Ring-opening polymerization of lactide using chiral salen aluminum complexes as initiators: High productivity and stereoselectivity

Article abstract of DOI:10.1039/c5nj00469a

A family of aluminum complexes bearing chiral salen ligands derived from (R,R)-1,2-diammoniumcyclohexane mono-(+)-tartrate salt and modified by salicylaldehyde were prepared. The complexes were characterized by ¹H, ¹³C NMR and elemen

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DOI: 10.1039/C5NJ00469A
Paper
Graphical Abstract
Chiral Salen Aluminum
complexes have been
synthesized and
Bo Gao, Dongni Li, Yanhui Li, Qian Duan,
Ranlong Duan and Xuan Pang
investigated as initiators for
Page No. – Page No.
L-lactide and rac-lactide
polymerization.
Ring-Opening Polymerization of Lactide using
Chiral Salen Aluminum Complexes as
Initiators: High Productivity and
Stereoselectivity

New Journal of Chemistry
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New Journal of Chemistry
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Paper
Ring-Opening Polymerization of Lactide using Chiral Salen Aluminum
Complexes as Initiators: High Productivity and Stereoselectivity
Bo Gao,*a, b Dongni Li,a Yanhui Li,a Qian Duan,*a Ranlong Duanb and Xuan Pangb
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A family of aluminum complexes bearing chiral Salen ligands derived from (R,R)-1,2-diammoniumcyclohexane mono-(+)-tartrate salt and
salicylaldehyde-modified were prepared. They were characterized by 1H, 13C NMR and elemental analysis. These complexes were used as
initiators for the ring-opening polymerization (ROP) of L-lactide and rac-lactide. Complex 2 (R1=R2=Cl) showed the highest activity
among these complexes for the ROP of L-lactide, and complex 3 (R1=tBu, R2=Cl) showed the highest stereoselectivity for the ROP of rac-
lactide with enriched isotactic polylactide with a Pm of 0.91. The kinetics’ data of the polymerization employed complex 3 as initiator
indicated that the polymeric rate was both first-ordered in lactide as well as initiator.
.
aluminum catalysts (Fig. 2) to prepare the multiblock
Introduction
copolymer (PLLA–PDLA)n. Coates[49] uncovered subse-quently
Today an increasing number of attentions to our environment have
brouht about looking for environment friendly and sustainable
polymers, which could substitude the normally used petroleum-
based polymers.[1-4] Biocompatible, biodegradable and sustainable
polylactide (PLA) starting from corn or sugar beet, is becoming
one of the most promising polymers as a replacement for
petroleum-based polymer.[5] PLA has been exploited for a wide
scope of applications such as disposal containers, controlled release
drug carriers, bio-absorbable bone nail, sutures, scaffolds, textiles
and tissue-engineering material, etc.[3, 4] Existence of double chiral
centers in the lactide (LA) leads to three species of LA
stereoisomers (L-lactide (L-LA), D-lactide (D-LA) and meso-
lactide (meso-LA), as shown in Fig. 1). The stereochemistry
configuration of the polymer chains dominates the physical and
chemical properties of PLA. Generally, PLA is prepared by the
that a number of chiral aluminum complexes bearing Salen-type
Schiff base could get the PLA of isotactic enrichment. Feijen[51, 52]
applied a chiral bulky (R,R)-CyclohexylSalenAlOiPr [(R,R)-1]
catalyst to get a polymer of isotactic enrichment. But the effect of
the phenol ring substituent on stereoselectivity in the ROP of the
rac-lactide initiated by salen aluminum complexes was not further
studied in detail. Our group[57-65] have reported various of
aluminum complexes based on Salen ligands. These complexes
were proved to be efficient living catalysts for the stero-controlled
ROP of LA. Intrigued by the success of aluminum complexes
based on Salen ligands in polymerization catalysis, we are very
interested in studying the catalytic behavior of aluminum
complexes based on Salen type Schiff base derived from (R,R)-
1,2-diaminocyclohexane and salicylaldehyde derivatives (para or
ortho positional substituents on phenyl rings are selected from H,
tBu or Cl: pro-ligand L1 R1=tBu, R2=H; L2 R1=R2=Cl; L3 R1=tBu,
R2=Cl, see Scheme 1). These complexes, to our knowledge, have
not been studied in the ROP of LA. In this work, we report a
series of Salen aluminum complexes with Schiff base ligands and
the preliminary application on initiating for the ROP of LA.
