Bimetallic Schiff-base aluminum complexes based on pentaerythrityl tetramine and their stereoselective polymerization of racemic lactide

Article abstract of DOI:10.1039/c4ra02092h

A series of Schiff base aluminum(iii) complexes with bimetallic active centers are synthesized. Their catalytic properties in the solution polymerization of racemic lactide (rac-LA) are examined. The modifications in the auxiliary ligand exhibited a dramatic influence on the catalytic performance. Among these complexes, 3a has the highest stereoselectivity (P _m = 0.91) owing to the bulky tert-butyl groups on the salicylaldehyde. Kinetic studies indicate that the polymerizations are both first-ordered with respect to the monomer and catalyst. Other factors that influence the polymerization such as the polymerization time and the temperature, as well as the monomer concentration, are discussed in detail. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02092h

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Bimetallic Schiff-base aluminum complexes based
on pentaerythrityl tetramine and their
Cite this: RSC Adv., 2014, 4, 22561
stereoselective polymerization of racemic lactide†
Xuan Pang, Ranlong Duan, Xiang Li, Bo Gao, Zhiqiang Sun, Xianhong Wang
*
and Xuesi Chen
A series of Schiff base aluminum(III) complexes with bimetallic active centers are synthesized. Their catalytic
properties in the solution polymerization of racemic lactide (rac-LA) are examined. The modifications in the
auxiliary ligand exhibited a dramatic influence on the catalytic performance. Among these complexes, 3a
has the highest stereoselectivity (Pm ¼ 0.91) owing to the bulky tert-butyl groups on the salicylaldehyde.
Kinetic studies indicate that the polymerizations are both first-ordered with respect to the monomer and
catalyst. Other factors that influence the polymerization such as the polymerization time and the
temperature, as well as the monomer concentration, are discussed in detail.
Received 10th March 2014
Accepted 31st March 2014
DOI: 10.1039/c4ra02092h
www.rsc.org/advances
racemic mixture of L-LA and D-LA, referred to as rac-LA, can be
traced back to 1996 by the pioneering studies of Spassky and
coworkers.6 They show that an enantiomerically pure Schiff
Introduction
In recent years many researchers have been focused on biode-
gradable polymers as petrochemical-based resources are grad-
ually depleted. Poly(lactic acid) (PLA) from bio-based resources
is particularly attractive as a biodegradable and biocompatible
polymer because the ester linkage is easy to hydrolyse to CO2
and water.1–3
Poly(lactic acid)s are generally synthesized by the ring-
opening polymerization (ROP) of lactide (LA), the cyclic dimer
of lactic acid. Due to the presence of two chiral centers in the
lactide monomer the different lactide stereoisomers are
distinguished, namely L-lactide (L-LA), D-lactide (D-LA) and meso-
lactide (Scheme 1). The stereochemistry of the monomeric units
in the polymer chains plays an important role in the mechan-
ical, physical and degradation properties of PLA materials.4,5
The synthesis of stereoregular PLA materials starting from a
base aluminum alkoxide catalyst-initiator preferably polymer-
ized L-LA over D-LA from rac-LA, leading to an isotactic type PLA
material with a gradient of L- and D-LA units in the polymer
chains. A few other studies have attempted to elucidate the
relationship between the monometallic aluminum salen Schiff
base complexes and the stereoselectivity.7–13 Among them,
Coates,14 Feijen15 and Duda16 used chiral salen-type Schiff base
catalysts via the enantiomorphic site control mechanism;
Nomura17 used achiral salen-type Schiff base catalysts via the
chain-end control mechanism.
Although monometallic salen Schiff base complexes have
been studied extensively as LA polymerization catalysts, little
research about Schiff base complexes with bimetallic active
centers has been reported for rac-LA polymerization.18–20 It is
postulated that these complexes with bimetallic active centers
might combine a high stereoselectivity and a high activity
because of the synergetic effect of the two salicylidene moieties.
Inspired by the successful preparation and application of
symmetrical and unsymmetrical bimetallic salen complexes,21
we are very interested in studies on the catalysis of bimetallic
aluminum salen complexes. The research described in this
paper is focused on the design and application of these
complexes with bimetallic active centers for the controlled and
stereoselective ROP of lactides.
Scheme 1 Stereoisomers of lactides.
Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, P. R. China.
E-mail: xschen@ciac.ac.cn
Experimental section
General
† Electronic supplementary information (ESI) available: Crystal structure of
ligand 2 and calculation of the entropy and enthalpy difference. See DOI:
10.1039/c4ra02092h
All experiments were carried out under argon using Schlenk
techniques. Starting materials for the synthesis of ligand 1–3
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2a. 1H NMR (300 MHz, CDCl3) d ¼ 8.01(m, NCH 4H), 6.95(m,
ArH 4H), 6.72(m, ArH 4H), 3.71(m, CCH2N 8H), 2.14(s, ArCH3
24H), 0.86(m, AlCH2CH3 6H), ꢁ0.09(m, AlCH2 4H). 13C NMR
(100 MHz, CDCl3) d ¼ 170.53(NCH), all benzene ring: 163.54,
137.65, 130.48, 128.89, 124.58, 117.09; 63.60(NCH2),
41.95((CH2)4C), 20.12(ArCH3), 15.73(ArCH3), 9.64(AlCH2CH3),
1.11(AlCH2CH3). Elem. anal.: calcd C 70.29, H 7.08, N 7.29%;
found C 70.26, H 7.10, N 7.25%.
were purchased from Aldrich Inc. and used without further
purication. Toluene was distilled from Na-benzophenone.
Ethyl acetate and 2-propanol were distilled from CaH2 under
the protection of argon. rac-Lactide (Purac) was puried by
recrystallization from ethyl acetate and dried under vacuum at
room temperature (RT) before use. NMR spectra were recorded
ꢀ
on Bruker AV 300 M and Bruker AV 400 M in CDCl3 at 25 C.
Chemical shis are given in parts per million from tetrame-
thylsilane. Gel permeation chromatography (GPC) measure-
ments were conducted with a Waters 515 GPC with CHCl3 as the
eluent (ow rate: 1 mL minꢁ1, at 35 ꢀC). The molecular weights
were calibrated against polystyrene (PS) standards.
3a. 1H NMR (300 MHz, CDCl3) d ¼ 8.19(m, NCH 4H), 7.43(m,
ArH 4H), 6.91(m, ArH 4H), 3.80(m, CCH2N 8H), 1.50(m,
ArC(CH3)3 36H), 1.32(m, ArC(CH3)3 36H), 0.70(m, AlCH2CH3
6H), ꢁ0.19(m, AlCH2 4H). 13C NMR (100 MHz, CDCl3): d ¼
176.79(NCH), all benzene ring: 168.11, 157.96, 140.58, 136.57,
130.65, 127.00, 126.12, 117.81; 62.17(NCH2), 45.11((CH2)4C),
35.09, 33.84(ArC(CH3)3), 31.34(C(CH3)3), 29.35(C(CH3)3),
8.64(AlCH2CH3), 1.34(AlCH2CH3). Elem. anal.: calcd C 74.96, H
9.30, N 5.07%; found C 75.01, H 9.27, N 5.09%.
Synthesis of ligands (general procedure)
A solution of pentaerythrityl tetramine (1 mmol) in ethanol (50
mL) was added dropwise to a stirred solution of substituted
salicylaldehyde (4 mmol) in ethanol (50 mL). The reaction
mixture was reuxed for 12 h before cooling to RT. Aer removal
of the solvent under vacuum, a crystalline solid was produced
and puried by recrystallization in ethanol–CHCl3 mixture.
1. 1H NMR (300 MHz, CDCl3) d ¼ 13.20(s, ArOH 4H), 8.40(s,
NCH 4H), 7.33(m, ArH 4H), 7.23(m, ArH 4H), 6.97(m, ArH 4H),
6.89(m, ArH 4H), 3.75(s, CCH2N 8H). 13C NMR (100 MHz, CDCl3)
d ¼ 166.92(NCH), all benzene ring 160.80, 132.39, 131.33,
118.60, 118.21, 116.55; 60.93(NCH2), 43.53((CH2)4C). Elem.
anal.: calcd C 72.24, H 5.88, N 10.21%; found C 72.28, H 5.84, N
10.19%.
