Efficient ring-opening polymerization of epsilon-caprolactone catalyzed by iron(III) chloride under mild reaction conditions

doi:10.21203/rs.3.rs-3312272/v1

As a biodegradable polyester, the commercialized synthesis protocol of poly(ε-caprolactone) (PCL) under mild conditions is of great significance and attractive. Herein, the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) using FeCl₃ catalyst and benzyl alcohol (BnOH) as the initiator was developed. FeCl₃ has been proved to be an efficient catalyst for the ROP of ε-CL under mild conditions. The conversion of ε-CL to PCL was 98.8% at 60 ^oC within 4 h, with number-average molecular weight (Mn) and dispersity (PDI) of 16500 g/mol and 1.28, respectively. Moreover, a coordination-insertion ring-opening polymerization mechanism mediated by FeCl₃ was proposed. Overall, this work is expected to provide a feasible industrial method for the preparation of PCL from the direct aerobic oxidation of cyclohexanone.

Due to ecological issues, poly (ε-caprolactone) (PCL) is becoming the most promising bioplastics, as well as sustainable and environmental-friendly alternatives to petroleum-derived plastics [1–3]. The ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) provides an efficient and convenient route to the direct synthesis of polyester [4–6]. Metal complexes are one of the most efficient catalysts for the ROP of ε-CL [7]. Various metal species, including Sn [8–10], Zn [11–13], Ti [14–16], Al [17–19] etc, have been reported.

Among them, iron complexes are excellent choices due to their abundance, low toxicity, and biocompatibility, matching up reasonably with bioplastics [20–22]. In addition, iron-based catalysts have unique advantages in a wide range of spin states, overall charges and oxidation states, which providing potential possibilities for the catalytic performance [23]. For example, iron-containing imidazolium-based ionic liquid 1-n-butyl-3-methylimidazolium heptachlorodiferrate was able to efficiently catalyze the ROP of ε-CL, in which polymerization yields higher than 80% were obtained after 4 h at 85°C [24]. Salen-iron(III) chloride complexes presented excellent performance for the ROP of ε-CL in the presence of propylene oxide for 12 h at 120 ^oC [21]. Cramer et al reported the efficient polymerization of ε-CL with iron bis(imino)pyridine complexes as well as the detail mechanism research [23]. N, N’-Bidentate iron(II) complexes have been reported to initiate the polymerization of ε-CL with 95% yield at 40 ^oC within 30 min ([CL]₀/[Fe] = 200) in toluene solution [22]. Recently, both anhydrous and hydrated FeCl₃ mediated ε-CL polymerization in tetrahydrofuran solution were reported. Polymerization by anhydrous FeCl₃ proceeded by active chain end (ACE) mechanism, and activated monomer (AM) was the mechanism for using hydrous FeCl₃ as catalyst [25].

As well known that the key method for preparing ε-CL is the Baeyer-Villiger oxidation of cyclohexanone using organic peroxy acids [26–28]. Our group reported the synthesis of ε-CL using molecular oxygen as oxidant in the presence of iron catalyst [29]. Inspired by the coupling and matching rules of reaction-separation process, we recognized the feasibility of the one-step preparation of PCL from the cyclohexanone oxidation through dismissing the separation process of iron catalyst.

With this idea in mind, the ring-opening polymerization of ε-CL using FeCl₃ and benzyl alcohol (BnOH) as initiator was developed in this work. It is expected to achieve the efficiency polymerization under solvent-free and mild conditions. The well-defined PCL was obtained with number-average molecular weight (M_n) and dispersity (PDI) of 16500 g/mol and 1.28, respectively. Our results demonstrate FeCl₃ is an efficient catalyst for ring-opening polymerization of epsilon-caprolactone. It is expected to realize the one-step preparing of PCL through the direct aerobic oxidation of cyclohexanone.

2.1 Materials

ε-CL from Aldrich was dried over calcium hydride (CaH₂) and distilled under reduced pressure before use. Benzyl alcohol (BnOH) was purchased from Aladdin with analytical purity. FeCl₃ and other anhydrous metal salts were used as received. All reagents were purchased commercially with the maximum purity available and used as received unless indicated.