ring-opening polymerization (ROP) of lactide initiated by metal
[6-8]
complexes, such as some alkoxides of tin,
aluminum,[9-14]
zinc,[15-21] magnesium, [22-24] iron,[25, 26] titanium,[27-33] indium,[34, 35]
rare-earth metals,[36-45] organo-catalysts,[46] and enzymes.[47]
Aluminum catalysts were regarded as competent catalysts
employed for ROP of LA because of their effectiveness in the
control the stereoregular degrees of polymers.[48-56] Many efforts
have been tried to get PLA with highly stereoregular degrees. This
PLA is derived from rac-lactide (rac-LA) and catalyzed by
stereoselective catalysts bearing Schiff base ligands. Lots of groups
had tried to clarify the connection among stereochemistry of PLA,
the aluminum catalysts based on Schiff base and rac-LA with
various endeavors [48-57, 58-66] (Fig.2).
Figure 1. Stereoisomers of lactides.
Scheme 1 The preparations of pro-ligands and aluminum
complexes.
One of the first catalysts employed for the
stereoselective polymerization of rac-LA into isotactic
PLA was reported by Spassky,[48] who utilized achiral Salen
1
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1 and Table 2. Experimental results revealeDdOthI:a1t0.t1h0e3s9e/Cc5oNmJp0l0e4x6e9sA
were efficient initiators for ROP of LA. Molecular weight of the
1
PLA polymers was measured by GPC. H NMR and GPC were
applied to calculate the monomer conversion and the number-
averaged molecular weights of the PLA. These aluminum
complexes showed moderate to high productivities (79–96%) at
70 °C. The number-averaged molecular weights (Mn were
calculated according to formula Mn = 0.58Mn(GPC)[68]) of PLA were
close to theoretical ones (Mn(calcd) calculated from the monomer-to-
catalyst molar ratio) with narrow molecular weight distributions
(PDI: 1.08–1.26). The data of L-LA conversions versus polymer-
ization time were shown in Figure 3. It is noteworthy that the
activities of these complexes reduced with the raise of substituent’s
bulk on the phenyl parts, while electron-withdrawing substituents
increased polymerization rate. Complex 2 displayed the highest
activity (93.7% monomer conversion Table 1, Entry 2) at the same
polymerization conditions among these complexes (Table 1,
Entries 1, 2 and 3). Similar situations also appeared in the previous
reports.[69-71] Moreover, the Salen ligands had certain ability to
affect the PDI of the PLA, and this ability depended on volume of
ligands. For instance, the PDI decreased from 1.26 to 1.08 with the
raise of the bulk of the para positional substituents on phenyl rings
from Cl to tBu (see Table 1, Entries 2, 3).
Figure 2 Salen aluminum catalysts for the ROP of LA.
Results and Discussion
Complexes synthesis and characterization
The syntheses of pro-ligands L1−L3 were outlined in Scheme 1,
these pro-ligands were synthesised through the condensation
reaction
diammoniumcyclohexane
salicylaldehyde-modified in high yields (86.5–91.4%) according to
starting
from
the
commercial
(R,R)-1,2-
mono-(+)-tartrate
salt and
52, 67]
a described procedure.[51,
Three singlet proton resonance
signals were observed at δ 8.37, 8.57 and 8.33 ppm, which were
assigned to the protons of -N=CH- in pro-ligands L1−L3,
respectively (see ESI). This suggested that these protons of imines
in ligands were equivalent. Complexes 1−3 were prepared by the
reaction of ligands L1−L3 with aluminum isopropoxide in toluene
according to a described procedure (see Scheme 1). [51, 52] All three
complexes were sensitive to air and moisture. Complexes 1−3 were
Figure 3. Kinetics of the ROP of L-LA by 3 at 70 °C in toluene.
