Tailoring Molecular Weight of Bioderived Polycarbonates via

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Tailoring molecular weight of bio-derived polycarbonates via bifunctional ionic liquids catalysts under metal-free conditions Congkai Ma, Fei Xu, Weiguo Cheng, Xin Tan, Qian Su, and Suojiang Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04284 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 7, 2018

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ACS Sustainable Chemistry & Engineering

Tailoring molecular weight of bio-derived polycarbonates via bifunctional ionic liquids catalysts under metal-free conditions Congkai Ma,†,‡,§ Fei Xu,† Weiguo Cheng,*,† Xin Tan,†, § Qian Su, †, § and Suojiang Zhang*,†,§

†

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, 1 North 2nd Street, Zhongguancun, Haidian District, Beijing 100190, China.

‡

Sino Danish College, University of Chinese Academy of Sciences, 380 Huaibeizhuang, Huairou District, Beijing 101408, China

§School

of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, Beijing 100049, China

Corresponding authors’ email address: *Weiguo Cheng: [email protected] *Suojiang Zhang: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Synthesis of bio-derived high-molecular-weight polycarbonates over metal-free catalysts is of great importance but also challenging. In this work, a series of 1-butyl-3-methylimidazolium (Bmim) ionic liquids (ILs) were prepared as catalysts for melt polycondensation reaction of isosorbide and diphenyl carbonate. By modifying the structures of ILs’ anions, the number-average molecular weight (Mn) of poly (isosorbide carbonate) (PIC) was effectively tailored. In the presence of trace amount (0.05 mol% based on isosorbide) of bifunctional [Bmim][CH3CHOHCOO], the synthesized PIC possessed high Mn of 61,700 g/mol and glass transition temperature of 174 °C, which both have been the highest so far to the best of our knowledge. Besides, it was found that the anions with stronger electronegativity and hydrogen bond formation ability, were more efficient for the formation of PIC with higher Mn. To modify the flexibility of PIC, poly(aliphatic diol-co-isosorbide carbonate)s with Mn ranging from 34,000 to 75,700 g/mol were also formulated by incorporating with various aliphatic diols. Additionally, based on the experimental results and nuclear magnetic resonance spectroscopy, a possible mechanism of cooperative nucleophilic–electrophilic activation through hydrogen bond formation and electrostatic interactions by the ILs catalyst was proposed.

KEYWORDS: Bio-derived polycarbonate, Bifunctional ionic liquids, Catalysis, Hydrogen bond


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INTRODUCTION Polycarbonate (PC) is highly versatile as a kind of engineering plastics due to its attractive properties, including high impact strength, high glass transition temperature (Tg), good transparency, broad temperature resistance, and good electrical insulation render.1 Bisphenol-A (BPA) is a critical monomer for the preparation of PC with high mechanical and excellent thermal properties. However, BPA, originated from petroleum, is increasingly subject to questioning because it adversely affects the health of human beings via acting as endocrine disruptor.2-4 As a consequence, the demand for seeking greener, safer and competitive alternatives to BPA has been raised. As sustainable materials, bio-derived polymers are drawing a rapid growing and widespread interest in recent years due to concerns regarding the likelihood of gradual depletion of petroleum resources and environmental problems.5-10 Among the renewable building blocks of the chemical platform, a few of them have shown great potential for industrialization, for instance, limonene oxide11, pinene oxide12, isosorbide13, and so forth. Isosorbide, derived from glucose by D-sorbitol dehydration, has drawn an increasing interest for polymerization processes owing to its attractive rigidity, chirality and non-toxicity.14-19 It has been widely used for the synthesis of polyester,20-22 polyurethane,23-24 polycarbonate,15,

19, 25-28

etc. As far as PC is

concerned, isosorbide is considered as an excellent candidate to replace BPA. However, the


