Ring opening polymerization of l-lactide by an electron-rich Schiff base zinc complex: An activity and kinetic study

Article abstract of DOI:10.1016/j.molcata.2011.10.012

A zinc complex with the Schiff base [(CH₃)₂NCH ₂CH₂N=CHC₆H₃(OH)(OMe)], LH, derived from 2-dimethylaminoethylamine and o-vanillin [Zn₂L ₂(OBn)₂], has been synthesized and its structure has been established by X-ray crystallography and NMR spectroscopy. This zinc complex efficiently initiated the ring-opening polymerization (ROP) of l-lactide (l-LA), and the polymerization runs were well controlled, giving polylactide end-capped with benzyl ester and hydroxyl groups. The structure of the ancillary ligands showed some influence on the catalytic activity, the introduction of electron-rich methoxy at ortho-phenoxy substituent resulted in a decrease of the polymerization rate. In addition, we also have found that the overall rate expression is d[lactide]/dt = k_p[lactide]¹[Zn ₂L₂(OBn)₂]¹ in the kinetic study with the ROP of l-LA.

Full text of DOI:10.1016/j.molcata.2011.10.012

Journal of Molecular Catalysis A: Chemical 352 (2012) 57–62
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Ring opening polymerization of l-lactide by an electron-rich Schiff base zinc
complex: An activity and kinetic study
Lihui Yao, Lei Wang, Jinfeng Zhang, Ning Tang, Jincai Wu∗
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical
Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2011
Received in revised form
29 September 2011
Accepted 11 October 2011
Available online 18 October 2011
A zinc complex with the Schiff base [(CH3)2NCH2CH2N CHC6H3(OH)(OMe)], LH, derived from 2-
dimethylaminoethylamine and o-vanillin [Zn2L2(OBn)2], has been synthesized and its structure has been
established by X-ray crystallography and NMR spectroscopy. This zinc complex efficiently initiated the
ring-opening polymerization (ROP) of l-lactide (l-LA), and the polymerization runs were well controlled,
giving polylactide end-capped with benzyl ester and hydroxyl groups. The structure of the ancillary
ligands showed some influence on the catalytic activity, the introduction of electron-rich methoxy at
ortho-phenoxy substituent resulted in a decrease of the polymerization rate. In addition, we also have
found that the overall rate expression is d[lactide]/dt = kp[lactide]1[Zn2L2(OBn)2]1 in the kinetic study
with the ROP of l-LA.
Keywords:
Schiff base zinc complex
Ring opening polymerization
l-Lactide
© 2011 Elsevier B.V. All rights reserved.
Catalytic activity
Kinetic study
1. Introduction
ization of cyclic esters and have exhibited highly active and living
properties.
Polylactide (PLA), prepared from natural renewable resources
has recently gained much attention due to its biodegradable and
biocompatible nature, and its wide use in the biomedical, phar-
maceutical and environmental fields as a potential alternative
to polyolefins [1–12]. Nowadays, the major chemical synthetic
methods for preparing high molecular weigh polylactides gener-
ally rely on the controlled ring opening polymerization (ROP) of
lactides initiated by well-defined metal complexes of aluminium
[13–18], calcium [19–24], magnesium [22,25–28], rare-earth met-
als [29–31] and zinc derivatives [22,32–37].
The industrial scale production of PLA relies on the use of Sn(II)
initiators. However, these are difficult to remove from the resultant
polymer and there are concerns over the toxicity of tin [38]. There-
fore, numerous zinc, magnesium and calcium complexes have been
used as “benign” polymerization initiators in the ROP processes.
At the same time, the development of new ancillary (mainly N-
and O-containing) ligand systems with the low toxicity is currently
attracting growing attention. As a result, many zinc complexes, sup-
ported by steric hindrance trispyrazolylborate [39], diketiminate
[32,40,41], Schiff base [36,42–44], aminophenolate [22,33–35] and
many other ligands [37,45–47] have been used for the polymer-
As Lin et al. [42] reported that a series of NNO-tridentate Schiff
base zinc alkoxides are good initiators for the ring opening poly-
merization of lactide, they also pointed out that the para-position
electron-donating groups on the phenol ring of NNO-tridentate
Schiff base ligand can increase the polymerization progress and
the bulky group at the ortho-position of phenol can decrease the
activity of the initiator. Therefore, there is an interesting question
about the activities of these kinds of initiators when the ortho-
position of phenol is an electron donor group. In this context, a new
electron-rich Schiff base [(CH3)2NCH2CH2N CHC6H3(OH)(OMe)]
zinc complex [Zn2L2(OBn)2] (Scheme 1) has been prepared and
characterized by X-ray crystallography and NMR spectroscopy.
