Living and stereoselective polymerization of rac-lactide by bimetallic aluminum Schiff-Base complexes

Article abstract of DOI:10.1002/pola.27123

A series of bimetallic aluminum Schiff-base complexes have been prepared and characterized. The complexes used as catalysts were applied in the lactide polymerization to test their activities and stereoselectivities. All polymerizations are living, as evidenced by the narrow polydispersities and the good fit between calculated and found number-average molecular weights of the isolated polymers. Isotactic enriched polylactide was obtained by using these complexes. Kinetic studies indicated that the polymerizations are both first-ordered with respect to lactide monomer and catalyst.

Full text of DOI:10.1002/pola.27123

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Living and Stereoselective Polymerization of Rac-Lactide by Bimetallic
Aluminum Schiff-Base Complexes
Zhi Qu,1,2 Ranlong Duan,1 Xuan Pang,1 Bo Gao,1 Xiang Li,1 Zhaohui Tang,1
Xianhong Wang,1 Xuesi Chen1
1Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, China
2Graduate University of Chinese Academy of Sciences, Beijing 100039, China
Correspondence to: X. Pang (E-mail: xpang@ciac.ac.cn)
Received 19 December 2013; accepted 27 January 2014; published online 17 February 2014
DOI: 10.1002/pola.27123
ABSTRACT: A series of bimetallic aluminum Schiff-base com-
plexes have been prepared and characterized. The complexes
used as catalysts were applied in the lactide polymerization to
test their activities and stereoselectivities. All polymerizations
are living, as evidenced by the narrow polydispersities and the
good fit between calculated and found number-average molec-
ular weights of the isolated polymers. Isotactic enriched poly-
lactide was obtained by using these complexes. Kinetic studies
indicated that the polymerizations are both first-ordered with
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KEYWORDS:
biomaterials;
bimetallic;
catalysts;
lactide;
polymerization
Sn,9 and Zn,10 have been explored for these purposes. Prob-
lems associated with the use of some of these nonligated
metal alkoxides are racemization and transesterification as
side reactions during the polymerization, which lead to the
disturbance of the polymer microstructure, unpredictable
molecular weights. It was found by Ikada that the stereo-
complex polymers formed by an equivalent mixture of PLLA
and PDLA have many advantages such as higher melting
temperature (Tm 5 230 C). Many efforts have been made
to obtain crystalline PLA via direct ROP of racemic lactide
(rac-LA; i.e., a 1:1 mixture of L-LA and D-LA) by the stereose-
lective catalysts.
INTRODUCTION Poly(lactic acid) (PLA) material have
attracted great interests because of their unique biodegrad-
ability and biocompatibility. They could be used in wide range
of applications, such as sutures, bone fracture fixation devices,
drug controlled release carriers, tissue engineering scaffolds,
and green plastics.1 PLA 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 LA monomer, the different LA stereoisomers are distin-
guished, namely (S,S)-LA (l-LA), (R,R)-LA (d-LA), and (R,S)-LA
(meso-LA) (Scheme 1).2 The chain chemical stereostructures
of PLA are one of the most important factors that influence
the physical, mechanical, and degradation properties of the
polymers. Highly stereoregular poly(L-lactide) (PLLA) and
poly(D-lactide) (PDLA) are crystalline polymers, with a high
melting temperature and good mechanical strength.3 Atactic
PLA with a random placement of S- and R-LA units in the
polymer chains are amorphous and brittle materials. Due to
the crystallinity of stereoregular PLAs, these materials slowly
degrade in a physiological environment, whereas atactic PLA
materials degrade much faster (Scheme 2).4
11
ꢀ
Spassky et al.12 discovered that Schiff-base salen complex
could give a highly stereocontrolled polymerization of rac-LA
to form isotactic and crystalline PLA with a higher Tm than
optically pure PLLA. A few other excellent studies have
attempted to elucidate the relationship between the alumi-
num Schiff-base complexes and stereoselectivity. Coates,13
Feijen,14 Duda,15 and our group16 used chiral salen- or
salan-type Schiff-base catalysts via the enantiomorphic site
control mechanism; Nomura17 and we18 used achiral
salen-type Schiff-base catalysts via the chain-end control
mechanism. Recently, we developed enolic complexes in the
stereocontrolled polymerization of rac-LA to give isotactic
PLAs are usually prepared by ROP of LA, generally applying
metal-based catalysis. A large number of nonligated metal
complexes, in particular, metal alkoxides of Al,5 Li,6 Ca,7 Fe,8
Additional Supporting Information may be found in the online version of this article.
