to obtain tridentate Schiff base ligands
5a-d in 88% to quantitative yield.
(S,E)-2,4-di-tert-butyl-6-((1-(dimethylamino)-3-phenylpropan-2-
ylimino)methyl)phenol (L1-H) 5a. Following the general procedure
for synthesis of tridentate Schiff base ligands, 3,5-di-tert-butyl-2-
hydroxybenzaldehyde (1.26 g, 5.38 mmol) in MeOH (30 mL) was
(121) Cameron, P. A.; Gibson, V. C.; Redshaw, C.; Segal, J. A.; Bruce, M.
D.; White, A. J. P.; Williams, D. J. Chem. Commun. 1999, 1883–1884.

Article
General Procedure for Synthesis of Tridentate Schiff Base
Inorganic Chemistry, Vol. 49, No. 5, 2010 2371
3.28 mmol) in pentane. The reaction mixture was stirred until a
yellow precipitate was formed and allowed to stir at room
temperature for an additional 3 h. The resulting yellow pre-
cipitate was then washed with cold pentane (3ꢁ 2 mL). The
volatile component was removed under reduced pressure to
Zinc Complexes (L1-4-H) 6a-d. A Tridentate Schiff base ligand
(1 equiv) was dissolved in pentane and was cannulated to a
solution of Zn[N(SiMe3)2]2 (1.0 equiv) in pentane. The reaction
mixture was stirred until a yellow precipitate was formed and
allowed to stir at room temperature for an additional 3 h. The
resulting yellow precipitate was then washed with cold pentane
(3ꢁ 2 mL). The volatile component was removed under reduced
pressure to obtain light yellow solid 6a-d.
21
obtain light yellow solid 6d in 52% yield. [R]D = 0.00 (c =
1
1.05, C6D6); H NMR (300 MHz, CDCl3) δ = 0.37 (s, 18H),
1.38 (s, 9H), 1.75 (s, 9H), 1.90 (s, 6H), 2.78 (t, J = 6.01 Hz, 2H),
3.11 (t, J = 6.01 Hz, 2H), 6.85 (d, J = 2.7 Hz, 1H), 7.40 (s, 1H),
7.63 (d, J = 2.7 Hz, 1H); 13C NMR (300 MHz, CDCl3) δ = 6.10,
30.06, 31.84, 34.06, 36.29, 45.64, 52.99, 57.22, 59.67, 60.33,
118.66, 129.79, 135.14, 142.26, 169.87. Anal. Calcd for
C25H49N3OSi2Zn: C, 56.73; H, 9.33; N, 7.94. Found: C, 57.91;
H, 8.61; N, 7.18.
Synthesis of [L1ZnOPh] 6e. A Tridentate Schiff base ligand 5a
(0.55 g, 1.82 mmol) and p-fluorophenol (0.24 g, 1.82 mmol) were
dissolved in pentane (3 mL) and was cannulated to a solution of
Zn[N(SiMe3)2]2 (0.74 g, 1.82 mmol) in pentane (1 mL). The
reaction mixture was stirred until a yellow precipitate was
formed and allowed to stir at room temperature for an addi-
tional 3 h. The resulting yellow precipitate was then washed with
cold pentane (3ꢁ 2 mL). The volatile component was removed
under reduced pressure to obtain light yellow solid 6e in 55%
Synthesis of [L1ZnN(SiMe3)2] 6a. A Tridentate Schiff base
ligand 5a (1.06 g, 2.68 mmol) was dissolved in pentane (3 mL)
and was cannulated to a solution of Zn[N(SiMe3)2]2 (1.04 g, 2.68
mmol) in pentane (1 mL). The reaction mixture was stirred until
a yellow precipitate was formed and allowed to stir at room
temperature for an additional 3 h. The resulting yellow pre-
cipitate was then washed with cold pentane (3ꢁ 2 mL). The
volatile component was removed under reduced pressure to
obtain light yellow solid 6a in 51% yield. [R]D21 = þ67.50 (c =
1.02, C6D6); 1H NMR (300 MHz, C6D6) δ = 0.43 (s, 18H), 1.36
(s, 9H), 1.57-1.66 (m, 1H), 1.76 (s, 9H), 1.82 (s, 6H), 2.35-2.47
(m, 2H), 2.67-2.74 (m, 1H), 3.14-3.24 (m, 1H), 6.75 (d, J =
2.95 Hz, 1H), 6.93 (d, 1H), 6.96 (d, 1H), 7.09-7.18 (m, 3H), 7.45
(s, 1H), 7.63 (d, J = 2.53, 1H); 13C NMR (300 MHz, C6D6) δ =
6.20, 30.04, 31.69, 34.02, 36.11, 39.91, 45.41, 61.89, 63.28,
118.24, 127.25, 128.33, 129.09, 129.63, 129.85, 137.17, 142.05,
168.56, 170.82, 165.63; Anal. Calcd for C32H55N3OSi2Zn: C,
62.05; H, 8.95; N, 6.78. Found: C, 62.48; H, 8.55; N, 6.07.
