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Characterization of a Double Metal Cyanide (DMC)-Type Catalyst in the Polyoxypropylation Process: Effects of Catalyst Concentration Arkadiusz Chruściel,†,‡ Wiesław Hreczuch,† Jarosław Janik,*,†,‡ Krystyna Czaja,‡ Katarzyna Dziubek,‡ Zygmunt Flisak,‡ and Andrzej Swinarew§ †

MEXEO Wiesław Hreczuch, ul. Energetyków 9, 47-225 Kędzierzyn-Koźle, Poland Faculty of Chemistry, Opole University, ul. Oleska 48, 45-052 Opole, Poland § Institute of Chemistry, University of Silesia, ul. 75 Pułku Piechoty 1A, 41-500 Chorzów, Poland ‡

S Supporting Information *

ABSTRACT: The alkaline catalysts commonly applied to alkoxylation are characterized by a limited spectrum of activity caused by an irreversible termination of the polyether chains. The presented results show that double metal cyanide (DMC) catalysts reduce or eliminate the aforementioned adverse rearrangement of hydroxyl groups. Moreover, DMC catalysts indicate high activity at low concentrations (ppm range), as expressed by high polymerization rates. It was demonstrated that decreased concentrations of DMC catalyst irreversibly influence its reactivity and the dispersity of the obtained products, as exemplified by the production and determination of selected polyoxypropylenediols at different concentrations of the catalyst. Because of their unique advantages, the DMC catalysts are a very attractive alternative to conventional alkaline catalysts for the polyaddition of oxiranes. The phenomenon was discussed and explained by an alteration of reaction rate coefficients at subsequent polyaddition stages.

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INTRODUCTION Although the catalytic properties of double metal cyanide (DMC) complex salts of transition metals in oxirane ring opening reactions have been known for several decades, and catalysts containing certain structural elements of such compounds are successfully used in chemical technology for making polyetherpolyols, the research interest in DMC catalysts does not decline. This is mainly because not enough experimental knowledge of the scope of structural conditions of catalytic activity for that particular group of substances has been disclosed, because the existing knowledge of the properties of the DMC catalysts is mainly of an applied nature, based predominantly on patent literature. Because of their unique advantages, the DMC catalysts are a very attractive alternative to conventional alkaline catalysts for the polyaddition of oxiranes. Polyaddition with the use of an initiator, followed by the oxirane ring catalytic opening step is usually applied to oxirane (1,2-epoxyethane, ethylene oxide) or methyloxirane (1,2-epoxypropane, propylene oxide). A general equation of the reaction, based on the example of the polyaddition of methyloxirane to the hydoxyderivative molecule as a starter, is shown in Scheme 1, where R = hydrogen or the other organic group. Although the oxirane ring opening step proceeds rather without difficulty, it is necessary to use catalysts under industrial

conditions. Conventional catalysts for epoxy ring opening have been known for a long time; typically alkaline compounds, they are usually introduced into a reaction medium as hydroxides (NaOH, KOH).1 The main commonly observed disadvantage of the alkaline catalyst is the significant number of unfavorable side processes occurring in their presence, leading both to a product with a broad distribution of homologues as well as the occurrence of competitive rearrangement reactions, leading in effect to increased levels of undesirable unsaturated groups in the product2 or increase of the functionality of the final polyol. The latter is particularly significant disadvantage in the aspect of further processing of the oxyalkylenation product, since a higher number of functional groups are available for further chemical changes and the product has certain undesirable physicochemical properties.1−7 The introduction of DMC catalysts in the 1960s5−7 was a milestone in polyoxypropylenation technology. Although the selectivity of the early used DMC catalysts was rather poor, the significant development has been achieved in decades; then improved catalysts with higher activity were obtained and the resulting polyetherols had much better selectivities.8−21 The DMC catalysts have a very high activity, which is their major characteristic. It makes a difference compared with conventional alkaline catalysts and allows the use of much lower catalyst concentrations in synthesis conditions (even as low as several ppm). This eliminates the troublesome need to remove any residual catalyst from the product in the technological process, making them more

Scheme 1

Received: Revised: Accepted: Published: © 2014 American Chemical Society

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January 3, 2014 March 21, 2014 March 27, 2014 March 27, 2014 dx.doi.org/10.1021/ie500031j | Ind. Eng. Chem. Res. 2014, 53, 6636−6646

Industrial & Engineering Chemistry Research

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an initiator and catalyst, followed by dehydration through a nitrogen purge at 130 °C over a period of 30 min. The polyoxypropylation reaction was performed using two methods: Method (a). The desired amount of methyloxirane was introduced gradually in order to maintain the overpressure in the range of 0.3−0.6 MPa, at the assumed reaction temperature. After introducing the desired amount of the monomer to the reactor, when the overpressure dropped to a constant level, the final product was kept for an additional 30 min at the reaction temperature. Then, the product was cooled and collected. Method (b). Kinetics of the polymerization was also investigated by a one-step drop of the monomer into the starter material at a molar ratio of 1:1. In this case, the desired amount of the monomer was introduced in one step into a mixture of the starter with the catalyst, and the reaction was continued until the overpressure in the reactor dropped to a constant level. The changes in overpressure reflected the kinetics of the monomer conversion. Preparation of DMC Catalyst. K3[Co(CN)6] (7.5 g) was dissolved in a round-bottom flask that was filled with a mixture of distilled water (40 mL) and tBuOH (50 mL) (Solution A). Solution B was prepared by dissolving ZnCl2 (75 g) in a mixture of water (75 mL) and tBuOH (2 mL). Solution B was added to Solution A over a period of 30 min, with stirring, using a mechanical stirrer. After their combination, stirring was continued for another 15 min. Two hundred milliliters (200 mL) of a 1% aqueous solution of a flocculentacrylic acid polyamide, which enables precipitates to be separated from aqueous solutionswas added to the prepared reaction mixture and stirred for an additional 10 min. The precipitation o f t h e vi r t ua l l y a m o r p h o u s p r e c i p i t a t e o f z i n c hexacyanocobaltate(III) followed. The mixture was centrifuged and the resulting catalyst cake was dissolved in a mixture of t BuOH (130 mL) and 1% aqueous solution of the flocculent (55 mL). The reaction mixture was stirred vigorously for 15 min. The described operation of rinsing and filtration was performed once more, and the obtained catalyst solid was dried at 50 °C to a constant weight. The described procedure was the subject of Polish Patent Application No. 398518 (March 19, 2012, before publication).29 Analysis of Prepared DMC Catalyst. Nitrogen, carbon, and chlorine were determined by elemental analysis, using the Elementarvario EL III apparatus for nitrogen and carbon. The method described by Bobrański30 was used for chlorine analysis. Zinc and cobalt quantification were determined by atomic absorption spectroscopy (AAS), using the Thermo Solar 6 M apparatus equipped with HCL tubes for Co and Zn at wavelengths of 240.7 and 213.9 nm, respectively. The measurements were performed three times in the air−C2H2 flame. Metal contents were determined using the standard curve method. Characterization of the Polyaddition Products. The hydroxyl value (OHV) is defined as the equivalent amount of KOH (mg) corresponding to the hydroxyl groups contained in 1 g of product. It was determined using standard methods (ASTM D-4274D, PN-85C-89052/03). Unsaturation numbers were determined in accordance with standard ASTM D4671D87. Gel permeation chromatography (GPC) analyses were performed using the GPC apparatus from Shimadzu, equipped with two LC-20AD pumps with a gradient, a degassing unit (Model DGU-20A3), and a fraction collector (Model FRC-

