Synthesis of aliphatic polycarbonates by irreversible polycondensation catalyzed by dilithium tetra-tert-butylzincate

Accepted Manuscript Synthesis of aliphatic polycarbonates by irreversible polycondensation catalyzed by dilithium tetra-tert-butylzincate Miyuki Oshimura, Tomoki Hirata, Tomohiro Hirano, Koichi Ute PII:

S0032-3861(17)30987-4

DOI:

10.1016/j.polymer.2017.10.022

Reference:

JPOL 20064

To appear in:

Polymer

Received Date: 20 July 2017 Revised Date:

30 September 2017

Accepted Date: 10 October 2017

Please cite this article as: Oshimura M, Hirata T, Hirano T, Ute K, Synthesis of aliphatic polycarbonates by irreversible polycondensation catalyzed by dilithium tetra-tert-butylzincate, Polymer (2017), doi: 10.1016/j.polymer.2017.10.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Synthesis of Aliphatic Polycarbonates by Irreversible Polycondensation Catalyzed

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by Dilithium Tetra-tert-Butylzincate

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Miyuki Oshimura*, Tomoki Hirata, Tomohiro Hirano, Koichi Ute

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Department of Chemical Science and Technology, Institute of Technology and Science, Tokushima University, 2-1 Minami-josanjima, Tokushima 770-8506, Japan

*Corresponding author. Tel.: +81-88-656-7404; fax: +81-88-656-7404

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Abstract

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E-mail address: [email protected] (M. Oshimura)

Diethyl carbonate was reacted with phenol or 1,6-hexanediol with 5 mol% dilithium tetra-tert-butylzincate (TBZL) as a catalyst at 25 or 70 °C. The exchange reaction with phenol did not occur under these conditions, while that with 1,6-hexanediol did. Thus, polycondensation of diphenyl carbonate with 1,6-hexanediol was investigated. As expected, poly(hexamethylene carbonate) was obtained without removal of the 1

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by-product phenol. Conversion reached over 99% with increasing polymerization temperature. Additionally, 1,5-pentandiol and 1,9-nonanediol were also suitable diols

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for the polycondensation. Aliphatic polycarbonates with number-average molecular weights of 10,000 or more were obtained under optimum polymerization conditions.

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Polycondensation catalyzed by TBZL proceeded faster than that by titanium

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tetraisopropoxide, which is used as a general catalyst for polycondensation of diesters, dialkylcarbonates, and diphenyl carbonate with diols. Polycondensations with TBZL proceeded under atmospheric pressure in the presence of the by-product phenol. This

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polymerization system is expected to be applicable to various diols.

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Keywords: Polycondensation; Irreversible reaction; Aliphatic polycarbonate

1. Introduction

Aliphatic polycarbonates (APCs) have received much attention for various applications in biology because of their biocompatibility and biological reactivity [1]. Moreover, APCs with hydroxy end chains can also be used as a soft segment for

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polyurethane synthesis [2]. APCs are generally prepared by 1) ring-opening polymerization of cyclic carbonate monomers [3], 2) alternating copolymerization of

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CO2 with glycidyl ether monomers [4], 3) polycondensation of phosgene with diols [5], and 4) polycondensation of a carbonate with diols [6]. The fourth procedure is

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particularly versatile because it uses easily available diols and not toxic phosgene.

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However, this procedure requires a relatively high polymerization temperature (~200 °C) or reduced pressure (~200 Pa) to remove by-products to prevent the reverse reaction. From the viewpoints of environmental benignity and industrial utility, development of polymerization procedures that involve polycondensation of carbonates

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with diols under mild conditions are greatly desired. Uchiyama and coworkers reported that dilithium tetra-tert-butylzincate

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(TBZL) catalyzed allylations of haloaromatics via halogen–zinc exchange reactions [7].

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Recently, we reported the ring-opening polymerization of ε-caprolactone [8] and acylation and transesterification of alcohols with vinyl acetate and carboxylic ester [9] in the presence of catalytic amounts of TBZL. Acylation in the presence of a catalytic amount (1–10 mol%) of TBZL proceeded quantitatively at 0 °C within 1 h. Furthermore, transesterification occurred for numerous combinations of esters and primary and secondary alcohols using TBZL at 0 to −40 °C. This was the first example of successful 3

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transesterification at such low temperatures. Based on these reports, we considered the use of TBZL as a catalyst in the exchange reaction of carbonates with alcohol.

