Synthesis and structural studies of copper(II) complex supported by -ONNO- tetradentate ligand: Efficient catalyst for the ring-opening polymerization of lactide

Article abstract of DOI:10.1016/S1872-2067(14)60293-X

The -ONNO- tetradentate Schiff base ligand N,N'-bis(2-hydroxy-3-methoxybenzaldehyde)benzene-1,2-diamine (HMBBD) has been synthesized. The ligand was attached to copper (Cu-HMBBD) in methanol under N₂ atmosphere to give a mononuclear complex. Th

Full text of DOI:10.1016/S1872-2067(14)60293-X

Chinese Journal of Catalysis 36 (2015) 764–770
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Synthesis and structural studies of copper(II) complex supported by
ONNO– tetradentate ligand: Efficient catalyst for the ring-opening
polymerization of lactide
–
a
a
a
b,
Anita Routaray , Nibedita Nath , Somanath Mantri , Tungabidya Maharana *,
a,#
Alekha Kumar Sutar
a
Catalysis Research Lab, Department of Chemistry, Ravenshaw University, Cuttack-3, Odisha, India
Department of Chemistry, National Institute of Technology, Raipur, India
b
A R T I C L E I N F O
A B S T R A C T
Article history:
The –ONNO– tetradentate Schiff base ligand N,N’-bis(2-hydroxy-3-methoxybenzaldehyde)benzene-
Received 12 December 2014
Accepted 12 January 2015
Published 20 May 2015
1,2-diamine (HMBBD) has been synthesized. The ligand was attached to copper (Cu-HMBBD) in
methanol under N atmosphere to give a mononuclear complex. The reactivity of this complex in the
2
ring-opening polymerization (ROP) of lactide has been studied. The complex has a square planner
geometry, as determined by X-ray diffraction studies. The copper complex is highly active towards
the ring-opening polymerization of lactide, and the rate of polymerization is heavily dependent on
the initiator used. The copper complex allows controlled ring-opening polymerization, as shown by
the linear relationship between the percentage conversion and the number average molecular
weight. Based on the literature, a mechanism for the ROP of lactide has been proposed.
Keywords:
Ring-opening polymerization
Lactide
Copper complex
Polylactic acid
©
2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1
.
Introduction
polymerization manner [3]. A wide variety of cyclic organic
compounds have been investigated via ROP to give the poly-
mers with novel architectures. The ROP of lactide has received
renewed attention in recent years as it allows for control of the
copolymer microstructures in ways that would be difficult to
achieve by conventional polymerization methods. Many poly-
mer chemists have polymerized lactide using different metal
complexes (transition and lanthanide) as catalysts/initiators,
incorporating bi-dentate, tri-dentate and tetra-dentate ligands.
Of the various methods that have been employed, metal con-
taining initiators have achieved increasing industrial im-
portance [4,5]. The limiting use in biomedical applications is
the extent to which the metal residues can be removed upon
quenching the polymerization. As the removal of metal can
The synthesis of polymers with accurately prepared archi-
tectures (controlled molecular weights) and promising proper-
ties has been a constant challenge for polymer scientists. Or-
ganic polymer chemists commonly adapt known organic reac-
tions to control the way compounds of low molecular weights
are converted into polymers of high molecular weights. How-
ever, living polymerization offers similar or better control and
was first discovered by Szwarc [1] and thereafter,
Matyjaszewski et al. [2] developed the controlled cationic and
radical vinyl polymerizations. Similarly, a number of coordina-
tion and metathesis polymerizations, as well as ring-opening
polymerizations (ROPs) were found to operate in a living
*
#
Corresponding author. Tel: +91-771-2254200; Fax: +91-771-2254600; E-mail: mtungabidya@gmail.com
Corresponding author. Tel: +91-671-2610060; Fax: +91-671-2610304; E-mail: dralekhasutar@gmail.com
This work was supported by DST, CSIR and UGC, New Delhi, India.
DOI: 10.1016/S1872-2067(14)60293-X | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 36, No. 5, May 2015

Anita Routaray et al. / Chinese Journal of Catalysis 36 (2015) 764–770
765
never be complete, it is preferred that a feasible industrial pro-
cess should employ metals that are not cytotoxic. When the
process uses environmentally benign metals that are part of the
mammalian anatomy, there should be no harm from the metal
Sigma-Aldrich and used as received. Other chemicals were of
analytical grade (>99.0 wt%) and used as received.
