Nickel(II) complex catalyzed ring-opening polymerization of lactide

Article abstract of DOI:10.1016/j.poly.2016.08.032

The present manuscript is focused on the synthesis of a series of nickel complexes [Ni-HMBED, Ni-HMBPD and Ni-HMBBD] based on Salen-type Schiff-base ligands and a study of their reactivity for the ring-opening polymerization (ROP) of lactide. These nickel

Full text of DOI:10.1016/j.poly.2016.08.032

Accepted Manuscript
Nickel(II) Complex Catalyzed Ring-Opening Polymerization of Lactide
Anita Routaray, Somanath Mantri, Nibedita Nath, Alekha Kumar Sutar,
Tungabidya Maharana
PII:
S0277-5387(16)30391-6
DOI:
http://dx.doi.org/10.1016/j.poly.2016.08.032
POLY 12169
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
25 April 2016
10 August 2016
10 August 2016
Please cite this article as: A. Routaray, S. Mantri, N. Nath, A. Kumar Sutar, T. Maharana, Nickel(II) Complex
Catalyzed Ring-Opening Polymerization of Lactide, Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.
2016.08.032
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Nickel(II) Complex Catalyzed Ring-Opening Polymerization of Lactide
Anita Routaray1, Somanath Mantri1, Nibedita Nath1, Alekha Kumar Sutar1# and Tungabidya
Maharana2*
1Catalysis Research Lab, Department of Chemistry, Ravenshaw University, Cuttack-3, Odisha,
India
2 Department of Chemistry, National Institute of Technology, Raipur, India
#Corresponding author: Dr. AK Sutar; Tel - +91-671-2610060; Fax: +91-671-2610304; E-mail:
dralekhasutar@gmail.com
∗ Corresponding author: Dr. T Maharana; Tel-+91-771-2254200; Fax: +91-771-2254600; E-mail:
mtungabidya@gmail.com
Abstract
The present manuscript is focused on the synthesis of a series of nickel complexes [Ni-HMBED,
Ni-HMBPD and Ni-HMBBD] based on Salen-type Schiff-base ligands and a study of their
reactivity for the ring-opening polymerization (ROP) of lactide. These nickel complexes were
prepared by the reactions of a nickel solution with one molar equivalent of the Salen-type Schiff-
base ligands in methanol under a nitrogen atmosphere. Characterization of these nickel
complexes have been carried out by different analytical techniques, which showed a square
planar geometry for the Ni atom. The nickel complexes have shown high activity towards the
ring-opening polymerization of lactide, with variation in the diamine group. The linear
relationship established between the percentage conversion and the number-average molecular
weight has confirmed the controlled ROP by the use of the nickel complexes as a catalyst.
Finally, a mechanism for the ROP of lactide has been proposed.
Keywords: Ring opening polymerization, Lactide, Schiff base, Nickel complex, PLA.
1

1. Introduction
In the last two decades, polymer scientists have been putting their efforts to search and find
new polymeric systems with new structures and promising properties. Poly(lactic acid) (PLA) is
an important biodegradable and biocompatible polyester, [1-3] which is mainly synthesized by
the ring-opening polymerization (ROP) of lactide (LA), and as it degrades to non-toxic
components it very useful for biomedical applications [4-7]. PLAs are also used in orthopedic
applications, tissue engineering and biodegradable internal fixation devices [8-9]. However, to be
used in a biomedical field, it is necessary to know the extent to which the metal residues are
removable. Further, removal of metal up to a zero level does not happen in reality, thus, non-
cytotoxic metals should be preferred. Thus, environmentally benign metals with no harmful
effects towards living beings should be considered [10]. However, different metal catalysts, such
as complexes of aluminum [11-13], lithium [1,14-16], magnesium [17-21], copper [22-23], iron
[24], tin [25-26], titanium [27-28] or zinc [19, 29-33] etc., have been used towards the ROP of
lactide. In recent times, Ca, Mg, Fe and Zn based initiators or catalysts has received great
attention because of their metabolic action in the body [34-35]. Nowadays, nickel complexes,
with high electron transfer ability and stability of reactive intermediates, should also be
considered as a part of in-depth investigations.
Furthermore, nickel initiators having nitrogen-containing polydentate ligands have not been
discussed much to date for their application in the ROP of lactide [36-40]. Nowadays, FDA-
approved stannous octanoate is also being used commercially for the synthesis of PLAs. Nickel
complexes [41] are less toxic and have almost similar catalytic characteristics as stannous
octanoate, which has been used for synthesis of PLA. Recently, our research group has
synthesized Schiff base copper complexes which resourcefully catalyzed the ROP of L-lactide
2

