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Organocatalytic Ring-Opening Polymerization of ε-Caprolactone Using bis(N-(N′-butylimidazolium)alkane Dicationic Ionic Liquids as the Metal-Free Catalysts: Polymer Synthesis, Kinetics and DFT Mechanistic Study

In this work, we successfully synthesized high thermal stable 1,n-bis(N-(N′-butylimidazolium)alkane bishexafluorophosphates (1,n-bis[Bim][PF6], n = 4, 6, 8, and 10) catalysts in 55–70% yields from imidazole which were applied as non-toxic DILs catalysts with 1-butanol as initiator for the bulk ROP of ε-caprolactone (CL) in the varied ratio of CL/nBuOH/1,4-bis[Bim][PF6] from 200/1.0/0.25–4.0 to 700/1.0/0.25–4.0 by mol%. The result found that the optimal ratio of CL/nBuOH/1,4-bis[Bim][PF6] 400/1.0/0.5 mol% at 120 °C for 72 h led to the polymerization conversions higher than 95%, with the molecular weight (Mw) of PCL 20,130 g mol−1 (Đ~1.80). The polymerization rate of CL increased with the decreasing linker chain length of ionic liquids. Moreover, the mechanistic study was investigated by DFT using B3LYP (6–31G(d,p)) as basis set. The most plausible mechanism included the stepwise and coordination insertion in which the alkoxide insertion step is the rate-determining step.


Introduction
Biodegradable polymeric materials with optimized physico-chemical and degradation properties have become more important in terms of new biomedical technologies in recent decades. As a result, a new generation of synthetic biodegradable polymers and comparable natural polymers have been modified specifically for biomedical applications [1,2]. Poly(ε-caprolactone) (PCL) is one of biologically relevant aliphatic polyesters widely used in various applications. Recently, Woodruff and Hutmacher reported the wide range of biomedical applications involving PCL, including drug delivery, medical devices, and tissue engineering of bone, cartilage, blood vessels, skin, and nerve [3][4][5].
Typically, the synthesis of biodegradable polymers, such as polylactones and polylactide, can be synthesized by several methods, e.g., polycondensation of hydroxycarboxylic acids or ring-opening polymerization (ROP) of cyclic ester monomers. However, the  6 ] as a metal-free catalyst with good conversions and narrow molecular weight distributions [53]. According to the aforementioned details, imidazolium ionic liquid has been commonly used as green solvent and catalyst in the ROP of cyclic monomers. These have a high catalytic efficiency in ROP.
In this work, we aim to investigate the synthesis, characterization, and more detail of mechanistic studies of the bulk ROP of ε-caprolactone (CL, 1) with the efficient dicationic ionic liquid catalyst system of 1,n-bis[N-(N -butylimidazolium)]alkane bishexafluorophosphate (1,n-bis[Bim][PF 6 ], n = 4, 6, 8 and 10) catalysts (2a-2d) as a metal-free catalysts and alcohols for the generation of PCL (3) of controlled molecular weight and polydispersity (Ð) (Scheme 1). In addition, we aim to understand the mechanistic aspect of the bulk ROP studies of CL by means of DFT calculations.
In this work, we aim to investigate the synthesis, characterization, and more detail of mechanistic studies of the bulk ROP of ε-caprolactone (CL, 1) with the efficient dicationic ionic liquid catalyst system of 1,n-bis[N-(N′-butylimidazolium)]alkane bishexafluorophosphate (1,n-bis [Bim][PF6], n = 4, 6, 8 and 10) catalysts (2a-2d) as a metal-free catalysts and alcohols for the generation of PCL (3) of controlled molecular weight and polydispersity (Đ) (Scheme 1). In addition, we aim to understand the mechanistic aspect of the bulk ROP studies of CL by means of DFT calculations. Scheme 1. Ring-opening polymerization of CL (1) using DILs (2a-2d) as catalysts.

Instruments
All melting points were determined on a Gallenkamp Electrothermal apparatus, UK. Fourier transform infrared (FTIR) spectra were recorded on a Bruker TENSOR 27 spectrometer (Karlsruhe, Germany) and the wavenumbers were recorded from 400 to 4000 cm -1 . The high-resolution mass spectra (HRMS, m/z values) were determined from the Scheme 1. Ring-opening polymerization of CL (1) using DILs (2a-2d) as catalysts.