see Table 1, Entries 4, 5 and 6. [LA]0 = 0.5 mol L–1; ■: [Al]0 =
0.005 mol L–1, [LA]0/[Al]0 = 100, kapp = 9.122 × 10-6 s–1; □: [Al]0 =
0.0067 mol L–1, [LA]0/[Al]0 = 75, kapp = 12.15 × 10-6 s–1; ▲: [Al]0
= 0.010 mol L–1, [LA]0/[Al]0 = 50, kapp = 18.47 × 10-6 s–1; where
kapp was the apparent polymerization rate constant, the slop of
curve.
1
characterized by H, 13C NMR in CDCl3 and elemental analysis.
The 1H and 13C NMR spectra of complexes 1−3 showed one Salen
ligand and one isopropoxy were coordinated to aluminum atom.
1
For example, H NMR spectrum of 3 showed the resonances at δ
Table 1 Representative polymerization data of L-LA with
complexes 1–3.[a]
0.87, 0.83 and 3.65 ppm were assigned to the methyl protons and
methine proton of the isopropoxy, respectively. Interestingly, the
1H NMR spectra of complexes 1–3 revealed that both the protons
of cyclohexyl as well as phenyl moieties are resolved into two
different chemical environments in these ligands. For instance, two
sets of resonances from imine protons (δ 8.32, 8.07 ppm) and the
α-protons of the cyclohexane moiety (δ ca. 3.94, 3.13 ppm) were
[f]
PDI[d]
Entry Com
-plex
T
h
[LA]0 Conv. Mn(calcd) Mn(GPC) Mn
kapp
[b]
/[Al]0
%
×10-4[c] ×10-4[d] ×10-4[e]
×10-6 s-1
N.A.
N.A.
1
2
3
4
5
6
56 100 79
56 100 94
56 100 85
90 100 95
1.13
1.35
1.22
1.37
0.72
0.67
1.90 1.10 1.11
2.26 1.31 1.26
1
2
3
3
3
3
2.12 1.23 1.08 N.A.
2.34 1.36 1.10 9.122
1.21 0.70 1.13 12.15
1.12 0.65 1.11 18.47
1
72 75
40 50
96
93
observed in the H NMR spectrum of complex 3 (see ESI). This
spectroscopic inequivalence showed that these complexes maybe
adopted distorted square pyramidal or distorted trigonal
bipyramidal geometries in solution. Similar observations were also
previously reported by the references.[51, 52]
[a] The polymerization reactions carried out in toluene solution at 70 °C;
[LA]0 = 0.5 mol L-1. [b] Measured by 1H NMR. [c] Calculated from the
molecular weight of LA × [LA]0/[Al]0 × conversion + Mwisopropoxy. [d]
Obtained from GPC analysis and calibrated against polystyrene standard.
[e] The calibration value of Mn could be calculated according to formula Mn
= 0.58Mn(GPC).
[68] [f] Apparent rate constant in the ROP of L-LA.
Ring-opening polymerization of L-LA and rac-LA
ROP of L-LA and rac-LA used complexes 1–3 as initiators had
been systematically examined in toluene or THF as shown in Table
2
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Table 2 Representative polymerization data of rac-LA with
complexes 1−3.[a]
[f]
PDI[d]
Pm
Entry Complex
T
h
Conv. Mn(
%
Mn(GPC) Mn
calcd
)
×10-4[c] ×10-4[d] ×10-4 [e]
[b]
1
2
3
4
5
6
56 48
56 63
56 55
90 64
240 92
180 26
0.70
0.90
0.79
0.92
1.32
0.37
1.19
1.62
1.31
1.62
2.20
0.60
0.69
0.94
0.76
0.94
1.28
0.35
1.15 0.85
1.22 0.57
1.16 0.88
1.17 0.87
1.14 0.88
1.10 0.91
1
2
3
3
3
3
[a] The polymerization reactions proceeded in toluene at 70 °C except that
a reaction, Entry 6, carried out in THF at 25 °C; [LA]0 = 0.5 mol L-1,
[LA]0/[Al]0 = 100. [b] Measured by 1H NMR. [c] Calculated from the
molecular weight of LA × [LA]0/[Al]0 × conversion. [d] Obtained from
Figure 5. kapp versus the concentration of 3 for the L-LA
polymerization in toluene at 70 °C ([LA]0 = 0.5 mol L–1, kp = 1.831
×10-3 L mol–1s–1).