[22-23]. Although, nickel Schiff base complexes have been widely studied for various chemical
transformations, their application in the ROP of L-lactide is still unexplored. Thus, in the current
context, study of the nickel complexes based on Salen type ligands, such as HMBED (N,N’-
bis(2-hydroxy-3-methoxybenzaldehyde)ethylenediamine, HMBPD (N,N’-bis(2-hydroxy-3-
methoxybenzaldehyde)propylenediamine
and
HMBBD
(N,N’-bis(2-hydroxy-3-
methoxybenzaldehyde)benzylenediamine), as initiators for ROP has been carried out. The main
advantages of these complexes are their stability in air and their ease of preparation. In the later
part, a kinetic study of the ROP of lactide was carried out to propose a reaction mechanism.
2. EXPERIMENTAL
2.1 Materials
The combination of a glove box and standard Schlenk techniques were used for the syntheses of
the Schiff bases, their nickel complexes and PLA. All syntheses were executed in dry nitrogen
ambiance. All solvents were of analytical grade and were dried and distilled prior to use.
Toluene and dichloromethane were dried and distilled from sodium benzophenone and P2O5
respectively. Anhydrous nickel chloride, ethylene diamine (ED), propylene diamine (PD) and
benzylene diamine (BD) were purchased from HiMedia Laboratories Pvt. Ltd., Mumbai, India.
2-Hydroxy-3-methoxybenzaldehyde (HMB) and benzyl alcohol were procured from E. Merck,
India. L-Lactide (LA) was obtained from Sigma Aldrich and was used as received. Other
chemicals were of analytical grade (>99.0 wt %) and were used as received.
2.2 Characterization of the Schiff bases and their nickel complexes
IR spectra were recorded on a Perkin-Elmer 1600 FTIR spectrophotometer. A Shimadzu
1601 PC UV–Vis spectrophotometer was used for determination of the electronic spectra. TGA
3

was carried out under a nitrogen atmosphere using a Perkin-Elmer Pyris, Diamond thermal
analyzer at a heating rate of 10ºC min-1. AAS was carried out with a Perkin-Elmer 3100 atomic
absorption spectrometer at λmax of the nickel ion. Analyses of compositions of the Schiff bases
and their nickel complexes were done on a Haraeus Carlo Ebra 1108 elemental analyzer. An FT-
NMR-Brucker 300 MHz spectrometer was used to record the NMR spectra of the samples, using
DMSO-d6 as a solvent and tetramethylsilane (TMS) as an internal reference. The room
temperature magnetic moment (µ) of the nickel complexes was measured with a Vibrating
Sample Magnetometer-155. Merck VAPRO 5600, Germany, A vapor pressure osmometer was
used to determine the molecular weights of the Schiff bases, their nickel complexes and PLA.
The crystal structure of the ligand was determined with a Bruker Kappa Apex-II diffractometer.
2.3 Synthesis of Characterization of Schiff bases and their nickel complexes
The N,N'-bis(2-hydroxy-3-methoxybenzaldehyde)ethylenediamine (HMBED), N,N'-
bis(2-hydroxy-3-methoxybenzaldehyde)propylenediamine (HMBPD) and N,N'-bis(2-hydroxy-3-
methoxybenzaldehyde)benzene-1,2-diamine (HMBBD) Schiff bases have been synthesized by
modification of a synthetic route reported in the literature [35]. A mixture of 2-hydroxy-3-
methoxybenzaldehyde (20.00 mmol, 3.04 g) and the diamine (10.00 mmol; 0.6g for ED, 0.74g
for PD and 1.08g for BD) in methanol were refluxed at 60 ºC for about 2 h. Upon cooling the
reaction mixture and recrystallizing with chloroform, colored crystals (yellow, light yellow and
orange colored for the ED, BD and PD ligands respectively) were obtained. Yield: 92.6, 91.2 and
94.4 % for ED, PD and BD respectively. The nickel complexes (Ni-HMBED, Ni-HMBPD and
Ni-HMBBD) were synthesized by refluxing a 100 ml methanolic solution of the respective
Schiff base (20.00 mmol; 6.56g for ED, 6.85g for PD and 7.53 g for BD) and 20.00 mmol (2.59
4

