Catalyst screening for the melt polymerization of isosorbide-based polycarbonate

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JIEC 2854 1–5 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx

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Catalyst screening for the melt polymerization of isosorbide-based polycarbonate Q1 Yong

Seok Eo a,b, Hee-Woo Rhee a, Seunghan Shin b,*

a

Department of Chemical & Biomolecular Engineering, Sogang University, 35 Baekbeom-ro, Mapo, Seoul 04107, Republic of Korea Green Materials and Process Group, Korea Institute of Industrial Technology (KITECH), 89 Yangdaegiro-gil, Ipjang-myeon, Cheonan 31056, Chungnam, Republic of Korea b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 3 November 2015 Received in revised form 1 March 2016 Accepted 1 March 2016 Available online xxx

Isosorbide, a renewable sugar-diol, is used to make isosorbide-based polycarbonates by melt polymerization with diphenyl carbonate. To make high-molecular-weight and optically clear isosorbide-based polycarbonates, several catalysts are selected from well-known promoters of polycondensation, and their catalytic activities are evaluated. Among those chosen, cesium carbonate shows the highest catalytic activity, and its optimum concentration is determined by taking into account the molecular weight, glass transition temperature and discoloration of the resulting polycarbonates. In this study, a small amount of cesium carbonate (0.2 ppm) is sufficient to produce a high-molecularweight (Mw = 32,600) isosorbide-based polycarbonate with a high glass transition temperature (164 8C) that shows the least discoloration. Furthermore, in the case of melt polymerization, the purities of isosorbide and diphenyl carbonate also play an important role in the discoloration of isosorbide-based polycarbonates. ß 2016 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.

Keywords: Isosorbide Polycarbonate Melt-polymerization Catalyst Cesium carbonate

8 9

Introduction

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Isosorbide (ISB), which can be produced from renewable resources such as glucose, has been regarded as a promising monomer for polycondensation [1–3] because of its attractive rigidity, chirality and non-toxicity [4,5]. Therefore, ISB has been used for the synthesis of polyesters [6–8], polyurethanes [9–11], poly(ester-co-carbonate)s [12,13] and polycarbonates [14–19]. As far as polycarbonates (PC) are concerned, ISB is an excellent candidate to replace petroleum-based bisphenol A (BPA), which causes chronic toxicity problems [20]. In addition, ISB can offer excellent optical properties such as high clarity, UV stability and low birefringence and can impart rigidity to the polymer backbone to induce an enhanced glass transition temperature (Tg) [21,22]. Therefore, ISB-based PCs have superior optical properties compared to the conventional BPA-based PCs and high resistance to heat and humidity. ISB-based PCs also have outstanding scratch resistance. Despite these advantages, it is difficult to polymerize

Q2 * Corresponding author. Tel.: +82 4158988422; fax: +82 415898580. E-mail address: [email protected] (S. Shin).

ISB-based PCs because ISB is highly hydrophilic and its acidity is lower than that of BPA. Several attempts have been made to produce PC with ISB by solution and melt polymerizations. Solution polymerization has mainly been attempted because it can precisely control chemical reaction heat and viscosity and can prevent auto-acceleration reactions. However, solution polymerization requires phosgene and chlorinated solvents, which are not environmentally benign, and additional processing is needed for their removal [18]. Conversely, melt polymerization is environmentally friendly compared with solution polymerization; therefore, it has recently attracted more interest from industry. However, melt polymerization demands severe reaction conditions for the removal of condensates, which is necessary to obtain a high-molecular-weight polymer. Because severe reaction conditions occasionally induce undesirable side reactions (e.g., degradation and colorization of the resulting polymers), catalysts that can easily shift the reaction equilibrium towards polymer formation under mild conditions become important. There are several papers on catalysts for melt polymerization of ISB. Betiku et al. reported several catalysts for the synthesis of a poly(isosorbide carbonate) and lactide copolymer by melt and solvent-assisted transesterifications [23]. Li et al. made an

