Ring-opening polymerization of lactide with Zr complexes of {ONSO} ligands: From heterotactically inclined to isotactically inclined poly(lactic acid)

Article abstract of DOI:10.1021/ma2023364

Six members of a new family of {ONSO}-type ligands - the tetradentate-diainionic imine-thiobis(phenolate) ligands - were prepared by a two-step synthesis. Their [{ONSO}Zr(O^tBu)₂] complexes formed as single diastereomers with differen

Full text of DOI:10.1021/ma2023364

Article
pubs.acs.org/Macromolecules
Ring-Opening Polymerization of Lactide with Zr Complexes of
{ONSO} Ligands: From Heterotactically Inclined to Isotactically
Inclined Poly(lactic acid)
Ayellet Stopper,† Jun Okuda,‡ and Moshe Kol†,*
†School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 69978,
Israel
‡Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, D-52056 Aachen, Germany
ABSTRACT: Six members of a new family of {ONSO}-type
ligandsthe tetradentate-diainionic imine-thiobis(phenolate)
ligandswere prepared by a two-step synthesis. Their
[{ONSO}Zr(OtBu)2] complexes formed as single diaster-
eomers with different degrees of fluxionality that depended on
the substitution pattern of the two phenolate rings. The
complexes were active in polymerization of L-lactide and rac-lactide. The tacticity of the poly(lactic acid) prepared from rac-
lactide changed gradually from heterotactically inclined through atactic to isotactically inclined. The most flexible complex and
the most rigid complex gave the highest degrees of heterotacticity and isotacticity, respectively.
Indeed, two types of heteroselective catalysts that have emerged
recently are the rigid Cs-symmetric catalysts operating by the
chain-end control mechanism9 and the fluxional-chiral catalysts
that may operate by the dynamic enantiomorphic site control
mechanism and are proposed to invert chirality between
consecutive insertions of enantiomeric lactide monomers.10,11
Herein, we introduce semirigid catalysts for lactide polymer-
ization based on zirconium complexes of newly designed
imine−thiobis(phenolate) ligands.12 We demonstrate that by
stepwise adjustment of the ligand character, the tacticity of the
PLA produced from rac-lactide may be gradually shifted from
heterotactically inclined to isotactically inclined.
INTRODUCTION
■
Poly(lactic acid) (PLA) is a biodegradable polyester derived
from annually renewable resources, which finds applications as
a commodity plastic and in biomedicine.1 The physical
properties and degradation tendency of PLA depend strongly
on the microstructure of the polymer chains, and, in particular,
on their stereochemistry.2 The most common method of
producing PLA is the ring-opening polymerization (ROP) of
lactide, the cyclic dimer of lactic acid.3 Lactide includes two
stereogenic centers, which are not altered in the polymerization
process by catalysts operating by the coordination−insertion
mechanism. The tacticity of the obtained PLA thus depends on
the constitution of the monomer and on the selectivity of the
catalyst.2a,4 Polymerization of the homochiral lactides, L-lactide
and D-lactide yields the isotactic enantiomeric polymers poly(L-
lactic acid), PLLA, and poly(D-lactic acid), PDLA, respectively,
having a melting point of 180 °C, irrespective of the catalyst
employed. For rac-lactide, the character of the catalyst plays a
crucial role. Non selective catalysts, like the industrially
employed tin octoate, give rise to atactic PLA, whereas selective
catalysts may favor consecutive insertions of either the same or
the opposite lactide enantiomer, with formation of isotactic or
heterotactic PLA, respectively. Isoselective catalysts are
relatively scarce and may operate by either the chain-end or
enantiomorphic-site control mechanism. Most of the iso-
selective catalysts reported to date are based on aluminum
complexes, which feature a low activity.5,6 A racemic mixture of
the isotactic PLLA and PDLA crystallizes as a stereocomplex
whose melting point is 50 °C above that of the homochiral
polymers, so an isoselective catalyst that can yield a
stereocomplex PLA or its stereoblock analogues from rac-
lactide is highly desirable.7 Heteroselective catalysts are more
abundant.8 Suitable such catalysts should be devoid of
permanent chirality that may favor a specific enantiomer.
RESULTS AND DISCUSSION
■
Ligand Design. Since fluxional-chiral catalysts often induce
heterotacticity and rigid-chiral catalysts possibly induce
isotacticity, we were intrigued by the possibility of merging
these two facets in the form of semirigid complexes, i.e.,
complexes containing a rigid segment and a flexible segment. A
possible way of achieving semirigidity is by designing a
sequential tetradentate dianionic ligand that features an internal
rigid anchor and an internal flexible anchor. In addition, it is
preferred that the complex fluctuation would interconvert
between enantiomers rather than diastereomers (so as to avoid
a possible bias toward a specific lactide enantiomer by a given
catalyst molecule). Therefore, upon binding to a metal, the
rigid donor should be nonstereogenic and the flexible donor
should be stereogenic. Suitable candidates for these distinct
roles are the imine donor that typically orients its neighboring
Received: October 18, 2011
Revised: December 12, 2011
Published: January 6, 2012
© 2012 American Chemical Society
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donors in a meridional arrangement,13 and the thio donor that
typically orients its neighboring donors in a facial arrange-
ment,14 respectively. The peripheral anionic arms were chosen
to be of the phenolate type, because their steric and electronic
Scheme 1. Synthesis of the {ONSO} Ligand Precursors and
their Zirconium Complexes
groups led to a mild decrease of the interconversion barrier to
ΔG‡ = 15.6 kcal/mol (Lig2Zr(OtBu)2), while the analogous
replacement on the flexible-side phenol led to a more
pronounced decrease of the interconversion barrier to ΔG‡ =
13.3 kcal/mol (Lig1Zr(OtBu)2). The same trend was found for
the complexes of the three other ligands, Lig4−6Zr(OtBu)2
having interconversion barriers of ΔG‡ = 15.5, 17.7, and >19.0
kcal/mol, respectively, with the highest barrier found for the
most crowded complex, containing ortho-adamantyl groups on
both phenolate rings (Figure 2).
Figure 1. {ONSO} ligand design and proposed structure of semirigid
octahedral complexes.
character may be accurately modified by substitutions (Figure
1).
