Introduction

Recently, ring-opening polymerization (ROP) of cyclic esters via coordination-insertion mechanism has become an important synthetic route for producing well-defined biodegradable polyesters such poly(ε-caprolactone) (PCL), poly(l-lactide) (PLLA), polyglycolide (PG), and copolyesters [1,2,3,4]. In this coordination-insertion mechanism, the effective initiator is the key to success in the control synthesis of biodegradable polyesters [4, 5]. These polymers are potentially prepared by the ROP of cyclic esters with the Sn-containing initiating systems [6,7,8,9,10]. Different Sn-containing initiating systems are recently utilized in the ROP of cyclic esters such as tin(II) carboxylate [11, 12], tin(II) alkoxide [6, 13, 14], tin(IV) alkoxide [15,16,17], tin(II) chloride [18, 19], n-butyltin(IV) derivatives [15, 20, 21]. Among them, tin(II) 2-ethylhexanoate (Sn(Oct)2) is mostly used as the initiating system for the production of biodegradable polyesters [22]. At present, it is known that Sn(Oct)2 is not a real initiator because it will react with hydroxyl-containing compounds converting to the tin(II) mono- and di-alkoxide prior initiate cyclic esters polymerization [23, 24]. This causes difficulty and uncontrollable true active species concentration. Our previous works have focused on the controlled ROP of ε-caprolactone (ε-CL) under the solvent-free condition using different Sn-containing initiators [7, 8, 10, 12, 13, 16, 17, 19, 20]. It was found that the tin chlorides initiator such as tin(II) chloride (SnCl2), tri-n-butyltin(IV) chloride (nBu3SnCl), and di-n-butyltin(IV) dichloride (nBu2SnCl2) could be considered as interesting initiator due to their commercially available, easy to handle, and soluble in monomer. From a kinetics study by the non-isothermal differential scanning calorimetry (DSC) [20], the steric hindrance around Sn active center causes the reduction of reactivity of tin chloride initiators in the ROP of ε-CL. From PCL synthesis by solvent-free polymerization, the slow initiators of nBu3SnCl and nBu2SnCl2 could not produce PCL by the conventional heating by silicone oil bath at 120 °C for 2 h. The synthesis condition was modified by increasing the polymerization temperature and time to 150 °C and 24 h, respectively. It was found that PCL was not obtained from the nBu3SnCl initiator. On the other hand, nBu2SnCl2 could produce high molecular weight PCL under a similar condition to nBu3SnCl.

From this point, improving of the synthesis of PCL by using the slow nBu3SnCl and nBu2SnCl2 is challenging. We found in the literature that the microwave irradiation can be used to assist the ROP of cyclic ester [25,26,27,28]. In 2002, Liao et al. [25] reported the microwave-assisted ROP of ε-CL with Sn(Oct)2 and zinc powder under vacuum conditions in the sealed ampule at different microwave powers. It was found that the high weight average molecular weight PCL was obtained from Sn(Oct)2 initiating system (Mw = 1.24 × 105 g/mol) at 680 W. The zinc powder seemed to produce a lower molecular weight of PCL than Sn(Oct)2. From their results, microwave irradiation was considered as the faster method in the production of PCL than the conventional thermal method. The microwave irradiation could greatly accelerate the ROP of ε-CL under solvent-free condition. Tan et al. [26] reported the ROP of ε-CL in the presence of hydrogen phosphate by microwave irradiation. ε-CL and dialkyl hydrogen phosphate (DHP) were mixed in the vacuum-sealed ampule. The microwave irradiation technique produced a higher molecular weight of PCL than the conventional heating method in a shorter time. The highest molecular weight of PCL (Mw = 8.1 × 103 g/mol) was obtained from diisopropyl hydrogen phosphate (DIPHP). Xu et al. [27] scaled up the microwave-assisted ROP of ε-CL with Sn(Oct)2 from 750 to 2450 g using their designed microwave oven. The highest value of PCL molecular weight (Mw = 1.22 × 105 g/mol) was obtained in 40 min at a microwave power/monomer mass ratio of 0.53 and monomer mass of 1600 g. The molecular weight distributions of the synthesized PCLs were in the range of 1.63–2.75. From the obtained results, it is possible to use microwave irradiation to synthesize PCL in an industrial scale. Yang et al. [28] studied the utilization of microwave irradiation in the ROP of ε-CL in the presence of modified halloysite nanotubes loaded with stannous chloride (APTES-P–h-HNTs-SnCl2). ε-CL and APTES-P-h-HNTs-SnCl2 were ultrasonically mixed, and the ROP was carried out in the MCR-3 atmospheric-pressure microwave chemical reactor at 400 W. The number average molecular weight (Mn) and dispersity (Đ) of PCL were in the range of 9.79 × 103–5.21 × 104 g/mol and 1.11–1.61, respectively. From the analysis of the thermal stability of the prepared PCL, the APTES-P-h-HNTs-SnCl2 showed a good dispersibility in the PCL matrix and could improve the thermal stability of the PCL composite.