Instruments
All melting points were determined on a Gallenkamp Electrothermal apparatus, UK. Fourier transform infrared (FTIR) spectra were recorded on a Bruker TENSOR 27 spectrometer (Karlsruhe, Germany) and the wavenumbers were recorded from 400 to 4000 cm −1 . The high-resolution mass spectra (HRMS, m/z values) were determined from the HR-TOF-MS Micromass model VQ-TOF2 (Manchester, UK) and Finnigan MAT 95 mass spectrometers (Waltham, MA, USA). The proton and carbon-nuclear magnetic resonance spectroscopy (500 MHz 1 H-NMR and 125 MHz 13 C-NMR) were recorded on a Bruker NEO TM 500 NMR spectrophotometer (Karlsruhe, Germany). Tetramethylsilane (TMS), or the residual signals, was used as an internal standard with the solvent resonance as the internal standard (CHCl 3 impurity in CDCl 3 , δ 7. 26 49.00; DMSO-d 6 , 2.50 and 39.52 ppm). The NMR data was reported in the following order: chemical shift, multiplicity and coupling constants (J, in hertz). Splitting patterns shown in NMR data were assigned as follows: singlet (s), doublet (d), triplet (t), and multiplet (m). The broad NMR peak was denoted by br prior to the chemical shift multiplicity. Gel permeation chromatograph (GPC) was carried out on a Waters 2414 GPC (Minneapolis, MN, USA) connected with the refractive index (RI) detector and equipped with Styragel HR5E 7.8 × 300 mm column (molecular weight ranging from 2000-4,000,000 g mol −1 ). THF was used as eluent with a flow rate of 1.0 mL/min at 40 • C. The thermal stability of dicationic liquids was investigated by thermogravimetric analysis (TGA) on a TG-DTA8122 thermo plus EVO2 (Tokyo, Japan). The thermal property of polymer samples was analyzed by the differential scanning calorimetry (DSC) on a METTLER-TOLEDO DSC-1 system (San Francisco, CA, USA).
HR-TOF-MS Micromass model VQ-TOF2 (Manchester, UK) and Finnigan MAT 95 mass spectrometers (Waltham, MA, USA). The proton and carbon-nuclear magnetic resonance spectroscopy (500 MHz 1 H-NMR and 125 MHz 13 C-NMR) were recorded on a Bruker NE-O TM 500 NMR spectrophotometer (Karlsruhe, Germany). Tetramethylsilane (TMS), or the residual signals, was used as an internal standard with the solvent resonance as the internal standard (CHCl3 impurity in CDCl3, δ 7.26 and 77.0 ppm; MeOH-d4, 3.31 and 49.00; DMSO-d6, 2.50 and 39.52 ppm). The NMR data was reported in the following order: chemical shift, multiplicity and coupling constants (J, in hertz). Splitting patterns shown in NMR data were assigned as follows: singlet (s), doublet (d), triplet (t), and multiplet (m). The broad NMR peak was denoted by br prior to the chemical shift multiplicity. Gel permeation chromatograph (GPC) was carried out on a Waters 2414 GPC (Minneapolis, MN, USA) connected with the refractive index (RI) detector and equipped with Styragel HR5E 7.8 × 300 mm column (molecular weight ranging from 2000-4,000,000 g mol -1 ). THF was used as eluent with a flow rate of 1.0 mL/min at 40 °C. The thermal stability of dicationic liquids was investigated by thermogravimetric analysis (TGA) on a TG-DTA8122 thermo plus EVO2 (Tokyo, Japan). The thermal property of polymer samples was analyzed by the differential scanning calorimetry (DSC) on a METTLER-TOLEDO DSC-1 system (California, USA).