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In the present paper, synthesis of APCs catalyzed by TBZL under mild conditions is investigated. First, diethyl carbonate is reacted with phenol in the presence

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of 5 mol% of TBZL to determine whether or not the exchange reaction of carbonate

to

transesterification

of esters

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with phenol occurs. As expected, this reaction did not procced in an analogous manner with

phenol.

From

this

result,

irreversible

polycondensation of diphenyl carbonate (DPC) with diols without a tedious

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polymerization procedure to remove the by-product phenol is devised (Scheme 1).

Scheme. 1 Irreversible polycondensation of DPC with diols in the presence of TBZL.

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2. Experimental 2.1. Materials

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A THF solution of TBZL (0.31–0.39 mol L−1) was provided by Tosoh Finechem Co., Ltd. (Yamaguchi, Japan). Phenol was purchased from Kishida Chemical

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Co., Ltd. (Osaka, Japan). Diethyl carbonate (DEC; >96%), DPC (>99.4%), and

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1,5-pentanediol (1,5-PD; >98%) were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan) and dried in a desiccator before use. Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan) supplied 1,6-hexanediol (1,6-HD; >97%) and 1,9-nonanediol (1,9-ND; >98%), which were dried in a desiccator before use. Titanium tetraisopropoxide (Ti(OiPr)4;

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>97%), magnesium acetate tetrahydrate (Mg(OAc)2; >99.3%), and magnesium carbonate hydroxide (MgCO3; >40%) were purchased from Kanto Chemical Co., Inc.

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Zinc acetate (Zn(OAc)2; >98%) and magnesium hydroxide (Mg(OH)2; >99.9%) were

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purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan).

2.2. Model reaction

DEC (1.18 g, 10.0 mmol) and phenol (0.94 g, 10.0 mmol) were added to a glass ampoule that had been degassed and filled with nitrogen. A solution of TBZL (0.5 mmol) in THF was added under a nitrogen atmosphere to the ampoule placed in a 5

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constant-temperature bath. The mixture was kept at 25 or 70 °C and stirred. After 1 h, the mixture was evaporated to remove excess THF. An aliquot of the residue was diluted

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with CDCl3 and transferred to an NMR tube. The progress of the reaction was confirmed by 1H NMR spectroscopy. A reaction of DEC with 1,6-HD (1.18 g, 10.0

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mmol) instead of phenol was also carried out.

2.3. Polycondensation

A typical polycondensation procedure was as follows. DPC (2.14 g, 10.0 mmol) and 1,6-HD (1.18 g, 10.0 mmol) were added to a glass ampoule that had been

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degassed and filled with nitrogen. A solution of TBZL (0.5 mmol) in THF (1.6 mL, 0.31 mol L-1) was added under a nitrogen atmosphere to the ampoule placed in a

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constant-temperature bath. The mixture was kept at 25, 40, 50, 60, or 70 °C and stirred.

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After a prescribed period, the mixture was evaporated to remove excess THF. An aliquot of the residue was diluted with CDCl3 and transferred to an NMR tube. Reaction progress was determined by 1H NMR spectroscopy. The resulting crude solid was diluted with CHCl3 (ca. 8 mL) and poured into methanol (ca. 300 mL). The precipitated polymer was collected by centrifugation, and dried in vacuo. The copolymer yield was determined gravimetrically. When tracking the reaction using time–conversion curves, 6

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additional THF (18.4 mL) was added to the glass ampoule (monomer concentration [M]0 = 0.5 mol L−1). Additional THF was not added to increase monomer concentration

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and close to bulk condition, later than kinetic study in Figure 3 on Results and

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Discussion.