From data reported in the literature, it seems to be possible to improve the performance of tin chloride initiators in the synthesis of PCL via microwave irradiation. Therefore, this work wants to apply the microwave irradiation to the solvent-free ROP of ε-CL with nBu3SnCl, nBu2SnCl2, and nBuSnCl3 and compare with the conventional initiating system of Sn(Oct)2. The effect of the chloride group presented in the tin chloride will be investigated and discussed for the first time. The effect of initiator concentration on the solvent-free polymerization of ε-CL will also be investigated. The synthesized PCL will be characterized by methods of Fourier transform infrared spectroscopy (FTIR), proton- and carbon-nuclear magnetic resonance spectroscopy (1H- and 13C-NMR), and gel permeation chromatography (GPC). The polymerization mechanism for the solvent-free ROP of ε-CL with all initiating systems will also be proposed and discussed.

Experimental

Materials preparation

Commercial ε-caprolactone (ε-CL, Sigma-Aldrich, 97.0%) was purified by vacuum distillation and stored in a 250-mL round bottom flask connected with a glass stopcock. The clear and colorless liquid of ε-CL was kept under vacuum condition before being used. Tri-n-butyltin(IV) chloride (nBu3SnCl, Sigma-Aldrich, 95.0%), di-n-butyltin(IV) dichloride (nBu2SnCl2, Sigma-Aldrich, 97.0%), n-butyltin(IV) trichloride (nBuSnCl3, Sigma-Aldrich, 97.0%), methanol (Qrec, 99.0%), and chloroform (LabScan, 99.5%) were used as received. Tin(II) 2-ethylhexanoate (Sn(Oct)2, Sigma-Aldrich) was purified by vacuum distillation yielding the viscous, clear, and colorless liquid before being utilized as initiating system.

Microwave-assisted solvent-free ring-opening polymerization of ε-caprolactone

Purified ε-CL (4 g) with different concentrations (0.025–0.200 mol%) of nBu3SnCl, nBu2SnCl2, nBuSnCl3, and Sn(Oct)2 were weighed into the dry vial (15 mL). These prepared vials were ultrasonicated for 30 min, placed in the 250 mL beaker, and then heated by a commercial microwave oven (Samsung, model MW81GN) at the electrical powers of 300 and 450 W (50 Hz). The vials were irradiated for 2 min, stopped for 1 min, and then reradiated until the irradiation time reached 20 or 30 min. After the complete heating process, the crude PCL samples were dissolved in CHCl3, precipitated in cold methanol, and dried in a hot air oven similar to our previous works [19, 20].

Polymer characterization

The functional groups of the synthesized PCL were characterized by the Fourier transform infrared spectroscopy (FTIR) on a Shimadzu Tracer 800 IR and KBr disk technique. The wavenumber was recorded from 400 to 4000 cm−1 at 4 cm−1/s. The chemical structure of the synthesized PCL was measured by the proton- and carbon-nuclear magnetic resonance spectroscopy (1H and 13C-NMR) on a Bruker Avance 400 using CDCl3 as solvent. The molecular weight average and dispersity of all synthesized polymers were investigated by the gel permeation chromatography (GPC) on a Waters 2414 GPC using THF (1.0 mL/min) and polystyrene (PS) as eluent and standard. GPC measurement was conducted at 40 °C using viscosity and refractive index detectors.

Results and discussion

From our previous works [20], it was found that the tin chlorides such as nBu3SnCl and nBu2SnCl2 acted as the interesting initiating system in the synthesis of biodegradable polyester from the ROP of cyclic ester. From the non-isothermal DSC kinetics study, the average activation energy (Ea) and frequency factor (A) values for the ROP of ε-CL initiated with nBu2SnCl2 (64.1 kJ/mol, 1.34 × 108 min−1) are lower than nBu3SnCl (70.6 kJ/mol, 2.64 × 108 min−1), respectively. The reactivity of nBu2SnCl2 in the ROP of ε-CL was higher than nBu3SnCl. This demonstrates that the steric hindrance around Sn–O active center plays an important role in the reactivity of the tin chloride initiator. From PCL synthesis via solvent-free polymerization, it was found that the tin(II) chloride (SnCl2) effectively produced a very high molecular weight of PCL in a short time due to its high reactivity. Interestingly, nBu2SnCl2 initiators produced low molecular weight PCL under a similar synthesis condition to SnCl2. The higher temperature and longer reaction time are required for the slow initiating systems of nBu3SnCl and nBu2SnCl2. At a synthesis temperature of 120 °C for 2 h, PCL could not obtain from both nBu3SnCl and nBu2SnCl2 initiators. Furthermore, when the temperature and time were increased to 150 °C and 24 h, PCL was only obtained from the nBu2SnCl2 initiator with Mn up to 5.6 × 104 g/mol and a yield of 83%.

