A non-phosgene process for bioderived polycarbonate with high molecular weight and advanced property profile synthesized using amino acid ionic liquids as catalysts

Article abstract of DOI:10.1039/d0gc00265h

The conversion of biomass and carbon dioxide to plastics is one of the key solutions to reduce the greenhouse effect and alleviate the petroleum resource depletion. However, there is still a lack of bioderived polymers with high molecular weights and excellent performance and their corresponding green synthesis processes, which limits the potential of bioderived polymers to replace petroleum-based polymers. In this study, an eco-friendly synthetic process for bioderived polycarbonate, catalyzed by amino acid ionic liquids, was developed by utilizing isosorbide (ISO) and diphenyl carbonate (DPC) as reactants, derived from a renewable resource and carbon dioxide, respectively. By using 1-ethyl-3-methylimidazole lysine ([Emim][Lys]) as a catalyst, poly(isosorbide carbonate) (PIC) with the weight average molecular weight of 150 000, the highest reported so far to the best of our knowledge, was synthesized, and the T_g of PIC was up to 174 °C. The reaction mechanism was investigated using nuclear magnetic resonance (NMR) spectroscopy, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF MS), and density functional theory (DFT) calculation. The remarkable catalytic performance was attributed to the fact that [Emim][Lys] could effectively activate the hydroxyl group of ISO and carbonyl group of DPC, and inhibit the formation of cyclic intermediates. Moreover, to overcome the brittleness of PIC, 1,4-butanediol (BD) and 1,4-cyclohexanedimethanol (CHDM) were introduced into PIC, and the copolycarbonate showed excellent Young's modulus, ultimate tensile strength, and elongation at break, which were 979 MPa, 57 MPa, and 145%, respectively, and were comparable to the commercial petroleum-based polycarbonates. Such a process provides further industrial prospects for the next generation of bioderived polycarbonate.

Full text of DOI:10.1039/d0gc00265h

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Paul T. Anastas et al.
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PAPER
A Non-phosgene Synthetic Process for Bioderived Polycarbonate
with High-molecular-weight and Advanced Property Profile
Catalyzed by Amino Acid Ionic Liquids
Received 00th January 20xx,
Accepted 00th January 20xx
Zhencai Zhanga,b, Fei Xu*b,c, Yaqin Zhangb, Chenhao Lib, Hongyan Heb,c, Zifeng Yangb, Zengxi Li*a,b
DOI: 10.1039/x0xx00000x
The conversion of biomass and carbon dioxide to plastics is one of the key solutions to reduce the greenhouse effect and
alleviate the petroleum resource depletion. However, there is still a lack of bioderived polymers with high molecular
weight, excellent performance and their corresponding green synthesis process, thus which limits the potential of
bioderived polymers to replace petroleum-based polymers. In this paper, an eco-friendly synthetic process for bioderived
polycarbonate catalyzed by amino acid ionic liquids was developed by utilizing isosorbide (ISO) and diphenyl carbonate
(DPC) as reactants, which are derived from renewable resource and carbon dioxide, respectively. By using 1-ethyl-3-
methylimidazole lysine ([Emim][Lys]) as catalyst, poly(isosorbide carbonate) (PIC) with the weight average molecular
weight of 150 000, the highest so far to the best of our knowledge, was synthesized and the Tg of PIC was up to 174 oC. The
reaction mechanism was investigated using nuclear magnetic resonance (NMR), matrix-assisted laser
desorption/ionization time-of-flight mass spectrometry (MALDI-ToF MS), density functional theory (DFT) calculation, and
the remarkable catalytic performance was attributable to the fact that [Emim][Lys] can effectively activate the hydroxyl
group of ISO and carbonyl group of DPC, and inhibit the formation of cyclic intermediates. Moreover, to overcome the
brittleness of PIC, 1,4-butanediol (BD) and 1,4-cyclohexanedimethanol (CHDM) were introduced into PIC, and the
copolycarbonate showed excellent Young’s modulus, ultimate tensile strength, and elongation at break, which were 979
MPa, 57 MPa, and 145%, respectively, which was comparable to the commercial petroleum-based polycarbonates. Such
process provides a further industrial prospect for the next generation of bioderived polycarbonate.
