Zn(ii) and Cu(ii) formamidine complexes: Structural, kinetics and polymer tacticity studies in the ring-opening polymerization of ?-caprolactone and lactides

Article abstract of DOI:10.1039/c5nj03159a

Treatment of N,N′-bis(2,6-dimethylphenyl)formamidine (L1), N,N′-bis(2,6-diisopropylphenyl)formamidine (L2), and N,N′-dimesitylformamidine (L3) with Zn(OAc)₂·2H₂O or Cu(OAc)₂·H₂O produced the corresponding Zn(ii)

Full text of DOI:10.1039/c5nj03159a

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Zn(II) and Cu(II) formamidine complexes:
structural, kinetics and polymer tacticity studies
in the ring-opening polymerization of
e-caprolactone and lactides†
Cite this:DOI: 10.1039/c5nj03159a
a
b
a
Ekemini D. Akpan, Stephen O. Ojwach,* Bernard Omondi* and
a
Vincent O. Nyamori
0
0
Treatment of N,N -bis(2,6-dimethylphenyl)formamidine (L1), N,N -bis(2,6-diisopropylphenyl)formamidine
0
(
L2), and N,N -dimesitylformamidine (L3) with Zn(OAc)
2
ꢀ2H
(L1)
] (4), respectively. While complex 1 is trinuclear, compounds 2–4 are dimeric in the
2
O or Cu(OAc)
2
ꢀH
2
O produced the corresponding
0
Zn(II) and Cu(II) N,N -diarylformamidine complexes [Zn
3
2
(OAc) ] (1), [Zn
6
2
(L2)
2
(OAc) ] (2), [Zn (L3) (OAc) ] (3)
4
2
2
4
and [Cu (L2) (OAc)
2
2
4
solid state. The X-band EPR spectra of complex 4 in solid and solution states are consistent with perfect
axial symmetry and confirm retention of the dinuclear paddle-wheel core in the solution state. Complexes
1
–4 formed active catalysts in the ring opening polymerization (ROP) of e-caprolactone (e-CL) and lactides
ꢁ1
ꢁ1
(LA). Complexes 1 and 3 exhibited higher rate constants of 0.1009 h and 0.0963 h compared to the rate
ꢁ1
ꢁ1
constants of 0.0479 h and 0.0477 h observed for 2 and 4, respectively, in the ROP of e-CL at
ꢁ
1
ꢁ1
110 1C. Higher rate constants of 0.5963 h and 1.2962 h were obtained for complexes 1 and 3 in the
ROP of LAs compared to those reported in the ROP of e-CL at 110 1C. Activation parameters were
Received (in Victoria, Australia)
‡
ꢁ1
‡
ꢁ1
ꢁ1
9th November 2015,
determined as DH = 25.08 kJ mol and DS = ꢁ201.7 J K mol for the ROP of e-CL using 3.
Investigation of the kinetics of polymerization of e-CL and LAs revealed first order dependence of
the polymerization reactions on monomer concentration. Moderate molecular weight polymers of up to
Accepted 10th February 2016
DOI: 10.1039/c5nj03159a
ꢁ1
2
1 286 g mol exhibiting relatively moderate molecular weight distributions and moderately heterotactic
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PLAs with Pr up to 0.65 were obtained.
of polycaprolactone (PCL) and polylactides (PLA) giving polymers
with high molecular weights, low polydispersity indices (PDI)
Polyesters obtained from renewable resources are fast replacing and specific stereo-microstructures for PLA. The development
petrochemical-based plastics due to their biocompatible and of new catalysts or initiators for ROP of esters has generated
biodegradable nature. As a result polyesters have found appli- great interest. Zn(II) and Cu(II) complexes have been shown to
Introduction
5
–7
1
2
3
cations in the fields of agriculture, packaging, biomedical,
produce effective catalysts for the ROP reactions of cyclic esters
4
and pharmaceutical industries. Ring-opening polymerization largely because they are easily synthesized, stable, less toxic and
8
(
ROP) has remained the most effective method for the synthesis more importantly biocompatible.
The catalytic activities of b-diketiminate complexes employed in
9
–11
a
b
the ROP reactions of cyclic esters have been studied extensively.
School of Chemistry and Physics, Westville Campus, University of KwaZulu-Natal,
Private Bag X54001, Durban, 4000, South Africa. E-mail: owaga@ukzn.ac.za
School of Chemistry and Physics, Pietermaritzburg Campus, University of
KwaZulu-Natal, Private Bag X01, Scottsville, 3209, South Africa.
E-mail: ojwach@ukzn.ac.za
Reports on amidine and amidinate complexes employed for ROP
catalysis of cyclic esters are extremely rare. Amidinate ligands
have found many applications in coordination chemistry and also
as ancillary ligands to form complexes as catalysts/initiators in
12
†
Electronic supplementary information (ESI) available: The supplementary
1
3,14
organic transformation and polymerization reactions.
The
figures and Tables S1–S19 represent NMR spectra of complexes 1–3, COSY and
NOESY NMR spectra of complex 1, DOSY NMR spectra of complexes 1 and 3, flexibility of amidine and amidinate ligands to coordinate either
2
graphs of percentage conversions of e-CL to PCL, lactides to PLAs, plots of ln[CL]
0
/
as monodentate or as chelating (Z ) ligands renders their
[
CL] vs. time at different concentrations and temperatures, GPC chromatograms,
t
15
respective complexes very promising for application in catalysis.
1
13
ESI-MS spectra of LA, and homonuclear decoupled H NMR and C NMR of
The steric bulk of the ligand affects the coordination mode around
the metal centers, and largely influences the control of polymer
microstructure, and as such, careful design of the ligand motif
PLAs, respectively, as discussed in this study. CCDC 1413137 (1), 1413165 (2),
1
413216 (3) and 1413168 (4). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5nj03159a
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1
6,17
could result in controlled ROP of cyclic esters.
The nature 130–140 1C. After 3 h, the temperature was increased to 150 1C
and chemical activities of the polymers obtained could be and all volatiles were removed via distillation. Upon cooling
controlled via modification of the electronic and steric properties to room temperature, the reaction mixture solidified. The crude
of amidine/amidinate ligands. Neutral mono (amidinate) and product was triturated with cold hexane and collected by vacuum
bi-functional ligands with two amidinate moieties bridged by a filtration. The solid obtained was recrystallized in minimal hot
linker with rare earth metal complexes show catalytic activity acetone and stored at 4 1C. Crystals formed were collected via
towards cyclic ester polymerization, giving high molecular filtration and dried in vacuo, yielding the pure products (Scheme 1).