In this present work, we aim to improve the synthesis of PCL from the ROP of ε-CL with nBu3SnCl, nBu2SnCl2, and nBuSnCl3 by using the microwave irradiation and compare with the conventional initiating system of Sn(Oct)2. First of all, the ROP of ε-CL with a constant nBu3SnCl concentration of 0.1 mol% is conducted at different microwave powers and times. The physical appearance of the obtained crude and purified PCLs is summarized in Fig. 1.

Fig. 1
figure 1

Physical appearance of crude and purified PCLs obtained from the ROP of ε-CL initiated by 0.1 mol% of nBu3SnCl at different microwave powers and times: a 300 W for 20 min, b 300 W for 30 min, c 450 W for 20 min, and d 450 W for 30 min

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Fig. 2
figure 2

Physical appearance of crude and purified PCLs obtained from the ROP of ε-CL initiated by different concentrations of nBu2SnCl2 at the microwave power of 450 W and 30 min: a 0.025, b 0.050, c 0.100 and d 0.200 mol%

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From Fig. 1, the crude PCLs are in solid form with pale yellow color. After being purified in cold methanol, the obtained PCLs are white solid with different appearances from powder (a, b) to tough fiber (c, d). GPC analysis of the purified PCLs shown in Fig. 1 is summarized in Table 1. From Table 1, microwave irradiation can improve the performance of the slow nBu3SnCl initiator in the synthesis of PCL. The molecular weight and %yield of PCL are higher than the conventional heating process reported in our previous work [20]. From these, the concentration of nBu3SnCl is varied to test the control ability of this initiator in the ROP of ε-CL by using the condition of entry 4, and the results are summarized in Table 2.

Table 1 GPC analysis for the purified PCLs obtained from the ROP of ε-CL initiated by 0.1 mol% of nBu3SnCl at different microwave powers and times
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Table 2 GPC analysis for the purified PCLs obtained from the ROP of ε-CL initiated by different concentrations of nBu3SnCl at microwave power of 450 W and 30 min
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From Table 2, it is found that the nBu3SnCl initiator can control the ROP of ε-CL. The molecular weight of PCL seems to be lower when the concentration of nBu3SnCl is lower than 0.100 mol% because of the very low amount of Sn active center in the system. The condition used in Table 2 will be further applied to the more reactive system of nBu2SnCl2, nBuSnCl3, and Sn(Oct)2. The physical appearances of crude and purified PCLs obtained from each initiating system are depicted in Figs. 2, 3, 4, and the GPC results are summarized in Table 3. It is important to note that the physical appearance of the purified PCLs changes from short fiber (Fig. 2) to powder (Fig. 4). This indicates that the initiator plays an important role in the property of the synthesized polymer which will be described later.

Fig. 3
figure 3

Physical appearance of crude and purified PCLs obtained from the ROP of ε-CL initiated by different concentrations of nBuSnCl3 at the microwave power of 450 W and 30 min: a 0.025, b 0.050, c 0.100 and d 0.200 mol%

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Fig. 4
figure 4

Physical appearance of crude and purified PCLs from the ROP of ε-CL initiated by different concentrations of Sn(Oct)2 at a microwave power of 450 W and 30 min: a 0.025, b 0.050, c 0.100 and d 0.200 mol%

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Table 3 GPC analysis for the purified PCLs obtained from the ROP of ε-CL initiated by different concentrations of nBu2SnCl2, nBuSnCl3, and Sn(Oct)2 at the microwave power of 450 W and 30 min
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From Table 3, it is found that the molecular weight of PCL seems to decrease when the highly reactive initiator is used. This may be related to the higher amount of side reactions such as transesterification occurring in the reactive initiating system. The control of ε-CL polymerization seems to be more difficult for the highly reactive initiator under the condition used in this work. From GPC analysis, it is found that the Đ values shown in Table 3 are in the range of 1.62–2.34 indicating the occurrence of the intramolecular transesterification reaction that can broaden the Đ value of the synthesized polymer [29, 30]. Furthermore, the ROP of ε-CL under microwave irradiation occurred at a very high temperature which can induce and accelerate more side reactions to be occurred than the conventional heating by an oil bath. Interestingly, the molecular weight of PCL seems to decrease with the increasing amount of chlorine atoms in the tin chloride initiator. From the results, the highly reactive initiating system seems to be not suitable for the polymerization of ε-CL at high temperatures.