2.2 Characterizations

Proton nuclear magnetic resonance (¹H NMR) spectra were conducted on a Bruker AV400 MHz spectrometer with deuterated chloroform (CDCl₃) as the solvent and tetramethylsilane (TMS) as the internal standard. FT-IR spectra were recorded from 400 to 4000cm^− 1 in a Bruker spectrophotometer (EQUINOX 55) by using potassium bromide pellets. Mn and PDI were measured by GPC (Gel Permeation Chromatography) using Waters-2414 equipped with a Waters styragel HT4 and use PS (polystyrene) as standards. THF was used as mobile phase at a rate of 1.0 mL/min and the column temperature of 25 ^oC. The polymerization reaction intermediate was detected by ESI-MS. Sample was dissolved in 5 mL methanol. The solution was filtered by organic filter membrane in 0.45 µm. The filtration was injected to MS analysis through direct injection mode. The ionization method is ESI, and the scanning range is 50-1500 m/z.

2.3 General procedure for polymerization of ε-CL

A typical procedure for the polymerization of caprolactone is as follows: ε-CL (9.12 g, 80.0 mmol) and BnOH (10.4 µL, 0.1 mmol) were placed in a flamed and argon purged round-bottom flask equipped with a magnetic stirrer. The flask was placed in an oil bath at 60 ^oC and FeCl₃ (0.0162 g, 0.1 mmol) was added under an argon atmosphere to start the polymerization. The polymerization was quenched by the addition of Amberlyst A21 after 4 h. During polymerization, small portions of the polymerization mixture were withdrawn and added to an amount of triethylamine to determine the monomer conversion by ¹H NMR measurements. The product was dissolved in dichloromethane and precipitated into a large excess of cold methanol to give solid PCL. After filtration, the product was dried under vacuum for 24 h prior to characterization.

3.1 Effect of metal salts catalyst on the polymerization of ε-CL

The efficiency of various metal sources catalyst was evaluated for the ring-opening polymerization of ε-CL with benzyl alcohol as initiator (Table 1). Obviously, no polymerization was observed in the absence of catalyst at 60 ^oC for 4 h (entry 1 in Table 1). Unsatisfactory activity could be observed when the polymerization of ε-CL catalyzed by some commonly used metal chlorides like RuCl₃, TiCl₃ or ZnCl₂ at [M]/[C]/[I] ratio of 800/1/1 (entries 3 ~ 5 in Table 1). Although AlCl₃ exhibited a relatively high activity under the same conditions, the wide distribution of polymers with PDI 1.57 was obtained (entry 2 in Table 1).

To our delight, FeCl₃ exhibited a much higher activity than other metal chlorides catalysts even the polymerization was conducted at room temperature (entries 6 ~ 7 in Table 1). As one kind of industrial catalyst in the polymerization of ε-CL, Sn(Oct)₂ presented its high efficiency only at high temperature (140 ^oC) (entries 8 ~ 9 in Table 1). It indicated that FeCl₃ catalyst is superior to industrial catalyst in terms of catalytic efficiency and polymer product distributions. An attempt was made to replace FeCl₃ with FeCl₂, but no polymerization products could be determined (entry 10 in Table 1). It demonstrated that iron(III) may be more easily coordinated with the substrate or initiator to form active intermediates, which then accelerated the polymerization. In addition, the catalytic performance of FeBr₃ was lower than that of FeCl₃, which indicated that the generation of the active intermediates also be affected by the anions of catalyst (entry 11 in Table 1).

Table 1

Ring-opening polymerization of ε-CL catalyzed by various metal salts ^a
Entry	Catalyst	Conv./%	T/^oC	PCL
Entry	Catalyst	Conv./%	T/^oC	M_w	M_n	PDI
1	-	0	60	-	-	-
2	AlCl₃	76.4	60	15200	9700	1.57
3	RuCl₃	20.5	60	4700	3800	1.24
4	TiCl₃	51.2	60	5100	4500	1.13
5	ZnCl₂	20.3	60	4500	4000	1.12
6	FeCl₃	98.8	60	21100	16500	1.28
7	FeCl₃	98.3	25	9300	8000	1.16
8	Sn(Oct)₂	10.2	60	5300	4400	1.20
9	Sn(Oct)₂	91.2	140	17800	9600	1.85
10	FeCl₂	0	60	-	-	-
11	FeBr₃	71.4	60	12300	9300	1.33

^a [M]:[C]:[I] (mol) = 800:1:1, [M]: ε-CL, [C]: catalyst, [I]: BnOH; 4 h.