GPC analysis and calibrated against polystyrene standard. [e] The
[68]
calibration value of Mn have to be calibrated by Mn= 0.58Mn(GPC)
.
[f]
1
Homonuclear decoupled H NMR spectra of the methine part of poly(rac-
LA).
Figure 4. Plots of PLA’ s Mn and PDI in the light of L-LA
conversion employing complex 3, [LA]0/[Al]0 = 100, in toluene at
70 °C.
Kinetic Studies
Figure 6. Homonuclear decoupled 1H NMR spectrum of the
methine part of poly(rac-LA) using 3 at 25 °C, Pm = 0.91, in
CDCl3 (Table 2, Entry 6).
Kinetic studies of L-lactide polymerization with representative 3 as
initiator was conducted in various monomer/initiator ratios ([LA]0
= 0.5 mol L–1, [LA]0/[Al]0 ranged from 50 to 100) in toluene at
70 °C, and the monomer conversion was monitored by 1H NMR as
a function of the polymerization time. The data of conversions
versus time were plotted in Fig. 3. First-order kinetics in monomer
was observed equality (1)
Stereoselective polymerization
Furthermore, the poly(rac-LA) (Table 2, Entry 6) with the
homonuclear decoupled 1H NMR spectrum of the methane
fragment[72] (Fig. 6) was also researched. The Pm value, 0.91,
manifested that the poly(rac-LA) obtained was predominantly
isotatic.[73] The experimental results showed that the Pm
selectivities increased obviously from 0.57 to 0.88 with the raise of
the bulk of the substitutes on ligands in these complexes at 70 °C
in toluene (see Table 2, Entries 2 and 3). For complex 3, the Pm
value increased lightly from 0.88 to 0.91 with the temperature from
70 to 25 °C (Table 2, Entries 3, 5).
–d[LA]/dt = kapp [LA]
(1)
(where kapp was the apparent polymerization rate constant, the slop
of curve). The molecular weight of the polymers propagated
linearly depending on the raise of the monomer transformation rate
as well as the PDI of these polymers were relatively narrow (1.08–
1.12, see Fig. 4), this manifested the living feature of the catalytic
system. In order to deduce the order of initiator, kapp was plotted
versus the concentration of 3. As shown in Fig. 5, kapp increased
linearly with the 3 concentration, manifesting that the order in
initiator was first-order. Hence, the polymerization of L-LA using 3
followed the kinetics equation (2)
Mechanism of lactide polymerization
In order to research the mechanism of polymerization initiated by
Salen aluminum complex, end group analysis of the oligomer,
which was prepared by the ROP of the L-LA at small monomer to
1
initiator ratio ([LA]0/[3]0 = 12/1) was measured by H NMR (Fig.
–d[LA]/dt = kp[LA][Al]
(2)
S1). Two groups of peaks appearing as a triplet at 1.24 ppm and the
quartet at 4.34 ppm, with an integral ratio of 6/1, were ascribed to
the methyl protons of the isopropoxycarbonyl end group and the
methane proton neighboring the hydroxyl end group. This clearly
revealed that the polymer was capped with one isopropyl ester
(where kp was the polymerization rate constant, kp = kapp/[Al]).