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48 ISB-based PC by melt polymerization with dimethyl carbonate 49 (DMC) and lithium acetylacetonate as a catalyst [24]. Ignatov et al. 50 reported the activities of different classes of transesterification 51 catalysts for BPA and diphenyl carbonate (DPC) melt transester52 ification [25], and Woo et al. reported that melt polymerization 53 of BPA and DPC in a semi-batch reactor [26]. However, there are 54 few papers on catalyst use for the melt polymerization of ISB and 55 DPC. 56 In this paper, we sought to synthesize high-molecular-weight 57 and high-Tg PC by melt polymerization of ISB and DPC. DPC is 58 chosen as a carbonyl source instead of DMC because DMC forms 59 azeotrope with methanol. This makes it difficult to remove 60 methanol from DMC and interrupts reaction equilibrium shift to 61 the polymer formation. Although DPC contains a carbonyl group 62 connected to two phenols, the transesterification of DPC and ISB 63 scarcely occurs because of the less reactive hydroxyl group on ISB. 64 To determine the proper catalyst, we tested several complexes, 65 including two representative base catalysts for polycondensation, 66 alkali metal carbonates, and lanthanum acetylacetonate 67 Q3 [La(acac)3], which shows good performance in the melt polymeri68 zation of BPA and DPC. Their catalytic activities for ISB and DPC 69 melt polymerization were evaluated, and the most promising 70 catalyst was proposed based on these results. ISB-based PCs were 71 characterized by 1H and 13C NMR spectroscopy; their average 72 molecular weights and Tgs were measured by gel permeation 73 chromatography (GPC) and differential scanning calorimetry 74 (DSC). We also investigated the effect of catalyst quantity and 75 identified the optimum catalyst content for melt polymerization of 76 ISB and DPC.

77

Experimental

78

Materials

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Isosorbide (ISB, 98%) and diphenyl carbonate (99%) were purchased from Sigma-Aldrich. Lithium hydroxide (LiOH, 98%), titanium isopropoxide [Ti(OCH(CH3)2)4, 99.999%], lanthanum acetylacetonate [La(acac)3, hydrate], lithium carbonate (LiCO3, 99%), sodium carbonate (NaCO3, 99.999%), potassium carbonate (KCO3, 99.995%), and cesium carbonate (CsCO3, 99.995%) were also purchased from Sigma-Aldrich. All reagents were used without further purification.

87

Preparation of ISB-based PCs

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ISB-based PCs were synthesized by a one-step melt polymerization method, where transesterification and polymerization occurred in the same reactor (see Fig. S1 in the ESI). Under a nitrogen atmosphere, ISB (10.484 g, 0.0717 mol), DPC (15 g, 0.0700 mol), and a catalyst (5 mg, 500 ppm based on ISB) were charged into a 100-mL reactor for polymerization that was equipped with a mechanical stirrer, a nitrogen inlet, and a Dean-Stark type condenser (for vacuum). In the transesterification stage, the reaction mixture was heated to 180 8C and stirred continuously for 30 min until the reagents presented a uniform melt. The temperature was then gradually increased to 200 8C and maintained for 30 min to remove phenol. In the polymerization stage, the temperature was gradually increased to 220 8C and maintained for 30 min under vacuum (100 Torr); then, the temperature was further increased to 240 8C and maintained for 1 h under high vacuum (<5 Torr). Before characterization, all ISBbased PCs were purified by dissolving in dichloromethane, followed by precipitation from methanol. The 1H and 13C NMR spectra of synthesized ISB-based PC are displayed in Fig. S2 in the ESI.

Catalytic activity measurements

108

First, 53.5 g (0.25 mol) of DPC and 35.78 g (0.25 mol) of ISB were introduced into a round-bottomed double-necked flask under a nitrogen blanket. The flask was then immersed in an oil bath and heated until the temperature was stabilized at 165 8C. Then, an appropriate amount of catalyst was added to the reaction melt and magnetically stirred. Small amounts of the reaction melt (0.2– 0.3 mL) were sampled throughout the reaction and then poured into small ampoules, sealed and stored immediately in a refrigerator. The concentrations of phenol in the samples obtained at different reaction times were determined by gas chromatography, and biphenyl was used as an internal standard.

