Highly active tridentate amino-phenol zinc complexes for the catalytic ring-opening polymerization of ε-caprolactone

Article abstract of DOI:10.1016/j.jorganchem.2018.12.013

Tridentate 2,6-bis(imino)phenol (L1?L4) and 2,6-bis(amino)phenol (L5?L8) ligands and their corresponding zinc chloride complexes (1–8) have been synthesized and well characterized by FT-IR, ¹H and ¹³C NMR, elemental analysis and single-crystal X-ray diffraction analysis. It was found that zinc complexes (1–4) with 2,6-bis(imino)phenol ligands were inactive for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) even if they were activated by methyllithium. Zinc complexes (5–8) with 2,6-bis(amino)phenol ligands were proved to be highly efficient initiators for the ROP of ε-CL in combination with 4 equiv. of methyllithium. The ROP of ε-CL in the absence or presence of benzyl alcohol, including the influence of monomer ratio, reaction temperature and the substituents in the ligands, were investigated. The structure and thermal properties of obtained polymers were analyzed by NMR and MALDI-TOF mass spectra, DSC and TGA, respectively.

Full text of DOI:10.1016/j.jorganchem.2018.12.013

Accepted Manuscript
Highly active tridentate amino-phenol zinc complexes for the catalytic ring-opening
polymerization of ε-caprolactone
Qian Hu, Suyun Jie, Pierre Braunstein, Bo-Geng Li
PII:
S0022-328X(18)30875-1
DOI:
https://doi.org/10.1016/j.jorganchem.2018.12.013
JOM 20670
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 9 October 2018
Revised Date: 27 November 2018
Accepted Date: 15 December 2018
Please cite this article as: Q. Hu, S. Jie, P. Braunstein, B.-G. Li, Highly active tridentate amino-phenol
zinc complexes for the catalytic ring-opening polymerization of ε-caprolactone, Journal of Organometallic
Chemistry (2019), doi: https://doi.org/10.1016/j.jorganchem.2018.12.013.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Highly active tridentate amino-phenol zinc complexes for the catalytic
ring-opening polymerization of ε-caprolactone
,‡
Qian Hu,† Suyun Jie, ,† Pierre Braunstein, Bo-Geng Li†
† State Key Laboratory of Chemical Engineering, College of Chemical and Biological
Engineering, Zhejiang University, Hangzhou 310027, China
‡ Université de Strasbourg, CNRS, CHIMIE UMR 7177, Laboratoire de Chimie de
Coordination, 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France
Abstract: Tridentate 2,6-bis(imino)phenol (L1−L4) and 2,6-bis(amino)phenol (L5−L7) ligands
and their corresponding zinc chloride complexes (1−7) have been synthesized and well
characterized by FT-IR, 1H and 13C NMR, elemental analysis and single-crystal X-ray diffraction
analysis. It was found that zinc complexes (1−4) with 2,6-bis(imino)phenol ligands were inactive
for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) even if they were activated
by methyllithium. Zinc complexes (5−7) with 2,6-bis(amino)phenol ligands were proved to be
highly efficient initiators for the ROP of ε-CL in combination with 4 equiv. of methyllithium. The
ROP of ε-CL in the absence or presence of benzyl alcohol, including the influence of monomer
ratio, reaction temperature and the substituent in the ligand, were investigated. The structure and
thermal properties of obtained polymers were analyzed by NMR and MALDI-TOF mass spectra,
DSC and TGA, respectively.
Keywords: zinc, ring-opening polymerization, ε-caprolactone
1. Introduction
Aliphatic polyesters, such as poly(ε-caprolactone) (PCL) and polylactide (PLA), are
important biodegradable materials, which have found wide applications in i.a. biomedical,
packaging, and agricultural areas [1,2]. For the moment, the catalytic ring-opening
polymerization (ROP) of cyclic esters is one of the most powerful and convenient methods for
the preparation of aliphatic polyesters [3,4]. A challenge is to develop more efficient catalysts
that also allow a better control of the ring-opening polymerization, the molecular weight and the
molecular weight distribution of the resulting polymers. ROP is commonly catalyzed by metal
coordination complexes [4,5] and improved activity, selectivity, and controllability, associated
with a lower toxicity are highly desirable to provide these polyesters with good environmental
* Corresponding author. E-mail: jiesy@zju.edu.cn
* Corresponding author. E-mail: braunstein@unistra.fr

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and biological compatibility and a broader potential for biomedical application. Numerous metal
complexes have been used to initiate the ROP of cyclic esters [4,5], mostly containing e.g. alkali
metals [6-10], alkaline earth metals [11-15], Group IVB metals [16-21], rare earth metals [22-27],
aluminum [28-34], copper [35,36], gallium [37,38]. However, traces of residual metal in the
polymers formed are very hard to be completely removed, which is harmful to the environment
or humans and consequently limits their applications of polymers. The use of harmless metal
ions in the catalytic systems is one approach to solve this problem [39]. Zinc is one of the
essential oligo-elements in the human bodies and has good biological activities. Therefore,
zinc-based metal catalysts are attracting increasing attention as green catalysts for the ROP of
cyclic esters. Zinc complexes bearing various organic ligands have been reported to be active
for the ROP of cyclic esters to form biodegradable polyesters with high molecular weight and
narrow molecular weight distribution [40-55].
Most reported zinc-based catalysts for the ROP of cyclic esters were zinc alkyl complexes
that are air-sensitive. There were only few examples of zinc chloride complexes used for the
ROP of lactones. For instance, zinc dichloride or dichlorozinc complexes were reported to
catalyze the ROP of ε-caprolactone or rac-lactide by a coordination-insertion mechanism [55,56].
