Ring-opening polymerization of lactones catalyzed by ion-exchanged clay montmorillonite

Abstract

Tin ion-exchanged montmorillonite catalyzed the ring-opening polymerization of δ-valerolactone in a solvent free system. The polymerization proceeded in a living manner, giving rise to poly(δ-valerolactone)s with controlled molecular weights. Aluminium and iron ion-exchanged montmorillonites were also effective for the polymerization. Furthermore, the copolymerization of δ-valerolactone with γ-butyrolactone took place in the presence of the montmorillonite to produce the corresponding copolymers.

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Green Context

The production of polymers represents a major class of chemical processes, and there are many opportunities for cleaner polymer synthesis. This contribution describes the use of a clay-based catalyst for a polymerisation leading to a polyester. The polymerisation proceeds smoothly, and a simple filtration is sufficient to recover the catalyst. This process means that separation of the catalyst is much more straightforward, leading to a simpler separation, and also opening up the possibility of better quality polymers, with the potential for reducing the amount of polymer required for a specific purpose.

DJM

Introduction

Clay montmorillonite is a layered silicate possessing ion-exchange ability. The acid property of montmorillonite can be easily altered by replacing interlayered cations in the montmorillonite without any change of the crystalline structure.¹ It has been reported that aluminium, iron and tin ion-exchanged montmorillonites are strongly acidic and efficient for several acid-catalyzed organic reactions, such as aldol and Michael reactions.^2,3

Aliphatic polyesters are of great interest for their applications in the medical field as biodegradable surgical sutures or as a delivery medium for controlled release of drugs due to their biodegradable, biocompatible and permeable properties.⁴ Recent development of living ring-opening polymerization of lactones has enabled us to obtain polyesters with controlled molecular weights and polydispersity ratios. One of the most useful methods to control ring-opening polymerization of lactones is ‘activated monomer cationic polymerization’, which was carried out using an acid catalyst and an alcohol initiator.⁵ In this type of the ring-opening polymerization, Lewis or protic acids are used as a catalyst, which activates the lactone monomer. Then, the alcohol initiates the polymerization via ring-opening. Mesoporous zeolite has also been used as a solid acidic catalyst for the controlled polymerization of lactones.⁶

In this paper, we report that tin(IV) ion-exchanged montmorillonite (Sn-mont) is a novel, efficient solid catalyst for the ring-opening polymerization of δ-valerolactone (δ-VL) to produce poly(δ-VL), which takes place by a living mechanism (Scheme 1). Aluminium and iron ion-exchanged montmorillonites (Al-mont and Fe-mont, respectively) are also employed for the polymerization of δ-VL. Furthermore, we demonstrate herein the copolymerization of δ-VL with γ-butyrolactone (γ-BL) catalyzed by Sn-mont, giving rise to copoly(δ-VL/γ-BL) (Scheme 2).

Scheme 1 Ring-opening polymerization of δ-VL.

Scheme 2 Ring-opening copolymerization of δ-VL with γ-BL.

Results and discussion

The polymerization of δ-VL initiated with ethanol in the presence of various amounts of Sn-mont proceeded at room temperature without solvent and the monomer conversion was quantitative after 24 h to give poly(δ-VL) (Table 1). For example, when a mixture of δ-VL and ethanol at a mole ratio of 10 was stirred in the presence of Sn-mont (0.20 g) under argon, the polymerization proceeded to attain 98.4% monomer conversion in 24 h (Table 1, entry 2). After the reaction, Sn-mont was removed by filtration and the filtrate evaporated under reduced pressure to obtain the product poly(δ-VL). The structure of the product was determined by its ¹H NMR spectrum (CDCl₃): δ 1.55–1.83 (m, O=CCH₂CH₂CH₂C, 4 H), 2.22–2.48 (m, O=CCH₂, 2 H), 3.98–4.23 (m, CH₂O, 2 H). In addition to these peaks due to the polymer chain, a set of small signals assignable to CH₃CH₂O (δ 1.26) and CH₂OH (δ 3.61) is observed. The relative intensity of these signals was 3∶2, indicating that the polymerization was initiated by ethanol with incorporation of the ethoxy end group at the initiating site of the polymer chain. The degree of polymerization (DP), therefore, was calculated by the integrated ratio of the signal due to the methyl end group to the signals due to the polymer main chain. Furthermore, the molecular weight distribution (M_w/M_n) of the product was estimated by gel permeation chromatography (GPC) measurement with chloroform as the eluent. Fig. 1 illustrates the relationships of DP and M_w/M_n in terms of the monomer conversion. When 0.20 g of Sn-mont was used with 0.50 g of δ-VL the DP values (Fig. 1, ●) were proportional to the monomer conversions, and close to the calculated values. Furthermore, the M_w/M_n values (Fig. 1, ■) were <1.25. These data indicate the living nature of the present polymerization process catalyzed by Sn-mont. An additional evidence of the living nature is given by a monomer-addition experiment (Fig. 2). When the same amount of monomer was added after the complete consumption of the first monomer at the final stage, the propagation still continued to give the polymer having a doubled DP value at the second final stage. The DP values obtained by using the smaller amount of the catalyst (0.025 g), however, were lower than the calculated values (Fig. 1, ○). The above data indicate that the use of 0.20 g of catalyst with 0.50 g of the monomer is more advantageous for the living polymerization process compared to the smaller amount of the catalyst (0.025 g). The following experiments, therefore, were carried out using 0.20 g of the catalyst and 0.50 g of monomer.