www.rsc.org/
the plastic industry.3, 4
Polycarbonate (PC) as
a
commercial thermoplastic
Introduction
engineering plastic has been widely applied in electronics,
automotive, medical equipment, food packaging, aerospace,
etc., because of its unique thermal, mechanical, and optical
properties.3 However, the traditional synthetic process of PC
from petroleum-derived bisphenol-A (BPA) with phosgene
(Scheme 1(a)) or diphenyl carbonate (DPC) (Scheme 1(b)) is
facing elimination, because phosgene is a highly toxic
As the plastics have become indispensable parts of our
modern life, from materials for clothing, housing, and
automobiles to those with sophisticated applications in
medicine, diagnostics, and electronics.1, However, the vast
2
majority of these polymers are derived from fossil feedstocks,
which have diminished drastically due to indiscriminate
extraction. As these have become international issues, many
scientists are now focusing on developing eco-friendly
polymers derived from renewable sources and their green
synthetic process to appreciably improve the sustainability of
(a)
O
C
CH3
CH3
O
Catalyst
*
O
O
*
n
+
+
Cl
Cl
O
HO
(b)
OH
OH
aSchool of Chemical Engineering, University of Chinese Academy of Sciences, Beijing
100049, China.
bCAS Key Laboratory of Green Process and Engineering, State Key Laboratory of
Multiphase Complex Systems, Beijing Key Laboratory of Ionic Liquids Clean Process,
Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190,
China.
cDalian National Laboratory for Clean Energy, Dalian 116023, China.
E-mail: zxli@ipe.ac.cn, fxu@ipe.ac.cn;
Fax: +86 10 8262 7080; Tel: +86 10 8262 7080
CH3
CH3
O
Catalyst
O
*
O
O
*
n
O
HO
(c)
HO
*
O
O
ILs
O
O
+
O
O
O
O
OH
O C
O
*
n
Scheme
1 Traditional routes for the synthesis of polycarbonate (a)
polymerization of BPA with phosgene and (b) polymerization of BPA with
DPC, green route for the synthesis of bioderived polycarbonate (c)
polymerization of ISO with DPC.
Electronic Supplementary Information (ESI) available: Detailed structure
characterization results of ILs, PIC and intermediates, including NMR, FTIR, DSC, MS,
etc. See DOI: 10.1039/x0xx00000x
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compound and BPA is a low-toxic substance that affects the
health of human beings, especially children.4-8 Therefore, the
development of green non-phosgene alternative routes for the
View Article Online
DOI: 10.1039/D0GC00265H
synthesis of PCs from renewable sources is urgently needed.9,
10
Isosorbide (ISO) is defined as a sustainable monomer that is
produced from renewable resources, such as glucose, terpene
derivatives, which can be employed in the polymerization.11, 12
Because of its rigid molecular structure, chirality and lack of
toxicity, ISO has been extensively utilized in preparing
Scheme
2
The structures of 1-ethyl-3-methylimidazolium-based AAILs.
17
19
polyesters,13-15 polyurethanes,16,
and polycarbonates.18,
([Emim][Lys]) achieved excellent catalytic activity for
synthesizing PIC with Mw of 150 000, which was the highest
molecular weight among reported PICs to the best of our
knowledge, possessing attractive thermal property (glass
transition temperature, Tg = 174 °C), higher than that of BPA-
based polycarbonate. It is attributed to the improved reactivity
of reactants and the effective inhibition of the formation of
cyclic intermediates by AAILs. Furthermore, 1,4-butanediol (BD)
and 1,4-cyclohexanedimethanol (CHDM) were introduced into
PIC through copolymerization to adjust the flexibility, and the
mechanical and thermal properties were observed. The report
here for PIC represents indeed a green synthetic route, and
the high polymerization degree along with excellent
mechanical performance of product led it to further industrial
application prospect.
ISO is considered to be one of the most promising BPA
substitutes among the commercial diols for the production of
polycarbonate. More importantly, the melt polymerization of
ISO and DPC derived from excessive carbon dioxide (CO2) is a
green and environmentally friendly process, which avoids the
use of highly toxic phosgene (Scheme 1(c)). However, there are
still some challenges in the development of a new green
synthetic process for poly(isosorbide carbonate) (PIC), such as
how to prepare high molecular weight and high performance
PICs due to the low reactivity and the rigid structure of ISO.
