Highly Active Heterogeneous Double Metal Cyanide Catalysts for Ring-Opening Polymerization of Cyclic Monomers

A series of heterogeneous Zn-Co double metal cyanide (DMC) catalysts were investigated for ring-opening polymerization (ROP) of various cyclic monomers. Notably, inexpensive and commonly used organic solvents such as acetone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, nitromethane, and 1-methylpyrrolidin-2-one were very effective complexing agents for the preparation of DMC catalysts, showing high catalytic activity for the ROP of propylene oxide, ε-caprolactone, and δ-valerolactone. The chemical structures and compositions of the resultant catalysts were determined using various techniques such as FT-IR, X-ray photoelectron spectroscopy, powder X-ray diffraction, and elemental analysis. α,ω-Hydroxyl-functionalized polyether and polyester polyols with high yields and tunable molecular weights were synthesized in the presence of various initiators to control functionality. Kinetic studies of the ROP of δ-valerolactone were also performed to confirm the reaction mechanism.


Introduction
Aliphatic polyesters are an important and versatile class of biocompatible and biodegradable polymers with distinct mechanical properties that are useful in a wide range of applications from the biomedical fields such as biodegradable implant materials, drug delivery systems, and tissue engineering [1,2], to non-biomedical areas such as environmental, packaging, textile, elastomer, and polyurethane industries [3][4][5][6][7]. The development of bio-based polyesters from renewable resources has been of great interest in terms of sustainable manufacturing and environmental issues [8]. Ring-opening polymerization (ROP) of lactones is one of the most effective approaches for producing polyesters with a highly controlled manner [9]. In this context, lactide, ε-caprolactone (CL), and δ-valerolactone (VL) derived from biomasses such as lactic acid, hydroxymethylfurfural, and furfural, respectively [10][11][12], are typical candidates, and their polymerizations have been widely established [13]. The ROP of lactones was conducted using homogeneous catalytic systems such as alkoxide or phenolate complexes of Lewis acidic metals and rare-earth metals [14,15]. The metalfree, catalyzed ROP of lactones using organocatalytic systems have also gained increasing attention in recent decades [16][17][18][19][20][21][22][23]. Stannous octoate (Sn(Oct) 2 ) catalyst has been used for the ROP of lactones due to the high efficiency and well-controlled polymerization [24]. However, potential toxicity induced by residual metals from the catalysts have prevented the practical use of the resultant polyester for biomedical and electronic applications [25]. Heterogeneous systems exhibit several advantages with respect to their homogeneous counterparts, notably, the ease of separation and reusability, thus diminishing the manufacturing cost and environmental issues. Nevertheless, solid catalysts for the ROP of lactones counterparts, notably, the ease of separation and reusability, thus diminishing the manufacturing cost and environmental issues. Nevertheless, solid catalysts for the ROP of lactones are still underexplored, as compared to homogeneous ones, thus effective catalytic systems for lactone polymerization are still in demand.
Double metal cyanide (DMC) complexes are inorganic coordination polymers generated by two metal atoms linked via cyanide bridges [26]. They are strictly Lewis acidic and highly stable solid materials [27] that have been employed as catalysts for the ROP of propylene oxide (PO) since the 1960s [28,29]. DMC catalysts have been typically prepared by reacting aqueous solutions of ZnCl2 and K3Co(CN)6 in the presence of tertbutyl alcohol (TBA) as a complexing agent (CA) and a polyvalent alcohol such as Pluronic P-123 ® (P123) as a co-CA [30]. Thereafter, numerous organic reactions catalyzed by DMC catalysts were reported such as carbon dioxide fixation [31,32], esterification [33], transesterification [34], hydroamination [35], Prins condensation [36], oxidation reactions [37], and lactone polymerizations [38,39]. Herein, homopolymerization of various cyclic monomers including PO, CL, and VL has been investigated using various DMC catalysts in the presence of different hydroxyl-functionalized initiators ( Figure 1). To improve the catalytic activity as well as to reduce the processing cost and environmental issues induced by a conventional CA such as TBA, readily available and inexpensive common solvents such as acetone (Ac), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), nitromethane (NMe), and n-methylpyrrolidin-2-one (NMP) were used as CAs to prepare a series of DMC catalysts. The catalytic activities of these catalysts were first evaluated and optimized for the ROP of PO. Then the optimized DMC catalysts were investigated for the ROP of CL and VL. The kinetic studies of the ROP of VL were also performed to confirm the reaction mechanism.

