Metal-size influence of alkali metal complexes for polymerization of rac-lactide

Article abstract of DOI:10.1016/j.poly.2017.11.046

A series of alkali metal complexes with 2,2′-ethylidenebis(4,6-di-tert-butylphenol) (EDBP-H₂) as a ligand have been synthesized. In the presence of benzyl alcohol as an initiator, all complexes exhibit good catalytic behaviors for the ring-opening polymerization (ROP) of rac-lactide, giving desirable molecular weights and narrow molecular weight distributions. The relative order of activities for this series of complexes is Li > Na > K > Rb, which is related to metallic ion radius; and Lewis acidity of these metal ions is also an important factor for the kinetics of the ROP. The relative order of stereoselectivities for this series of complexes is Rb ≈ K > Na ? Li in this system. The first example of Rb complex in the ROP of rac-lactide in this paper was also described, showing a modest activity and a nice polymerization control, even with a certain isoselectivity.

Full text of DOI:10.1016/j.poly.2017.11.046

Accepted Manuscript
Metal-Size Influence of Alkali Metal Complexes for Polymerization of rac-
Lactide
Jiao Xiong, Yangyang Sun, Jitao Jiang, Changjuan Cheng, Xiaobo Pan, Cheng
Wang, Jincai Wu
PII:
S0277-5387(17)30774-X
https://doi.org/10.1016/j.poly.2017.11.046
POLY 12948
DOI:
Reference:
Polyhedron
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Accepted Date:
4 October 2017
28 November 2017
Please cite this article as: J. Xiong, Y. Sun, J. Jiang, C. Cheng, X. Pan, C. Wang, J. Wu, Metal-Size Influence of
Alkali Metal Complexes for Polymerization of rac-Lactide, Polyhedron (2017), doi: https://doi.org/10.1016/j.poly.
2017.11.046
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Metal-Size Influence of Alkali Metal Complexes for Polymerization of
rac-Lactide
Jiao Xiong,a Yangyang Sun,a Jitao Jiang,a Changjuan Cheng,a Xiaobo Pan,a Cheng
Wang,b* Jincai Wua*
aState Key Laboratory of Applied Organic Chemistry, College of Chemistry and
Chemical Engineering, Key Laboratory of Nonferrous Metal Chemistry and Resources
Utilization of Gansu Province, Lanzhou University, Lanzhou 730000, People’s Republic
of China.
bThe Second Affiliated Hospital of Lanzhou University, Lanzhou 730000, China.
* Corresponding authors. Tel.: +86 931 8912552; fax: +86 931 8912582.
E-mail addresses: wujc@lzu.edu.cn (J. Wu).
ABSTRACT
A series of alkali metal complexes with 2, 2’-ethylidenebis(4,6-di-tert-butylphenol)
(EDBP-H2) as a ligand have been synthesized. In the presence of benzyl alcohol as an
initiator, all complexes exhibit good catalytic behaviors for the ring-opening
polymerization (ROP) of rac-lactide, giving desirable molecular weights and narrow
molecular weight distributions. The relative order of activities for this series of
complexes is Li > Na > K > Rb, which is related to metallic ion radius; and Lewis
acidity of these metal ions is also an important factor for the kinetics of the ROP. The
relative order of stereoselectivities for this series of complexes is Rb ≈ K > Na >> Li
in this system. The first example of Rb complex in the ROP of rac-lactide in this paper
was also described, showing a modest activity and a nice polymerization control, even
with a certain isoselectivity.
Keywords: polylactide; ring-opening polymerization; alkali metal complexes;
isoselectivity.
1