Zinc chloride complexes with various ligands were also reported to be activated by
methyllithium or LiOCHMe2 to generate in situ zinc methyl species or zinc isopropoxide
derivatives, respectively, which were active for the ROP of rac-LA [57-62]. We previously
showed that zinc chloride complexes containing (benzimidazolyl)pyridine alcohol ligands were
also activated by methyllithium and then used in the ROP of ε-CL with high efficiency, in the
absence or presence of benzyl alcohol [63,64]. In this study, zinc complexes bearing tridentate
bis(amino)phenol ligands were designed, synthesized and applied in combination with
methyllithium, to the catalytic ROP of ε-CL, in the absence or presence of benzyl alcohol.
2. Results and discussion
2.1. Synthesis and characterization of the ligands and their zinc complexes
The 2,6-bis(imino)phenol ligands (L1–L4) were synthesized through the Schiff-base
condensation reaction of 4-substituted 2,6-diformylphenols with 2,6-dimethylaniline or
2,6-diisopropylaniline in refluxing ethanol according to literature procedures and characterized
1
by FT-IR and H NMR spectroscopy (Scheme 1) [65–67]. The 2,6-bis(amino)phenol ligands
(L5–L7) were prepared by reduction of the imine groups of ligands L1–L3 with sodium
borohydride and characterized by FT-IR, 1H and 13C NMR spectroscopy, and elemental analysis.
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However, ligand L4 was difficult to reduce using NaBH4 because its imino groups are surrounded
and protected by the bulky isopropyl groups.
Scheme 1. Synthesis of ligands L1–L7 and Zn complexes 1–7
The zinc complexes 1–7 were readily prepared by reaction of the corresponding ligand with
anhydrous zinc chloride in absolute ethanol at room temperature and isolated as air-stable solids
(Scheme 1). All the characterizing data for these complexes, including FT-IR, 1H and 13C NMR
spectra as well as elemental analyses, were fully consistent with their structures. In the NMR
spectra, the substituents on the two phenyl ring show slightly different chemical shifts, which is
consistent with the asymmetrical structure of the zinc complexes. This was unambiguously
established by single-crystal X-ray diffraction analysis of complex 4.
Single crystals of complex 4 suitable for X-ray diffraction analysis were obtained by slow
diffusion of n-hexane into an ethanol solution. As shown in Fig. 1, the coordination geometry
around the zinc center is distorted tetrahedral, with bond angles in the range of 93.14(9) –
114.47(8)°. The structure is similar to that of the corresponding cobalt and nickel complexes [65–
67]. The bond lengths C7–N1 (1.279(4) Å) and C14–N2 (1.286(4) Å) are consistent with the
presence of C=N double bonds.
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Fig. 1. Molecular structure of complex 4 with thermal ellipsoids at the 50% probability level.
All the hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°)
as follows: Zn1–N1: 2.031(3), Zn1–O1: 1.973(2), Zn1–Cl1: 2.2164(11), Zn1–Cl2: 2.2260(11),
C7–N1: 1.279(4), C14–N2: 1.286(4); Cl1–Zn1–Cl2: 110.36(5), O1–Zn1–Cl1: 113.54(7), O1–
Zn1–Cl2: 110.44(7), O1–Zn1–N1: 93.14(9), N1–Zn1–Cl1: 113.94(8), N1–Zn1–Cl2: 114.47(8).
2.2. Ring-opening polymerization of ε-caprolactone
It is worth mentioning that the zinc dichloride complexes alone had no activity in the
ring-opening polymerization (ROP) of ε-CL, whether benzyl alcohol (BnOH) was involved or
not. However, addition of methyllithium activated the zinc dichloride complexes by generating
zinc methyl species in situ. We [63,64] and others [57–59] have already used this approach.
Unexpectedly, even after the addition of 2 equiv. of MeLi, complexes 1−4 bearing
2,6-bis(imino)phenol ligands remained inactive for the ROP of ε-CL, in the presence or not of
BnOH, owing to the influence of the imino groups. However, after these imino groups were
reduced to amino groups, the corresponding zinc complexes 5−7, associated with 4 equiv. of
MeLi, showed excellent catalytic activity for the ROP of ε-CL, both in the presence or not of
BnOH (Tables 1 and 2).
2.2.1. ROP of ε-CL in the absence of BnOH
In the absence of BnOH, the 5−7/MeLi catalytic systems were highly active for the ROP of
ε-CL in toluene and the monomers were completely converted into polymers within 3 min in
most cases, giving a relatively broad molecular weight distribution (Ð) (Table1). The
number-average molecular weight (Mn) of the polymers increased gradually with the
[CL]0/[Zn]0 ratio, and the solution quickly became very sticky, preventing magnetic stirring.
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The catalytic systems remained highly active over a broad temperature range (30−90 °C), but
higher reaction temperatures resulted in slightly lower Mn of PCLs (entries 3−5 in Table 1).
Table 1 ROP of ε-CL catalyzed by zinc complexes 5−7/MeLia
c
d
time
Yieldb
(%)
100
100
99.0
100
100
100
100
91.4
100
100
100
100
100
Mn,cal
Mn,obs
Entry Complex T (°C)
[CL]0/[Zn]0
Ðd
(min)
(kg/mol)
5.71
11.4
(kg/mol)
6.75
15.8
28.4
20.9
22.0
18.2
42.7
6.89
21.0
23.4
26.8
30.0
20.2
1
2
30
30
30
60
90
60
30
30
3
3
50
100
1.68
1.59
1.63
1.57
1.58
1.59
1.31
1.34
1.47
1.27
1.65
1.51
1.32
5
5
5
5
5
5
5
6
6
6
6
6
7
3
3
300
34.2
34.2
34.2
57.1
114.1
11.4
4
3
300
5
3
300
6
3
500
7
90
15
3
1000
100
8
9
200
22.8
34.2
57.1
57.1
57.1
30
30
10
11
12e
13
3
300
30
30
30
3
500
3+3
3
200+300
500
a Polymerization conditions: zinc complex: 0.02 mmol, [Zn] = 0.058 M, [MeLi]/[Zn] = 4:1, solvent: toluene.
b Determined by weight of isolated polymer.
c Mn,cal = [CL]0/[Zn]0 × 114.14 × Yield%, assuming one propagation chain per zinc atom.
d Measured by GPC in THF, calibrated with polystyrene standards and corrected using a correction factor of
0.56 [68].
e 200 equiv. of CL was first added, and after 3 min, a second batch of 100 equiv. of CL was added and the
polymerization reaction was continued for an additional 3 min.