Preparation and Optimization of DMC Catalysts
Zn-Co DMC catalysts bearing common solvents as CAs were prepared and optimized as follows. Typically, ZnCl 2 (1.23 g, 9 mmol), NMe (0.1-1.5 mL), and 2.5 mL of distilled water were added to a 20 mL vial (solution 1). In another vial, K 3 Co(CN) 6 (0.5 g, 1.5 mmol) was dissolved in 2.5 mL of distilled water (solution 2). A mixture of NMe (1.0 mL) and P123 (0.1 g, 0.017 mmol) was prepared in a third vial as solution 3. Solution 1 was then reacted with solution 2 for 30 min at 50-90 • C under vigorous stirring. Subsequently, solution 3 was added to the reaction mixture and stirred for 10 min. After removal of the filtrate, the catalyst slurry was washed with excess distilled water and dried under vacuum to obtain white powder catalyst. DMC-NMe catalysts were optimized by modifying the amount of NMe in solution 1 (V = 0.1, 0.5, 1.0, and 1.5 mL) and the reaction temperature (T = 50 • C, 70 • C, and 90 • C). Likewise, DMC-DMAc, DMC-DMF, DMC-DMSO, and DMC-NMP were prepared using DMAc, DMF, DMSO, and NMP, respectively, as Cas. The pure DMC compound, DMC-1, was simply prepared by reacting aqueous ZnCl 2 with aqueous K 3 Co(CN) 6 at room temperature in the absence of both CAs and P123 ([Zn]/[Co] ratio of 1.5:1). DMC-TBA using TBA as a CA was prepared following the literature procedure for comparison [29].

Polymerization of PO
The semi-batch ROP of PO was conducted according to the previous reported protocol [40]. Typically, DMC catalyst (100 mg) and PPG-400 initiator (20 g) have been added to a 500 mL stainless steel reactor (Parr Instrument Company, Moline, IL, USA) and evacuated for 30 min to remove any trace of water. The polymerization was performed at 115 • C under continuous addition of PO monomer until the monomer consumption reach 200 g. The induction and reaction time were monitored to determine the catalytic activity.

Polymerization of Lactones
The batch ROP of lactone was performed in a 10 mL round-bottom flask. Prescribed amounts of the DMC catalyst, initiator, and a magnetic stirrer bar were added to the reactor and purged with nitrogen for 30 min at 90 • C. Then lactone monomers were introduced, and the polymerization was investigated under different conditions. The reaction mixture was sampled (at 30-60 min intervals) and subjected to 1 H NMR and GPC analyses for evaluating of catalytic activity and reaction kinetics. The resultant polylactones were dissolved in chloroform, and the solid catalyst could be readily separated by centrifugation.

