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Ind. Eng. Chem. Res. 2010, 49, 4107–4116

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Tuning of the Activity and Induction Period of the Polymerization of Propylene Oxide Catalyzed by Double Metal Cyanide Complexes Bearing β-Alkoxy Alcohols as Complexing Agents Sang Hyun Lee,† In Kyu Lee,† Ju Young Ha,† Jung Kyu Jo,† Inha Park,‡ Chang-Sik Ha,† Hongsuk Suh,§ and Il Kim*,† The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Department of Polymer Science and Engineering, Pusan National UniVersity, Busan, Korea, Technical SerVice & DeVelopment Center, SKC Company, Ulsan, Korea, and Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National UniVersity, Busan, Korea

For the development of commercially viable catalyst systems for the polymerization of propylene oxide (PO), a series of double metal cyanide (DMC) complexes have been developed by reacting aqueous zinc(II) chloride with aqueous potassium hexacyanocobaltate(III) in the presence of various complexing agents (CAs) such as 2-methoxyethanol, 2-ethoxyethanol, 2-propoxyethanol, 2-butoxyethanol (BE), 1-methoxypropan-2-ol, 1-ethoxypropan-2-ol, 1-propoxypropan-2-ol, and 1-butoxypropan-2-ol (BP) with different polarities and tert-butyl alcohol (tBuOH). The catalyst composition was estimated by combining elemental analysis with thermogravimertic analysis. The coordination of the CA can be identified by employing X-ray photoelectron spectroscopy, X-ray diffraction patterns, and infrared spectroscopy of the catalysts. Pursuing a catalyst system showing favorable polymerization behavior for commercial aspects of both catalytic activity and reaction control, the kinetics of PO polymerizations were investigated by using 4 and 8 L reactors equipped with pilot programmable logic controller systems. While a DMC catalyst bearing tBuOH as a CA showed a long induction period, it gave a very high overall rate of polymerization once activated, resulting in severe exothermic reactions that are hard to control. This problem could be successfully overcome by using the DMC catalysts bearing BE or BP as a CA. All polyether polyols were analyzed by measuring their unsaturation level, molecular weight, and distribution. All of the polyols produced featured a low level of unsaturation (0.014-0.024 mequiv g-1), moderate number-average molecular weights (900-3400), and very narrow polydispersities (1.04-1.19). Introduction The double metal cyanide (DMC) catalyst discovered in the early 1960s by General Tire Inc. is very effective for the production of polyether polyols, core raw materials for polyurethane (PU) applications.1 Compared with conventional alkaline catalysts, the typical DMC catalysts are highly active and do not significantly promote the isomerization of propylene oxide (PO). As a result, DMC catalysts give polyether polyols of a very low level of unsaturation and high molecular weights (MWs).2-6 Although DMC catalysts have been developed for their performance and applicability during the past decade, they must be activated over several hours before the epoxides start to be polymerized.7-9 Figure 1 summarizes the typical rate curves that can be obtained by the DMC-catalyzed polymerizations of PO. In general, the DMC catalysts are activated for a long period of time at high temperature (above 100 °C) together with a low-MW poly(propylene glycol) (PPG) starter. This long induction period at high temperature increases manufacture costs, and the activity of DMC catalysts is easily reduced or deactivated because of exposure to high temperature for a long period of time (Figure 1b).7 Because the induction period depends on the types and amounts of catalysts, the reaction * To whom correspondence should be addressed. Tel: +82-51-5102466. Fax: +82-51-513-7720. E-mail: [email protected]. † The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Department of Polymer Science and Engineering, Pusan National University. ‡ SKC Company. § Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University.

temperature, and the contents of water and other impurities, improvement of the reaction techniques is expected to shorten the induction period, as shown in Figure 1a.8,9 In a typical procedure, the reaction of an aqueous solution of zinc(II) halide with potassium hexacyanocobaltate(III) in the presence of complexing agent (CA) results in an insoluble DMC salt of the general formula Zn3[Co(CN)6]2 · xH2O · yCA. As expected, the type and amount of CA play a major role in determining the catalytic behavior. Although many types of

Figure 1. Typical rate profiles of DMC-catalyzed polymerizations of PO: (a) very high activity with a short induction period; (b) very high activity with a long induction period; (c) moderate but steady activity with a negligible induction period.

10.1021/ie1000967  2010 American Chemical Society Published on Web 03/26/2010

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Figure 2. Series of alkoxy alcohols used as CAs for DMC catalysts.