competitive, compared to conventional alkaline catalysts. On the other hand, heterogenic coordination catalysts from the DMC group have the disadvantage of being highly sensitive to poisoning in the presence of electrodonor compounds (strong Lewis bases, OH− ions, amines, low molecular hetero-organic compounds), which is typical of coordination catalysts. A typical, state-of-the-art composition of a DMC catalyst may be defined with Scheme 2.where MeI and MeII denote transition Scheme 2

metals (most effective Zn and Co), L1···Ln stand for suitable ligands (most frequently alcohols and ethers), and x, y, a1, ..., an are appropriate integers. Generally, the DMC catalysts are obtained by reacting a selected metal salt with a cyanide derivative of another metal in the presence of an electrodonor organic complexing ligand.8,22−28 In addition to the correct selection of the types of salt and the complexing ligand, as mentioned above, relatively complex preparatory methods are required to obtain the DMC catalysts, involving several critical factors that determine the resulting catalytic activity, kinetic path, and quality of the resulting polyadducts. Moreover, when obtaining a catalyst in its active form, with a sufficiently high content of the amorphous phase, it is necessary to handle suspensions with nearly colloidal dispersities, which poses significant difficulties in technical-scale unit processes and operations. According to a patent by the authors of this paper, a method is proposed to obtain the DMC catalyst, which involves the flocculation of the colloidal suspension of the catalyst, much improving the conditions of the catalyst’s separation from a solution.29 The purpose of this paper is to provide a characterization of the DMC catalyst obtained in a modified process, with respect to polyoxypropylenation; such characterization will include an analysis of the effect of the concentration of the catalyst on the reaction kinetics and on the composition of the resulting products, compared with the activity of the conventional alkaline catalyst in the form of KOH.

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EXPERIMENTAL SECTION Materials. tert-Butyl alcohol (tBuOH, >99% pure) was obtained from Brenntag Sp. z o. o., Ked̨ zierzyn−Koźle, Poland. K3[Co(CN)6] was a commercial product prepared in MEXEO, Ked̨ zierzyn-Koźle, Poland. Acrylic acid polyamide−Rokrysol WF2 (content of 5.6% m/m), as a flocculant, was purchased from Brenntag Sp. z o. o., Ked̨ zierzyn−Koźle, Poland. Polyoxypropylenediol PPG 450 (Mn = 450), which was applied as the starter material, was obtained as a commercial product from PCC Rokita S.A., Brzeg Dolny, Poland. It was prepared using KOH as an alkaline catalyst with its subsequent removal below 1 ppm potassium content. Propylene oxide (PO) was commercially produced by PCC Rokita, Brzeg Dolny, Poland, and KOH (85% pure) was purchased from Brenntag Sp. z o. o., Ked̨ zierzyn−Koźle, Poland. Methods of Polyoxypropylation. Polyaddition of methyloxirane was carried out using a 1-L laboratory autoclave equipped with a programmable logic control system (PLCS) controlling its operation, as well as a mechanical stirrer, a heating jacket, and a cooling coil. The reactor was charged with 6637

dx.doi.org/10.1021/ie500031j | Ind. Eng. Chem. Res. 2014, 53, 6636−6646


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coordination links between heteroatoms in the starter molecules and active sites in the catalyst. In the DMC catalysts, which are coordination systems as well, this must lead to difficulties in the direct alkoxylation of starters having low molecular weights (Mn < 100), which is a problem that has been frequently reported in the literature and is also confirmed by the present authors. Although some of the reports suggest the possibility of omitting the problem by means of the acidification of the reaction medium using a strong acid,13,29 numerous tests carried out by the present authors indicate an insufficient repeatability of the results of the syntheses that were carried out in that manner. The DMC coordination catalyst is often observed to be deactivated by low-molecular-weight alcohols. Therefore, polyoxypropylenediol, with an average molecular weight of Mn = 450 (PPG 450), was used as the starter. This provided a product with a desirable average molecular weight of Mn = 2700 (PPG 2700), later to be used as the starter in the reaction to obtain the polymer with an average molecular weight of Mn = 10000 (PPG 10000). Reactions that used the two compared catalysts were performed under same conditions, according to the description given in the Experimental Section. When testing several catalysts obtained as described in the above procedure and characterized in Table S1 in the Supporting Information, it was found that small differences in their compositions did not have a significant effect on the course of propoxylation or on product characteristics. The results in Table S2 in the Supporting Information indicate that the calculated average polyaddition rate (Rp) for the product PPG 2700, obtained in the presence of the DMC catalyst, based on experimental measurements, is ∼440 times higher than that for KOH (Rpav(DMC)/Rpav(KOH) ≈ 440). The corresponding value in the synthesis of PPG 10000 is Rpav(DMC)/Rpav(KOH) ≈ 350. The two results above indicate that, despite a 100-fold decrease in the effective concentration of the DMC catalyst in comparison with the conventional one, the rate of polyaddition involving the former is 400 times as high. These data confirm the high activity of the DMC catalyst developed by the present authors and its essentially superior features, in this respec,t over the conventional alkaline KOH catalyst. Figure 1 shows the conversion rates for the compared DMC catalysts at different concentrations in the test syntheses of PPG 2700 and PPG 10000.