Polymerization of rac-LA
In a typical polymerization experiment, complexes 1a–3a (10
mmol), the required amount of rac-LA in toluene were loaded in
a ame-dried vessel containing a magnetic bar. The vessel was
placed in a thermostated oil bath. Conversion of the monomer
was determined on the basis of 1H NMR spectroscopic studies.
Aer a certain reaction time, the polymer was isolated by
precipitation with cold methanol. The precipitate was collected
and dried under vacuum at RT for 36 h.
2. 1H NMR (300 MHz, CDCl3) d ¼ 13.21(s, ArOH 4H), 8.34(s,
NCH 4H), 7.05(s, ArH 4H), 6.88(s, ArH 4H), 3.78(s, CCH2N 8H),
2.29(s, ArCH3 24H). 13C NMR (100 MHz, CDCl3)
d
Results and discussion
¼
167.03(NCH), 156.58, 134.58, 128.91, 126.98, 125.18, 117.19 all
benzene ring; 61.08(NCH2), 43.29((CH2)4C), 20.09(ArCH3),
15.02(ArCH3). Elem. anal.: calcd C 74.52, H 7.32, N 8.48%;
found C 74.50, H 7.38, N 8.51%.
Ligand synthesis
Pentaerythrityl tetramine was synthesized by a three-step
procedure as shown in Scheme 2. The versatile ligand family
was synthesized from pentaerythrityl tetramine with
substituted salicylaldehyde. Ligands 1–3 had identical aliphatic
backbones but the different salicylaldehyde substituents: H for
1, CH3 for 2 and C(CH3)3 for 3. The 1H NMR spectra of ligand 2
shows signals at d 8.34 and 3.78 which were attributed to the
N]CH protons and NCH2 protons of the pentaerythrityl tetra-
mine, respectively. The intensity ratio of the signals at 8.34 and
3.78 ppm was 1 : 2, which conrmed the structure of ligand 2.
3. 1H NMR (300 MHz, CDCl3) d ¼ 13.71(s, ArOH 4H), 8.55(s,
NCH 4H), 7.46(s, ArH 4H), 7.18(s, ArH 4H), 3.84(s, CCH2N 8H),
1.53(s, ArC(CH3)3 36H), 1.35(s, ArC(CH3)3 36H). 13C NMR (100
MHz, CDCl3) d ¼ 167.87(NCH), all benzene ring 157.70, 139.96,
136.34, 126.93, 125.86, 117.32; 61.16(NCH2), 43.79((CH2)4C),
34.74, 33.78(ArC(CH3)3), 31.11(C(CH3)3), 29.11(C(CH3)3). Elem.
anal.: calcd C 78.27, H 9.70, N 5.62%; found C 78.30, H 9.68, N
5.67%.
Synthesis of complexes
AlEt3 (0.2 mmol) in toluene (5 mL) was added to the stirred 1 mL
toluene solution of ligands 1–3 (0.1 mmol) at RT. The reaction
was maintained at 80 ꢀC for 12 h, and the reaction mixture was
then slowly cooled to RT. The toluene was removed under
vacuum.
1a. 1H NMR (300 MHz, CDCl3) d ¼ 8.05(m, NCH 4H), 7.36(m,
ArH 4H), 7.01(m, ArH 4H), 6.80(m, ArH 4H), 6.54(m, ArH 4H),
3.75(m, CCH2N 8H), 0.62(m, AlCH2CH3 6H), ꢁ0.12(m, AlCH2 4H).
13C NMR (100 MHz, d8-THF) d ¼ 173.13(NCH), all benzene ring:
168.38, 136.92, 134.60, 123.35, 120.69, 117.07; 64.70(NCH2),
44.14((CH2)4C), 10.21(AlCH2CH3), 2.19(AlCH2CH3). Elem. anal.:
calcd C 67.67, H 5.83, N 8.53%; found C 67.71, H 5.85, N 8.55%. Scheme 2 Synthetic pathway for the preparation of ligands.
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Further evidence came from the single crystal of ligand 2 equation of the form shown in eqn (2), where kp was the poly-
(see ESI, Fig. S1†).
merization rate constant and kp ¼ kapp/[cat].
ꢁd[LA]/dt ¼ kp[LA][cat]
(2)
Complex formation
Bimetallic Schiff base aluminum complexes 1a–3a were
obtained via the reaction of ligands 1–3 with stoichiometric
AlEt3 in toluene, respectively (Scheme 3). All these complexes
Table 2 shows kapp and kp values using 1a–3a at different
temperature. The rst-order kinetics in monomer was further
conrmed as 3a had almost the same kp values at various
monomer concentrations (Table 2, entry 7, 8 and 9).