1. Introduction
Polylactide (PLA), as a leading biodegradable polymer, has been widely used in daily
life because of its biorenewable and biocompatible properties, and expected to be an
alternative for petroleum-based products since the existence of serious pollution of the
ecosystem in the nowadays world [1]. The major method employed to synthesize PLA is
the ring-opening polymerization (ROP) of lactide because of well-controlled molecular
weight and low molecular weight distribution (Đ) compared to the direct condensation
of lactic acid [2]. Thus, many metal complexes have been synthesized to catalyze the
ROP of lactide, including Mg [3], Ca [4], Zn [5], Al [6], Ti [7], lanthanide complexes [8]
alkali metal complexes [9] and so on. Although some of these metal complexes are
excellent initiators/catalysts, most of them are still suffered from some demerits. For
example, Salen aluminum complexes show impressive degrees of stereocontrol but
usually with low activities [2e, 6a, 6q]; lanthanide complexes also show good
isoselectivity, but are cytotoxic [10]; calcium is essential element in the human body
and innocuous, while good stereoselective catalysts have rarely been reported [4a]; thus,
there is still a lot of room to improve the ROP of polylactide mediated by metal
complexes. Alkali metal complexes, like sodium and potassium complexes, are one of
our main research subject due to non-toxicity and high reactivity [11]. Recently, a series
of sodium and potassium crown ether complexes were reported to efficiently catalyze
the ROP of rac-lactide with high activities and high isoselectivities [12], the highest
isoselectivity even can reach to Pm = 0.94 at -70 °C [12e] and the molecular weight can
2

be high up to 107 kg/mol [12h]. The research of stereoselective synthesis of polylactide
is valuable because the physical and chemical properties of polylactide are highly
dependent on their stereo microstructures [13]. As pointed out in our previous work, a
confined space around metal center of alkali metal complexes is important for the
controllable isoselective ring-opening polymerization of rac-lactide. The confined space
or steric effect around the metal center does not only depend on the substituted group of
ligand but also the metal ionic radii. The effective ionic radii of six-coordinate Na+, K+,
and Rb+ are 116, 152, and 166 pm, respectively [14]. Considering spherical volume,
which is of a cubic function, the size ratio of these three ion radii will be 1:2.25:2.93,
Rb+ is nearly the three times size of Na+. To deeply understand the effect caused by
different ionic radii, herein, we explore the catalytic performance of a series of lithium,
sodium, potassium, and rubidium complexes, which enables us to understand the
influence of atomic size and coordination environment on the activity of metal
complexes for the isoselective polymerization of rac-lactide. In addition, to our
knowledge, the rubidium phenolate reported in this work is the first example of
rubidium complex utilized as a catalyst for the ROP of cyclic ester.
2. Experimental Section
2.1. General Considerations
All complexes were synthesized and crystallized under a dry argon atmosphere, and
all the polymerization reaction were performed in a glovebox using standard Schlenk
techniques. The 1H NMR and 13C NMR spectra were recorded on a Varian mercury plus
3

300M or JNM-ECS 400M MHz spectrometer; The elemental compositions of the four
metal complexes were measured using an Elemental Vario EL series CHN analyzer
under a nitrogen atmosphere. The GPC measurements were performed using a Waters
1525 binary system that was equipped with a Waters 2414 RI detector and two Styragel
columns (102-106 kg/mol) by using THF (HPLC grade) as an eluent running at 1 mL
min−1. Molecular weights (Mn) and molecular weight distributions (Đ) were calculated
using polystyrene as the standard. Each measuring value was corrected using a factor of
0.58 for polylactide according to the literature [15]. The mass spectroscopic data were
obtained using a Thermo Scientific Orbitrap Elite MS (LTQ Orbitrap Elit).
2.2. Materials
Toluene, THF, and hexane were dried by refluxing several hours with sodium, and
benzophenone serve as an indicator. CH2Cl2 was distilled with P2O5; rac-LA was
purchased from Daigang BIO Engineer Ltd. of China and was recrystallized from
toluene at least three times. CDCl3 was purchased from J&K Scientific, Ltd. in Beijing
n
and dried with activated molecular sieves. KN(SiMe3)2, NaN(SiMe3)2, BuLi, Rb, were
purchased from J&K Scientific Ltd. 18-crown-6 and 12-crown-4 were purchased from
local companies and were used as received.
2.3. Synthesis
2.3.1. Synthesis of Rb(Ot-Bu)
Add element Rb (0.095 g, 1.10 mmol) to a solution of tert-butyl Alcohol (0.22 mL,
4