10A). Separation was carried out on the Shodex GPC column (Model KF-805L). The flow rate of the eluent (tetrahydrofuran) was 1 cm3/min at a temperature of 40 °C. The molecular weights of the polymers were determined on the basis of calibration performed for polymethacrylate standards with narrow-range distributions of molecular weights (Sigma−Aldrich). The results were processed using the Shimadzu LC Solution software, which was used afterward to determine the molecular weights of the analyzed systems, their dispersity, and purity.

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RESULTS AND DISCUSSION Analytical Characterization of the DMC Catalyst. Table S1 in the Supporting Information shows the elemental compositions and estimated contents of the ligand (tBuOH) in representative samples of several catalyst lots, obtained as described above and, for comparison, in two samples of zinc hexacyanocobaltate(III) that differ only in their water content. The expected content of the ligand (tBuOH), as given in the last column in Table S1 in the Supporting Information, was calculated from the stoichiometric dependence defined by eq 1: w − 0.8571wN w tBuOH(%) = C × 100 (1) 0.6487 The zinc-to-cobalt and carbon-to-nitrogen content ratios (by weight; wZn/wCo) and wC/wN, respectively) are given in Table S1 in the Supporting Information (columns 4 and 7, respectively). For the zinc hexacyanocobaltate samples (DMC 1 and DMC 2) the ratios were found to be in agreement with the stoichiometric ratio (i.e., 1.66 and 0.857, respectively), regardless of the absolute content of the elements, which result from that of water. The content of chlorine in the materials is not higher than 1%, which results from the incomplete removal of Cl− ions from the precipitate during the course of its washing. The calculated content of tBuOH in the DMC 1 and DMC 2 samples is near zero, as expected. In the case of the catalyst samples (DMC 3−5), the contents of the respective elements, as well as the diagnostic ratios Zn/Co and C/N, are different from the respective values determined for samples DMC1 and DMC2; for this, there is an obvious justification in the intentional excess of Zn, with respect to Co and the content of the ligand, which is the carbon source. The ligand contents, as calculated from the stoichiometric dependence, are virtually similar. Any minor differences that do exist are of little importance for the chemical properties of the catalyst and they result from the evaporation of the physically bonded tBuOH. The catalyst samples have an increased amount of chlorine, which was not removed during vigorous rinsing; this is the result of bonds that exist between Cl and Zn atoms. Examination of the Conversion Rates. Comparative reactions of methyloxirane polyaddition in the presence of KOH and the DMC catalysts were carried out. The syntheses were carried out as described above (Method (a)), gradually adding the required amount of the monomer until its desired concentration was reached. The course of the reaction was monitored by observing the overpressure drop in the reactor. Because of considerable differences between the reactivities of the compared types of catalysts, they were used at different concentrations, selected on the basis of prior experience and a knowledge of patents and literature on this subject. Coordination catalysts have the common feature of being highly sensitive to the presence of strongly electrodonor compounds. They deactivate the catalyst by forming permanent

Figure 1. Relationship between the average polyaddition rate (Rp and Rp′) for the produced polyoxypropylenediol (PPG) and DMC catalyst concentrations of 30, 50 and 100 ppm. 6638



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The synthesis processes were conducted according to Method (b), keeping the reactants at a molar ratio of 1:1. In connection with the specific sigmoidal curve shape of the empirical dependences obtained in the experiments with the use of the DMC catalyst, a formal model of the type of transition function was selected, having a shape similar to that of the asymmetrical cumulative log-normal distribution; the model is commonly used for the approximation of stochastic dependences with similar courses and is expressed by the following equation (eq 2):

The shape of curves illustrating the relationship between the propoxylation rate index Rp′ and the catalyst concentration does not show any significant variability over the catalyst concentration range of 30−100 ppm. The values of Rp′ in that range increase only slightly in the range 550−600 gPO/gPPG h for propoxylation of the starter PPG having a molecular weight of 450 and in the lower range of propoxylation rates (i.e., 300− 350 gPO/gPPG h for the starter having a molecular weight of 2700). Even though the values of Rp′ do indicate a slight dependence on the catalyst concentration in the experimental concentration range, major increments of the parameter per concentration unit should rather be expected in the range 0−20 ppm, assuming that the curve of Rp′ versus catalyst concentration should intersect the origin of coordinates (Figure 1; see dashed line). The shape of the curves, which shows the dependence of the relative polyaddition rate index, with respect to the catalyst concentration (Rp′), shows a significant decrease in its value with concentration, which is both an obvious mathematical consequence of the relationship between Rp and Rp′, and an illustration of the actual properties of the catalyst and the process kinetics. Assuming that the rate Rp, which indicates changes in the polyaddition rate in relation to the catalyst concentration, is a measure of its efficiency, the course of the curves shown indicates a decrease in the catalyst efficiency accompanying the increase in its concentration. The phenomenon, which is observed for highly active coordination catalysts in the polymerization of various monomers, is typical for increased contributions of diffusion processes in higher concentration ranges for catalysts with high activities. Kinetics of Polyoxypropylation with DMC Catalyst. Induction and Activation Period. Figure 2 shows the results of

pPO (t ) = a + b erfc[c ln(dt )]

(2)

where a, b, c, and d are constants for the model and the term “erfc” denotes the error function, which is defined by eq 3: erfc(x) = 1 −

2 π

∫0

x

exp( −t 2) dt

(3)

Figure 2 also shows the curve of methyloxirane pressure variations, recorded during a similar experiment, which was carried out with the use of the conventional catalyst (KOH) at a concentration of 3000 ppm in the DMC concentration range of 0−100 min and in its extended range 100−300 min (incorporated plot). In this case, the model used was typical of the substrate decay in a first-order reaction, as expressed by eq 4: pPO (t ) = a exp( −bt )