The polymerization rate was so high that no influence of the substituent R1 at the phenol ring
on the activity could be detected, which is consistent with the substituent R1 being far away from
the metal center. Therefore, all the zinc complexes showed high activities for the ROP of ε-CL;
however, they produced PCLs with different Mn and Ð.
The catalytic systems exhibited the “immortal” and controllable properties for ROP of ε-CL in
the absence of BnOH although the Ð was relatively broad. In one experiment (entry 12 in Table
1), the ROP of a first batch of monomer (200 equiv.) was initiated with complex 6/MeLi and after
3 min, a second batch of monomer (300 equiv.) was added to the reaction mixture and the
polymerization reaction was continued for an additional 3 min. This resulted in chain extension
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and the Ð of polymers was narrower than when one batch of 500 equiv. of monomer was
converted (entries 11 and 12 in Table 1, Fig.2).
entry 9
entry 11
entry 12
20 21 22 23 24 25 26 27 28
Elution time (min)
Fig. 2. GPC traces before and after the addition of a second batch of monomer (entries 9, 11
and 12 in Table 1)
2.2.2. ROP of ε-CL in the presence of BnOH
In the presence of BnOH, the current catalytic systems also showed high activity for the ROP
of ε-CL and 100% yields of polymers were obtained in most cases (Table 2). It is worthy to note
that the PCLs obtained in the presence of BnOH had much narrower molecular weight
distributions than those obtained in the absence of BnOH, which suggested that the
polymerization reactions under these conditions proceeded in a more controllable fashion. The
BnOH not only served as an initiator but also as a chain transfer agent, leading to the decrease in
the Mn of polymers. Increasing the amount of BnOH resulted in the decrease of yield and Mn of
PCLs. For instance, the yields of PCLs with lower [CL]0/[BnOH]0 values could not reach 100%
even though the polymerization reactions proceeded for a longer time and the Mn of PCLs were
also much lower (entries 1 and 8 in Table 2). Varying the polymerization temperature had only a
slight influence on the yields and Mn of the polymers; however, higher temperatures resulted in
slightly broader molecular weight distributions (entries 3−5 in Table 2). With an increase of the
[CL]0/[Zn]0/[BnOH]0 ratio, the Mn of polymers increased gradually and the Ð of polymers
changed slightly, in the range 1.08−1.58. A similar trend was observed for each complex.
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Similarly, the catalytic activity and the Mn and Ð of the polymers were only slightly affected by
the substituent R1 at the phenol ring and all the complexes (5−7) were highly active for the ROP
of CL in the presence of BnOH. In order to verify the living character of the polymerization in
the presence of BnOH, the ROP of a first batch of CL (200 equiv.) was initiated with complex
5/MeLi/BnOH. After 3 min, a second batch of CL (100 equiv.) was added to the reaction mixture
and the polymerization reaction was continued for an additional 3 min (entry 7 in Table 2).
Extension of polymer chains was observed, but both Mn and Ð of PCLs were lower than those of
the PCLs obtained in a one-batch reaction involving 300 equiv. of monomer (entries 5−7 in Table
2 and Fig. 3).
Table 2 ROP of ε-CL catalyzed by zinc complexes 5−7/MeLi in the presence of BnOHa
c
d
Entry Complex
T
(°C)
30
30
30
60
90
30
30
30
30
30
30
30
30
30
30
Time
[CL]0/[Zn]0/[BnOH]0
Yieldb
(%)
31.3
100
100
100
100
100
100
65.7
99
Mn,cal
Mn,obs
Ðd
(min)
(kg/mol)
1.89
5.82
22.9
22.9
22.9
34.4
34.4
4.61
56.6
115.2
34.4
57.2
22.5
34.4
57.2
(kg/mol)
1.58
6.77
9.86
10.4
9.22
21.6
15.6
3.17
21.7
59.2
14.8
26.0
7.60
11.5
23.5
1
2
90
3
50:1:1
100:1:1
200:1:1
200:1:1
200:1:1
300:1:1
(200+100):1:1
300:1:5
500:1:1
1000:1:1
300:1:1
500:1:1
200:1:1
300:1:1
500:1:1
1.24
1.08
1.18
1.51
1.45
1.44
1.31
1.07
1.27
1.58
1.26
1.36
1.16
1.13
1.30
5
5
5
5
5
5
5
5
5
5
6
6
7
7
7
3
3
4
3
5
3
6
7e
3
3+3
30
3
8
9
10
11
12
13
14
15
3
100
100
100
98.3
100
100
3
3
3
3
3
a Polymerization conditions: zinc complex: 0.02 mmol, [Zn] = 0.058 M, [MeLi]/[Zn] = 4:1, solvent: toluene.
b Determined by weight of isolated polymer.
c Mn,cal = [CL]0/[Zn]0 × 114.14 × Yield% + 108.14, assuming one propagation chain per zinc atom.
d Measured by GPC in THF, calibrated with polystyrene standards and corrected using a correction factor of
0.56 [68].
e 200 equiv. of CL was added in the first stage, and after 3 min, the second batch of 100 equiv. of CL was added
and the polymerization reaction was continued for an additional 3 min.