Interestingly, electromagnetic radiation seems to be an attractive method for producing of biodegradable polyester. As reported by Bener et al. [31], PCL could be synthesized by using the photoinduced ROP method. They used the photocatalyst without the heavy metal in the structure to catalyze the ROP of ε-CL under UV irradiation at room temperature. From the results, it was found that this system could produce PCLs with Mn and Đ of 1400–7800 g/mol and 1.15–1.36, respectively. The Mn values of PCL seem to be lower than the microwave irradiation reported in this present work. However, the Đ values of our synthesized PCLs are higher because the polymerization occurred at a very high temperature that induces the intramolecular transesterification as described above. An example of the intramolecular transesterification mechanism of PCL catalyzed by nBu2SnCl2 is displayed in Fig. 5.

Fig. 5
figure 5

The proposed intramolecular transesterification of PCL catalyzed by nBu2SnCl2

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By applying the transesterification mechanisms reported in the literature [30], the intramolecular transesterification of PCL in the presence of nBu2SnCl2 can be described as follow: (i) the Sn-Cl active center is coordinated with the carbonyl oxygen from the PCL chain resulting in the more reactive carbonyl group, (ii) oxygen atom at the suitable position attacks the carbonyl carbon of others carbonyl group resulting in the PCL chain cleavage, and (iii) the macrocycle is formed and the cleaved PCL chain is shorter than the starting PCL. This intramolecular transesterification process causes the reduction of PCL molecular weight and broadens the molecular weight distribution [29, 30]. In the case of Sn(Oct)2, it acted as an excellent transesterification catalyst in the ROP of cyclic esters [1, 30, 32]. The higher temperature and long reaction time are not suitable for this type of initiator. As mentioned in our previous work [30], Sn(Oct)2 acted as a highly reactive intramolecular transesterification catalyst for the high molecular weight poly(L-lactic acid) (PLA). It causes the fast reduction of PLA molecular weight in CHCl3 solvent at 35 °C with an apparent transesterification rate constant (kapp) of 0.297 d−1.

After obtaining the molecular weight and Đ data, the functional groups of the synthesized PCLs are investigated by the FTIR technique and the examples of FTIR spectra are depicted in Fig. 6. From Fig. 6, the vibrational assignments to the FTIR spectra of all synthesized PCLs consist of: (i) the weak absorption peak of –OH stretching of PCL end group at 3700 cm−1, (ii) the moderate signal of –CH2 stretching around 2900–3000 cm−1, (iii) the sharp signal of C = O stretching at 1700 cm−1, (iv) the weak signal of –CH2 bending at 1450 cm−1, and (v) the moderate signal of C-O stretching at 1200 cm−1. The obtained results are close to the FTIR spectra reported in the literature [33, 34].

Fig. 6
figure 6

FTIR spectra (KBr disk) of the synthesized PCLs obtained from the ROP of ε-CL initiated by 0.200 mol% of nBu3SnCl (entry 7) and nBu2SnCl2 (entry 11) at a microwave power of 450 W and 30 min

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To support the results from FTIR analysis, the 1H and 13C-NMR techniques are used. The 400 MHz 1H-NMR spectra of the synthesized PCLs are shown in Fig. 7. The 1H-NMR spectra of all PCLs show a similar pattern, and the signals are listed as follows: (i) the multiplets signal of methylene proton connected to the carbonyl oxygen of PCL chain at 4.20 ppm (f), (ii) the triplet signal of methylene proton connected to chloride end group at 3.65 ppm (a) [35], (iii) the triplet single of methylene proton connected to carbonyl carbon at 2.30 ppm (e), and (iv) the multiplets signal of methylene proton of PCL chain around 1.40–1.60 ppm (b,c,d,g,h,i). The obtained 1H-NMR spectra of PCLs are similar to the literature [19, 25, 28].