The internal hydroxyl group (endo-OH) of ISO tends to form
intramolecular hydrogen bonding, which leads to its lower
activity than that of the external hydroxyl group (exo-OH). The
growth of chains in polymerization was severely inhibited by
different reactivity of endo-OH and exo-OH. Recently, Sun et al.
synthesized PIC with the weight average molecular weight (Mw)
of 25 600 using quaternary ammonium ionic liquid as a
catalyst.19 Yong et al. reported PIC with Mw of 32 600 was
synthesized using cesium carbonate as the catalyst.20 Zhang et
al. further improved the Mw of PIC to 66 900.21 Despite these
remarkable progresses, PICs with high polymerization degree
are still rarely reported, thus it is of great significance to study
the micro interaction between raw materials and catalyst and
Results and Discussion
Screening of the AAIL catalysts
Considering the development of a non-phosgene route of
bioderived polycarbonate to alleviate resource shortage and
environmental deterioration, the AAILs were developed as
efficient catalysts for the polymerization of ISO and DPC, and
propose
a
new design method for high efficiency
polymerization catalyst. Moreover, considering the industrial
application of bioderived PCs, the mechanical properties of PIC
synthesized from ISO and DPC are inferior. The introduction of
flexible functional groups is an important way to regulate and
improve the mechanical properties of PIC. Although some
researches on the modification of PICs have been reported,
excellent mechanical properties have rarely been investigated.
Ionic liquids (ILs) can be designed for task-specific functions
by modifying cations and anions.22-28 Due to its high thermal
stability and excellent catalytic activity, ILs have been explored
for the synthesis of polymers. In our previous work,
carboxylate anion-based IL catalysts have demonstrated to be
highly effective in activating the hydroxyl group of ISO and
carbonyl group of DPC to synthesize PIC.21 Inspired by this, a
series of amino acid ionic liquids (AAILs) containing both the
carboxylate anion and hydrogen bond donating groups such as
amino group and hydroxyl group were developed as catalysts
for the synthesis of PIC from ISO and DPC as shown in Scheme
2, which are expected to form intermolecular hydrogen
bonding with the reactant and further promote the
polymerization via activating the carbonyl and hydroxyl groups
of DPC and ISO. As expected, 1-ethyl-3-methylimidazole lysine
Table
1 Effects of the structures of AAIL catalysts on the catalytic
performancea
Molecular weightb
Yield
(%)
Entry
Catalysts
Mw
PDI
1
2
3
4
5
6
7
[Emim][Lys]
[Emim][Thr]
[Emim][Val]
[Emim][Ala]
[Emim][Ser]
[Emim][His]
[Emim][Asp]
99%
98%
96%
98%
97%
96%
91%
150 000
130 000
122 000
117 000
114 000
99 000
97 000
1.63
1.41
1.47
1.36
1.35
1.35
1.42
a(1) Transesterification: 1 atm, 150 oC, 0.5 h. (2) Polycondensation: 100
Pa, 240 oC, 0.5 h, n(DPC): n(ISO)=1: 1. bMeasured by DMF-GPC using
polystyre
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the experimental results were discussed as follows.
and streric hindrance of anions.
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DOI: 10.1039/D0GC00265H
The synthesis of bioderived PIC from ISO and DPC using
AAILs as catalysts was investigated. Firstly, the effect of acid-
base properties of anions on the catalytic performance was
investigated. The AAILs containing different amino acid anions,
including basic anions ((lysine ([Lys]-), histidine ([His]-)), neutral
anions (alanine ([Ala]-), valine ([Val]-), serine ([Ser]-), threonine
([Thr]-)), and acidic anion (aspartate [Asp]-), were applied in
the polymerization as catalyst (Table 1, entries 1-7). It was
found that the catalytic activity of AAILs containing basic
anions or neutral anions was significantly higher than that of
acidic anions. In general, the catalytic activity of AAILs for the
synthesis of PIC decreased in the following order: [Lys]-> [Thr]-
> [Ala]-> [Ser]-> [Asp]-. Considering the isoelectric point (pI)
values of [Lys]- (9.74), [Thr]- (6.53), [Ala]- (6.02), [Ser]- (5.68),
and [Asp]- (2.93), the lower activity of [Asp]- can be attributed
to its acidity, which is not conducive to polymerization. This is
consistent with the reported literature that the basicity of the
anion plays an important role in the catalytic activity of ILs.19
However, the catalytic activity of ILs containing the basic [His]-
anion is lower than that of other neutral anions, indicating that
the steric hindrance and substituted amino and hydroxyl
groups of anion may also influence the catalytic activity of ILs.
Surprisingly, [Emim][Lys] containing two amino groups
exhibited excellent catalytic activity for this polymerization,
achieving PIC with the Mw value of 150 000 and the yield up to
99%.