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relatively poor reactivity of the secondary hydroxyl groups of isosorbide, makes it difficult to synthesize high molecular weight polymers.6, 8, 15 Several attempts have been made to obtain poly (isosorbide carbonate) (PIC) by developing various catalysts. Eo et al. screened the catalysts for the melt polycondensation of isosorbide with diphenyl carbonate (DPC) and cesium carbonate (Cs2CO3) showed the best catalyst performance, with number-average molecular weight (Mn) and Tg of PIC as high as 26,700 and 164 °C, respectively.26 Reaction of isosorbide with dimethyl carbonate (DMC) catalyzed by lithium acetylacetonate (LiAcac) was successfully conducted and the Mn of the obtained PIC ranged from 15,700 to 28,800.25 Zinc acetate (Zn(OAc)2), magnesium oxide (MgO) and sodium alkoxide also showed great promise to prepare PC from DPC or DMC and aliphatic diols.29 As it can be seen above, in most cases, the use of alkali (earth) metal-based catalysts is required for PIC preparation by polycondensation process because of their high efficiency. However, it is generally thought that metal-containing catalysts are likely to cause potential environmental concerns as reported in many literatures.30-32 To the best of our knowledge, reports involving high-molecular-weight PIC synthesis from isosorbide over metal-free catalysts are rare. Ionic liquids (ILs) have attracted intense interest in the areas of scientific and engineering research in the past few decades.33 More importantly, ILs show outstanding performance in catalysis and are regarded as promising alternatives for traditional alkali (earth) metal-based catalysts owing to their excellent properties of tunability and low toxicity.34-40 Particularly, imidazolium-based ILs are the most commonly studied group


4

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and one of the first to be applied in large scale facilities.39 They have been extensively used as efficacious catalysts in polymerization reactions.41-43 In the previous work of our group, several quaternary ammonium ILs had been applied as catalysts to prepare PIC.27 However, as most of the organocatalysts do, the corresponding obtained PIC showed unsatisfying Mn (lower than 20,000

g/mol)

compared

with

metal

catalysts

in

this

reaction.44

Interestingly,

1-butyl-3-methylimidazolium-2-carboxylate (Bmim-2-CO2) was reported to be used as an efficient catalyst precursor to synthesize aliphatic polycarbonates.44-45 However, the molecular weight and Tg of the obtained polycarbonate are not sufficiently high to meet the essential properties demand in engineering plastic’s applications, which have been reported that polycarbonates with weight-average molecular weight (Mw) greater than 70, 000 g/mol enable to possess useful mechanical properties.46 Additionally, the molecular weight of polycarbonates is supposed to be as high as possible to maintain favorable mechanical strength in the case of being used as rigid plastics.47 Therefore, it is necessary to develop a complete metal-free, green and highly efficacious catalytic system to synthesize PIC with high Mn and Tg by melt polycondensation approach. Herein, diverse imidazolium-based ILs with various anions were designed and synthesized. We intended to tailor the Mn of PIC over ILs by hydrogen bond formation and electrostatic interactions. After modifying the structure of anions, the Mn of obtained PIC was tailored from 27,500 to 61,700 g/mol, which offered a wider window of molecular weights valuable concerning partial replacement of BPA-based polycarbonate in various applications.


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Additionally, with cycloaliphatic diol (1,4-cyclohexane dimethanol) and linear aliphatic diols (1,4-butanediol,

1,6-hexanediol

and

diethylene

diol)

as

comonomers,

poly(aliphatic

diols-co-isosorbide carbonate)s (PAICs) were also synthesized to promote the processability of PIC. Based on the experimental results and nuclear magnetic resonance spectroscopy, a possible cooperative nucleophilic–electrophilic mechanism in the presence of catalysts was preliminarily proposed.

EXPERIMENTAL SECTION

Materials Isosorbide (98%), diethylene glycol (99%, DEG), 1,4-butanediol (99%, BD), 1,6-hexanediol (97%, HD), 1,4-cyclohexanedimethanol (cis+trans, 99%, CHDM), LiAcac (99%), Cs2CO3 (99%) and anhydrous Zn(OAc)2 (99%) were purchased from Alfa Aesar. DPC (99%), L(+)-lactic acid, formic acid, acetic acid, propionic acid, butyric acid, benzoic acid, glycolic acid, L-(+)-tartaric acid, succinic acid, citrate, biphenyl, imidazole (99%), 1-butylimidazole (98%), dimethyl carbonate (DMC, 99%) and 4-dimethylaminopyridine (DMAP, 99%) were supplied by Shanghai Aladdin Biological Technology Co., Ltd. 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD, 98%) and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD, 98%) were supplied by Sigma-Aldrich. The 1-butyl-3-methylimidazolium carboxylate ILs were synthesized and the others were purchased from Linzhou Ke Neng Co. Ltd. and Shanghai Cheng Jie Chemical Co. Ltd.