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entry 7
entry 6
entry 5
21 22 23 24 25 26 27 28 29
Elution time (min)
Fig. 3. GPC traces before and after the addition of a second batch of monomer in the
presence of BnOH (entries 5−7 in Table 2)
2.3. Characterization of PCLs
2.3.1 NMR analysis
The PCLs produced by complex 5/MeLi in the absence (entry 1 in Table 1) or presence (entry
8 in Table 2) of BnOH were analyzed by 1H and 13C NMR spectroscopy as representative samples
to determine the terminal groups in the chain ends (Figs. 4 and 5). In the 1H NMR spectra, the
typical signals of characteristic methylene protons in the CL units around 1.36 (CH2), 1.62 (CH2),
2.28 (CH2C=O), 4.03 (OCH2) ppm were observed for both samples (Fig. 4). No signals
corresponding to any end group could be found in the 1H NMR spectra of PCLs obtained in the
absence of BnOH. However, in the 1H NMR spectra of PCLs obtained in the presence of BnOH,
the triplet at 3.72 ppm was assigned to the protons of –CH2OH groups in the chain ends and the
resonances at 7.33 ppm and 5.09 ppm were assigned to the phenyl-H and PhCH2O– protons in
the benzyl ester group, respectively.
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e
b
c
d
CDCl3
*
*
g
h
*
f
a
ppm
1
7
6
5
4
3
2
Fig. 4. 1H NMR spectra of PCLs produced by complex 1/MeLi in the absence (top, entry 1 in
Table 1) or presence (bottom, entry 8 in Table 2) of BnOH. *, impurity.
The 13C NMR spectra of both samples showed signals at 24.5 (CH2), 25.5 (CH2), 28.3 (CH2),
34.1 (CH2C=O), and 64.1 (OCH2) ppm for the methylene-C atoms and 173.5 ppm (C=O) for the
carbonyl-C atom in the CL repeating units (Fig. 5). There were no other signals in the 13C NMR
spectra of the PCLs obtained in the absence of BnOH. For the PCLs obtained in the presence of
BnOH, the peaks at 128.5, 128.1 and 67.9 ppm were assigned to the phenyl-C and methylene-C
(PhCH2O) atoms of the benzyl ester group, respectively. The resonances of the terminal CH2OH
group in the chain ends appeared at 62.5 ppm. Compared with the main signals of repeating CL
units, the signals of carbon atoms in the last CL unit were slightly shifted. In summary, both 1H
13
and C NMR analyses indicated that the PCLs obtained in the absence of BnOH were mainly
cyclic, whereas the polymers obtained in the presence of BnOH were linear PCLs with benzyl
alcohol as an end group. This implies that the monomer inserted into the Zn−OBn bond formed
after the addition of BnOH, thus initiating the polymerization. However, in the absence of BnOH,
monomer insertion occurred into the Zn−L bond and was followed by repeated monomer
insertion. The final intramolecular transesterification led to the formation of cyclic polymers.
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c,d,e
b
f
*
a
g
h
i
ppm
25
175
150
125
100
75
50
Fig. 5. 13C NMR spectra of PCLs produced by complex 5/MeLi in the absence (top, entry 1 in
Table 1) or presence (bottom, entry 8 in Table 2) of BnOH
2.3.2 MALDI-TOF mass analysis
The MALDI-TOF mass spectra of PCLs obtained by complex 5/MeLi in the absence (entry 1
in Table 1) or presence (entry 8 in Table 2) of BnOH are shown in Figs. 6 and 7. As shown in Fig.
6, the major signals at m/z 6200.8 (n = 54), 6314.8 (n = 55), 6428.7 (n = 56), ect. were attributed
to the cyclic PCLs ((CL)n) clustered with K+ and the adjacent signals were separated by a CL
repeating unit. There are two major series of signals in Fig. 7, located at m/z 2739.1 (n = 23),
2853.1 (n = 24), 2967.2 (n = 25), etc. and m/z 2755.0 (n = 23), 2869.1 (n = 24), 2983.2 (n = 25),
etc., which correspond to linear PCLs capped with BnO and hydroxyl end groups (BnO(CL)nOH)
clustered with Li+ and Na+, respectively. The successive series showed that there was a difference
of 114 in m/z between every neighbor peaks, which corresponds to the molecular weight of the
CL monomer. The other small peaks were probably produced by trace impurities generated
during the polymerization process. Gratifyingly, the MALDI-TOF mass analysis was consistent
with the results of NMR analysis.
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(CL)n, K+
4000
5000
6000
m/z
7000
8000
Fig. 6. MALDI-TOF mass spectrum of PCL samples prepared with complex 5/MeLi
(entry 1 in Table1)
BnO(CL)nH, Li+
BnO(CL)nH, Na+
2000
2400
2800
3200
3600
4000
m/z
Fig. 7. MALDI-TOF mass spectrum of PCL samples prepared with complex 5/MeLi in the
presence of BnOH (entry 8 in Table 2)
2.3.3. Thermal properties
Differential scanning calorimetry (DSC) was applied to analyze the thermal properties of
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cyclic and linear PCLs with similar molecular weight (entry 10 in Table 1 and entry 15 in Table
2). The DSC curves in Fig. 8 showed that the two samples had similar melting point around 55 °C
(Tm, determined on the second heating scan). In order to determine the thermal stability of the
polymers, a thermo-gravimetric analysis (TGA) of the cyclic and linear PCLs was carried out
under a nitrogen atmosphere (Fig. 9). The results indicated that the cyclic PCLs exhibited better
thermal stability than linear PCLs, with an initial degradation temperature of ca. 290 °C and
269 °C (5% weight loss), respectively. There were two thermal degradation stages observed for
both samples and a similar complete degradation temperature was found at ca. 420 °C, with a
fastest degradation rate at ca. 392 °C.