18–20
weight and narrow molecular weight distribution polymers.
2
1
Phomphrai and coworkers reported rapid polymerization of
e-caprolactone and also found that bis(amidinate) tin(II) complexes
with electron donating groups accelerated the polymerization
reactions and enhanced the catalytic activities of the complexes.
In this paper, we report the synthesis of Zn(II) and Cu(II)
formamidine complexes and their application as catalysts in
the ROP of e-CL and LAs. The expected structural rigidity offered
by these formamidine ligands as opposed to the more flexible
Synthesis of Zn(II) and Cu(II) formamidine complexes
Zn (L1) (OAc) ] (1). To a solution of Zn(OAc) 0 (0.087 g,
ꢀH
.396 mmol) in ethanol a solution of L1 (0.200 g, 0.79 mmol) in
ethanol was added dropwise. The resulting solution was stirred
at room temperature for 24 h. The solvent was removed under
vacuum, and the crude product was washed with hexane and
[
3
2
6
2
2
0
recrystallized from the DCM/hexane solvent mixture to afford
1
complex 1 as a white solid (0.281 g, 67.2%). H NMR (CDCl
3
,
(pyrazolylmethy)pyridine synthons previously used may result in
4
6
7
00 MHz): d (ppm) 1.99 (s, 18H, Ac CH
3
), 2.32 (s, 24H, CH
3
),
better control of the ROP reactions. A detailed structural study
of the complexes and evaluation of the kinetics and polymer
tacticity of these ROP reactions have been performed and are
herein discussed.
3
.07–6.04 (d, J = 10.9 Hz, 1H, NH), 7.22–7.04 (m, 12H, Ar), 7.74–
3
13
3
.71 (d, J = 9.8 Hz, 1H, NQCH). C NMR (CDCl , 400 MHz) d
(ppm) 180.2, 133.7, 129.0, 129.0, 128.5, 127.6, 22.9, 18.4, 17.7.
IR (Nujol): n = 3333 (w), 1630 (s), 1588 (s), 1471 (m). ESI-TOF
+
+
MS: m/z (%); 391.28 [LZn + Li] (90), 569.26 [L Zn + H] (35).
2
2
Anal. calcd for C H N O Zn : C, 52.16; H, 5.90; N, 5.29.
Experimental section
General procedures
46 58
4
12
3
Found: C, 52.20; H, 6.10; N, 5.24.
Zn (L2) (OAc) ] (2). The reaction of compound L2 (0.150 g,
.410 mmol) and Zn(OAc) 0 (0.045 g, 0.021 mmol) in ethanol
ꢀH
afforded complex 2 as a crystalline white solid (0.150 g, 68%).
[
2
2
4
All experiments were carried out under argon using Schlenk
techniques. All solvents were obtained from Sigma-Aldrich.
Reagent grade ethanol was distilled and dried from magnesium
turnings; dichloromethane (DCM) and hexane were dried from
a sodium–benzophenone mixture. Metal salts (Cu(OAc) ꢀH O,
0
2
2
1
H NMR (CDCl
3
, 400 MHz): d (ppm) d (ppm) 1.18–1.19 (d,
3
J = 6.80 Hz, 48H, CH ), 1.89 (s, Ac CH , 6H), 3.33–3.30 (m,
3
3
3
2
2
8H, CH), 7.24–7.11 (m, 12H, Ar), 7.38–7.35 (d, J = 11.7 Hz, 1H,
NQCH), 5.72–5.70 (d, J = 11.8 Hz, 1H, NH). IR (Nujol): n = 2960
3
Zn(OAc) ꢀ2H O), and the monomers (e-caprolactone and lactides)
2
2
1
13
were obtained from Sigma-Aldrich. H and C NMR spectra were
(
m), 1661 (s), 1587 (m), 1382 (w). ESI-TOF MS: m/z (%); 731.58
+
+
measured at room temperature using a Bruker 400 MHz spectro-
[LZn
2
(OAc)
4
2
ꢁ H] (45); 792 [L Zn] (100). Anal. calcd for
1
meter. H NMR data were recorded in CDCl
3
listed as residual
(d 7.26). Similarly, C NMR data were recorded in
listed as residual internal CDCl (d 77.00). IR spectra were
C
58 84 4 8 2
H N O Zn : C, 63.55; H, 7.72; N, 5.11. Found: C, 63.30; H,
1
3
internal CDCl
CDCl
3
7
.66; N, 5.12.
4
Zn (L3) (OAc) ] (3). The reaction of compound L3 (0.130 g,
3
3
[
2
2
obtained on a PerkinElmer Universal ATR spectrum 100 FTIR
spectrometer. Mass spectra of compounds were obtained from a
Water synapt GR electrospray positive spectrometer.
0
.464 mmol) and Zn(OAc) ꢀH 0 (0.050 g, 0.023 mmol) in ethanol
2
2
afforded the crude product. The crude product was washed with
hexane and recrystallized from the DCM/hexane solvent mixture
to afford complex 3 as a white solid (0.176 g, 82%). H NMR
1
General procedure for ligand syntheses
(CDCl
3
, 400 MHz): d (ppm) 1.97 (s, 12H, Ac CH
3
), 2.27–2.18 (m,
3
Acetic acid (1.5 mole equivalents) was added to a round bottom 36H, CH
flask charged with aniline (2 mole equivalents) and triethyl Ar), 7.67–7.65 (d, J = 9.5 Hz, 1H, NQCH). C NMR (CDCl
3
), 5.99–5.96 (d, J = 11.9 Hz, 1H, NH), 6.93–6.84 (m, 8H,
3
13
3
,
orthoformate (1 mole equivalent). The reaction mixture was 400 MHz) d (ppm) 137.3, 133.7, 129.0, 129.0, 22.9, 20.8, 18.4,
heated under reflux and the temperature was maintained at 18.3, 17.6. IR (Nujol): n = 3147 (w), 2915 (m), 1641 (s), 1556 (m),
Scheme 1 Formamidine ligands used in this study.