2.3 mmol) in THF and stir at room temperature under an argon atmosphere for 24 h.
The light-yellow liquid formed was dried under vacuum and a white powder was
obtained. Yield: 0.17 g (97 %).
2.3.2. Synthesis of complex 1
To a solution of EDBP-H2 (0.438 g, 1.0 mmol) and 12-crown-4 (0.194 g, 1.1 mmol)
in toluene (50.0 mL), nBuLi (1.1 mmol, 2.4 M in hexane) was added dropwise. After stir
at room temperature under an argon atmosphere for 12 h, a white turbid liquid was
obtained. The precipitate formed was separated by filtration, washed with 20 mL hexane
and dried in vacuum to give a white powder (0.58 g, 94 %). Colorless crystals of 1
suitable for X-ray diffraction studies were obtained from a toluene solution at room
temperature. 1H NMR (400 MHz, CDCl3): δ 7.20 (d, J = 2.5 Hz, Ar-H, 2H), 6.98 (d, J =
2.4 Hz, Ar-H, 2H), 5.14 (q, J = 7.5 Hz, Ar-H, 1H), 3.98 (b, -CH2-, 8H), 3.64 (b, -CH2-,
8H), 1.65 (d, J = 7.4 Hz, -CH3, 3H), 1.41 (s, -C(CH3)3, 18H), 1.30 (s, -C(CH3)3, 18H).
13C NMR (100 MHz, CDCl3): 157.14, 135.69, 134.75, 134.53, 120.02, 119.87 (Ph),
66.95 (-CH2-), 35.00, 34.12 (-C(CH3)3), 31.94, 30.05 (-C(CH3)3), 29.48 (CHCH3), 19.97
(CH3). Anal. Calcd for C38H61O6Li: 73.47, H 9.86. Found: C 73.52, H 9.90.
2.3.3. Synthesis of complex 2
According to the procedure described above for 1, EDBP-H2 (0.438 g, 1.0 mmol) and
18-crown-6 (0.290 g, 1.1 mmol) was converted to complex 2 as a white powder (0.65 g,
90 %) by reacting with NaN(SiMe3)2 (0.55 mL, 1.1 mmol, 2.0 M in THF) in toluene.
5

Colorless crystals of 2 suitable for X-ray diffraction studies were obtained from a
toluene solution at room temperature. 1H NMR (400 MHz, CDCl3): δ 7.13 (d, J = 2.4 Hz,
Ar-H, 2H), 6.93 (d, J = 2.5 Hz, Ar-H, 2H), 5.19 (q, J = 7.4 Hz, -CH-, 1H), 3.67 (s,
-CH2-, 24H), 1.59 (d, J = 7.5 Hz, -CH3, 3H), 1.39 (s, -C(CH3)3 18H), 1.28 (s, -C(CH3)3
18H). 13C NMR (100 MHz, CDCl3): 158.60, 135.21, 134.77, 134.55, 119.62, 119.44
(Ph), 68.98 (-CH2-), 34.79, 34.05 (-C(CH3)3), 31.98, 30.06 (-C(CH3)3), 19.83 (CH3).
Anal. Calcd for C42H69O8Na: C 69.63, H 9.61. Found: C 69.58, H 9.59.
2.3.2. Synthesis of complex 3
According to the procedure described above for 1, EDBP-H2 (0.438 g, 1.0 mmol) and
18-crown-6 (0.290 g, 1.1 mmol) was converted to complex 3 as a white powder (0.64 g,
86 %) by reacting with KN(SiMe3)2 (1.1 mL, 1.1 mmol, 1.0 M in THF) in toluene.
Colorless crystals of 3 suitable for X-ray diffraction studies were obtained from a
toluene solution at room temperature. 1H NMR (400 MHz, CDCl3): δ 7.19 (d, J = 2.5 Hz,
Ar-H, 2H), 6.94 (d, J = 2.6 Hz, Ar-H, 2H), 5.23 (q, J = 7.4 Hz, -CH-, 1H), 3.66 (s,
-CH2-, 24H), 1.66 (d, J = 7.5 Hz, -CH3, 3H), 1.39 (s, -C(CH3)3 18H), 1.27 (s, -C(CH3)3
18H). 13C NMR (100 MHz, CDCl3): 158.89, 135.28, 134.46, 134.23, 119.64, 119.59
(Ph), 69.74 (-CH2-), 35.02, 34.03 (-C(CH3)3), 31.97 (-C(CH3)3), 30.24 (CHCH3), 29.99
(-C(CH3)3), 19.68 (CH3). Anal. Calcd for C42H69O8K: C 68.01, H 9.48. Found: C 68.07,
H 9.38.
2.3.4. Synthesis of complex 4
6