3
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group and one hydroxyl group,[58, 59, 74, 75] so the polymerization
1H, PhH), 6.90–6.72 (m, 2H, PhH), 3.88–D3O.8I0: 10(m.10,319H/C,5NNJ-0C0H46-)9,A
3.72 (sept, J = 6.0 Hz, 1H, OCH(CH3)2), 3.11–3.06 (m, 1H, N-CH-
), 2.35–2.00 (m, 4H, (CH2)4), 1.47 (s, 9H, C(CH3)3), 1.44 (s, 9H,
C(CH3)3), 1.40–1.34 (m, 4H, (CH2)2), 0.86 (d, J = 6.0 Hz, 3H,
OCH(CH3)2), 0.82 ppm (d, J = 6.0 Hz, 3H, OCH(CH3)2). 13C NMR
(75 MHz, CDCl3): δ 165.34 (1C, N=CH), 163.04 (1C, N=CH),
154.89 (1C, PhC), 152.24 (1C, PhC), 138.54 (1C, PhC), 138.06
(1C, PhC), 128.99 (1C, PhC), 127.80 (1C, PhC), 127.00 (1C, PhC),
126.25 (1C, PhC), 120.13 (1C, PhC), 119.83 (1C, PhC), 118.00
(1C, PhC), 116.42 (1C, PhC), 63.06 (1C, N-CH-), 62.24 (1C,
OCH(CH3)2), 61.81 (1C, N-CH-), 35.02, 34.43 (2C, C(CH3)3),
29.87 (1C, CH2), 28.03 (1C, CH2), 27.55 (3C, C(CH3)3), 27.02 (3C,
C(CH3)3), 26.17 (1C, CH2), 26.03 (1C, CH2), 24.39 (1C,
OCH(CH3)2), 23.36 ppm (1C, OCH(CH3)2). Anal. Calcd for
C31H43AlN2O3 (%): C, 71.79; H, 8.36; N, 5.40. Found: C, 71.76; H,
8.34; N, 5.43.
possibly selected a coordination insertion mechanism.[76-79]
Conclusions
In conclusion, a number of novel Salen aluminum complexes were
synthesized and characterized. All these complexes were viable
initiators for lactide polymerization with high monomer conversion.
Kinetics studies revealed polymerizations were living with the
narrow molar mass distributions. Within the series of Salen ligands,
the electron withdrawing ligand promoted polymerization rate.
Microstructural analysis of polylactide initiated by these complexes
indicated that the chiral Salen ligands had competent ability to
control the stereoregular degrees of the polymer, and this sort of
ability varied with the volume of substituents on phenyl rings in
ancillary ligands.
1
Experimental
Complex 2: H NMR (300 MHz, CDCl3) δ 8.30 (s, 1H, N=CH),
8.09 (s, 1H, N=CH), 7.56 (s, 1H, PhH), 7.53 (s, 1H, PhH), 7.20 (s,
1H, PhH), 7.16 (s, 1H, PhH), 4.07–3.90 (m, 1H, N-CH-), 3.73
(sept, J = 6.0 Hz, 1H, OCH(CH3)2), 3.26–3.11 (m, 1H, N-CH-),
2.68–1.42 (m, 8H, (CH2)4), 0.94 (d, J = 6.0 Hz, 3H, OCH(CH3)2),
0.89 ppm (d, J = 6.0 Hz, 3H, OCH(CH3)2). 13C NMR (75 MHz,
CDCl3): δ 168.67 (1C, N=CH), 166.80 (1C, N=CH), 157.97 (1C,
PhC), 155.23 (1C, PhC), 139.58 (1C, PhC), 136.44 (1C, PhC),
132.04 (1C, PhC), 129.77 (1C, PhC), 128.53 (1C, PhC), 127.84
(1C, PhC), 126.23 (1C, PhC), 123.35 (1C, PhC), 121.90 (1C, PhC),
116.93 (1C, PhC), 67.64 (1C, OCH(CH3)2), 65.17 (1C, N-CH-),
64.02 (1C, N-CH-), 30.38 (1C, CH2), 29.03 (1C, CH2), 27.95 (1C,
CH2), 26.40 (1C, CH2), 26.07 (1C, OCH(CH3)2), 25.89 ppm (1C,
OCH(CH3)2). Anal. Calcd for C23H23AlCl4N2O3 (%): C, 50.76; H,
4.26; N, 5.15. Found: C, 50.79; H, 4.28; N, 5.19.