electron-donating organic molecules can be utilized as CAs, tertbutyl alcohol (tBuOH) is most widely used because the DMC catalyst bearing it gives a very high activity once activated (Figure 1a,b). Although such a highly active system, especially shown in Figure 1a, looks favorable at first glance, the high activity makes it difficult to control the reactor temperature, possibly resulting in an autoacceleration effect. From an actual process control point of view, the polymerization system showing slow but steady activity (like Figure 1c) might be a virtue of the DMC catalyst. However, it is not so easy to achieve this type of kinetic profile by employing tBuOH as a CA because the polymerization behavior of the DMC catalyst is intrinsically dependent upon the nature of the active sites formed by the complexation of CA to zinc. In order to overcome the drawbacks of the DMC/tBuOH catalyst, we have modified the nature of the active sites by employing a series of β-alkoxy alcohols (Figure 2) with different polarities. All of the catalysts are characterized by elemental analysis, thermogravimetric analysis (TGA), X-ray diffraction (XRD), and spectroscopic techniques. In order to search for the catalyst systems showing the polymerization rate profiles shown in Figure 1c, detailed kinetics of PO polymerizations have been investigated in small pilot-scale reaction systems with 4 and 8 L reactors. In this way, we have attempted to find suitable candidate catalyst systems for commercialization. The unsaturation level and MW of the resulting polyether polyols were also investigated. Experimental Section Materials. All reagents such as potassium hexacyancobaltate(III) [K3Co(CN)6] and zinc(II) chloride (ZnCl2) were purchased from Aldrich and used without further purification. β-Alkoxy alcohols, 2-methoxyethanol (ME), 2-ethoxyethanol (EE), 2-propoxyethanol (PE), 2-butoxyethanol (BE), 1-methoxypropan-2-ol (MP), 1-ethoxypropan-2-ol (EP), 1-propoxypropan-2-ol (PP), and 1-butoxypropan-2-ol (BP), were purchased from Aldrich and used without further purification. A trifunctional PPG starter [number-average molecular weight (Mn) ) 550] and a polymerization-grade PO monomer were donated by SKC (Korea). Preparation of Catalysts. Typical DMC catalysts complexed with tBuOH and various β-alkoxy alcohols have been prepared according to the literature procedures.7 For example, K3Co(CN)6 (5.61 g, 0.0169 mol) was dissolved in distilled water (120 mL) in a beaker (solution 1). ZnCl2 (22.98 g, 0.169 mol) was dissolved in distilled water (200 mL) and CA (70 mL) in a second beaker (solution 2). Solution 3 was also made in a third beaker with a mixture of distilled water (10 mL) and CA (240 mL). Solution 2 was added to solution 1 over 60 min at 50 °C, agitating with a mechanical stirrer. Solution 3 was then added, and the mixture was stirred for 10 min. The mixture was centrifuged to get the first catalyst cake and then reslurried in a CA (300 mL)/water (30 mL) mixture with vigorous agitation

over 60 min at 50 °C. The mixture was centrifuged to yield the second catalyst cake and then reslurried as described above. The third catalyst cake separated by centrifugation was dried at 70 °C under a vacuum to a constant weight. We designate DMC/ME, DMC/EE, DMC/PE, DMC/BE, DMC/MP, DMC/EP, DMC/PP, and DMC/BP for the DMC catalysts bearing ME, EE, PE, BE, MP, EP, PP, and BP as CAs, respectively, and DMC/tBuOH for the DMC catalyst bearing t BuOH as a CA. Polymerization. Polymerizations of PO were carried out by using 4 L and 8 L pilot-scale systems, with a personal computer monitoring the PO consumption, temperature, and pressure of the reactor. Figure 3 shows the polymerization setup with a 4 L reactor. For polymerization of PO, the reactor was charged with 550 g of the PPG starter and then purged with nitrogen three times. The starter was heated to reaction temperature and evacuated for 3 h to remove the trace of residual water possibly existing in the reaction system. The reactor was filled with nitrogen, charged with 0.30 g of DMC catalyst (∼100 ppm of residue in the final product), and then evacuated for 30 min. A prescribed amount of the initial PO monomer was introduced into the reactor at a desired temperature (Tp). In this step, the pressure in the reactor was increased from a vacuum to 2-4 bar, based on the added amount of initial PO. Once an accelerated pressure drop by 1 bar occurred, indicating activation of the catalyst, the continuous addition of additional monomer was started, keeping the pressure of the reactor constant. The polymerization was stopped when the total amount of added monomer reached 2450 g (or 4900 g for the 8 L reactor system). The polymerization rate was continuously recorded by measuring the amount of monomer introduced into the reactor, and the pressure and temperature of the reactor was continuously monitored with a pilot programmable logic controller (PLC) system. Vapor Pressure of PO. To obtain the kinetic data of the induction period and reaction rate, the monomer conversion should be monitored carefully. Considering that the PO monomer is characterized by a low boiling point and a high vapor pressure, careful monitoring of the pressure of the reactor can provide a good tool to measure the residual PO in the reactor. In addition, the initial pressure of the reactor can be controlled by regulating the initial amount of PO added before the catalyst is activated. For this, the pressure of reactor (4 L) was recorded at a temperature range between 110 and 130 °C according to the amount of PO mixed with 550 g of the PPG starter (Figure 4). The regression resulted in linear expressions: P (bar) ) 0.0212 × 0.8721 (at 110 °C); P (bar) ) 0.0266 × 0.8178 (at 120 °C); P (bar) ) 0.0354 × 0.7729 (at 130 °C). Characterizations. X-ray photoelectron spectroscopy (XPS) analysis of the catalysts was performed on an ESCALAB 250 induced electron emission spectrometer with Al KR (1486.6 eV, 12 mA, and 20 kV). XRD patterns of the catalysts were obtained with a wide-angle goniometry using Cu KR radiation at 40 kV and 30 mA. Slit sizes were 1° (for the divergence slit), 0.05° (for the monocrometer slit), and 0.15° (for the detector slit). The data were collected from 2° to 60° with a step of 2θ ) 0.02° and a counting time of 3-6 s per step (Rigaku RAD3C). Infrared (IR) spectra of the catalysts were obtained in transmission mode by a Nicolet 380 (Thermo Fisher Scientific). There were 32 scans per experiment at a resolution of 2 cm-1. Elemental analysis of Zn and Co was carried out using an inductively coupled plasma optical emission spectrometry (ICPOES; Varian ICP720-OES). TGA was carried out with a TA