To a solution of EDBP-H2 (0.438 g, 1.0 mmol) and 18-crown-6 (0.290 g, 1.1 mmol)
in toluene (50.0 mL), Rb(Ot-Bu) (0.158 g, 1.1 mmol in toluene) was added dropwise.
After stir at room temperature under an argon atmosphere for 12 h, a white turbid liquid
was obtained. The precipitate formed was separated by filtration, washed with 20 mL
hexane and dried in vacuum to give a white powder (0.65 g, 83%). Colorless crystals of
4 suitable for X-ray diffraction studies were obtained from a CDCl3 solution at room
temperature. 1H NMR (400 MHz, CDCl3): δ 7.23 (d, J = 2.2 Hz, Ar-H, 2H), 6.99 (d, J =
2.4 Hz, Ar-H, 2H), 5.19 (q, J = 7.5 Hz, -CH-, 1H), 3.67 (s, -CH2-, 24H), 1.71 (d, J = 7.4
13
Hz, -CH3, 3H), 1.46 (s, -C(CH3)3 18H), 1.34 (s, -C(CH3)3, 18H). C NMR (100 MHz,
CDCl3): 158.95, 135.41, 134.58, 134.44, 119.74, 119.66 (Ph), 69.84 (-CH2-), 35.05,
34.06 (-C(CH3)3), 31.98 (-C(CH3)3), 30.71 (CHCH3), 30.01 (-C(CH3)3), 19.64 (CH3).
Anal. Calcd for C42H69O8Rb: C 64.02, H 8.81. Found: C 64.06, H 8.83.
2.3.5. General procedure for polymerization of rac-lactide
A typical polymerization procedure is illustrated by the synthesis of PLA
([LA]0/[cat.]0/[BnOH]0 = 100:1:1, Table 1, entry 1). In a glove box, to a solution of 3
(7.4 mg, 0.01 mmol) and BnOH (100 µL, 0.1 M in toluene) in 5 mL toluene, rac-lactide
(0.144 g, 1.0 mmol) was added. The mixture was then rapidly stirred at ambient
temperature for 10 h. The reaction was quenched by one drop of water. Resulting
polymers were precipitated in hexane and dried to give final products.
2.4. Crystallographic studies
7

The data were collected using a SuperNova (Dual) X-ray diffractometer equipped
with a graphite-monochromated Cu/Mo Kα radiation source (λ = 1.54184/0.71073 Å).
The structure was solved by direct methods using the Siemens SHELXTL PLUS
program [16]. Non-hydrogen atoms were refined with anisotropic displacement
parameters during the final cycles. All hydrogen atoms were placed by geometrical
considerations and were added to the structure-factor calculations. No. CCDC
1533900-1533903 contain the supplementary crystallographic data for this paper.
3. Results and Discussion
3.1. Synthesis and Structures of alkali metal complexes 1−4.
Scheme 1. Preparation of compounds 1-4.
2,2’-ethylidenebis(4,6-di-tert-butylphenol) (EDBP-H2) was applied as a ligand in this
work due to its non-toxicity, which had been approved by the U.S. Food and Drug
Administration as an indirect food additive. So stereoselective researches utilizing
EDBP complexes in ROP reactions seem to be of potential values. Thus, four alkali
metal complexes, 1 (EDBP-H)Li(12-crown-4), 2 (EDBP-H)Na(18-crown-6), 3
8

(EDBP-H)K(18-crown-6), and 4 (EDBP-H)Rb(18-crown-6) were prepared by treatment
of EDBP-H2 with the same equivalents of 12-crown-4/18-crown-6 and
nBu-Li/NaN(SiMe3)2/KN(SiMe3)2/tBu-ORb in toluene at ambient temperature (Scheme
1), all these four complexes were isolated as white solids in high yields (> 80%), and
were confirmed by the elemental analysis, NMR spectra, and single crystal X-ray
diffraction.
Fig. 1. Molecular structures of complexes 1-4 with probability ellipsoids at 30% (most of the
hydrogen atoms are omitted for clarity). Selected bond lengths (Å): complex 1, Li1−O1 =
1.833(4), Li1---O2 = 3.512(42), O2---O1 = 2.594(21); complex 2, Na1-O1 = 2.266(19),
Na1---O2 = 3.549(19), O2---O1 = 2.528(21); complex 3, K1−O1 = 2.677(19), K1---O2 =
3.942(20); O2---O1 = 2.513(26); complex 4, Rb1−O1 = 2.871(5), Rb1---O2 = 3.233(6),
O2---O1 = 2.433(81).
9