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Characterizations

120

The 1H and 13C NMR spectra were recorded on a Bruker AVANCE 600 NMR spectrometer using deuterated chloroform (CDCl3) and tetramethylsilane (TMS) as an internal reference. The numberaverage (Mn) and weight-average (Mw) molecular weights and their distribution (Mw/Mn) were estimated by high-temperature chromatography (PL-GPC220) using m-cresol as an eluent and polystyrene as a standard. The glass transition temperatures (Tgs) of the polymers were measured using a differential scanning calorimeter (DSC Q20 TA Instruments) at a heating rate of 10 8C/ min with a nitrogen gas purge (50 mL/min). The concentrations of phenol in samples from different reaction times were determined by gas chromatography (Shimadzu QP 2010 ultra). Samples were diluted in dichloromethane, and biphenyl was used as an internal standard.

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Results and discussion

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Catalyst screening for ISB-based PC

136

The relatively low nucleophilicity of the hydroxyl group in ISB makes it difficult to attack the carbonyl group of DPC, which means that the transesterification of DPC and ISB scarcely occurs in the absence of a catalyst. We chose four different catalysts, namely, Ti(OCH(CH3)2)4, LiOH, La(acac)3 and CsCO3, as per previous reports [25]. Ti(OCH(CH3)2)4 and LiOH are representative Lewis acid and base catalysts for polycondensation, especially for polyester, and La(acac)3 showed good reactivity for BPA and DPC polymerization in the work of Betiku et al. [23] Based on patent claims and our preliminary results, CsCO3 was also tested as a catalyst for ISB and DPC polymerization [27]. Table 1 summarizes the results of typical transesterifications. Catalysts such as Ti(OCH(CH3)2)4 and LiOH are not effective for the transesterification of ISB and DPC in the melt, whereas La(acac)3 induces formation of a high-molecular-weight polymer from ISB and DPC, as observed in the reaction between BPA and DPC. Interestingly, CsCO3 generates a polymer with the highest molecular weight among these catalysts, and the Tg of the resulting polymer was 164 8C. The Tgs of the ISB-based PCs were determined from DSC thermograms (see Fig. S3 in the ESI). Based on the

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Table 1 Molecular weights and glass transition temperatures of ISB-based PCs prepared by melt polymerization with different catalysts (catalyst amount was 500 ppm). Catalyst

Mn (g/mol)

Mw (g/mol)

PDI

Tg (8C)

Titanium isopropoxide Lithium hydroxide Lanthanum acetylacetonate Cesium carbonate

5,960 14,100 21,800 26,700

9,100 19,900 30,500 39,500

1.51 1.64 1.40 1.48

120 154 158 164

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JIEC 2854 1–5 Y.S. Eo et al. / Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx Table 2 Molecular weights and glass transition temperatures of ISB-based PCs prepared by melt polymerization with alkali metal carbonate catalysts (catalyst amount was 500 ppm). Catalyst

Mn (g/mol)

Mw (g/mol)

PDI

Tg (8C)

Lithium carbonate Sodium carbonate Potassium carbonate Cesium carbonate

17,900 14,740 14,970 26,700

32,600 30,450 30,230 39,500

1.82 2.07 2.03 1.48

163 160 161 164

0

k

observed molecular weight and Tg, CsCO3 is a good catalyst for the melt polymerization of ISB and DPC. Considering the weak polarization of the DPC carbonyl group and the low nucleophilicity of the hydroxyl group in ISB, the Cs+ ion appears to fortify the polarization of the DPC carbonyl group by forming metal-carbonyl coordination. To understand the alkaline metal ion effect in greater detail, four different alkali metal carbonate catalysts were tested, and the results are given in Table 2. Among the four different alkali metal carbonates, CsCO3 yields a polymer with the highest molecular weight and a relatively narrow PDI value. A narrow PDI implies that CsCO3 can suppress the side reactions caused by severe reaction conditions. To understand Tables 1 and 2, kinetics studies on the transesterification of ISB and DPC with different catalysts were performed, and the catalytic activities of these catalysts were calculated.