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1
478 (s), 1432 (s), 1369 (m). ESI-TOF MS: m/z (%); 561 system controller (run by Breeze Version 3.30 SPA) and a Waters
+
+
[LZn
2 2 2 2 2
(OAc) + K] (100), 812.28 [L Zn (OAc) + Na] (20). Anal. in-line Degasser AF. A Waters 2414 differential refractometer
calcd for C H N O Zn : C, 59.42; H, 6.72; N, 6.03. Found: C, was used at 30 1C in series along with a Waters 2487 dual
4
6
60
4
8
2
5
0
9.07; H, 6.52; N, 6.19.
wavelength absorbance UV/Vis detector operating at variable
[
Cu (L2) (OAc) ] (4). The reaction of compound L2 (0.200 g, wavelengths. Tetrahydrofuran (THF, HPLC grade, stabilized with
2
2
4
ꢁ1
.55 mmol) and Cu(OAc)
2
ꢀ2H
2
0 (0.050 g, 0.250 mmol) in ethanol 0.125% BHT) was used as the eluent at flow rates of 1 ml min .
afforded the crude product. The crude product was washed with The column oven was kept at 30 1C and the injection volume was
hexane and recrystallized from the DCM/hexane solvent mixture 100 ml. Two PLgel (Polymer Laboratories) 5 mm Mixed-C (300 ꢂ
to afford complex 4 as a light green solid (0.200 g, 74%). IR (Nujol): 7.5 mm) columns and a pre-column (PLgel 5 mm Guard, 50 ꢂ
n = 3264 (w), 2963 (m), 2866 (w), 1652 (s), 1621 (s), 1436 (s), 1412 (s). 7.5 mm) were used. Calibration was done using narrow poly-
+
6
ꢁ1
ESI-TOF MS: m/z (%); 791.50 [M ꢁ 4OAc] (100). Anal. calcd for styrene standards ranging from 580 to 2 ꢂ 10 g mol . All
C H N O Cu : C, 63.54; H, 8.09; N, 5.11. Found: C, 63.20; H, molecular weights were reported as polystyrene equivalents.
5
8
84
4
8
2
7.79; N, 5.19.
X-ray crystallography
General procedure for bulk polymerization of e-caprolactone
The crystal evaluation and data collection of 1, 2, 3, and 4 were
done on a Bruker Smart APEXII diffractometer with Mo Ka
radiation (l = 0.71073 Å) equipped with an Oxford Cryostream
low temperature apparatus operating at 100 K for all samples.
Reflections were collected at different starting angles and the
Bulk polymerization reactions were performed by introducing
an appropriate amount of the complex and the e-CL monomer
(1.14 g, 0.01 mol) was added to a Schlenk tube immersed in a
pre-heated oil bath at 110 1C and the reaction was initiated by
stirring. Kinetics experiments were carried out by withdrawing
samples at regular intervals using a syringe and quenching
22
APEXII program suite was used to index the reflections. Data
23
reduction was performed using the SAINT software and the
quickly by rapid cooling into an NMR tube containing CDCl
3
scaling and absorption corrections were applied using the
solvent using ice water. The quenched samples were analyzed
24
SADABS multi-scan technique. The structures were solved
1
by H NMR spectroscopy for the determination of polymerization
25
by the direct method using the SHELXS program and refined.
of e-CL to PCL. The percentage conversion of [PCL]/[CL]
where [CL] is the initial concentration of the monomer and
PCL] is the concentration of the polymer at time t, was evaluated
by integration of the peaks for CL (4.2 ppm, OCH signal) and PCL
0
ꢂ 100,
Graphics of the crystal structures were drawn using OLEX2
26
0
software. Non-hydrogen atoms were first refined isotropically
[
and then by anisotropic refinement with the full-matrix least
2
25
2
squares method based on F using SHELXL. All hydrogen
atoms were positioned geometrically, allowed to ride on their
parent atoms and refined isotropically. Disorder was found for
one of the isopropyl methyl groups of complex 2. The electron
density was observed in the difference map and used to model
the disorder using PART instructions resulting in 58% occupancy
of the major component. The C–C bond distance in the disordered
methyl group was subjected to distance restraint, DFIX. Disorder
was also found for one of the oxygen atoms of the acetate anions
in complex 3 in which the electron density was observed in the
difference map. The disorder was modelled using PART instruc-
tions resulting in an B60% occupancy of the major component.
Distant restraints (DFIX, DANG and SADI) were used to improve
the CQO bond length in the disordered oxygen atoms of the acetate
anions. Crystal data collection and structural refinement parameters
for complexes 1–4 are provided in the ESI (Table S1, ESI†).
(
4.0 ppm, OCH signal) according to the equation [PCL]/[CL] = I4.0/
4.2 + I4.0), where I4.2 is the intensity of the CL monomer signal at
.2 ppm and I4.0 is the intensity of the PCL signal at 4.0 ppm for
2
0
(
I
4
OCH
2
protons. The observed rate constants were extracted from
/[CL] vs. time.
the slope of the line of best fit of the plot of ln[CL]
0
t
Polymerization of D,L-LA and L-LA
A suitable lactide (1.44 g, 0.01 mol) was dissolved in toluene in
a Schlenk tube equipped with a magnetic stirrer under argon
and the required amount of the complex was added. The
reaction mixture was stirred at 110 1C. Kinetics experiments
were carried out by withdrawing samples at regular intervals
using a syringe and quenching quickly by rapid cooling into an
NMR tube containing CDCl
3
solvent using ice water. The
1
quenched samples were analyzed by H NMR spectroscopy for
the determination of polymerization of lactides to PLA. The
integration values of the methine proton of the monomer and that
of the polymer were used to calculate the percentage conversion
using the equation ICH monomer/(ICH monomer + ICH polymer) ꢂ 100.
Results and discussion
0
Synthesis of N,N -diarylformamidine Zn(II) and Cu(II)
Polymer characterization by size exclusion chromatography
complexes
(
SEC)
0
0
The N,N -diarylformamidine ligands, N,N -bis(2,6-dimethylphenyl)-
0
The samples were analyzed by size exclusion chromatography formamidine (L1), N,N -bis(2,6-diisopropylphenyl)formamidine
at Stellenbosch University. The samples were dissolved in BHT (L2), and N,N -dimesitylformamidine (L3), were synthesized
stabilized THF (2 mg ml ). Sample solutions were filtered via a following the literature procedure.
syringe through 0.45 mm nylon filters before being subjected to two molar equivalents of Zn(OAc)
analysis. The SEC instrument consists of a Waters 1515 isocratic afforded the corresponding trinuclear and dinuclear Zn(II) and
0
ꢁ1
27,28
Reactions of L1–L3 with
O or Cu(OAc) O salts
2
ꢀ2H
2
ꢀH
2 2
3 2 6 2 2 4
HPLC pump, a Waters 717plus auto-sampler, a Waters 600E Cu(II) complexes [Zn (L1) (OAc) ] (1), [Zn (L2) (OAc) ] (2),
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Scheme 2 Synthesis of Zn(II) and Cu(II) formamidine complexes 1–4.