2.2. Characterization of the HMBBD Schiff base and its copper
complex
[
6].
Most recently, metal alkoxides (e.g., Al [7–9], Li [10–12], Mg
13–18], Fe [19], Sn [20,21], Ti [22,23] or Zn [24–27]) have
[
Fourier transform infrared (FT-IR) spectra were recorded
as a KBr pellet using a Perkin-Elmer 1600 FTIR Spectropho-
tometer. Electronic spectra were recorded on a Shimadzu 1601
PC UV-Vis Spectrophotometer as a sample mull in a cuvette.
Thermo gravimetric analysis (TGA) was carried out using a
proved to be efficient catalysts for the ROP of lactide, because
they initiate the living ring-opening polymerization and pro-
duce polymers with both well-controlled molecular weights
and narrow molecular weight distributions. Copper complexes
have high electron transfer ability, moderate Lewis acidity and
are stable with respect to the reactive intermediates of ROP,
and are a prime target for ROP investigations. Cu is also a bio-
compatible metal and is required for life. To the best of our
knowledge there have been few reports on copper initiators
involving N-containing polydentate ligands in ROP of lactide.
This group of ligand is one of the most versatile ligand classes
in both main group and transition metal coordination chemis-
try and allows for the facile modulation of steric and electronic
factors [28]. The copper complexes that have been reported
include those derived from pyrazole [29], phenoxy-ketimine
Perkin-Elmer Pyris, Diamond Thermal Analyzer under a N
2
atmosphere at a heating rate of 10 °C/min. The loading of metal
ions in the Schiff base was determined by analyzing the loading
solution with a Perkin-Elmer 3100 Atomic Absorption Spec-
trometer at λmax of copper ions. The composition of the HMBBD
Schiff base and its copper complex was estimated using a
Haraeus Carlo Ebra 1108 Elemental Analyzer. The 1H-NMR
spectra were recorded on an FT-NMR Brucker 300 MHz Spec-
6
trometer using DMSO-d as the solvent and tetramethylsilane
(TMS) as an internal reference. The magnetic moment (µ) of
metal complexes was measured using a Vibrating Sample
Magnetometer-155. The molecular weight of the HMBBD Schiff
base and its copper complex was determined using a Vapor
Pressure Osmometer (Merck VAPRO 5600, Germany). Suitable
crystals of the ligand were analyzed on a Bruker Kappa Apex-II
diffractometer.
[30], salicylaldimine [31,32], salen ligands [33] and diketimi-
nate [34], which are active in the polymerization of lactides and
produced polymers with a moderate number average molecu-
lar weights and narrow molecular weight distribution. Ap-
pavoo et al. [29] reported the polymerization of D,L-lactide in
toluene at a 100:1 ratio using copper pyrazole at 110 °C, giving
M
w
= 1780 Da and PDI = 1.84 and this shows the typical for-
2.3. Synthesis of HMBBD Schiff base and its copper complex
mation of low molecular weight polymers. Similarly, ROP of
L-lactide at a 200:1 ratio at 160 °C produced low molecular
weight polymers [30]. The literature has many examples of
Schiff base complexes that have multiple applications in differ-
ent organic transformation reactions [35,36], but there have
been no systematic studies of nitrogen-containing polydentate
ligand complexes like the copper complex of N,N’-bis(2-hy-
droxy-3-methoxybenzaldehyde)benzene-1,2-diamine (HMBBD).
Although the lactide polymerization by transition metal
complexes with N-containing polydentate ligands has been
reported [28], the copper complex of HMBBD has not been
reported; hence, we report the synthesis, structural character-
ization and catalysis studies of the Cu complex of HMBBD for
the ROP of lactide.