g) of nickel chloride in a round bottom flask at 60 ºC for 5 h. All the reactions were carried out
under a nitrogen atmosphere. Finally, the nickel complexes were recovered in methanol and
dried in a vacuum desiccator. Yield: 86.1, 84.3 and 88.8 % for Ni-HMBPD, Ni-HMBED and Ni-
HMBBD, respectively
The thermal stabilities of the Ni-HMBED, Ni-HMBPD and Ni-HMBBD catalysts were
analyzed to provide proof for the complexation and also to study their applications in high-
temperature reactions. The TGA of HMBED, HMBPD and HMBBD Schiff bases showed a
weight loss of 50.4, 46.3 and 40.1 wt% at 500 ºC, but their nickel(II) ion complexes showed a
weight loss of 39.5, 38.4 and 35.5 wt%, which indicates that the Ni-HMBED, Ni-HMBPD and
Ni-HMBBD complexes were more stable in comparison to the free ligands, and among all the
complexes Ni-HMBPD was found to be the most stable (Figure 1) [42]. FTIR and UV
techniques for the HMBED, HMBPD and HMBBD Schiff bases and their nickel complexes have
provided evidence for the formation of Ni-HMBED, Ni-HMBPD and Ni-HMBBD, and their
structures and geometries were also confirmed by elemental analysis and a magnetic property
study.
The absorption bands for the >C=N and >C–O phenolic groups and a broad band for the
phenolic OH group were observed at 1613, 1256 and 3100-2800 cm-1 for HMBED, 1634, 1258
and 3000-2850 cm-1 for HMBPD and 1609, 1260 and 3050-2850 cm-1 for HMBBD, respectively.
Elemental analysis data for HMBED showed (wt %): C 66.13, N 8.42, H 6.21; Calcd(%): C
65.84, N 8.53, H 6.14; for HMBPD (wt %): C 66.35, N 7.89, H 6.70; Calcd (%): C 66.65, N
8.18, H 6.48; for HMBBD (wt %): C 70.35, N 7.02, H 5.70; Calcd (%): C 70.20, N 7.44, H 5.36,
which correspond to the formula C18H20N2O4, C22H22N2O4 and C22H20N2O4, respectively [43-
44]. The molecular weights of the HMBED, HMBPD and HMBBD Schiff bases were 329.23
5

(Calcd 328.36), 340.63 (Calcd 342.39) and 377.68 g mol-1 (Calcd 376.41 g mol-1), respectively.
The absorption bands at 264 and 330 nm for HMBED, 262 and 330 nm for HMBPD and 260 and
330 nm for HMBBD (Figure 3), represent π → π* and n → π* transitions respectively (Figure
S1-S3) [22-23,45-47].
The complexation of the nickel ion with the HMBED, HMBPD and HMBBD Schiff
bases was carried out by refluxing a mixture of the Schiff bases and nickel chloride at 60 °C for
5 h (Scheme 1) in methanol. The elemental analysis for the Ni-HMBED complex showed (wt
%): C 56.64, N 7.21, H 4.81; Calcd(%): C 56.15, N 7.18, H 4.71; for Ni-HMBPD (wt %): C
57.71, N 7.09, H 5.19; Calcd(%): C 57.18, N 7.02, H, 5.05; for Ni-HMBBD (wt %): C 60.51, N
6.19, H 4.13; Calcd(%): C 61.01, N 6.47, H 4.19, which correspond to the formula
C18H18NiN2O4, C19H20NiN2O4 and C22H18NiN2O4, respectively. The observed molecular weights
of Ni-HMBED, Ni-HMBPD and Ni-HMBBD were 385.47, (Calcd 385.04), 399.34, (Calcd
399.07) and 434.10 g mol-1, (Calcd 433.08 g mol-1).
A considerable difference in the IR bands for the >C=N and >C–O groups were observed
due to complexation and also two new absorption bands at 520 and 421 cm-1 for Ni-HMBED,
513 and 429 cm-1 for Ni-HMBPD and 525 and 422 cm-1 for Ni-HMBBD were observed due to
the formation of Ni–O and Ni–N bonds (Table S1 and Figure S4-S6). Further, the phenolic OH
band between 2800 and 3100 cm-1 for HMBED, 2850 and 3000cm-1 for HMBPD and 2850 and
3050 cm-1 for HMBBD disappeared, which provides proof for the formation of the complexes
Ni-HMBED, Ni-HMBPD and Ni-HMBBD [43].
The Ni-HMBED, Ni-HMBPD and Ni-HMBBD complexes showed a hypsochromic shift in the π
*
→ π transition from 264 to 251, 262 to 251, 260 to 246 nm respectively and for the n→ π*
transition from 330 to 300, 330 to 278 and 330 to 292 nm, respectively. Two additional bands at
6

331 and 424nm for Ni-HMBED, 301 and 381nm for Ni-HMBPD, and 312 and 403nm for the Ni-
HMBBD complex were observed for the C→T and d→d transitions (Table S1). These electronic
6
2
transitions correspond to t2g eg configuration for the nickel(II) ion in these complexes. The
magnetic moments (µ) of the Ni-HMBED, Ni-HMBPD and Ni-HMBBD complexes were found
to be 0.69, 0.67 and 0.84 BM, which indicates the occurrence of a low-spin nickel(d8) center and
the slight variation from diamagnetism may be due to the presence of small amounts of
paramagnetic impurities [48-50]. All the nickel complexes have a square planar geometry with
dsp2 hybridization.
2.4 Nickel complexes in the ring opening polymerization of L-lactide
A representative polymerization process was demonstrated by the synthesis of PLA-150
([LA]/[Ni] = 150) at room temperature. To a rapidly stirred solution of L-lactide (2.88 g, 20
mmol), Ni-HMBED (0.051 g, 0.133 mmol) in toluene (30 mL) was added along with the
requisite amount of benzyl alcohol. An increase in viscosity was observed and finally stirring
was ceased after 25 h. Volatile materials were removed under vacuum, and the residue was
extracted with THF (30 mL). The extraction was dried again and the white precipitate was
washed with n-hexane three times and dried under vacuum overnight, giving a crystalline white
solid. Yield: 2.1 g (72.9%)
The molecular weights (Mn and Mw) and polydispersity index (Mw/Mn) were determined by
using gel permeation chromatography (GPC) manufactured by Waters. The GPC instrument was
equipped with a Waters 1525 Binary HPLC pump. Two columns, namely Waters Styragel HR4
7.8×300 mm WAT10573 and Waters Styragel HR3 7.8×300 mm WAT044223, were used in
series. THF was used as the mobile phase. The flow rate of THF was 1.0 ml/min. An ELS
detector, Waters 2420, was used for the detection of different molecular weight fractions.
7