2 2 4 2 2 4
[Zn (L3) (OAc) ] (3) and [Cu (L2) (OAc) ] (4) in moderate to
high yields (Scheme 2). Complexes 1–3 were isolated as white
solids, while complex 4 was obtained as a green solid.
1
13
Complexes 1–3 were characterized using H and C NMR
spectroscopy (Fig. S1 to S5, ESI†), mass spectrometry, IR spectro-
scopy, elemental analyses and single crystal X-ray crystallography.
COSY spectra of complex 1 showed correlation between aromatic
protons (7.16–7.04 ppm) and the CH
ring. A perfect correlation also existed between the NH proton
6.07–6.04 ppm) and the CHQN methine proton (7.74–7.71 ppm)
Fig. S6, ESI†). The cross peaks in the NOESY spectrum (Fig. S7,
ESI†) could be due to cross relaxation between neighboring
3
protons on the phenyl
(
(
Fig. 1 X-ray crystal structure of complex 1 with thermal ellipsoids drawn
at 40% probability level. Hydrogen atoms have been omitted for clarity.
Selected bond lengths (Å) and angles (1): Zn(1)–Zn(2) 3.2099(3), N(1)–Zn(1)
2
9
protons in the ligand motif. A distance of 2.779 Å between the
two types of protons was measured in the crystal structure of
complex 1, affirming the NOESY NMR spectra. Mass spectra 1.9932(13), O(2)–Zn(2) 2.0312(11), O(1)–Zn(1) 1.9581(12), N(1)–C(9)
1
1
.299(2), N(2)–C(9) 1.325(2), O(3)–Zn(2) 2.1441(10), Zn(1)–O(3)–Zn(2)
of complexes 1–4 showed m/z peaks corresponding to various
02.97(4), O(3)–Zn(1)–O(1) 104.24(5), O(2)–Zn(2)–O(3) 90.39(4), O(3)–
fragments of the parent compounds. For example, complex 3
i
Zn(1)–N(1) 123.34(5), O(1)–Zn(1)–N(1) 108.04(5), and O(6) –Zn(1)–N(1)
+
showed a m/z = 645.13 corresponding to [LZn
2
(OAc)
4
+ H] . FTIR
1
01.75(5). Symmetry transformations used to generate equivalent atoms:
spectra of complexes 1–4 displayed the characteristic carbonyl
bands assignable to the nasym (OCO) and nsym (OCO) of the acetate
(
i) = ꢁx + 1, ꢁy + 1, ꢁz + 1.
30
ligands, respectively. For example, the IR spectrum of complex 1
ꢁ
1
ꢁ1
ꢁ1
revealed two bands at 1639 cm and 1588 cm assignable to n
(
Zn atom. Dinuclear complex 2 has one full molecule and two half
molecules in the asymmetric unit and the molecules in 2 are
asym
ꢁ1
ꢁ
1
OCO) while the bands at 1471 cm , 1404 cm , and 1373 cm
could be attributed to nsym (OCO). The measured effective magnetic related by a d-glide. Dinuclear complex 3 has two half molecules in
moment of the paramagnetic complex 4 at room temperature was its asymmetric unit with the other halves generated by a center
31
1
.97 BM, signifying significant CuꢀꢀꢀCu interaction.
of inversion located at h1 1 1i in one half and at h1/2 1/2 1/2i in
the other half. The two full molecules are related by an n-glide
but are however having different acetate environments as
Molecular structures of complexes 1, 2, 3 and 4
X-ray quality crystals of complexes 1–4 were obtained by slow discussed later in this section. Complex 4 has half a molecule
diffusion of hexane into dichloromethane solutions of the in the asymmetric unit.
respective complexes. Fig. 1–4 show the molecular structures
The coordination to the metal centers in all four complexes
of complexes 1–4, respectively, while selected bond distances is through the imine N atom of the formamidine ligands (in a
and angles are under the captions of each figure. The asymmetric monodentate fashion) and O atoms of acetate anions in the
unit of the trinuclear complex 1 has half a molecule of the complex remaining coordination sites in monodentate, bidentate or
with the other half generated by a center of inversion at the central bridging fashions. The three Zn(II) centers in complex 1 are
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Fig. 2 X-ray crystal structure of complex 2 with thermal ellipsoids drawn
at 40% probability level. Hydrogen atoms and the minor component of the
disordered methyl groups have been omitted for clarity. Only one of the
dimers in the asymmetry unit is shown for clarity. Selected bond lengths (Å)
and angles (1): Zn(1)–Zn(2) 3.7648(6), N(1)–Zn(2) 2.0515(13), Zn(1)–O(6)
Fig. 4 X-ray crystal structure of complex 4 with thermal ellipsoids drawn
at 30% probability level. Hydrogen atoms have been omitted for clarity.
Selected bond lengths (Å) and angles (1): Cu(1)–Cu(1) 2.6797(5), N(1)–Cu(1)
2
.0256(12), Zn(2)–O(1) 2.0389(12), O(3)–Zn(2)–N(1) 97.05(5), O(1)–Zn(2)–
O(3) 113.43(6), O(8)–Zn(2)–O(2) 91.50(5), N(1)–Zn(2)–O(1) 100.22(5).
2
1
.1821(13), O(4)–Cu(1) 1.9891(13), O(3)–Cu(1) 1.9654(14), N(1)–Cu(1)–O(2)
00.55(5), O(1)–Cu(1)–O(4) 166.70(5), O(4)–Cu(1)–O(3) 87.21(6), symmetry
code: (i) = ꢁx + 1, ꢁy + 1, ꢁz.
arranged in a linear fashion revealing the two bridging modes.
In total six acetate anions are involved in the bridging environ-
ment where four are in the bidentate while two are in the centers, one in an anisodentate fashion (Fig. 3a) and the other
monodentate bridging mode. The acetate anions in the dinuc- in a monodentate fashion (Fig. 3b). The conformations of the
lear complex 2 coordinate to the Zn(II) centers in two modes. bi-metallacycle moieties in 3 are different from that of complex
Two are bridging in a bidentate mode resulting in an eight 2 and so is the coordination of the acetate anions probably due
member bi-metallacycle with two Zn atoms and two O–C–O to steric influences. Complex 4 exhibits a familiar dinuclear
moieties of the acetate anion. Each of the remaining two are paddle wheel conformation in which all four acetate anions
chelating to the Zn(II) centers in a bidentate manner based on bridge the two Cu(II) centers in a bidentate bridging mode.