Experimental results show that this complex is an excellent initia-
tor toward the controlled polymerization of l-LA and its activity
decrease by the introduction of electron-rich methoxy at ortho-
phenoxy substituent to the ancillary ligand. To extend the deepness
of our polymerization studies, herein we also report that the overall
rate expression is d[lactide]/dt = kp[lactide]1[Zn2L2(OBn)2]1 in the
kinetic study with the ROP of l-LA.
2. Experimental
2.1. General considerations
∗
All manipulations were carried out under
gen atmosphere. Solvents were dried by refluxing for at least
a dry nitro-
Corresponding author. Tel.: +86 931 8912552; fax: +86 931 8912582.
E-mail address: wujc@lzu.edu.cn (J. Wu).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.10.012

58
L. Yao et al. / Journal of Molecular Catalysis A: Chemical 352 (2012) 57–62
Scheme 1. Synthesis of [Zn2L2(OBn)2].
24 h over sodium/benzophenone (n-hexane, toluene, and THF),
phosphorus pentoxide (CH2Cl2) and calcium hydride (benzyl
alcohol). l-Lactide (Jinan Daigang Co., Ltd.) was recrystallized
in toluene twice for using. o-Vanillin was purchased from
Acros. Others reagents were purchased from Tianjin Chemical
Reagent Co., Ltd. and used without further purification. Schiff
base [(CH3)2NCH2CH2N CHC6H3(OH)(OMe)][HL] was synthesized
according to the literature [48]. 1H and 13C NMR spectra were
recorded on a Bruker ADVANCE DRX-200 (200 MHz for 1H and
50 MHz for 13C) or a Bruker AVANCE III (400 MHz for 1H and
100 MHz for 13C) spectrometer. Chemical shifts are reported in ı
(ppm) using residual proton impurities in CDCl3 as a reference.
Monomer conversion was determined from the relative intensi-
ties of the OCH2 signals for the monomer (multiplet at ı 5.05 ppm)
and polymer (multiplet at ı 5.25 ppm). Mass Spectra were obtained
from the Trace DSQ GC–MS(EI) or the LC–MS(ESI). Elemental anal-
yses were carried out on an Elementar Vario EL element analyzer.
X-ray structural analyses were carried out on a Bruker SMART
APEX II diffractometer, using graphite monochromated Cu K␣ or
Mo K␣ radiation. The molecular weight and polydispersity of PLA
were measured by the Waters gel permeation chromatograph
(GPC) using THF (HPLC grade) as an eluent and calculated using
polystyrene as a standard reference. The GPC system was equipped
with a Waters 2707 plus autosampler, a Waters 1515 isocratic HPLC
pump, a Waters 2414 refractive index detector. THF was used as
eluentat a flowrateof 1.0 mL/min. Thetemperaturesof thecolumns
and detector were both 30 ◦C. Mn values of PLAs were corrected
with a Mark-Houwink factor of 0.58 [49].
Fig. 1. ORTEP drawing of [Zn2L2(OBn)2] with thermal ellipsoids drawn at the 30%
probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths
˚
(A) and bond angles (deg): Zn–O(1) 2.041(2), Zn–O(2) 2.001(2), Zn–O(2A) 1.977(2),
Zn–N(1) 2.057(3) and Zn–N(2) 2.369(3). O(1)–Zn–N(2) 158.44(10), O(2)–Zn–N(1)
147.06(10), O(2A)–Zn–N(1) 129.14(10) and (2A)–Zn–O(2) 83.03(8).
polymerization conversion was analyzed by 1H NMR spectroscopic
studies. l-Lactide (0.216 g, 1.5 mmol) was added to a solution
of [Zn2L2(OBn)2] (0.012 g, 0.015 mmol) in toluene (10 mL). After
the solution was stirred at 60 ◦C for 2 h, the reaction was then
quenched by the addition of water. Hexane (40 mL) was then
added to the above mixture to give a white crystalline solid. The
resulting solid was washed with hexane twice and then dried
under vacuum. Yield: 0.132 g (62%). 1H NMR (400 MHz, CDCl3,
25 ◦C, ppm): ı 7.37–7.34 (m, 5H, ArH-OBn), 5.16 (q, 2H, OCH2Ph
and nH, CH-PLA, J = 7.2 Hz), 4.35 (dd, 1H, CH-PLA, J = 5.6 Hz), 1.98
(s, 1H, OH-PLA) and 1.58 (d, 3H, CH3-PLA, J = 7.2 Hz).