Crystals suitable for single-crystal X-ray diffraction were isolated from toluene
solution for complexes 2, 3, and 4, and from CDCl3 solution for complex 1, respectively.
The molecular structures of 1-4 are monomeric species wherein the metal center is
coordinated by one EDBP-H anion ligand and one crown ether auxiliary ligand (Fig. 1).
Compared to complexes 3 and 4, the crown ether auxiliary ligand in complex 2 shows a
severely distorted geometry, which is inconsistent with our previously work [12d]. We
think the shorter Na1-O1 bond distance (2.266 Å versus 2.677 Å (K1-O1) and 2.871 Å
(Rb1-O1)) leads to a closer distance and a stronger repulsion between crown ether and
tert-butyl on EDBP-H ligand, and this repulsion force the crown ether to adjust its
configuration with a distorted geometry to give a whole stable structure. Unlike Li1,
Na1, and K1 atoms, Rb1 atom in complex 4 almost locates in the middle of the two
oxygen atoms (O1 and O2) of EDBP-H with a shortest distance of 3.233 Å between
Rb1 and O2 of hydroxyl group despite the longest ionic radius of Rb. While Li1, Na1,
K1 atoms are strongly biased towards one side and cause longer M-O2 distances of
3.516, 3.549 and 3.942 Å, respectively. Therefore, the shortest distance of M(Rb)-O2
may suggest this repulsion between crown ether and tert-butyl on EDBP-H is the
smallest in complex 4 because the radius of Rb is large enough to separate crown ether
auxiliary ligand and EDBP-H ligand; the coordination of Rb to O2 can make complex 4
more stable. Since Li has the shortest ionic radii, we change 18-crown-6 to 12-crown-4
to avoid seriously distorted configuration. As showed in Fig. 1, the 1.833 Å bond
distance of Li1-O1 is comparable to other Li-O bind in phenolate lithium complexes
10

which indicate there is no remarkable repulsion between crown ether and the tert-butyl
groups of EDBP-H [17].
3.2. Controllability of Catalysts 1-4 in the ROP of rac-Lactide.
The ring-opening polymerizations of rac-lactide catalyzed by 1-4 were systematically
examined under room temperature with the reaction concentration of 1-4 was 2.0 mM
(Table 1). It was found that complexes 1-4 were efficient catalysts for the ROP of
rac-lactide in toluene solution. 86% of rac-LA was converted into PLA in toluene
within 10 hours when complex 3 was used (Table 1, entry 1). However, less than 30%
of rac-LA can be converted into polymer within the same time when CH2Cl2 or THF
was applied as a solvent (Table 1, entries 2 and 3), which may be caused by solvent
polarities to form loose or tight ion paired complexes [12d] or competitive coordination
of THF as we reported in our previous works [12d,12e]. However, the polymerization
catalysed by complex 3 with just one equivalent of alcohol as initiator showed only
modest controllability as inferred from the relatively lower molecular weight of polymer
than calculated value (Table 1, entry 1). In alkali metal phenolate system, an
uncontrollable ROP reaction of lactide giving cyclic polylactide can happens sometimes
via a coordination-insertion mechanism with phenoxy as initiator directly, which has
been reported by Cano and us [9h, 12e]. Upon addition of excess benzyl alcohol as the
co-initiator (two, three, and four equivalents, respectively) to suppress this side reaction,
the molecular weights become to agree with the calculated values (Table 1, entries 4-6).
The polymerizations catalyzed by complexes 1, 2 and 4 are also living, which can be
11