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Isosorbide was purified for three times by recrystallization in acetone, and DPC was purified once by recrystallization in anhydrous ethanol. The other chemicals were used without any further purification.

Characterization 1

H NMR and

13

C NMR spectra were recorded by a Bruker AVANCE III 700 MHz nuclear

magnetic resonance spectrometer in deuterated chloroform (CDCl3) or deuterated water (D2O) with tetramethylsilane (TMS) as internal standard. Collection of high resolution mass spectra of synthesized ILs was performed on a Bruker QTOF-Q II mass spectrometer. The number-average molecular weight, weight-average molecular weight (Mw) and its distribution (PDI=Mw/Mn) were determined by Gel Permeation Chromatography (GPC) performing on Agilent PL-GPC 50 device equipped with one guard column (PLgel 10 µm, 50 × 7.5 mm) and two mixed columns (PLgel 10 µm MIXED-B, 300 × 7.5 mm). Dimethyl formamide (DMF) was used as eluent with a flow rate of 1.0 mL/min at 40 °C and polystyrene was used to establish a calibration curve. Thermogravimetric Analysis (TGA) was performed on a DTG-60H thermal analyzer (Shimadzu, Japan) at a heating rate of 10 °C/min under a nitrogen atmosphere using an empty crucible as the reference. The glass transition temperatures of polycarbonates were estimated by a Mettler-Toledo Differential Scanning Calorimeter (DSC) instrument at a heating rate of 10 °C/min with a nitrogen gas purging (50 ml/min) and the information obtained from the second heating was used as the results. Dynamic mechanism analysis (DMA) was performed on a


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DMA-Q800 instrument. All samples were measured under the single cantilever mode at a heating rate of 3 °C/min from -140 °C with a frequency of 1 Hz. Specimens for DMA were prepared by a Haake Minijet. The temperature of cylinder and mold, the filling time and the pressure of injection were 250 °C, 80 °C, 10 s and 800 bar, respectively. The models of anion ions were built and subjected to geometry optimization based on Density Functional Theory (DFT), using the B3LYP functional and 6-31+G(d,p) basis set. The calculations of all the structures were achieved with the Gaussian 09 software package48 and the electrostatic potential fit (ESP) charges of all the atoms of the optimized structure were calculated with the Merz– Kollmann (MK) method.

ILs Catalysts Synthesis The 1-butyl-3-methylimidazolium carboxylate ILs in Table 1 were synthesized and purified based on the method reported by Xu et al.49-51 The typical procedure was as follows: the ethanol solution of 1-butyl-3-methylimidazolium hydroxide ([Bmim][OH]) was neutralized with equimolar carboxylic acids which were added dropwise. Afterwards, the mixture was stirred constantly for 24 h at room temperature. After removing water (by product) and ethanol by evaporation under reduced pressure, the obtained viscous liquid was thoroughly washed by anhydrous ether for several times followed by drying under vacuum for 24 h at 70 °C. The synthetic

methods

of

1-butyl-3-methylimidazolium-2-carboxylate


(Bmim-2-CO2),

8

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tetraethylammonium imidazolate (TEAI) and characterization results of all the ILs were shown in Supporting Information. Table 1. The structures of anions of used 1-butyl-3-methylimidazolium ILs

O

O

HCOO-

CH3CHOHCOO-

BF4-

CH3COO-

C2H5COO-

OHCH2COO-

N(CN)2-

C3H7COO-

[Tartrate]-

NTf2-

HSO4-

C6H5COO-

[Succinate]-

H2PO4-

PIC Synthesis The PIC was synthesized by one-pot melt polycondensation method, where transesterification and polycondensation reactions were conducted in the same reactor continuously (Scheme 1).25-26 Typically, isosorbide (4.38 g, 0.03 mol) and DPC (6.42 g, 0.03 mol) were charged into a three-necked 250 ml round-bottom flask equipped with a nitrogen inlet, a mechanical stirrer and a reflux condenser. During the transesterification stage, the reactants were gradually heated to 100 °C followed by addition of the catalyst. The melt mixture was stirred intensively for 5 h under ordinary pressure. In the polycondensation stage, the temperature was gradually increased to 240 °C under vacuum (< 5 kPa) and maintained for 30 min. When the reaction was complete,