1
were isolated as pale yellow powder. The H NMR spectrum of
compound 2a shows signals at d 0.86 and ꢁ0.09 ppm, which are
attributed to the methyl protons and methylene protons of the
aluminum ethyl group, respectively. Comparing with ligand 2
which the signals of N]CH protons and NCH2 protons were at
8.34 and 3.78, the corresponding signals of complex 2a moved
to higher elds at 8.01 and 3.71, respectively. The intensity ratio
of the signals at 8.01, 3.71, 0.86 and ꢁ0.09 ppm was 2 : 4 : 3 : 2,
which conrmed the structure of 2a.
1H NMR spectrum of PLA oligomers at a low monomer-to-
initiator ratio showed 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 (Fig. 4). These peaks were assigned to the methyl protons
of the isopropoxycarbonyl end group and the methine proton
neighboring the hydroxyl end group, respectively. This clearly
indicates that the oligomer was systematically capped with one
isopropyl ester group and one hydroxyl group. This conrmed
that the aluminum isopropoxides were the actual active species
in LA polymerizations when applying aluminum ethyls/2-prop-
anol as catalyst/initiator systems.
Ring-opening polymerization of rac-LA in solution
Bimetallic complexes 1a–3a were investigated in the rac-LA
polymerization. All polymerizations were carried out in toluene,
and the levels of conversion were monitored by 1H NMR
determination of samples withdrawn from the reaction
mixtures at a certain time interval. In the presence of propan-2-
ol as an initiator, 1a–3a catalyzed PLAs with similar number
average molecular weights as calculated, as well as narrow
molecular weight distributions, indicating well-controlled
polymerization. Representative polymerization results are
summarized in Table 1. The data of conversions vs. time were
collected (Fig. 1).
As for a certain complex, the polymerization rate was largely
determined by the reaction temperature (Fig. 5). An increase in
the temperature from 70 ꢀC to 110 ꢀC led to a 256% increase in
kp value (for 3a, from 0.70 to 2.49 in Table 2). The activation
energy of the polymerization was calculated by tting kp values
determined at different temperatures to the Arrhenius equation
(kp ¼ AeꢁE /RT) as shown in Table 2. An activation energy Ea of
a
34.83 kJ molꢁ1 was deduced by plotting ln kp versus 1/T (Fig. 6).
This value was much lower compared to that by tin(II) octanoate
(70.9 ꢂ 1.5 kJ molꢁ1).24
First-order kinetics in monomer was observed in each case
[eqn (1)], where kapp was the apparent polymerization rate
constant.
Complex structure and stereoselective polymerization
The substituents on the ligand phenolate rings also affect the
polymerization rate signicantly. Polymerization data revealed
that 2a had the highest activity: kp value was 1.3 L molꢁ1 minꢁ1
for 1a, 5.2 L molꢁ1 minꢁ1 for 2a and 0.7 L molꢁ1 minꢁ1 for 3a.
Complexes 1a–3a had the same pentaerythrityl tetramine
backbone, but the different substituted salicylaldehydes: H for
1a, –CH3 for 2a and –tBu for 3a. The more bulky substituents
with more steric hindrance would keep active species from
being approached by lactide monomer, as a result, slowing
down the polymerization rate. Introducing methyl groups
resulted in a remarkable increase in kp value. It is worth noting
that in our recent study of monometallic salan or bis(pyrroli-
dene) Schiff base complexes, the introduction of methyl groups
resulted in a remarkable decrease in kp.25,26 It was postulated
that the substituents took a complicated way to affect the
complexes behavior.
ꢁd[LA]/dt ¼ kapp[LA]
(1)
The number-average molecular weight (Mn) also followed a
linear relationship in monomer conversion (Fig. 2). To deter-
mine the order in catalyst, kapp was plotted vs. the concentration
of 3a (Fig. 3). kapp increased linearly with 3a concentration,
manifesting that there was only one initiating species for all the
bimetallic complexes during the polymerization process. This
indicated that the order in 3a was rst-order too. Therefore, the
polymerization of rac-LA using 3a followed an overall kinetic
The stereochemical microstructures of the resultant PLAs
were determined from the methine region of the homonuclear
1
decoupled H NMR spectra (Fig. 7). The Pm value was 0.65 for
1a, 0.73 for 2a and 0.91 for 3a. 3a had the highest stereo-
selectivity among the three complexes (Table 1, entry 1, 4 and 8).