Polystyrene standards with a low dispersity were used to generate a calibration curve. The GPC
chromatograms were analyzed through Breeze version 3.3 software.
3. RESULTS AND DISCUSSION
3.1 Nickel complexes in the ring opening polymerization of L-lactide
On the basis of the “immortal” property of lactide polymerization [51] and the evidence of the in
situ formation of metal alkoxides in the presence of alcohol [11-33], the complexes Ni-HMBED,
Ni-HMBPD and Ni-HMBBD were expected to behave as catalysts toward the ROP of lactide in
the presence of benzyl alcohol. The nickel complexes Ni-HMBED, Ni-HMBPD and Ni-HMBBD
in the presence of benzyl alcohol initiate the ring-opening polymerization of L-lactide (LA) in
dichloromethane at 30 °C (Scheme 2). The polymerization results are listed in Tables 1, 2 and 3.
All the runs displayed good activities for the polymerization of L-lactide and showed great
control of molecular weight, and the presence of benzyl alcohol has a significant influence on the
polymerization behavior of the corresponding complexes.
The ring-opening polymerization of L-lactide using nickel complexes, with a monomer to
benzyl alcohol ratio 150/1, has been systematically studied at 30 °C (Table 1). It is worth noting
that the nickel complexes are more active in CH2Cl2 than in toluene or THF. The difference in
activity is probably due to higher solubility of the complexes in CH2Cl2 in comparison to
toluene. The slowest polymerization rate is found with THF, which may be attributed to the
coordination ability of THF with the nickel metal, which retards the reaction rate [22-23].
By comparing the polymerization results listed in Tables 2 and 3, several structure-
activity trends may be drawn. The experimental results indicate that the nickel complexes are
efficient catalysts for the ROP of L-lactide in the presence of BnOH when the [M]0/[I]0 ratio
8

ranges from 50 to 200. The polymerization is well controlled and the “living” character is
demonstrated by the low polydispersity index (PDI), ranging from 1.07 to 1.16 for the polymers,
and by the linear relationship between Mn and the [Ni]0/[BnOH]0 ratio (Figure 2). It is interesting
to note that the ROP of L-lactide catalyzed by the Ni complexes in the presence of benzyl
alcohol (BnOH) shows immortality. The ‘immortal’ character was examined using two or four
equivalent ratios of benzyl alcohol as the chain transfer agent (Table 4). It was found that for
polymerization without the use of BnOH, there is almost negligible conversion (<5%), (Tables 2
and 3), but the use of BnOH changes the activities of the catalyst drastically. The reaction time
decreases from 24 to 15 h with the use of a greater amount of BnOH (Table 3). This may be due
to active participation of the initiator in the polymerization reaction. The molecular weight of the
polymers was also affected by the ratio of BnOH used. By the addition of two or four equivalents
of benzyl alcohol in the polymerization reactions, the molecular weights became half or one
fourth, respectively (Table 3). It was also observed that for polymerization using only the ligand
or nickel chloride, there is almost zero conversion (Table 2). To observe the influence of the
substituents towards the ROP of lactide, a series of nickel complexes have been prepared.
Among all the nickel complexes, Ni-HMBED showed better activity based on percentage of
conversion in 24 hours (Tables 1, 2 and 3) [30]. This may be due to the creation of more
electrophilicity at the nickel center in comparison to the other nickel complexes, Ni-HMBPD and
Ni-HMBBD, and is favorable for the coordination and insertion of LA monomers. The catalytic
activity was also found to decrease with the increased steric hindrance offered by the diamine
substituent (Tables 1, 2 and 3).
1H NMR (Figure 3 and Figure S7-S8) studies on PLA catalyzed by the nickel complexes in
1
the presence of BnOH was carried out to determine the process of initiation. The H NMR
9