Characterization
FTIR spectra of the DMC catalysts were measured by an IR Prestige 21 spectrometer (Shimadzu Co., Tokyo, Japan) using the standard KBr disc method at a resolution of 1 cm −1 over a wavelength region from 4000 to 400 cm −1 . X-ray photoelectron spectroscopies (XPS) analysis was performed by an ESCALAB 250 induced electron emission spectrometer with monochromatic Al Kα radiation (hν = 1486.6 eV) from an X-ray source operating at 12 mA and 20 kV (Thermo Fisher Scientific Solutions LLC, Seoul, Korea). The elemental analysis of the catalysts was performed using an inductively coupled plasma (ICP) optical emission spectrometer (OES) (Varian ICP720-OES) using aqua regia digestion method and a CHNS/O analyzer (Vario-Micro Cube, Elementar Analysensysteme GmbH). Thermogravimetric analysis (TGA) was conducted with a TGA Q50 analyzer (TA Instruments, New Castle, DE, USA) from 20 • C to 800 • C with a heating rate of 10 • C min −1 . X-ray powder diffraction (XRD) analysis was performed using a RINT2000 (Rigaku Co., Tokyo, Japan) wide-angle goniometer 185 with Cu Kα radiation. The molecular weight (MW) and dispersity (Ð) were measured by gel permeation chromatography (GPC) using a Waters 150 (Agilent Technologies, Inc., Santa Clara, CA, USA) instrument operating at 40 • C with 104, 103, and 500 Å columns in tetrahydrofuran, using polystyrene standards for calibration. The hydroxyl functionality (F) of the polyols was determined by titration according to the ASTM standard (E1899-97) [41]. The 1 H NMR (400 MHz) analysis were performed using an INOVA 400 NMR (Varian Inc., Palo Alto, CA, USA) spectrometer and referenced to the residual solvent signal of CDCl 3 (7.25 and 77.16 ppm, respectively). The chemical shifts are presented in parts per million (ppm) relative to the residual solvent peaks as the internal standard. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was measured by a Voyager-DETM STR Biospectrometry Workstation (Applied Biosystems Inc., Waltham, MA, USA) equipped with a nitrogen laser delivering 3 ns laser pulses at 337 nm, using dithranol as the matrix.

Characterization of DMC Catalysts
The structure of all the DMC catalysts were initially determined by FTIR ( Figure 2). The characteristic signal of the v(C≡N) vibrations of the DMC catalysts bearing organic Cas had shifted to higher frequencies (2195-2190 cm −1 ), as compared to that of DMC-1 (2178 cm −1 ) bearing no organic CAs. For DMC-Ac and DMC-DMF, the characteristic signal of v(C=O) vibrations was located at 1701 and 1660 cm −1 , respectively. The δ(Co−CN) bands of the DMC catalysts bearing organic CAs had also shifted to higher frequencies (470-474 cm −1 ), as compared to that of DMC-1 (448 cm −1 ). The signals at 1639-1615 cm −1 were attributed to the OH bending of the absorbed water, whereas the v(C−O−C) bands of P123 were observed at 1081−1089 cm −1 for all the catalysts, except DMC-1. Details of the FTIR vibrations are summarized in Table 1. cm −1 over a wavelength region from 4000 to 400 cm −1 . X-ray photoelectron spectroscopies (XPS) analysis was performed by an ESCALAB 250 induced electron emission spectrometer with monochromatic Al Kα radiation (hν = 1486.6 eV) from an X-ray source operating at 12 mA and 20 kV (Thermo Fisher Scientific Solutions LLC, Seoul, Korea). The elemental analysis of the catalysts was performed using an inductively coupled plasma (ICP) optical emission spectrometer (OES) (Varian ICP720-OES) using aqua regia digestion method and a CHNS/O analyzer (Vario-Micro Cube, Elementar Analysensysteme GmbH). Thermogravimetric analysis (TGA) was conducted with a TGA Q50 analyzer (TA Instruments, New Castle, DE, USA) from 20 °C to 800 °C with a heating rate of 10 °C min −1 . X-ray powder diffraction (XRD) analysis was performed using a RINT2000 (Rigaku Co., Tokyo, Japan) wide-angle goniometer 185 with Cu Kα radiation. The molecular weight (MW) and dispersity (Ð) were measured by gel permeation chromatography (GPC) using a Waters 150 (Agilent Technologies, Inc., Santa Clara, CA, USA) instrument operating at 40 °C with 104, 103, and 500 Å columns in tetrahydrofuran, using polystyrene standards for calibration. The hydroxyl functionality (F) of the polyols was determined by titration according to the ASTM standard (E1899-97) [41]. The 1 H NMR (400 MHz) analysis were performed using an INOVA 400 NMR (Varian Inc., Palo Alto, CA, USA) spectrometer and referenced to the residual solvent signal of CDCl3 (7.25 and 77.16 ppm, respectively). The chemical shifts are presented in parts per million (ppm) relative to the residual solvent peaks as the internal standard. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was measured by a Voyager-DETM STR Biospectrometry Workstation (Applied Biosystems Inc., Waltham, MA, USA) equipped with a nitrogen laser delivering 3 ns laser pulses at 337 nm, using dithranol as the matrix.