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In this work, we reported the preliminary results of the syn-
thesis and characterization of bimetallic salen complexes
which acting as catalysts to polymerize LA in a controlled
manner under mild conditions to give isotactic enriched
polylactide.
SCHEME 1 Stereoisomers of lactides.
EXPERIMENTAL
enriched polylactide.19 So far, the monometallic salen com-
plexes have been studied extensively as the homogeneous
catalysts in the LA polymerization. To the best of our knowl-
edge, the multimetallic salen complex has not been explored
in great detail in LA polymerization, despite their great
potential structure varieties of ancillary ligands.20 The multi-
metallic structure may function cooperatively which could
result in an increase of catalytic activity and stereoselectivity
in comparison to the monometallic complexes.21
General
All experiments were carried out in a dry nitrogen atmos-
phere using standard Schlenk techniques or in a glovebox.
Starting materials for the synthesis of ligands were pur-
chased from Aldrich Inc. and used without further purifica-
tion. Toluene and hexane were distilled from Na-
benzophenone before use. Ethyl acetate and 2-propanol were
distilled from CaH2 under the protection of argon. Rac-Lac-
tide (Purac) was purified by recrystallization from ethyl
SCHEME 2 Stereostructures of PLA.
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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 shifts
were given in parts per million from tetramethylsilane. Gel-
permeation chromatography (GPC) measurements were con-
ducted with a Waters 515 GPC with CHCl3 as the eluent
(flow rate: 1 mL min21 at 35 ꢀC). The molecular weights
were calibrated against polystyrene standards.
in ethanol/CHCl3 mixture (Supporting Information contains
detailed information of ligands 4, 5, and 6).
Complex Synthesis
General Procedure
For complexes 4a26a, AlEt3 (0.2 mmol) in toluene (5 mL)
was added to the stirred 1 mL toluene (THF for 6a) solution
of ligand precursors 426 (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.
Synthesis of Compounds 1, 2, and 3
1: Benzene-1,4-dicarbaldehyde (13 g), 2,2-bis-bromomethyl-
propane-1,3-diol (52 g), and p-toluenesulfonic acid (0.3 g) in
200 mL of toluene were placed in a flask equipped with a
Dean–Stark apparatus, CaCl2 drying tube, and a magnetic
stirrer. The solid–liquid reaction mixture was heated to
reflux until 3.6 mL of H2O evolved. The reaction mixture was
then cooled to RT, filtered, and purified by ether then dried
in vacuo at RT. The final products were white powder.
Complex 4a. 4a was obtained as a yellow solid in 92% yield.
1H NMR (400M, CDCl3): d 5 8.15(s, 2H, NCH), 8.02(s, 2H,
NCH), 7.55(s, 4H, ArH), 7.48(s, 4H, ArH), 7.09(d, 4H, ArH),
5.53(s, 2H, ArCH), 4.15(s, 4H, CCH2O), 4.07(d, 4H, CCH2N),
3.88(m, 4H, CCH2O), 3.73(d, 4H, CCH2N), 0.89(t, 6H,
AlCH2CH3), 20.03(q, 4H, AlCH2). 13C NMR (100 MHz, d6-
THF) d 5 171.01 (NCH), all benzene ring 161.02, 139.60,
134.43, 130.91, 126.48, 122.36, 120.92, 119.97, 119.11,
Yield: 92%. 1H NMR (300.00 MHz, CDCl3) d 5 7.49(s, 4H,
ArH), 5.40(s, 2H, ArCH), 4.26(d, 4H, CCH2O), 3.96(s, 4H,
CCH2N), 3.75(d, 4H, CCH2O), 3.31(s, 4H, CCH2N).
102.27(ArCO),
72.19(CCH2O),
64.50,
59.41(CCH2N),
38.07(OCH2CCH2N), 8.72(AlCH2CH3), 1.15(AlCH2CH3). ELEM.
ANAL.: Calcd. C 51.48, H 3.80, N 4.80; Found C 51.55, H 3.91,
N 4.75.
2: Sodium azide (6.5 g) was added to a stirred solution of
compound 1 (12.4 g)ꢀ in DMSO (500 mL), the mixture was
slowly heated to 110 C and kept for 24 h and then isolated
by precipitation into H2O (600 mL), filtered, and purified by
ether before dried in vacuo at RT. The final products were
white powder.