Effect of reaction parameters
To obtain the optimization of the reaction conditions, the
effect of various reaction parameters, including the
polycondensation temperature, the polycondensation time
and the catalyst amount on the polymerization of ISO and DPC
using [Emim][Lys] as a catalyst was investigated. Generally, the
reaction temperature is of a great importance factor that
Fig. 2 Effect of the polycondensation temperature on the synthesis of
PIC.
Fig. 3 Effect of the catalyst amount on the synthesis of PIC.
Fig. 1 Effect of the electronegativity of anions on catalytic activity.
To further investigate the influence of anions on the
catalytic performance of ILs, their electronegativity was also
studied. The Merz-Kollmann (MK) charges distribution of
anions was calculated by density functional theory (DFT) and
the charges of different anions on the group (-COOCHNH2) are
shown in Fig. 1. In the case of AAILs with [Asp]-, [His]-, [Ser]-,
[Ala]- anions, the order of molecular weight of PIC was
increased as the increase of electronegativity. However, the
catalytic activity of [Lys]- anion was the highest while its
electronegativity was lower than that of [Ala]- anion, which
indicated that higher electronegativity is helpful in promoting
the polymerization. However, the catalytic activity of AAILs
was also affect by the basicity, hydrogen bond donating groups,
Fig. 4 Effect of the polycondensation time on the synthesis of PIC.
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affects the molecular weight of PIC. Thus, the effect of the
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DOI: 10.1039/D0GC00265H
o
o
polycondensation temperature from 220 C to 260 C was
investigated. As shown in Fig. 2, when the reaction
o
temperature was 220 C, the Mw value was 70 000. The Mw
value increased significantly with rising reaction temperature
and reached up to 150 000 at 240 oC. While when the
o
temperature exceeded 240 C, the corresponding Mw value
decreased. The reason for this phenomenon may be that too
high temperature led to the thermal decomposition of PIC.29
Furthermore, the reaction system exhibited a little light yellow
color at 260 oC. This indicated that too high temperature could
aggravate side reactions and the optimized polycondensation
temperature for the synthesis of PIC was 240 oC.
The amount of the catalyst also played a significant effect on
the Mw value of PIC. The amount of the catalyst varied from
1×10-5 and 1×10-4 (the molar ratio of [Emim][Lys] to ISO). As
shown in Fig. 3, with the ascension of the [Emim][Lys] amount
from 1×10-5 to 5×10-5, the Mw value significantly increased and
the maximum Mw value was achieved up to 150 000. However,
the increase in the Mw value was not observed with the
amount of catalyst beyond 5×10-5. On the contrary, the Mw
value decreased sharply, when the amount of the catalyst was
further increased from 5×10-5 to 7.5×10-5. It probably means
that the Mw value decrease due to the decomposition of the
resulting PIC, indicating excessive catalyst is not conducive to
polymerization.30 Therefore, considering the reaction
equilibrium and the molecular weight of PIC, the optimum
amount of catalyst was found to be 5×10-5 (the molar ratio of
[Emim][Lys] to ISO).
The influence of polycondensation time on the molecular
weight of PIC catalyzed by [Emim][Lys] was investigated
through the polymerization reaction in different reaction
times. As shown in Fig. 4, the Mw values were increased with
prolonged reaction time from 0 min to 30 min. It is interesting
that the Mw and Mn values increased at the beginning, and
then the Mn value slightly changed with reaction time from 15
min to 45 min. Meanwhile, the maximum Mw value could
reach 150 000 at 30 min. However, when the reaction time
1
Fig. 5 The H NMR (a) and 13C NMR (b) spectra of PIC synthesized by
[Emim][Lys] (CDCl3).
o
connected carbonate groups.
exceeded 30 min at 240 C, the Mw value slightly decreased.
Synthesis and characterization of copolycarbonates
The obtained PIC showed slightly yellowish with reaction time
up to 60 min. The results demonstrated that prolonged time at
high temperatures resulted in an increase in side reaction and
thermal degradation of resultant PIC at the presence of the
catalyst.30
The scope of AAILs is further extended for polymerization of
DPC, ISO, CHDM, and BD. Although the copolycarbonates of
DPC, CHDM and ISO have rarely been reported through one-
pot melt polymerization, the introduction of bioderived BD to
adjust the flexibility of copolycarbonates, especially using IL
catalysts, has not been reported yet, as far as we know.