The HMBBD Schiff base was synthesized by a modification
of a procedure reported in the literature [37,38]. A mixture of
2-hydroxy-3-methoxybenzaldehyde (20.00 mmol, 3.04 g) and
1,2-diaminobenzene (10.00 mmol, 1.08 g) in methanol was
refluxed at 60 °C for about 2 h. The reaction mixture was cooled
to low temperature, producing orange crystals, which were
filtered and recrystallized from chloroform. Yield = 94.4%. El-
em. Anal.: Found (%): C = 70.35, N = 7.02 and H = 5.70; Calcd
C
22
H
20
N
2
O (%): C = 70.20, N = 7.44 and H = 5.36. The copper
4
complex of the HMBBD Schiff base was prepared by taking 100
mL of a methanolic solution of the Schiff base (20.00 mmol,
7.53 g) and 20.00 mmol (2.69 g) of the copper salt in a round
bottom flask and refluxing at 60 °C for 5 h. All reactions were
performed under a nitrogen atmosphere. The copper complex
was recrystallized from methanol and dried in a vacuum desic-
cator. Elem. Anal.: Found (%): C = 61.75, N = 6.24 and H = 4.21;
2
.
Experimental
.1. Materials
Syntheses were performed under a dry N
2
Calcd C22
H
18CuN
2
O (%): C = 60.34, N = 6.40 and H = 4.14. The
4
loading of copper ions in HMBBD was calculated as the com-
plexation of metal ion using the following equation: Complexa-
tion of copper ion = amount of copper loaded/amount of Schiff
base used for loading × 100%.
2
atmosphere us-
ing a combination of a glove box and standard Schlenk tech-
niques. All solvents were of analytical grade and were dried
and distilled prior to use. Toluene and CH
distilled from sodium benzophenone and P
2
Cl
2
were dried and
, respectively.
2
O
5
2.4. Cu-HMBBD complex in the ring-opening polymerization of
L-lactide
Anhydrous copper chloride and 1,2-diaminobenzene were
purchased from HiMedia Laboratories Pvt. Ltd., Mumbai, India.
2
-Hydroxy-3-methoxybenzaldehyde and benzyl alcohol were
The synthesis of polylactic acid (PLA-150) at room temper-
ature shows a typical polymerization procedure. L-lactide (0.72
procured from E. Merck, India. L-lactide was obtained from

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Anita Routaray et al. / Chinese Journal of Catalysis 36 (2015) 764–770
OCH
3
OCH
3
OCH
3
OCH
3
OH
HO
O
O
100
Metal salt
solution
Cu
HMBBD
Cu-HMBBD
N
N
N
N
9
8
7
6
0
0
0
0
Cu-HMBBD
HMBBD
Scheme 2. Coordination of the copper ion to HMBBD.
The HMBBD Schiff base was obtained in excellent yield from
the simple reaction of the two starting materials (94.4%)
Scheme 1). The elemental analysis of HMBBD corresponds to
the empirical formula (C22
) of HMBBD [37,38]. The
molecular weight of the Schiff base was 377.68 g/mol (Calcd.:
76.41 g/mol). The copper ions were coordinated to the
(
100
200
30o0
400
500
20 2 4
H N O
Temperature ( C)
3
Fig. 1. Thermal stability of the HMBBD Schiff base and its copper com-
plex.
HMBBD Schiff base by refluxing a mixture of the two at 60 °C
for 7 h (Scheme 2). The copper complex after separation and
purification was analyzed for the metal ions loading by atomic
absorption spectroscopy and was found to be 86.27 wt%. The
elemental analysis of Cu-HMBBD complex corresponded to the
g, 20 mmol) and a requisite amount of benzyl alcohol were
added to a rapidly stirred solution of Cu-HMBBD (0.058 g,
0.133 mmol) in toluene (20 mL). A rise in the viscosity was
empirical formula of the Cu-HMBBD complex (C22
H
18CuN
2
O ).
4
observed, and the stirring was ceased after 24 h. Volatile mate-
rials were removed under vacuum, and the residue was ex-
tracted with THF (30 mL). The extraction was dried and the
white precipitate washed with n-hexane three times before
being dried under vacuum overnight, giving a crystalline white
solid (0.58 g, yield: 81%).
The IR spectrum (Fig. 2) of HMBBD has absorption bands at
609 cm–1 (>C=N), 1260 cm–1 (>C–O) phenolic and a broad
1
HMBBD
3.