Complex 5a. 5a was obtained as a yellow solid in 95% yield.
1H NMR (400M, CDCl3): d 5 8.14(s, 2H, NCH), 7.98(s, 2H,
NCH), 7.57(s, 4H, ArH), 7.11(s, 4H, ArH), 6.82(d, 4H, ArH),
5.52(s, 2H, ArCH), 4.12(s, 4H, CCH2O), 3.97(d, 4H, CCH2N),
3.85(m, 4H, CCH2O), 3.68(d, 4H, CCH2N), 2.21(d, 24H,
ArCH3), 0.94(t, 6H, AlCH2CH3), 20.06(q, 4H, AlCH2). 13C
NMR (100 MHz, CDCl3) d 5 176.97 (NCH), all benzene ring
161.73, 139.77, 132.05, 129.71, 126.26, 125.93, 124.78,
117.73, 101.33 (ArCO), 70.03(CCH2O), 62.71, 59.25(CCH2N),
39.74(OCH2CCH2N), 20.37(ArCH3), 16.07(ArCH3), 9.02
(AlCH2CH3), 1.05(AlCH2). ELEM. ANAL.: Calcd. C 69.44, H 6.83,
N 5.59; Found C 69.48, H 6.75, N 5.67.
Yield: 82%. 1H NMR (300.00 MHz, CDCl3). d 5 7.48(s, 4H,
ArH), 5.41(s, 2H, ArCH), 4.05(d, 4H, CCH2O), 3.81(s, 4H,
CCH2N), 3.71(d, 4H, CCH2O), 3.24(s, 4H, CCH2N).
3: Under the protection ofꢀargon, LiAlH4 (1.5 g) was slowly
added to 200 mL THF at 0 C. Compound 2 (4.7 g in 150 mL
THF) was then added to the LiAlH4 solution. The reaction
vessel was kept for at 0 ꢀC for 2 h and at RTꢀfor another 2
h. H2O (1.5 g) was added to the mixture at 0 C and stirred
for 15 min. Then, 1.5 g NaOH solution (15% in H2O) was
added and stirred for another 15 min at 0 ꢀC. H2O (4.5 g)
was added to the reaction vessel and kept for 30 min at 0
ꢀC. The reaction mixture was then warm to RT, filtered, and
dried in vacuo at RT. The final products were white powder.
Complex 6a. 6a was obtained as a white solid in 93% yield.
1H NMR (400M, CDCl3): d 5 8.34(s, 2H, NCH), 8.07(s, 2H,
NCH), 7.56(s, 4H, ArH), 7.09(s, 4H, ArH), 6.97(d, 4H, ArH),
5.54(s, 2H, ArCH), 4.10(m, 4H, CCH2O), 3.96(s, 4H, CCH2N),
3.80(m, 4H, CCH2O), 3.60(s, 4H, CCH2N), 1.41(d, 36H,
ArC(CH3)3), 1.27(d, 36H, ArC(CH3)3), 1.02 (t, 6H, AlCH2CH3),
20.03(q, 4H, AlCH2). 13C NMR (100 MHz, CDCl3) d 5
177.73(NCH), all benzene ring 162.51, 140.83, 139.39,
133.26, 129.74, 128.30, 126.25, 118.38; 101.22(ArCO),
70.15(CCH2O), 59.11, 58.39(CCH2N), 39.78(OCH2CH2N),
35.44, 34.23(ArC(CH3)3), 31.38(C(CH3)3), 29.44(C(CH3)3),
9.12(AlCH2)CH3), 0.88(AlCH2). ELEM. ANAL.: Calcd. C 73.51, H
8.73, N 4.18; Found C 73.46, H 8.71, N 4.24.
1
Yield: 47%. H NMR (300.00MHz, d6-DMSO). d 5 7.38(s, 4H,
ArH), 5.37(s, 2H, ArCH), 3.94(d, 4H, CCH2O), 3.58(d, 4H,
CCH2N), 2.84(d, 4H, CCH2O), 2.51(d, 4H, CCH2N).