0.5
0.0
linear PCLs
-0.5
cyclic PCLs
-1.0
-1.5
-2.0
55.6 °C
-2.5
-3.0
55.2 °C
-3.5
-60
-40
-20
0
20
40
60
80
100
Temperature (°C)
Fig. 8. DSC curves of PCLs obtained from entry 10 in Table 1 and entry 15 in Table 2
100
cyclic PCLs
80
60
40
20
0
linear PCLs
100
200
300
400
500
Temperature (°C)
Fig. 9. TGA of polymers PCLs obtained from entry 10 in Table 1 and entry 15 in Table 2
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3. Conclusions
Two series of tridentate ligands (imino-phenoxy ligands and amino-phenoxy ligands) and their
corresponding zinc complexes 1−7 were synthesized and characterized. Complexes 1−4 bearing
2,6-bis(imino)phenol ligands were inactive for the ROP of ε-CL in the presence or absence of
BnOH. However, in combination with 4 equiv. of methyllithium, complexes 5−7 with
2,6-bis(amino)phenol ligands were highly active for the ROP of ε-CL both in the presence and
in the absence of BnOH. The “living” polymerization mode was observed in both situations and
much narrower Ð of PCLs were obtained in the presence of BnOH. The structural analysis by
NMR and MALDI-TOF mass spectra demonstrated that the PCLs obtained in the absence of
BnOH were mainly cyclic, whereas the addition of BnOH led to the formation of linear PCLs.
The cyclic and linear PCLs with similar molecular weight had a similar melting point, but the
former had higher thermal stability.
4. Experimental section
4.1. General consideration and materials
All the manipulations of air- or moisture-sensitive compounds were carried out under an
atmosphere of nitrogen using standard Schlenk techniques. Hexamethylenetetramine,
4-methylphenol, 4-chlorophenol, 4-tert-butylphenol and sodium borohydride (from Sinopharm
Chemical Reagent Co., Ltd.), and 2,6-dimethylaniline and 2,6-di-iso-propylaniline (from
Aladdin) were directly used without further purification. Toluene from J&K Scientific Ltd. was
purified by passing through solvent purification systems (MB-SPS, MBRAUN). Anhydrous zinc
chloride (ZnCl2) and methyllithium (MeLi, 1.6 M in diethylether) were purchased from J&K
Scientific Ltd. Benzylalcohol (from J&K Scientific Ltd.) and ε-caprolactone (ε-CL, from Energy
Chemical Ltd.) were dried over 5 Å molecular sieves and CaH2, and further purified by vacuum
distillation. All other chemicals were used directly without further purification unless otherwise
stated. The starting materials, 4-methyl-2,6-diformylphenol, 4-chloro-2,6-diformylphenol and
4-tert-butyl-2,6-diformylphenol, were prepared from the corresponding 4-substituted phenols
via the Dull reaction [69]. The 2,6-bis(imino)phenol ligands (L1–L4) were synthesized through
the Schiff-base condensation reaction of 4-substituted 2,6-diformylphenols with
2,6-dimethylaniline or 2,6-diisopropylaniline according to the literatures [65-67].
1H NMR and 13C{1H} NMR spectra were recorded on a Bruker DMX-400 instrument in
d6-DMSO (for zinc complexes) or CDCl3 (other compounds and PCLs) with TMS as the internal
13

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standard. Splitting patterns are designated as follows: s, singlet; d, doublet; dd, doublet of
doublets; t, triplet; m, multiplet. FT-IR spectra were recorded on a Perkin-Elmer FT-IR 2000
spectrometer by using KBr disks in the range of 4000−400 cm-1. Elemental analysis was
performed on a Flash EA1112 microanalyzer and were the average of a minimum of two
independent measurements. The molecular weight and molecular weight distribution of PCLs
were determined by GPC using Waters 2414 series system in THF at 25 °C calibrated with
polystyrene standards. MALDI-TOF mass spectra of PCLs were performed on a Bruker ultraflex
TOF mass spectrometer equipped with a modified Nd:YAG laser (355 nm). The mass spectra
were recorded in purified THF (2 mg ml-1) at 25 °C in a linear mode using 2,5-dihydroxybenzoic
acid (DHB) as the matrix. Thermogravimetric analysis (TGA) was carried out under nitrogen
atmosphere on a PE TGA analyzer from 50 to 550 °C. The differential scanning calorimetry
(DSC) analysis was recorded on a NETZSCH 214 Polyma DSC (NETZSCH, Germany)
thermal analyzer under nitrogen atmosphere. The samples were first cooled from ambient
temperature to –100 °C at 50 °C/min and then heated to 100 °C at 10°C/min to remove the
thermal history, the samples were subsequently cooled to –100 °C at 10 °C/min and reheated to
100 °C at 10 °C/min.
4.2. Synthesis of ligands L5–L7
4.2.1. Synthesis of 2,6-bis(2,6-dimethylphenylaminomethyl)-4-methylphenol (L5)
The compound L1, 2,6-bis(2,6-dimethylphenyliminomethyl)-4-methylphenol (1.52 g, 4.1
mmol) was dissolved in anhydrous methanol (25 ml). To the solution, 4.0 equiv. of sodium
borohydride (0.62 g, 16.4 mmol) were slowly added at 0 °C in an ice-water bath. The reaction
solution was slowly warmed to room temperature and stirred for 4 h. The color of solution
changed from orange to light yellow and a pale yellow powder precipitated. A small amount of
water was added slowly to quench the sodium borohydride in excess. Methanol was removed
under reduced pressure and the residue was extracted with dichloromethane (30 ml × 4), and the
combined organic extracts were washed with saturated brine (100 ml × 2) and then dried over
anhydrous sodium sulfate and filtered. After all the volatiles were removed under reduced
pressure, a pale yellow powder (L5) was obtained. Yield 1.45 g (94.7%). FT-IR (KBr, cm-1): 3358,
3314, 2961, 2916, 2847, 1625, 1589, 1472, 1438, 1374, 1340, 1262, 1215, 1200, 1159, 1094,
1
1068, 1023, 990, 862, 839, 804, 774, 737. H NMR (400 MHz, CDCl3), δ (ppm): 7.09 (d, 4 H, J
= 7.6 Hz, Ar-H), 6.97 (s, 2 H, Ar-H), 6.96 (t, 2 H, J = 7.6 Hz, Ar-H), 4.17 (s, 4 H, CH2NH), 2.43
(s, 12 H, CH3), 2.30 (s, 3 H, CH3). 13C NMR (100 MHz, CDCl3), δ (ppm): 153.7, 144.9, 130.9,
14

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129.0, 128.9, 128.4, 124.8, and 123.4 (Ar-C), 50.3 (CH2NH), 20.34 and 18.3 (Ar-CH3). Anal.