Ligands and Complexes Synthesis and Character-
ization. Six imine-thiobis(phenolate) ligand precursors were
targeted for this exploratory work. They include different
combinations of bulky alkyl groups and electron withdrawing
chloro groups in the ortho and para positions of the two phenol
rings. As one of the phenol rings is bound to the flexible donor
and the other is bound to the rigid donor, the placement of the
substituents may play different roles on the degree of
fluxionality of the corresponding complexes and on their
activity in lactide polymerization. To investigate this possibility,
the targeted ligands included the isomeric pairs Lig1H2/Lig2H2
and Lig4H2/Lig5H2 having inversely linked substituted phenols.
The ligands were prepared in moderate yields by a two step
synthesis starting from 2-aminoethanethiol: condensation of
the amine functionality with the appropriate salicylaldehyde,15
followed by nucleophilic reaction of the thiol functionality with
the given bromomethyl phenol. The six {ONSO}H2 ligand
precursors were reacted with 1 equiv of Zr(OtBu)4 yielding the
corresponding [{ONSO}Zr(OtBu)2] complexes in high to
quantitative yields, as described in Scheme 1.
‡
Figure 2. The most fluxional complex (Lig1Zr(OtBu)2; ΔG = 13.3
‡
kcal/mol) and the most rigid complex (Lig6Zr(OtBu)2; ΔG > 19.0
kcal/mol) described in this work.
Lactide Polymerization. The performance of these
[{ONSO}Zr(OtBu)2] complexes in polymerization of L-lactide
and rac-lactide was investigated. The complexes exhibited high
catalytic activities in the polymerization of both monomers at
140 °C. As may be appreciated from Tables 1 and 2, high
conversions of monomer to polymer were usually obtained
within a few minutes, and in certain cases almost complete
conversions were attained (entries 1 and 3 in Tables 1 and 2).
The complex of the bulkiest {ONSO} ligandLig6Zr(OtBu)2
seemed to be somewhat slower requiring longer times to reach
high conversions of both L-lactide and rac-lactide. For L-lactide,
the molecular weight distributions were relatively narrow (PDI
as low as 1.10) which seems to support a living polymerization,
however, in several polymerizations, the molecular weights
were higher than calculated based on a single growing polymer
chain for each catalyst molecule. For example, Lig1Zr(OtBu)2
led to PLLA with Mn = 72 600 g mol−1 whereas the calculated
molecular weight for 87% conversion of 300 equiv of L-lactide is
Mn,calc = 37 500 g mol−1. This is consistent with partial
activation of the catalyst. Slightly broader molecular weight
1H NMR characterization of the complexes revealed that
they had all formed as single diastereomers, and supported their
mononuclear structures. As expected, the complexes were
fluxional. Having a C1-symmetry on the slow-exchange regime,
and an average Cs-symmetry of the fast-exchange regime, the
geometry of these complexes is consistent with the proposed
meridional (imine)/facial (thio) wrapping of the {ONSO}
ligand around the octahedral metal center (Figure 1). Variable
temperature NMR experiments revealed that the barriers for
interconversion ranged from ΔG‡ of 13.3 to >19.0 kcal/mol.
Bulkier substituents led to higher rigidity, especially if they were
located in the ortho position of the flexible-segment phenol. For
example, Lig3Zr(OtBu)2 having two tert-butyl groups on both
phenol arms exhibited a ΔG‡ of 17.1 kcal/mol. Replacing the
two tert-butyl groups on the rigid-side phenol with chloro
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Table 1. Polymerization of L-Lactide at 140 °C
a
b
c
entry
initiator
L-lactide (g)
time (min)
PLA obtained (g)
convn
Mn (g mol−1)
Mn,calc (g mol−1)
PDI
1
2
3
4
5
6
Lig1Zr(O-tert-Bu)2
Lig2Zr(O-tert-Bu)2
Lig3Zr(O-tert-Bu)2
Lig4Zr(O-tert-Bu)2
Lig5Zr(O-tert-Bu)2
Lig6Zr(O-tert-Bu)2
0.62
0.58
0.60
0.58
0.58
15
14
10
13
6
0.54
0.18
0.55
0.41
0.40
0.28
0.87
0.31
0.92
0.71
0.69
0.57
72 600
22 300
49 200
20 300
23 000
29 100
37 500
13 400
39 700
30 600
30 000
24 500
1.10
1.12
1.39
1.16
1.47
1.17
0.49
54
a
b
c
10 mg of catalyst were employed. Correction parameter 0.58 × Mn polystyrene standards. calculated from 144.13 × (LA/I) × conversion of
monomer.
Table 2. Polymerization of rac-Lactide at 140 °C
a
b
c
d
entry
initiator
rac-lactide (g)
time (min)
PLA obtained (g)
convn
Mn (g mol−1)
Mn,calc (g mol−1)
PDI
Pr/Pm
1
2
3
4
5
6
Lig1Zr(O-tert-Bu)2
Lig2Zr(O-tert-Bu)2
Lig3Zr(O-tert-Bu)2
Lig4Zr(O-tert-Bu)2
Lig5Zr(O-tert-Bu)2
Lig6Zr(O-tert-Bu)2
0.62
0.61
0.63
0.58
0.58
6
5
0.58
0.46
0.61
0.42
0.41
0.32
0.92
0.75
0.97
0.72
0.70
0.61
16 300
17 700
20 300
9500
39 500
32 500
42 000
31 000
30 000
26 500
1.55
1.38
1.57
1.44
1.44
1.34
0.63/0.37
0.58/0.42
0.50/0.50
0.50/0.50
0.50/0.50
0.55/0.45
7
8
6
7000
0.52
18
20 300
a
b
c
10 mg of catalyst were employed. Correction parameter 0.58 × Mn polystyrene standards. Calculated from 144.13 × (LA/I) × conversion of
d
1
monomer. Pr and Pm are the probability for heterotactic and isotactic enchainment calculated from homonuclear decoupled H NMR spectrum.