Characterization of DMC Catalysts
The structure of all the DMC catalysts were initially determined by FTIR ( Figure 2). The characteristic signal of the v(C≡N) vibrations of the DMC catalysts bearing organic Cas had shifted to higher frequencies (2195−2190 cm −1 ), as compared to that of DMC-1 (2178 cm −1 ) bearing no organic CAs. For DMC-Ac and DMC-DMF, the characteristic signal of v(C=O) vibrations was located at 1701 and 1660 cm −1 , respectively. The δ(Co−CN) bands of the DMC catalysts bearing organic CAs had also shifted to higher frequencies (470-474 cm −1 ), as compared to that of DMC-1 (448 cm −1 ). The signals at 1639−1615 cm −1 were attributed to the OH bending of the absorbed water, whereas the v(C−O−C) bands of P123 were observed at 1081−1089 cm −1 for all the catalysts, except DMC-1. Details of the FTIR vibrations are summarized in Table 1.  Table S1 in the Supporting Information.

Catalyst
Vibration The XPS spectra of all the catalysts indicated that Zn, Co, O, N, and C atoms were the main components of the DMC surface ( Figure 3). The binding energy of the zinc atoms in DMC-Ac, DMC-DMAc, DMC-DMF, DMC-DMSO, DMC-NMe, and DMC-NMP catalysts were slightly increased (1047.5/1024.4−1048.0/1025.1 eV), as compared to that of free ZnCl2 (1046.6/1023.7 eV). The binding energy of Zn 2p spin-orbital components are summarized in Table S1 in the Supporting Information.   The composition of the resultant DMC catalysts was determined by combining several elemental analysis techniques. The contents of Zn and Co (wt%) were estimated based on ICP-OES analysis, whereas CHNS/O analysis and TGA were used to determine content of the organic components. With the exception of the pure DMC compound (DMC-1), all the catalysts showed a higher content of Zn (0.48−3.37 mol per 1.0 mol of Zn3[Co(CN)6]2) than the stoichiometric amount of Co (1.5) due to the use of excess of ZnCl2. The TGA results showed that the content of the matrix water had significantly decreased from 12% for DMC-1 bearing no organic CA to 1.6−2.8% for the DMC catalysts using different organic CAs, due to the substitution of water molecules within DMC framework by CA molecules during the preparation stage ( Figure S1, Supporting Information). The percentage of CAs within the catalyst structures varied from 4.8 to 7.2%, depending on the CAs used. The estimated formulae of the resultant catalyst are summarized in Table 2.

Catalyst Vibration Frequency (cm −1 ) v(OH) v(C≡N) v(C=O) δ(H−O−H) v(C−O−C) δ(Co−CN)
As shown in Figure 4, the crystallinity of the DMC catalysts was significantly reduced by the substitution of the matrix water molecules for organic CAs, which was evidenced  The composition of the resultant DMC catalysts was determined by combining several elemental analysis techniques. The contents of Zn and Co (wt%) were estimated based on ICP-OES analysis, whereas CHNS/O analysis and TGA were used to determine content of the organic components. With the exception of the pure DMC compound (DMC-1), all the catalysts showed a higher content of Zn (0.48-3.37 mol per 1.0 mol of Zn 3 [Co(CN) 6 ] 2 ) than the stoichiometric amount of Co (1.5) due to the use of excess of ZnCl 2 . The TGA results showed that the content of the matrix water had significantly decreased from 12% for DMC-1 bearing no organic CA to 1.6-2.8% for the DMC catalysts using different organic CAs, due to the substitution of water molecules within DMC framework by CA molecules during the preparation stage ( Figure S1, Supporting Information). The percentage of CAs within the catalyst structures varied from 4.8 to 7.2%, depending on the CAs used. The estimated formulae of the resultant catalyst are summarized in Table 2.
As shown in Figure 4, the crystallinity of the DMC catalysts was significantly reduced by the substitution of the matrix water molecules for organic CAs, which was evidenced by the disappearance of the signals corresponding to the cubic phase (Fm-3m) at 2θ = 14.9 • , 17.1 • , 24.3 • , 35.1 • , and 39.1 • . The DMC-NMe catalyst showed only a singlet peak at 23.7 • 2θ that could be assigned to the monoclinic phase (P11m), whereas signals corresponding to both cubic and monoclinic phases were observed for the other DMC catalysts. These results indicated that the organic CAs had a great impact on the formation of the crystal structure and the degree of crystallinity in DMC complexes.