spectrum of PLA-200, prepared from a [LA]0/[BnOH] ratio of 200, indicates that the polymer
chain is capped with a benzyl ester group on one end and a hydroxyl group on the other end,
suggesting that the initiation occurred through the insertion of the benzyl alkoxy group into L-
lactide giving an intermediate, which further reacts with an excess of L-lactide yielding
polyesters. The polymerization procedures agree well with the processes found in other metal
alkoxides [11-33]. A literature report [36] for the polymerization of L-lactide using a nickel
complex at 130 °C shows PLA with Mn = 1200 g mol-1 has been achieved, which suggests that
the results obtained in the present investigations are superior in terms of Mn (Tables 2 and 3).
Further, considerably higher molecular weight PLA has been obtained in all entries (Tables 2
and 3) which indicates that the propagation is faster than the initiation or partial hydrolysis of the
initiating functionality.
3.2 Kinetics of the ROP of lactide
Kinetic studies for the polymerization of L-lactide using the nickel complexes (Figures
S9-S11) as catalysts in the presence of the initiator BnOH with a [LA]o/[Ni]/[BnOH] ratio of
100/1/1 has been performed. The ln[M]o/[M]t vs time plots were found to be linear, where [M]o is
the initial lactide monomer concentration and [M]t is the lactide concentration at a given reaction
time [52-53]. It has been concluded from these plots that the polymerization reactions obey first
order kinetics. The values of the apparent rate constant (kapp) may be calculated from the slope of
these plots. The value of kapp and the standard deviation (SD) for L-lactide polymerization in the
presence of the initiator were found to be 00.08003 h−1 and 0.06316, 0.01056 h−1 and 0.01466
and 0.06317 h−1 and 0.02638 for Ni-HMBED, Ni-HMBPD and Ni-HMBBD respectively [22-
23].
3.3 Mechanism of the ROP of lactide using the nickel complexes
10

The mechanism for the polymerization of L-lactide proceeds as outlined in Scheme 3.
Coordination of L-lactide with the nickel complexes yields a monomeric penta-coordinated
intermediate I, which by the insertion of a benzyl alkoxy group into the L-lactide produces a new
initiator II. Increasing the steric hindrance and including an electron-donating group on the imine
carbon atom tends to stabilize the monomeric intermediate and hence increases the reaction rate.
Finally, subsequent addition of lactide produce PLA.
4. CONCLUSIONS
Nickel complexes supported on Salen type ligands were shown to be efficient catalysts
for the ring opening polymerization of lactide when benzyl alcohol is taken as an initiator. Use of
benzyl alcohol plays a vital role towards the ROP. Without benzyl alcohol, the nickel complexes
show much lower activity. However, the presence of a two or four fold excess of benzyl alcohol
has produced PLA with half or one fourth of the molecular weight, respectively. The PLA thus
produced by the ROP of L-lactide shows a moderate molecular weight with a narrow PDI,
ranging from 1.07 to 1.16. Among the diamine groups, the polymerization rate follows the order:
ethylene group > propylene group > benzene group, based on the percentage conversion.
Supporting Information
1
This includes FTIR spectra of the Schiff bases, H NMR spectrum of PLA, Semi-logarithmic
plots of L-lactide conversion catalyzed by the nickel complexes, FTIR frequencies and electronic
transitions of the Schiff bases and their nickel complexes (Table S1).
Acknowledgments
11

The authors are thankful to CSIR and UGC, New Delhi, India for funding. The authors are also
grateful to Ravenshaw University and the National Institute of Technology, Raipur for providing
research facilities.
REFERENCES
[1].
A.K. Sutar, T. Maharana, S. Dutta, Chen. C.-T, C.-C. Lin, Chem. Soc. Rev. 39 (2010)
1724.
[2].
[3].
[4].
[5].
J. Wu, T.-L. Yu, C.-T. Chen, C.-C. Lin, Coord. Chem. Rev. 250 (2006) 602.
O. Dechy-Cabaret, B. Martin-Vaca, D. Bourissou, Chem. Rev. 104 (2004) 6147.
T. Maharana, B. Mohanty, Y.S. Negi, Prog. Polym. Sci. 24 (2009) 99.
K.E. Uhrich, S.M. Cannizzaro, R.S. Langer, K.M. Shakesheff, Chem. Rev. 99 (1999)
3181.
[6].
[7].
B. Jeong, Y.H. Bae, D.S. Lee, S.W. Kim, Nature, 388 (1997) 860.
R. Gref, Y. Minamitake, M. T. Peracchia, V. Trubetskov, V. Torchilin, R. Langer,
Science, 263 (1994) 1600.
[8].
[9].
R. Langer, Science, 249 (1990) 1527.
D.J. Darensbourg, W. Choi, C.P. Richers, Macromolecules, 40 (2007) 3521.
[10]. G.B. Kharas, F. Sanchez-Riora, D.K. Soverson, 2004 Polymers of lactic acid, D.P.
Mobley (Ed.), Plastics from Microbes, Hanser Publishers, Munchen, Germany.
[11]. A. Pilone, K. Press, I. Goldberg, M. Kol, M. Mazzeo, M. Lamberti, J. Am. Chem. Soc.
136 (2014) 2940.
[12]. Y. Zhang, A. Gao, Y. Zhang, Z. Xu, W. Yao, Polyhedron, Accepted (2016)
DOI:10.1016/j.poly.2016.03.042
12