2.4. Kinetic study of l-lactide polymerization with [Zn2L2(OBn)2]
1
2.2. Preparation of Schiff base zinc complex [Zn2L2(OBn)2]
A kinetic study [50] was conducted to establish the reaction
order with respect to monomer and initiator 1. l-Lactide (0.216 g,
1.5 mmol) was added to a solution of 1 (1.50, 1.88, 2.39 or 2.78 mM)
in toluene (10 mL). The solution was then stirred at 60 ◦C under an
atmosphere of nitrogen. At the indicated time intervals, 0.05 mL
aliquots were removed, trapped with liquid N2, stripped of solvent,
and analyzed by 1H NMR in CDCl3. The lactide concentration [LA]
was determined by integrating the quartet methine peak of lac-
tide at 5.00 ppm and the quartet methine peak of polylactic acid at
5.25 ppm.
ZnEt2 (2.2 mL, 2.2 mmol) was slowly added to an ice cold toluene
(20 mL) solution of benzyl alcohol (0.23 mL, 2.2 mmol) under a
N2 atmosphere. After the mixture was stirred for 1 h at 0 ◦C, this
mixture was transferred to HL (0.44 g, 2 mmol), and then the mix-
ture was stirred for 12 h at room temperature. The solvent was
removed under vacuum to afford a yellow solid, and the residue
was recrystalled from a toluene-THF solution. Yield 0.38 g (48%).
1H NMR (400 MHz, CDCl3, 25 ◦C, ppm): ı 8.16, 7.94 (s, 1H, N CH),
7.34–7.32, 7.18–6.99 (m, 5H, ArH-OBn), 6.83–6.78, 6.71–6.67,
6.66–6.42 (m, 3H, ArH-L), 4.85 (br, 2H, OCH2Ph), 3.80 (s, 3H, OCH3),
3.64 (br, 2H, C NCH2), 3.24 (t, 2H, C NCH2, J = 5.6 Hz), 2.55 (br,
2H, C NCH2CH2), 2.40 (t, 2H, C NCH2, J = 5.6 Hz), 2.06 (s, 6H,
N(CH3)2). 13C NMR (100 MHz, CDCl3, 25 ◦C, ppm): ı 170.69 (C N),
162.46, 152.66, 127.89, 127.39, 126.86, 117.87, 113.94, 112.21
(ArC), 59.42, 59.18 (OCH2Ph), 57.77 (OCH3), 55.90 (C NCH2), 45.94
(C NCH2CH2), 45.43(N(CH3)2). Anal. calc. for C38H48N4O6Zn2: C,
57.95; H, 6.14; N, 7.11. Found: C, 54.94; H, 6.30; N, 6.56.
2.5. X-ray crystallographic studies
Suitable crystal of [Zn2L2(OBn)2] complex were sealed in thin-
walled glass capillaries under a nitrogen atmosphere and was
mounted on a Bruker AXS SMART 1000 diffractometer. Intensity
data were collected in 1350 frames with increasing w (width of 0.3◦
per frame). The absorption correction was based on the symmetry-
equivalent reflections using the program SADABS [51]. The space
group determination was based on a check of the Laue symme-
try and systematic absence and was confirmed using the structure
solution. The structures were solved by direct methods using the
SHELXTL package [52,53]. All non-H atoms were located from suc-
cessive Fourier maps, and hydrogen atoms were refined using a
2.3. Typical polymerization procedures
A
typical polymerization procedure was exemplified
by the synthesis of PLA (the number 100 indicates the
designed [LA]0/[complex]0) at 60 ◦C (Table 1, entry 1). The

L. Yao et al. / Journal of Molecular Catalysis A: Chemical 352 (2012) 57–62
59
Table 1
Ring-opening polymerization of l-Lactide by [Zn2L2(OBn)2].a
.
Entry
Solvent
[M]0 /[Cat]0
Temperature (◦C)
Time (h)
Convb. (%)
Mn(cal)c (g mol−1
)
Mn(obs)d (g mol−1
)
PDId
1
2
3
4
5
Toluene
Toluene
Toluene
Toluene
Toluene
100/1
200/1
300/1
400/1
100/1
60
60
60
60
25
2
4
8
10
16
94.0
92.3
97.6
99.5
89.3
6876
13,399
21,190
28,764
6538
5644
10,045
21,255
24,908
6145
1.21
1.14
1.26
1.12
1.14
a
Conditions: [Zn2L2(OBn)2] (0.015 mmol), toluene (10 mL), 60 ◦C.