Calcd. for C25H30N2O (374.53): C, 80.17; H, 8.07; N, 7.48; found: C, 79.66; H, 7.92; N, 7.30.
4.2.2. Synthesis of 2,6-bis(2,6-dimethylphenylaminomethyl)-4-tert-butylphenol (L6)
By using the similar procedure described for L5, ligand L6 was obtained in the yield of 86.3%
by the reduction reaction of compound L2. FT-IR (KBr, cm-1): 3379, 3313, 2960, 2863, 1624,
1
1593, 1474, 1392, 1363, 1290, 1260, 1201, 1096, 1064, 1026, 984, 879, 820, 766. H NMR
(400 MHz, CDCl3), δ (ppm): 7.13 (s, 2 H, Ar-H), 7.05 (d, 4 H, J = 7.6 Hz, Ar-H), 6.96 (t, 2 H, J =
7.6 Hz, Ar-H), 4.19 (s, 4 H, CH2NH), 2.41 (s, 12 H, CH3), 1.30 (s, 9 H, C(CH3)3). 13C NMR (100
MHz, CDCl3), δ (ppm): 153.7, 144.9, 130.8, 129.0, 128.2, 125.4, 124.3, and 123.3 (Ar-C), 50.6
(CH2NH), 33.9 (C(CH3)3), 31.5 (C(CH3)3), 18.4 (Ar-CH3). Anal. Calcd. for C28H32N2O (416.61):
C, 80.73; H, 8.71; N, 6.72; found: C, 80.81; H, 8.69; N, 6.75.
4.2.3. Synthesis of 2,6-bis(2,6-dimethylphenylaminomethyl)-4-chlorophenol (L7)
By using the similar procedure described for L5, ligand L7 was obtained in the yield of 88.4%
by the reduction reaction of compound L3. FT-IR (KBr, cm-1): 3360, 3312, 2962, 2915, 2853,
1624, 1592, 1468, 1434, 1377, 1339, 1261, 1236, 1198, 1095, 1067, 1022, 913, 867, 839, 803,
1
775, 730. H NMR (400 MHz, CDCl3), δ (ppm): 7.13 (s, 2 H, Ar-H), 7.07 (d, 4 H, J = 7.6 Hz,
Ar-H), 6.95 (t, 2 H, J = 7.6 Hz, Ar-H), 4.14 (s, 4 H, CH2NH), 3.68 (br, 2 H, CH2NH), 2.40 (s, 12
H, CH3). 13C NMR (100 MHz, CDCl3), δ (ppm): 154.7, 144.4, 130.9, 129.0, 127.9, 126.5, 123.8,
and 123.7 (Ar-C), 49.9 (CH2NH), 18.3 (Ar-CH3). Anal. Calcd. for C24H27ClN2O (394.94): C,
72.99; H, 6.89; N, 7.09; found: C, 72.79; H, 6.59; N, 7.06.
4.3. Synthesis of Zinc Complexes (1–7)
One equivalent of anhydrous Zinc chloride (0.25 g, 1.8 mmol) was added to a stirred solution
of the corresponding ligand (1.8 mmol) in anhydrous ethanol (40 mL), and the reaction mixture
was stirred at room temperature for 3 h. The resulting precipitate was collected by filtration, and
the filtrate was washed three times with n-hexane then dried in vacuo to give the corresponding
zinc complex. All the complexes were prepared in this manner.
Complex 1: a yellow solid in 59.7% yield. FT-IR (KBr, cm-1): 3448, 3041, 2969, 2921, 1634,
1605, 1588, 1543, 1498, 1471, 1386, 1351, 1324, 1277, 1232, 1184, 1093, 1070, 1047, 1000, 875,
810, 776. 1H NMR (400 MHz, CDCl3), δ (ppm): 8.43 (s, 1 H, CH=N), 8.08 (s, 1H, CH=N), 7.56
(s, 1 H, phenol-H), 7.46 (s, 1 H, phenol-H), 7.25–7.06 (m, 6 H, Ar-H), 2.45 (s, 6 H, Ar-CH3), 2.36
15

ACCEPTED MANUSCRIPT
(s, 6 H, Ar-CH3), 2.29 (s, 3 H, Ar-CH3). 13C NMR (101 MHz, CDCl3), δ (ppm): 172.4 (CH=N),
170.9 (CH=N), 165.7, 148.2, 147.6, 141.7, 135.1, 131.9, 130.1, 129.7, 129.5, 128.6, 126.7, 126.1,
121.9, and 116.8 (Ar-C), 19.7 (Ar-CH3), 18.9 (Ar-CH3), 18.8 (Ar-CH3). Anal. Calcd. for
C25H26Cl2N2OZn (506.78): C, 59.25; H, 5.17; N, 5.53; found: C, 59.48; H, 5.66; N, 5.55.
Complex 2: a yellow solid in 58.0% yield. FT-IR (KBr, cm-1): 3443, 2961, 1635, 1540, 1465,
1
1370, 1329, 1296, 1234, 1185, 1091, 1058, 1029, 990, 885, 830, 781. H NMR (400 MHz,
DMSO): δ 10.43 (s, 1 H, Ar-OH), 8.74 (s, 1 H, CH=N), 8.68 (s, 1 H, CH=N), 8.09–7.85 (m, 2 H,
Ar-H), 7.15–6.98 (m, 4 H, Ar-H), 6.79–6.39 (m, 2 H, Ar-H), 2.17–2.06 (m, 12 H, Ar-CH3), 1.32
(d, J = 15.2 Hz, 9 H, C(CH3)3). 13C NMR (101 MHz, DMSO), δ (ppm): 168.1 and 161.7 (CH=N),
146.8, 143.7, 141.3, 136.5, 128.14, 128.4, 128.05, 127.81, 127.76, 127.3, 125.3, 124.4, 123.0,
120.8, 119.7, and 116.1 (Ar-C), 34.0 (C(CH3)3), 31.1 and 30.9 (C(CH3)3), 18.12, 18.06, and 17.8
(CH3). Anal. Calcd. for C28H32Cl2N2OZn (548.86): C, 61.27; H, 5.88; N, 5.10; found: C, 61.04; H,
5.91; N, 5.00.