32
bond distances and angles. Complex 3 as mentioned earlier has
The three Zn(II) centers in complex 1 adopt two coordination
two half molecules with the other halves generated by inversion. geometries in which the two axial Zn atoms are in a distorted
Both molecules form a similar eight member bi-metallacycle tetrahedral environment with a N atom and three O atoms from
through two of the four acetate anions as in complex 2. However the formamidine and the acetate anions, respectively, while the
the remaining two anions coordinate differently to the Zn(II) middle Zn atom is in a distorted octahedral environment in
Fig. 3 X-ray crystal structure of complex 3 with thermal ellipsoids drawn at 30% probability level. Hydrogen atoms and the minor component of the
disordered O atom have been omitted for clarity. Selected bond lengths (Å) and angles (1): (a) molecule with anisodentate coordination acetates, Zn(1)–
N(1) 2.022(2), Zn(1)–O(2) 1.984(2), Zn(1)–O(1) 1.960(3), Zn(1)–O(4A) 2.34(2), O(1)–Zn(1)–O(2) 112.26(11), O(2)–Zn(1)–N(1) 98.58(9), O(3)–Zn(1)–O(2)
1
11.28(10), symmetry transformations used to generate equivalent atoms: (i) = ꢁx + 2, ꢁy + 2, ꢁz + 2. (b) Molecule with monodentate coordination
acetate, Zn(2)–N(3) 2.018(2), Zn(2)–O(5) 1.923(2), Zn(2)–O(7) 1.964(2), O(7)–Zn(2)–O(5) 121.90(11), O(7)–Zn(2)–N(3) 98.20(9), O(8)–Zn(2)–O(5) 111.61(11),
symmetry transformations used to generate equivalent atoms: (ii) = 1 ꢁx + 1, ꢁy + 1, ꢁz + 1.
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which the equatorial and axial coordination site are occupied perfectly isotropic and is characterized by a single line (g = 2.1062).
by O atoms. In complex 2 the geometries around the Zn atoms This indicates that there is a total symmetric environment where
adopt a severely distorted trigonal bipyramidal geometry due to the electrons in the different d-orbitals have equal interactions in
the coordination of one of the acetate anions being anisobidentate all directions (all the principal g-factors are the same).
in manner, while the Zn(II) atoms in complex 3 adopt a distorted
tetrahedral geometry in one of the molecules and a distorted and occur at a resonant frequency governed by g . The significant
trigonal pyramidal geometry in the second molecule. The distortions g-shift of g = 2.1062 to g (2.0025) reflects the spin–orbit coupling
e
from a regular arrangement and the selective formation of either effect in the paddle-wheel complex. The solid state EPR spectrum
trinuclear and dinuclear complexes could be due to the flexibility (Fig. 5b) shows axial symmetry such that the total magnetic
The spin-allowed Dm = ꢃ1 transitions are thus degenerate
s
iso
iso
34
in binding modes of the acetate ligands, in addition to steric moment in the Z-direction is rather large (g
x
= g
y z
a g ). The
1
7,33
restriction imposed by L1–L3, respectively.
magnetic parameters for the exchange-coupled copper–copper
The trigonal planar nature of the secondary amine group pairs are g = 2.3554 and g> = 2.0840. A consistent interpretation
J
suggests conjugation of the amine lone pair with the p-electron of these data is presented on the basis of a weak metal–metal
4
4
system of the aryl ring, possibly accounting for its apparently interaction. The similarity in the solution and solid state EPR
rather poor s-donor strength, and could justify its inability to spectra of complex 4 confirms the retention of the paddle-wheel
3
4
facilitate chelation of the metal ion. The Zn–O bond distances structure in solution.
in complexes 1–3 fall within the ranges of similar complexes in
3
5–42
Ring opening polymerization of e-CL and LAs
the literature.
Only one of these seems a little longer as seen
in complex 2 and complex 3a, respectively, resulting in aniso-
The ROP reactions of e-CL using complexes 1–4 as catalysts
were initially investigated at 110 1C using the [M]/[catalyst] ratio
of 200. Under these conditions, complexes 1–4 exhibited significant
catalytic activities giving maximum conversions between 48 h and
2
0
bidentate chelating behavior. The observed metal to metal
distances in complexes 1–3 are greater than the sum of the van
der Waals radii of Zn (1.39 Å) indicating the absence of any
meaningful Zn–Zn interatomic metal bond vis- a` -vis metal–metal
76 h (Fig. S9a, ESI†). Complexes 1 and 3 were also investigated in
3
4
interaction. Complex 4 exhibit a Cuꢀ ꢀ ꢀCu distance of 2.6796(3)
the ROP of D,L-LA and L-LA at 110 1C using the [M]/[catalyst] ratio
of 200 in toluene and afforded conversions of 97% and 99%
within 3.5 and 9 h, respectively (Fig. S9b, ESI†). Tables 1 and 2
contain a summary of the ROP data of e-CL and LAs for
complexes 1–4, respectively.
Having established that complexes 1–4 form effective catalysts
in the ROP of e-CL and LAs, we carried out detailed kinetic and
polymer property studies to gain insight into the influence of
catalyst structure and reaction conditions on the kinetics of the
polymerization reactions and the nature of polymers obtained.
Å typical of binuclear copper(II) acetates that possess N-donating
4
3
apical ligands.
In the crystals of all four complexes fairly strong intramolecular
hydrogen bonds and interactions exist in which the acetate oxygen
atoms are the acceptor atoms and the ligand N atoms the donor
atoms. These interactions seem to play a significant role in the
37,40
packing of the molecules in crystals.
Electron paramagnetic resonance spectra of complex 4
In order to understand the coordination environment of the
paddle-wheel Cu(II) complex in both solid state and solution,
Kinetics of ROP reactions of e-CL and LAs
X-band EPR spectra of complex 4 were acquired in methanol Kinetic studies of the ROP of e-CL were investigated for complexes
1
solution and in solid state at 295 K (Fig. 5). In methanol 1–4 and monitored by H NMR spectroscopy. The rates of the
0 t
solution, the EPR spectrum of complex 4 (Fig. 5a) is almost reaction were determined from the plot of ln[CL] /[CL] versus time
Fig. 5 (a) Room temperature EPR spectrum of complex 4 in methanol solution (9.786 GHz) and (b) solid state EPR spectrum of complex 4 (295 K,
.870 GHz).