Determined by 1H NMR.
b
c
Calculated from Mn(cal) = [M]/2[I] × M[M] × conv(%) + M(BnOH).
d
Observed by GPC in THF using polystyrene standards and a correction factor of 0.58 [49].
riding model. Anisotropic thermal parameters were used for all
non-H atoms, and fixed isotropic parameters were used for H
atoms.
O(2A)–Zn–(2) 83.03(8)◦. The distances from the Zn atom to O(1),
O(2), O(2A), N(1), and N(2) were 2.041(2), 2.001(2), 1.977(2),
2.057(3) and 2.369(3) A. These bond lengths and bond angles are
˚
all within a normal range for zinc Schiff base complexes [42,43].
The stoichiometric structure of [Zn2L2(OBn)2] was further con-
firmed on the basis of 1H and 13C NMR spectroscopy. In the 1H
NMR spectrum of this complex (Fig. 2) in CDCl3 at room tem-
perature, two sets of resonance peaks (N CH, ı 8.16, 7.94 ppm;
N–(CH3)2, ı 2.02, 1.87 ppm) indicate the existence of both dimeric
and monomeric species instead feature in solution at this tem-
perature. Furthermore, monomeric species is more than dimeric
species [42].
3. Results and discussion
3.1. Synthesis and structural characterization of [Zn2L2(OBn)2]
The Schiff base [HL] was synthesized according to the liter-
ature [48] [Zn2L2(OBn)2] was obtained via the reaction of HL
and ZnEt(OBn) in 1:1.1 molar ratio in toluene at room tem-
perature (Scheme 1). Single crystals of [Zn2L2(OBn)2] suitable
for X-ray structural determination was obtained from toluene
and THF solution. As depicted in Fig. 1, in the solid state
3.2. Ring-opening polymerization of l-lactide
[Zn2L2(OBn)2] [54] has
a dimeric structure with two sym-
metrical pentacoordinated zinc centre bridging through the
oxygen atom of the benzyl alkoxy group. And the geometry
around Zn atoms were distorted trigonal bipyramidal with an
average compressed axial O(1)–Zn–N(2) 158.44(10)◦ and equa-
torial O(2)–Zn–N(1) 147.06(10)◦, O(2A)–Zn–N(1) 129.14(10)◦ and
As can be seen from the data compiled in Table 1, [Zn2L2(OBn)2]
1(1.5 mM) is active initiators for the ring opening polymerization
of l-lactide. Experimental results show that the polymerization
can reach to completion within 10 h at 60 ◦C in the monomer-to-
initiator ratio ([M]0/[I]0) range of 100–400. The polymerization is
Fig. 2. The 1H NMR spectrum of [Zn2L2(OBn)2].

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L. Yao et al. / Journal of Molecular Catalysis A: Chemical 352 (2012) 57–62
polymerization takes 30–80 min at 25 ◦C while using complexes (A
and E) as an initiator, but the polymerization reaction completed
within 240 min at 80 ◦C for C. It is found that the presence of sub-
stituent, at ortho-position of the phenoxide unit of the ligand, plays
an important role in determining the polymerization rate. When
the ortho-hydrogen of phenoxide is replaced by sterically hindered
tert-butyl group and electron-donating methoxyl group, the poly-
merization rate both decreases dramatically as compared to the
reaction rate of 1, A and E, and the polymerization rate decreases
more for 1 than A. It is revealed that electron-donating effect com-
bined with steric effect plays an important role in decreasing the
reactivity of zinc alkoxide. It attributes to the bond intensity of zinc
and oxygen atom of the alkoxy group (1 > A, E) in their single crys-
tal structure, such as Zn–O(2) 2.001(2), Zn–O(2A) 1.977(2) for 1,
Zn–O(2) 2.016(1), Zn–O(2A) 1.978(2) for A and Zn–O(2) 1.989(3),
Zn–O(2A) 2.089(3) for E. And the stronger these bonds intensity is,
the more difficultly the coordination insertion occurs.
Fig. 3. (A) Ring-opening polymerization of l-LA initiated by [Zn2L2(OBn)2] under
different [LA]0/[initiator]0, ꢀ: plot of Mn vs. l-LA to initiator ratio, and ꢀ: PDI.