Results and discussion
3.1. Synthesis and characterization of the HMBBD Schiff base
and its copper complex
he thermal stability of the Cu-HMBBD catalysts was ana-
lyzed for their applications in high-temperature reactions, and
to provide proof for the complexation of copper ion to the
HMBBD Schiff base. The TGA of the HMBBD Schiff base showed
a 40.1% weight loss at 500 °C, but its copper complex showed a
3500
3000
2500
2000
1500
1000
500
−
1
Wavenumber (cm )
Cu-HMBBD
32.0% weight loss at the same temperature (Fig. 1). The copper
complex started to decompose at 150 °C whereas the ligand
was only stable until 100 °C. This shows that the copper com-
plex is more stable [39]. In addition to the thermal analysis,
HMBBD and its copper complex were also characterized by IR
and UV spectroscopies to provide proof metal ions had com-
plexed to the ligand. These would also help to determine the
structure and geometry of the copper complex based primarily
on the elemental analysis and magnetic properties of metal
complex.
3000
2500
2000
Wavenumber (cm )
Fig. 2. FTIR spectra of HMBBD and Cu-HMBBD.
1500
1000
500
−1
OCH3
OH
OCH3
OCH3
OH
OCH3
HO
HO
Reflux in EtOH
+
+
o
at 60 C for 1 h
O
H2N
NH2
O
N
N
2
-Hydroxy-
-methoxybenzaldehyde
HMB)
2-Hydroxy-
3-methoxybenzaldehyde
(HMB)
3
(
N,N'-Bis (2-hydroxy-3-methoxy benzaldehyde)
Benzene-1,2-diamine
BD)
benzene-1,2-diamine Schiff base
(
(HMBBD)
Scheme 1. Synthesis of N,N′-bis(2-hydroxy-3-methoxybenzaldehyde)benzene-1,2-diamine.

Anita Routaray et al. / Chinese Journal of Catalysis 36 (2015) 764–770
767
Table 1
FTIR frequencies and electronic transitions in HMBBD and its copper complex.
–1
Absorption frequencies (cm )
Frequencies λmax (nm)
Compound
υ
1609
C=N
υ
1260
C–O
υ
OH
υ
—
M–O
υ
—
M–N
π→π∗
260
n→π∗
330
C→T
—
d→d
—
HMBBD
2850–3050
Cu-HMBBD
1627
1251
—
568
444
250
326
347
443
–
1
band between 3050 and 2850 cm , which was assigned to the
phenolic OH. Complexation of the copper ion to HMBBD re-
sulted in significant shifts in the >C=N and >C–O absorptions
C(7)–N(1)–C(1)
C(6)–C(1)–N(1)
123.5(2);
124.7(2);
C(2)–C(1)–N(1)
N(1)–C(7)–C(15)
116.5(2);
121.3(2);
O(3)–C(17)–C(18) 125.7(2); O(3)–C(17)–C(16) 114.6(2);
C(3)–C(2)–N(2) 121.4(2); C(1)–C(2)–N(2) 118.5(2);
O(1)–C(10)–C(9) 122.3(2); O(1)–C(10)–C(11) 117.7(2);
O(4)–C(11)–C(12) 125.3(2); O(4)–C(11)–C(10) 15.1(2);
–1
(
18 and 9 cm , respectively) and new absorption bands due to
–1
–1
the formation of Cu–O (568 cm ) and Cu–N bonds (444 cm ).
The disappearance of the phenolic absorption band on com-
plexation was additional evidence for the inclusion of metal
ions in the complex (Table 1) [37,38].
Table 2
Crystal data of HMBBD and structure refinement parameters.
The complexation of metal ions to the Schiff base was fur-
ther confirmed by comparing the electronic spectra of metal
complexes and pure HMBBD. The electronic spectra of HMBBD
Emp. form.