Evaluation of Catalytic Activity of DMC Catalysts for the Polymerization of PO
The DMC catalysts bearing organic solvents were briefly investigated for the ROP of PO. The optimized condition for the preparation of the DMC catalysts was determined by modifying parameters such as the amount of CA (V) used in solution 1 and the preparation temperature (T). As shown in Figure 5, DMC-DMAc prepared at 70 °C using 1 mL of CA exhibited the highest activity for the ROP of PO. Likewise, the optimized condition for making other DMC catalysts was determined. The DMC catalysts prepared in this work exhibited exceptionally high activity in the semi-batch polymerization of PO with extremely short induction times (2−11 min) and high polymerization rates, as compared 2 Theta (degree) 10 20

Evaluation of Catalytic Activity of DMC Catalysts for the Polymerization of PO
The DMC catalysts bearing organic solvents were briefly investigated for the ROP of PO. The optimized condition for the preparation of the DMC catalysts was determined by modifying parameters such as the amount of CA (V) used in solution 1 and the preparation temperature (T). As shown in Figure 5, DMC-DMAc prepared at 70 • C using 1 mL of CA exhibited the highest activity for the ROP of PO. Likewise, the optimized condition for making other DMC catalysts was determined. The DMC catalysts prepared in this work exhibited exceptionally high activity in the semi-batch polymerization of PO with extremely short induction times (2-11 min) and high polymerization rates, as compared to the benchmark DMC-TBA ( Figure 6). The activity of the DMC catalysts for the ROP of PO (g-PPO g-cat −1 h −1 ) decreased in the following order: DMC-DMAc (7200) > DMC-NMe (6350) > DMC-NMP (5970) > DMC-DMF (5100) > DMC-DMSO (4480) >> DMC-TBA (2200). The resultant polyether polyols were characterized by extremely low unsaturation levels (0.0030-0.0110 meq g −1 ) and narrow dispersity (1.08-1. 18) values. An overview of the catalyst preparation condition and polymerization results is given in Table 3. The 1 H NMR spectra of the resultant PPO polyols are given in Figures S2-S7 in the Supporting Information.