A change of substituents from less bulky group (H) to more
Scheme 3 Synthetic pathway for the preparation of complexes 1a–3a. bulky one (tBu) led to a 40% increase in stereoselectivity.
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Table 1 Polymerization data of rac-LA with complexes 1a–3aa
Conv.b (%)
Mn(calcd) ꢃ 10ꢁ3
Mn(NMR) ꢃ 10ꢁ3
Mn(GPC) ꢃ 10ꢁ3
PDIe
Pm
c
d
e
f
Entry
Complex
Temp. (ꢀC)
t (min)
[M]0/[cat]
1
2
3
4
5
6
7
8
1a
1a
1a
2a
2a
2a
3a
3a
3a
3a
3a
70
70
70
70
70
70
70
70
70
90
110
300
410
512
90
147
185
674
648
751
287
194
100
150
200
100
150
200
100
150
200
100
100
88
82
91
90
89
94
92
89
86
88
90
6.3
8.9
13.1
6.5
9.6
13.5
6.5
9.6
12.4
6.3
6.8
9.5
14.0
6.6
11.1
15.7
7.8
11.4
14.2
8.1
5.9
9.3
12.4
5.7
9.2
12.0
6.2
8.2
10.8
7.2
1.14
1.18
1.15
1.20
1.16
1.14
1.15
1.16
1.14
1.29
1.35
0.65
0.65
0.65
0.73
0.72
0.73
0.90
0.91
0.91
0.78
0.68
9
10
11
6.5
8.4
7.6
a
b
All polymerizations were carried out in toluene solution, [LA]0 ¼ 0.5 mol Lꢁ1
.
Measured by 1H NMR. c Calculated from the molecular weight of
LA ꢃ [M/2]0/[cat] ꢃ conversion + Mw(iPrOH). d Obtained from 1H NMR analysis. e Obtained from GPC analysis and calibrated against polystyrene
f
22
standard. The true value of Mn could be calculated according to formula Mn ¼ 0.58MnGPC
.
Pm.23
Fig. 1 Kinetic plots of the rac-lactide conversion vs. the reaction time.
(a) Complex 2a, [M]0/[cat] ¼ 100; (b) complex 1a, [M]0/[cat] ¼ 100; (c)
complex 3a, [M]0/[cat] ¼ 100; (d) complex 3a, [M]0/[cat] ¼ 150; (e)
complex 3a, [M]0/[cat] ¼ 200.
Fig. 3 kapp vs. the concentration of 3a for the rac-LA polymerization.
Table
2 Kinetic results of rac-LA polymerization at different
temperatures
Temp. [M]0/ kapp
kp
Entry Complex (ꢀC)
[cat]
(minꢁ1 ꢃ 10ꢁ3
)
(L molꢁ1 minꢁ1
)
1
2
3
4
5
6
7
8
1a
1a
1a
2a
2a
2a
3a
3a
3a
3a
3a
70
70
70
70
70
70
70
70
70
90
110
100
150
200
100
150
200
100
150
200
100
100
6.65
4.33
3.38
26.20
17.33
12.93
3.65
2.33
1.78
8.05
12.45
1.33
1.30
1.35
5.24
5.20
5.17
0.73
0.70
0.71
1.61
2.49
9
10
11
Fig. 2 Plot of PLA Mn (-) and polydispersity (:) as a function of rac-
lactide conversion using (a) complex 1a, [M]0/[cat] ¼ 100; (b) complex
2a, [M]0/[cat] ¼ 100; (c) complex 3a, [M]0/[cat] ¼ 100.
It was consistent with previous results in the monometallic and 110 ꢀC using 3a led to a reduction in Pm value of 14% and
Schiff base systems17,27 that the enhancement of stereo- 25%, respectively (from 0.91 to 0.78 and 0.68). Pm values are
selectivity requires bulky substituents at the ortho positions for collected in Table 1.
the stereoselective polymerization adopting chain end control
mechanism.