Form. wt
Temp. (K)
λ (Å)
Crystal system
Space group
a (Å)
C
22
H
20
N
2
O
4
376.41
293(2)
0.71073
monoclinic
P2 /c
(
Fig. 3) show absorption bands at 260 and 330 nm, which were
*
*
assigned to π → π and n → π transitions. The Cu-HMBBD com-
plex exhibits a hypsochromic shift in the π → π transition from
*
1
*
6.6546 (2)
2
60 to 250 nm, and in the n→ π transition from 330 to 326 nm
Table 1, Fig. 3). Two additional bands, a C→T transition and a
d→d transition, appeared at 347 and 443 nm, respectively.
b (Å)
16.8579 (7)
(
c (Å)
17.1749 (7)
α (°)
90
6
3
These electronic transitions correspond to the t2g and e con-
g
β (°)
93.958 (3)
figurations for the copper(II) ion in this complex. The magnetic
moment (µ) of the Cu-HMBBD complex was found to be 1.89
BM, which indicated that it was paramagnetic and most likely
γ (°)
90
3
V (Å )
1908.17 (13)
Z
4
3
ρ (mg/m )
µ (Mo Kα) (mm )
F(000)
Crystal size (mm )
θ range (°C)
Index ranges
Reflections collected
Independent reflections
R(int)
Completeness to θ = 25.242°
Refinement method
GOF
Final R indices (I > 2ρ(I))
R indices
Extinction coefficient
1.310
0.091
792
2
in a square planar structure with dsp hybridization.
–
1
Single crystals of HMBBD suitable for XRD measurement
3
1
crystallized in the monoclinic space group P2 /c. Crystallo-
0.52 × 0.50 × 0.23
1.701–25.739
–8 ≤ h ≤ 7, –20 ≤ k ≤ 20, –20 ≤ l ≤ 20
graphic data and the results of structure refinements are sum-
marized in Table 2, and the crystal structure is shown in
Scheme 3. This shows that each 2-hydroxy-3-methoxybenzal-
dehyde binds to one of the two diaminobenzene nitrogens.
Selected bond lengths (Å) and bond angles (deg): O(2)–C(16)
20366
3614
0.0713
99.9%
2
Full-matrix least-squares on F
1
1
.348(2); O(1)–C(10) 1.354(3); N(2)–C(8) 1.274(3); N(2)–C(2)
.425(3); O(3)–C(17) 1.364(3); O(3)–C(21) 1.417(3);
0.991
R
R
1
= 0.1183, wR
= 0.1183, wR
n/a
2
= 0.1454
= 0.1454
N(1)–C(7) 1.279(3); N(1)–C(1) 1.410(3); O(4)–C(11) 1.365(3);
O(4)–C(22) 1.423(3); C(15)–C(7) 1.436(3); C(9)–C(8) 1.457(3);
O(2)–C(16)–C(15) 122.5(2); O(2)–C(16)–C(17) 117.6(2);
C(8)–N(2)–C(2) 118.4(2); C(17)–O(3)–C(21) 117.8(2);
1
2
–
3
ρmin – ρmax (e Å )
0.143 and –0.189
Cu-HMBBD
HMBBD
2
00
300
400
Wavelength (nm)
Fig. 3. UV-Vis spectra of HMBBD and its copper complex.
500
600
700
800
Scheme 3. Crystal structure of HMBBD.

768
Anita Routaray et al. / Chinese Journal of Catalysis 36 (2015) 764–770
O
O
O
HO
N
N
O
Cu-HMBBD
BnOH
H
O
n
N
N
O
Cu
O
O
OBn
O
O
O
Cu
O
O
OCH
3
OCH
3
O
n
Step-3
O
OCH
OCH
3
3
Step-2
OH
Scheme 4. Ring-opening polymerization of lactide.
O
O
O
O
O
Step-1
N(2)–C(8)–C(9) 122.2(2).
3
.2. Cu-HMBBD complex in the ring-opening polymerization of
N
N
Cu
O
L-lactide
O
O
O
OCH
3
OCH
3
O
O
O
H
O
Step-5
Step-4
O
On the basis of the “immortal” property of lactide polymeri-
OH
zation, demonstrated by several copper complexes [29–34],
and evidence of in situ formation of a metal alkoxide in the
presence of an alcohol [7–27], the complex Cu-HMBBD was
expected to behave as a catalyst in the ROP of lactide in the
presence of benzyl alcohol. Cu-HMBBD does initiate the ROP of
L-lactide (LA) in the presence of benzyl alcohol in dichloro-
methane at 30 °C (Scheme 4). The polymerization results are
listed in Tables 3 and 4. All runs displayed high activities for the
polymerization of L-lactide and great control of the molecular
weight. The presence of benzyl alcohol has a significant influ-
ence on the polymerization behavior of the Cu-HMBBD.