9
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Paper
a
Table 1 Summary of polymerization data of e-CL by complexes 1–4
Order of ROP of e-CL reaction with respect to catalysts 1 and 3
Time
(h)
Conversion
(%)
k
(h
app
M
(GPC)
w
Polymerization reactions at different catalyst concentrations
at constant e-CL monomer concentration were carried out to
ꢁ
1
b
b
c
Catalyst
)
PDI
IE
1
2
3
4
48
72
53
76
98
96
96
98
0.1009
0.0479
0.0963
0.0441
12 632
5426
10 342
3838
2.11
1.86
2.14
1.66
0.56 determine the order of reaction with respect to catalysts 1 and 3
0.25
0.47
0.17
(
Table 3). A plot of ln kapp versus ln[1/3] gave linear relationships
which allowed us to determine the order of reaction with
respect to 1 and 3 (Fig. 6). The order of reaction with respect
to 1 and 3 was extracted from the gradients of the lines of best fit
a
Reaction conditions, bulk polymerization, 110 1C, [CL]
0
/[catalyst] = 200.
Molecular-weight average and polydispersity index (PDI) determined by
GPC relative to polystyrene standard values, the values obtained from
b
(Fig. 6) and was obtained as 2.1420 E 2 and 0.6343 respectively.
33 c
GPC ꢂ correction factor of 0.56.
Initiator efficiency (IE) = Mwexp/Mwcalc
,
Fractional orders of reaction with respect to catalysts have been
previously reported and largely attributed to catalyst aggregation
where Mwcalc = Mw(monomer) ꢂ [CL]
0
/[I] ꢂ [PCL]/[CL]
0
+ Mw(chain-endgroup).
15,49
especially in bulk polymerization reactions.
experiment was carried out in order to ascertain if complexes 1 and
indeed undergo aggregation (Fig. S8, ESI†). The DOSY spectrum
The DOSY NMR
a
Table 2 ROP of D,L-LA and L-LA using complexes 1 and 3
3
b
Time Conversion
k
app
M
w
is consistent with the existence of one species in solution thereby
negating the possibility of complex aggregation. A slight difference
ꢁ
1
c
c
d
Entry Catalyst (h)
(%)
(h
)
(GPC) PDI IE
ꢁ
9
2
ꢁ1
e
f
1
2
3
4
1
1
3
3
3.5
5
97
96
99
99
1.2765 10 376 2.25 0.37 in the diffusion co-efficient of 1.666 ꢂ 10 m s and 1.112 ꢂ
ꢁ9 2 ꢁ1
0.8737 21 286 1.79 0.77
0.6412 15 867 1.99 0.55
0.5963 18 054 1.85 0.63
10
m s was observed at 30 1C for 1 and 3, respectively. More
e
f
9
9
intriguing is the large difference in reaction orders with respect
to catalysts 1 and 3. This might be associated with the nuclearity
and structures of the complexes; while complex 1 is trinuclear,
complex 3 is dinuclear.
a
Reaction conditions: [CL]
0
/[I] = 200; solvent, toluene; temperature,
10 1C. Maximum conversion achieved. Molecular-weight average
and polydispersity index (PDI) determined by GPC relative to polystyr-
b
c
1
ene standard values, the values obtained from GPC ꢂ correction factor
It is therefore conceivable to conclude that the active catalytic
species produced from complexes 1 and 3 are structurally different
and that the solid state structures are likely to be retained in
solution. The overall order of the polymerization reactions
catalyzed by complexes 1 and 3 could thus be described according
to eqn (2) and (3) respectively:
33 d
of 0.58.
Initiator efficiency (IE) = Mwexp/Mwcalc, where Mwcalc =
e f
M
w(monomer) ꢂ [CL]
0
/[I] ꢂ [PCL]/[CL]
0
+ Mw(chain-end group)
.
D,L-LA. L-LA.
(
Fig. S10a, ESI†). Linear relationships consistent with pseudo-first-
order dependency on the monomer were observed in all cases
Fig. S10a, ESI†). Thus, the rate of e-CL polymerization can be
written as shown in eqn (1):
(
d½CLꢄ
2
¼
k½CLꢄ½1ꢄ
(2)
(3)
dt
d½CLꢄ
¼
k½CLꢄ
(1)
dt
d½CLꢄ
dt
0:6
¼
k½CLꢄ½3ꢄ
x
where k = k [I] ; k is the rate of chain propagation, I is the
p p
initiator, and x is the order of reaction.
The apparent rate constants for catalysts 1–4 in the ROP of
e-CL were extracted from Fig. S10a (ESI†) and are given in
Table 1. More discerning was the drastic decrease in catalytic
Effect of solvent and temperature on the ROP kinetics of e-CL
To understand the influence of solvent on the polymerization
kinetics of e-CL, we compared the activity of complex 3 in
bulk and solution polymerization reactions. Table 4 shows a
ꢁ1
activity observed for catalyst 2 (0.0479 h ) bearing isopropyl
ꢁ1
groups relative to catalyst 1 (0.1009 h ) containing the less bulky
methyl groups. Reduction of catalytic activity with an increase in
the steric bulk of the ligand framework may be rationalized by Table 3 Effect of catalyst concentrations on the polymerization kinetics
45
a
inhibition of monomer coordination to the metal center. Similar of e-CL
trends have been observed for bis(thiophosphinic amine) yttrium
b
Time Conversion
k
(h
app
M
w
46
ꢁ
1
c
c
d
catalyst systems.
Kinetic studies of the ROP of LAs were also investigated using
complexes 1 and 3. A linear relationship of the plot of ln[CL] /[CL]
Catalyst [CL]
0
/[Cat] (h)
(%)
)
(GPC) PDI IE
1
1
1
1
3
3
3
3
100
150
250
300
100
150
250
300
18
30
48
68
30
48
48
48
97
97
96
82
97
98
95
95
0.2894
0.1409
7666 2.01 0.69
7879 2.36 0.47
0
t
versus time was also obtained (Fig. S10b, ESI†) consistent with a
pseudo-first-order dependency on LA concentration. The apparent
rate constants for complexes 1 and 3 in the ROP of LAs were
extracted from Fig. S10b (ESI†) and are given in Table 2. We
observed that the reaction rates of ROP of e-CL were much
slower than those of LA reactions. This is in good agreement
with literature findings and has largely been attributed to the
0.0450 13 252 2.47 0.48
0.0252 13 646 2.15 0.49
0.1557
0.1097
0.0797 13 506 1.92 0.50
0.0781 14 124 2.14 0.43
5141 1.91 0.46
7864 2.41 0.47
a
b
Reaction conditions, bulk polymerization, 110 1C. Maximum conversion
achieved. Molecular-weight average and polydispersity index (PDI) deter-
mined by GPC relative to polystyrene standard values, the values obtained
c
4
7
larger ring size of e-CL. The six-membered ring in LA increases
33 d
from GPC ꢂ 0.56.
Initiator efficiency (IE) = Mwexp/Mwcalc, where Mwcalc
=
4
8
the strain resulting in rapid ROP reactions.