Thermal Decomposition Analysis and Cytotoxicity Testing of the Synthesized Dicationic Ionic Liquids (2a-2d)
The mass loss profile of the synthesized DIL catalysts (2a-2d) was recorded from 20.0 to 600.0 • C at a heating rate of 10.0 • C/min on a TG-DTA8122 thermo plus EVO2 (Rigaku, Tokyo, Japan), which could obtain the mass loss and mass loss derivative of DILs during thermal decomposition. Heating rates was 10.0 • C/min from 25.0 to 600.0 • C. The synthesized DIL catalysts (2a-2d) were submitted to preliminary cytotoxicity assays against vero cells (African green monkey kidney fibroblast) at the cytotoxicity test Service Center at Scientific Instruments Center, King Mongkut's Institute of Technology Ladkrabang University (KMITL), Bankok, Thailand. The synthesized DILs solution at 100 µg/mL concentration were determined using the colorimetric MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) assay in 96-well microtiter plates. DMSO was used as the reference substance. Cell lines were grown at 1 × 10 5 cell/well and seeded to 96-well plates and further incubated at 37 • C with 5% of CO 2 atmosphere for 24 h. Then, media was extracted from each well, and 100 µg/mL of sample solution (100 µL/well) was added. After that, 10 µL/well of the MTT solution with concentration of 5 mg/mL was added and incubated at 37 • C with 5% CO 2 atmosphere for 4 h. Then, the MTT solution was aspirated, and the 100 µL/well of 100% DMSO with 10% of SDS at a ratio 9:1 was added. The quantity of formazan (presumably directly proportional to the number of viable cells) is measured by recording changes in absorbance at 570 nm. The percentages of cytotoxicity were calculated using Equation (1) [58].
where A is the control wells' absorbance (wells with cells in cultured food), and B is the absorbance of the wells containing the cells. The values of A and B must be subtracted by the absorbance of blank (well with DMSO and SDS solution).  , 3) was then purified by precipitation in cold methanol, and the chemical structure and molecular weight averages of the purified PCL was further characterized by 1 H-NMR and GPC techniques. The thermal property of PCLs was investigated by DSC technique. 5 mg of PCL sample was weighed into the aluminum pan and sealed. The sample was heated from 20 to 250 • C at a heating rate 10 • C/min and then held at 250 • C for 1 min. Then, the molten sample was cooled down from 250 to 20 • C at a cooling rate of 10 • C/min. For the second heating, the sample was heated from 20 to 250 • C at a heating rate 10 • C/min.   (2) [53].
where I α and I β are the proton integral from the ethylenic protons of PCL (3) (4.14 ppm) and monomer (4.26 ppm), respectively.

Computational Study by Density Functional Theory (DFT)
Density functional theory (DFT) calculations were performed at B3LYP level with 6-31G(d,p) basis set to study the ROP of ε-caprolactone (CL) triggered by 1,4-bis[Bim][PF 6 ] (2a) catalyst [59,60]. The local lowest point was found via structural optimizations of reactants (R), complexes (COM), intermediates (INT), and products (P). Using the usual Berny transition-state optimization method, all transition states (TS) were estimated to be saddle points. Furthermore, vibrational frequency calculations were performed at the same level of theory to verify optimal structures by zero imaginary frequency stretching along the reaction path for R, INT, and P, and one imaginary frequency stretching along the reaction path for all TS structures. By including zero-point vibration energy contributions, Gibbs free energy profiles for all systems were adjusted. Thermal adjustments were also tested at 298 • F. The Gaussian 09 suite of programs was used for all calculations [61].

1,n-Bis[Bim][PF
The preliminary cytotoxicity evaluation of these DILs (2a-2d) was tested using the conventional MTT assay. In order to find out whether these DILs (2a-2d) are toxic to normal cells, their anti-proliferative activity against African green monkey kidney fibroblast (Vero) were evaluated, and the results are shown in Table S1. Percentage cell viability (% living cells) showed that all DILs (2a-2d) exhibited less cytotoxicity against tested cell lines with a concentration of 100 µg/mL. Furthermore, the percentage of cell viability from cytotoxicity testing of the synthesized DILs was higher than that of the control experiment of 1% DMSO.