Complex 3: a yellow solid in 46.4% yield. FT-IR (KBr, cm-1): 3442, 3044, 2969, 1627, 1584,
1540, 1471, 1385, 1347, 1315, 1255, 1224, 1181, 1091, 1051, 885, 782. 1H NMR (400 MHz,
CDCl3), δ (ppm): 8.65 (s, 1 H, CH=N), 8.10 (s, 1 H, CH=N), 7.77 (d, J = 2.4 Hz, 1 H, Ar-H), 7.61
(d, J = 2.4 Hz, 1 H, Ar-H), 7.28 (t, J = 8.0 Hz, 1 H, Ar-H), 7.17–7.12 (m, 5 H, Ar-H), 2.43 (s, 6 H,
Ar-CH3), 2.40 (s, 6 H, Ar-CH3). 13C NMR (100 MHz, CDCl3), δ (ppm): 172.2, 169.8, 164.5,
147.3, 145.7, 139.8, 134.6, 131.9, 130.1, 129.9, 128.8, 127.1, 123.4, 120.9, and 117.8 (CH=N and
Ar-C), 19.0, and 18.9 (Ar-CH3). Anal. Calcd. for C24H23Cl3N2OZn (527.19): C, 54.68; H, 4.40; N,
5.31; found: C, 54.25; H, 4.58; N, 4.73.
Complex 4: an orange powder in 43.3% yield. FT-IR (KBr, cm-1): 3462, 2964, 2922, 2866,
1628, 1579, 1537, 1456, 1361, 1325, 1293, 1257, 1182, 1104, 1055, 1009, 931, 856, 791, 758,
726. 1H NMR (400 MHz, CDCl3), δ (ppm): 8.23 (s, 1 H, CH=N), 8.16 (s, 1 H, CH=N), 7.70, (s, 1
H, Ar-H), 7.55 (s, 1 H, CH=N), 7.40 (d, J = 8.0 Hz, 1 H, Ar-H), 7.35–7.27 (m, 5 H, Ar-H), 3.39
(sept, 1 H, J = 6.8 Hz, CH(CH3)2), 3.18 (sept, 1 H, J = 6.8 Hz, CH(CH3)2), 3.06 (sept, 2 H, J = 6.8
Hz, CH(CH3)2), 1.43 (d, 3 H, J = 6.8 Hz, CH(CH3)2), 1.40 (d, 3 H, J = 6.8 Hz, CH(CH3)2), 1.35 (d,
3 H, J = 6.8 Hz, CH(CH3)2), 1.28 (d, 12 H, J = 6.8 Hz, CH(CH3)2), 1.18 (d, 3 H, J = 6.8 Hz,
13
CH(CH3)2). C NMR (101 MHz, CDCl3), δ (ppm): 172.8, 168.6, 166.7, 160.0, 146.0, 144.5,
143.0, 140.8, 139.7, 132.40, 130.8, 127.7, 124.7, 124.4, 124.1, 123.8, 123.4, 123.2, 122.7, 120.9,
118.5, and 117.6 (CH=N and Ar-C), 28.9, 28.3, 28.0, 27.9 (Ar-CH), 25.9 (Ar-CH3), 24.0, 23.6,
23.5, 23.0, and 22.4 (CH3). Anal. Calcd. for C33H42Cl2N2OZn (618.99): C, 64.03; H, 6.84; N,
4.53; found: C, 64.16; H, 6.99; N, 4.53.
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Complex 5: a beige powder in 45.4% yield. FT-IR (KBr, cm-1): 3443, 2918, 1633, 1483, 1378,
1260, 1186, 1155, 1099, 1025, 864, 769. 1H NMR (400 MHz, DMSO), δ (ppm): 7.07–6.90 (m, 8
H, Ar-H), 4.31 (s, 4 H, CH2NH), 2.45 (s, 12 H, Ar-CH3), 2.24 (s, 3 H, Ar-CH3). 13C NMR (100
MHz, CDCl3), δ (ppm): 139.6, 131.8, 131.7, 131.1, 130.6, 130.1, 129.6, 129.3, 129.1, 128.7,
128.5, 128.4, 126.7, 124.6, and 121.1 (Ar-C), 52.3 (CH2NH), 20.4, 18.6, 17.9. Anal. Calcd. for
C25H30Cl2N2OZn (510.81): C, 58.78; H, 5.92; N, 5.48; found: C, 58.74; H, 6.00; N,5.43.
Complex 6: a light brown powder in 53.8% yield. FT-IR (KBr, cm-1): 3444, 2960, 2864, 1637,
1486, 1441, 1363, 1299, 1213, 1181, 1103, 1027, 883, 821, 766. 1H NMR (400 MHz, CDCl3), δ
(ppm): 7.19 (s, 2 H, Ar-H), 6.92 (d, 4 H, J = 7.6 Hz, Ar-H), 6.74 (t, 2 H, J = 7.6 Hz, Ar-H), 4.40 (s,
4 H, CH2NH), 2.46 (s, 12 H, Ar-CH3), 1.26 (s, 9 H, C(CH3)3). 13C NMR (100 MHz, CDCl3), δ
(ppm): 142.5, 138.9, 131.8, 129.6, 128.4, 127.2, 125.1, and 121.6 (Ar-C), 52.5 (CH2NH), 34.0
(C(CH3)3), 31.4 (Ar-CH3), 18.4 (C(CH3)3). Anal. Calcd. for C28H36Cl2N2OZn (552.89): C, 60.83;
H, 6.56; N, 5.07; found: C, 61.00; H, 6.07; N, 4.92.