3.2 Effect of initiator on the polymerization of ε-CL

The effect of initiator on the polymerization of ε-CL was also investigated at [M]/[C]/[I] ratio of 800/1/1. Table 2 presented the results for obtained PCL initiated by various alcohols using FeCl₃ as catalyst. A high molecular weight (M_w>20000 g/mol) could be obtained by using benzyl alcohol, phenylethanol, 1-butanol and ethanediol as initiator, respectively (entries 1–4 in Table 2). Although the high molecular weights (M_w=26400 g/mol) was obtained using ethanediol as initiator, the other chain polymerizations were also significant with a high PDI value (1.50). However, low molecular weight of PCL (M_w=14100 g/mol) could be obtained using isopropanol (entry 5 in Table 2). Therefore, it could be known that the polymerization was closed related with the steric hindrance of the active site of initiator [30].

Table 2

The effect of different initiators on polymerization of ε-CL^a
Entry	Initiator	Conv. (%)	M_n,theo^b	PCL
Entry	Initiator	Conv. (%)	M_n,theo^b	M_w	M_n	PDI
1	Benzyl alcohol	98.8	90300	21100	16500	1.28
2	2-Phenylethanol	96.7	88400	21200	15500	1.37
3	1-Butanol	98.3	89800	22800	17100	1.33
4	Ethanediol	98.0	89500	26400	17600	1.50
5	Isopropanol	99.6	91000	14100	10700	1.32

^a[M]:[C]:[I] = 800:1:1, 4h, 60 ^oC, [M]: ε-CL, [C]: catalyst, FeCl₃, [I]: initiator.

3.3 Effect of molar ratio of monomer to initiator on the polymerization

Effect of the amount of initiator on the ring-opening polymerization of ε-CL was tested as shown in Fig. 1. As the molar ratio of [M]/[I] increased from 200 to 800, M_w of PCL increased from 14400 g/mol to 21100 g/mol. In addition, the declined PDI value was also obtained with the decreased amount of initiator. However, the PDI value and M_w of PCL increased significantly when the molar ratio of [M]/[I] was higher than 800. Due to a significant excess of ε-CL, the polymerization time was also extended. The wide molecular weight distribution of products could be ascribed to the long reaction time, which leaded to the occurrence of other chain polymerizations such as intermolecular and intramolecular transesterification.

3.4 Effect of temperature on the polymerization of ε-CL

The influence of temperature on the ROP of ε-CL catalyzed by FeCl₃ in the presence of benzyl alcohol as an initiator was further investigated (Fig. 2). As the reaction temperature increased from 30 ^oC and 60 ^oC, the molecular weights of PCL increased sharply. When the temperature was further increased, the increase trend of molecular weights was not significant, and even showed a downward trend when reaction temperature was higher than 100 ^oC. On the other hand, the PDI value increased continuously with the gradual increased temperature. It could be suggested that the high temperature promoted the other chain polymerizations or intramolecular transesterification [31].

3.5 Kinetic and mechanism of polymerization

The profiles of the ring-opening polymerization of ε-CL catalyzed by FeCl₃ was shown in Fig. 3. In the first hour, ε-CL conversion slowly increased. Followed, the reaction rate accelerated rapidly. There is an obvious induction period in the catalytic polymerization system. After 1.5 hours of reaction, the conversion of ε-CL reached up to 83.3%, and Mw was 10400 g/mol.

Kinetic measurements of ε-CL polymerization catalyzed by FeCl₃ were carried out. The kinetic plots for FeCl₃-catalyzed ROP of ε-CL is shown in Fig. 4. A distinct first-order relationship between ln([CL]₀/[CL]) versus time was observed, obtaining a constant reaction rate related to the monomer consumption (the apparent rate constant, k_p = 1.16 h^− 1). It also could be known that the polymerization has a short induction period. It indicated that FeCl₃ presented excellent catalytic performance for the ROP of ε-CL.

