Non-symmetrical aluminium salen complexes: Synthesis and their reactivity with cyclic ester

Article abstract of DOI:10.1016/j.polymer.2015.09.036

Non-symmetrical aluminium salen complexes that contained different substituents were designed and synthesized. All the ligands and their complexes were characterized by ¹H, ¹³C NMR and elemental analysis. These complexes can be used

Full text of DOI:10.1016/j.polymer.2015.09.036

Accepted Manuscript
Non-Symmetrical Aluminium Salen Complexes: Synthesis and Their Reactivity with
Cyclic Ester
Ranlong Duan, Zhiqiang Sun, Xuan Pang, Chenyang Hu, Huili Shao, Xuesi Chen,
Xianhong Wang
PII:
S0032-3861(15)30241-X
10.1016/j.polymer.2015.09.036
JPOL 18122
DOI:
Reference:
To appear in: Polymer
Received Date: 7 July 2015
Revised Date: 7 September 2015
Accepted Date: 15 September 2015
Please cite this article as: Duan R, Sun Z, Pang X, Hu C, Shao H, Chen X, Wang X, Non-Symmetrical
Aluminium Salen Complexes: Synthesis and Their Reactivity with Cyclic Ester, Polymer (2015), doi:
10.1016/j.polymer.2015.09.036.
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.

ACCEPTED MANUSCRIPT
Graphical Abstract
Title: Non-Symmetrical Aluminium Salen Complexes: Synthesis and Their
Reactivity with Cyclic Ester
5
1

ACCEPTED MANUSCRIPT
Non-Symmetrical Aluminium Salen Complexes: Synthesis and Their
Reactivity with Cyclic Ester
Ranlong Duan,[a] Zhiqiang Sun,[a] Xuan Pang,* [a] Chenyang Hu,[a] Huili Shao,[b] Xuesi Chen[a] and
Xianhong Wang[a]
a
5
Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese
Academy of Sciences, 5625 Renmin Street, Changchun 130022, P. R. China
b
State Key Laboratory for Modification of Chemical Fibers and Polymeric Materials, Donghua
University, Shanghai 20051, China
* Correspondence to: Xuan Pang (E-mail: xpang@ciac.ac.cn.)
10 Abstract: Non-symmetrical aluminium salen complexes that contained different substituents were
1
designed and synthesized. All the ligands and their complexes were characterized by H, 13C NMR and
elemental analysis. These complexes can be used as catalysts to produce polylactide and
poly-ε-caprolactone. All polymerizations were living and molar mass distributions were narrow. The
Mn(obsd) of the isolated polymers were in good agreement with Mn(calcd). The polymerization rate of
15 electrophilic substituted complex was higher than the non-electrophilic substituted analogies. The
bulky substituents with more steric hindrance of the complexes had relatively lower activity. Kinetic
studies showed that the polymerizations were both first-ordered with respect to lactide and
ε-caprolactone monomers.
Keywords: half-Salen, ring-opening polymerization, lactide
20 1. Introduction
1

ACCEPTED MANUSCRIPT
Fossil-based polymers are extensively used in our life. Millions of tons of fossil-based polymers are
produced and disposed of every year.[1] As fossil resources are being depleted gradually, and the growing
amount of waste created by these polymers have generated serious pollution of our ecosystem, recent
research efforts have been focused on the development of biodegradable alternatives.[2-4] Due to the
biocompatible and biodegradable properties, poly(lactic acid) (PLA), whose starting materials are from
corn or sugar beets, becomes a leading candidate in this respect.[5-10] PLA can degrade via hydrolytic
cleavage of the ester bonds of the polymer backbone. The ring-opening polymerization (ROP) of lactide
(LA) is the general way to produce PLA.[11-13] Because of the presence of two chiral centers in the lactide
monomer, different lactide stereoisomers are distinguished, namely, (S,S)-LA (L-LA), (R,R)-LA (D-LA),
5
10 and (R,S)-LA (meso-LA). The stereochemistry of the monomeric units in the polymer chains plays an
important role in mechanical, physical, and degradation properties of PLA materials.[14-16] The
polymerization process is typically catalyzed or initiated by Lewis acidic metal alkoxide complexes of
tin,[17-18] zinc,[19] or the rare-earth metals.[20-21] Aluminum catalysts were effective initiators in the
preparation of PLA for their high Lewis acidity and low toxicity.[12, 22-25]
15
Symmetric salen Schiff base aluminum catalysts have achieved considerable progress in the synthesis
of PLA polymers.[11, 26-29] As far as we know, researches on non-symmetrical salen Schiff base complex
were still not fully exploited.[30] Recently, our research group reported two types of aluminum complexes:
one derived from enolic ligands and another from symmetric salen Schiff base ligands.[31-33] Enolic
ligands could be obtained from the reaction of β-diketone and diamine. Ideal Schiff base ligands and their
20 aluminum complexes can be obtained through keto-enol tautomerism. These complexes proved to be
highly effective single-site living initiators for the controlled ROP of lactones. Intrigued by the success of
2