Complex 7: a beige solid in 40.9% yield. FT-IR (KBr, cm-1): 3422, 3297, 3248, 2961, 2919,
1637, 1618, 1584, 1473, 1437, 1380, 1260, 1234, 1176, 1093, 1073, 1028, 871, 800, 768. 1H
NMR (400 MHz, DMSO), δ (ppm): 6.96–6.77 (m, 6 H, Ar-H), 6.39 (t, J = 7.6 Hz, 2 H, Ar-H),
4.48 (s, 4 H, CH2NH), 2.06 (s, 12 H, Ar-CH3). 13C NMR (100 MHz, DMSO), δ (ppm): 144.1,
127.7, 120.4, 115.7 (Ar-C), 17.8 (Ar-CH3). Anal. Calcd. for C24H27Cl3N2OZn (531.22): C, 54.26;
H, 5.12; N, 5.27; found: C, 54.52; H,5.84; N, 5.51.
4.4. Typical procedure for ring-opening polymerization of ε-CL
In a typical polymerization experiment, a suspension of zinc complex (0.02 mmol) in purified
toluene was charged in a 100 mL sealed Schlenk flask with a magnetic stir bar under nitrogen. To
this solution, a certain amount of methyllithium solution (MeLi, 0.04 mol/L in toluene,
[MeLi]0/[1–4]0 = 2 or [MeLi]0/[5–7]0 = 4) was added dropwise at a temperature of –10 °C
controlled with a thermostat. The mixture was slowly warmed to room temperature and stirred for
2 h; it consisted of a yellow solution and white precipitate. After that, BnOH (if needed) and
ε-caprolactone were added to the above suspension in sequence at a certain temperature and the
polymerization time was measured from that point. After polymerization, the reaction was
quenched with deionized water (0.5 mL) and then poured into n-hexane to precipitate the polymer.
The white precipitate was collected by filtration under reduced pressure, dissolved by a few drops
of CH2Cl2 and then n-hexane (3ml) was added to precipitate the polymer; this process was
repeated three times. The polymer was finally dried under vacuum.
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Cyclic PCLs: 1H NMR (400 MHz, CDCl3): δ (ppm): 4.04 (t, J = 6.8 Hz, 2n H, −OCH2−), 2.28
(t, J = 7.6 Hz, 2n H, −CH2C=O), 1.66−1.58 (m, 4n H, −CH2−), 1.40−1.32 (m, 2n H, −CH2−). 13C
NMR (100 MHz, CDCl3), δ (ppm): 173.5 (C=O), 64.1 (OCH2), 34.1, 28.3, 25.5, and 24.5
(−CH2−).
Linear PCLs: 1H NMR (400 MHz, CDCl3): δ (ppm): 7.34−7.29 (m, 5 H, Ph-H), 5.09 (s, 2 H,
PhCH2O−), 4.03 (t, J = 6.8 Hz, 2n H, −OCH2−), 3.72 (t, J = 6.4 Hz, 2 H, terminal −CH2OH), 2.28
(t, J = 7.6 Hz, 2n H, −CH2C=O), 1.67−1.58 (m, 4n H, −CH2−), 1.39−1.32 (m, 2n H, −CH2−). 13C
NMR (100 MHz, CDCl3), δ (ppm): 173.5 (C=O), 136.2, 128.5, 128.14, and 128.11 (Ph-C), 67.9
(PhCH2O), 64.1 (OCH2), 34.0, 28.3, 25.5, and 24.5 (−CH2−).
4.5. X-ray crystallographic analysis
Brown crystals of the zinc complex 4 suitable for single X-ray diffraction analysis were
obtained by slow diffusion of n-hexane into their ethanol solution. The crystallographic data for 4
were collected at 296 K on a Bruker Smart Apex II CCD diffractometer with graphite
monochromated Mo-Kα radiation (λ = 0.71073 Å). Cell parameters were obtained by global
refinement of the positions of all collected reflections. Intensities were corrected for Lorentz and
polarization effects and empirical absorption. The structure was solved by direct methods and
refined by full-matrix least-squares on F2. All non-hydrogen atoms were refined anisotropically.
All the hydrogen atoms were placed in calculated positions. The crystallographic data have been
deposited within the Cambridge Crystallographic Data Center (CCDC number 1868711) and are
accessible free of charge via www.ccdc.cam.ac.uk/data_request/cif.
Crystal data for complex 4. C33H42Cl2N2OZn, Mw = 618.95 g mol-1, T = 293(2) K,
monoclinic crystal system, space group P21/n, a = 15.0619(12) Å, b = 16.7063(13) Å, c =
18.6514(12) Å, α = γ = 90 º, β = 92.587(7) º, V = 4688.4(6) Å3, Z = 4, Dcalc = 0.877 Mg m-3, µ =
0.656 mm-1, F(000) = 1304, 8543 reflections collected, 4983 unique which were used in all
calculations. GOF = 1.033, Final R indices [I>2σ(I)] R1 = 0.0523, wR2 = 0.1350, R indices (all
data) R1 = 0.0933, wR2 = 0.1609. Largest diff. peak and hole 0.452 and –0.267 e Å–3.
Acknowledgements
This study was financially supported by the National Key R&D Program of China (No.
2016YFC1100801) and the State Key Laboratory of Chemical Engineering (No.
SKL-ChE-18D01).
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Highlights
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ó
2,6-Bis(amino)phenol ligands and their zinc complexes were synthesized.
In combination with MeLi, zinc complexes were highly efficient for the ROP of
ε-CL.
ó
ó
The PCLs obtained in the absence of BnOH were mainly cyclic.
The linear PCLs were produced in the presence of BnOH.