Synthesis of Ligands
General Procedure
The ligand precursors 4, 5, and 6 were obtained by conden-
sation between the tetraamine compound 3 and substi-
tuented salicylaldehyde. A solution of 3 (0.1 mol L21) in
ethanol (50 mL) was added dropwise to a stirred solution of
substituted salicylaldehyde (0.4 mol L21) in ethanol (50
mL). The reaction mixture was refluxed for 14 h before cool-
ing to RT. After removal of the solvent under vacuum, a crys-
talline solid was produced and purified by recrystallization
LA Polymerization
In a glovebox, rac-LA (1.00 g, 6.94 mmol), 2-propanol (4.33
mg, 0.072 mmol) in 2 mL of toluene, and 4a26a (0.072
mmol) dissolved in 2 mL of toluene and another 9 mL of tol-
uene were added successively into a flame-dried reaction
vessel equipped with a magnetic stirring bar. The vessel was
removed from the glovebox and placed in an oil bath
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SCHEME 3 Ligands prepared for the stereoselective polymerization of lactides.
thermostated at 70 ꢀC. At certain time intervals, an aliquot
of the reaction mixture was taken out using a syringe to
determine the monomer conversion by H NMR. A few drops
of acetic acid were added to quench the polymerization after
it reached a certain conversion. The polymer was isolated by
precipitation into cold methanol, filtering, and drying under
vacuum at RT for 24 h.
Kinetic Studies
The polymerization processes were systematically investi-
gated by kinetic studies using bimetallic complexes 4a26a.
Polymerization data were collected in Table 1. Polymeriza-
1
1
tion was monitored by H NMR spectroscopy until monomer
consumption was reached. The data of conversions versus
time were collected in Figure 1. The conversions increased
linearly with the reaction time for complexes 4a, 5a, and 6a
(Fig. 1, left). The linear relationship also remained at various
concentrations for a certain complex (Fig. 1, right). First-
order kinetics in monomer was observed [eq (1)], where
kapp was the apparent polymerization rate constant. The
first-order kinetics as shown in eq (1) implied that the con-
centration of active species remained unchanged, or in other
RESULTS AND DISCUSSION
Ligand Synthesis
The bimetallic ligand family (Scheme 3) was synthesized
from readily available starting materials by a four-step pro-
cedure. First, compound 1 was obtained by an acetalization
of benzene-1,4-dicarbaldehyde, and 2,2-bis-bromomethyl-
propane-1,3-diol. Subsequently, reaction of compound 1 with
sodium azide afforded compound 2. Compound 3 was
obtained by the reduction of compound 2 in the presence of
LiAlH4. The ligand precursors 4, 5, and 6 were obtained by
condensation between the tetraamine compound 3 and sub-
stituted salicylaldehyde. Ligands 4, 5, and 6 had the identical
imine backbones but the different substituents at the salicyli-
dene phenolate rings: ACl for 4, ACH3 for 5, and A tBu
for 6.
Complexes Formation and Characterization
Reaction of ligands 426 with stoichiometric AlEt3 formed
bimetallic salen complexes 4a26a, respectively (Scheme 4).
All these complexes were isolated as solid powder. The 1H
NMR spectrum of compound 6a showed signals at d –0.03
and 1.02 ppm, which were attributed to the methylene pro-
tons and methyl protons of the aluminum ethyl group,
respectively. The CCH2N protons displayed two singlets at
3.60 and 3.96 ppm; the NCH protons showed two singlets at
8.07 and 8.34 ppm. The intensity ratio of NCH, CCH2N, and
AlCH2 was 1:2:1, which confirmed the structure of 6a.
SCHEME 4 Synthetic pathway for the preparation of com-
plexes 4a–6a.
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TABLE 1 Polymerization Data of Rac-LA Using Complexes 4a–6a.a
T/ (ꢀC)
Time (h)
[M]0/[Cat]
Conv (%)b
M
n(calcd)/103c
MnGPC /103d
PDId
Pme
4a
4a
4a
5a
5a
5a
6a
6a
6a
6a
6a
70
70
70
70
70
70
70
70
70
90
110
2
100
160
200
100
160
200
100
160
200
200
200
94
96
96
98
97
96
96
95
92
85
90
6.8
11.9
19.8
23.2
12.4
19.7
23.1
11.5
19.7
23.6
20.8
23.0
1.06
1.07
1.03
1.08
1.10
1.09
1.05
1.07
1.04
1.15
1.20
0.66
0.67
0.67
0.76
0.78
0.76
0.87
0.88
0.85
0.71
0.61
4
11.1
13.8
7.1
6
10
12
15
16
18
24
10
5
11.2
13.8
6.9
11.0
13.3
12.3
13.0
a
e
The polymerizations were carried out in toluene solution. [LA]0 5 0.5
The parameter Pm is the probability of meso meso-linkages. According
mol/ L21
b
.