Copolycarbonates were synthesized via the transesterification
of DPC, ISO, CHDM, and BD at 150 °C followed by
polymerization at 240 °C utilizing [Emim][Lys] as the catalyst,
and named it as PIBCC as depicted in Scheme 3. In this study,
the content of CHDM was chosen as 40 mol% since the
amorphous polymer with 30-50 mol% CHDM showed
enhanced toughness.11 Different mole ratios of three diols
were selected to examine the influences on copolycarbonates
properties. The copolycarbonate with 1:5:4 mole ratios of ISO,
BD and CHDM was referred to as PIBCC-10 and other
abbreviations were similar to PIBCC-20, PIBCC-30, PIBCC-40,
Characterization of PIC
The chemical structure of the synthesized PIC sample was
1
measured by H NMR and 13C NMR spectroscopy, as shown in
Fig. 5. The resonances related to PIC at δ 3.99, δ 5.08, δ 4.88, δ
4.56, δ 5.11, δ 3.92, and δ 3.91 ppm corresponded to the
protons of the ISO moiety, which was assigned to H-1, H-2, H-
3, H-4, H-5, H-6, respectively. The peaks in the 13C NMR spectra
of PIC at δ 70.5, 73.0, 76.9, 80.9, 81.5 and 85.7 ppm were
assigned to C-1, C-2, C-3, C-4, C-5, C-6, respectively. Due to the
two different hydroxyl groups (exo-OH and endo-OH) in ISO,
the C=O group displayed three signals with intensity ratios of
1:2:1 at δ153.9, 153.6 and 153.2 ppm, attributed to the endo-
endo (a1), endo-exo/exo-endo (a2), and exo-exo (a3) irregularly
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PIBCC-50, respectively (Table 2). The chemical structure of
PIBCC-30 was characterized by 1H NMR and 13C NMR
spectroscopy as shown in Fig. 6. The peaks at δ 3.90, 1.85 ppm
from the protons of the BD moiety were assigned to H-a, H-b,
respectively. Peaks at δ 4.05, δ 5.09, δ 4.88, δ 4.54, δ 5.06, δ
3.96 ppm from the protons of the ISO moiety were assigned to
H-1, H-2, H-3, H-4, H-5, H-6, respectively. The protons of the
CHDM moiety were observed at δ 4.16, δ 1.81, δ 1.71, δ 1.66,
δ 1.51, and δ 1.44 ppm and labeled as H-7, H-8 cis (8(c)), H-9a
trans (9a(t)), H-8 trans (8(t)), H-9a cis (9a(c)), H-9b cis (9b(c)),
H-9b trans (9b(t)), respectively. Additionally, the compositions
of the copolymers contained ISO, CHDM, and BD were
calculated from the relative peak intensities of the protons on
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DOI: 10.1039/D0GC00265H
HO
OH
O
O
OH
+
+
+
HO
HO
O
O
O
OH
Transesterification 150 o
C
IL
Polymerization
240 o
C
Fig. 6 The 1H NMR (A) and 13C NMR (B) spectra of the PIBCC-30 (CDCl3).
O
O
O
O
O
O
C
O
O
C
O

O
C

O
l
m
n
Scheme 3 Copolycondensation of ISO, CHDM and BD with DPC.
Table 2 Molar composition, molecular weight and thermal properties of
copolymersa
Feed ratio
Actual ratiob
c
d
Entry
Mw
PDIc
Tg
(ISO/BD/CHDM)
(ISO/BD/CHDM)
PIBCC-10
PIBCC-20
PIBCC-30
PIBCC-40
PIBCC-50
10/50/40
20/40/40
30/30/40
40/20/40
50/10/40
14.5/28.8/56.8
24.9/42.2/32.6
31.9/37.5/30.5
41.9/18.9/39.1
49.2/9.6/41.1
15 900
19 800
73 600
53 800
105 900
1.76
1.81
1.55
1.72
1.57
9
30
50
71
98
a(1)Transesterification: 1 atm, 150 oC, 1 h (2) Polycondensation: 100 Pa, 240
oC, 1 h. bMeasured by 1H NMR. cMeasured by DMF-GPC using polystyrene
standards (RI detector). dMeasured by DSC.
each diols, and the results were summarized in Table 2.
Thermal properties
The thermal properties of the resulting PICs and
copolycarbonates were measured by DSC and TGA, as depicted
in Fig. 7(a), and the results were summarized in Table 2. The
o
glass transition temperature (Tg) of PIC reached 174 C (Fig.