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the system was cooled under nitrogen. The product was dissolved in chloroform, followed by precipitation from anhydrous methanol. The obtained white powder or floccule was dried in vacuum oven at 70 °C for 24 h.

a

Transesterification stage: Catalyst, 1 atm, 5h, 100 °C; bPolycondensation stage: Catalyst, high vacuum (< 5 kPa), 0.5h, 100 ~ 240 °C. The catalyst amount is 0.05 mol% based on isosorbide. Scheme 1. Melt polycondensation of isosorbide and DPC

PAICs Synthesis Similarly, the PAICs were synthesized by isosorbide and various aliphatic diols via one-pot method (Scheme 2). Typically, isosorbide (2.19 g, 0.015 mol), aliphatic diol (0.015 mol) and DPC (6.42g, 0.03 mol) were introduced into a 250 ml three necked flask in the transesterifiaction stage. The reaction temperature was gradually elevated to 100 °C and then the catalyst was injected into the melt mixture. The following procedures were identical to those of the PIC synthesis. The aliphatic diols applied in this work incorporated with linear diols (1,4-butanediol, 1,6-hexanediol and diethylene diol) and cycloaliphatic diols (1,4-cyclohexane dimethanol). As shown in Scheme 2, the obtained corresponding copolycarbonates were labeled as PBIC, PHIC, PGIC and PCIC, respectively.


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a

Transesterification stage: Catalyst, 1 atm, 5h, 100 °C; bPolycondensation stage: Catalyst, high vacuum (< 5 kPa), 0.5h, 100 ~ 240 °C. The catalyst amount is 0.05 mol% based on isosorbide. Scheme 2. Melt polycondensation of DPC, isosorbide and aliphatic diols

RESULTS AND DISCUSSION

ILs Catalysts Screening A variety of 1-butyl-3-methylimidazolium carboxylate ILs (0.05 mol% based on isosorbide) were used as the catalysts in the polymerization of isosorbide and DPC. The results were summarized in Figure 1 (specific data was listed in Table S1 in Supporting Information), implying that the catalytic efficiency was significantly influenced by the anions of the ILs. The ILs in Figure 1, reported to be basic ones,52 were efficient for the polymerization reaction. By adjusting the structures of anions, the Mn of PIC was tailored from 27,500 to 61,700 g/mol, offering a wider window of molecular weights useful concerning partially replace BPA-based polycarbonate in various applications. All of the obtained PIC samples displayed narrow


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dispersity with PDI in the range of 1.57 to 1.80 (Table S1). More importantly, among all these ILs, bifunctional [Bmim][CH3CHOHCOO] was the most effective with yield of the synthesized PIC reaching 99 % and Mn up to 61,700 g/mol, which has been the highest molecular weight for PIC at present to the best of our knowledge. To figure out how the anions influenced the catalytic performance, their electronegativity and structure were surveyed. All of the ILs could be separated into two series, without hydroxyl groups (ILs 1-6) and with hydroxyl groups (ILs 7-9) in Figure 1. Firstly, the effect of electronegativity in each series was investigated, respectively. The ESP charge distribution of anion ions offers desirable representation of the electrostatic potential and deformation electron density. The charges on the oxygen of COO- were also shown in Figure 1 and it was apparent that the anions with greater electronegativity trended to be more effective in each series (Figure 1, ILs 1-6 and ILs 7-9). This was because the greater electronegativity of the COO- in the anions, the more facile to activate the carbonyl groups in DPC and hydroxyl groups in isosorbide, which made the transesterification process easier. Then the influence of the hydroxyl groups on the catalytic performance was also studied. [Bmim]2[Tartrate], [Bmim][OHCH2COO] and [Bmim][CH3CHOHCOO], containing hydroxyl groups in the anions, were justified to be more efficient (the corresponding PIC with higher Mn) compared with [Bmim]2[Succinate], [Bmim][CH3COO] and [Bmim][CH3CH2COO], respectively. This was probably owing to the OH in these ILs expected to form hydrogen bonds with the carbonyl groups, which induced the electrophilic activation of DPC. Additionally, according to the catalytic performance of these six