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2.4. Measurements

H NMR spectra were obtained in CDCl3 at 25 °C using an ECX-400,

ECA-400, or ECA-500 spectrometer (JEOL Ltd., Tokyo, Japan). The number-average molecular weights (Mn) and polydispersity indexes (Mw/Mn) of the polymers were

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determined using size-exclusion chromatography (SEC) calibrated against polystyrene standards. SEC was performed using an HLC 8220 chromatograph (Tosoh) equipped

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with TSK gel columns [SuperHM-M (6.5 mm ID × 150 mm) and SuperHM-H (6.5 mm

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ID × 150 mm), Tosoh] with THF as the eluent at 40 °C and a flow rate of 0.35 mL min−1. The initial polymer concentration was 1.0 mg mL−1. Matrix-assisted laser desorption/ionization–time-of-flight mass spectrometry (MALDI-TOF MS) was performed using an Autoflex Speed-TK spectrometer (Bruker Daltonics Inc.) with dithranol and sodium iodide as the matrix and ionizing agent, respectively.

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3. Results and Discussion

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3.1. Model reaction

Equimolar amounts of DEC and phenol were added to a glass ampoule. After

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adding a solution of TBZL in THF, the mixture was kept at 25 °C for 1 h. The 1H NMR

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spectrum of the reaction mixture after 1 h contained no phenyl signal of DPC at 7.4 ppm nor methylene signal of ethanol at 3.6 ppm. Although a similar reaction was also carried out at 70 °C for 1 h, DPC and ethanol were not produced even at 70 °C (Figure 1). These results suggest that the exchange reaction of carbonate with phenol does not

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procced in an analogous manner to transesterification of esters with phenol. In contrast, an exchange reaction of DEC with 1,6-HD occurred under similar reaction conditions. It

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was assumed that as a result of lower nucleophilicity and/or bulkiness of phenol than

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that of aliphatic alcohol. Namely, irreversible polycondensation of DPC with diols catalyzed by TBZL without a tedious polymerization procedure to remove phenol produced as a by-product might be possible.

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FIGURE 1 1H NMR spectra of a DEC and phenol mixture (a) before and (b) after

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addition of TBZL and reaction at 70 °C for 1 h.

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3.2. Polycondensation

FIGURE 2 1H NMR spectra of a DPC and 1,6-HD mixture (a) before and (b) after

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addition of TBZL and reaction at 25 °C for 72 h.

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The polycondensation of DPC with 1,6-HD was examined to investigate the catalytic activity of TBZL (Figure 2). Irreversible polycondensation proceeded with 5 mol% TBZL at 25 °C. Reaction conversions were determined from the ratio of the signals from the phenyl group adjacent to the carbonate group (signal b’) to the phenol signals (signal i) in 1H NMR spectra. Increasing the reaction temperature to 50 °C caused faster polymerization and higher conversion (Figure 3a). Moreover, increasing 10

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the reaction temperature caused higher n and equilibrium constant (K) values (Figure 3b). The n increased linearly with reaction time. This data is reasonable for

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polycondensation using catalysts. The MALDI-TOF MS results indicated that poly(hexamethylene carbonate) (PHC) obtained under these conditions includes cyclic

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oligomers with low molecular weight, so extra THF was not added in subsequent

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experiments to increase monomer concentration. Figure 4 shows 1H NMR spectra of PHC prepared by polycondensation of DPC with 1,6-HD catalyzed by 5 mol% TBZL reacted at 50 °C for 72 h. The numbers in parentheses indicate integral values for signals. Signals from the phenyl group of DPC (signal b’), methylene group adjacent to

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the carbonate at the chain end (signal d’), methylene group adjacent to the carbonate in the main chain (signal d), and methylene group adjacent to the hydroxy group at the

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chain end (signal d’’) were observed at 7.4, 4.3, 4.2, and 3.6 ppm, respectively. The

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signals marked with a cross originate from a small amount of residual phenol. Polymerization degree (n) and Mn of PHC determined from signal d, d’, and d’’ in the 1

H NMR spectrum were 19.1 and 3.1×103, respectively. These results show that TBZL

catalyzed polycondensation of DPC with 1,6-HD at 25–50 °C.

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FIGURE 3 (a) Time–conversion and (b) time–polymerization degree curves for polycondensation of DPC with 1,6-HD catalyzed by 5 mol% TBZL ([M]0 = 0.5 mol

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L−1).