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Figure 3. Polymerization setup with a 4 L reactor equipped with a pilot PLC system: T-110 ) monomer storage tank; WT-110 ) electric balance; 110 and 120 ) liquid flowmeters; CWS ) cooling water supply; sv-210, sv-211, sv-240, and sv-250 ) solenoid valves; VP-150 ) vacuum pump; P-200 ) heating oil pump; R-4100 ) stirred tank reactor; PI-100 ) pressure indicator; RPM-100 ) rotation speed indicator; TIC-100E, TIC-100S, and TI-C210 ) temperature indicators.

other organic CAs than water yields frameworks with a 3:2 stoichiometry and a higher concentration of vacancies than those in Prussian blue (Fe4[Fe(CN)6]3 · 14H2O), which is readily accomplished by the addition of ferric ions to an aqueous solution containing ferrocyanide ions: 3[Zn(H2O)6]2+ + 2[Co(CN)6]3- f Zn3[Co(CN)6]2·12H2O (1)

Figure 4. Vapor pressure of the reactor (4 L) according to the residual amount of PO dissolved in 550 g of the PPG starter at various temperatures (°C): (a) 110; (b) 120; (c) 130.

Instrument (model TGA Q50) from 20 to 500 °C with heating rate of 2 °C min-1. The total degree of unsaturation of polyols was measured by a titration method according to ASTM D2847. The MW and polydispersity index (PDI) were measured using a Waters 2695 instrument operated at 40 °C, with a set of 104, 103, and 500 °C columns in a tetrahydrofuran solvent. Poly(ethylene glycol) standards were used to generate a calibration curve. Results and Discussion Characterization of Catalysts. The reaction of dicationic zinc with trianionic hexacyanocobaltate in the absence of any

Here, water molecules coordinated to the ferric ions are displaced by N atoms from the ferrocyanide ions to generate linear ZnII-CN-CoIII bridges and an extended framework based on the face-centered-cubic unit cell.10,11 This Prussian blue analogue compound is stable and highly crystalline; however, they gave no activity in PO polymerizations. In order to activate them for PO polymerizations, complexation of suitable organic CAs other than water molecules is needed. Eight different DMC compounds were prepared by reacting an aqueous ZnCl2 with aqueous K3Co(CN)6 ([Zn]/[Co] ) 10) in the presence of various CAs such as ME, EE, PE, BE, MP, EP, PP, and BP. The resulting DMC catalysts are expected to have a general formula: Zna[Co(CN)6]b · cCA · dH2O · eCl- because an excess amount of ZnCl2 is used together with CA. Combining elemental analysis using ICP-OES and TGA analysis to measure the amount of water and organic CA, the catalyst formulation could be estimated and the results are summarized in Table 1, together with the approximate empirical formulas of the resulting DMC complexes. The weight loss due to the existence of coordinated water is recorded in the temperature range from 50 to 120 °C, and that due to organic CA is detected between 120 and 280 °C. The observed [Zn]/[Co] ratio (2.19-2.74) is different from the theoretical value (1.5), demonstrating the existence of

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Table 1. Elemental Analysis of the DMC Complex of the General Formula Zna[Co(CN)6]b · cCA · dH2O · eClICP-MS catalyst

CA

Zn (wt %)

DMC/ME DMC/EE DMC/PE DMC/BE DMC/MP DMC/EP DMC/PP DMC/BP

2-methoxyethanol 2-ethoxyethanol 2-propoxyethanol 2-butoxyethanol 1-methoxypropan-2-ol 1-ethoxypropan-2-ol 1-propoxypropan-2-ol 1-butoxypropan-2-ol

31.78 26.56 27.38 28.77 28.51 31.1 29.09 25.87

TGA analysis

calculation

Co (wt %)

H2O (wt %)

CA (wt %)

(CN)6(wt %)

Cl(wt %)

estimated catalyst formulation

10.74 10.08 9.35 10.52 11.75 10.23 10.14 9.91

0.99 0.93 1.38 0.65 0.94 0.41 1.45 0.56

18.30 20.55 16.16 19.67 17.93 17.03 18.49 17.49

28.54 26.68 24.80 27.92 31.20 27.14 26.83 26.30

9.65 15.20 20.93 12.47 9.67 14.09 14.00 19.87

Zn2.67[Co(CN)6]1.0 · 0.24ME · 0.30H2O · 1.49ClZn2.38[Co(CN)6]1.0 · 0.23EE · 0.30H2O · 2.51ClZn2.64[Co(CN)6]1.0 · 0.16PE · 0.48H2O · 3.71ClZn2.47[Co(CN)6]1.0 · 0.17BE · 0.20H2O · 1.97ClZn2.19[Co(CN)6]1.0 · 0.20MP · 0.22H2O · 1.37ClZn2.74[Co(CN)6]1.0 · 0.16EP · 0.13H2O · 2.29ClZn2.59[Co(CN)6]1.0 · 0.16PP · 0.47H2O · 2.30ClZn2.35[Co(CN)6]1.0 · 0.13BP · 0.18H2O · 3.32Cl-