ACCEPTED MANUSCRIPT
these two types of aluminum complexes, it was believed that the combination of these two types of
aluminum complexes would produce a new family of non-symmetrical complexes. These complexes are
regarded as promising catalysts for the ROP of lactones. Herein we report a number of aluminum
non-symmetrical salen complexes and the preliminary application on their use as catalysts for the ROP of
lactides and ε-caprolactone.
5
2. Experimental
General
All experiments were carried out in a dry nitrogen atmosphere in a glovebox. Starting materials for the synthesis
of ligands, rac-LA and ε-caprolactone were purchased from Aldrich Inc. Ethyl acetate and 2-propanol were
10 distilled from CaH2 under the protection of argon. rac-Lactide was purified by recrystallization from ethyl
acetate and dried under vacuum at room temperature before use. NMR spectra were recorded on Bruker AV 400
M. Chemical shifts were given in parts per million from tetramethylsilane. Gel permeation chromatography
(GPC) measurements were conducted with a Waters 515 GPC with CHCl3 as the eluent (flow rate: 1 mL min-1
at 35 oC). The molecular weights were calibrated against polystyrene (PS) standards.
15 Synthesis of ligands L1-L5
Ligand family was prepared as shown in Scheme 1. A solution of 2,4-pentanedione (15 mmol) in
dichloromethane (15 mL) was added to a solution of 1,2-diaminoethane (30 mmol) in dichloromethane (15 mL)
slowly, and the solution was refluxed for 1 h. The excess 1,2-diaminoethane was removed under vacuum at
60 °C.[34] A solution of the residue in dichloromethane (20 mL) was added dropwise to a solution of
3

ACCEPTED MANUSCRIPT
corresponding substituted salicylaldehyde (15 mmol) in ethanol (15 mL). The corresponding ligands were
obtained after purification by flash column chromatography.
1
L1: H NMR (300MHz, CDCl3): δ = 13.01(s ArOH 1H), 10.93(s COH 1H), 8.36(s NCH 1H), 7.32(m ArH
1H), 7.25(d J=5.0Hz ArH 1H), 6.97(t J=7.3Hz ArH 1H), 6.89(t J=7.5Hz ArH 1H), 4.96(s CHCOH 1H),
3.77(t J=5.8Hz NCH2CH2N 2H), 3.61(dd J=12.1,6.0Hz NCH2CH2N 2H), 1.99, 1.91(s NCCH3, HOCCH3
6H). 13C NMR (100MHz, CDCl3): δ = 195.34(COH), 167.06(NCH), 162.29(CH3CN), all benzene ring:
160.96, 132.57, 131.70, 118.81, 118.65, 117.03; 95.92(CHCOH), 55.24, 50.37(NCH2CH2N), 28.91
(HOCCH3), 18.97(NCCH3). Elem. Anal.: Calcd. C 68.27, H 7.37, N 11.37; Found C 68.26, H 7.37, N
5
11.35.
1
10 L2: H NMR (300MHz, CDCl3): δ = 13.00(s ArOH 1H), 10.89(s COH 1H), 8.29(s NCH 1H), 7.01, 6.89(s
ArH 2H), 4.94(s CHCOH 1H), 3.73(t J=6.0Hz NCH2CH2N 2H), 3.58(dd J=12.2,6.1Hz NCH2CH2N 2H),
2.25(d J=5.3Hz ArCH3 6H), 1.98, 1.90(s NCCH3, HOCCH3 6H). 13C NMR (100MHz, CDCl3): δ =
195.32(COH), 167.29(NCH), 163.00(CH3CN), all benzene ring: 157.04, 134.70, 129.36, 127.11, 125.68,
117.60; 95.93(CHCOH), 60.04, 43.66(NCH2CH2N), 28.94(HOCCH3), 19.03(NCCH3), 20.36,
15 15.48(ArCH3). Elem. Anal.: Calcd. C 70.04, H 8.08, N 10.21; Found C 70.06, H 8.10, N 10.22.
1
L3: H NMR (300MHz, CDCl3): δ = 13.95(s ArOH 1H), 10.86(s COH 1H), 8.27(s NCH 1H), 7.40(d
J=2.4Hz ArH 1H), 7.15(dd J=5.9,2.2Hz ArH 1H), 4.95(s CHCOH 1H), 3.78(t J=5.6Hz NCH2CH2N 2H),
3.62(dd J=11.7,5.8Hz NCH2CH2N 2H), 2.06, 2.01(s NCCH3, HOCCH3 6H). 13C NMR (100MHz, CDCl3):δ
= 195.73(COH), 165.73(NCH), 165.69(CH3CN), all benzene ring: 162.88, 156.27, 132.55, 129.45, 123.11,
20 119.47; 96.28(CHCOH), 59.04, 43.26(NCH2CH2N), 28.98(HOCCH3), 19.06(NCCH3). Elem. Anal.: Calcd.
C 53.35, H 5.12, N 8.89; Found C 53.32, H 5.09, N 8.90.
4