Fig. 7 (a) DSC and (b) TGA curves of the copolycarbonates.
S1), which is higher than that of the traditional PC5, and the Tg
o
o
of copolycarbonates varied from 9 C to 98 C. To our delight,
the introduction of diols into PIC can effectively regulate the Tg
of copolycarbonates. It is obvious that the intermolecular
interactions and the rigid structure of ISO dramatically affect
the Tg of PIBCCs. It was found that TGA analyses of the PIC and
copolycarbonate samples could be decomposed by a single-
step thermal process (as shown in Fig. 7(b)). The results
showed that the thermal stability of the PIC and
copolycarbonates had good thermal stability under 300 C,
with 5% degradation occurring at 310-337 oC.
o
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materials will be carried out in the future.
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DOI: 10.1039/D0GC00265H
Tensile properties
Plausible reaction mechanism
To investigate the effect of flexible functional groups on
mechanical properties of PIC, the tensile stress-strain was
performed in the universal testing machines as described in
Fig. 8, and tensile strength, elongation at break and Young’s
modulus were shown in Table 3. For PIBCC-10% and PIBCC-
20%, it was very difficult to make a spline because they were
too soft. For general polymeric materials, mechanical
properties are primarily affected by the rigidity and molecular
weight.31 For PIC with relatively high ISO content, tensile tests
were not performed due to the rigid structure of the
isosorbide molecule, which limited its prospect of industrial
application. However, for copolycarbonates, a “brittle-to-
ductile” transition occurred with the decrease of ISO content.
When the content of ISO in the copolycarbonate was 50%, the
flexibility of copolycarbonate was significantly increased,
which indicated that the introduction of comonomers can
effectively adjust the flexibility of polycarbonate. For PIBCC-30
and PIBCC-50, Young’s modulus was 667 and 979 MPa,
elongation at break was 162% and 145%, respectively. As an
expected observation, PIBCC-30 exhibited lower mechanical
strength than PIBCC-50, because mechanical strength was
mainly affected by molecular weight.32 However, although a
common feature of brittleness was caused by ISO,
copolycarbonate containing 30% ISO exhibited better
flexibility, which was also consistent with the reported
conclusion that the rigidity of polymer structure will affect the
properties of polymer. Further studies on the relationship
between rigidity and molecular weight effect for polymeric
Based on the above experiment, it was believed that the
cations and anions of ionic liquids played a critical role in the
Scheme 4 Possible mechanism of polymerization of ISO and DPC catalyzed
by [Emim][Lys].
Fig. 9 1H NMR spectra of the [Emim][Lys], DPC and their mixture (DMSO,
298 K).
Fig. 8 Stress-strain curves of PIBCC-30 and PIBCC-50 at 25 °C.
Table 3 Mechanical properties of PICs
Tensile strength
(MPa)
Elongation at
break (%)
Young’s modulus
(MPa)
Sample
PIBCC-30%
PIBCC-50%
PIC
31
57
-
162
145
-
667
979
-
Fig. 10 MALDI-TOF MS of the reaction intermediate of PIC at 180 oC.
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chain propagation process, and a plausible mechanism was
proposed for the cation-anion synergistic catalytic system,
which was summarized in Scheme 4. Initially, the [Emim]+
cation activated the carbonyl groups on DPC. Meanwhile, the
hydroxyl groups of ISO were activated by lysine anion and then
nucleophilic oxygen attacked the activated carbonyl group on
DPC. Afterward, the intermediate is formed by removing one
molecule of phenol, and then the polymer unit is formed by
removing phenol continuously. Finally, the high molecular
weight PIC was obtained.
View Article Online
DOI: 10.1039/D0GC00265H
To deeply understand the interaction between the ILs and
DPC, the mixture of an equivalent of [Emim][Lys] with DPC was
1
characterized by NMR analysis. From the H NMR spectra of
[Emim][Lys] and its mixture with DPC (Fig. 9), it is obvious that
the chemical shifts of the C-2 hydrogen of [Emim][Lys] shifted
from 9.72 to 9.26 ppm due to mixing with DPC. Meanwhile,
the peaks of the protons on the phenyl ring of DPC were
drastically shifted from 7.48, 7.39, 7.33 ppm to 7.13, 6.78, 6.72
ppm, respectively, as shown in Fig. 9. These shifts indicated
the existence of the interaction between [Emim][Lys] and DPC,
and [Emim][Lys] can activate the carbonyl group on DPC by
forming hydrogen bonding, which promotes nucleophilic
attack of intermediate on the carbonyl group of DPC. To
further investigate the different activity of the ILs with the
same cation, various anions of the ILs including [Thr]-, [Val]-,
Scheme 5 DFT calculation of interaction energy between anions and endo-
OH in ISO (B3LYP/6-311++g**).