Table 1 Ring-opening polymerization of δ-VL initiated with ethanol in the presence of ion-exchanged montmorillonites^a

			Before precipitation			After precipitation
Entry	Montmorillonite	[δ-VL]0/ [Ethanol]0	Conversion^b (%)	DP^b	Mw/Mn^c	Yield^d (%)	DP^b	Mw/Mn^c
a δ-VL; 0.50 mmol, montmorillonite; 0.20 g, at room temperature for 24 h.b Determined by ^1H NMR spectroscopy.c Determined by GPC analyses with chloroform as the eluent.d Cyclohexane insoluble fraction.
1	Sn-mont	5	97.6	6.2	1.22	86.8	7.7	1.17
2	Sn-mont	10	98.4	9.2	1.27	95.4	9.6	1.24
3	Sn-mont	15	98.6	11.5	1.24	96.1	12.3	1.24
4	Sn-mont	20	98.8	13.4	1.27	98.0	14.3	1.27
5	Al-mont	10	98.6	9.1	1.26	95.5	9.1	1.20
6	Fe-mont	10	98.6	9.0	1.29	96.6	9.1	1.24

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Fig. 1 DP and M_w/M_n values vs. monomer conversions in the polymerization of δ-VL; [δ-VL]₀/[ethanol]₀ = 10.0: (●and ■) Sn-mont, 0.20 g, δ-VL, 0.50 g; (○ and □) Sn-mont, 0.025 g, δ-VL, 0.50 g. The dashed line represents the calculated DP value

Fig. 2 DP values obtained in a monomer-addition experiment; [δ-VL]₀/[ethanol]₀ = 5.0.

The product polymer can be purified further by precipitation into cyclohexane. Some polymerization results are summarized in Table 1. When the ratios of monomer to initiator are <10, the DP values of the product polymers are in good agreement with the calculated values (entries 1 and 2). With increasing the ratios of monomer to initiator than 10, the DP values become smaller than the calculated values (entries 3 and 4). These observations are probably due to the presence of the moisture in the polymerization system at the larger monomer/initiator ratios. Because the integrated ratios of the CH₃ peak to the CH₂OH peak were smaller than 3/2 in the ¹H NMR spectra of the polymers of entries 3 and 4, indicating that the polymerizations were initiated by not only ethanol but also water. Al-mont and Fe-mont were also effective as catalysts for the polymerization (entries 5 and 6).

After the polymerization, the Sn-mont catalyst was removed from the reaction mixture by filtration. The recovered Sn-mont was dried at 120 °C for 3 h under reduced pressure, and used as the catalyst again for the polymerization of δ-VL under the same conditions as above. The yields and DP values were comparable to those shown in Table 1. For example, when the polymerization was carried out using the recovered Sn-mont under the conditions as in entry 2, poly(δ-VL) was obtained in 96.6% yield. The DP and M_w/M_n values were 9.2 and 1.29, respectively. When the catalyst after the 2nd run was recovered further and used once more for the polymerization under the same conditions, poly(δ-VL) with DP = 9.2 and M_w/M_n = 1.28 was obtained in 98.9% yield. These experimental results indicate that the catalytic activity did not decrease for the 2nd and 3rd runs.