unreacted ZnCl2. In fact, we have prepared a series of catalysts by changing the [Zn]/[Co] ratio from 0.5 to 20 and found that the maximum polymerization activity was recorded around the ratio of 10. In addition, only negligible activities were recorded for catalysts prepared by using a [Zn]/[Co] ratio of less than 2. Even though the exact role of free ZnCl2 is still not known, its presence plays an important role in achieving a highly active catalyst system. Much interest has thus been devoted to zinc derivatives as potential nontoxic catalysts for the ring-opening polymerization of lactide and glycolide.12 Zinc powder itself is a relatively good polymerization catalyst that is used industrially. Numerous zinc salts have also been investigated.13 Binary initiating systems that consist of a protonic acid and ZnCl2 have been shown to induce living cationic polymerization of vinyl ethers.14 Thus, the presence of free ZnCl2 may activate the PO monomer via coordination to Zn and thus promote PO polymerization. The polarity of eight CAs decided by the dielectric constant15-18 decreases in the order ME > EE > MP > PE > EP > PP > BE > BP; however, this order is not perfectly in line with the total amount of CA loaded into the catalyst. The molar ratio of [CA]/[Zn] is in the range between 0.06 and 0.1 and that of [H2O]/[Zn] is between 0.05 and 0.18, demonstrating that 6-10 molecules of organic CA are coordinated to 100 Zn atoms. Alteration of the crystal structure caused by the complicated composition of the DMC catalyst can be recorded by XRD patterns. XRD analysis of the Prussian blue analogue, Zn3[Co(CN)6]2 · 12H2O, prepared by equimolar amounts of aqueous ZnCl2 and K3Co(CN)6 solutions in the absence of any other organic CAs according to eq 1, showed broad peaks at 17.6° (200), 24.8° (220), 35.2° (400), 39.6° (420), and 43.5° (422), which can be indexed as the Prussian blue cubic space group Fm3m.11 However, the DMC catalysts prepared in the presence of various alkoxy alcohol CAs and tBuOH (shown for comparison) are characterized by much broader peaks, and some of the characteristic peaks are absent (Figure 5). These results are most probably induced by the complexation of CA to the catalyst matrix and by the nonequivalent amount of Zn to Co. Note that an excess amount of Zn ([Zn]/[Co] ) 10) was used for the preparation of the catalyst to enhance the catalytic activity via prescreening tests. Even though the DMC catalysts become substantially amorphous, they are not fully amorphous because the XRD patterns of the DMC catalysts exhibit broad signals at d spacings of 5.75, 5.07, 4.59, 2.54, and 2.28 Å, ascribed to a cubic lattice structure of Zn3[Co(CN)6]2.19 Even though it is not so easy to explain the differences among the catalysts, all of the catalysts prepared in the presence of eight CAs are characterized by a broad peak at about 2θ ) 23.6° (d ) 3.7 Å) and a sharp peak at 2θ ) 7.0° (d ) 13.6 Å). This means that the complexation of CA may collapse the catalyst framework and make the crystalline Zn3[Co(CN)6]2 substantially amorphous. The crystallinity of the catalyst may strongly influence

the catalytic activity. Note that the highly crystalline Zn3[Co(CN)6]2 · 12H2O showed no activity in the PO polymerization.7-9,20,21 The DMC catalysts were also analyzed using XPS analysis, detecting the chemical composition of the catalyst surface. The results of XPS analysis of the DMC catalysts prepared by using eight different CAs are summarized in Table 2. The binding energy (BE; 1023.7 eV) of the Zn atom in free ZnCl2 shifts toward lower values by 1.5-3.9 eV after formation of the DMC complexes. These chemical shifts result from coordination of an O atom to Zn by the reaction of ZnCl2 with K3Co(CN)6 in the presence of CA. By complexation of CAs, the amount of O atoms existing on the surface of the catalyst increases, as the O/Zn ratio indicates in Table 2. The types and amounts of O atoms coordinated to Zn play an important role in the polymerization of PO because O atoms coordinated to Zn ions are believed to be real active centers.22 Sreeprasanth and co-workers studied acidic sites of Zn/Fe DMC catalysts complexed with only water (K4Zn4[Fe(CN)6]3 · 12H2O) and with water and t BuOH (K4Zn4[Fe(CN)6]3 · 6H2O · 2tBuOH).23 CO2 did not adsorb onto these catalysts, indicating that there are no basic sites on their surfaces. Pyridine adsorption followed by DRIFT spectroscopic studies and NH3 temperature-programmed desorption studies revealed the presence of strong Lewis acid sites. Bro˜nsted acid sites were absent. The concentration of the strong acidic sites was higher for the K4Zn4[Fe(CN)6]3 · 6H2O · 2tBuOH catalyst than for K4Zn4[Fe(CN)6]3 · 12H2O. Thus, considering the similarity between these catalysts and the catalysts of the present study, it is assumed that coordination of CA to Zn should change the acidity of the catalyst and therefore influence the catalytic activity. The decrease of the electron-withdrawing

Figure 5. XRD patterns of the DMC catalysts prepared by using various CAs: (a) ME, (b) EE, (c) PE, (d) BE, (e) MP, (f) EP, (g) PP, (h) BP, and (i) tBuOH.

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Table 2. XPS Analysis of the DMC Catalysts Bearing Ethers as CAs Zn 2p3 catalyst ZnCl2 K3Co(CN)6 DMC/ME DMC/EE DMC/PE DMC/BE DMC/MP DMC/EP DMC/PP DMC/BP a

Co 2p3

O 1s

N 1s

C 1s

C 12p

BEa(eV) [AT]a(%) BE (eV) [AT] (%) BE (eV) [AT] (%) BE (eV) [AT] (%) BE (eV) [AT] (%) BE (eV) [AT] (%) O/Zn 1023.0 1019.1 1020.9 1021.1 1021.2 1021.0 1021.2 1021.3 1021.5