(4)

where a and b are constants for the assumed kinetic model. In the two cases of use of the model functions as defined above, a very good fit was obtained for the experimental time ranges (R2 > 0.99). As expected from an observation of the general properties of the DMC catalysts in the propoxylation process, as described in the previous section of this paper, the curve family for methyloxirane pressure variations during the catalytic reaction in the presence of DMC are very different from the kinetic curve for a process running in the presence of the KOH catalyst. Such differences are seen both in their range of variability, corresponding to the total rate of the process, and in their shape, which is connected with differences in the monotonicity of the curve shape. Although, in the case of curves corresponding to processes running in the presence of DMC, complete conversions are observed in the range from 15 min (for DMC levels between 100 ppm) to ∼60 min (for 30 ppm DMC), the curve showing the PO pressure drop in the case of the conventional catalyst reaches only 100 kPa (ca. 85% of conversion) after as long as 300 min (see incorporated plot for the expanded reaction time scale). A characteristic feature illustrating the qualitative differences between the two types of curves is their monotonicity. In contrast with the curve for the experiment where the conventional catalyst was used, and which had the features of a monotonically decreasing function, the curves pPO = f(t), which describe the process running in the presence of the DMC catalyst, clearly have an inflection point in the vicinity of which the curve pPO = f(t) takes a different course, and the reaction rate (s) is different as well. Propoxylation rate variation curves are shown in Figure 3. The curves were found analytically from the equation defining the reaction rate (eq 5), as used with respect to eqs 3 and 4:

Figure 2. Pressures of methyloxirane in the propoxylation of the substrate PPG 450 in the presence of the DMC catalyst at various concentrations (30−100 ppm) and in the presence of the conventional catalyst (KOH) at a concentration of 3000 ppm.

measurements of propylene oxide pressure in the course of propoxylation of the substrate PPG 450 in the presence of various concentrations of the DMC catalyst (30−100 ppm) and in the presence of the conventional catalyst (KOH) at a concentration of 3000 ppm. Vectors for the measurement points were approximated using formal model functions to carry out an analysis of the empirical values obtained. 6639



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Figure 3. Variations in propoxylation rate versus time.

rPO (kPa/min) = −

Figure 4. Comparison of the induction time and activation time during the conversion of methyloxirane, at 130 °C, depending on the catalyst concentration.

dpPO (t ) dt

(5)

reactions had frequently been observed to stop at such DMC concentrations, which, although adequately low, were definitely different from zero. Therefore, a function given by the following equation was arbitrarily adopted for the mathematical description: a ti , ta = [DMC] − [DMC]0 (6)

All curves that describe variations in the rate of propoxylation in the presence of the DMC catalyst were found to have a twostep shape and a maximum reaction rate point. The first step of the catalytic reaction running in the presence of DMC is the step where the reaction rate grows to its maximum value, despite a decrease in the substrate concentration. The observed quality is typical of catalytic processes having a catalyst activation step. In contrast with the terminology proposed in the literature, the present authors propose the term activation period to emphasize a difference between that step and the actual induction period, which is understood as the “initial slow phase of a chemical reaction which later accelerates”.31 The occurrence of activation and the activation period in polymerization reactions may be justified by the time needed to form catalytic reaction sites, especially under heterogenic catalysis conditions in the case of DMC. Therefore, the activation time (ta), which is defined as the time from the reaction start to the reaction rate maximum, is characterized by the course of the activation process. How the activation time is determined is shown in Figure 3 in the reaction rate curve corresponding to the catalyst concentration of 30 ppm. What deserves attention in the activation step region is the induction step, characterized by induction time (ti), in which the reaction accelerates. The values of induction time for the respective experiments were found as the point of intersection of a tangent to the arm of the propoxylation rate curve and the axis corresponding to a zero reaction rate. Furthermore, it can be seen that the polyaddition induction time and activation time are clearly dependent on the catalyst concentration. The induction time and activation time values of the test reaction vs DMC catalyst concentration, as determined from kinetic curves (Figure 3), are shown in Figure 4. As expected, both the induction period and the activation period were observed to decrease with the catalyst concentration. The simple reciprocal model was used for approximation of the dependence of both induction period and activation period on the concentration of DMC catalyst. Based on our earlier experiences in investigating the properties of the DMC catalysts, it was assumed that the function that approximated the above results should have its improper limit in infinity for a nonzero value of concentration of the DMC catalyst, since

where [DMC]0 is the limiting concentration of the catalyst, satisfying the condition given by eq 7: lim[DMC] → [DMC]0 ti , ta = ∞

(7)

For both cases of dependence between ti and ta on the catalyst concentrations, a good fit with experimentally determined points was obtained for the models given by eqs 8 and 9, where ti =

163.93 [DMC] − 12.3

(8)

and ta =

416.67 [DMC] − 12.5

(9) 2

The coefficient of determination (R ) was 0.99 in both cases. The values of [DMC]0, obtained as parameters of the respective regression curves were similar, reaching 12.3 and 12.5 ppm, respectively. At the same time, the test catalyst was experimentally confirmed to have no activity below a certain limiting concentration in the synthesis of polyoxypropylenediol. The existence of a limiting concentration of DMC can be justified by the presence, in the reaction medium, of small amounts of electrodonor substances (e.g., oxygen, water) which contaminate the reactants; competing with the monomer, such contaminants form coordination bonds with the active site, resulting in durable complexes that irreversibly deactivate the catalyst. The effect of such small amounts of contaminants is especially visible for catalysts used in very small concentrations. Dependence of Process Rate on DMC Concentration. The various processes that are operating during the activation of the DMC catalyst are very complex. They are comprised of the following: induction of catalytic reaction, formation of active sites of the DMC catalyst, as well as the catalytic polyaddition reaction itself. Therefore, it is quite a problem to 6640