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ILs above, it could also be concluded that the effect of hydrogen bonds outweighed that of the electronegativity. [Bmim][BF4] and [Bmim][NTf2], reported to be neutral ILs,52 showed inferior catalytic activity compared with the basic ILs (Table 2, entries 1-4). [Bmim][HSO4] and [Bmim][H2PO4], with relatively strong acidity,53-54 were found to be inactive for the reaction (Table 2, entries 5-6). In addition, the catalytic activities of several representative metal catalysts (Table 3, entries 2-4) and organocatalysts (Table 3, entries 5-9), reported to be effective for polycarbonate preparation,25-27, 44, 55 were also investigated. The performance of [Bmim][CH3CHOHCOO] was even better when compared with that of metal catalysts and it showed a distinct advantage over the studied organocatalysts. The kinetic study using [Bmim][CH3CHOHCOO], [Bmim][HCOO] and [Bmim][Succinate] as catalysts were also conducted respectively for illustration in Figure S1 and Table S2 (Supporting Information) and it was confirmed that [Bmim][CH3CHOHCOO] possessed the best reactivity owing to its ultimate reactivity rate. Overall, it was concluded that the non-acidic 1-butyl-3-methylimidazolium ILs, whose anions with stronger electronegativity and hydrogen bond formation ability, were more efficient to acquire PIC with higher Mn. Besides, lactate was considered as non-toxic pharmaceutically acceptable

anions,

therefore

[Bmim][CH3CHOHCOO]

was

considered

to

be

environmentally-friendly.56 So it was chosen for further investigation.


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Figure 1. Effect of electronegativity and hydrogen bond formation ability of ILs’ anions on catalytic performance. (Reaction conditions: transesterification stage, 5 h at 100 °C under ordinary pressure; polycondensation stage, 0.5 h at 240 °C under high vacuum (< 5 kPa); catalyst amount, 0.05 mol% based on isosorbide) Table 2. Effect of ILs’ acidity and basicity on the catalytic performancea Entry

Sample

Catalysts

Yield (%)

Mn (g/mol)

Mw (g/mol)

PDI

1

PIC-1

[Bmim][CH3CHOHCOO]

99

61,700

105,800

1.71

2

PIC-2

[Bmim][N(CN)2]

95

45,400

75,400

1.66

3

PIC-3

[Bmim][BF4]

92

21,600

36,000

1.66


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4

PIC-4

[Bmim][NTf2]

96

12,200

20,600

1.69

5

—

[Bmim][HSO4]

trace

—

—

—

6

—

[Bmim][H2PO4]

trace

—

—

—

a

Reaction conditions: transesterification stage, 5 h at 100 °C under ordinary pressure; polycondensation stage, 0.5 h at 240 °C under high vacuum (< 5 kPa); catalyst amount, 0.05 mol% based on isosorbide). Table 3. Comparison of the catalytic performance of [Bmim][CH3CHOHCOO] with that of reported metal catalysts and organocatalysts for polycarbonates synthesisa Entry

Catalysts

Yield (%)

Mn (g/mol)

Mw (g/mol)

PDI

1

[Bmim][CH3CHOHCOO]

99

61,700

105,800

1.71

2

LiAcac

93

53,000

102,200

1.93

3

Zn(OAc)2

90

45,800

83,500

1.82

4

Cs2CO3

95

27,400

47,700

1.74

5

TBD

94

38,000

70,100

1.84

6

MTBD

93

29,300

54,900

1.87

7

Bmim-2-CO2

94

26,300

48,400

1.84

8

TEAI

93

12,000

23,600

1.97

9

DMAP

65

4,600

6,600

1.43

a

Reaction conditions: transesterification stage, 5 h at 100 °C under ordinary pressure; polycondensation stage, 0.5 h at 240 °C under high vacuum (< 5 kPa); catalyst amount, 0.05 mol% based on isosorbide).


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Effect of reaction parameters The effects of transesterification time, catalyst amount and polycondensation temperature on Mn and PIC yield were investigated (Figure 2). The Mn values increased gradually from 44,900 g/mol to 61,700 g/mol with the transesterification time increasing from 1 h to 5 h. However, the Mn value decreased sharply to 49,400 g/mol when the time was extended to 6 h and further dropped to 47,100 g/mol for 7 h (Figure 2a). Besides, the polymer became brownish when the time was more than 5 h, indicating that the side reactions or polymer degradation occurred, which probably contributed to the decrease of molecular weight. The yield showed similar trend with that of the molecular weight. The results revealed that the optimum transesterification time was 5 h. To clarify the effect of catalyst amount on the polymer properties, a series of PIC samples catalyzed by different amount of [Bmim][CH3CHOHCOO] were synthesized (Figure 2b). Mn of PIC increased slightly with the increasing of catalyst amount from 0.01 mol% to 0.05 mol% based on isosorbide. However, any further catalyst accumulation failed to make the molecular weight

or

PIC

yield

increase.