Synthesis of Poly(ε-Caprolactone) via the ROP of ε-Caprolactone using the Synthesized 1,4bis[Bim][PF6] (2a) Catalyst with 1-Butanol Initiator
Generally, it is known that the hydrogen atom at the C-2 position of the imidazolium salt (H-2) is a weak Brønsted acid that can catalyze many organic reactions. We first eval-

Synthesis of Poly(ε-Caprolactone) via the ROP of ε-Caprolactone using the Synthesized 1,4-bis[Bim][PF 6 ] (2a) Catalyst with 1-Butanol Initiator
Generally, it is known that the hydrogen atom at the C-2 position of the imidazolium salt (H-2) is a weak Brønsted acid that can catalyze many organic reactions. We first  Table 1.  6 ] ratio of 400/1.0/0.50. mol% The molecular weight of PCL showed the increasing trend in the molar ratio of CL/nBuOH ranging from 200/1.0 to 400/1.0. However, when the molar ratio of CL/nBuOH higher than 400/1.0, the molecular weight of the obtained PCL tended to decrease. Furthermore, the Ð values for all synthesized PCLs were in the range of 1.16-1.88, indicating the low amount of transesterification occurred in our polymerization systems. From these obtained results, the most suitable synthetic condition from Table 1 would be applied to other catalysts, as shown in the following section.

Synthesis of Poly(ε-Caprolactone) via the ROP of ε-Caprolactone using the Synthesized DILs (2a-2d) and Sn(Oct) 2 Catalyst with 1-Dodecanol Initiator
After obtaining the optimized synthetic condition for PCL (3) (at the CL (1)/nBuOH/1,4bis[Bim][PF 6 ] ratio of 400/1.0/0.50 mol%), as in previous section, it was applied to other DILs catalysts and Sn(Oct) 2 . In this section, the 1-dodecanol (nC 12 H 25 OH) initiator was used as a substituent for nBuOH in the synthesis at a higher temperature range. The ROP of CL catalyzed by DILs (2a-2d) and Sn(Oct) 2 with nC 12 H 25 OH initiator was carried out at 150, 160, and 170 • C for 72 h. After complete polymerization, the obtained crude PCL was dissolved in CHCl 3 and precipitated in cold methanol yielding purified PCL. The obtained purified PCL was further characterized by 1 H-NMR and GPC technique, and the results are summarized in Table 2.
From Table 2, the % conversion for the ROP of CL with all catalysts were higher than 96%. Moreover, it was found that the molecular weight of PCL seemed to be higher than the molecular weight of PCL shown in Table 1. This suggested that the molecular weight of PCL could be improved by increasing the polymerization temperature. Furthermore, the molecular weight of PCL slightly increased with the increasing of DIL chain length. When comparing the performance of DILs with the conventional system of Sn(Oct) 2 , it was found that the DILs showed the equivalent performance to Sn(Oct) 2 in terms of PCL molecular weight. Additionally, the Ð values for the synthesized PCL were higher than PCL shown in Table 1. This clearly demonstrated that the increasing of synthesis temperature could result in the broadening of PCL molecular weight distribution due to the higher amount of transesterification reactions.
The thermal property of the synthesized PCL was subsequently identified by DSC technique. An example of thermal characterization of PCL sample (entry 25) is displayed in supporting data ( Figure S41). It was found that the crystalline melting temperature (T m ) of PCL was found at around 40-58 • C for the first heating. From the cooling step, the crystallization exotherm was observed at around 30-40 • C. This indicated that the synthesized PCL could crystallize under the synthetic condition used in this work. For the second heating, the T m of PCL was found ca. 45-75 • C. It was important that the one melting peak of PCL was observed, suggesting one shape and size of PCL crystal which was obtained.