ACCEPTED MANUSCRIPT
1
L4: H NMR (300MHz, CDCl3): δ = 13.45(s ArOH 1H), 10.90(s COH 1H), 8.35(s NCH 1H), 7.39(t
J=5.6Hz ArH 1H), 7.08(d J=2.4 Hz ArH 1H), 4.95(s CHCOH 1H),3.77(t J=6.1Hz NCH2CH2N 2H), 3.61(q
J=6.1Hz NCH2CH2N 2H), 1.99, 1.92(s NCCH3, HOCCH3 6H), 1.43, 1.30(s C(CH3)3 18H). 13C NMR
(100MHz, CDCl3): δ = 195.34(COH), 168.20(NCH), 163.16(CH3CN), all benzene ring: 158.16, 140.54,
136.74, 127.37, 126.36, 117.83; 96.00(CHCOH), 60.06, 43.75(NCH2CH2N), 35.12, 34.24(ArC(CH3)3),
31.60, 29.52(C(CH3)3), 28.96(HOCCH3), 19.01(NCCH3). Elem. Anal.: Calcd. C 73.70, H 9.56, N 7.81;
Found C 73.72, H 9.54, N 7.79.
5
1
L5: H NMR (300MHz, CDCl3): δ = 13.23(s ArOH 1H), 10.89(s COH 1H), 8.32(s NCH 1H), 7.43(dd
J=7.2,1.7Hz ArH 1H), 7.23(d J=1.6Hz ArH 1H), 6.87(t J=7.4Hz ArH 1H), 4.93(s CHCOH 1H), 3.75(t
10 J=5.9Hz NCH2CH2N 2H), 3.62(dd J=12.3,6.1Hz NCH2CH2N 2H), 1.98, 1.89(s NCCH3, HOCCH3 6H),
0.91(s SiC(CH3)3 9H), 0.33(s Si(CH3)2 6H). 13C NMR (100MHz, CDCl3): δ = 195.37(COH), 167.49(NCH),
165.99(CH3CN), all benzene ring: 163.16, 139.72, 133.21, 125.07, 118.33, 117.70; 95.99(CHCOH), 60.17,
43.60(NCH2CH2N),
28.96(HOCCH3),
27.17(SiC(CH3)3),
18.96(NCCH3),
17.73(SiC(CH3)3),
-4.68(Si(CH3)2). Elem. Anal.: Calcd. C 66.62, H 8.95, N 7.77; Found C 66.64, H 8.93, N 7.74.
15 Synthesis of complexes 1-5
AlEt3 (0.1 mmol) in toluene (5 mL) was added to the stirred 1mL toluene solution of ligand L1-L5 (0.1 mmol) at
RT. The reaction was maintained at 80 ºC for 16 h, and the reaction mixture was then slowly cooled to RT. The
toluene was removed under vacuum (Scheme 1).
Complex 1: 1H NMR (300MHz, CDCl3): δ = 8.21(s NCH 1H), 7.35(m ArH 1H), 7.20(d J=7.7Hz ArH 1H),
20 7.08(d J=8.5Hz ArH 1H), 6.72(t J=7.2Hz ArH 1H), 5.10(s CHCOAl 1H), 4.07(dd J=18.0,11.6Hz
NCH2CH2N 1H), 3.66(dd J=13.1,6.2Hz, NCH2CH2N 1H), 3.56(dd J=12.0,5.7Hz NCH2CH2N 1H), 3.34(m
5

ACCEPTED MANUSCRIPT
NCH2CH2N 1H), 2.08, 2.02(s NCCH3, AlOCCH3 6H), 0.86(t J=8.0Hz AlCH2CH3 3H), -0.30(m AlCH2CH3
2H). 13C NMR (100MHz, CDCl3): δ = 181.48(COAl), 174.63(NCH), 166.02(CH3CN), all benzene ring:
165.60, 134.99, 132.51, 122.86, 119.17, 116.37; 99.57(CHCOAl), 54.07, 47.88(NCH2CH2N),
26.08(AlOCCH3), 22.92(NCCH3), 9.99(AlCH2CH3), 0.41(AlCH2CH3). Elem. Anal.: Calcd. C 63.99, H
7.05, N 9.33; Found C 64.03, H 7.04, N 9.35.
5
Complex 2: 1H NMR (300MHz, CDCl3): δ = 8.13(s NCH 1H), 7.11, 6.82(s ArH 2H), 5.09(s CHCOAl 1H),
4.06(m NCH2CH2N 4H), 3.63(dd J=14.0,5.7Hz NCH2CH2N 4H), 3.53(ddd J =12.2,6.0,1.8 Hz NCH2CH2N
4H), 3.31(m NCH2CH2N 4H), 2.30, 2.24(s ArCH3 6H), 2.05, 2.01(s NCCH3, AlOCCH3 6H), 0.86(t J=8.1Hz
AlCH2CH3 3H), -0.30(m AlCH2CH3 2H). 13C NMR (100MHz, CDCl3): δ = 181.28(COAl), 174.15(NCH),
10 165.44(CH3CN), all benzene ring: 162.69, 136.73, 130.37, 129.40, 124.30, 117.78; 99.19(CHCOAl), 53.83,
47.71(NCH2CH2N), 25.83(AlOCCH3), 22.66(NCCH3), 20.23, 16.28(ArCH3), 10.00(AlCH2CH3),
0.35(AlCH2CH3). Elem. Anal.: Calcd. C 65.84, H 7.67, N 8.53; Found C 65.81, H 7.71, N 8.50.
Complex 3: 1H NMR (300MHz, CDCl3): δ = 8.16(s NCH 1H), 7.46(m ArH 1H), 7.20(dd J=17.6,4.8Hz ArH
1H), 5.11(s CHCOAl 1H), 4.07(m NCH2CH2N 1H), 3.70(d J=7.3Hz NCH2CH2N 1H), 3.65(d J=7.9Hz
15 NCH2CH2N 1H), 3.40(m NCH2CH2N 1H), 1.98, 1.89(s NCCH3, AlOCCH3 6H), 0.80(m AlCH2CH3 3H),
-0.34(m AlCH2CH3 2H). 13C NMR (100MHz, CDCl3): δ = 182.00(COAl), 175.99(NCH), 171.63(CH3CN),
all benzene ring: 164.05, 136.66, 134.00, 131.92, 129.66, 119.82; 99.59(CHCOAl), 54.13,
47.39(NCH2CH2N), 25.90(AlOCCH3), 22.83(NCCH3), 9.78(AlCH2CH3), 0.54(AlCH2CH3). Elem. Anal.:
Calcd. C 52.05, H 5.19, N 7.59; Found C 52.07, H 5.21, N 7.58.
1
20 Complex 4: H NMR (300MHz, CDCl3): δ = 8.18(s NCH 1H), 7.47(d J=2.5Hz ArH 1H), 7.00(d J=2.5Hz
ArH 1H), 5.06(s CHCOAl 1H), 4.10(m NCH2CH2N 1H), 3.65(dd J=13.2,6.8Hz NCH2CH2N 1H), 3.50(dd
6