The

results

demonstrated

that

the

optimum

[Bmim][CH3CHOHCOO] amount was 0.05 mol% based on isosorbide.


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Figure 2. (a) Effect of transesterification time (catalyst amount of 0.05 mol% based on isosorbide and polycondensation temperature at 240 °C); (b) Effect of catalyst amount (transesterification time of 5 h and polycondensation temperature at 240 °C); (c) Effect of polycondensation temperature (catalyst amount of 0.05 mol% mol based on isosorbide and transesterification time of 5 h) on Mn and PIC yield.

Concerning the effect of polycondensation temperature, the Mn values increased gradually with the temperature rising from 190 to 205 °C and increased drastically from 205 to 240 °C, followed by decreasing in the range of 240 to 250 °C (Figure 2c). High temperature contributed


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to reduced viscosity of the melt mixture, which might accelerate the diffusion-limited polycondensation kinetics and resulted in higher Mn.29 But too high temperature probably led to thermal degradation of polymer, resulting in decline of the Mn value from 240 to 250 °C. The change of the yield was not apparent. Therefore, the optimized polycondensation temperature was 240 °C.

Structure Characterization of PIC and PAICs The structure of the synthesized sample PIC-1 (in Table 2) was characterized by 1H NMR and 13

C NMR (Figure S2 in Supporting Information). As shown in Figure S2a, the signals on the 1H

NMR spectrum were in good correspondence with protons in the PIC molecular chain. The two multiplet signals between 7.16 and 7.43 ppm were attributed to the terminal phenol groups. The 13

C NMR of sample PIC-1 with assigned peaks was presented in Figure S2b. Interestingly, three

carbonyl signals with intensity ratio of 1:2:1 were found between 153 and 154 ppm, which resulted from that there were two asymmetric hydroxyl groups at C(2) and C(5) position (exo and endo) in isosorbide. The intensity ratio of 1:2:1 indicated the formation of endo-endo, endo-exo, exo-endo, and exo-exo irregularly connected carbonate groups in polymer, which corresponded with the reported structure.19, 25 The results of copolycondensation were summarized in Table 4. On the whole, [Bmim][CH3CHOHCOO] was found to be applicable to effectively catalyze the synthesis of PAICs as well. It showed that the molecular weight of PCIC and PHIC was higher than that of


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PIC. But the Mn and Mw of the other two copolycarbonates (PGIC and PBIC) were lower than those of PIC. The results demonstrated that the reactivity of hydroxyl groups from various monomers were disparate and 1,4-cyclohexane dimethanol seemed to have the most reactive hydroxyl groups among the four aliphatic diols. Furthermore, the comonomer molar compositions of PAICs were determined by 1H NMR. Figure 3 showed the spectra of PCIC for illustration and all the peaks were assigned. The NMR spectra of other PAICs were shown in Figure S3-S5 (Supporting Information). The compositions of PAICs samples were obtained by the integration values of individual proton peaks for repeating units. The results showed that compositions were almost consistent with the feed ratios. Additionally, the synthesized PAICs were all random copolymers (see Table S3 and Figure S6 in Supporting Information). Table 4. Results of copolycondensation of isosorbide and aliphatic diols with DPCa Sampleb

Diols

Is/Diolc

Yield (%)

Mn (g/mol)

Mw (g/mol)

PDI

PCIC

CHDM

1/1.02

99

75,700

116,100

1.53

PHIC

HD

1/0.98

95

64,200

104,500

1.63

PGIC

DEG

1/0.99

92

40,200

67,500

1.68

PBIC

BD

1/0.99

91

34,000

58,800

1.73

a

Reaction conditions: transesterification stage, 5 h at 100 °C under ordinary pressure; polycondensation stage, 0.5 h at 240 °C under high vacuum (< 5 kPa); catalyst amount, 0.05 mol% based on isosorbide). bThe feed ratio of isosorbide and aliphatic diols was 1. cComonomer molar composition was determined by 1H NMR spectroscopy.