Table 3. Polymerization of rac-Lactide at 70 °C in Toluene
a
b
c
d
entry
initiator
rac-lactide (g)
PLA obtained (g)
convn
Mn (g mol−1)
Mn,calc (g mol−1)
PDI
Pr/Pm
1
2
3
4
5
6
Lig1Zr(O-tert-Bu)2
Lig2Zr(O-tert-Bu)2
Lig3Zr(O-tert-Bu)2
Lig4Zr(O-tert-Bu)2
Lig5Zr(O-tert-Bu)2
Lig6Zr(O-tert-Bu)2
0.64
0.59
0.62
0.57
0.59
0.56
0.63
0.51
0.50
0.40
0.44
0.09
0.98
0.87
0.81
0.70
0.75
0.18
8500
14 700
21 300
11 200
8700
42 000
37 500
35 000
30 000
32 500
8000
1.45
1.50
1.70
1.42
1.56
1.17
0.72/0.28
0.65/0.35
0.50/0.50
0.50/0.50
0.38/0.62
0.33/0.67
18 000
a
b
10 mg of catalyst and 5 mL of toluene were employed. Polymerization time was 20 h. Correction parameter 0.58 × Mn polystyrene standards.
c
d
Calculated from 144.13 × (LA/I) × conversion of monomer. Pr and Pm are the probability for heterotactic and isotactic enchainment calculated
1
from homonuclear decoupled H NMR spectrum.
distributions and lower molecular weights were found for
polymerizations of rac-lactide with these catalysts at 140 °C,
and the molecular weights of the PLA did not exceed the
calculated values (Table 2). The solution polymerizations of
rac-lactide were pursued for 20 h (Table 3). Most catalysts,
except for Lig6Zr(OtBu)2, exhibited high conversions after that
period. The molecular weights and molecular weight
distributions were generally in line with the values obtained
for the polymerizations run in the melt. The polymerization of
rac-lactide versus time by Lig2Zr(OtBu)2 in toluene-d8 at 90 °C
was monitored by 1H NMR spectroscopy. Following an
induction period of a few minutes (that may be caused by
catalyst activation as well as monomer dissolution), the
polymerization was found to be first order with respect to
lactide up to a conversion of 88% (8 h), as evident from a linear
relationship of ln([lactide]t=0/[lactide]t) versus time (Figure 3).
We found that the microstructure of the PLA obtained in the
polymerization of rac-lactide could be gradually shifted by
variation of the phenolate substituents. Significantly, the
Figure 3. Semilogarithmic plot of rac-LA conversion versus time
character of the substituents on the flexible segment phenol
played a major role, and the character of the substituents on the
rigid segment phenol played a minor role, in parallel to their
effects on the fluxionality of the complexes. This phenomenon,
employing Lig2Zr(OtBu)2 in toluene-d8 at 90 °C. [rac-LA]t=0 = 0.34 M.
[rac-LA]t=0: [Zr] = 80. r2 = 0.994.
phenol and tert-butyl substituents on the rigid segment phenol
led to PLA having the highest degree of heterotacticity, with Pr
= 0.72 at 70 °C. The degree of heterotacticity was diminished
to Pr = 0.63 at 140 °C. Lig4Zr(OtBu)2, the analogous complex
1
clearly observable in the homodecoupled H NMR spectra of
the PLA was more apparent for the solution polymerizations
run at 70 °C. The most flexible complexLig1Zr(OtBu)2
which includes chloro substituents on the flexible segment
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1
Figure 4. Homo-decoupled H NMR of the polymer samples prepared from catalysts Lig1−6Zr(OtBu)2 at 70 °C in toluene.
which includes chloro substituents on the flexible segment
phenol and an o-adamantyl substituent on the rigid segment
phenol led to PLA with a slightly lower heterotacticity of Pr =
0.65 at 70 °C and Pr = 0.58 at 140 °C. Lig2Zr(OtBu)2 and
Lig3Zr(OtBu)2, the two complexes featuring tert-butyl groups
on the flexible segment phenol, yielded atactic PLA at both 70
°C and at 140 °C. Most unusually, Lig5Zr(OtBu)2 and
Lig6Zr(OtBu)2, the two complexes featuring the bulky 1-
adamantyl ortho-substituent on the flexible segment phenol led
to clear isoselective polymerization at 70 °C, which was more
apparent for the more rigid of the twoLig6Zr(OtBu)2 that
yielded PLA with Pm = 0.67. The isoselectivity was reduced for
the two complexes at 140 °C. The transition between tacticity
CONCLUSIONS
■
In summary, zirconium complexes of tetradentate ligands that
combine a rigid segment and a flexible segment were described.
These tailor-made complexes exhibited broad ranges of steric
congestion on each of the different phenol arms that was
manifested in their degree of fluxionality. Notably, the
substitution pattern on the phenolate rings was shown to
play equivalent roles in the degree of fluxionality of the
complexes and in their tendencies to insert the same or the
opposite lactide enantiomer, i.e., their tendency to form
isotactic or heterotactic PLA from rac-lactide. It still needs to
be established whether the reversal of stereoselectivity is a
direct consequence of the change of ligand substitution pattern,
or results from the change of complex fluxionality. Current
efforts in our groups include the development of rigid
analogues of the above systems aiming at catalysts of higher
isoselectivity based on the findings introduced herein.
1
types can be clearly appreciated in the homodecoupled H
NMR spectra of the corresponding polymers in Figure 4.
Notably, reverse tacticity induction could be attained by merely
inverting the substitution patterns on the two different phenol
arms, as apparent for the isomer pair Lig4Zr(OtBu)2 (Pr = 0.63)
and Lig5Zr(OtBu)2 (Pm = 0.63). To our knowledge, the only
precedence for reversal of tacticity of PLA in rac-lactide
polymerization upon a change of the substitution pattern of a
given ligand bound to a given metal involved Salan and related
ligands bound to aluminum.16 While the tacticities reported
herein are not exceptionally high, the ability to manipulate
them by ligand modifications, and the relatively high catalytic
activities indicate that such design motifs may be important for
future stereoselective catalysts.
EXPERIMENTAL SECTION
■
General Information. All reactions with air- and/or water
sensitive compounds were carried out under dry nitrogen atmosphere
in a glovebox. Ether was purified by distillation under dry argon
atmosphere from purple Na/benzophenone solution. Pentane was
washed with HNO3/H2SO4 prior to distillation from Na/benzophe-
none/tetraglyme. Toluene was refluxed over Na and distilled. 3,5-
Dichloro-2-hydroxybenzaldehyde, 3,5-di-tert-butyl-2-hydroxybenzalde-
hyde, 2-aminoethanethiol, triethylamine, and Zr(tert-butoxide)4 were
purchased from Aldrich and used as received. D-Lactide and L-lactide
were obtained by Purac and used as received, rac-lactide was prepared
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by mixing equal molar amounts of L-lactide and D-lactide and
crystallizing from toluene. 3-Adamantyl-2-hydroxy-5-methylbenzalde-
hyde,17 2-(bromomethyl)-4,6-dichlorophenol,18 2-(bromomethyl)-4,6-
di-tert-butylphenol,18 and 2-(bromomethyl)-4-methyl-6-adamanthyl-
phenol19 were synthesized according to published procedures.