[13]. A. Routaray, N. Nath, T. Maharana, A. K. Sutar, Catal. Sci. Tech., Accepted (2015)
DOI: 10.1039/C5CY00454C
[14]. Y. Sun, L. Wang, D. Yu, N. Tang, J. Wu, J. Mol. Catal. A-Chem. 393 (2014) 175.
[15]. Z. Liang, M. Zhang, X. Ni, X. Li, Z. Shen, Inorg. Chem. Commun. 29 (2013) 145.
[16]. R.K. Deana, A.M. Recklinga, H. Chen, L.N. Dawe, C. M. Schneider, C. M. Kozak,
Dalton. Trans. 42 (2013) 3504.
[17]. Y. Gao, Z. Dai, J. Zhang, X. Ma, N. Tang, J. Wu. Inorg. Chem, 53 (2014) 716.
[18]. J. Wojtaszak, K. Mierzwicki, S. Szafert, N. Gulia, J. Ejfler, Dalton. Trans. 43 (2014)
2424.
[19]. M. H. Chisholm, N. W. Eilerts, J. C. Huffman, S. S. Iyer, M. Pacold, K. Phomphrai, J.
Am. Chem. Soc. 122 (2000) 11845.
[20]. A. P. Dove, V.C. Gibson, E.L. Marshall, H.S. Rzepa, A.J.P White, D.J. Williams, J. Am.
Chem. Soc. 128 (2006) 9834.
[21]. C.K. Williams, L.E. Breyfogle, S.K. Choi, W. Nam, V.G. Young, M.A. Hillmyer, W.B.
Tolman, J. Am. Chem. Soc. 125 (2003) 11350.
[22]. A. Routaray, N. Nath, S. Mantri, T. Maharana, A. K. Sutar, Chinese. J. Catal. 36 (2015)
764.
[23]. A. Routaray, N. Nath, S. Mantri, T. Maharana, A. K. Sutar, J. Macromol. Sci, A: Pure.
Appl. Chem. 52 (2015) 1.
[24]. A.B. Bnesser, B. Li, J. A. Byers, J. Am. Chem. Soc. 135 (2013) 16553.
[25]. E.J. Lee, K.M. Lee, J. Jang, E. Kim, J.S. Chung, Y. Do, S.C. Yoon, S.Y. Park, J. Mol.
Catal. A-Chem. 385 (2014) 68.
13

[26]. C. Sattayanon, W. Sontising, J. Jitonnom, P. Meepowpan, W. Punyodom, N. Kungwan,
Comput. Theor. Chem, 1044 (2014) 29.
[27]. H.-Y. Chen, M.–Y. Liu, A.K. Sutar, C.-C. Lin, Inorg. Chem. 49 (2010) 665.
[28]. C.-Y. Tsai, H.-C. Du, J.-C. Chang, B.-H. Huang, B.-T. Ko, C.-C .Lin, RSC. Adv. 4
(2014) 14527.
[29]. R. Petrus, P. Sobota, Organometallics, 31 (2012) 4755.
[30]. H.-Y. Chen, Y.-L. Peng, T.-H. Huang, A. K. Sutar, S.A. Miller, C.- C. Lin, J. Mol. Catal.
A-Chem. 339 (2011) 61.
[31]. Z. Mou, B. Liu, M. Wang, H. Xie, P. Li, L. Li , S. Li, D. Cui, Chem. Commun. 50
(2014) 11411.
[32]. S. Abbina, G. Du, ACS. Macro. Lett. 3 (2014) 689.
[33]. C. Fliedel, D. Vila-Viçosa, M.J. Calhorda, S. Dagorne, T. Aviles, Chem. Cat. Chem. 6
(2014) 1357.
[34]. L. Piao, M. Deng, X. Chen, L. Jiang, X. Jing, Polymer, 44 (2003) 2331.
[35]. K.C. Gupta, A.K. Sutar, Coord. Chem. Rev. 252 (2008) 1420.
[36]. A. John , V. Katiyar, K. Pang, M. Shaikh, H. Nanavati, P. Ghosh, Polyhedron 26 (2007)
4033.
[37]. B. Gao, L. Xin, W. Gao, Z-Q. Hao, X-Z Xin, Q-L. Wu, Y. Mu, Polyhedron 63 (2013) 91.
[38]. B. Gao, R. Duan, X. Pang, X. Li, Z. Qu, H. Shao, X. Wang, X. Chen, Dalton Trans. 42
(2013) 16334.
[39]. B. Gao, D. Zhao, X. Li, Y. Cui, R. Duan, X. Pang, RSC Adv. 5 (2015) 440.
[40]. B. Gao, X. Li, R. Duan, X. Pang, New J. Chem. 39 (2015) 2404.
14