General considerations
All experiments were performed under a dry nitrogen atmosphere
using standard Schlenk techniques or an inert-gas glovebox. 1H
NMR and 13C NMR spectra were performed on Bruker AV 300M
apparatus at 25 °C in CDCl3 for compounds and polymers.
Elemental analysis was perfomed by a Varian EL microanalyzer.
The monomer conversions were confirmed by the integral signals
at 1.65 ppm standed for LA monomer and 1.59 ppm standed for
PLA in CDCl3. Pm values were computed from different tetrad
intensities measured by homonuclear decoupled 1H NMR spectrum.
Gel permeation chromatography (GPC) measurements were
conducted with a Waters 515 GPC with CHCl3 as the eluant (flow
rate: 1 mLmin–1, at 35 °C). The molecular weight was adjusted
through PS standard. Toluene and THF used for polymerization
were distilled from sodium benzophenone ketyl solution just before
use. Pentane, ethyl acetate and chloroform-d were distilled over
calcium hydride. Lactides were stored under nitrogen at -20 °C and
recrystallized from dry ethyl acetate and dried under reduced
pressure. (R,R)-1,2-diammoniumcyclohexane mono-(+)-tartrate
salt was purchased from Aldrich and stored under dry nitrogen at -
20 °C. Other reagents such as aluminum isopropoxide, 3-tert-
butylsalicylaldehyde, 3-tert-butyl-5-chlorosalicylaldehyde and 3,5-
dichlorosalicylaldehyde were purchased from J&K company.
General procedure for lactide polymerization
The substrate and solvent were prepared in sealed glass ampoules
using the standard Schlenk techniques. In
a representative
polymerization reaction, aluminum complex (0.50 mmol) and
required quantities of lactides in toluene (100 mL) were placed in a
dry ampoule with a magnetic bar at 70 °C. To the vigorously
stirred solution, a required quantities of lactides were added,
resulted in [LA]0 = 0.50 mol L–1 and [LA]0 : [Al]0 = 100 : 1. At
selected reaction time intervals, ca. 0.60 mL aliquots were removed
and the conversion was determined by 1H NMR. At high
conversion, the reaction was terminated by adding glacial acetic
acid, the polymer was precipitated by cold methanol or refrigerated
centrifuge. The polymers were collected and dried in vacuo at
40 °C for 24 h.
General procedure for the synthesis of Salen aluminum
complexes
The pro-ligand (2.85 mmol), aluminum isopropoxide (0.583 g, 2.85
mmol), and toluene (20 mL) were added into a flamed-dried
Schlenk tube equipped with a magnetic stirring bar. The mixture
was stirred for 3 days at 80 °C and then slowly cooled to ambient
temperature, the solution was transferred to a glovebox and the
solvent was removed under reduced pressure. The residual solid
was then washed with cold pentane (1.0 mL×2), filtered, and dried
under reduced pressure. Yield: 87.8–92.6%. Complex 3 was
prepared according to the reference.[80]
Acknowledgements
We are grateful to National Natural Science Foundation of China
(Nos. 21204082 and 51173183); National High Technology
Research and Development Program of China (No.
2011AA02A202); China Postdoctoral Science Foundation (No.
2014M561268); the Project for Science and Technology
Development of Jilin Science & Technology Department, Jilin
Province, China (No. 20140204017GX).
Complex 1：1H NMR (300 MHz, CDCl3) δ 8.24 (s, 1H, N=CH),
8.03 (s, 1H, N=CH), 7.24 (d, J = 7.9 Hz, 1H, PhH), 7.17 (d, J = 7.9
Hz, 1H, PhH), 7.08 (d, J = 7.4 Hz, 1H, PhH), 7.01 (d, J = 7.4 Hz,
Notes and references
4
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