Fig. 7
figure 7

400 MHz 1H-NMR spectra of the synthesized PCLs obtained from the ROP of ε-CL initiated by 0.200 mol% of nBu3SnCl (entry 7), nBu2SnCl2 (entry 11), and Sn(Oct)2 (entry 19) at a microwave power of 450 W and 30 min

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To support the results from Fig. 7, the 100 MHz 13C-NMR spectra of the synthesized PCLs are depicted in Fig. 8. The assignments of 13C-NMR spectra of the synthesized PCLs consist of: (i) the sharp signal of carbonyl carbon is found around 173.55 ppm (f, l) (Fig. 8a), (ii) the singlet signal of the carbon of methylene group connected to oxygen is found around 64.15 ppm (a, g) (Fig. 8b), and (iii) the singlet signal of the carbon of methylene group in PCL chain is observed around 34.2 (e, k), 28.5 (b, h), 25.5 (c, i), and 24.5 (d, j) ppm (Fig. 8c). The obtained 13C-NMR spectra of PCLs are similar to the literature [36]. From Figs. 7 and 8, it is shown that the PCL is synthesized from the ROP of ε-CL with all initiating systems by using microwave irradiation.

Fig. 8
figure 8

100 MHz 13C-NMR spectra of the synthesized PCLs obtained from the ROP of ε-CL initiated by 0.200 mol% of nBu3SnCl (entry 7), nBu2SnCl2 (entry 11), and Sn(Oct)2 (entry 19) at a microwave power of 450 W and 30 min

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Polymerization mechanism

The obtained results confirm that the PCL is completely synthesized from the ROP of ε-CL. The ROP of ε-CL with n-butyltin(IV) chloride initiators can be explained by coordination-insertion mechanism as displayed in Scheme 1. By using the data reported in the literature [35], the ROP of ε-CL with nBuSnCl3 initiator is started by the coordination of the nBuSnCl3 with the carbonyl oxygen of the ε-CL ring. Then, the Cl will attack the alkyl-oxygen bond (–CH2–O–) of ε-CL resulting in the alkyl-oxygen bond scission in the ε-CL ring. The broken ε-CL ring will insert into the nBuSnCl3 to form the propagating species. At this stage, the Sn-Cl active site of nBuSnCl3 is changed to the reactive Sn–O bond in propagating species. After that, another ε-CL molecule will react with this active site via the same mechanism resulting in the formation of a long PCL chain.

Scheme 1
figure 1

Plausible coordination-insertion mechanism for the solvent-free ROP of ε-CL initiated by nBuSnCl3 under microwave irradiation

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In the ROP of ε-CL with reactive Sn(Oct)2, it is understood that the polymerization involves the formation of the true active species such as tin(II) mono and di-alkoxides (OctSnOR and Sn(OR)2) [23, 24]. Sn(Oct)2 will react with hydroxyl impurities to form the OctSnOR and Sn(OR)2 before initiating the ROP of ε-CL. After forming the true initiator, the polymerization is started by the coordination of ε-CL with the reactive Sn–O bond of an initiator. The O from the reactive Sn–O bond will attack the carbonyl carbon of the ε-CL ring and be followed by the acyl-oxygen bond cleavage as shown in Scheme 2. After the complete propagation step, the long PCL chain is obtained.

Scheme 2
figure 2

Coordination-insertion mechanism for the solvent-free ROP of ε-CL initiated by Sn(Oct)2 under microwave irradiation.

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Conclusions

The solvent-free ROP of ε-CL with nBu3SnCl and nBu2SnCl2 initiators was improved from our previous work by using microwave irradiation. The bulk polymerization of ε-CL with all initiators was completed in a short time. The molecular weight of PCL obtained from nBu3SnCl was higher than nBu2SnCl2, nBuSnCl3 ≈ Sn(Oct)2. The high steric hindrance and slow initiator could produce high molecular weight PCL under suitable conditions. This may be related to the lower amount of side reaction that reduces the PCL molecular weight. The highest molecular weight (Mn = 3.70 × 104 g/mol and Mw = 6.31 × 104 g/mol) of PCL was obtained from the slowest nBu3SnCl initiator with %yield and Đ of 73% and 1.71, respectively. The increasing number of chlorine atoms in n-butyltin(IV) chloride caused the reducing of PCL molecular weight and increasing Đ value that might be corresponded to the increasing amount of intramolecular transesterification. Under the condition used in this work, the highly reactive Sn(Oct)2 was not suitable for the solvent-free synthesis of PCL under microwave irradiation. The polymerization mechanism of the ROP of ε-CL with all n-butyltin(IV) chloride initiators was proposed through coordination-insertion mechanism. The results from this work were recently applied to other metal-containing initiators and scale-up of the synthesis of biodegradable polymers that still working in our laboratory.