FIGURE 4 1H NMR spectra of PHC prepared by polycondensation of DPC with 1,6-HD catalyzed by 5 mol% TBZL reacted at 50 °C for 72 h after reprecipitation.

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Figure

5

shows

MALDI-TOF

MS

data

for

PHC

prepared

by

polycondensation of DPC with 1,6-HD catalyzed by 5 mol% TBZL reacted at 70 °C for

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72 h. Multiple series of peaks at intervals of 144 m/z, corresponding to the repeating unit mass, were observed. The m/z values for the series with higher mass numbers

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(series a and c) agreed well with the monoisotopic masses of the Na+ adducts of PHCs

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with a phenyl group and a hydroxy group at each chain end (n = 15, Mtheor = 2278.212, Mobs = 2278.388), and with two hydroxy groups at both chain ends (n = 15, Mtheor = 2302.260, Mobs = 2302.451), respectively. PHCs with two phenyl groups at both chain ends (series b) were not observed in this MALDI-TOF MS analysis (n = 15, Mtheor =

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2398.233). It is assumed that due to the difference of ionization efficiency. The m/z values for the series with lower mass numbers (series d) agreed well with the

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monoisotopic masses of the Na+ adducts of cyclic PHCs (n = 5, Mtheor = 743.383, Mobs =

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743.470). Although cyclization was not completely suppressed under these polymerization conditions, the progress of the polycondensation was indicated by MALDI-TOF MS analysis. Cyclic PHCs were observed not only by MALDI-TOF-MS analysis, but also by SEC analysis (Figure 6). SEC chart of PHC before reprecipitation had bimodal distribution, not simple tailing. The second distribution was eliminated, and the first distribution indicate symmetry after reprecipitation These results show that 13

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a few cyclic PHCs were generated in the polycondensation condition, and was able to

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be eliminated from linear PHCs easily.

FIGURE 5 MALDI-TOF MS analysis of PHC prepared by polycondensation of DPC with 1,6-HD catalyzed by 5 mol% TBZL reacted at 70 °C for 72 h after reprecipitation.

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FIGURE 6 SEC chart of PHC prepared by polycondensation of DPC with 1,6-HD catalyzed by 5 mol% TBZL reacted at 70 °C for 72 h before (blue line) and after (black

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line) reprecipitation.

The results of polycondensation of DPC with different diols in the presence of TBZL are summarized in Table 1. Increasing the reaction temperature from 25 to

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70 °C caused conversion, yield, and Mn to improve gradually (Run 1–6). Low yields and

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narrow Mw/Mn were obtained at low polymerization temperature, despite moderate to relatively high conversions. This was as a result of some of the PHC dissolving in methanol used as a poor solvent during reprecipitation. The reproducibility of the polycondensation was confirmed (Run 5 and 6). PHC with Mn of 6.5–7.8 ×103 was obtained in sufficient conversion and yield by polycondensation of DPC with 1,6-HD in the presence of 5 mol% TBZL at 70 °C for 72 h. 15

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Table 1. Polycondensation of DPC with diols in the presence of TBZL for 72 ha Temp. (°C)

1d 1,6-HD

TBZL

25

79

7

8.8

2

1,6-HD

TBZL

40

89

33

11.4

3

1,6-HD

TBZL

50

96

57

19.1

4

1,6-HD

TBZL

60

98

67

29.7

5

1,6-HD

TBZL

70

>99

93

39.1

6

1,6-HD

TBZL

70

>99

88

34.9

7

1,5-PD

TBZL

70

>99

85

8

1,9-ND

TBZL

70

>99

94

17

9

1,6-HD

Ti(O Pr)4

70

10

1,6-HD

Zn(OAc)2

70

11

70

12

1,6-HD Mg(OAc)2 1,6-HD Mg(OH)2

13

1,6-HD

MgCO3

70 70

a

Mnb×10−3 Mnc×10−3 Mw/Mnc 1.7

2.8

1.09

2.0

3.1

1.16

3.1

5.3

1.21

4.7

6.4

1.55

6.0

6.5

2.00

5.4

7.8

1.60

39.9

5.5

5.8

1.89

6.3 –

11.1

1.72

–

31.3 –

–

–

69

–

–

–

–

–

4

–

–

–

–

–

3

–

–

–

–

–

2

–

–

–

–

–

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i

nb

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Diol

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Conv.b Yield (%) (%)

Catalyst

Run

DPC (10 mmol), diol (10 mmol), Catalyst (5 mol%). Determined by 1H NMR spectroscopy. c Determined by SEC using polystyrene standards (eluent: THF). d Reaction time of 96 h.