ACCEPTED MANUSCRIPT
J=12.1,6.4Hz NCH2CH2N 1H), 3.34(m NCH2CH2N 1H), 2.00, 1.99(s NCCH3, AlOCCH3 6H), 1.49, 1.32(s
C(CH3)3 9H), 0.88(t J=8.1Hz AlCH2CH3 3H), -0.29(q J=7.8Hz AlCH2CH3 2H). 13C NMR (100MHz,
CDCl3): δ = 181.54(COAl), 174.11(NCH), 166.01(CH3CN), all benzene ring: 163.31, 140.98, 137.32,
129.74, 126.37, 118.15; 98.77(CHCOAl), 57.31, 47.82(NCH2CH2N), 35.40, 34.09(ArC(CH3)3), 31.59,
29.41(C(CH3)3), 25.78(AlOCCH3), 22.62(NCCH3), 10.21(AlCH2CH3), 0.51(AlCH2CH3). Elem. Anal.:
Calcd. C 69.87, H 9.04, N 6.79; Found C 69.85, H 9.05, N 6.82.
5
Complex 5: 1H NMR (300MHz, CDCl3): δ = 8.15(s NCH 1H), 7.49(d J=7.0Hz ArH 1H), 7.17(m ArH 1H),
6.71(t J=7.3Hz ArH 1H), 5.05(s CHCOAl 1H), 4.08(dd J=18.3,11.6Hz NCH2CH2N 1H), 3.60(dd J=13.2,6.7Hz,
NCH2CH2N 1H), 3.49(dd J=11.9,6.3Hz NCH2CH2N 1H), 3.33(m NCH2CH2N 1H), 2.00(s NCCH3, AlOCCH3
10 6H), 0.96(s SiC(CH3)3 9H), 0.83(t J=8.0Hz AlCH2CH3 3H), 0.36(s Si(CH3)2 6H), -0.35(q J=7.6Hz AlCH2CH3
2H). 13C NMR (100MHz, CDCl3): δ = 181.75(COAl), 174.36(NCH), 171.12(CH3CN), all benzene ring: 165.36,
142.59, 133.91, 130.38, 118.23, 115.81; 98.76(CHCOAl), 53.79, 47.78(NCH2CH2N), 27.62(SiC(CH3)3),
25.58(AlOCCH3), 22.79(NCCH3), 17.91(SiC(CH3)3), 10.21(AlCH2CH3), 0.21(AlCH2CH3), -4.61(Si(CH3)2).
Elem. Anal.: Calcd. C 63.73, H 8.51, N 6.76; Found C 63.77, H 8.48, N 6.74.
15 Polymerization procedure
In a typical polymerization experiment, complexes 1–5 (10 mmol), the required amount of LA or CL in toluene
were loaded in a flame-dried vessel containing a magnetic bar. The vessel was placed in a thermostated oil bath.
1
Conversion of the monomer was determined on the basis of H NMR spectroscopic studies. After a certain
reaction time, the polymer was isolated by precipitation with cold methanol. The precipitate was collected and
20 dried under vacuum at RT for 36 h.
3. Result and discussion
7

ACCEPTED MANUSCRIPT
Synthesis of ligands and aluminum complexes
The synthetic pathway of non-symmetrical ligands and aluminum complexes family was described in
1
Scheme 1. All ligands and complexes were characterized by H, 13C NMR spectroscopy and elemental
analysis. The ligand precursors L1-L5 were obtained by the condensation reaction between acetylacetone,
5
1,2-diaminoethane and the corresponding substituted salicylaldehyde.[34] The analytical results were in
1
accord with their respective formulas. For example, H NMR spectrum of L1 showed signals at about δ 8.36
and 4.96 ppm, which were attributed to the N=CH proton from salicylaldehyde part and CHCOH proton
from acetylacetone part, respectively. The intensity ratio of the signals at 8.36 and 4.96 ppm was 1:1. This
was consistent with the structure of ligand L1. Complexes 1-5 were obtained via the reaction of ligands
1
10 L1-L5 with AlEt3 under nitrogen atmosphere . The H NMR spectra of complex 1 showed signals at about δ
0.86 and –0.30 ppm, which were attributed to the methyl protons and methylene protons of the aluminum
ethyl group, respectively. The N=CH proton displayed signal at 8.21 ppm. The intensity ratio of the three
signals at 0.86, –0.30 and 8.21 ppm was 3:2:1, which confirmed the structure of complex 1.
15 Scheme 1 Synthetic pathway for the preparation of ligands and complexes.
Ring-opening polymerization of L-LA and rac-LA
Complexes 1-5 were investigated as catalysts for the ROP of L-LA and rac-LA in the presence of 2-propanol.
8