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construct such a kinetic equation for the first step which reflects its physical sense. Hence, the formal models used for analytical purposes in the previous section were abstracted from their physical sense, although approximate with respect to monotonicity and adequacy of approximation. Furthermore, becauseas was shown abovethe process might proceed in the diffusion regime, the diffusion phenomena should be taken into account in consideration of examined propoxylation process kinetics. Assuming that the process in a batch reactor operates under non-steady-state conditions, the continuity equation should be used to quantitatively define the decay of propylene oxide: ∂C PO = −div NPO + rPO ∂t

where HPO denotes Henry’s constant for propylene oxide in reaction mixture. Although the exact solutions of the parabolic second-order linear partial differential equations for given initial conditions are known,32 the solution of eq 16 would require knowledge of both several chemical reaction rate constants kj as well the dependence of DPO and HPO on the reaction mixture composition, which makes solution of the given equation difficult. Therefore, for the scale-up purposes, the simple exponential approximation was used to express the dependence of the propylene oxide pressure on concentration of DMC catalyst and polyaddition time. Assuming, as a working hypothesis, that once the activation time is reached, the reaction kinetics is determined mainly by the course of polyaddition, quantitative relationships that describe the kinetics of catalytic polyaddition in the presence of DMC may be determined from characteristics of the course of reaction after reaching a maximum rate corresponding to the inflection points of the curves, which illustrate the dependence of PO pressure on time. After the activation time is reached, contribution of the processes connected with the commencement of the catalytic polyaddition reaction, such as induction, formation, and activation of the catalyst, is less and less pronounced while the process rate is determined by the mechanism of polyaddition, undisturbed by other processes and diffusion phenomena. For the curves shown in Figures 3 and 4, the process rate decreases monotonically, which is consistent with the decrease in the substrate concentration, after reaching an activation time determined by the reaction rate maximum. Figure 5 shows the PO substrate pressure drop curves, comprising the time interval above the activation time

(10)

where CPO denotes the molar concentration of propylene oxide in liquid phase, NPO is the vector of propylene oxide molar flux (given in units of kmol/m2 s), and then rPO denotes the total rate of propylene oxide consumption. Referring eq 10 to the batch reactor case and assuming onedimensional mass transfer, the molar propylene oxide material balance for the phenomenon of mass transfer, followed by the chemical reaction, may be written in scalar form (see eq 11): ∂C PO ∂ 2C PO = DPO − rPO ∂t ∂x 2

(11)

where DPO stands for the diffusivity of propylene oxide, and x is the path of propylene oxide diffusion. If we assume that the propylene oxide is consumed in the following parallel set of reactions: k0

HO−(PO)m −OH + PO → HO−(PO)m + 1−OH k1

HO−(PO)m + 1−OH + PO → HO−(PO)m + 2 −OH

(12) (13)

⋮ kn

HO−(PO)m + n − 1−OH + PO → HO−(PO)m + n −OH (14)

where HO−(PO)m−OH denotes the initial glycol substrate containing m propylene oxide molecules (e.g., PPG 450), the overall propylene consumption rate may be expressed in the following form: j=n

rPO =

∑ rPOj j=0 j=n

=

∑ kjC HO−(PO)

C PO

m + j −OH

Figure 5. PO substrate pressure drop curves for a time interval above the reaction rate maximum.

j=0 j=n

= C PO ∑ kjC HO−(PO)m+j −OH (15)

j=0

corresponding to the reaction rate maximum. The plots are shown in a semilogarithmic scale, with respect to the axis of coordinates. The curves shown in Figure 5 have the features of a straightline course of the exponential-type dependence between PO pressure and time which, given by eq 17:

Substitution of eq 15 into eq 11 gives j=n

∂C PO ∂ 2C PO = DPO − C PO ∑ kjC HO−(PO)m+j −OH ∂t ∂x 2 j=0 = DPO

∂ 2C PO ∂x 2

−

pPO HPO

j=n

∑ kjC HO−(PO)

m + j −OH

j=0

DMC pPO (t ) = A exp( − k′t )

(16) 6641

(17)



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where A denotes the pre-exponential factor and k′ is the process rate constant. The observation does not conflict with commonly recognized kinetic characteristics of the steps of catalytic propagation of oxirane polyaddition in the presence of conventional catalysts, which enable a first-order kinetics model to be adopted to describe the kinetics of such reactions, with respect to PO, as defined by eq 18:3,33,34 rPO = −

d[PO] = k[kat][PO] dt

reaction to stop as soon as it is reached. It was postulated in the previous section that it was necessary to introduce this parameter. The value of the limiting concentration was found by transforming eq 20, which describes the curve shown in Figure 6, k′ = 0.0043[DMC] − 0.0562

to its equivalent form (see eq 21): k′ = 0.0043([DMC] − 13.1)

(18)

(21)

where the concentration value of 13.1, expressed in units of ppm, is the limiting concentration of the DMC catalyst for which the polyaddition rate constant (k′) approaches 0; this is identical to an infinitely long catalyst activation time, as postulated by eq 7. The value of limiting concentration, found independently from an analysis of eq 20, is close to 12.4 ppm, which is the value that results from eqs 8 and 9; this indicates that the adopted assumptions regarding the kinetic descriptions are correct. In view of that regularity, the kinetic equation that describes the course of polyaddition step II in the presence of the DMC catalyst can be described by eq 22:

to which the solution, assuming a constant concentration of the catalyst and the following initial condition:t = 0 ⇒pDMC PO (t) = pDMC PO (0) leads to the form shown by eq 19, where k′ = k[DMC]

(20)

(19)

Table S3 in the Supporting Information shows the values of the coefficients pDMC PO (0) and k′. Should be emphasized, despite the similarity of eq 17 to the first-order kinetics equation form, that k′ could not be interpreted as a first-order chemical reaction rate constant, because of the supposed diffusion phenomena contribution in the process and eq 17 is to be only the empirical approximation model. Instead, k′ has the sense of the overall process rate. The proposed models relate to the process step, which immediately follows its induction and activation, the values of parameter pDMC PO (0), as found by regression analysis, have a formal sense only and are values of methyloxirane pressure after their extrapolation to a hypothetical state of the polyaddition reaction running without the induction/activation step. Verification of the dependence of the determined overall process rate constants, k′, on the concentration of the DMC catalyst help justify the consistency and adequacy of the adopted model. Figure 6 shows the curve of dependence of the process rate constant, k′, on the concentration of the DMC catalyst.