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b

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Polycondensations of DPC with 1,5-PD and 1,9-ND were also carried out (Run 7 and 8). Similar results were obtained using 1,5-PD instead of 1,6-HD. Poly(nonamethylene carbonate) (PNC) with Mn of 11.1 ×103 was obtained by polycondensation of DPC with 1,9-ND. Figure 6 shows the 1H NMR spectrum of PNC, which illustrates the polycondensation of DPC with 1,9-ND.

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FIGURE 7 1H NMR spectrum of PNC prepared by polycondensation of DPC with

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1,9-ND catalyzed by 5 mol% TBZL reacted at 70 °C for 72 h after reprecipitation.

Finally, other ester-exchange catalysts, Ti(OiPr)4, Zn(OAc)2, Mg(OAc)2,

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Mg(OH)2, and MgCO3, were used instead of TBZL for the polycondensation (Run 9–

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13). The polycondensation of DPC with 1,6-HD catalyzed by Ti(OiPr)4, Mg(OAc)2, Mg(OH)2, or MgCO3 proceeded more slowly than that catalyzed by TBZL. Although the polycondensation using Zn(OAc)2 proceeded with moderate conversion, PHC was not obtained after purification. These results show that TBZL is a better catalyst in polycondensation of DPC with diols than those reported previously.

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4. Conclusions Synthesis of APCs catalyzed by TBZL under mild conditions was realized.

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The polycondensation proceeded in the presence of phenol as a by-product under atmospheric pressure. The mechanism of polymerization was not clarified now. It is

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assumed that the hydrogen abstraction reaction of the hydroxy group of alcohol with

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tert-butyl group of TBZL occurred at first step similar to ring-opening polymerization of ε-caprolactone with TBZL [8]. Essential research for TBZL is now under way using X-ray structure analysis and inductively coupled plasma optical emission spectroscopy. The irreversible polycondensation of DPC with diols without requiring a tedious

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polymerization procedure to remove the by-product phenol is expected to be suitable for

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synthesis of various APCs.

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Acknowledgment

This work was financially supported by a Grant-in-Aid for Young Scientists (B) of JSPS KAKENHI (No. 16K17915), Japan.

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Chemistry of Phosgene, Chemical reviews 73(1)

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[5] H. Babad, A.G. Zeiler, The

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solvent-free conditions, Polymer Chemistry 3(6) (2012) 1475–1480; b) J.H. Park, J.Y. Jeon, J.J. Lee, Y. Jang, J.K. Varghese, B.Y. Lee, Preparation of High-Molecular-Weight Aliphatic Polycarbonates by Condensation Polymerization of Diols and Dimethyl

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Uchiyama, Development of highly chemoselective bulky zincate complex, tBu4ZnLi2: design, structure, and practical applications in small-/macromolecular synthesis, Chemistry - A European Journal 14(33) (2008) 10348–14356; b) M. Uchiyama, T. Furuyama, M. Kobayashi, Y. Matsumoto, K. Tanaka, Toward a protecting-group-free

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ɛ-caprolactone with dilithium tetra-tert-butylzincate under mild conditions, Polymer [9] M. Oshimura, Y. Oda, K. Kondoh, T. Hirano, K. Ute, Efficient acylation and transesterification catalyzed by dilithium tetra-tert-butylzincate at low temperatures,

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Tetrahedron Letters 57(19) (2016) 2070–2073.

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Highlights  The polycondensation of diphenyl carbonate with diols catalyzed by TBZL were investigated.

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 Aliphatic polycarbonates were obtained without removal of the by-product phenol.  The irreversible polycondensations proceeded under atmospheric pressure.

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 Aliphatic polycarbonates with Mn of 10,000 or more were obtained.