proved by the controllable molecular weights and low molecular weight distributions (Đ
< 1.17, Table 1, entries 7−13) and can be confirmed further by the fact that the
molecular weights of polymers with complex 4 as a catalyst increase linearly with the
ratio of [rac-LA]0:[BnOH]0 (Table 1, entries 9-13, Fig. 2). Particularly, it is worth to
note that this is the first example of rubidium phenolate complex used for catalyzing the
ROP reaction of lactide. And the large radius of rubidium ion provides more diversity to
design alkali metal complexes for the ROP of cyclic esters, as well as potential different
catalytic characteristic.
Table 1. rac-Lactide Polymerization Catalyzed by
1
-
4a
Mn(calcd)c( Mn(obsd)d( Đd
.
e
Ent Cat.
ry
[LA]0/[Cat.]0 t/h
Conv.b(%)
Pm
/[BnOH]0
g/mol)
12500
3700
4500
6200
4600
3300
3300
3400
3100
6600
10000
14100
17700
2300
g/mol)
8500
3400
4100
6100
4700
3200
3100
3800
3300
6500
9700
13900
17000
2300
1
100:1:1
100:1:1
100:1:1
100:1:2
100:1:3
100:1:4
100:1:4
100:1:4
100:1:4
200:1:4
300:1:4
400:1:4
500:1:4
20:1:1
10
10
10
8
86
23
28
85
93
90
90
91
82
90
92
97
1.06
1.05
1.10
1.05
1.05
1.06
1.12
1.17
1.04
1.03
1.03
1.03
1.03
1.05
0.64
0.60
0.61
0.70
0.71
0.71
0.55
0.60
0.71
0.71
0.72
0.72
0.71
0.67
3
3
3
3
3
3
1
2
4
4
4
4
4
4
2f
3g
4
5
5.5
4
6
7
1
8
1.5
5.5
6.5
9
9
10
11
12
13
14
10
11.5 98
76
6
aReactions were performed in 5 mL of toluene at room temperature, [Cat.]0 = 2.0 mM. bDetermined
1
c
by H NMR spectroscopy. Calculated from the molecular weight of rac-LA × [LA]0/[BnOH]0 ×
d
conversion yield + MBnOH. Experimental molecular weight (Mn) and molecular weight distribution
(Ð) were measured by gel permeation chromatography (GPC) at 40 °C using THF as a solvent, an
eluent flow rate of 1 mL/min, and calibrated with standard polystyrene samples and corrected using
e
the Mark−Houwink factor of 0.58.[15]. Determined by analysis of all of the tetrad signals in the
1
f
methine region of the homonuclear-decoupled H NMR spectrum [8e, 18]. In 5 mL CH2Cl2. gIn 5
mL THF.
12

Fig. 2. Polymerization of rac-LA catalyzed by 4 in toluene at room temperature. The
relationships between Mn(■), Đ (●) of the polymer and the initial mole ratios [LA]0/[BnOH]0
(Table 1, entries 9-13) is shown.
3.3 Activity of Catalysts 1-4 in the ROP of rac-Lactide.
Based on the time consumed to convert rac-LA into PLA with similar conversions
(Table 1, entries 6-9), we got the order of activities is 1 > 2 > 3 > 4, which is coincide
with the Lewis acidities of alkali metals. Generally, the metal centers with higher Lewis
acidity are postulated to result in stronger binding to activation of monomer and give
higher polymerization rates in a ligand-assisted monomer-activated mechanism for
alkali metal phenolate complex system in the presence of alcohol. However, increased
Lewis acidity of the metal center may also induce stronger binding of the phenoxy to
the metal, retarding the ability of phenoxy group to activate alcohol [9d]. But in this
system, the shortest distance of O1---O2 of EDBP-H in rubidium complex 4 illustrates
the intramolecular hydrogen bond of O1---H-O2 is stronger than that in lithium, sodium
and potassium complexes 1-3 (O2---O1, 2.594 Å (Li) > O2---O1, 2.528 Å (Na) >
13

O2---O1, 2.513 Å (K) > O2---O1, 2.433 Å (Rb)). The stronger intramolecular hydrogen
bond can lead the negative charge on O1 decrease. Thus, the basicity of oxygen atom
O1 in rubidium complex 4 is slightly weaker than that in complexes 1-3. DFT at
M062X level with 6311+g(d) basis set for C, H, O, Li, Na, K, and ADZP for Rb [19]
were performed and natural bond orbital (NBO) analysis showed the negative charges
of O1 atoms in these four complexes are -0.95, -0.922, -0.914, and -0.865 respectively,
which confirms that the basicity order of EDBP-H anions is 1 > 2 > 3 > 4. And NBO
analysis results also confirm the hydrogen bond becomes stronger when metal ion
changes from Li+ to Rb+ because the hydrogen bond order between O1---H—O2 is
0.0989, 0.1421, 0.1548, and 0.2251 in complexes 1-4 respectively; as a result, the strong
hydrogen bond can reduce the negative charge on oxygen atom of O1. Therefore, the
Lewis acidity of the cation in complex 1 is the strongest, and the Lewis basicity of anion
in complex 1 also is strongest. Both the activations of monomer and alcohol in complex
1 are easier than that in complexes 2-4, and the activity order of these four complexes
seems to be reasonable (Figure 3). It is worth mentioning that, besides the Lewis
acidities, the steric hindrance of the active center sometimes may be another factor for
the order of rates. Usually the less crowdedness around the metal center can facilitate
the ROP reaction because the monomer of lactide can reach the metal center easier to be
activated. As we initially designed, rubidium complex 4 should be less crowded because
of its large ion radius compared with sodium and potassium complexes with the same
EDBP-H and 18-crown-6 ligands. But herein, this situation becomes more complicated
14