All NMR data were recorded on a Bruker Avance-400 spectrometer.
C6D6 (impurities in benzene-d6 at δ 7.15, and 13C chemical shift of
benzene at δ 128.70 were used as reference) and C7D8 (impurities in
toluene-d8 at δ 2.09, 6.98, 7.00, 7.09) were used as NMR solvents for
the metal complexes. CDCl3 was used as NMR solvent for the PLA
samples (chemical shift of TMS at δ 0.00 as reference). Elemental
analyses were performed in the microanalytical laboratory at the
Hebrew University of Jerusalem. PLA molecular weights were
determined by gel permeation chromatography (GPC) using TSKgel
GMHHR-M and TSKgel G 3000 HHR columns set on a Jasco
instrument equipped with a refractive index detector. Molecular weight
determination was carried out relative to polystyrene standards using
THF (high-performance liquid chromatography grade, distilled and
filtered under vacuum prior to use) as the eluting solvent.
Synthesis of 2-(Mercaptoethylimino)methyl-4,6-dichlorophenol.
2-Aminoethanethiol (0.99 g, 12.9 mmol) was added to a solution of
3,5-dichloro-2-hydroxybenzaldehyde (2.46 g, 12.9 mmol) in ethanol
and stirred for 2 h at room temperature. The solvent was removed
under reduced pressure yielding a yellow solid (2.88 g, 89%).
MS(APPI): calcd for C9H9Cl2NOS, 250.1; found, 250.0.
Synthesis of Lig3H2. A solution of 2-(bromomethyl)-4,6-di-tert-
butylphenol (1.52 g, 5.1 mmol) in THF (20 mL) was added dropwise
to a solution of 2-(mercaptoethylimino)methyl-4,6-dichlorophenol
(1.27 g, 5.1 mmol) and triethylamine (0.70 mL) in THF (20 mL) and
stirred for 2 h. The reaction mixture was worked up as described above
1
for Lig1H2 giving Lig3H2 in a final yield of 53%. H NMR (400 MHz,
CDCl3): δ 8.18 (s, 1H, NCH), 7.41 (d, 1H, J = 2.5 Hz, ArH), 7.28 (d,
1H, J = 2.4 Hz, ArH), 7.13 (d, 1H, J = 2.5 Hz, ArH), 6.92 (d, 1H, J =
2.7 Hz, ArH), 3.83 (s, 2H, ArCH2S), 3.68 (t, 2H, J = 6.4 Hz, CH2),
2.74 (t, 2H, J = 6.6 Hz, CH2), 1.42 (s, 9H, C(CH3)3), 1.28 (s, 9H,
C(CH3)3). 13C NMR (100.66 MHz, CDCl3), δ 164.5 (CN), 156.6
(C), 151.9 (C), 142.5 (C), 137.3 (C), 132.4 (CH), 129.1 (CH), 125.3
(CH), 123.9 (CH), 122.9 (C), 122.7 (C), 121.4 (C), 119.3 (C), 58.1
(CH2), 34.9 (C), 34.4 (CH2), 34.2 (C), 31.6 (CH3), 31.3 (CH2), 29.8
(CH3). MS(APPI): calcd for C24H31Cl2NO2S, 468.5; found, 506.2
(MK+). Anal. Calcd for C24H31Cl2NO2S: C, 61.53; H, 6.67; N, 2.99.
Found: C, 60.94; H, 6.58; N, 2.69.
Synthesis of 2-(Mercaptoethylimino)methyl-4,6-ditert-butylphe-
nol. 2-Aminoethanethiol (0.88 g, 11.4 mmol) was added to a solution
of 3,5-di-tert-butyl-2-hydroxybenzaldehyde (2.68 g, 11.4 mmol) in
benzene and refluxed for 2 h. The solvent was evaporated yielding a
yellow solid (3.14 g, 93%). MS(APPI): calcd for C17H27NOS, 293.5;
found, 294.1.
Synthesis of Lig4H2. A solution of 2-(bromomethyl)-4,6-di-tert-
butylphenol (1.46 g, 4.9 mmol) in THF (20 mL) was added dropwise
to a solution of 2-(mercaptoethylimino)methyl-4,6-di-tert-butylphenol
(1.43 g, 4.9 mmol) and triethylamine (0.68 mL) in THF (20 mL) and
stirred for 2 h. The reaction mixture was worked up as described above
Synthesis of Lig1H2. A solution of 2-(bromomethyl)-4,6-dichlor-
ophenol (1.40 g, 5.5 mmol) in THF (20 mL) was added dropwise to a
solution of 2-(mercaptoethylimino)methyl-4,6-di-tert-butylphenol
(1.61 g, 5.5 mmol) and triethylamine (0.75 mL) in THF (20 mL)
and stirred for 2 h. The solid that had formed was filtered out, the
solvent was removed under vacuum. The crude product was purified
by flash chromatography over Silica gel 60 with a mixture of petroleum
ether/dichloromethane in increasing polarity as eluent. The pure
product was obtained as a yellow solid in a final yield of 40%. 1H NMR
(400 MHz, CDCl3): δ 8.32 (s, 1H, NCH), 7.39 (d, 1H, J = 2.4 Hz,
ArH), 7.23 (d, 1H, J = 2.4 Hz, ArH), 7.15 (d, 1H, J = 2.4 Hz, ArH),
7.09 (d, 1H, J = 2.4 Hz, ArH), 3.76 (s, 2H, ArCH2S), 3.74 (t, 2H, J =
6.7 Hz, CH2), 2.80 (t, 2H, J = 6.7 Hz, CH2), 1.44 (s, 9H, C(CH3)3),
1.31 (s, 9H, C(CH3)3). 13C NMR (100.66 MHz, CDCl3), δ 167.3
(CN), 158.1 (C), 148.6 (C), 140.2 (C), 136.8 (C), 129.1 (CH), 127.7
(CH), 127.3 (C), 127.2 (CH), 126.1 (CH), 125.3 (C), 121.1(C),
117.7 (C), 59.2 (CH2), 35.1 (C), 34.2 (C), 32.6 (CH2), 31.5 (CH3),
31.3 (CH2), 29.5 (CH3). MS(APPI): calcd for C32H49NO2S, 511.8;
found, 534.3 (MNa+). Anal. Calcd for C24H31Cl2NO2S: C, 61.53; H,
6.67; N, 2.99. Found: C, 61.95; H, 6.65; N, 2.82.