[41]. G. Xiao, B. Yan, R. Ma, W. J. Jin, X. Q. Leu, L. Q. Ding, C. Zeng, L. L. Chen, F. Bao,
Polym. Chem. 2 (2011) 659
[42]. J.M. Fraile, J.A. Mayoral, A.J. Royo, R.V. Salvador, B. Altava, S.V. Luis, M.I. Burguete,
Tetrahedron, 52 (1996) 9853.
[43]. G. Grivani, A. Akherati, Inorg. Chem. Commun. 28 (2013) 90.
[44]. A.K. Sutar, Y. Das, S. Pattnaik, A. Routaray, N. Nath, P. Rath, T. Maharana, Chinese. J.
Catal. 35 (2014) 1701.
[45]. W.W. Fee, B. O. West, Aust. J. Chem. 16 (1963) 788
[46]. N. Hall, B. Willeford, J. Am. Chem. Soc. 73 (1951) 5419
[47]. S-Y. Lee, A. Hille, C. Frias, B. Kater, B. Bonitzki, S. Wolfl, H. Scheffler, A. Prokop, R.
Gust, J. Med. Chem. 53 (2010) 6064
[48]. R. Atkins, G. Brewer, E. Kokot, G. M. Mockler, E. Sinn, Inorg. Chem. 24(1985) 127
[49]. A. Bottcher, H. Elias, L. Miiller, H. Paulus, Angew. Chem. Int. Ed. Engl. 31 (1992)623
[50]. A. Bottcher, H. Elias, E-G. Jiiger, H. Langfelderova, M. Mazur, L. Miiller, H. Paulus, P.
Pelikan, M. Rudolph, M. Valko, Inorg. Chem. 32 (1993) 4131
[51]. Y-C. Liu, B-T Ko, C-C. Lin, Macromolecules 34(2001) 6196
[52]. J. Castro-Osma, C. Alonso-Moreno, I. Márquez-Segovia, A. Otero, A. Lara-Sánchez, J.
Fernández-Baeza, A. R. Luis, S. Barba, J. García-Martínez, Dalton Trans. 42 (2013) 9325
[53]. R. Gowda, D. Chakraborty, J. Mol. Cat. A. Chem 349 (2011) 86
[54]. H. Alamri, J. Zhao, D. Pahovnik, N. Hadjichristidis, Polym. Chem. 5 (2014) 5471.
15

OCH3
OCH3
OH
OCH3
OCH3
HO
O
O
Nickel chloride
solution
Ni
R
N
N
N
N
R
Nickel Complexes
Ni-HMBED (R= ED),
Ni-HMBBD (R= BD)
Ni-HMBPD (R= PD)
Scheme 1. Nickel complexes
O
O
Nickel complexes
H
O
O
n
O
OBn
O
BnOH
n
O
O
Scheme 2. ROP of lactide using the nickel complexes.
R
R
N
N
N
N
Ni
O
O
O
BnOH
Ni
OCH3
OCH3
O
O
R
OCH3
OCH3
O
N
O
N
Lactide
Ni
O
OH
I
O
O
O
O
OCH3
OCH3
O
O
O
H
O
O
O
O
O
O
O
LA
O
O
O
H+
II
O
H
O
n
O
PLLA
Scheme 3. Proposed mechanism for the ring opening polymerization of L-lactide
16

Table 1. ROP of L-lactide using the nickel complexes at 30 °C.
Entry
1
Complex
Parameter
Solvents
Toluene
CH2Cl2
91.3
THF
Ni-HMBED Conversion(%)a
Mn (Theory)b (g mol-1)
80.4
17400
12200
18000
1.09
64.2
19800
22300
27600
1.16
13900
8000
Mn (VPO)c (g mol-1)
Mn(GPC)d
14200
1.14
PDI
2
Ni-HMBPD Conversion(%)a
Mn (Theory)b (g mol-1)
Mn (VPO)c (g mol-1)
Mn(GPC)d
88.3
80.0
62.0
19200
21700
24200
1.10
17500
19500
21300
1.07
13500
16500
18100
1.12
PDI
3
Ni-HMBBD Conversion(%)a
Mn (Theory)b (g mol-1)
Mn (VPO)c (g mol-1)
Mn(GPC)d
86.1
80.0
61.3
18700
21300
23000
1.11
17400
19200
21100
1.17
13400
16100
17400
1.08
PDI
Conditions: [L-LA] = 20 mmol, room temperature, reaction time 24 h, [M]0/[Ni]/[BnOH] =
0
150/1/1
a
Percentage conversion of the monomer [(weight of polymer recovered/weight of monomer) ×
100].
b Calculated by [([LA]0/[BnOH]) × 144.13 × conversion% + 108.14]
c Determined by VPO.[54]
d
Obtained from GPC analysis and calibrated by a polystyrene standard. Values are obtained
from GPC times 0.58.
17