because the EDBP-H should adjust its configuration to adapt different alkali metal ions.
The surrounding of rubidium ion seem to be more crowded than that in complexes 2 and
3 when we compared the O1---O2 distances in these four complexes (O2---O1, 2.594 Å
(Li) > O2---O1, 2.528 Å (Na) > O2---O1, 2.513Å (K) > O2---O1, 2.433Å (Rb)), but
considering the large radius of the rubidium ion and flexible shift of crown ether on
rubidium ion, we believe the surrounding of rubidium ion should not be more crowded
remarkably than that of sodium and potassium complexes 2 and 3. It is interesting that
this activity order is contrast to similar sodium and potassium monophenolates
previously reported by us which are more active than complex 1-4. The intramolecular
hydrogen bond should be a reason for this remarkably different activity order. Possibly,
the intramolecular hydrogen bond in this system make the Lewis basicity of O1 weak
and lead the activation of alcohol at a low level; consequently, the Lewis acidities of
metal ions with different abilities to activation of monomer become a unique important
factor for the ROP reaction rate.
Fig. 3. The activations of monomer and alcohol mechanism for complexes 1-4.
15

3.4. Stereoselectivity of Catalysts 1-4 in the ROP of rac-Lactide.
The stereo microstructures of resulting poly(rac-lactide)s were evaluated by
1
homonuclear-decoupled H NMR spectra [18c, 20]. Compared to previous works of us,
Wang, and Ma [12], all of the complexes 1-4 based on EDBP-H ligand also show
isoselectivities which proves again the crown ether is important for this isoselective
system because no isoselective catalysts of EDBP-H-alkali metal complexes without
crown ether auxiliary ligand was reported for the ROP of rac-lactide [21]. Complexes 3
and 4 show a similar Pm value of 0.71 when the ratio of [rac-LA]0:[1]0:[BnOH]0 is
100:1:4, (Fig. 4; Table 1, entries 6, 9), which are better than that of complexes 1 (Pm =
0.55) and 2 (Pm = 0.60) under the same reaction conditions (Table 1, entries 7 and 8 ). It
is worth noting that when complex 3 was used as a catalyst and just one equivalent of
benzyl alcohol was used as initiator, the Pm value decreased from 0.71 to 0.64. But
when the amounts of benzyl alcohol are more than two equivalents, the Pm values
almost do not change (Table 1, entries 4-6). The low isoselectivity of complex 3 at low
equivalents of BnOH may results from the side cyclization reaction initiated by
phenoxy directly via a coordination-insertion mechanism as mentioned in the above
discussion. The low isoselectivity also may result from transesterification, because
transesterification can increase the stereo errors. The transesterification in the presence
of just one equivalent of BnOH is more serious than that in a large amount of alcohol
can be proved by the noticeable stronger peak at 5.215 ppm in homonuclear decoupled
1H NMR spectrum (Fig. S1), which was resulting from the overlap of mrm tetrad with
16

rrr, rrm and mrr tetrads, the later tetrads were enhanced by transesterification side
reaction. The isoselectivity of lithium complex 1 is low can be attribute to the shortest
radius of lithium and the shortest Li-O1 bond which will give a small confined space
between EDBP-H ligand and crown ether. We believe that only a suitable confined
space can give a high isoselectivity.
Fig. 4. Pm values determined for all tetrads based on Bernoulli statistics. Their average
values were used (Table 1, entry 6) [8e, 18].
The ESI mass spectrum of low molecular weight polymer obtained with complex 4 as
catalyst and 4 equivalent of benzyl alcohol as initiator (Table 1, entry 9) shows a series
of main peaks at 144n + 108 + 23 + 23 with charge of +2 and a series of main peaks at
144n + 108 + 23 + 23 + 19 with charge of +3 (Fig. 5), which can be assigned to
n(C6H8O4) + BnOH + 2Na+ and n(C6H8O4) + BnOH + 2Na+ + H3O+ separately,
demonstrating the existence of benzyl alcohol on the polymer chain. Some
17