ACCEPTED MANUSCRIPT
The representational polymerization data were listed in Table 1 and Table 2. Molecular weight of the PLAwas
determined by 1H NMR spectroscopy and gel permeation chromatography (GPC).
Fig. 1 Kinetic plots of the L-LA conversion vs. the reaction time. Left: [M]0/[cat]=60, (a) complex 3; (b)
complex 1; (c) complex 2; (d) complex 4; (e) complex 5. Right: (a) [M]0/[cat]=40; (b) [M]0/[cat]=60; (c)
[M]0/[cat]=80; (d) [M]0/[cat]=120 by using complex 4.
5
Kinetic studies of L-LA polymerization
Polymerization of L-LA was investigated in detail. Polymerization data indicated that the number-averaged
molecular weights Mn(GPC) of PLA were close to theoretical ones (Mn(calcd) calculated from the
10 monomer-to-catalyst molar ratio). The polydispersity index (PDI) of PLAs ranged from 1.1 to 1.2. Plots of
L-LA monomer conversions versus reaction time with complexes 1-5 were showed in Figure 1. For all the five
complexes, L-LA monomer conversions increased linearly with reaction time (Figure 1, left). Obviously the
activity of 1-5 was remarkably influenced by the substituent groups on the salicylaldehyde. Complex 3 (with
chloride substituent) showed the highest conversion under the same reaction condition. The activities of these
15 complexes reduced accompanied with the raise of substituent’s bulk on the phenyl parts, kp values were 1.26
Lmol−1min−1 for 1, 0.86 Lmol−1min−1 for 2, 0.20 Lmol−1min−1 for 4 and 0.12 Lmol−1min−1 for 5 (Table 1, entry 1,
9

ACCEPTED MANUSCRIPT
2, 7 and 12). kp value for complex 1 (R1=H) was more than 10 times higher than that of complex 5
(R1=Si(CH3)2C(CH3)3, TBDMS). While electron-withdrawing substituents increased polymerization rate as kp
value was 1.68 Lmol−1min−1 for 3 (Table 1, entry 3). Similar situations also appeared in our previous reports as
for the symmetric salen Schiff base aluminum catalysts system.[26,28] The more bulky substituents with more
steric hindrance may keep active species from being approached by lactide monomer, as a result, slowing down
the polymerization rate. While Lewis acidity as well as electrophilicities of metal centers was enhanced by the
electron-withdrawing substituents of chlorine, so LA monomer was easy to nucleophilic addition, as a result,
increase the polymerization rate.[13] At certain reaction time, took complex 4 as example, the reaction with
lower monomer/catalyst ratio showed higher monomer conversion (Figure 1, right). This was consistent with
5
10 our previous reports of enolic and/or symmetric salen Schiff base complexes.[26,28,32] First-order kinetics in
monomer was observed [equation (1)], and kapp was the apparent polymerization rate constant.
−d[LA]/dt = kapp[LA]
equation (1)
Fig. 2 Plot of PLA Mn (▄) and polydispersity (▲) as a function of L-LA conversion using complex 1,
15 [M]0/[cat]=60.
10

ACCEPTED MANUSCRIPT
Fig. 3 kapp vs. the concentration of complex 4 for the L-LA polymerization.
Linear relationship between number-average molecular weight (Mn) and monomer conversion was observed
(Figure 2). kapp was plotted versus the concentration of 4 to determine the order in catalyst (Figure 3). From
this plot, kapp increased linearly with complex 4 concentrations, manifesting that the order in 4 was
first-order too. Therefore, the polymerization of L-LA using 4 followed an overall kinetic equation of the
form as equation (2), kp was the polymerization rate constants, kp = kapp/[4].
5
−d[LA]/dt = kp[LA][4]
equation (2)
Table 1 Polymerization data of L-LA with complexes 1-5.a
e
Entry Complex Temp (°C)
t
(h) [M]0/[Cat] Convb (%) Mn(calcd)c×10-3 Mn (NMR) d ×10-3 Mn ×10-3 PDIe
1
2
1
2
3
4
4
4
4
4
4
4
4
5
90
90
6
7
60
60
60
40
40
40
60
80
120
40
40
60
95
90
94
82
85
91
87
90
85
90
93
86
8.3
7.8
8.2
4.8
5.0
5.3
7.6
10.4
14.8
5.2
5.4
7.5
8.1
7.5
7.9
4.5
4.7
5.6
8.0
10.9
15.1
5.7
5.8
7.4
8.5
8.0
8.7
4.3
5.2
5.8
7.8
10.6
14.1
5.4
6.2
7.9
1.18
1.14
1.17
1.11
1.14
1.19
1.17
1.15
1.19
1.21
1.24
3
90
4
4
70
24
16
17
20
30
42
8
5
80
6
90
7
90
8
90
9
90
10
11
12
100
110
90
6
25
1.10
a
1
c
10 All polymerizations were carried out in toluene solution, [LA]0 = 0.5 mol L-1. b Measured by HNMR. Calculated
from the molecular weight of LA × [M]0/[Cat] × conversion + Mw(iPrOH). d Obtained from H NMR analysis. e The
1
values of Mn were calculated according to formula Mn = 0.58MnGPC[35].
11

ACCEPTED MANUSCRIPT
o
Fig. 4 Kinetics of the L-LA polymerization using complex 4 at the reaction temperatures of (a) 110 C; (b)
100oC; (c) 90oC; (d) 80 oC; (e) 70 oC, [M]0/[Cat]=40.
Investigation of the influence of temperature on the polymerization of LA monomer was proceeded (Figure 4).
Decrease the temperature from 90oC to 80oC would result in a decrease in kp value from 0.22 Lmol−1min−1 to
0.16 Lmol−1min−1, further decrease to 70oC led to a reduction in kp to 0.11 Lmol−1min−1, a 50% reduction. At the
same time, an increase in the temperature to 110oC led to a 191% increase in kp value (from 0.22 Lmol−1min−1 to
0.64 Lmol−1min−1) using complex 4.
5
10 Fig. 5 Plot of Lnkp vs 1/T for the polymerization of L-LA with complex 4.
The activation energy of the polymerization was calculated by fitting kp values determined at different
temperatures to the Arrhenius equation (kp=Ae−Ea/RT), an activation energy Ea of 48.5 kJmol−1 was deduced
by plotting lnkp versus 1/T (Figure 5).
12