DMC DMC pPO (t ) = pPO (0) exp( − k[DMC]e t )

(22)

where [DMC]e denotes the effective concentration of the DMC catalyst in its active form, as expressed by eq 23: [DMC]e = [DMC] − [DMC]0

(23)

Characteristics of the Reaction Selectivity. Hydroxyl Value. Hydroxyl values were determined for the obtained products in order to examine the reaction selectivity in the presence of the DMC catalyst. Assuming that the reaction is selective toward polyaddition, the hydroxyl value is related to the number of molecules of the generated polymer in 1 g of the sample, and it is defined by eq 24: ∞

OHV = 56100∑ nifi i=1

(24)

where ni is the number of moles of homologue i and f i is the functionality of homologue i. Assuming a constant value of functionality of f = 2 for glycol and the defining equation of number-average molecular weight, as defined by eq 25, ∞

M̅ n =

∑i = 1 ni Mi ∞

∑i = 1 ni

(25)

the dependence between the number-average molecular weight and the value of OHV is shown by the following relationship (see eq 26):

Figure 6. Curve of dependence of reaction rate constant (k′) on the concentration of the DMC catalyst.

M̅ n (g/mol) =

Although the obtained dependence clearly has the features of a straight-line course, indicating a proportional dependence of the constant, k′, on concentration, the above relationship is different from the form given by eq 19 and results directly from the adopted model, as well as from the constant value of the catalyst concentration. As shown further, the reason for the above divergence is the catalyst’s limiting concentration [DMC]0, which causes the

56100f OHV

(26)

Assuming the above, the value of OHV found for PPG with a molecular weight of 2700 is 41.55 mg KOH/g; for a respective product with a molecular weight of Mn = 10 000, OHV = 11.22 mg KOH/g. The values of OHV, determined for propoxylation products obtained with various concentrations of the DMC catalyst and in the presence of the alkaline catalyst, are shown in Figure 7. 6642



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Figure 7. Comparison of the results of a measurement of hydroxyl values (OHV) for polyoxypropylenediols, depending on the type and concentration of the catalyst used and on the molecular weight of the products obtained.

Figure 8. Comparison of GPC chromatograms for polyoxypropoxylation products PPG 2700, depending on the concentration of the DMC catalyst.

It can be seen that hydroxyl values (OHV) for products obtained in the presence of the alkaline catalyst (KOH) were much higher than the theoretical ones, both for PPG 2700 and for PPG 10000. This indicates a significant contribution of side reactions, including termination of the polyoxyalkylene chain, which generate additional hydroxyl groups in the product. On the other hand, products obtained in the presence of the DMC catalyst were characterized by hydroxyl values which were very similar to the theoretical ones, indicating that selectivity of the synthesis was high in that case. Unsaturation Number. The content of double-bond macromolecules in the product is a second selectivity criterion in the synthesis of polyoxypropylenation. This is connected with the termination of the polyether chain or the conversion of methyloxirane to allyl alcohol. Such phenomena are well-known and undesirable, because they affect product quality and usefulness.17−19 The level of generated unsaturated bonds, found in the test products obtained in the presence of the DMC catalyst, was 1 order of magnitude lower, compared to that of the alkaline catalyst (see Table S4 in the Supporting Information). Moreover, there is a rather noticeable growing tendency for higher concentrations of the DMC catalyst in the test range, and their content in products with higher molecular range was also slightly higher. Higher concentrations of the catalyst indicate a larger number of active sites of the reaction, which may account for the increased number of double bonds generated, as well as the longer reaction time, connected with the synthesis of polyethers with higher molecular weights. Polydispersity of the Product. The polydispersity of polyethers, measured by the value of MWD = Mw/Mn, where Mw = ∑NiMi2/∑NiMi denotes a mass-average molecular weight and Mn = ∑NiMi/∑Ni is the number-average molecular weight, which is is an important parameter in industrial applications. Narrow polydispersity is usually a desirable property; its required value (MWD) is ∼1, which indicates a uniformity of macromolecules in the product. It was determined, rather unexpectedly, that the concentration of the DMC catalyst had a significant effect on the fractional composition of the obtained products (see Figure 8). Dispersity (MWD = Mw/Mn) of the obtained products clearly decreases, which is consistent with an increase in the concentration of the catalyst (Figure 8). Polyoxypropylenediol products with higher average molecular weights have an

improved uniformity of macromolecules (lower values of MWD; see Table S5 in the Supporting Information). The Natta−Mantica relationship can be used for kinetic interpretation of the content of the discussed products.35 It represented a mathematical model of polyaddition based on the assumption that the ratio of the reaction rate constants of the ith propagation step (ki) to the reaction rate constant of the first PO molecule addition to starter substrate (k0) can vary each from the other. The relationship between the molar fraction of the single component xi values (measured) and the Natta−Mantica distribution coefficient, ci = ki/k0 is given by the following formula: i−1

i

xi = ( −1) ∏ cj ∑ j=1

j=0

xocj ∏ik = 0 (cj − ck) k≠j

where k 0 ≠ k1 ≠ ... ≠ ki

(27)

The distribution coefficients c1, c2, ..., ci can be computed using eq 1, from experimentally determined molar fractions x0, x1, x2, ..., xi, according to the appropriate interval bisection method. Based on the discussed mathematical model, it was demonstrated that the increased polydispersity of the polyaddition product is accounted for by a relative increase in the reaction rate constants, ki, in the consecutive propagation steps, whereki/k0 > 1.36 In effect, higher catalyst concentrations (more active sites) relatively improve the probability of effective collisions between the particles that take part in the reaction, in the early steps of polyaddition. If the reaction rate constants tend to decrease at lower steps of polyaddition then, in effect, the rate of conversion of methyloxirane, as calculated for products obtained with a relatively higher number of active sites, is lower while the molecular weight distribution (MWD) for the obtained product is narrower than in the case of the same products obtained in the presence of lower concentrations of the DMC catalyst. Respectively, in the presence of relatively fewer active sites, the successively generated polyadducts with higher propagation rate constants will be more favored already at the beginning of the reaction, leading to higher MWD values for the product obtained. 6643



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Figure 9. An apparent reaction route during the addition of an adduct in the presence of different catalyst concentrations (ki/ki+1 < 1. (The term “Cat” denotes the single catalyst active center.)

the presence of higher concentrations of the catalyst. The dependence of polydispersity of polypropoxylation products on the DMC catalyst concentration ought to hold true for all catalysts of that type, although absolute relationships in that scope may differ for various methods to obtain such catalysts (that is, on the number of available active sites per weight unit).