Kinetic Studies of the ROP of ε-Caprolactone Catalyzed by the Synthesized DIL Catalysts (2a-2d) with 1-Dodecanol Initiator
The reactivity of all synthesized DILs (2a-2d) in the ROP of CL was investigated at the temperature of 150 • C for 4 h using the 1 H-NMR technique. An example of the 1 H-NMR spectra of crude PCL obtained from the ROP of CL catalyzed by 0.50 mol% of 1,4-bis[Bim][PF 6 ] (2a) and 1.0 mol% of nC 12 H 25 OH at 150 • C are illustrated in Figure 3. The 1 H-NMR spectra of crude PCL were obtained from the ROP of CL catalyzed other DILs (2b-2d) are depicted in the supporting data (see Figures S38-S40).
From Figure 3, the intensity of triplet proton of the methylene group connected to carbonyl carbon of PCL chain at 2.30 ppm (1 and 6) increased with increasing polymerization time, but the intensity of triplet proton of the methylene group adjacent to carbonyl carbon of CL ring at 2.65 ppm (1 ) decreased similar to other DIL catalysts.
Furthermore, all spectra showed the multiplet proton of methyl chain end of butyl group (CH 3 ) at 0.90 ppm (a). The multiplet proton of methylene group of PCL and CL was found at 1.20-1.65 ppm (2, 3, 4, 7, 8, 9, 2 , 3 , 4 , b, c). The triplet proton of methylene group connected to -OH end group was found at 3.65 ppm (10). Finally, the triplet proton of methylene groups adjacent to oxygen atom of CL and PCL were found at 4.12 (5, d) and 4.25 ppm (5 ), respectively. From the spectra, the monomer conversion was determined from the Equation (2). The plots of % monomer conversion against polymerization time for the ROP of CL catalyzed by 0.50 mol% of DILs (2a-2d) with 1.0 mol% of nC 12 H 25 OH at 150 • C are depicted in Figure 4.
NMR spectra of crude PCL obtained from the ROP of CL catalyzed by 0.50 mol% of 1,4bis [Bim][PF6] (2a) and 1.0 mol% of nC12H25OH at 150 °C are illustrated in Figure 3. The 1 H-NMR spectra of crude PCL were obtained from the ROP of CL catalyzed other DILs (2b-2d) are depicted in the supporting data (see Figures S38-S40).
From Figure 3, the intensity of triplet proton of the methylene group connected to carbonyl carbon of PCL chain at 2.30 ppm (1 and 6) increased with increasing polymerization time, but the intensity of triplet proton of the methylene group adjacent to carbonyl carbon of CL ring at 2.65 ppm (1′) decreased similar to other DIL catalysts. Furthermore, all spectra showed the multiplet proton of methyl chain end of butyl group (CH3) at 0.90 ppm (a). The multiplet proton of methylene group of PCL and CL was found at 1.20-1.65 ppm (2, 3, 4, 7, 8, 9, 2′, 3′, 4′, b, c). The triplet proton of methylene group connected to -OH end group was found at 3.65 ppm (10). Finally, the triplet proton of methylene groups adjacent to oxygen atom of CL and PCL were found at 4.12 (5, d) and 4.25 ppm (5′), respectively. From the spectra, the monomer conversion was determined from the Equation (2). The plots of % monomer conversion against polymerization time

Mechanistic Study Using DFT Method
From the obtained results, the synthesized 1,4-bis[Bim][PF6] (2a) acted as the most efficient catalyst in the ROP of CL in terms of PCL molecular weight and polymerization rate. To investigate the polymerization mechanism, the computation detail by DFT method was utilized. The mechanism of the ROP of CL catalyzed by 1,4-bis[Bim][PF6] (2a) and initiated by nBuOH was described and used as the template for other DIL catalysts. In this part, the abbreviations of COM, TS, INT, and P were the complex, the transition state, the intermediate, and the product, respectively. The polymerization started with the coordination between CL monomer, 2a, and nBuOH that could be classified into three mechanisms. Starting with the coordination of two units of CL with the acidic protons on the imidazole ring of 2a, resulting in the formation of COM1, is depicted in Scheme 3. The coordination of monomer was a spontaneous process that could be confirmed by the determined Gibbs free energy (ΔG) of −45.21 kcal mol -1 .