to chain-end control mechanism, the expressions for the tetrad
probabilities of poly(rac-LA) are: [mmm] 5 Pm2 1 (1 2 Pm)Pm/2, [mmr] 5
[rmm]
Measured by 1H NMR.
c
2
Calculated from the molecular weight of LA 3 [M/2]/[Cat] 3 conver-
5
(1
2
P
m)Pm
,
[rmr]
5
(1
2
P
m)2, [mrm]
5
((1
2
Pm)
1
sion 1 Mwisopropanol
.
Pm(1 2 Pm))/2.
d
Obtained from GPC analysis and calibrated against polystyrene stand-
ard. The true value of Mn could be calculated according to formula Mn
5
5 0.58MnGPC
.
words, the growing polymer chains remained alive during
the entire polymerization.
was postulated that both the bimetallic centers in 6a were
active.
2d½LAꢁ=dt5kapp½LAꢁ
(1)
To determine the order in catalyst, kapp was plotted versus
the concentration of catalyst (Fig. 3) using 6a. The linear
relationship of kapp versus [Cat]0 revealed a first-order in
catalyst. Therefore, the polymerization of rac-LA using 6a
followed an overall kinetic law of the following form as eq 2,
The number-average molecular weight (Mn) also followed a
linear relationship in monomer conversion (Fig. 2 and Table
1). The molecular control and the low polydispersity indi-
cated that the polymerization had a characteristic of con-
trolled propagation. The theoretical Mn of 6a close to the
determined value by GPC (Table 1, entry 7–9), and, thus,
reflecting the nature of living and controlled polymerization
catalyzed by these complexes. It was worth noting that the
calculation of the theoretical Mn was operated based on the
hypothesis that bimetallic centers would involve in the prop-
agation of polymer chains. Therefore, there would have two
propagating species on 6a, the actual concentration of mono-
mer would be only half of the original. Fortunately, the good
consistencies between MnGPC and Mn(calcd) verified this. So it
where kp was the polymerization rate constants and kp
app/[Cat].
5
k
2d½LAꢁ=dt5kp½LAꢁ½Catꢁ
(2)
We could extrapolate this equation to other complexes in the
rac-LA polymerization. The 1H NMR spectrum of oligomer
prepared using 6a at a low concentration ratio of monomer
to catalyst was shown in Figure 4. The triplet of two over-
lapping doublets at 1.24 ppm and the quartet at 4.34 ppm,
with an integral ratio close to 6:1, were assigned to the
FIGURE 1 Kinetic plots of the rac-lactide conversion versus the reaction time: (a) 4a, [M]0/[Cat] 5 200; (b) 5a, [M]0/[Cat] 5 200; (c)
6a, [M]0/[Cat] 5 200; (d) 6a, [M]0/[cat] 5 160; (e) 6a, [M]0/[Cat] 5 100.
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FIGURE 2 Plot of PLA Mn and polydispersity (Mw/Mn) as a function of rac-lactide conversion using complex 4a, 5a, and 6a, [M]0/
[Cat] 5 200.
methyl protons of the isopropoxycarbonyl end group and the
methine proton neighboring to the hydroxyl end group. This
indicated that the polymer chains were systematically end-
capped with an isopropyl ester and a hydroxyl group (Fig.
4). The Mn determined by end-group analysis is 1600, close
to the theoretical value of 1700. This manifested that the
ring-opening occurred through a so-called coordination–
6a/2-propanol was deduced by fitting lnkp versus 103/T
according to the Arrhenius equation (kp5 Ae2E =RT) (Fig. 6).
a
The activation energy Ea was 54.3 kJ mol21 by 6a, a 23.4%
reduction compared to that by tin(II) octanoate (70.9 6 1.5
kJ mol21).23
Complex Structure and Stereochemistry
insertion mechanism (Scheme 5).17(b) 22
,
Determination of the stereochemical microstructures of PLA
was achieved through inspection of the methine region of
homonuclear decoupled 1H NMR spectra of the resultant
polymers.24 The stereoselectivity decreased with the increas-
ing temperature. For example, an increase in the temperature
The influence of temperature on the polymerization rate was
also investigated (Fig. 5). The polymerization rate increased
with the increasing temperature. The apparent polymeriza-
tion rate constant kapp and polymerization rate constants kp
values were cꢀollected inꢀTable 2. An increase in the tempera-
ture from 70 C to 110 C led to a 630% increase in kp value
(0.59 at 70 ꢀC, 1.63 at 90 ꢀC, 4.3 at 110 ꢀC). From these
three kp values determined at different temperatures in
Table 2, the activation energy of the polymerization using
25
from 70 ꢀC to 110 ꢀC using 6a led to a reduction in Pm
value from 0.87 to 0.61, a 30% reduction. Pm values were
collected in Table 2.