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Figure 3. 1H NMR spectrum (a) and 13C NMR spectrum (b) of PCIC.

Thermal Properties The thermal properties of PICs and PAICs were determined by DSC and TGA. The PIC samples with different molecular weights were evaluated. The results were summarized in Table 5 and the DSC and TGA curves were shown in Figure 4. All of the PIC samples presented noticeably high Tg ranging from 152 to 174 °C, which has been the highest by now as far as we know. Besides, the Tg values of PIC with identical structure tended to be higher with increasing molecular weight.25 However, the glass temperatures of PAICs were lower than those of PIC because of the addition of aliphatic diols. The Tg of PCIC was much higher than PHIC, PGIC and PBIC owing to the rigidity of its cyclohexane group. Figure 4b showed that all the PIC and PAIC samples could be decomposed by a single-step thermal process and they were thermally stable up to 310-342 °C. The TGA curve of PIC-1 was presented for illustration. The temperatures at 5 % weight loss (Td-5%) and the maximum degradation rate (Td-max) were also displayed in Table 5. Td-5% tended to increase while Td-max


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values were almost unchanged with increasing molecular weight. In general, the thermal stability of PIC and PCIC was better than that of the PHIC, PGIC and PBIC, in which linear diols were incorporated. More importantly, a Td-max of 367-385 °C is sufficiently high for the general application of PIC and PAICs in the plastics industry.28

Figure 4. DSC curves (a) and TGA curves (b) of PIC and PAICs samples.

Dynamic Mechanical Properties The dynamic mechanical properties of the PIC-1 and PAICs were evaluated by DMA. Figure 5 presents the storage modulus (E’), loss modulus (E’’) and tan δ of the PIC-1 and PAICs samples (except for PBIC) with temperature scanning at a frequency of 1Hz. The synthesized PBIC sample was too brittle to be tested by DMA. It was noticeable that the combination of linear or cyclohexane group with PIC backbone substantially changed the dynamic mechanical properties.


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Table 5. Thermal properties of the PIC and PAICs samples

Sample

Tga(°C)

Td-5%b(°C)

Td-maxb(°C)

PIC-1

174

342

380

PIC-2

170

340

380

PIC-3

165

338

379

PIC-4

152

332

381

PCIC

109

340

385

PGIC

62

331

381

PBIC

68

310

367

PHIC

52

316

371

a

Evaluated by DSC at a heating rate of 10 °C /min (second heating). bTemperature at 5 % weight loss (Td-5%) and the maximum degradation rate (Td-max) evaluated by TGA under N2 atmosphere at a heating rate of 10 °C/min. As shown in Figure 5a, the storage modulus of PIC was much higher than that of PAICs in the whole scanned temperature range and the rigidity was in the sequence of PIC > PGIC > PCIC > PHIC, which was in good consistence with the literature.25 The results proved that the flexibility of PIC could be successfully improved by copolymerization with proper aliphatic diols. Additionally, the E’ of all the samples decreased sharply in the glass transition range. Figure 5b and 5c revealed that there were two relaxation peaks for the PIC and PAICs, corresponding to αand β-relaxation. The α-relaxation peak was related to the glass transition which was owing to the motion of long segments of polymer chains and the β-relaxation in the range of -100 °C to


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0 °C seemed to be associated with the motion of carbonate groups and end groups.25, 28 The Tg obtained by DMA in Figure 5c, defined as the temperature where tan δ achieved the maximum, almost coincided with that detected by DSC in Figure 4a.

Figure 5. Temperature dependence of E’ (a), E’’ (b) and tan δ (c) for PIC-1 and PAICs.