As an extension of the polymerization reaction catalyzed by Sn-mont, copolymerization of δ-VL with γ-BL was carried out under the conditions as used for the homopolymerization of δ-VL. It is well known that γ-BL on its own does not polymerize under such polymerization conditions. In fact, when the polymerization of γ-BL was performed in the presence of Sn-mont, only an oligomeric product with low DP (2.4) was obtained in low yield (5.5%) (Table 2, entry 6). The copolymerization of δ-VL with γ-BL gave the corresponding copolymers consisting of δ-VL and γ-BL units. The structure of the copolymer was confirmed by ¹H NMR spectroscopy. As shown in Fig. 3, both the signals ascribed to δ-VL units (signals a, d, and f) and γ-BL units (a, c, and e) are observed, accompanied with the signals g and b due to CH₃CH₂O and CH₂OH of the end groups, respectively. The results of the copolymerization at various monomer feed ratios are shown in Table 2. The contents of γ-BL units in the copolymers are always less than the γ-BL ratios in the monomer feeds, attributed to the very low polymerizability of γ-BL. The yields and DP values decrease with increasing the γ-BL ratios in the monomer feeds (entries 1–3). Longer reaction times gave the copolymer with higher DP in higher yield (entry 4). A higher reaction temperature, however, was not effective for improving the DP and yield (entry 5).

Table 2 Copolymerization of δ-VL with γ-BL initiated with ethanol in the presence of Sn-mont^a

Entry	Feed ratio δ-VL∶γ-BL	Temperature/°C	Time/h	Yield^b (%)	DP^c	Mw/Mn^d	Composition^c δ-VL∶γ-BL
a δ-VL; 1.0 mmol, ethanol; 0.10 mmol, Sn-mont; 0.20 g.b Residual percentage after evaporation.c Determined by ^1H NMR spectroscopy.d Determined by GPC analyses with chloroform as the eluent.e γ-BL; 5.0 mmol, ethanol; 0.50 mmol, Sn-mont, 0.20 g.
1	1∶1	rt	3	49.3	6.6	1.33	1∶0.19
2	1∶5	rt	3	15.8	5.0	1.34	1∶0.25
3	1∶10	rt	3	6.7	3.9	1.29	1∶0.34
4	1∶10	rt	24	9.6	4.7	1.28	1∶0.43
5	1∶10	50	3	5.7	4.3	1.25	1∶0.39
6^e	0∶1	rt	3	5.5	2.4	1.21	0∶1

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Fig. 3 ¹H NMR spectrum of the copolymer between δ-VL and γ-BL (CDCl₃).

In conclusion, we have found that ion-exchanged montmorillonites were effective as acidic catalysts for the ring-opening polymerization of δ-VL. The polymerization catalyzed by Sn-mont proceeded in a living manner to give poly(δ-VL)s with controlled molecular weights. Furthermore, copolymerization of γ-BL with δ-VL catalyzed by Sn-mont also took place, giving rise to the corresponding copolymers.

Experimental

Ion-exchanged montmorillonites were prepared according to the literature and dried at 120 °C for 3 h under reduced pressure (0.20 mmHg) prior to use.⁷ Monomers, δ-VL and γ-BL, and the initiator ethanol were purified by distillation. NMR spectra were recorded on a Varian Mercury 200 spectrometer. GPC analyses were performed by using a Hitachi 655A-11 apparatus with a refractive index detector under the following conditions: Tosoh TSKgel G3000H_XL column with chloroform as eluent at a flow rate of 1.0 mL min⁻¹.

Typical procedure

Under argon, to a suspension of δ-VL (0.50 g, 5.0 mmol) and Sn-mont (0.20 g) was added ethanol (0.023 g, 0.50 mmol) with stirring at room temperature to start the polymerization. After the solution was stirred for 24 h, the reaction mixture was diluted with chloroform and Sn-mont was separated by filtration. The chloroform solution was washed with aqueous sodium hydrogen carbonate and dried over sodium sulfate. The solution was evaporated and dried under reduced pressure to give a white polymeric material (0.52 g). The product was dissolved in chloroform and the solution poured into a large amount of cyclohexane to precipitate the polymer. The precipitate was isolated by decantation and dried in vacuo to give the purified polymer in 95.4%.

Acknowledgement

We are indebted to Ms Masa Karasu for technical assistance.

References

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