In an effort to understand the preference of isotactic ster-
eosequence, further investigations were applied. It was antici-
The stereoselectivity decreased with increasing temperature. pated that the stereoselectivity in the polymerization of rac-LA
For example, an increase in the temperature from 70 ꢀC to 90 ꢀC took place via a chain-end control mechanism, since the ligands
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were achiral in this research. As for the so-called chain-end
control mechanism, the initiation reaction occurred without
any differentiation between the lactide enantiomers, and the
last unit in the growing polymer chain inuenced which
enantiomer form of the monomers would incorporate into the
molecular chain in the next step. If the stereogenic center in the
last unit favors a meso-enchainment, the isotactic PLA was
obtained; if the stereogenic center in the last unit favors a
racemic-enchainment, the syndiotactic PLA would be obtained.
As there is a preference for isotactic addition during the
ROPs of rac-LA in research, the intensity values of the individual
stereosequences do not obey Bernoullian statistics, so a
Markovian statistics and absolute reaction rate theory would be
preferred to interpret the stereosequence distribution.28–30
According to rst-order Markovian statistics,31,32 the entropy
and enthalpy difference between homo-propagation and cross-
propagation were calculated as ꢁ23.23 cal Kꢁ1 molꢁ1 and ꢁ9.42
kcal Kꢁ1 molꢁ1, respectively, which explained the preference of
isotactic stereosequence (for detailed calculation please see
ESI†).
Fig. 4 1H NMR spectrum of oligomers of rac-LA.
Conclusion
A series of new bimetallic Schiff base aluminum complexes were
designed and prepared from pentaerythrityl tetramine with
different substituted salicylaldehydes. The catalytic behavior of
the complexes in rac-lactide polymerization was examined. The
polymerization data manifested that their performance varied
remarkably due to the modications in the auxiliary ligand. The
Pm value was 0.65 for 1a, 0.73 for 2a and 0.91 for 3a. 3a had the
highest stereoselectivity owing to the bulky tert-butyl groups on
the salicylaldehyde. Polymerization data revealed that 2a had
the highest activity: kp value was 1.3 L molꢁ1 minꢁ1 for 1a, 5.2
L molꢁ1 minꢁ1 for 2a and 0.7 L molꢁ1 minꢁ1 for 3a. Activation
energy, Ea, of 34.83 kJ molꢁ1 was deduced by using 3a, which
was much lower compared to that of tin(II) octanoate (70.9 ꢂ 1.5
kJ molꢁ1). The different performance of these complexes was
attributed to the different substituent groups on auxiliary
ligands. As the complexes in this paper had no chirality, it was
presumed that the polymerization followed a so-called chain-
end control mechanism. Research toward the origin of the
activity and stereoselectivity of bimetallic Schiff base aluminum
complexes is currently in progress.
Fig. 5 Kinetics of the rac-LA polymerization using 3a at the reaction
temperatures of (a) 110 ꢀC; (b) 90 ꢀC; (c) 70 ꢀC, [M]0/[cat] ¼ 100.
Fig. 6 Plot of ln Kp vs. 1/T for the polymerization of rac-lactide with 3a.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (nos 21204082, 51173183, 51103058,
51203155 and 51321062) and the Ministry of Science and
Technology of China (no. 2011AA02A202).
Notes and references
1 K. E. Uhrich, S. M. Cannizzaro, R. S. Langer and
K. M. Shakesheff, Chem. Rev., 1999, 99, 3181–3198.
2 D. J. Mooney, G. Organ, J. P. Vacanti and R. Langer, Cell
Transplant., 1994, 3, 203–210.
Fig. 7 Homonuclear decoupled 1H NMR spectra by using 3a.
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3 J. L. Eguiburu, M. J. Fernandez-Berridi, F. P. Cossio and 18 A. W. Kleij, Chem.–Eur. J., 2008, 14, 10520–10529.
J. San Roman, Macromolecules, 1999, 32, 8252–8258.
4 S. A. Krouse, R. R. Schrock and R. E. Cohen, Macromolecules,
1987, 20, 903–904.
5 A. Belbella, C. Vauthier, H. Fessi, J. P. Devissaguet and
F. Puisieux, Int. J. Pharm., 1996, 129, 95–102.
6 N. Spassky, M. Wisniewski, C. Pluta and A. LeBorgne,
Macromol. Chem. Phys., 1996, 197, 2627–2637.