1
for Lig1H2 giving Lig4H2 in a final yield of 30%. H NMR (400 MHz,
CDCl3): δ 8.28 (s, 1H, NCH), 7.38 (d, 1H, J = 2.0 Hz, ArH), 7.27 (d,
1H, J = 2.1 Hz, ArH), 7.06 (d, 1H, J = 1.9 Hz, ArH), 6.95 (d, 1H, J =
2.0 Hz, ArH), 3.85 (s, 2H, ArCH2S), 3.67 (t, 2H, J = 6.7 Hz, CH2),
2.72 (t, 2H, J = 6.6 Hz, CH2), 1.43 (s, 9H, C(CH3)3), 1.42 (s, 9H,
C(CH3)3), 1.30 (s, 9H, C(CH3)3), 1.27 (s, 9H, C(CH3)3). 13C NMR
(100.66 MHz, CDCl3), δ 167.2 (CN), 158.0 (C), 152.0 (C), 142.3
(C), 140.2 (C), 137.2 (C), 136.8 (C), 127.2 (CH), 125.9 (CH), 125.3
(CH), 123.8 (CH), 121.6 (C), 117.7 (C), 59.2 (CH2), 35.0 (CH2),
34.5 (CH2), 34.2 (C), 34.1 (C), 31.6 (CH3), 31.5 (CH3), 31.3 (C),
29.8 (CH3), 29.4 (CH3), 29.3 (C). MS(APPI): calcd for C32H49NO2S,
511.8; found, 534.3 (MNa+). Anal. Calcd for C32H49NO2S: C, 75.10;
H, 9.65; N, 2.74. Found: C, 75.26; H, 9.36; N, 2.44.
Synthesis of Lig5H2. A solution of 2-(bromomethyl)-4-methyl-6-
adamanthylphenol (1.00 g, 3.0 mmol) in THF (20 mL) was added
dropwise to a solution of 2-(mercaptoethylimino)methyl-4,6-dichlor-
ophenol (0.75 g, 3.0 mmol) and triethylamine (0.42 mL) in THF (20
mL) and stirred for 2 h. The reaction mixture was worked up as
Synthesis of 2-(Mercaptoethylimino)methyl-4-methyl-6-ada-
mantylphenol. 2-Aminoethanethiol (0.30 g, 3.88 mmol) was added
to a solution of 3-adamantyl-2-hydroxy-5-methylbenzaldehyde (1.05 g,
3.88 mmol) in benzene and refluxed for 2 h. The solvent was removed
under reduced pressure yielding a yellow solid (0.91 g, 71%).
MS(APPI): calcd for C20H27NOS, 329.5; found, 330.1.
1
described above for Lig1H2 giving Lig5H2 in a final yield of 31%. H
NMR (400 MHz, CDCl3): δ 8.15 (s, 1H, NCH), 7.41 (d, 1H, J = 2.4
Hz, ArH), 7.14 (d, 1H, J = 2.4 Hz, ArH), 6.99 (d, 1H, J = 1.6 Hz,
ArH), 6.73 (s, 1H, ArH), 3.78 (s, 2H, ArCH2S), 3.66 (t, 2H, J = 6.3
Hz, CH2), 2.74 (t, 2H, J = 6.5 Hz, CH2), 2.24 (s, 3H, ArCH3), 2.12
(bs, 6H, adamantyl), 2.07 (bs, 3H, adamantyl), 1.77 (bs, 6H,
adamantyl). 13C NMR (100.66 MHz, CDCl3): δ 165.3 (CN), 157.3
(C), 152.9 (C), 138.9 (C), 133.1 (CH), 129.9 (C), 129.8 (CH), 129.4
(CH), 128.4 (CH), 123.7 (C), 123.4 (C), 122.8 (C), 120.1 (C), 58.7
(CH2), 41.4 (CH2), 37.8 (CH2), 37.6 (C), 34.8 (CH2), 32.1 (CH2),
29.8 (CH), 21.5 (CH3). MS(APPI): calcd for C27H31Cl2NO2S, 504.5;
found, 526.1 (MNa+). Anal. Calcd for C27H31Cl2NO2S: C, 64.28; H,
6.19; N, 2.78. Found: C, 63.83; H, 6.11; N, 2.74
Synthesis of Lig2H2. A solution of 2-(bromomethyl)-4,6-dichlor-
ophenol (0.60 g, 2.3 mmol) in THF (20 mL) was added dropwise to a
solution of 2-(mercaptoethylimino)methyl-4-methyl-6-adamantylphe-
nol (0.76 g, 2.3 mmol) and triethylamine (0.32 mL) in THF (20 mL)
and stirred for 2 h. The reaction mixture was worked up as described
1
above for Lig1H2 giving Lig2H2 in a final yield of 50%. H NMR (400
MHz, CDCl3): δ 8.28 (s, 1H, NCH), 7.25 (d, 1H, J = 2.3 Hz, ArH),
7.16 (d, 1H, J = 2.4 Hz, ArH), 7.08 (d, 1H, J = 1.9 Hz, ArH), 6.90 (d,
1H, J = 3.3 Hz, ArH), 3.75 (s, 2H, ArCH2S), 3.75 (t, 2H, J = 8.3 Hz,
CH2), 2.80 (t, 2H, J = 6.7 Hz, CH2), 2.28 (s, 3H, ArCH3), 2.16 (bs,
6H, adamantyl), 2.07 (bs, 3H, adamantyl), 1.78 (m, 6H, adamantyl).