Mechanistic Study Using DFT Method
From the obtained results, the synthesized 1,4-bis[Bim][PF 6 ] (2a) acted as the most efficient catalyst in the ROP of CL in terms of PCL molecular weight and polymerization rate. To investigate the polymerization mechanism, the computation detail by DFT method was utilized. The mechanism of the ROP of CL catalyzed by 1,4-bis[Bim][PF 6 ] (2a) and initiated by nBuOH was described and used as the template for other DIL catalysts. In this part, the abbreviations of COM, TS, INT, and P were the complex, the transition state, the intermediate, and the product, respectively. The polymerization started with the coordination between CL monomer, 2a, and nBuOH that could be classified into three mechanisms. Starting with the coordination of two units of CL with the acidic protons on the imidazole ring of 2a, resulting in the formation of COM1, is depicted in Scheme 3.
The coordination of monomer was a spontaneous process that could be confirmed by the determined Gibbs free energy (∆G) of −45.21 kcal mol −1 .
rate. To investigate the polymerization mechanism, the computation detail by DFT method was utilized. The mechanism of the ROP of CL catalyzed by 1,4-bis[Bim][PF6] (2a) and initiated by nBuOH was described and used as the template for other DIL catalysts. In this part, the abbreviations of COM, TS, INT, and P were the complex, the transition state, the intermediate, and the product, respectively. The polymerization started with the coordination between CL monomer, 2a, and nBuOH that could be classified into three mechanisms. Starting with the coordination of two units of CL with the acidic protons on the imidazole ring of 2a, resulting in the formation of COM1, is depicted in Scheme 3. The coordination of monomer was a spontaneous process that could be confirmed by the determined Gibbs free energy (ΔG) of −45.21 kcal mol -1 . Then, COM1 was further coordinated with nBuOH via two steps before the ringopening of CL ring: (i) the coordination of nBuOH with the carbonyl group of CL ring forming COM2 with G value of -31.57 kcal mol −1 and (ii) the coordination of nBuOH with the acyl oxygen atom of CL ring forming COM3 with ∆G value of −31.28 kcal mol −1 . For the third mechanism, the flexible C−C bond of 2a could also be rotated to produce Twisted-2a. The imidazole rings were placed on the same side due to the C−C bond rotation that affected the coordination of monomer. From this, one unit of CL could be used to coordinate Twisted-2a yielding COM4 with ∆G of −22.94 kcal mol −1 . Although the ∆G value of COM4 (22.94 kcal mol −1 ) is a less negative value than that of COM2 and COM3, it still indicated that the coordination process was spontaneous. Because of the steric hindrance around the imidazole rings, nBuOH could only coordinate at the acyl oxygen atom of CL yielding COM5 with ∆G of −13.74 kcal mol −1 .
Firstly, we discussed the ROP mechanism in the first pathway that was calculated as the stepwise mechanism as shown in Scheme 4. After the coordination of CL with COM1, the carbonyl oxygen of CL was coordinated with nBuOH, producing the stimulated monomer as COM2 with the more electrophilic carbonyl carbon. The oxygen atom attacked the weak carbonyl carbon, and the proton of nBuOH was transferred to the oxygen atom on the carbonyl carbon via the four-membered ring to form transition state 1 (TS1-8) with ∆G of 14.64 kcal mol −1 , indicating the non-spontaneous process and required ∆G ‡ of 46.24 kcal mol −1 . After that, the planar carbonyl carbon was rearranged to the tetrahedral geometry yielding intermediate 1 (INT1-9). At TS1-10, the proton was transferred to acyl oxygen, and the C−O of acyl bond was broken, and that required the ∆G ‡ of 25.94 kcal mol −1 . Finally, the product of C−O bond cleavage was given as product 1 (P1-11) with the ∆G of −21.82 kcal mol −1 . The initiation of another CL molecule was also proceeded by a similar mechanism as described. The results of the second initiation revealed that the ∆G of TS1- 12  The second mechanism for the ROP of CL catalyzed by the 1,4-bis[Bim][PF6] (2a) with nBuOH initiator is illustrated in Scheme 5. The acyl oxygen atom of CL was coordinated with nBuOH after the coordination between CL and 2a yielding COM3. The ΔG of TS2-16 was 16.14 kcal mol -1 and required the ΔG ‡ of 47.42 kcal mol -1 . Then, the carbonyl carbon of CL was attacked by the oxygen atom of nBuOH, and the proton of nBuOH was transferred to the acyl oxygen atom of CL, leading to formation of P2-17. Although the ΔG (18.02 kcal mol -1 ) of the TS2-18 was higher than TS2-16, TS2-18 showed a lower value of ΔG ‡ than that of TS2-16. This indicated that TS2-16 was the rate-determining step for the second ROP mechanism of CL catalyzed by the 1,4-bis[Bim][PF6] (2a) with nBuOH initiator. The second mechanism for the ROP of CL catalyzed by the 1,4-bis[Bim][PF 6 ] (2a) with nBuOH initiator is illustrated in Scheme 5. The acyl oxygen atom of CL was coordinated with nBuOH after the coordination between CL and 2a yielding COM3. The ∆G of TS2-16 was 16.14 kcal mol −1 and required the ∆G ‡ of 47.42 kcal mol −1 . Then, the carbonyl carbon of CL was attacked by the oxygen atom of nBuOH, and the proton of nBuOH was transferred to the acyl oxygen atom of CL, leading to formation of P2-17. Although the ∆G (18.02 kcal mol −1 ) of the TS2-18 was higher than TS2-16, TS2-18 showed a lower value of ∆G ‡ than that of TS2-16. This indicated that TS2-16 was the rate-determining step for the second ROP mechanism of CL catalyzed by the 1,4-bis[Bim][PF 6 ] (2a) with nBuOH initiator.