781.0 9.56 9.36 8.83 8.47 8.83 9.38 8.41 9.31

779.8 780.3 780.5 780.8 780.6 780.7 780.7 780.8

3.74 3.63 3.55 2.77 3.77 2.72 2.83 3.69

529.2 530.8 532.2 530.6 531.8 532.3 532.1 532.2

10.36 13.30 11.22 11.61 9.10 12.35 13.05 9.60

396.1 397.1 397.6 399.1 397.6 397.36 398.0 379.9

21.24 21.22 19.99 21.57 21.46 20.33 19.04 18.13

284.6 284.6 284.6 284.6 284.6 284.6 284.6 284.6

6.19 3.00 7.76 3.91 5.77 4.72 5.99 5.20

196.5 197.3 199.6 198.6 197.4 198.3 199.0 198.6

48.9 49.49 48.65 51.67 51.07 50.51 50.69 54.07

1.08 1.42 1.27 1.37 1.03 1.32 1.55 1.03

BE ) binding energy. AT ) atomic ratio.

power of the ligands bound to the Zn atom due to the substitution of Cl- for CN- ligands and at the same time the coordination of CAs to coordinatively unsaturated Zn metal centers led to a chemical shift to lower values. Chemical shifts (1021.0-1021.5 eV) of Zn coordinated with propylene glycol ethers bearing secondary alcohols are smaller than those (1019.1-1021.2 eV) of Zn coordinated with ethylene glycol ethers bearing primary alcohols. The degree of shift decreases in the order of DMC/ME > DMC/EE >DMC/PE >DMC/BE in a series of DMC complexes using ethylene glycol ethers and of DMC/MP > DMC/EP >DMC/PP >DMC/BP in a series of DMC complexes using propylene glycol ethers. It is interesting to note that the degree of BE shift of Zn is roughly in line with the degree of polarity of the CA. The C 1s peak coming from the C atom was taken as an internal reference, and its BE value is at 284.6 eV. The peak assigned to Co 2p3 in K3Co(CN)6 shifts from 781 eV to a slightly lower BE by 0.2-1.2 eV for the catalysts. These red shifts of the BE indicate a change of the microenvironments for the corresponding metal elements due to the formation of Zn2+-NC-Co3+ complexes in the presence of CAs. The IR spectra of the DMC catalysts exhibited characteristic peaks attributed to the CdON stretching in Zn2+-CN-Co3+ bonds, the C-O stretching of ether units, and the hydroxyl stretching vibrations from alcohol fragments in CA (Figure 6). Cyano complexes can be identified easily because they exhibit sharp ν(CdON) at 2200-2000 cm-1. The ν(CdON) band of free CN- is 2080 cm-1 (aqueous solution).24 Coordination to the metals shifts the ν(CdON) band to higher frequencies according to the electronegativity, the oxidation state, and the coordination number of the metals. Thus, the ν(CdON) band of K3Co(CN)6 is observed at 2127 cm-1, and the ν(CdON) bands of eight DMC catalysts prepared by using different CAs shift further to 2191-2195 cm-1, i.e., to 2195.1, 2194.5, 2193.8, and 2193.2 cm-1 for DMC/ME, DMC/EE, DMC/PE, and DMC/BE, respectively, and to 2194.4, 2193.5, 2192.5, and 2192.2 cm-1 for DMC/MP, DMC/EP, DMC/PP, and DMC/ BP, respectively. The ν(CdON) shift to higher frequencies demonstrates that the CN- ion acts as not only a σ donor by donating electrons to Co but also an electron donor by chelating to Zn metal. Electron donation tends to raise the ν(CdON) band because electrons are removed from the 5σ orbital, which is weakly antibonding, while π back-bonding tends to decrease the ν(CdON) band because the electrons enter into the antibonding 2pπ* orbital. In general, CN-1 is a good s donor and a poorer p acceptor. Because the electronegativity of Zn becomes smaller as the electronegativity of the coordinated CA agent increases, electron donation from cyanide ligands to Zn becomes smaller; the ν(CdON) band is expected to be lower. Thus, according to the degree of shift of the ν(CdON) band, the coordination strength of Zn to CA is in the order of DMC/

Figure 6. IR spectra of (a) K3Co(CN)6 and DMC catalysts prepared by using eight different CAs: (b) DMC/ME; (c) DMC/EE; (d)DMC/PE; (e)DMC/BE; (f) DMC/MP; (g) DMC/EP; (h) DMC/PP; (i)DMC/BP.

ME > DMC/EE >DMC/PE ≈ DMC/BE in a series of DMC complexes using ethylene glycol ethers and DMC/MP > DMC/ EP >DMC/PP ≈ DMC/BP in a series of DMC complexes using propylene glycol ethers. These results are in good agreement with the data obtained from XPS analysis. In addition to ν(CN), the cyano complexes exhibit ν(Co-C) and (Co-CN) bands in the low-frequency region. The ν(Co-C) band in K3Co(CN)6 shifts from 560.2 to 612.3 cm-1 for the DMC catalysts, and the (Co-CN) band in K3Co(CN)6 shifts from 412.7 to 470.1-471.5 cm-1 for the DMC catalysts. These results indicate that the Co-C π bonding of K3Co(CN)6 is increased by the formation of a Zn2+-NC-Co3+ complex because the degree of Co-C π bonding might be proportional to the number of d electrons in the F2g electronic level. Polymerization of PO. The semibatch polymerizations of PO using the DMC catalysts have been carried out in 4 and 8 L reactors equipped with pilot PLC systems (Figure 3) by changing various parameters such as the polymerization temperature, the initial amount of PO existing in the reactor before activation, and the reactor pressure. Unlike a conventional base catalyst, a prolonged induction period is needed in DMCcatalyzed polymerization: i.e., DMC catalysts must be activated before PO can be added continuously to the reactor. In order to investigate the polymerization behavior of a DMC catalyst prepared in the presence of tBuOH, which is the most widely utilized CA, PO polymerizations were carried out with DMC/

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Figure 7. Polymerization rate curves obtained by using the DMC/tBuOH catalyst at 110, 120, and 130 °C in a 4 L reactor equipped with a pilot PLC system. The initial pressure was controlled to 4 bar (parts a, d, and g), 3 bar (parts b, e, and h), and 2 bar (parts f, h, and i). Conditions: PPG starter ) 550 g and catalyst ) 0.3 g (corresponding to ∼100 ppm in the final product).