O
O
N
N
O
Cu
O
O
OCH
3
OCH
3
O
H
O
O
O
O
O
O
O
H
O
Final Step
n
O
O
O
O
Scheme 5. Reaction steps in the ROP of L-lactide.
should give a higher polymerization rate.
This ROP of L-lactide using Cu-HMBBD with a monomer to
benzyl alcohol ratio of 150/1 has been systematically studied at
From the polymerization results listed in Table 4, several
structure-activity trends may be drawn. These results indicate
that Cu-HMBBD is an efficient catalyst for ROP of L-lactide in
the presence of BnOH, when the [LA]/[BnOH] ratio is 50–200.
The polymerization is well controlled and the “living” character
30 °C (Table 3). It is worth noting that Cu-HMBBD is more ac-
tive in CH Cl than in toluene or in THF. The slowest polymeri-
2
2
zation rate was found in THF, probably because of coordination
of the THF to the copper metal, thus slowing the reaction rate.
is demonstrated by the linear relationship between M
LA]/[BnOH] ratio (Fig. 4). The “immortal” character of this
n
and the
Toluene is a non-polar solvent and CH
2
Cl is a polar aprotic
2
[
solvent. Based on the mechanism (Scheme 5), the polar solvent
reaction was examined by using two or four equivalents of
benzyl alcohol as a chain transfer agent (Table 4, entries 6 and
7). It was found that for the polymerization without BnOH,
there is almost negligible conversion (<5%), but the use of
BnOH drastically changes the activity of the catalyst. The use of
more BnOH (entries 6 and 7) decreases the reaction time from
Table 3
Polymerization of L-lactide using Cu-HMBBD in CH
2
Cl at 30 °C.
2
Conversion a
(Theory) b
(VPO) c
M
n
M
n
Solvent
CH Cl
(%)
(g/mol)
20100
17500
12300
(g/mol)
22900
19400
17100
2
2
92.4
80.7
56.5
Toluene
THF
24 to 11 h. This may be because the initiator is actively partici-
pating in the polymerization reaction. The molecular weight of
the polymers was also affected by the ratio of BnOH used. By
adding two or four equivalents of benzyl alcohol to the
Reaction conditions: [LA]
0
= 20 mmol, room temperature, 24 h,
/[Cu]/[BnOH] = 150/1/1.
Percentage conversion of the monomer calculated by (weight of pol-
[
M]
0
a
ymer recovered/weight of monomer) × 100%.
b
Calculated by ([LA]
Determined by VPO [40].
0
/[BnOH]) × 144.13 × conversion + 108.14.
c
30000
Table 4
Polymerization of L-lactide by copper complex (Cu-HMBBD) in the
presence of benzyl alcohol.
25000
2
1
1
0000
5000
0000
[
LA]
0
/
Time
(h)
24
24
24
24
24
20
15
Conversion
(%)
M
n
(Theory)
(g/mol)
– a
5800
12900
20100
27400
6700
3500
M (VPO)
(g/mol)
– a
7300
15100
22900
28600
7900
4100
n
Entry
[
Cu]/[BnOH]
100:1:0
50:1:1
100:1:1
150:1:1
200:1:1
100:1:2
100:1:4
1
2
3
4
5
6
7
<5
78.6
88.9
92.4
94.6
91.2
93.3
5
000
40
60 80 100 120 140 160 180 200 220
LA]/[BnOH]
Fig. 4. Polymerization of L-lactide catalyzed by Cu-HMBBD in CH
room temperature.
[
Reaction conditions: [LA]
Data not available.
0
= 20 mmol, room temperature, CH
Cl
2 2
30 mL.
2
Cl at
2
a

Anita Routaray et al. / Chinese Journal of Catalysis 36 (2015) 764–770
769
2.5
2.0
1.5
1.0
0.5
0.0
Y = -0.00741 + 0.0918 X
R = 0.99997
2
0
5
10
15
20
25
Time (h)
Fig. 6. Semi-logarithmic plot of L-lactide conversion catalyzed by
Cu-HMBBD in the presence of BnOH: [LA] /[Cu]/[BnOH] = 100/1/1.
0
BnOH as initiator in the ratio [LA]/[Cu]/[BnOH] = 100/1/1.