Mw(monomer) ꢂ [CL]
0
/[Cat] ꢂ [PCL]/[CL]
0
+ Mw(chain-end group).
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Fig. 6 (a) Plot of ln kapp vs. ln[1] and (b) ln kapp vs. ln[3] for the determination of order of reactions with respect to catalysts 1 and 3.
summary of the bulk and solution ROP data of e-CL and at The low energy barrier hints at a greater number of active sites
different reaction temperatures. The rate of polymerization in the system at lower temperatures. From the Eyring plot in Fig. 7b,
‡
reactions recorded in toluene solvent was comparable to that the enthalpy of activation DH and the entropy of activation
‡
ꢁ1
ꢁ1
ꢁ1
in the bulk experiment (Table 3). This contrasts previous DS were obtained as 25.08 kJ mol and ꢁ201.7 J K mol
reports where we observed higher catalytic activities in bulk respectively, for complex 3 at [CL] /[I] = 200. These results are
consistent with highly ordered transition state systems and are
,
0
1
6
reactions in comparison to solution experiments.
The dependence of the kinetics of the ROP of e-CL reaction in good agreement with those reported for coordination-insertion
51,52
on reaction temperature was studied by determination of the mechanisms in the ROP of e-CL.
rate constants at various temperatures (60 1C to 110 1C) using
complex 3 (Fig. S11, ESI†). After induction periods observed
at lower temperatures, linear relationships consistent with
pseudo first-order dependency on the monomer were observed.
Molecular weight and molecular weight distribution of polymers
The molecular weight and molecular weight distribution of
polymers obtained were determined by GPC and compared with
ꢁ
1
1
A significant decrease in the rate constant from 0.0963 h to
the theoretical values calculated from H NMR spectra (Tables 1–4).
ꢁ1
ꢁ1
0.064 h
was recorded with the decrease in the reaction
Generally higher molecular weights of up to 21 286 g mol were
ꢁ1
temperature from 110 1C to 90 1C. The observed rate constants
extracted from the semi-logarithmic plots (Fig. S11, ESI†) are
shown in Table 4.
obtained for PLA compared to the maximum value of 14 124 g mol
reported for PCL. ESI-MS spectra of poly(D,L-LA) showed some minor
signals in addition to the main peaks (Fig. S15, ESI†) while that of
poly(L-LA) showed mainly one signal corresponding to the mass
of the lactide repeat unit (72 Da) (Fig. 8). The minor signals in
the mass spectrum of poly(D,L-LA) are believed to originate from the
The overall activation energy of the ROP of e-CL using complex 3
ꢁ1
calculated from the slope of the Arrhenius plot of ln k vs. T was
ꢁ1
found to be 28.05 kJ mol (Fig. 7a). This value is lower than that
reported for ROP reactions using the lanthanide tris(2,4,6-tri-
53–55
transesterification process (back-biting) occurring in the system.
ꢁ
1 50
tert-butylphenolate) catalyst of 39.3 kJ mol
but higher than
The ring-opening polymerization of cyclic esters by metal-
based catalysts is likely to proceed via either the coordination-
insertion mechanism (CIM) or the activated-monomer mechanism
ꢁ1
49
the value of 12.05 kJ mol reported by Mei and co-workers.
5
6
(
AMM). In the CIM route, the polymer end chain bears the
Table 4 Effect of solvents and reaction temperature on the polymerization
kinetics of e-CL using complex 3
nucleophile on one end and the metal center on the other end.
However, hydrolysis of one end of the polymer chain to form an
–OH end group could be promoted by chain transfer agents
a
Time Conversion
k
(h
app
M
w
(GPC)
ꢁ
1
b
b
c
Entry [CL]
0
/[I] (h)
(%)
)
PDI
IE
57
such as water or alcohols. To establish the nature of the initiating
d
1
1
2
3
4
5
200e
200
52
96
156
168
240
98
97
97
93
95
0.0778 7073
0.0640 4108
0.0371 2186
0.0283 4319
0.0257 2153
2.02 0.32 and chain-end groups in our system, H NMR and ESI-MS spectra
1.74 0.19
1.46 0.10
1.76 0.20
1
f
of poly(L-LA) obtained were analyzed. H NMR spectra of all the
200g
polymers revealed the absence of acetate methyl signals at about
200
200
h
1.43 0.10 2.00 ppm (Fig. 8). Similarly, no signals associated with the ligand
a
b
moiety in the complexes were observed. However, the ESI-MS
Maximum conversion achieved. Molecular-weight average and poly-
dispersity index (PDI) determined by GPC relative to polystyrene spectra of poly(L-LA) showed signals indicating the presence of
3
3 c
standard values, the values obtained from GPC ꢂ 0.56.
Initiator
/[I] ꢂ [PCL]/
Zn–OH end groups, associated with the hydrolysis and ligand
dissociation of the zinc complex (Fig. 9). These results are in
efficiency (IE) = Mwexp/Mwcalc, where Mwcalc = Mw(monomer) ꢂ [CL]
0
d
e
f
[
CL]
0
+ Mw(chain-end group)
.
Solvent, toluene. Temp., 90 1C. Temp.,
g
h
58
8
0 1C. Temp., 70 1C. Temp., 60 1C.
agreement with those reported by Pilone and co-workers in
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ꢁ1
Fig. 7 (a) Arrhenius plot of ln k vs. T for the bulk polymerization of e-CL initiated by 3, M/I = 200. (b) Eyring plot of temperature dependence of the rate
constant.
1
Fig. 8 H NMR spectrum of poly(L-LA) produced by complex 1.
ꢁ1
which water molecules hydrolyzed the polymer end chains. 22 550 g mol was obtained using complex 1 bearing the less
From these data, the polymerization reactions in our system sterically demanding methyl substituents on the phenyl ring
ꢁ
1
can thus be said to proceed through the coordination insertion compared to the molecular weight of 9689 g mol observed for
mechanism followed by hydrolysis of the acetate end groups.
complex 2, containing the bulkier isopropyl groups (Table 1,
6
1,62
A linear relationship of the plot of the molecular weight entries 1 and 2).
versus monomer conversion (Fig. 10) established the living
We also noted the dependence of PCL molecular weight and
5
9,60
polymerization behavior of these catalysts.
The observed molecular weight distributions on the identity of the solvent
increase in polymer weight with the increase in [CL]/[catalyst] used in the ROP of e-CL. Interestingly, PCL obtained in methanol
ratio (low catalyst concentration) further supported this living solvent exhibited a narrow PDI of 1.12 compared to a PDI of 2.02
polymerization nature (Table 3) and is consistent with a small recorded in toluene solvent.
number of active sites at lower concentration of the catalyst.