13C NMR (100.66 MHz, CDCl3): δ 167.7 (CN), 159.1 (C), 149.3
(C), 138.2 (C), 131.5 (CH), 130.3 (CH), 129.8 (CH), 128.5 (CH),
127.8 (C), 127.5 (C), 126.0 (C), 121.8 (C), 118.9 (C), 59.9 (CH2),
41.0 (CH2), 37.9 (CH2), 37.7 (C), 33.1 (CH2), 32.0 (CH2), 29.8
(CH), 21.4 (CH3). MS(ESI): calcd for C27H31Cl2NO2S, 504.5; found,
504.2. Anal. Calcd for C27H31Cl2NO2S: C, 64.28; H, 6.19; N, 2.78.
Found: C, 63.14; H, 5.89; N, 2.51.
Synthesis of Lig6H2. A solution of 2-(bromomethyl)-4-methyl-6-
adamanthylphenol (0.93 g, 2.8 mmol) in THF (20 mL) was added
dropwise to a solution of 2-(mercaptoethylimino)methyl-4-methyl-6-
adamantylphenol (0.91 g, 2.8 mmol) and triethylamine (0.38 mL) in
THF (20 mL) and stirred for 2 h. The reaction mixture was worked up
as described above for Lig1H2 giving Lig6H2 in a final yield of 20%. 1H
NMR (400 MHz, CDCl3): δ 8.23 (s, 1H, NCH), 7.08 (d, 1H, J = 2.0
Hz, ArH), 6.98 (d, 1H, J = 1.9 Hz, ArH), 6.88 (d, 1H, J = 1.8 Hz,
ArH), 6.76 (d, J = 1.7, 1H, ArH), 3.77 (s, 2H, ArCH2S), 3.64 (t, 2H, J
= 6.4 Hz, CH2), 2.72 (t, 2H, J = 6.6 Hz, CH2), 2.28 (s, 3H, ArCH3),
2.24 (s, 3H, ArCH3), 2.17 (m, 6H, adamantyl), 2.13 (m, 6H,
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adamantyl), 2.07 (bs, 6H, adamantyl) 1.77 (bs, 12H, adamantyl). 13C
NMR (100.66 MHz, CDCl3): δ 167.8 (CN), 159.1 (C), 153.1 (C),
138.8 (C), 138.2 (C), 131.5 (CH), 130.2 (CH), 129.7 (C), 129.6
(CH), 128.2 (CH), 127.5 (C), 122.9 (C), 118.9 (C), 59.9 (CH2), 41.3
(CH2), 41.0 (CH2), 37.9 (CH2), 37.8 (CH2), 37.7 (C), 37.6 (C), 34.9
(CH2), 32.1 (CH2), 29.8 (CH), 29.8 (CH), 21.5 (CH3), 21.4 (CH3).
MS(APPI): calcd for C38H49NO2S, 583.9; found, 606.3 (MNa+). Anal.
Calcd for C38H49NO2S: C, 78.17; H, 8.46; N, 2.40. Found: C, 77.89;
H, 8.16; N, 2.12.
126.4 (CH), 124.7 (CH), 123.1 (CH), 121.1 (C), 76.9 (C), 75.8 (C),
60.1 (CH2), 36.7 (CH2), 36.3 (CH2), 35.9 (C), 34.7 (C), 33.6 (CH3),
32.4 (CH3), 32.0 (CH3), 31.9 (C), 31.6 (C), 30.6 (CH3). Anal. Calcd
for C40H65NO4SZr: C, 64.29; H, 8.77; N, 1.87. Found: C, 63.36; H,
8.59; N, 1.54.
Synthesis of Lig5Zr(O-tert-Bu)2. Lig5H2 (49 mg, 0.10 mmol) was
dissolved in ca. 2 mL of ether and was added dropwise to a solution of
Zr(OtBu)4 (38 mg, 0.10 mmol) at room temperature. The solution
was stirred for 2 h after which the solvent was removed under vacuum,
Synthesis of Lig1Zr(O-tert-Bu)2. Lig1H2 (47 mg, 0.10 mmol) was
dissolved in ca. 2 mL of ether and was added dropwise to a solution of
Zr(OtBu)4 (38 mg, 0.10 mmol) at room temperature. The solution
was stirred for 2 h after which the solvent was removed under vacuum
and the resulting yellow solid was washed with pentane (49 mg, 70%).
1H NMR (400 MHz, C6D6): δ 7.71 (d, 1H, J = 1.8 Hz, ArH), 7.29 (d,
1H, J = 1.8 Hz, ArH), 7.24 (s, 1H, NCH), 6.94 (d, 1H, J = 2.0 Hz,
ArH), 6.63 (d, J = 2.4, 1H, ArH), 3.50 (bs, 2H, CH2), 2.85 (bs, 2H,
CH2), 1.91 (m, 2H, CH2), 1.75 (s, 9H, C(CH3)3), 1.29 (s, 27H,
C(CH3)3). 13C NMR (100.66 MHz, C6D6): δ 167.8 (CN), 160.7 (C),
158.4 (C), 139.1 (C), 138.6 (C), 129.7 (CH), 129.6 (CH), 128.5
(CH), 128.1 (CH), 124.9 (C), 123.4 (C), 122.1 (C), 119.8 (C), 59.2
(CH2), 35.5 (CH2), 34.5 (CH2), 33.9 (C), 32.5 (CH3), 31.3 (CH3),
31.1 (C), 29.7 (CH3). Anal. Calcd for C32H47Cl2NO4SZr: C, 54.60; H,
6.73; N, 1.99. Found: C, 53.82; H, 6.43; N, 1.75.
giving a yellow solid quantitatively (76 mg). H NMR (400 MHz,
1
C6D6): δ 7.33 (d, 1H, J = 2.5 Hz, ArH), 7.07 (d, 1H, J = 2.4 Hz, ArH),
6.86 (s, 1H, NCH), 6.61 (d, 1H, J = 2.6 Hz, ArH), 6.50 (d, 1H, J = 1.8,
ArH), 3.89 (d, 1H, J = 13.9, CH), 3.36 (d, 1H, J = 13.9, CH), 3.08 (m,
1H, CH), 2.40 (m, 1H, CH), 2.24 (s, 3H, ArCH3), 2.14 (m, 6H,
adamantyl), 1.86 (m, 6H, adamantyl), 1.70 (m, 3H, adamantyl), 1.54
(s, 9H, C(CH3)3), 1.45 (m, 1H, CH), 1.22 (m, 1H, CH), 1.14 (s, 9H,
C(CH3)3). 13C NMR (100.66 MHz, C6D6): δ 164.0 (CN), 160.9 (C),
139.0 (C), 134.5 (CH), 131.7 (CH), 129.4 (CH), 126.9 (C), 125.4
(C), 123.9 (C), 121.4 (C), 120.9 (C), 77.2 (C), 76.2 (C), 61.3 (CH2),
41.2 (CH2), 37.7 (CH2), 37.5 (C), 36.5 (CH2), 33.3 (CH3), 33.2
(CH3), 31.8 (CH2), 30.1 (CH) 21.4 (CH3). Anal. Calcd for
C35H47Cl2NO4SZr·Et2O: C, 57.54; H, 7.06; N, 1.72. Found: C,
57.60; H, 6.61; N, 1.64.