The results are depicted in Fig. 6.
This plot implies that the polymerization reaction obeys
first order kinetics as the ln(M
0
/M ) vs time plot is linear. The
t
apparent rate constant (kapp) may be calculated from the slope
of this plot. The kapp for the L-lactide polymerization was found
to be 0.0918 h .
7
6
5
4
3
2
−1
1
Fig. 5. H NMR spectrum of PLLA-150 (150 indicates a [LA] /[BnOH]
0
ratio of 150).
3.4. Mechanism of polymerization
Considering the experimental findings for the ROP of
L-lactide under the conditions mentioned, the polymerization
may proceed by the mechanism outlined in Scheme 5. The
copper complex (Cu-HMBBD) produces active species in step 2
through fast interactions with lactide and benzyl alcohol. The
active species subsequently forms an intermediate through a
rearrangement in step 3, and then in step 4, ring opening of
lactide occurs to give a benzyl ester at one end. Ring opening of
another molecule of lactide occurs in step 5 as in step 2. Sub-
sequent additions of lactide produce PLLA.
polymerization reactions, the molecular weights were halved
or quartered, respectively.
1
To understand the initiation process, H NMR studies on the
poly-L-lactide (PLLA) as formed by Cu-HMBBD with BnOH as
1
an initiator have been performed. The H NMR spectrum of
PLLA-150 (Fig. 5), prepared from a [LA] /[BnOH] ratio of 150,
0
indicates that the polymer chain is capped with a benzyl ester
group on one end and a hydroxyl group on the other. This sug-
gests that the initiation occurred through the insertion of the
benzyl alkoxy group into L-lactide, giving a copper alkoxide
intermediate, which further reacts with an excess of L-lactide to
yield the product polyesters. This agrees with processes found
for other metal alkoxides [15–18]. Our polymerization results
are superior to those reported for copper complexes with more
elaborate ligands [30–33]. Literature reports for the polymeri-
zation of L-lactide using three different copper phenoxy-
4.
Conclusions
The copper complex Cu-HMBBD was synthesized and char-
acterized and is a good catalyst for the ring-opening polymeri-
zation of L-lactide in the presence of benzyl alcohol. The copper
complex Cu-HMBBD is inactive in the absence of benzyl alcohol
and, in the presence of two or four equivalent benzyl alcohol in
the polymerization reaction, the molecular weights reduce to
one half and one quarter, respectively. All the PLLA produced
by the ROP of L-lactide has a high molecular weight and greater
than 80% conversion rate.
ketimine complexes at 160 °C have produced PLLA of M =
n
2200, 4500 and 6500 g/mol [30], which suggests that our re-
sults are superior in terms of M (Table 4). Also, the molecular
n
weights of these polymers are much lower than PLLA from
common initiators such as [Al(OiPr) ] (M = 90000 g/mol) [9].
3
n
This may be due to the higher electrophilicity of the metal cen-
ter when compared with Cu-HMBBD and more favorable coor-
dination and insertion into the L-lactic acid monomers. There-
fore, for the production of higher molecular weight PLLA, more
electropositive metal centers and electron withdrawing groups
attached to the Schiff base are required.
Acknowledgments
The authors are grateful to Ravenshaw University and Na-
tional Institute of Technology, Raipur for providing research
facilities.
3.3. Kinetics of polymerization
References
We have performed kinetic studies for the polymerization of
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770
Anita Routaray et al. / Chinese Journal of Catalysis 36 (2015) 764–770
Graphical Abstract
Chin. J. Catal., 2015, 36: 764–770 doi: 10.1016/S1872-2067(14)60293-X
Synthesis and structural studies of copper(II) complex supported by –ONNO– tetradentate ligand: Efficient catalyst for the
ring-opening polymerization of lactide
Anita Routaray, Nibedita Nath, Somanath Mantri, Tungabidya Maharana*, Alekha Kumar Sutar *
Ravenshaw University, India; National Institute of Technology, India
O
ROP
O
O
O
O
*
*
O
n
PLA
Catalyst
This perspective describes the synthesis, characterization and catalytic studies of copper complex supported by –ONNO– tetradentate
Schiff base, towards ring-opening polymerization of lactide.
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