An accurate quantitative determination of the homologues concentration in the discussed high-molecular-weight products is not possible, to use them for relevant computer calculation. However, a qualitative explanation of the obtained differences in the products distributions can be carried out as follows. The differences in the product contents, caused by the differences in catalyst concentration prove that an effective simultaneous collision of substrate (polypropoxylate), adduct (methyloxirane), and the catalyst is a condition to generate any successive propagation step. The effectiveness of the collisions are further controlled by the relative values of reaction rate constants at the following addition steps. To simulate the synthesis, a simplified model of the reaction can be assumed, where two particles of substrate react with two particles of adduct and two particles of catalyst, as compared to the same reaction in the presence of one particle of catalyst, while ki/ki+1 < 1 (see Figures 9a−d). Figures 9a and 9b show the reaction under conditions of greater concentration of the catalyst, which favors the formation of the shorter polyoxypropylene chains, because of high concentrations of the catalyst active center. In the case of a lower concentration of the catalyst (Figures 9c and 9d), the propagation of polyoxypropylene chain occurs on small quantities of the active centers, thus, the final chains are longer. It was demonstrated that varying the catalyst concentrations generates different product contents when the reaction rate constants are different at the following addition steps, while narrower homologue distributions are obtained in

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CONCLUSIONS An improvement to the process of obtaining a DMC catalyst by adding a polyelectrolyte to the reaction medium in order to provide the necessary conditions for the flocculation of strongly dispersed catalyst precipitate has no negative impact on the high activity of the DMC catalystthe latter remains unaffected. A procedure that includes the flocculation step provides a catalyst that has a very reproducible composition, as indicated by elemental balance, based on the results of elemental analysis. The reaction rates for polyaddition in the presence of the DMC catalyst obtained by the method which comprises flocculation are roughly 400 times higher than those for the conventional alkaline catalysts. The results of the study of the catalyst concentration on its efficiency, as measured by the coefficients Rp and Rp′, which are applied in the polyoxypropylenation process engineering, indicate a slight growth of the polyaddition rate (in grams of propylene oxide per hour per gram of the starter) and a decrease in the catalyst efficiency per unit weight of the catalyst; this is related to the higher contribution of diffusive mass transfer in the 6644



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polyaddition process. In contrast with the kinetic characteristics of the polyaddition process in the presence of the conventional alkaline catalyst (KOH), the test reactions which were carried out in the presence of the DMC catalyst were found to have a two-phase course of polyaddition. The occurrence of the first step, leading to the growth of reaction rate to a maximum, was justified by the occurrence of the step of induction of polyaddition and the formation of active sites of the catalyst. A second step of the reaction, after exceeding the maximum reaction rate, can be described by the empirical equation, characterized by an exponential decay of methyloxirane substrate. Both the induction time and activation time, which characterize the induction and activation steps, respectively, are reversely proportional to the catalyst concentration. The dependence of induction time, activation time, and reaction rate constant on the concentration of the DMC catalyst enabled the authors, independently, to demonstrate that a nonzero limiting concentration of the catalyst below which catalytic polyaddition will not occur did exist. The existence of the limiting concentration was justified by the presence of trace amounts of poisons, which deactivate part of the catalyst. The selectivity of the DMC catalyst, which is the subject of the study described in this paper, expressed with the use of hydroxyl number and unsaturation number, is much better than that of the conventional catalyst. The hydroxyl values, obtained for polyaddition products in the presence of the DMC catalyst, were consistent with stoichiometric assumptions, unlike the conventional catalyst, for which the hydroxyl values were higher than the assumed values. The above results confirm earlier observations: a low selectivity of the KOH catalyst, as manifested by the formation of undesirable side products. Unsaturation numbers of products obtained by polyaddition with the use of DMC were much lower than products obtained in the presence of KOH. The DMC catalyst is characteristic in that polyoxypropylenediol macromolecules, obtained in the presence of DMC, have higher uniformities for higher catalyst concentrations. Using an appropriate mathematical model, it was demonstrated that lower dispersity (higher uniformity of product) is a consequence of a relative reduction of the values of reaction rate constants in the successive steps of propagation.

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Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +48 77-487-38-10. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by the program Science and Business in the Implementation of Innovative Projects INNOTECH II, 2012−2015 (Project ID K2/IN2/21/ 181982/NCBR/12). Jarosław Janik is a recipient of a Ph.D. scholarship under a project financed out of the European Social Fund.

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REFERENCES

(1) Schick, M. Nonionic Surfactants; Marcel Dekker: New York, 1967. (2) Huang, Y.-J.; Qi, G.-R.; Wang, Y.-H. Controlled ring-opening polymerization of propylene oxide catalyzed by double metal-cyanide complex. J. Polym. Sci. 2002, A40, 1142−1150. (3) Bailey, F. E., Jr.; Koleske, J. V. Alkylene Oxides and Their Polymers; Surfactant Science Series, Vol. 35; Marcel Dekker: New York, 1991 (4) MaterialScience. Presented at 13th Annual Green Chemistry & Engineering Conference; University of Maryland, College Park, MD, June 2009. (5) Herold, R. J. Method of making a polyether using a double metal cyanide complex compound. U.S. Patent 3,278,459, Oct. 11, 1966. (6) Milgrom, J. Method of making a polyether using a double metal cyanide complex compound. U.S. Patent 3,278,457, Oct.11, 1966. (7) Milgrom, J. Production of polyether diols using water as a telogen. U.S. Patent 3,404,109, Oct. 1, 1968. (8) Le-Khac, B.; Bowman, P. T.; Hinney, H. R. Highly active double metal cyanide complex catalyst. U.S. Patent5,712,216, Jan. 27, 1998. (9) Hayes, J. E.; McDaniel, K. G.; Perry, M. J.Direct polyoxyalkylation of glycerine with double metal cyanide catalysis. U.S. Patent 6,077,978, June 20, 2000. (10) Suzuki,C.; Suzuki, T.; Ikai, S. Process for purifying polyether. U.S. Patent20090203875, Aug. 13, 2009. (11) Eleveld, M. B.; Meurs, J. H. H.; Raney, K. H.; Wiersma, R. J.; Van, Z. A. Preparation of an alkoxylate composition. World Patent WO2005005360, Jan. 20, 2005. (12) Browne, E. P. Starter feed stream acidification in dmc-catalyzed process. U.S. Patent 2010234647, Sept. 16, 2010. (13) Browne, E. P.Starter feed stream acidification in DMC-catalyzed process for the production of polyether polyols. Eur. Patent EP 1,577,334, Feb. 20, 2005. (14) Ooms, P.; Hofmann, J.; Gupta, P.; Groenendaal, L. Double metal cyanide catalysts for preparing poly-etherpolyols. U.S. Patent6,204,357, March 20, 2001. (15) Hofmann, J.; Ooms, P.; Gupta, P.; Schäfer, W. Double metal cyanide catalysts for producing polyether polyols, U.S. Patent6,291,388, Sept. 18, 2001. (16) Hofmann, J.; Ooms, P.; Gupta, P.; Schäfer, W.; Lohrenz, J. Double metal cyanide catalysts for producing polyether polyols. U.S. Patent7,008,900, March 7, 2006. (17) Kim, I.; Ahn, J.-T.; Lee, S.-H.; Ha, C.-S.; Park, D.-W. Preparation of multi-metal cyanide catalysts and ring-opening polymerization of propylene oxide. Catal. Today 2004, 93−95, 511− 516. (18) Chen, S.; Zhang, P.; Chen, L. Fe/Zn double metal cyanide (DMC) catalyzed ring-opening polymerization of propylene oxide. Part 3. Synthesis of DMC catalysts. Prog. Org. Coat. 2004, 50, 269− 272. (19) Huang, Y. J.; Qi, G. R.; Chen, L. S. Effects of morphology and composition on catalytic performance of double metal cyanide complex catalyst. Appl. Catal., A 2003, 240, 263−271.