ISO was lower than that of the exo-OH. Therefore, activation
of the endo-OH of ISO is of great significance. To gain deep
insight into the interaction between AAILs and reactants, DFT
calculations were performed on the basis of the experimental
results to understand the role of the activities of different
anions with ISO (Scheme 5). To our delight, it is obvious that
the hydrogen bonding between [Lys]- and the endo-OH in ISO
is stronger than that between [Asp]- and the endo-OH in ISO,
which is consistent with the molecular weight of the
synthesized PIC. Besides, the interaction energy was also
investigated. The interaction energy between [Emim][Ala] and
isosorbide was the highest one, which was consistent with the
results of electronegativity. However, the best catalytic
performance was observed with [Emim][Lys], which further
indicated that the hydrogen bond donating group and steric
hindrance of anion also play an important role on the catalytic
1
[Ala]-, [Ser]-, [His]- and [Asp]- were performed on H NMR
analysis. As shown in Fig. S2, nearly all the proton peaks of DPC
shifted to high field in the case of [Emim][Lys], while only part
of the proton peaks of DPC shifted to high field in the case of
[Emim][Asp], which was also consistent with the experimental
results. Combined with the NMR and experimental results, it
indicated that strong interaction between catalyst and
reactant is benefited for the polymerization reaction and
affected the catalytic activity of catalysts.
performance. Finally,
a
possible synergistic catalytic
In order to better understand the reaction mechanism
extensively, MALDI-ToF MS was employed to identify the
possible intermediates during the transesterification reaction
(Fig. 10). The reaction intermediates were observed in the
MALDI-ToF MS, defined as A, B, C and D, respectively. The
repeating unit of the molecular weight of PIC was 172. As
expected, the main intermediate A was detected. Cyclic
intermediate B was also obtained which was consistent with
reported literature, which suggested that the cyclization
reaction would be continuously in competition with the chain
growth continually regardless of the transesterification stage
or the polymerization stage.15 Surprisingly, compared with the
mechanism of the cation-anion combined with the hydrogen
bond effect was proposed.
Conclusions
In this study, high-molecular-weight PIC using ISO and DPC
as raw materials were synthesized in the presence of the
[Emim][Lys] ionic liquid catalyst. The Mw of the prepared PIC
o
was 150 000 and the Tg of PIC reached up to 174 C. The
interaction between ionic liquid and reactant was confirmed
by NMR and the reaction intermediates were analyzed by
MALDI-TOF spectrometry. The results revealed that
carboxylate anion-based ionic liquids with hydrogen donating
group might efficiently inhibit the production of cyclic
intermediates and activate the carbonyl and hydroxyl groups
of reactants, which provided a new pathway for the further
design of catalyst. Furthermore, based on the experimental
results and DFT calculations, a probable cooperative cation-
anion-hydrogen bond catalytic mechanism was proposed.
Moreover, the copolymerization of BD, CHDM with PIC was
introduced to adjust the flexibility of PIC. When the mole
ratios of resulting copolycarbonate with 30% (ISO):30%
cyclic
intermediate
B
of
PIC
catalyzed
by
tetrabutylphosphonium acetate ionic liquid under the identical
reaction condition (Fig. S3), the relative content of cyclic
intermediate B catalyzed by [Emim][Lys] was much lower than
that of tetrabutylphosphonium acetate. This suggested that
the substituted group in [Emim][Lys] probably play an
important role on inhibiting the formation of cyclic
intermediate, thus synthesizing high molecular weight PIC. The
detailed study on the relationship between catalyst structure
and cyclization reaction will be carried out in the future.
According to previous reports, the activity of the endo-OH of
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ARTICLE
Journal Name
(BD):40% (CHDM), excellent tensile properties were achieved the addition of methanol.
with Young’s modulus of 667 MPa and elongation at break of Characterizations
162 %, which greatly broaden the application prospect of 1H NMR and 13C NMR spectroscopy were performed on a
View Article Online
DOI: 10.1039/D0GC00265H
bioderived polycarbonate.