7 N. Nomura, R. Ishii, Y. Yamamoto and T. Kondo, Chem.–Eur.
J., 2007, 13, 4433–4451.
8 H. Du, X. Pang, H. Yu, X. Zhuang, X. Chen, D. Cui, X. Wang
and X. Jing, Macromolecules, 2007, 40, 1904–1913.
19 S. J. Wezenberg and A. W. Kleij, Angew. Chem., Int. Ed., 2008,
47, 2354–2364.
20 D. J. Darensbourg and O. Karroonnirun, Organometallics,
2010, 29, 5627–5634.
21 S. Curreli, E. C. Escudero-Adan, J. Benet-Buchholz and
A. W. Kleij, J. Org. Chem., 2007, 72, 7018–7021.
22 J. Baran, A. Duda, A. Kowalski, R. Szymanski and S. Penczek,
Macromol. Rapid Commun., 1997, 18, 325–333.
23 Pm is the probability of meso linkages, according to [mmm] ¼
Pm2 + (1 ꢁ Pm)Pm/2, [mmr] ¼ [rmm] ¼ (1 ꢁ Pm)Pm/2, [rmr] ¼
(1 ꢁ Pm)2/2, [mrm] ¼ [(1 ꢁ Pm)2 + Pm(1 ꢁ Pm)]/2.
9 X. Pang, H. Du, X. Chen, X. Wang and X. Jing, Chem.–Eur. J., 24 D. R. Witzke, R. Narayan and J. J. Kolstad, Macromolecules,
2008, 14, 3126–3136. 1997, 30, 7075–7085.
10 Z. H. Tang, X. S. Chen, X. Pang, Y. K. Yang, X. F. Zhang and 25 H. Du, A. H. Velders, P. J. Dijkstra, J. Sun, Z. Zhong, X. Chen
X. B. Jing, Biomacromolecules, 2004, 5, 965–970. and J. Feijen, Chem.–Eur. J., 2009, 15, 9836–9845.
11 Z. Y. Zhong, P. J. Dijkstra and J. Feijen, Angew. Chem., Int. 26 H. Du, A. H. Velders, P. J. Dijkstra, Z. Zhong, X. Chen and
Ed., 2002, 41, 4510–4513. J. Feijen, Macromolecules, 2009, 42, 1058–1066.
12 X. L. Zhuang, H. Y. Yu, Z. H. Tang, K. Oyaizu, H. Nishide and 27 A. Amgoune, C. M. Thomas, T. Roisnel and J. F. Carpentier,
X. S. Chen, Chin. J. Polym. Sci., 2011, 29, 197–202. Chem.–Eur. J., 2006, 12, 169–179.
13 Z. Qu, L. Xiang, X. Pang, R. L. Duan, B. Gao and X. S. Chen, 28 K. A. M. Thakur, R. T. Kean, E. S. Hall, J. J. Kolstad and
Chem. J. Chin. Univ., 2014, 35, 626–632.
14 T. M. Ovitt and G. W. Coates, J. Am. Chem. Soc., 2002, 124,
1316–1326.
15 Z. Y. Zhong, P. J. Dijkstra and J. Feijen, J. Am. Chem. Soc.,
2003, 125, 11291–11298.
16 K. Majerska and A. Duda, J. Am. Chem. Soc., 2004, 126, 1026–
1027.
17 N. Nomura, R. Ishii, M. Akakura and K. Aoi, J. Am. Chem.
Soc., 2002, 124, 5938–5939.
E. J. Munson, Macromolecules, 1998, 31, 1487–
1494.
29 K. A. M. Thakur, R. T. Kean, M. T. Zell, B. E. Padden and
E. J. Munson, Chem. Commun., 1998, 1913–1914.
30 M. H. Chisholm, S. S. Iyer, M. E. Matison, D. G. McCollum
and M. Pagel, Chem. Commun., 1997, 1999–2000.
31 M. Wisniewski, A. LeBorgne and N. Spassky, Macromol.
Chem. Phys., 1997, 198, 1227–1238.
32 J. E. Kasperczyk, Macromolecules, 1995, 28, 3937–3939.
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