The polymerization data indicated that the introduction of
substituent groups on the auxiliary ligand surroundings
FIGURE 3 kapp versus the concentration of 6a for the rac-LA
polymerization.
FIGURE 4 1H NMR spectrum of oligomers of rac-LA.
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SCHEME 5 Proposed mechanism for the polymerization of lactides.
significantly affected the stereoselectivity and activity. Com-
plexes 4a, 5a, and 6a had the same tetraimine backbone, dif-
ferent phenolate substituents, ACl for 4a, ACH3 for 5a, and
active species from being approached by LA monomer, as a
result, slowing down the polymerization rate. A change of
substituents from less bulky group to more bulky one led to
an increase in stereoselectivity. The Pm value was 0.87 for
6a and 0.72 for 5a. The compound 6a had the highest ste-
reoselectivity. This was consistent with a previous study that
the enhancement of stereoselectivity requires bulky substitu-
ents at the ortho positions for the stereoselective polymer-
t
A Bu for 6a. Consequently, they displayed different catalytic
behaviors. Polymerization data revealed that 4a had the
highest activity; 4a had its kp value more than sevenfolds
higher than that of 6a, that is, kp value was 4.24 L mol21
min21 for 4a and 0.59 L mol21 min21 for 6a. The presence
of the substituent on the phenolic ring played an important
role in determining the polymerization rate. The electron-
withdrawing chlorine substituents on ligand increased Lewis
acidity of metal centers, enhanced metal electrophilicities.
This made the nucleophilic addition through the metal–alk-
oxide bond easier, resulting in higher polymerization rates.
Similar results were found in our previous studies of mono-
metallic enolic and salen-type Schiff-base aluminum complex-
es.18(a),19(b) In our previous systems, fluorin substituted
enolic complexes and chlorine substituted salen complexes
exhibited greater polymerization activities than did their
non-electron-withdrawing substituted counterparts. It was
also reported by Gibson26 that chloro substituents in the
phenoxide unit had higher activity than their dimethyl ana-
logues for salan-type catalysts. The more bulky tert-butyl
(AtBu) substituents with more steric hindrance may keep
ization adopting chain-end control mechanism.17(b) 22
,
Stereochemistry of Rac-LA
Since the ligands and complexes reported here were achiral,
and there was a preference for isotactic addition during the
ROPs of rac-LA, we considered the chain-end control mecha-
nism.17(b) 22
In such a reaction, the ligand and the complex
,
were achiral, the initiation reaction occured without any dif-
ferentiation between the two enantiomers, and the last unit
in the growing polymer chain influenced which enantiomer
form of the monomers would incorporate next.
In the case of random propagation (Bernoullian statistics),
the additions of different enantiomeric monomers were inde-
pendent events, so the rates of addition of L- and D-mono-
mers to the growing chain-ends were not affected by the
configuration of the last repeating unit. Atatic poly(rac-LA)s
were most probably prepared under this condition. Because
there was a preference for isotactic addition during the
ROPs of rac-LA in this research, the intensity values of the
TABLE 2 Kinetic Results of Rac-LA Polymerization at Different
Temperatures Using 4a-–6a, [M]/[Cat] 5 200
Entry Cat T/ (ꢀC) kapp (min21
)
kp (L mol21min21
)
Pm
1
2
3
4
5
4a
5a
6a
6a
6a
70
70
70
90
110
0.01060
0.00225
0.00147
0.00408
0.01075
4.24
0.90
0.59
1.63
4.30
0.68
0.72
0.87
0.69
0.61
FIGURE 5 Kinetics of the rac-LA polymerization using 6a at the
reaction temperatures of (a) 110 ꢀC; (b) 90 ꢀC; (c) 70 ꢀC, [M]0/
[Cat] 5 200.
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This work was supported by the National Natural Science
Foundation of China (Nos. 21204082, 51173183, 21004061,
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