Bruker Avance III-600 spectrometer using tetramethylsilane
(TMS) as the internal reference. FT-IR spectra equipped with
Thermo Nicolet 380 spectrophotometer (KBr pellets). The
Experimental Section
AXIMA-Performance
Matrix-assisted
laser
desorption/ionization time-of-flight mass spectrometry
(MALDI-ToF MS) (Shimadzu, Japan) equipped with a pulsed
nitrogen laser (337 nm) and 2,5-dihydroxybenzoic acid (DHB)
solution as the matrix. The molecular weight distributions
(Mw/Mn) were carried out by Gel permeation chromatography
(GPC) (Agilent PL GPC-50) based on polystyrene standards.
TGA performed on a DTG-60 (Shimadzu, Japan) from room
temperature to 600 °C at the rate of 10 °C/min under nitrogen
(N2) atmosphere. Glass transition temperatures (Tg) was
measured by DSC on Mettler Toledo DSC at the heating rate of
10 °C/min under N2 atmosphere. Tensile testing was measured
by universal testing machine of Instron 3365 instrument and
the crosshead speed was 5 mm/min at room temperature.The
optimal conformation of molecular structures was calculated
by the Gaussian37 program based on the B3LYP/6-311G(d,p)
level, and the Merz-Kollmann (MK) charges distribution of
anions was calculated by DFT. Moreover, the calculation of the
energy of intra- and intermolecular hydrogen bonds was
performed by the Multiwfn code.
Materials
Isosorbide (98%), N,N-dimethyl formamide (DMF, 99.7% for
HPLC) were bought from Alfa Aesar, and diphenyl carbonate
(98%), 1,4-butanediol (99%), 1,4-cyclohexanedimethanol
(99%), lysine (98%), histidine (99%), serine (98%), valine
(99%), threonine (99%), alanine (99%), aspartic acid (99%), 1-
ethyl-3-methylimidazolium bromide ([Emim][Br], 98%) were
purchased from Shanghai Aladdin Bio-chem Technology. ISO
and DPC were purified by recrystallization before use,
respectively.
Synthesis of amino acid ionic liquids
The 1-ethyl-3-methylimidazole alanine ([Emim][Ala]) was
synthesized according to the reported literatures.33-36 The 1-
ethyl-3-methylimidazolium hydroxide ([Emim][OH]) in ethanol
solution was prepared from [Emim][Br] using anion-exchange
resin, and slightly excess equimolar alanine was added in the
ethanol solution of [Emim][OH]. The mixture was stirred for 24
h. Then, water was evaporated at 60 °C. The proportion of
acetonitrile and methanol in the mixed solution was 9:1, which
was added to the mixture and washed to remove impurities.
The mixture was evaporated to remove solvents. The product
was dried in vacuo for 48 h at 60 °C. The structure of the
resulting [Emim][Ala] was confirmed by NMR spectroscopy.
Other AAILs were conducted using the same procedure. The
Acknowledgments
This work is financially supported by National Natural Science
Foundation of China (21878316), Strategic Fundation of
Chinese
Academy
of
Sciences
(Y941081135),
structures of the ILs were determined by H NMR, 13C NMR
1
"Transformational Technologies for Clean Energy and
Demonstration", Strategic Priority Research Program of the
Chinese Academy of Sciences, Grant No. XDA21031000,
National Science Fund for Excellent Young Scholars
(21922813), the DNL Cooperation Fund of CAS (DNL 180202),
and the fund of Youth Innovation Promotion Association of
CAS (2017066).
and Fourier transform infrared (FT-IR) in the Fig. S4-S24, and
the obtained spectra were in good accordance with the
anticipated chemical structures.
Synthesis of bioderived polycarbonate and copolycarbonates
The synthesis of PIC refers to the reported literature.21 The ISO
(0.03mol) and DPC (0.03mol) were added into 250 mL three-
neck flask under nitrogen (N2) atmosphere. The reaction
mixture was heated to 150 °C and held at this temperature for
0.5 h. Then the reaction temperatures increased from 150 to
180 °C while by-product, phenol, is removed through pressure
reduction. Thereafter, the temperature gradually increased to
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8 | J. Name., 2012, 00, 1-3
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Journal Name
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Bio-based polycarbonate with high molecular weight and advanced property profile was synthesizeDdObI:y10a.m10i3n9Vo/ieDw0AGrtCic0le0O2n6l5inHe
acid ionic liquids.