ACCEPTED MANUSCRIPT
Fig. 6 1H NMR spectrum of oligomer of L-LA
.
In order to explore the mechanism of initiation, 1H NMR spectrum of PLA oligomers was investigated. A triplet
of two overlapping doublets at 1.24 ppm and a quartet at 4.34 ppm with an integral ratio close to 6:1 (Figure 6)
were assigned to the methyl protons of the isopropoxycarbonyl and the methine proton neighboring the hydroxyl
end groups, respectively. This clearly indicated that the polymer was capped with one isopropyl ester group and
one hydroxyl group.[11,33,35] The actual initiator was the aluminium alkoxide propagating species. The ROP
selected a coordination insertion mechanism (Scheme 2).[24,36-37]
5
Stereoselective polymerization of rac-LA
10 To test the stereo-controllability of complexes 1-5, the ring opening polymerizations of rac-LA by 1-5 were also
studied. The stereochemical microstructures of the poly(rac-LA) were determined from the methine fragment of
1
the homonuclear decoupled H NMR spectra by 1-5 (Figure 7). Similar activity trends were found as in L-LA
polymerization (Figure 8).
13

ACCEPTED MANUSCRIPT
Table 2 Polymerization data of rac-LA with complexes 1-5.a
t
Convb
(%)
e
Entry
Complex
[M]0/[Cat]
Mn(calcd)c×10-3
Mn (NMR)d ×10-3
Mn ×10-3
PDIe
Pm
(h)
1
2
3
4
5
6
7
8
9
1
2
3
4
4
4f
4
4
5
4.5
8
60
60
60
40
60
40
80
120
60
90
96
95
92
85
93
83
80
83
7.8
8.4
8.3
5.4
7.4
5.4
9.6
13.9
7.2
7.6
8.7
8.1
8.2
8.4
5.8
7.7
5.9
9.9
14.4
7.5
1.16
1.15
1.19
1.19
1.20
1.12
1.15
1.17
1.12
047
0.50
0.43
0.64
0.65
0.68
0.63
0.64
0.67
4
8.8
18
20
36
24
36
24
6.0
8.0
5.7
10.7
14.2
7.0
a All polymerizations were carried out in toluene solution at 90 oC, [LA]0 = 0.5 mol L-1. b Measured by 1HNMR. c
Calculated from the molecular weight of LA × [M]0/[Cat] × conversion + Mw(iPrOH). d Obtained from 1H NMR analysis. e
The values of Mn were calculated according to formula Mn = 0.58MnGPC[35]..f Reaction temperature 70 oC
.
5
Scheme 2 Proposed mechanism for the polymerization of LA.
Fig. 7 Homonuclear decoupled 1H NMR spectrum of the methane part of poly(rac-LA) using complex 5.
14

ACCEPTED MANUSCRIPT
Fig. 8 Kinetic plots of the rac-LA conversion vs. the reaction time, [M]0/[cat]=60. (a) complex 3; (b) complex 1;
(c) complex 2; (d) complex 4; (e) complex 5.
Parameter Pm was used to describe the controllability.[37] The substituent groups on the salicylaldehyde showed
remarkable influence. Complexes with small steric hindrance showed no stereoselectivity (complexes 1 and 2
had Pm values of 0.47 and 0.50). Complex 5 had the highest stereoselectivity among the five complexes (Table
2, entry 1, 2, 3, 5 and 9). The enhancement of stereoselectivity was attributed to the bulky substituents of the
salicylaldehydes of ligand.[24] Lowered the temperature from 90 °C to 70 °C, the Pm value raised, i.e., from
0.65 to 0.68 for complex 4 (Table 2, entry 5, 6).
5
10
Fig. 9 Kinetic plots of the ε-CL conversion vs. the reaction time [M]0/[cat]=100. (a) complex 3; (b) complex 1;
15

ACCEPTED MANUSCRIPT
(c) complex 2; (d) complex 4; (e) complex 5.
Table 3 Polymerization data of ε-CL with complexes 1-5.a
t
Convb
(%)
e
kp/Lmol−1min−1
Entry
Complex
[M]0/[Cat]
Mn(calcd)c×10-3
Mn (NMR)d ×10-3
Mn ×10-3
PDIe
(min)
1
2
3
4
5
6
7
8
1
2
3
4
4
4
4
5
55
100
40
100
100
100
50
93
90
95
80
90
91
80
85
10.7
10.0
10.9
4.6
10.9
9.7
11.0
10.2
11.4
4.9
1.41
1.42
1.54
1.48
1.50
1.51
1.57
1.40
10.40
4.81
16.01
3.42
3.38
3.34
3.39
1.44
11.3
5.1
55
150
170
200
155
100
150
200
100
10.3
15.6
18.3
9.8
9.8
10.6
16.2
19.3
10.5
16.7
19.4
10.8
a All polymerizations were carried out in toluene solution at 60 °C, [CL]0 = 0.5 mol L-1. b Measured by 1HNMR. c
Calculated from the molecular weight of CL × [M]0/[Cat] × conversion + Mw(iPrOH). d Obtained from 1H NMR analysis. e
The values of Mn were calculated according to formula Mn = 0.56MnGPC[39].
5
Recently, our research lab has successfully developed some enolic Schiff base aluminum complexes; they
were used as catalysts to prepare isotactic enriched PLA from rac-LA.[32] In comparison, complexes 1-5 in
this work had effectively one half of the enolic complexes structurally, and this would offer the opportunity
for the comparison of the two types of complexes, as we only need to compare another half part from salen
10 Schiff base. Comparing with our previous result of enolic Schiff base complex 1a (see reference 32, with
symmetrical enolic ligand surrounding), complexes1 and 2 had lower Pm. This may indicate that the steric
hindrance of the corresponding half Schiff base with H or CH3 substituent group was small. This was also
consistent with the activity data shown in Table 2 that a less bulky structure due to smaller substitutent led
to complex having higher catalytic activity. Considering the large steric hindrance, it was not surprise that
15 complex 5 with TBDMS substituent group showed higher Pm and lower kp comparing with 1a in reference
32. It is worth noting that complex 4 and 1a had similar Pm and kp, this might suggest their similar ligand
surroundings. The reason for these phenomena is still not very clear and is under investigation.
16