ASSOCIATED CONTENT

* Supporting Information S

Supporting Information file includes analytical characteristics of the DMC catalysts: DMC1 and DMC2 (zinc hexacyanocobaltate(III) with different water contents) and DMC 3−5 (DMC catalysts obtained according to the procedure (samples of independent product lots)) (Table S1); average conversion rates for PO during the synthesis of PPG 2700 and PPG 10000, in the presence of KOH and DMC DMC (0) andk′ versus catalysts (Table S2); coefficients pPO concentration of DMC catalyst (Table S3); results of an assessment of unsaturation number for various types and concentrations of catalyst for the propoxylation products PPG 2700, PPG 10000, and PPG 50000 (Table S4); and dependence of polydispersity (MWD) of the tested polyoxypropylenediols with molecular weights of 2700 and 10000, depending on the catalyst concentration (Table S5). This material is available free of charge via the Internet at http:// pubs.acs.org. 6645



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(20) Liu, H.; Wang, X.; Gu, Y.; Guo, W. Preparation and characterization of double metal cyanide complex catalysts. Molecules 2003, 8, 67−73. (21) Lee, S.; Baek, S. T.; Anas, K.; Ha, C.-S.; D.-W., P.; Lee, J. W.; Kim, I. Tuning of activity, induction period and polymer properties of double metal cyanide catalyzed ring-opening polymerization of propylene oxide by using quaternary ammonium salts. Polymer 2007, 48, 4361−4367. (22) Hofmann,J.; Ehlers, S.; Klinksiek, B.; Fechtel, T.; Ruhland, M.; Scholz, J.; Foehles, F.; Esser, U. Method for producing double metal cyanide (dmc) catalysts. World Patent WO01/80994, Nov. 1, 2000. (23) Hofmann, J.; Klinksiek, B.; Ehlers, S.; Fechtel, T.; Foehles, F.; Ooms, P. Method for producing dmc catalysts. World Patent WO01/ 39883, June 7, 2001. (24) Zhang, X.-H.; Hua, Z.-J.; Chen, S.; Liu, F.; Sun, X.-K.; Qi, G.-R. Role of zinc chloride and complexing agents in highly active double metal cyanide catalysts for ring-opening polymerization of propylene oxide. Appl. Catal., A 2007, 325, 91−98. (25) Yoon, J. H.; Lee, I. K.; Choi, H. Y.; Choi, E. J.; Yoon, J. H.; Shim, S. E.; I., K. Double metal cyanide catalysts bearing lactate esters as eco-friendly complexing agents for the synthesis of highly pure polyols. Green Chem. 2011, 13, 59−63. (26) Huang, Y.-J.; Zhang, X.-H.; Hua, Z.-J.; Chen, S.-L.; Qi, G.-R. Ring-opening polymerization of propylene oxide catalyzed by a calcium-chloride-modified zinc-cobalt double metal-cyanide complex. Macromol. Chem. Phys. 2010, 211, 1229−1237. (27) Herold, J. R. Polyethers and method for making the same. U.S. Patent 3,829,505, Aug. 13, 1974. (28) Herold, J. R. Polyethers and method for making the same. U.S. Patent 3,941,849, March 2, 1976. (29) Chruściel, A.; Hreczuch, W.; Wojdyła, H.; Czaja, K.; Janik, J.; Domarecka D.; Domarecki, W. Skład i sposób otrzymywania katalizatora oksyalkilenowania. Pol. Patent Application 398518, March 19, 2012. (30) Bobrański, B. Ü ber ein neues Zentigramm-Verfahren zur Bestimmungder Halogene in organischen Substanzen. Z. Anal. Chem. 1931, 84, 225−239. (31) McNaught, A. D.; Wilkinson, A. Compendium of Chemical Terminology, 2nd Ed. (The “Gold Book”); Blackwell Scientific Publications: Oxford, 1997. (32) White, R. E.; Subramanian, V. R. Computational Methods in Chemical Engineering with Maple; Springer−Verlag, Berlin Heidelberg, 2010. (33) Ionescu, M. Chemistry and Technology of Polyurethane and Polyols; Rapra Technology Ltd.: Shawbury, Shrewsbury, Shropshire, U.K., 2005; pp 77−78. (34) Dubois, P.; Coulembier, O.; Raquez, J. Handbook of RingOpening Polymerisation; Wiley−VCH Verlag GmbH&Co KGaA: Weinheim, Germany, 2009. (35) Natta, G.; Mantica, E. The Distribution of Products in a Series of Consecutive Competitive Reactions. J. Am. Chem. Soc. 1952, 74, 3152−3156. (36) Hreczuch, W.; Szymanowski, J. Synthesis of surfactans with narrow range distribution of the polyoxyethylene chain. J. Am. Oil Chem. Soc. 1996, 73, 73−78.

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