BuOH at 110, 120, and 130 °C (Figure 7). The initial pressure (Pinit) of the reactor was controlled to 2, 3, and 4 bar at each polymerization temperature (Tp) by changing the initial amount of PO in the reactor (see Figure 3). In general, the DMC/tBuOH catalyst is characterized by a long induction period and a very high activity once activated. For example, it takes 167 min to reach the maximum rate (Rp,m) at 110 °C and Pinit ) 2 bar. It is interesting to note that the time to reach Rp,m becomes shorter as Pinit increases. Evidently, this long induction period undercuts the economic advantage of the DMC-catalyzed polymerizations, and a too fast rate after activation makes temperature control of the reactor difficult. As Tp and Pinit increase, the time to reach Rp,m is shortened: i.e., from 167 min (Pinit ) 2 bar), 109 min (3 bar), and 64 min (4 bar) recorded at 110 °C to 54 min (Pinit ) 2 bar), 38 min (3 bar), and 26 min (4 bar) at 130 °C, respectively. Although the DMC/tBuOH catalyst shows a high activity and a short induction period at higher Tp and Pinit, a too fast initial polymerization rate after the induction period causes a severe exothermic reaction. As was already mentioned earlier, a polymerization behavior showing a negligible induction period and a steady rate (see Figure 1c) might be advantageous for practical application on an industrial scale. We found that it was not so easy to achieve this type of rate curve by employing tBuOH as a CA. In order to overcome this problem, we prepared a series of DMC catalysts by diversifying the CA, as shown in Figure 2. Figures 8-11 show detailed kinetic curves of PO polymerizations employing various catalysts. Figures 8 and 10 are kinetic curves of PO polymerizations using DMC catalysts (DMC/ME, DMC/ EE, DMC/PE, and DMC/BE) bearing ethylene glycol ethers and those (DMC/MP, DMC/EP, DMC/PP, and DMC/BP) bearing propylene glycol ethers as CAs, respectively, obtained t

Figure 8. Polymerization rate curves obtained using various DMC catalysts bearing ethylene glycol ethers as CAs at an initial reactor pressure of 4 bar and at different temperatures: at 130 °C (parts a, d, g, and j); at 120 °C (parts b, e, h, and k); at 110 °C (parts c, f, i, and l). Conditions: PPG starter ) 550 g, catalyst ) 0.3 g, and reactor ) 4 L.

at different temperatures after the initial reactor pressure was fixed at 4 bar. On the contrary, Figures 9 and 11 are kinetic curves of PO polymerizations using DMC catalysts (DMC/ME, DMC/EE, DMC/PE, and DMC/BE) bearing ethylene glycol ethers and those (DMC/MP, DMC/EP, DMC/PP, and DMC/BP) bearing propylene glycol ethers as CAs, respectively, obtained at different initial reactor pressures after Tp was fixed at 120 °C. Compared with the kinetic profiles (Figure 7) obtained by a DMC/tBuOH catalyst at similar conditions, the DMC catalysts complexed with glycol ethers feature a short induction period and a slow rate of polymerization after activation. Thus, it is possible to choose suitable catalyst fittings with industrialization conditions. As summarized in Table 3, for ease of comparison, the DMC/ t BuOH catalyst is characterized by a long induction period (tind) and a high activity (as Rp,m). The time to reach Rp,m after activation, tmax - tind, where tind is the time needed until additional monomer is fed into the reactor after the catalyst is activated and tmax is the time to reach Rp,m, is only 5-9 min, representing the polymerization behaviors shown in Figure 1a and, more likely, Figure 1b. Evidently, this high rate of polymerization must be a pro factor; however, it can be a con factor in that a rapid increase in the overall rate of reaction leads to a possible reaction runaway and alters the characteristics of the polymers produced, especially considering the industrial scale of the reaction. On the contrary, most of the catalysts bearing alkoxy alcohols as CAs feature a negligible induction period and a slow increase in the overall rate like the polymerization behavior shown in Figure 1c. Note that it is one of

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Figure 9. Polymerization rate curves obtained by using various DMC catalysts bearing ethylene glycol ethers as CAs at 120 °C and at different initial reactor pressures: at 4 bar (parts a, d, g, and j); at 3 bar (parts b, e, h, and k); at 2 bar (parts c, f, i, and l). Conditions: PPG starter ) 550 g, catalyst ) 0.3 g, and reactor ) 4 L.

our main goals of this study to achieve this type of rate profile. In summary, the kinetic studies obtained by changing the catalyst formulations in different polymerization conditions give us a good clue for commercialization of the DMC catalyst because we may choose commercially viable catalysts together with controllable conditions, once the commercial process is decided. During investigation of the Rp,m values obtained by the DMC catalysts coordinated by alkoxy alcohols at 110, 120, and 130 °C, the activity decreases in the order of DMC/BP > DMC/ BE > DMC/PP ∼ DMC/EP > DMC/MP ∼ DMC/ME > DMC/PE ∼ DMC/EE. In addition, catalysts bearing CAs of low polarity such as PE, BE, PP, and PP show no induction period. These results demonstrate that less polar alkoxy alcohols are more effective CAs for the DMC catalyst. The relatively long induction period of DMC/ME containing the most polar ME as a CA might be caused by the relatively strong complexation of the CA to the dormant active sites of Zn. Too strong complexation prevents either PPG starter molecules or incoming monomers from substituting for the CA molecules. If the CA molecules remain coordinated to the Zn atoms, there are no sufficient spaces for coordination of the monomers, resulting in a delay in propagation or making it impossible. In a series of polymerizations according to changes in Tp and the initial reactor pressure, it is found that (1) the tind value decreases as Tp and the initial reactor pressure increase, (2) the Rp,m value increases as the initial reactor pressure increases, and (3) the best activity is observed at 120 °C for all catalysts. In order to investigate the effect of scale-up on the polymerization behaviors, which is one of the most important