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Catalytic activity analysis

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Catalytic activity significantly influences the equilibrium constant for the transesterification reaction and the reaction time. Consequently, the choice of catalyst is crucial for the success of this polymerization reaction. In this study, catalytic activities were determined by monitoring the condensate generated during the

½P ; ½A2o ½P½Ao 0 k 2 ðmL2 =mol minÞ ¼ ½Cat

where k’ is the effective reaction rate constant; t is the reaction time, [P] is the concentration of phenol at time t, [A]o is the initial concentration of a monomer, k is the catalytic reactivity, and [Cat] is the initial concentration of the catalyst. Fig. 1 shows the catalytic activities determined for four different catalysts. The catalytic activity (k) of LiOH is 133.1, and that of CsCO3 is 1899.3. Based on Table 1 and Fig. 1, high-molecularweight PC was obtained when a catalyst with a large catalytic activity was used. Furthermore, by removing as much phenol as possible in 50 min, the resulting polymers exhibit high molecular weights. This means that a high initial rate results in a high degree of polymerization. The initial reaction rate of CsCO3 is higher than that of the other catalysts, per its catalytic reactivity value. After 50 min, the catalytic activities become constant for all catalysts (see Fig. S4 in the ESI). Therefore, a long reaction time has little effect on the phenol volume, and the initial rate is more important for phenol removal and the degree of polymerization. Although LiOH seems to have a high catalytic activity (Fig. 1), its k value is lower than those of the other catalysts when considering the catalyst concentration. Fig. 2 shows the catalytic activities of several alkali metal carbonates towards the ISB-based PC melt polymerization. Among the alkali metal carbonates, CsCO3 shows the best catalytic activity. In terms of cationic radius and electronic polarizability, the Cs+ ion stands out against the other alkali metal ions [28,29]. It is also known that the Cs+ ion has the lowest degree of solvation and ionpairing when compared with analogous alkali metal ions. In the ISB and DPC polymerization system, CsCO3 will dissociate to yield a free Cs+ ion, which has the highest reactivity in nucleophilic

Lan(acac)3 CsCO3 10

5

0

0

10

20

30

40

50

Time (min)

k' -3 (10 (mL/mol min)

[Cat] -3 (10 (mol/mL)

k 2 2 (mL /mol min)

Lithium hydroxide

277.8

2.087

133.1

Titanium isopropoxide

176.4

0.176

1002.2

Lanthanum acetylacetonate

144.4

0.114

1266.7

Cesium carbonate

290.6

0.153

1899.3

Catalyst

180 181

(1)

LiOH Ti[OCH(CH3)2]4

15

[P]/([A]02-[P][A0])( mL/mol)

transesterification reaction and calculated according to the following equation, as suggested by Ignatov et al. [25] kt¼

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3

Fig. 1. Kinetic curves for the melt transesterification of ISB and DPC with various catalysts. The reaction rate constant was calculated as the slope of the plot obtained from the equation reported by Ignatov et al.

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[P]/([A]02-[P][A0])( mL/mol)

15

LiCO3 NaCO3 KCO3 CsCO3

10

5

0

0

10

20

30

40

50

Time (min)

k' -3 (10 (mL/mol min)

[Cat] -3 (10 (mol/mL)

k 2 2 (mL /mol min)

Lithium carbonate

115.4

0.677

170.5

Sodium carbonate

169.7

0.472

359.5

Potassium carbonate

143.3

0.362

395.9

Cesium carbonate

290.6

0.153

1899.3

Catalyst

Fig. 2. Kinetic curves for the melt transesterification of ISB and DPC using alkali metal carbonate catalysts. The reaction rate constant was calculated by the slope of a plot obtained from the equation reported by Ignatov et al.