Synthesis of [L2ZnN(SiMe3)2] 6b. A Tridentate Schiff base
ligand 5b (1.08 g, 2.97 mmol) was dissolved in pentane (3 mL)
and was cannulated to a solution of Zn[N(SiMe3)2]2 (1.15 g, 2.97
mmol) in pentane (1 mL). The reaction mixture was stirred until
a yellow precipitate was formed and allowed to stir at room
temperature for an additional 3 h. The resulting yellow pre-
cipitate was then washed with cold pentane (3ꢁ 2 mL). The
volatile component was removed under reduced pressure to
obtain light yellow solid 6b in 50% yield. [R]D21 = þ89.82 (c =
1.00, C6D6); 1H NMR (300 MHz, CDCl3) δ = 0.4 (s, 18H), 0.72
(dd, J = 11.25, 5.62 Hz, 6H), 1.38 (s, 9H), 1.78 (s, 9H), 1.82 (bs,
2H), 1.94 (s, 6H), 2.16 (m, 2H), 2.42 (bs, 1H), 3.13-3.27 (m, 1H),
7.03 (d, J = 3.03 Hz, 1H), 7.66 (d, J = 3.03 Hz, 1H), 7.86 (s, 1H);
13C NMR (300 MHz,C6D6) δ = 2.78, 6.19, 22.20, 23.26, 24.54,
30.09, 31.80, 34.15, 36.28, 41.83, 46.10, 58.26, 64.88, 118.66,
129.97, 135.09, 142.35, 168.59; Anal. Calcd for C29H57N3O-
Si2Zn: C, 59.50; H, 9.81; N, 7.18. Found: C, 59.63; H, 9.63; N, 6.66.
Synthesis of [L3ZnN(SiMe3)2] 6c. A Tridentate Schiff base
ligand 5c (1.03 g, 2.73 mmol) was dissolved in pentane (3 mL)
and was cannulated to a solution of Zn[N(SiMe3)2]2 (1.05 g, 2.73
mmol) in pentane (1 mL). The reaction mixture was stirred until
a yellow precipitate was formed and allowed to stir at room
temperature for an additional 3 h. The resulting yellow pre-
cipitate was then washed with cold pentane (3ꢁ 2 mL). The
volatile component was removed under reduced pressure to
obtain light yellow solid 6c in 55% yield. [R]D21 = þ115.70 (c =
1
yield. H NMR (300 MHz, CDCl3) δ = 1.39 (s, 9H), 1.76 (s,
9H), 1.99 (s, 6H), 2.80 (t, J = 5.72 Hz, 2H), 3.11 (t, J = 5.72 Hz,
2H), 6.70 (t, J = 8.78 Hz, 2H), 6.91 (bs, 2H), 7.02 (d, J = 2.72
Hz, 1H), 7.77 (m, 2H); 13C NMR (300 MHz, CDCl3) δ = 30.10,
30.36, 31.77, 34.12, 36.05, 45.50, 46.31, 54.44, 57.17, 59.57,
115.75, 116.04, 118.10, 119.50, 129.82, 135.49, 141.75, 169.67,
171.81, Anal. Calcd for C25H35FN2O2Zn: C, 62.56; H, 7.35; N,
5.84; F, 3.96. Found: C, 62.83; H, 7.54; N, 5.89; F, 3.84.
Polymerization Procedure. In a typical experiment (Table 3,
entry 1), in the glovebox, a Schlenk flask was charged with a
solution of complex 6a (8.58 mg, 13.87 μmol) in CHCl3 (2 mL).
To this solution was added rac-lactide (100 mg, 0.69 mmol, 50
equiv). The mixture was stirred at room temperature for 60 min.
After a small sample of the crude solution was removed with a
syringe to be characterized by 1H NMR spectroscopy, the
product was isolated and purified by precipitation from dichloro-
methane by the addition of 5% hydrochloric acid in methanol.
The polymer was collected and dried under vacuum to constant
weight.
Kinetic Studies. Polymerizations of L- or D-lactide using
complexes 6a-d were monitored by 1H NMR spectroscopy.
Each monomer and corresponding zinc complex were dissolved
in C6D6, and the % conversion was investigated from the
integration of polymer and monomer signals. The chemical
shifts of polylactide are 5.01 (q, H) and 1.32 (d, CH3), and
the chemical shifts of lactide monomer are 3.79 (q, H) and 1.16
(d, CH3).
Acknowledgment. We gratefully acknowledge the financial
support of the National Science Foundation (CHE-05-43133)
and the Robert A. Welch Foundation (A-0923). We also thank
PURAC America, Inc. for providing lactide samples for this
research. Stephanie Wilson is also acknowledged for some of the
preliminary studies she carried out on ligand synthesis during an
NSF REU program.
1
1.21, C6D6); H NMR (300 MHz, CDCl3) δ = 0.39 (s, 18H),
1.36 (s, 9H), 1.40-1.51 (m, 2H), 1.70 (s, 3H), 1.75 (s, 9H), 1.90 (s,
6H), 2.04-2.27 (m, 4H), 3.11-3.24 (m, 1H), 6.98 (d, J = 2.73
Hz, 1H), 7.64 (d, J = 2.73 Hz, 1H), 7.87 (s, 1H) 13C NMR (300
MHz, CDCl3) δ = 6.14, 15.23, 30.06, 30.26, 31.78, 32.50, 34.11,
36.18, 45.78, 59.33, 63.64, 118.46, 128.49, 129.99, 135.10, 142.14,
168.65, 170.03; Anal. Calcd for C28H55N3OSSi2Zn: C, 55.73; H,
9.91; N, 6.69; S, 5.31. Found: C, 55.68; H, 8.89; N, 6.05; S, 5.28.
Synthesis of [L4ZnN(SiMe3)2] 6d. A Tridentate Schiff base
ligand 5d (1.00 g, 3.28 mmol) was dissolved in pentane (3 mL)
and was cannulated to a solution of Zn[N(SiMe3)2]2 (1.27 g,
Supporting Information Available: X-ray crystallographic
files in CIF format for the structural determinations of com-
plexes 6a-e. This material is available free of charge via the
Internet at http://pubs.acs.org.