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Figure 10. Polymerization rate curves obtained by using various DMC catalysts bearing propylene glycol ethers as CAs at an initial reactor pressure of 4 bar and at different temperatures: at 130 °C (parts a, d, g, and j); at 120 °C (parts b, e, h, and k); at 110 °C (parts c, f, i, and l). Conditions: PPG starter ) 550 g, catalyst ) 0.3 g, and reactor ) 4 L.

considerations for commercialization, we carried out a kinetic study with a 8 L reactor equipped with a pilot PLC system and compared the data with those obtained in a 4 L reactor system. Figure 12 shows kinetic profiles of PO polymerizations from both reactor systems. All polymerizations were controllable within (2 °C. When tind and tmax - tind values obtained in a 4 L reactor are compared with those obtained in a 8 L reactor using the same catalyst, similar shapes of the rate profiles are observed for both systems with the same catalyst. These results demonstrate again that the type of CA in the DMC catalyst is the most decisive factor determining the type of kinetics of PO polymerizations. In addition, no conspicuous variations of the rate profiles are observed by the scale-up of the reactor, demonstrating that all systems using DMC catalysts bearing ethers as CAs are commercially more viable in that (1) they show only a negligible induction period, (2) they give a high enough activity to be commercially attractive, (3) the polymerization systems are readily controllable with the common reactor-regulating system, as shown in Figure 3, and (4) the scale-up of the reactor generates no unexpected problems. Note that Tp rose above 200 °C in the comparative polymerization runs in a 8 L reactor at 120 °C (Pinit ) 4 bar) by using a DMC/ t BuOH catalyst. The similar polymerization behavior was also observed in a 4 L reactor at 120 °C (Pinit ) 4 bar; see Figure 7d for the rate profile). It was almost impossible to control Tp within a commercially permitted range (say below 140 °C) with the polymerization setup shown in Figure 3. A special cooling unit may be needed for this. Although this system looks favorable only if we think about the overall activity, evidently it is not a commercially viable one because there exists a long

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Figure 11. Polymerization rate curves obtained by using various DMC catalysts bearing propylene glycol ethers as CAs at 120 °C and at different initial reactor pressures: at 4 bar (parts a, d, g, and j); at 3 bar (parts b, e, h, and k); at 2 bar (parts c, f, i, and l). Conditions: PPG starter ) 550 g, catalyst ) 0.3 g, and reactor ) 4 L.

induction period, and especially the ease of control of the reaction system is a prerequisite factor for commercialization. Analysis of Polymer. Regulating polymerization parameters and controlling the catalyst formulations demonstrate that all of the catalysts of the present study have been demonstrated to be active enough to allow the final catalyst concentration to be at a very low level ( DMC/BE ∼ DMC/EE ∼ DMC/PP > DMC/PE (0.015 mequiv g-1). Although it is too premature, with the data collected here, to conclude the factor driving different unsaturation levels, the type of CA coordinated to the catalyst plays a very important role in limiting undesirable side reactions. In this sense, if the lower level of unsaturation of polyol is a research target, it is necessary to tune the type of CA influencing it. All of the polyols produced in this study feature moderate MW and very low PDI values (1.04-1.19). All catalysts produce polyols with similar Mn values except the DMC/PE catalyst.

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Table 3. Results of the 4 L Pilot Scale of PO Polymerizations Catalyzed by DMC Catalysts with Different CAs (Polymerization Conditions: PPG Starter ) 450 g and Catalyst ) 0.3 g) Rp,mc (g of catalyst min-1)

tinda (min)/tmaxb (min) catalyst

110 °C, 4 bar

120 °C, 4 bar

130 °C, 4 bar

120 °C, 2 bar

120 °C, 3 bar

110 °C, 4 bar

120 °C, 4 bar

130 °C, 4 bar

120 °C, 2 bar

120 °C, 3 bar

DMC/ME DMC/EE DMC/PE DMC/BE DMC/MP DMC/EP DMC/PP DMC/BP DMC/tBuOH

22/63 0/99 0/65 0/34 3/41 8/39 0/32 0/22 58/64

12/46 0/65 0/46 0/26 3/36 7/34 0/26 0/16 19/27

8/39 0/45 0/31 0/23 3/24 6/24 0/26 0/13 17/26

0/104 0/81 0/84 0/77 4/61 2/65 0/59 0/26 81/90

0/66 0/71 0/56 0/56 3/39 12/44 0/40 1/20 50/55

3.42 1.79 2.11 4.67 3.55 4.03 3.86 6.11 22.4

3.71 2.28 2.08 5.89 3.35 3.97 4.09 8.44 26.9

2.74 1.83 2.44 5.80 4.52 3.89 3.18 6.99 36.0

0.27 0.27 0.37 0.28 0.32 0.32 0.38 0.52 2.7

1.02 0.84 0.91 1.17 1.56 1.48 1.62 2.95 17.6

a tind ) induction period, time needed until additional monomer is fed after the catalyst is activated. b tmax ) time to reach the maximum rate. c Rp,m ) maximum rate.