For the third mechanism, the ROP of CL catalyzed by the 1,4-bis[Bim][PF 6 ] (2a) with nBuOH initiator proceeded via the one TS3-20, as displayed in Scheme 6. In this mechanism, the CL monomer was coordinated with Twisted-2a, yielding COM4. Then, the acyl oxygen atom of the activated CL was coordinated with nBuOH, resulting in the COM5. Then, the carbonyl carbon of activated CL was attacked by the oxygen atom of nBuOH and followed by the acyl bond scission through the four-membered ring to form TS7 with ∆G and ∆G ‡ of 29.29 and 44.62 kcal mol −1 , respectively.
The first and second ROP mechanisms demonstrated that the ∆G values of COM2 and COM3 were found to be −31.57 and −31.28 kcal mol −1 , respectively. However, the ∆G ‡ values for the rate-determining step of the first and second ROP mechanisms were 46.24 and 47.42 kcal mol −1 , respectively. For the third ROP mechanism, the COM5 displayed a higher ∆G value than COM2 and COM3. Furthermore, the ∆G ‡ value for the third ROP mechanism was the highest. Therefore, based on ∆G and ∆G ‡ for these three ROP mechanisms, the first ROP mechanism showed the lowest in both of ∆G and ∆G ‡ values. Therefore, the ROP mechanism of CL catalyzed by the synthesized 1,4bis[Bim][PF 6 ] (2a) with nBuOH initiator occurred and proceeded through the first ROP mechanism. The overall ∆G profile for the three mechanisms of the ROP of CL catalyzed by the synthesized 1,4-bis[Bim][PF 6 ] (2a) with nBuOH initiator using the calculation from the B3LYP/6-31G(d,p) level is illustrated in Figure 5. For the third mechanism, the ROP of CL catalyzed by the 1,4-bis[Bim][PF6] (2a) with nBuOH initiator proceeded via the one TS3-20, as displayed in Scheme 6. In this mechanism, the CL monomer was coordinated with Twisted-2a, yielding COM4. Then, the acyl oxygen atom of the activated CL was coordinated with nBuOH, resulting in the COM5. Then, the carbonyl carbon of activated CL was attacked by the oxygen atom of nBuOH and followed by the acyl bond scission through the four-membered ring to form TS7 with ΔG and ΔG ‡ of 29.29 and 44.62 kcal mol -1 , respectively. The first and second ROP mechanisms demonstrated that the ΔG values of COM2 and COM3 were found to be −31.57 and −31.28 kcal mol -1 , respectively. However, the ΔG ‡ values for the rate-determining step of the first and second ROP mechanisms were 46.24 and 47.42 kcal mol -1 , respectively. For the third ROP mechanism, the COM5 displayed a higher ΔG value than COM2 and COM3. Furthermore, the ΔG ‡ value for the third ROP mechanism was the highest. Therefore, based on ΔG and ΔG ‡ for these three ROP mechanisms, the first ROP mechanism showed the lowest in both of ΔG and ΔG ‡ values. Therefore, the ROP mechanism of CL catalyzed by the synthesized 1,4-bis[Bim][PF6] (2a) with nBuOH initiator occurred and proceeded through the first ROP mechanism. The overall ΔG profile for the three mechanisms of the ROP of CL catalyzed by the synthesized 1,4bis [Bim][PF6] (2a) with nBuOH initiator using the calculation from the B3LYP/6-31G(d,p) level is illustrated in Figure 5. For the third mechanism, the ROP of CL catalyzed by the 1,4-bis[Bim][PF6] (2a) with nBuOH initiator proceeded via the one TS3-20, as displayed in Scheme 6. In this mechanism, the CL monomer was coordinated with Twisted-2a, yielding COM4. Then, the acyl oxygen atom of the activated CL was coordinated with nBuOH, resulting in the COM5. Then, the carbonyl carbon of activated CL was attacked by the oxygen atom of nBuOH and followed by the acyl bond scission through the four-membered ring to form TS7 with ΔG and ΔG ‡ of 29.29 and 44.62 kcal mol -1 , respectively. The first and second ROP mechanisms demonstrated that the ΔG values of COM2 and COM3 were found to be −31.57 and −31.28 kcal mol -1 , respectively. However, the ΔG ‡ values for the rate-determining step of the first and second ROP mechanisms were 46.24 and 47.42 kcal mol -1 , respectively. For the third ROP mechanism, the COM5 displayed a higher ΔG value than COM2 and COM3. Furthermore, the ΔG ‡ value for the third ROP mechanism was the highest. Therefore, based on ΔG and ΔG ‡ for these three ROP mechanisms, the first ROP mechanism showed the lowest in both of ΔG and ΔG ‡ values. Therefore, the ROP mechanism of CL catalyzed by the synthesized 1,4-bis[Bim][PF6] (2a) with nBuOH initiator occurred and proceeded through the first ROP mechanism. The overall ΔG profile for the three mechanisms of the ROP of CL catalyzed by the synthesized 1,4bis [Bim][PF6] (2a) with nBuOH initiator using the calculation from the B3LYP/6-31G(d,p) level is illustrated in Figure 5.