Table 2. ROP of L-lactide by the nickel complexes.
Entry Complex [L-LA]0 / [Ni Time Conversiona
or Schiff base]
/[BnOH]
Mn
Mn
Mn
PDIe
(h)
(%)
(Theory)b (VPO)c
(GPC)e
(g mol-1) (g mol-1) (g mol-1)
1
2
3
4
HMBED
HMBPD
HMBBD
NiCl2
100:1:0
100:1:0
100:1:0
100:1:0
100:1:0
50:1:1
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
d
d
d
d
Ni-HMBED
<5
-
-
-
-
5
6
7
78.2
87.3
91.3
92.7
<5
5700
12700
19800
26800
8100
15900
22300
30600
8600
22100
27600
32800
1.12
1.12
1.16
1.10
100:1:1
150:1:1
200:1:1
100:1:0
50:1:1
d
d
d
d
Ni-HMBPD
-
-
-
-
75.1
85.1
88.3
91.5
<5
5500
12400
19200
26500
7600
15300
21700
27600
8600
18300
24200
30600
1.09
1.07
1.10
1.09
100:1:1
150:1:1
200:1:1
100:1:0
50:1:1
d
d
d
d
Ni-HMBBD
-
-
-
-
73.9
82.4
86.1
89.3
5400
11900
18700
25800
7100
15400
21300
27100
8000
18000
23000
28200
1.12
1.11
1.11
1.13
100:1:1
150:1:1
200:1:1
Conditions: [L-LA] 0 = 20 mmol, room temperature, solvent 30 mL of CH2Cl2
a
Percentage conversion of the monomer [(weight of polymer recovered/weight of monomer) ×
100].
b Calculated by [([LA]0/[BnOH]) × 144.13 × conversion% + 108.14]
c Determined by VPO.[54]
d Data not available
e Obtained from GPC analysis and calibrated by a polystyrene standard.
18

Table 3. ROP of L-lactide by the nickel complexes with variation of the BnOH equivalents.
Entry Complex
[L-LA]0 /
[M]/[BnOH]
Time Conversiona(%)
(h)
Mn
Mn
Mn
Mn
PDIe
(Theory)b (VPO)c (GPC)e (NMR)f
(g mol-1) (g mol-1)
d
d
d
d
d
1
2
3
Ni-
100:1:0
100:1:1
100:1:2
100:1:4
100:1:0
100:1:1
100:1:2
100:1:4
100:1:0
100:1:1
100:1:2
100:1:4
24
24
20
15
24
24
20
15
24
24
20
15
<5
-
-
-
-
-
HMBED
87.3
90.1
92.0
<5
12700
6600
3400
15900
8200
4600
22100 11200 1.12
10200
6300
8200 1.10
4050 1.09
d
d
d
d
d
Ni-
-
-
-
-
-
HMBPD
85.1
88.4
90.3
<5
12400
6500
3400
15300
8100
4000
18300 10000 1.07
10300
5200
7200 1.07
3600 1.10
d
d
d
d
d
Ni-
-
-
-
-
-
HMBBD
82.4
85.2
89.4
11900
6200
3300
15400
8000
3800
18000
9600
4300
8800 1.11
6400 1.09
3000 1.10
Conditions: [L-LA] 0 = 20 mmol, room temperature, solvent 30 mL of CH2Cl2
a
Percentage conversion of the monomer [(weight of polymer recovered/weight of monomer) ×
100].
b Calculated by [([LA]0/[BnOH]) × 144.13 × conversion% + 108.14]
c Determined by VPO [54]
d Data not available
e Obtained from GPC analysis and calibrated by a polystyrene standard.
f Obtained from 1H NMR analysis
19

Figure 1. TGA curve of Schiff bases and their nickel complexes
20

2.0
1.8
1.6
1.4
1.2
1.0
Ni-HMBED
30000
25000
20000
15000
10000
5000
40
60
80
100
120
140
160
180
200
220
[LA]/[BnOH]
2.0
1.8
1.6
1.4
1.2
1.0
Ni-HMBPD
25000
20000
15000
10000
5000
40
60
80
100
120
140
160
180
200
220
[LA]/[BnOH]
2.0
1.8
1.6
1.4
1.2
1.0
Ni-HMBPD
25000
20000
15000
10000
5000
40
60
80
100
120
140
160
180
200
220
[LA]/[BnOH]
Figure 2. ROP of L-lactide catalyzed by the nickel complexes in CH2Cl2 at room temperature.
21

a
a
O
O
e
d
c
a
c
a
H
O
O
O
O
O
b
c
n-1
O
O
1
Figure 3. H NMR spectrum of PLLA-200 (200 indicates [LA]0/[BnOH] = 200) catalyzed by
Ni-HMBED
22

Graphical Abstract Image
23

Short Abstract
Salen-type Schiff-base ligands and their nickel complexes have been synthesized and fully
characterized. All of the nickel complexes were tested for the ring-opening polymerization of
lactide. The PLA produced by the ROP of L-lactide shows a moderate molecular weight with a
narrow PDI range.
24