Table 4. Analytical Results of Polyether Polyols Produced by Using Eight DMC Catalysts Bearing Different Organic CAs (Conditions: PPG Starter ) 450 g and Catalyst ) 0.3 g)a Mnb catalyst DMC/ME DMC/EE DMC/PE DMC/BE DMC/MP DMC/EP DMC/PP DMC/BP

unsaturationc (mequiv g-1)

PDIb

110 °C, 120 °C, 130 °C, 120 °C, 120 °C, 110 °C, 120 °C, 130 °C, 120 °C, 120 °C, 110 °C, 120 °C, 130 °C, 120 °C, 120 °C, 4 bar 4 bar 4 bar 2 bar 3 bar 4 bar 4 bar 4 bar 2 bar 3 bar 4 bar 4 bar 4 bar 2 bar 3 bar 3300 2600 1300 3400 3300 3300 3400 3100

3300 2600 1300 3400 3200 3200 3400 2800

3100 2300 900 3300 3100 2600 3200 2500

3600 2500 1200 3200 3400 3200 2900 3000

3300 2500 1100 3400 3200 3000 3100 3000

1.19 1.19 1.08 1.19 1.15 1.12 1.19 1.15

1.16 1.15 1.05 1.20 1.16 1.18 1.16 1.17

1.17 1.15 1.04 1.18 1.17 1.16 1.13 1.16

1.24 1.18 1.07 1.19 1.21 1.22 1.15 1.25

1.19 1.18 1.07 1.17 1.14 1.17 1.19 1.21

0.021 0.017 0.015 0.018 0.021 0.020 0.017 0.022

0.023 0.019 0.015 0.019 0.024 0.022 0.018 0.023

0.024 0.019 0.015 0.021 0.024 0.023 0.020 0.024

0.020 0.018 0.014 0.018 0.021 0.018 0.016 0.020

0.022 0.019 0.014 0.017 0.020 0.019 0.016 0.021

a Polymerizations were carried out at a temperature range between 110 and 130 °C and at an initial reactor pressure range between 2 and 4 bar. b Mn and PDI were measured by GPC. c The total degree of unsaturation of polyols measured by titration method according to ASTM D2847.

The Mn values decrease monotonously as Tp increases in a series of polymerizations by the same catalyst; however, no clear trend is observed for the initial reactor pressure. The very narrow PDI value of polyol might be obtained by controlled ionic polymerization in a living mode. In this sense, it is interesting to note a mechanistic pathway of DMC catalysis resulting in such a narrow PDI. Because we used a large excess amount of the initiator (550 g) in comparison with the catalyst (0.3 g), an initiator-to-catalyst molar ratio as high as 103 or more was reached. Accordingly, on the basis of simple calculations, it is impossible to assume that all chains grow in a living mode like controlled anionic polymerization. It is therefore reasonable to assume that a rapid exchange between the dormant and active sites accounts for control of the MW and narrowing of PDI. As a result of the exchange reaction, the dormant sites and transient dead-polymer chains (PnOH) that can be reactivated are generated. If the exchange reaction is faster than the propagation reaction, the number of growing polymer chains remains constant depending on the initial amount of initiator, resulting in polymers of narrow PDI. A slow exchange reaction should result in the generation of polymer chains with different chain lengths. Like the case of an unsaturation level, the type of CA is likely to be one of the most important factors in deciding the level of MW and polydispersity because it changes the character of the active sites of the DMC catalyst. Conclusions A series of DMC catalysts of the general formula Zna[Co(CN)6]b · cCA · dH2O · eCl- were prepared by using various alkoxy alcohols such as ME, EE, PE, BE, MP, EP, PP, and BP as CAs. When elemental analysis is combined with the TGA results, the DMC formulation could be estimated. Analyses based on XPS, powder XRD, and IR spectroscopy showed that

the CAs were coordinated to the DMC matrix, resulting in a sharp decrease of crystallinity from the stable Zn3[Co(CN)6]2 · 12H2O. Kinetic studies of PO polymerizations carried out in 4 and 8 L reactors equipped with pilot PLC systems showed that the type of CA is a decisive factor in determining the polymerization rate behaviors including the induction period and overall polymerization rate. As a result, it was possible to develop catalyst systems showing controllable polymerization behavior, as shown in Figure 1c. Especially, the DMC/BE and DMC/BP catalysts bearing low polar CAs appear to be acceptable candidates for commercialization in that they show only a negligible induction period and a sufficient overall polymerization rate without appreciable exothermic reactions. The DMC/tBuOH catalyst bearing the most commonly employed tBuOH as a CA showed a much longer induction period and a much higher activity, showing rate profiles in Figure 1a,b, than the DMC catalysts bearing alkoxy alcohols as CAs; thus, it was difficult to control the reaction. The type of CA coordinated to the DMC catalysts plays a decisive role in determining the properties of the resulting polyols. Although the unsaturation MW and PDI values of polyols change to some degree according to polymerization parameters, their levels were decided by the type of CA. All of the polyols produced by the DMC catalysts at 120 °C and 4 bar of the initial reactor pressure were featured by a low level of unsaturation (0.015-0.024 mequiv g-1), moderate MW values (Mn ) 1300- 400), and very narrow PDI values (1.05-1.18). Acknowledgment This work was supported by the World Class University Program (No. R32-2008-000-10174-0), the National Core Research Center Program from MEST (No. R15-2006-022-

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ReceiVed for reView January 15, 2010 ReVised manuscript receiVed March 6, 2010 Accepted March 14, 2010 IE1000967