To get understanding the polymerization mechanism of ε-CL catalyzed by FeCl₃, some controlling experiments were carried out (Table 3). No polymerization of ε-CL occurred when the reaction was conducted in the absence of FeCl₃ catalyst (entry 1 in Table 3). FeCl₃ could promote the solvent-free ring-opening polymerization of ε-CL in the absence of BnOH initiator, but the conversion of ε-CL was only 18.6% (entry 2 in Table 3). When BnOH was used as initiator, the conversion of ε-CL reached up to 98.8%, and the molecular weights of PCL was 21100 g/mol.

Table 3

Effects of initiator on polymerization of CL catalysted by FeCl₃^a
Entry	[M]:[C]:[I]	Initiator	Conv. /%	M_n,theo^b	PCL
Entry	[M]:[C]:[I]	Initiator	Conv. /%	M_n,theo^b	M_w	M_n	PDI
1	800:0:1	BnOH	0	-	-	-	-
2	800:1:0	-	18.6	17100	1900	1700	1.12
3	800:1:1	BnOH	98.8	90300	21100	16500	1.28

^a[M]: CL, [C]: FeCl₃, [I]: BnOH, 60 ^oC, 4 h.

^bCalculated from ([M]/[I]) × Conv. × Mw(CL) + Mw (Initiator)

Based on the above results, it could be speculated that there are two reaction pathways for FeCl₃-catalyzed ROP of ε-CL. Firstly, in the absence of initiator, FeCl₃ may directly coordinate with ε-CL. The ε-CL monomer molecule was activated and to occur the ring opening polymerization reaction. The speculation could be confirmed through UV-Vis spectroscopy characterization. As shown in Fig. 5, the carbonyl group absorption peak of ε-CL at 240 nm shifted to 247 nm in the presence of FeCl₃. It indicated the occurrence of the coordination between FeCl₃ catalyst and ε-CL [32].

The interaction between FeCl₃ and BnOH results caused another reaction pathway when the polymerization of ε-CL was conducted in the presence of initiator. As shown in ¹H-NMR spectra (Fig. 5), the chemical shift of the hydrogen peak of BnOH at 4.56 ppm shifted to 4.62 ppm. It indicates that FeCl₃ may interact with BnOH to form metal alkoxy compounds.

Furthermore, the intermediate molecule during the ROP of ε-CL was captured by ESI-MS (electro spray ionization mass spectrum) (Fig. 6). The peak with m/z of 269.0 could be ascribed to the metal alkoxy compound generated from the reaction between FeCl₃ and BnOH. The peaks with m/z of 383.1, 497.1 and 611.1 belong to the small molecule PCL with monomers number of 1, 2 and 3. These results support a coordination-insertion mechanism as proposed in Fig. 7. The process begins with the monomer activation by the active ferrous alkoxy compound which is generated from the reaction of FeCl₃ catalyst and BnOH [33–35].

In summary, the ring-opening polymerization of ε-CL using FeCl3 as catalyst and benzyl alcohol as the initiator was developed. FeCl3 has been proved to be an efficient catalyst for the ROP of ε-CL under mild conditions. Under the optimized reaction conditions ([CL]/[BA]/[BnOH] = 800/1/1 (mol), 60 ^oC, 4 h), the well-defined PCL was obtained with M_n and PDI of 16500 g/mol and 1.28, respectively. Moreover, a coordination-insertion ring-opening polymerization mechanism mediated by FeCl₃ was proposed.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (No. 22278451 and 21938001), Guangdong Basic and Applied Basic Research Foundation (2022B1515120057, 2019A1515110979).

Conflicts of interest: The authors have no conflicts of interest to declare.

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Efficient ring-opening polymerization of epsilon-caprolactone catalyzed by iron(III) chloride under mild reaction conditions

Status:

Journal Publication

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Abstract

Figures

1. Introduction

2. Experimental