Synthesis of Lig6Zr(O-tert-Bu)2. Lig6H2 (34 mg, 0.06 mmol) was
dissolved in ca. 2 mL of ether and was added dropwise to a solution of
Zr(OtBu)4 (22 mg, 0.06 mmol) at room temperature. The solution
was stirred for 2 h after which the solvent was removed under vacuum,
Synthesis of Lig2Zr(O-tert-Bu)2. Lig2H2 (34 mg, 0.07 mmol) was
dissolved in ca. 2 mL of ether and was added dropwise to a solution of
Zr(OtBu)4 (26 mg, 0.07 mmol) at room temperature. The solution
was stirred for 2 h after which the solvent was removed under vacuum,
1
giving a yellow solid quantitatively (55 mg). H NMR (400 MHz,
1
C6D6): δ 7.33 (s, 1H, NCH), 7.26 (d, 1H, J = 2.2 Hz, ArH), 7.15 (s,
1H, ArH), 6.58 (d, 2H, J = 1.8 Hz, ArH), 4.05 (d, 1H, J = 13.7, CH),
3.48 (d, 1H, J = 13.7, CH), 3.11 (m, 1H, CH), 2.57 (m, 2H, CH), 2.48
(bs, 6H, adamantyl), 2.40 (m, 1H, CH), 2.31 (s, 3H, ArCH3), 2.26 (m,
6H, adamantyl), 2.15 (s, 3H, ArCH3), 2.10 (m, 3H, adamantyl), 1.85
(m, 12H, adamantyl), 1.64 (m, 3H, adamantyl), 1.55 (s, 9H,
C(CH3)3), 1.16 (s, 9H, C(CH3)3). 13C NMR (100.66 MHz, C6D6):
δ 166.5 (CN), 161.7 (C), 161.1 (C), 139.8 (C), 139.1 (C), 134.2
(CH), 132.7 (CH), 129.4 (CH), 128.9 (CH), 126.3 (C), 124.6 (C),
123.7 (C), 120.9 (C), 76.0 (C), 75.5 (C), 61.5 (CH2), 41.8 (CH2),
41.6 (CH2), 38.2 (C), 38.0 (CH2), 37.9 (CH2), 37.7 (C), 36.6 (CH2),
33.7 (CH3), 33.3 (CH3), 32.0 (CH2), 30.1 (CH), 30.0 (CH), 21.4
(CH3), 21.1 (CH3). Anal. Calcd for C46H65NO4SZr: C, 67.43; H, 8.00;
N, 1.71. Found: C, 67.48; H, 7.52; N, 1.25.
yielding a yellow solid quantitatively (55 mg). H NMR (400 MHz,
C6D6): δ 7.34 (s, 1H, ArH), 7.30 (s, 1H, ArH), 7.28 (s, 1H, NCH),
6.65 (s, 1H, ArH), 6.63 (s, 1H, ArH), 3.68 (d, 1H, J = 11.7, CH), 3.08
(d, 1H, J = 12.6, CH), 3.02 (m, 1H, CH), 2.69 (m, 1H, CH), 2.47 (s,
3H, ArCH3) 2.22 (m, 6H, adamantyl) 2.01 (m, 6H, adamantyl), 1.92
(m, 1H, CH), 1.86 (m, 3H, adamantyl), 1.67 (m, 1H, CH), 1.38 (s,
9H, C(CH3)3), 1.25 (s, 9H, C(CH3)3). 13C NMR (100.66 MHz,
C6D6): δ 167.7 (CN), 139.9 (C), 139.4 (C), 134.5 (CH), 132.5 (CH),
130.4 (CH), 126.6 (C), 125.8 (C), 124.0 (C), 123.5 (C), 120.3 (C),
59.7 (CH2), 41.2 (CH2), 38.0 (CH2), 37.4 (C), 35.2 (CH2), 33.4
(CH3), 31.6 (CH2), 30.1 (CH), 21.2 (CH3). Anal. Calcd for
C35H47Cl2NO4SZr: C, 56.81; H, 6.40; N, 1.89. Found: C, 56.98; H,
6.41; N, 1.85.
Synthesis of Lig3Zr(O-tert-Bu)2. Lig3H2 (46 mg, 0.09 mmol) was
dissolved in ca. 2 mL of ether and was added dropwise to a solution of
Zr(OtBu)4 (37 mg, 0.09 mmol) at room temperature. The solution
was stirred for 2 h after which the solvent was removed under vacuum,
resulting in a yellow solid (62 mg, 90%). 1H NMR (400 MHz, C6D6):
δ 7.57 (d, 1H, J = 2.5 Hz, ArH), 7.40 (d, 1H, J = 2.7 Hz, ArH), 6.95 (d,
1H, J = 2.4 Hz, ArH), 6.69 (s, 1H, NCH), 6.52 (d, J = 2.7, 1H, ArH),
4.31 (d, 1H, J = 12.2, CH), 3.67 (m, 1H, CH), 3.56 (d, 1H, J = 12.5,
CH), 2.11 (m, 3H, CH, CH2), 1.63 (s, 9H, C(CH3)3), 1.58 (s, 9H,
C(CH3)3), 1.38 (s, 9H, C(CH3)3), 1.23 (s, 9H, C(CH3)3). 13C NMR
(100.66 MHz, C6D6): δ 165.9 (CN), 159.8 (C), 159.5 (C), 139.4 (C),
137.0 (C), 133.9 (CH), 130.3 (CH), 126.5 (C), 126.3 (CH), 124.1
(CH), 123.1 (C), 121.4 (C), 119.6 (C), 76.9 (C), 60.0 (CH2), 35.9
(CH2), 35.3 (CH2), 34.0 (C), 32.5 (CH3), 31.7 (CH3), 30.5 (C), 29.7
(CH3). Anal. Calcd for C32H47Cl2NO4SZr: C, 54.60; H, 6.73; N, 1.99.