Polymerization of CL and VL Using DMC Catalysts
Inspired by the exceptionally high activity for the ROP of PO, the performances of these catalysts were then investigated for the ROP of CL and VL. The ROP of CL was conducted at 160 • C using EG as initiator ([CL] 0 /[EG] 0 = 10). As shown in Table 4, DMC-1 and DMC-DMSO exhibited relatively low activity (conversion of 25.9% and 38.7%, respectively), as compared to other catalysts. Among catalysts, DMC-NMP and DMC-NMe showed the highest activities for the ROP of CL with 95.3% and 95.4% monomer conversion, respectively, after 5 h of reaction time. The 1 H NMR spectra of the PCL polyols are shown in Figures S8-S13 in the Supporting Information. After reaction, the resultant PCLs were dissolved in chloroform so that the solid catalyst could be removed by centrifugation. Leaching testing also indicates that there was no change in the monomer conversion after removal of the solid catalyst ( Figure S14, Supporting Information). All the DMC catalysts bearing organic CAs exhibited high performances for the polymerization of VL, as calculated from 1 H NMR spectra ( Figures S16-S22, Supporting Information). Notably, the pure DMC compound (DMC-1) also showed high activity (81.3%), even higher than that of the conventional DMC-TBA (62.7%). It is worth noting that the DMC-1 compound showed no catalytic activity toward the ROP of PO. The activity of the DMC catalysts for the ROP of VL decreased in the following order: DMC-NMe > DMC-DMAc > DMC-DMF > DMC-NMP > DMC-DMSO > DMC-1 >> DMC-TBA. The results obtained above indicated that the crystal structure of the catalysts and the type of CAs most probably accounted for the differences in catalytic activity among the DMC catalysts. As summarized in Table 5 Figure 7 shows the 1 H NMR spectra of the EG-PVL and GL-PVL produced by EG and GL initiators, respectively. For the EG-PVL sample, the signal corresponding to the methylene protons (a) of the EG initiator were located at 4.28 ppm while those attributed to the methylene protons of the polyester backbones appeared at 1.68, 2.34, and 4.08 ppm. The triplet signal at 3.65 ppm could be assigned to the methylene protons next to the hydroxyl end groups. For GL-PVL, the signals of the GL initiator were almost overlapped by the signals of the polymer chains. The difference in activity of the primary and secondary hydroxyl groups of the GL initiator could result in PVLs with star-like architectures, instead of linear, as obtained for the EG initiator [42].
The copolymerization of PO with CL and VL were then investigated at 140 • C using the DMC-NMe catalyst. Notably, copolymers with MWs up to 20,000 g mol −1 and a narrow Ð of 1.27 were achieved even in the absence of an initiator (Figures S27-S29 and Table S3).
Kinetic studies of the ROP of VL were also conducted to corroborate the reaction mechanism (see Supporting Information, Figures S30-S39), and the results are summarized in Table 6. In the ROP of epoxides, the active sites of the DMC catalysts were generated by the cationic coordination between the Zn 2+ sites and the monomers that could propagate via both the coordinative and cationic routes [43,44]. Accordingly, a coordinative pathway took place on the catalyst surface during the first stage of the polymerization with typical first-order dependence upon the monomer concentration. On the other hand, the cationic polymerization, induced by the internal Zn 2+ sites and adsorbed monomers, occurred within the interstitial sites of the catalyst, resulting in catalyst fragmentation. This process generated new active sites for accelerating the polymerization rate and thus was one of the important factors to achieve high catalytic activity in the DMC catalysts. However, such cationic features were not observed in the DMC-catalyzed ROP of CL and VL. The kinetic results demonstrated that the polymerization of CL and VL simply proceeded via a coordination mechanism and mostly occurred on the surface of the DMC catalysts (Scheme S1).
This process generated new active sites for accelerating the polymerization rate a was one of the important factors to achieve high catalytic activity in the DMC c However, such cationic features were not observed in the DMC-catalyzed ROP o VL. The kinetic results demonstrated that the polymerization of CL and VL sim ceeded via a coordination mechanism and mostly occurred on the surface of t catalysts (Scheme S1).

Conclusions
We have developed a feasible procedure for producing highly active heterogeneous DMC catalysts for the ROP of various cyclic monomers. Notably, the DMC catalysts were prepared using inexpensive, common solvents such as Ac, DMAc, DMF, DMSO, NMe, and NMP as alternative CAs in place of the more costly TBA. All the catalysts prepared in this work showed excellent performance for the ROPs of PO, CL, and VL, as compared to the conventional DMC-TBA. The catalytic activities for the ROP of VL were in the following order: DMC-NMe > DMC-DMAc > DMC-DMF > DMC-NMP > DMC-DMSO > DMC-1 > DMC-TBA. Subsequently, DMC-NMe were used for producing PVLs with tunable MWs (650-6230 g mol −1 ) and narrow Ð values (1.05-1.57) as well as PPO-PCL and PPO-PVL copolymers with MWs of 11,060 and 20,680 g mol −1 , respectively. The polymerization kinetics were also investigated at different temperatures and various GL initiator concentrations using the DMC-NMe catalyst. The results of the kinetic studies indicated that the reaction rate was first-order dependent on the monomer and initiator concentrations, and the polymerization of VL proceeded via a coordination mechanism. The polyester polyols produced by DMC catalysts are potential precursors for the preparation of high-performance and biodegradable thermoplastic polyester and polyurethane elastomers, expanding the scope of DMC catalysis.