intermolecular transesterification may occur during reaction, as a cluster of weak peaks
with a molecular mass interval of ∼72 Da were observed in both series of peaks. When
complex 4 and 1 equivalent of benzyl alcohol was used (Table 1, entry 1), the ESI mass
spectrum of resulting polymer only shows a series of main peaks at 72n + 108 + 23 + 18
with charge of +2 and a series of peaks at 72n + 108 + 18 with charge of +1 (Fig. S3),
+
which can be assigned to n(C3H4O2) + BnOH +Na+ + NH4 and n(C3H4O2) + BnOH +
+
NH4 , respectively. Those main peaks with a molecular mass interval of ∼72 Da
demonstrate the existence of serious transesterification reaction during this ROP
reaction, which can explain the reason for molecular weights of polymer are small and
selectivity was low when 1 equivalent of benzyl alcohol was used as initiator.
1445.9248
1517.9459
1589.9672
z=2
z=2
z=2
1661.9880
z=2
1734.0093
z=2
1373.9032
z=2
1258.3813
z=3
1211.6833
z=3
1162.3531
z=?
1308.0460
z=3
1115.6554
z=3
1065.9901
z=?
1806.0299
z=2
1019.6271
z=3
1878.0515
z=2
971.6130
z=3
1950.0720
z=2
803.5453
z=1
936.7229
z=1
859.3868
z=?
923.5985
z=3
2022.5970
z=2
2058.6072
z=2
2166.6362
z=2
2238.6672
z=?
800
900
1000
1100
1200
1300
1400
1500
m/z
1600
1700
1800
1900
2000
2100
2200
Fig. 5. ESI-MS spectrum of poly (rac-LA) prepared by catalyst 4 when [rac-LA]0:[4]0:[BnOH]0
is 100:1:4 (Table 1, entry 9, Z stand for the charge of polymer chain).
4. Conclusions
18

In summary, a series of lithium, sodium, potassium, and rubidium complexes for the
ROP of rac-lactide are reported in this study. In the presence of multiple equivalents of
benzyl alcohol, all complexes can exhibit good behaviors for the ROP of rac-lactide and
give desired molecular weights and narrow molecular weight distributions. Both the
order of rates and the degree of stereoselectvities show a correlation with the metallic
covalent radius and corresponding structures. A smaller metal ion causes a higher
polymerization rate resulting in a relative order of rates: Li > Na > K > Rb, which
agrees well with the activation of monomer and the activation of alcohol by EDBP-H
anion. The lithium and sodium complexes with smaller metal center show low
steroselectivities comparing to potassium and rubidium complexes; this may result from
the steric hindrance surrounding of the active center, which is indirectly influenced by
the metallic atomic radius. Rubidium complex shows a medium activity and a nice
polymerization control for the ROP of rac-lactide, offering more diversity for the design
of excellent metal catalysts for the stereoselective ROP of cyclic esters.
Acknowledgments
Financial support from the National Natural Science Foundation of China (No.
21671087, 21771094 and 21401161), the Science Foundation of Gansu Province of
China (No. 1308RJ2A121 and 1506RJZA213), the Fundamental Research Funds for the
Central Universities (lzujbky-2017-k07), the project for the National Basic Science
Personnel Training Fund (J1103307), and the ScGrid of the Supercomputing Center of
the Chinese Academy of Sciences are gratefully acknowledged.
19

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23

Graphic abstract
Graphic abstract
A series of alkali metal complexes with 2, 2’-ethylidenebis(4,6-di-tert-butylphenol)
(EDBP-H2) as a ligand have been synthesized. The relative order of activities for this
series of complexes is Li > Na > K > Rb. The relative order of stereoselectivities for this
series of complexes is Rb ≈ K > Na >> Li. The first example of Rb complex in the
ROP of rac-lactide in this paper was also described, showing a medium activity, a nice
polymerization control, and a certain isoselectivity.
24