Possible reaction mechanism Based on the experiment results and the previous reports,44, 55, 57-58 a possible cooperative nucleophilic–electrophilic mechanism was proposed (Scheme 3). The C-2 hydrogen of the


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[Bmim]+ was expected to prompt electrophilic activation of DPC through a hydrogen bond, similar as what reported in other literature.57 This proposal was supported by the 1H NMR chemical shift of the C-2 hydrogen of [Bmim]+ which moved from 9.37 to 9.32 and further to 9.22 ppm when the molar ratios of DPC to ILs were 0:1, 1:1 and 3:1, respectively (Figure 6a). This was opposite to the expected downfield shift caused by formation of a hydrogen bond, which resulted from that the strong intermolecular hydrogen bonds of the ILs were replaced by relatively weak ones between the C-2 hydrogen of [Bmim]+ and the O-atom of DPC.59-60

Figure 6. Chemical Shift of the C-2 hydrogen of [Bmim][CH3CHOHCOO] after the addition of different amount of DPC (a) and chemical shift of DPC after the addition of different amount of [Bmim][CH3CHOHCOO] (b) in the 1H NMR spectra.


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More importantly, the COO- of the lactate enabled to induce the activation of both carbonyl group in DPC and hydroxyl groups in isosorbide. The evidence was derived from the 1H NMR of DPC (Figure 6b and Figure S7 in Supporting Information). As increasing the ratio of [Bmim][CH3CHOHCOO], the signals corresponding to part of protons on the phenyl ring of DPC were conspicuously shifted upfield from 7.48, 7.38, 7.33 ppm to 7.10, 6.80, 6.70 ppm, respectively, as shown in Figure 6b. This indicated that COO- of the lactate underwent nucleophilic attack on the carbonyl carbon of DPC. Noteworthy, the portion of shifted protons became larger when more ILs was added. The signals of the two protons of OH in isosorbide at 5.11 and 4.72 ppm could not be detected anymore once [Bmim][CH3CHOHCOO] was added (Figure S7), suggesting the two protons of OH were activated through formation of hydrogen bond with O-atom of COO-. In addition, the OH in lactate could also assist to activate the carbonyl group by formation of hydrogen bond, which could be speculated from the experiment results in Figure 1. Overall, [Bmim][CH3CHOHCOO] played a role of “electrophile nucleophile dual activation”57 through hydrogen bond formation and electrostatic interactions, making the carbon of carbonyl group in DPC more susceptible to be attacked by OH in isosorbide (Step 1). Then along with the transfer of electron, phenol was generated continually (Step 2). The molecular chain kept growing while the phenol kept being removed. Consequently, the product PIC could be obtained with more monomers involved into the system (Steps 3 & 4). During the entire process, [Bmim][CH3CHOHCOO] was highly favorable for the reaction to proceed smoothly.


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Scheme 3. Possible mechanism of the transesterification of isosorbide and DPC catalyzed by [Bmim][CH3CHOHCOO]

CONCLUSIONS In this work, diverse metal-free and environmentally friendly 1-butyl-3-methylimidazolium ILs were successfully prepared as catalysts to synthesize PIC via melt polycondensation method from isosorbide and DPC. By modifying the structures of ILs’ anions, the molecular weight of PIC was efficiently tailored. [Bmim][CH3CHOHCOO] exhibited the highest reactivity with the corresponding PIC bearing pretty high Mn of 61,700 g/mol and yield of 99 %. It was found the Bmim-based ILs with stronger electronegativity and hydrogen bond formation ability were more


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efficient. The Tg values of synthesized PIC were remarkably high (152 to 174 °C) and tended to be higher with the increasing of molecular weight. To improve the flexibility of PIC, the copolycarbonates PAICs were synthesized in the presence of [Bmim][CH3CHOHCOO] with isosorbide and equimolar aliphatic diols. Among all the PAICs, PCIC possessed the highest Tg and was more thermally stable than the other PAICs which were incorporated with linear diols. According to the experimental results and NMR spectra, a possible cooperative nucleophilic– electrophilic mechanism was proposed, which would be further studied in future research.

Supporting Information Kinetic Measurements, Characterization of ILs, PIC and PAICs, Chemical shift of isosorbide after the addition of [Bmim][CH3CHOHCOO]

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (W. Cheng). *E-mail: [email protected] (S. Zhang). ORCID Suojiang Zhang: 0000-0002-9397-954X Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (21676274 and 21506226) and National Key R&D Program of China (2016YFB0600903).

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For Table of Contents Use Only:

Synopsis: Bifunctional ionic liquid, [Bmim][CH3CHOHCOO], is highly efficient and green catalyst to prepare bio-derived polycarbonate with excellent performances.


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Tailoring Molecular Weight of Bioderived Polycarbonates via

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