ACCEPTED MANUSCRIPT
Fig. 10 Plot of PCL Mn (▄) and polydispersity (▲) as a function of ε-CL conversion using complex 1,
[M]0/[cat]=100.
Ring-opening polymerization of ε-CL
5
Complexes 1-5 can also efficiently control the ring-opening polymerization of ε-caprolactone. Polymerization
was systematically examined and the data were collected in Table 3. Kinetic studies were explored to further
understand the nature of ε-CL polymerization process. As for the ε-CL monomer, first-order kinetics was
proofed by the good linear relationship of the ln([ε-CL]0/[ε-CL]) vs. reaction time (Figure 9).
10 Fig. 11 kapp vs. the concentration of complex 4 for the ε-CL polymerization.
17

ACCEPTED MANUSCRIPT
Similar conclusions were found as in L-LA and rac-LA polymerization: the activity of complex decreased
with the increase of substituents’ bulk on salicylaldehyde (Table 3, entry 1, 2, 3, 5 and 8). Complex 1 (with
t
less bulky group, H) had its kp value more than 3 times greater than complex 4 (with bulky group, Bu), i.e.,
10.40 Lmol−1min−1 vs. 3.41 Lmol−1min−1; and even more than 7 times greater than complex 5 (with more
bulky group, TBDMS), i.e., 10.40 Lmol−1min−1 vs. 1.44 Lmol−1min−1. Electron-withdrawing substituents on
salicylaldehyde as complex 3 increased polymerization rate remarkable (kp value 16.0 Lmol−1min−1). This
principle was also consistent with previous reports about symmetric salen Schiff base aluminum
catalysts.[24] The number-average molecular weight (Mn) increased linearly along with the ε-CL monomer
conversion (Figure 10). Complexes 1-5 showed lower control efficiency in the polymerization of ε-CL as
5
10 slightly wide polydispersity of PCL than that of PLAs. This may suggest that ε-CL was less sterically
demanding monomer when comparing with LA monomer. [40] To determine the order in catalyst, kapp was
plotted against [catalyst]0 (Figure 11). In this plot, kapp increased linearly with the catalyst concentration,
indicating a first order in catalyst.
4. Conclusion
15 In conclusion, a series of non-symmetrical aluminium salen complexes 1-5 with half enolic and half salen Schiff
base structure were prepared. These complexes were active on the polymerization of cycle ester. Kinetic study
disclosed that 1-5 showed differentiation upon LA and ε-CL polymerization due to the different substituents on
salicylaldehyde part. Larger substituents with more steric hindrance had relatively lower activity. The
electron-withdrawing substituents enhanced the Lewis acidity, therefore increased the polymerization rate of LA
20 and ε-CL. Complexes 4 and 5 even showed moderate selectivity to rac-lactide to give isotactic enriched
polylactide. All the polymerizations of LA and ε-CL proceeded with first-order rate dependence on both
18

ACCEPTED MANUSCRIPT
monomer and catalyst concentrations.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (Nos. 51173183, 51103058,
51203155 and 51321062) and the Ministry of Science and Technology of China (No. 2011AA02A202).
5
References
1. R. Mehta, V. Kumar, H. Bhunia and S. N. Upadhyay, J. Macromol. Sci. Polym. Rev, 2005, C45, 325-349.
2. K. E. Uhrich, S. M. Cannizzaro, R. S. Langer and K. M. Shakesheff, Chem. Rev., 1999, 99, 3181-3198.
3. D. J. Mooney, G. Organ, J. P. Vacanti and R. Langer, Cell. Transplantat., 1994, 3, 203-210.
4. C. Wang, Y. Xiao, A. Heise and M. D. Lang, J. Polym. Sci., Part A: Polym. Chem., 2011, 49, 5293-5300.
10 5. J. L. Eguiburu, M. J. Fernandez-Berridi, F. P. Cossio and J. San Roman, Macromolecules, 1999, 32,
8252-8258.
6. A. C. Albertsson and I. K. Varma, Biomacromolecules, 2003, 4, 1466-1486.
7. M. H. Chisholm, S. S. Iyer, M. E. Matison, D. G. McCollum and M. Pagel, Chem. Commun., 1997,
1999-2000.
15 8. A. B. Biernesser, B. Li and J. A. Byers, J. Am. Chem. Soc., 2013, 135, 16553-16560.
9. X. Wang, A. Thevenon, J. L. Brosmer, I. Yu, S. I. Khan, P. Mehrkhodavandi and P. L. Diaconescu, J. Am.
Chem. Soc., 2014, 136, 11264-11267.
10. I. Yu, A. Acosta-Ramirez and P. Mehrkhodavandi, J. Am. Chem. Soc., 2012, 134, 12758-12773.
11. Z. Y. Zhong, P. J. Dijkstra and J. Feijen, J. Am. Chem. Soc., 2003, 125, 11291-11298.
20 12. Z. Y. Zhong, P. J. Dijkstra and J. Feijen, Angew. Chem., Int. Ed., 2002, 41, 4510-4513.
19