3.3. Kinetic studies of polymerization of l-lactide by
[Zn2L2(OBn)2] 1
well controlled and the “living” character is demonstrated by the
low polydispersity index (PDIs) ranging from 1.12 to 1.26 of the
polymers and by the linear relationship between Mn and [M]0/[I]0
ratio (Fig. 3). In order to realize the initiating process, 1H NMR stud-
ies on the PLLA-100 (Table 1, entry 1) indicates that the polymer
chain is capped with a benzyl ester group on one end and a hydroxyl
group on the other end (Fig. 4) According to the integration ratio
close to 5:1 between Ha and He, it is suggested that the initiation
occurred through the insertion of the benzyl alkoxy group from 1
into l-lactide, giving a zinc alkoxide intermediate, which further
reacts with excess of l-LA yielding polyesters [42,43].
Compared with the zinc analogue complexes (A–E) (Fig. 5)
reported by Lin et al. [42] the reactivity of initiator 1 toward the
ROP of l-lactide is lower than that of complexes A–E (except for
C). Polymerization goes to completion within 960 min at 25 ◦C
for 1 (Table 1, entry 5), and the polymerization conversion only
reaches to 10% within 80 min at 25 ◦C for 1. For instance, complete
A kinetic study [50] was conducted to establish the reaction
order with respect to monomer and initiator 1. l-Lactide (0.216 g,
1.5 mmol) was added to a solution of 1 (1.50, 1.88, 2.39, or 2.78 mM)
in toluene (10 mL). The solution was then stirred at 60 ◦C under an
atmosphere of nitrogen. At the indicated time intervals, conversion
of l-lactide was analyzed by 1H NMR in CDCl3.
As expected, plots of ln([lactide]0/[lactide]) vs. time for a wide
range of [1] are linear, indicating the usual first order dependence
on monomer concentration (Fig. 6). Thus, the rate expression can
be written as d[lactide]/dt = kp[lactide]1[1]n = kobs[lactide]1, where
kobs = kp[1]n. A plot of kobs vs. [1] (Fig. 7) allows the determi-
nation of kp and n. The linear relationship between kobs vs. [1]
indicates the first-order in initiator. The slope of the best fit
line (75.97) equals kp and thus the polymerization rate constant,
kp, is 75.97 M−1 min−1. Therefore, the overall rate expression is
d[lactide]/dt = kp[lactide]1[1]1.
Fig. 4. 1H NMR spectrum of PLLA-100 (Table 1, entry 1).

L. Yao et al. / Journal of Molecular Catalysis A: Chemical 352 (2012) 57–62
61
Fig. 5. The structure of zinc alkoxides (A–E).
4. Conclusion
A
zinc complex with the Schiff base, derived from 2-
dimethylaminoethylamine and o-vanillin [Zn2L2(OBn)2], has been
synthesized and its structure has been established by X-ray crys-
tallography and NMR spectroscopy. This zinc complex efficiently
initiated the ROP of l-LA, and the polymerization runs were bet-
ter controlled. The structure of the ancillary ligands showed some
influence on the catalytic activity, the introduction of electron-
rich methoxy at ortho-phenoxy substituent resulted in a decrease
of the polymerization rate. In addition, we also have found that
the polymerization reaction proceeds with first-order rate in both
monomer and initiator.
Acknowledgments
Financial supports by the National Natural Science Foundation
of China (No. 21071069), the Scientific Research Foundation for
the Returned Overseas Chinese Scholars, State Education Ministry
and the Fundamental Research Funds for the Central Universities
of China (lzujbky-2009-26) are gratefully acknowledged.
Fig. 6. First-order kinetic plots for l-lactide polymerizations with time in toluene
with different concentration of [Zn2L2(OBn)2] as an initiator.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2011.10.012.
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[54] Crystal data for [Zn2L2(OBn)2] complex: C38H48N4O6Zn2, M = 787.54,
˚
˚
Orthorhombic,
space
group
Pbca,
a = 12.562(2) A,
b = 16.541(3) A,
3
˚
˚
c = 17.953(3) A, ˛ = 90.00, ˇ = 90.00, ꢀ = 90.00, V = 3730.4(12) A , T = 296(2) K,
Z = 4, Dc = 1.402 g/cm3, F000 = 1648, 2ꢁmax = 28.30, 18662 reflections col-
lected, 4502 unique (Rint = 0.0831), no. of observed reflections 4502 (I > 2(I));
R1 = 0.0474, wR2 = 0.0813.