Generally, the polymers obtained in this study exhibited narrow to
Stereochemistry of polylactides
moderate molecular weight distributions, 1.12–2.47 and 1.79–2.25 Poly(LA) tacticity was studied by inspecting the methine regions
1
for PCLs and PLAs, respectively, indicating some degree of control and tetrad sequences in the homonuclear decoupled H NMR
1
3
63
of the ROP and minimal esterification and epimerization reactions. and C NMR spectra of the polymers. Fig. S16–S19 (ESI†) show
1
13
The catalyst structure was also found to influence the molecular the methine resonances of the homonuclear decoupled H and
C
3
6
weights of the polymers obtained. Contrary to expectations,
increasing the steric bulk of the ligands resulted in decreased to the appropriate tetrads in accordance with literature reports.
PCL molecular weights. For example, the molecular weight of The iii tetrad is the predominant peak in the spectrum, thereby
NMR spectra of poly(L-LA) and poly(D,L-LA). The peaks were assigned
57
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Fig. 9 ES-MS of the crude PLA (from L-LA) from catalyst 3, [CL]
0
/[3] = 200, 9 h, showing the distribution of signal corresponding to the mass of the
lactide repeat unit.
6
4
yielding moderate isotactic poly(L-LA). However, minor signals or the defect content was estimated from the total iii cored
1
in the decoupled H NMR spectra could be attributed to the tetrad intensities by assuming a certain defect fraction and then
epimerization of the chiral centers, thus randomizing their absolute continually corrected for changes in the defect fraction calculated
65
configuration and therefore resulting in the loss of control of from iis and sis/sii intensities. Based on the composition analysis,
5
7
polymer stereo-regularity. We attempted to use homonuclear we estimated the presence of 97.02% of poly(L-LA) and 2.98% of
1
decoupled H NMR to quantify the number of defects in the defects resulting from epimerization reactions. As reported in
1
65,66
poly(L-LA) chain. In the homonuclear decoupled H NMR spectra the literature,
a polymer sequence showing a characteristic
1
of the methine region of poly(L-LA) (Fig. S16, ESI†), the poly(L-LA) resonance at d = 5.21 ppm in a decoupled H NMR spectrum
contains a single defect, whereas the stereoregular sequence
shows a resonance at d = 5.17 ppm. In addition, double stereo-
defects show two characteristic resonances at d = 5.22 and d =
1
5
.23 ppm, respectively. Considering the H homonuclear decoupled
NMR of the methine region of poly(L-LA) formed with complexes
and 3, respectively (Fig. S16, ESI†), it is therefore conceivable
1
to state that the poly(L-LA) chain has double stereodefects
arising from epimerization reactions. The core tetrad stereo-
sequences in poly(D,L-LA) as given in Fig. S18 and S19 (ESI†) are
well resolved and peak assignments are consistent with the
6
4,66
literature
and the production of predominantly moderate
heterotactic poly(D,L-LA) with Pr of up to 0.65.
Conclusions
This work demonstrates the coordination chemistry and the
application of Zn(II) and Cu(II) formamidine complexes in the
ring-opening polymerization of e-caprolactone and lactides.
Fig. 10 Plot of experimental molecular weight against % conversion,
showing the living polymerization nature of complex 3 in the bulk ROP
of e-CL at 110 1C, [CL] /[I] = 200.
0
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We have shown that the substituents on the phenyl ring of the 15 S. Schmidt, S. Schulz, D. Bl ¨a ser, R. Boese and M. Bolte,
ligand backbone significantly affect the coordination chemistry
Organometallics, 2010, 29, 6097–6103.
of the complexes to afford dinuclear and trinuclear complexes. 16 C. M. Thomas, Chem. Soc. Rev., 2010, 39, 165–173.
Complexes 1–4 formed active and stable catalysts in the ring- 17 S. O. Ojwach, T. T. Okemwa, N. W. Attandoh and B. Omondi,
opening polymerization of e-caprolactone and D,L-lactide and
L-lactide to produce polymers with moderate weights and 18 J. Wang, Y. Yao, Y. Zhang and Q. Shen, Inorg. Chem., 2009,
molecular weight distributions. The catalytic activities of the
48, 744–751.
complexes were largely controlled by the ligand architecture 19 J. Wang, T. Cai, Y. Yao, Y. Zhang and Q. Shen, Dalton Trans.,
and metal atoms. The kinetics of the ROP reactions was pseudo-
2007, 5275–5281.
first order with respect to both e-caprolactone and lactide 20 Y. Luo, P. Xu, Y. Lei, Y. Zhang and Y. Wang, Inorg. Chim.
monomers. Both the temperature and solvent significantly
Acta, 2010, 363, 3597–3601.
influenced the ring-opening polymerization of e-caprolactone 21 K. Phomphrai, C. Pongchan-o, W. Thumrongpatanaraks,
Dalton Trans., 2013, 42, 10735–10745.
ꢁ1
and an overall activation energy of 28.5 kJ mol was obtained. The
catalysts display a reasonable degree of control of polymer stereo-
regularity producing predominantly heterotactic poly(D,L-lactide).
P. Sangtrirutnugul, P. Kongsaeree and M. Pohmakotr, Dalton
Trans., 2011, 40, 2157–2159.
22 Bruker, APEXII Bruker AXS Inc., 2009, Madison, Wisconsin, USA.
2
2
3 Bruker, SAINT Bruker AXS Inc., 2009, Madison, Wisconsin, USA.
4 Bruker, Bruker SADABS Bruker AXS Inc., 2009, Madison,
Wisconsin, USA.
Conflict of interest
2
2
2
5 G. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr.,
The authors declare no competing financial interest.
2008, 64, 112–122.
6 O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard
and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339–341.
7 T. Elkin, N. V. Kulkarni, B. Tumanskii, M. Botoshansky,
L. J. W. Shimon and M. S. Eisen, Organometallics, 2013, 32,
Acknowledgements
The authors would like to thank the College of Agriculture, Science
and Engineering, University of KwaZulu-Natal, and National
Research Foundation (NRF), South Africa for financial support.
6
337–6352.
8 E. C. Taylor and W. A. Ehrhart, J. Org. Chem., 1963, 28,
108–1112.
2
2
1
9 J. Zhang, K. Higashi, K. Ueda, K. Kadota, Y. Tozuka,
W. Limwikrant, K. Yamamoto and K. Moribe, Int. J. Pharm.,
2014, 465, 255–261.
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