ACCEPTED MANUSCRIPT
13. B. J. O'Keefe, L. E. Breyfogle, M. A. Hillmyer and W. B. Tolman, J. Am. Chem. Soc., 2002, 124,
4384-4393.
14. M. S. Reeve, S. P. McCarthy, M. J. Downey and R. A. Gross, Macromolecules, 1994, 27, 825-831.
15. J. R. Sarasua, R. E. Prud'homme, M. Wisniewski, A. Le Borgne and N. Spassky, Macromolecules, 1998, 31,
3895-3905.
5
16. N. Spassky, M. Wisniewski, C. Pluta and A. LeBorgne, Macromol. Chem. Phys., 1996, 197, 2627-2637.
17. A. Kowalski, J. Libiszowski, A. Duda and S. Penczek, Macromolecules, 2000, 33, 1964-1971.
18. A. Duda, S. Penczek, A. Kowalski and J. Libiszowski, Macromol. Symp., 2000, 153, 41-53.
19. G. Schwach, J. Coudane, R. Engel and M. Vert, Polym. Int., 1998, 46, 177-182.
10 20. B. Liu, T. Roisnel, L. Maron, J.-F. Carpentier and Y. Sarazin, Chem-Eur J., 2013, 19, 3986-3994.
21. Z. Mou, B. Liu, X. Liu, H. Xie, W. Rong, L. Li, S. Li and D. Cui, Macromolecules, 2014, 47, 2233-2241.
22. R. K. Iha, K. L. Wooley, A. M. Nystrom, D. J. Burke, M. J. Kade and C. J. Hawker, Chem. Rev., 2009, 109,
5620-5686.
23. N. Nomura, R. Ishii, Y. Yamamoto and T. Kondo, Chem-Eur J., 2007, 13, 4433-4451.
15 24. N. Nomura, R. Ishii, M. Akakura and K. Aoi, J. Am. Chem. Soc., 2002, 124, 5938-5939.
25. T. M. Ovitt and G. W. Coates, J. Am. Chem. Soc., 2002, 124, 1316-1326.
26. Z. H. Tang, X. S. Chen, X. Pang, Y. K. Yang, X. F. Zhang and X. B. Jing, Biomacromolecules, 2004, 5,
965-970.
27. K. Majerska and A. Duda, J. Am. Chem. Soc., 2004, 126, 1026-1027.
20 28. X. Pang, R. Duan, X. Li and X. Chen, Polym. Chem., 2014, 5, 3894-3900.
29. X. Pang, R. Duan, X. Li, B. Gao, Z. Sun, X. Wang and X. Chen, Rsc Adv., 2014, 4, 22561-22566.
20

ACCEPTED MANUSCRIPT
30. X. Pang, R. Duan, X. Li, Z. Sun, H. Zhang, X. Wang and X. Chen, Polym. Chem., 2014, 5, 6857-6864.
31. Z. H. Tang, X. S. Chen, Y. K. Yang, X. Pang, J. R. Sun, X. F. Zhang and X. B. Jing, J. Polym. Sci., Part A:
Polym. Chem., 2004, 42, 5974-5982.
32. X. Pang, H. Du, X. Chen, X. Wang and X. Jing, Chem-Eur J., 2008, 14, 3126-3136.
33. X. Pang, H. Z. Du, X. S. Chen, X. L. Zhuang, D. M. Cui and X. B. Jing, J. Polym. Sci., Part A: Polym.
Chem., 2005, 43, 6605-6612.
5
34. S. Supasitmongkol and P. Styring, Catal. Sci. Technol., 2014, 4, 1622-1630.
35. J. Baran, A. Duda, A. Kowalski, R. Szymanski and S. Penczek, Macromol. Rapid Commun, 1997, 18,
325-333.
10 36. A. Amgoune, C. M. Thomas, T. Roisnel and J. F. Carpentier, Chem-Eur J., 2006, 12, 169-179.
37. B. M. Chamberlain, M. Cheng, D. R. Moore, T. M. Ovitt, E. B. Lobkovsky and G. W. Coates, J. Am. Chem.
Soc., 2001, 123, 3229-3238.
2
38. Pm is the probability of meso linkages, according to [mmm]=Pm +(1-Pm)Pm/2, [mmr]=[rmm]=(1-Pm)Pm/2,
[rmr]=(1-Pm)2/2, [mrm]=[(1-Pm)2+Pm(1-Pm)]/2.
15 39. M. Save, M. Schappacher and A. Soum, Macromol. Chem. Phys., 2002, 203, 889-899.
40. M. Lamberti, I. D'Auria, M. Mazzeo, S. Milione, V. Bertolasi and D. Pappalardo, Organometallics, 2012,
31, 5551-5560.
21

ACCEPTED MANUSCRIPT
Highlights
ꢀ A number of Non-Symmetrical Aluminium Salen complexes were synthesized.
ꢀ These complexes were active in lactide and caprolactone polymerization.
ꢀ First-ordered kinetics with respect to lactide and caprolactone were observed.