Conclusions
The DILs with different chain lengths (2a-2d) were successfully synthesized using the commercially available imidazole as a starting material with % yield in a range of 55-70%. The synthesized DILs were completely characterized by FITR, 1 H-NMR, 13 C-NMR Figure 5. Calculated Gibbs free energy profiles for three ROP mechanism pathways of CL initiated by 2a and 1-butanol calculated at B3LYP/6-31G(d,p) level.

Conclusions
The DILs with different chain lengths (2a-2d) were successfully synthesized using the commercially available imidazole as a starting material with % yield in a range of 55-70%. The synthesized DILs were completely characterized by FITR, 1 H-NMR, 13 C-NMR HRMS, and mass spectrometry. From TGA analysis, the synthesized DILs showed high thermal stability with starting degradation temperature of around 260-320 • C. Based on cytotoxicity testing, the synthesized DILs exhibited less cytotoxicity against monkey kidney epithelial cells with a concentration of 100 µg/mL. From bulk polymerization of CL catalyzed by 1,4bis[Bim][PF 6 ] (2a) with nBuOH initiator, the PCL with M w of 20,130 g mol −1 was obtained at the molar ratio of CL/nBuOH/1,4-bis[Bim][PF 6 ] of 400/1.0/0.50. The chain length of the synthesized DILs slightly affected the molecular weight of PCL. The molecular weight of PCL could be improved by increasing the polymerization temperature. The highest M w (32,227 g mol −1 ) of PCL was obtained from the ROP of CL catalyzed by 1, 6 The effectiveness of the synthesized DILs was equivalent to the conventional system of Sn(Oct) 2 under the condition used in this work. From a computational study by DFT using the B3LYP/6-31G(d,p) level, the ROP of CL started by coordination of CL monomer with 1,4-bis[Bim][PF 6 ] (2a), resulting in more electrophilic carbonyl carbon of CL and followed by the attack of nBuOH to the carbonyl carbon of CL. Then, the proton of nBuOH is transferred to acyl oxygen, resulting in the acyl bond of CL scission. The ROP of CL with other DILs has been proposed through a similar mechanism to 1,4-bis[Bim][PF 6 ] (2a). Finally, the mechanistic results obtained from this work may be applied to describe the catalytic behavior of other organocatalytic systems in the ROP of cyclic esters.