Found: C, 54.90; H, 6.68; N, 1.70.
General Polymerization Procedure. Bulk polymerizations of L-
lactide and rac-lactide were carried out by heating the monomer and
the catalyst in a closed glass vessel to 140 °C for a period of time by
which the melt had become viscous. Here, 10 mg of catalyst and a
300:1 lactide to catalyst molar ratio were employed for all
polymerizations. The polymerization runs were terminated by the
addition of 1 mL of methanol. The reaction mixture was dissolved in
dichloromethane, followed by removal of the volatiles under reduced
pressure. The resulting polymer was purified by stirring with excess of
methanol overnight to remove any unreacted monomer, filtered, and
1
dried under vacuum for 2 h. H NMR analysis of the PLA samples
obtained by polymerization of L-lactide (400 MHz, CDCl3) indicated
that no epimerization occurred during the polymerization, and that the
polymer was isotactic, as was evident from a quartet (5.16 ppm) and a
doublet (1.58 ppm) in a ratio of 1:3.
Synthesis of Lig4Zr(O-tert-Bu)2. Lig4H2 (42 mg, 0.08 mmol) was
dissolved in ca. 2 mL of ether and was added dropwise to a solution of
Zr(OtBu)4 (32 mg, 0.08 mmol) at room temperature. The solution
was stirred for 2 h after which the solvent was removed under vacuum
and the resulting yellow solid was washed with pentane affording the
Solution polymerization runs of rac-lactide in 5 mL of toluene were
carried out at 70 °C employing 10 mg of catalyst and a 300:1 lactide to
catalyst molar ratio. After 20 h the reaction was terminated by the
addition of 1 mL of methanol and the volatiles were removed under
vacuum. The resulting polymer was purified by stirring with excess of
methanol overnight to remove any unreacted monomer, filtered, and
1
product quantitatively (63 mg). H NMR (400 MHz, C6D6): δ 7.69
1
(d, 1H, J = 2.5 Hz, ArH), 7.54 (d, 1H, J = 2.2 Hz, ArH), 7.23 (s, 1H,
NCH), 6.93 (d, 1H, J = 2.4 Hz, ArH), 6.89 (d, J = 2.4, 1H, ArH), 4.25
(d, 1H, J = 12.7, CH), 3.58 (d, 1H, J = 13.3, CH), 3.47 (m, 1H, CH),
2.50 (m, 1H, CH), 2.25 (m, 2H, CH2), 1.78 (s, 9H, C(CH3)3), 1.61 (s,
9H, C(CH3)3), 1.37 (s, 9H, C(CH3)3), 1.35 (s, 18H, C(CH3)3), 1.31
(s, 9H, C(CH3)3). 13C NMR (100.66 MHz, C6D6): δ 167.8 (CN),
160.9 (C), 139.2 (C), 138.9 (C), 138.6 (C), 138.1 (C), 130.3 (CH),
dried under vacuum for 2 h. The homonuclear-decoupled H NMR
spectrum of the PLA samples (400 MHz, CDCl3) of the methine
region was consistent with the formation of chains that are
predominantly heterotactic, isotactic or atactic estimated from the
relative intensities of the rmr (δ 5.23 ppm) and the mrm (δ 5.16 ppm)
tetrads for heterotactic sequences and the mmm (δ 5.17 ppm) tetrad
for isotactic sequences vs other tetrads (rmr δ 5.23 ppm, mrm δ 5.16
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ppm, rmm/mmr δ 5.22 and 5.18 ppm, mmm δ 5.17 ppm).20−22
Molecular weight determination and PDI analysis were done by GPC
measurements.
(c) Liu, X.; Shang, X.; Tang, T.; Hu, N.; Pei, F.; Cui, D.; Chen, X.;
Jing, X. Organometallics 2007, 26, 2747. (d) Clark, L.; Cushion, M. G.;
Dyer, H. E.; Schwarz, A. D.; Duchateau, R.; Mountford, P. Chem.
Commun. 2010, 273.
(10) (a) Ma, H.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed.
2006, 45, 7818. (b) Ma, H.; Spaniol, T. P.; Okuda, J. Inorg. Chem.
2008, 47, 3328. (c) Chmura, A. J.; Chuck, C. J.; Davidson, M. G.;
Jones, M. D.; Lunn, M. D.; Bull, S. D.; Mahon, M. F. Angew. Chem., Int.
Ed. 2007, 46, 2280. (d) Chmura, A. J.; Davidson, M. G.; Frankis, C. J.;
Jones, M. D.; Lunn, M. D. Chem. Commun. 2008, 1293.
(11) (a) Zelikoff, A. L.; Kopilov, J.; Goldberg, I.; Coates, G. W.; Kol,
M. Chem. Commun. 2009, 6804. (b) Sergeeva, E.; Kopilov, J.;
Goldberg, I.; Kol, M. Inorg. Chem. 2010, 49, 3977.
(12) For a report on different{OSNO} ligands, see: Meppelder, G-J.
M.; Fan, H.-T.; Spaniol, T. P.; Okuda, J. Inorg. Chem. 2009, 48, 7378.
(13) (a) Yeori, A.; Gendler, S.; Groysman, S.; Goldberg, I.; Kol, M.
Inorg. Chem. Commun. 2004, 7, 280. (b) Press, K.; Cohen, A.;
Goldberg, I.; Venditto, V.; Mazzeo, M.; Kol, M. Angew. Chem., Int. Ed.
2011, 50, 3591.
AUTHOR INFORMATION
Corresponding Author
*E-mail: moshekol@post.tau.ac.il.
Author Contributions
The manuscript was written through contributions of all
authors.
■
ACKNOWLEDGMENTS
■
We thank the German Israeli Foundation (G.I.F.) and the
Britain-Israel Research and Academic Exchange Partnership
(BIRAX) foundation for funding. We thank Purac for a
generous gift of D- and L-lactide.
(14) Cohen, A.; Yeori, A.; Goldberg, I.; Kol, M. Inorg. Chem. 2007,
46, 8114.
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dx.doi.org/10.1021/ma2023364 | Macromolecules 2012, 45, 698−704