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substitution reactions. Due to the high polarizability of the Cs+ ion, the carbonyl group of DPC is easily polarized and becomes more susceptible to an attack by secondary hydroxyl groups on ISB (see Scheme 1). OH stretching band observed at around 3500 cm1 in FTIR spectrum means resulting polymer has hydroxyl groups as its end groups (see Fig. S5 in the ESI). Catalyst solubility is also integral when attempting to understand catalytic activity. Because CsCO3 has superior solubility in aprotic polar solvents [28,29], it is expected that CsCO3 will have good solubility in the hydrophilic molten mixture of ISB and DPC.

Cs+ CO3O O

Cs+ CO3-

O

O

O HO O O

HO

O

O OH

O

OH

Cs+ CO3O

Cs+ CO3-

O

HO

O O O

OH

O

O HO O O O OH

Scheme 1. Polymerization mechanism of ISB-based PC with CsCO3 catalyst.

Higher solubility leads to greater accessibility to the monomers and thus to a higher reactivity.

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Effect of cesium carbonate content

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Although CsCO3 exhibits good catalytic activity, it causes discoloration as its concentration increases. To avoid this problem, the molecular weight and color of synthesized PCs were monitored while varying the amount of CsCO3. Table 3 shows that there are no remarkable changes in the molecular weight and Tg of ISB-based PC, despite a decrease in the catalyst to 0.2 ppm. When the concentration of CsCO3 was less than 10 ppm, the CsCO3 was added as an aqueous solution to ensure homogeneous mixing. Fig. 3 shows that the color of the polymer solution changes to light brown from dark brown as the amount of CsCO3 decreases to 10 ppm. In addition, the polymer solution becomes a brighter and more transparent yellow when 99.8% purity ISB and 99.9% purity DPC are used instead of 98% purity ISB and 99% purity DPC. In our work, 0.2 ppm CsCO3 can produce a highmolecular-weight (Mw = 32,600) and high-Tg (164 8C) ISB-based PC, and its color is also markedly improved, as displayed in Fig. 1(e). Thin film cast from 20 wt% of ISB-based PC (Fig. 1(e))

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Table 3 Cesium carbonate volume effects on the melt polymerization of ISB and DPC. Catalyst

Mn (g/mol)

Mw (g/mol)

PDI

Tg (8C)

5000 1000 500 50 10 0.2*

21,300 19,800 26,700 18,800 17,200 18,300

37,200 31,600 39,500 33,000 32,100 32,600

1.72 1.60 1.48 1.84 1.93 1.79

165 161 164 163 163 164

*

Aqueous solution.

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Fig. 3. Colors of the ISB-based PC dissolved in methylene chloride with different amounts of CsCO3 catalyst, different reaction times, and different purities of ISB and DPC: (a) CsCO3 0.05 g, 25 min; (b) CsCO3 0.05 g, 125 min; (c) CsCO3 0.01 g, 25 min; and (d) CsCO3 0.01 g, 125 min. The ISB-based PC was prepared with 0.2 ppm CsCO3 after precipitation (e).

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solution has 40 mm thickness and 91% transmittance at 550 nm (see Fig. S6 in the ESI).

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Conclusions

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An ISB-based PC was successfully synthesized by melt polymerization using ISB and DPC. Depending on the catalyst, the ISB-based PC displayed Mw values that ranged from 9100 to 39,500 g/mol, and our results showed that CsCO3 was the most effective in achieving a high-molecular-weight polymer. This result is believed to be due to the large electronic polarizability of the Cs+ ion and the good solubility of CsCO3. In our work, 0.2 ppm CsCO3 was sufficient to produce a high-molecular-weight (Mw = 32,600) and high-Tg (164 8C) ISB-based PC. Discoloration, which is critical to the melt polymerization of ISB-based PC, was influenced not only by the CsCO3 concentration but also by the purities of ISB and DPC.

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Acknowledgements

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This research was supported by the ‘‘Fusion Research Program for Green Technologies (2012 M3C1A1054496)’’ Q4 through the National Research Foundation of Korea (NRF). It Q5 was also funded by the National Research Council of Science and Technology (NST) as ‘‘Creative Convergence Research Project (CAP-11-1-KIST)’’.

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Appendix A. Supplementary data

265

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jiec.2016.03.007.

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