Melt Transesterification of Polycarbonate Catalyzed by DMAP

The molecular weight distribution of the PC product had the most probable distribution. The concentration of the catalyst reasonably affected the hue ...
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Melt Transesterification of Polycarbonate Catalyzed by DMAP Jyh-Ping Hsu* Department of Chemical Engineering, National Taiwan UniVersity, Taipei, Taiwan 10617

Jinn-Jong Wong Union Chemical Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan 300

The melt transesterification of polycarbonate (PC) by diphenol carbonate (DPC) and 4,4′-dihydroxy-diphenyl2,2-propane (BPA) using 4-(dimethylamino)pyridine (DMAP) as the catalyst was studied experimentally. It was observed that the operating conditions markedly influenced the properties of the resulting polymer. Narrow temperature control in the heat-transfer operation reduced the unwanted thermal effects. The ratio of DPC/ BPA markedly influenced the intrinsic viscosity of the resulting polymer. A ratio in the range 1.03-1.05 was found to yield polymer with a high intrinsic viscosity. The molecular weight distribution of the PC product had the most probable distribution. The concentration of the catalyst reasonably affected the hue of the PC resin, and we suggest that it should not exceed 50 ppm when DMAP is used as the catalyst. Introduction Polycarbonate (PC) is an important engineering thermoplastic material with good mechanical and optical properties for various industrial applications.1 It can be produced by an interfacial phosgenation process and melt transesterification.2 In recent years, the melt transesterification of PC with the starting materials 4,4′-dihydroxy-diphenyl-2,2-propane (BPA) and diphenol carbonate (DPC) is becoming more important, since it is regarded as an environmentally benign production technique.3 However, the PC prepared by conventional melt transesterification generally yields resins of poor quality. It is still not widely used in the industry. Therefore, the key for improving the process is improving the properties of the polymer. The melt transesterification process involves a transesterification stage and a melt polymerization stage. The transesterification is an equilibrium reaction. Most of the byproduct phenol must be split off at 150-200 °C at reduced pressure. Unfortunately, the thermal instability of BPA may appear, and a side reaction of the Kolbe-Schmitt type reaction might occur in the presence of an alkaline catalyst at a temperature greater than 150 °C.4 Following the transesterification stage, the reaction occurs at a higher temperature under a stronger vacuum to remove residual phenol from the high-viscosity polymer melt to yield PC with a high molecular weight. However, in the presence of a strongly alkaline catalyst of transesterification and at a sufficiently high temperature, some of the DPC and some of the formed PC are decomposed.4 Clearly, the thermal effects in the process critically determine the properties of the polymer. Such information has not previously been available in the literature. Basic compounds, such as alkali metals, alkaline earth metals, and their oxides, hydrides, and amides, as well as basic metaloxides, are well-known to accelerate transesterification markedly. However, an effective transesterification catalyst not only accelerates the reaction but also prevents side reactions, as much as possible, at high temperature. Therefore, a weak basic compound is favored as a catalyst. Additionally, the catalytic reaction of the melt transesterification of DPC and BPA follows a reaction mechanism that is based on the nucleophilic substitution of a carbonyl group.5 4-(Dimethylamino)pyridine (DMAP) * To whom correspondence should be addressed. Tel: 886-223637448. Fax: 886-2-23623040. E-mail: [email protected].

was selected as the catalyst; it is a weak alkaline compound and is generally a good nucleophilic agent.6 The use of DMAP as the catalyst in the melt transesterification of DPC and BPA, however, has not been reported in the literature. DMAP is expected to accelerate transesterification and improve the production of high-quality PC. Stoichiometrically, the preparation of PC by melt transesterification requires equal moles of DPC and BPA. The same amounts of hydroxy end groups and phenyl carbonate end groups are consumed during polycondensation. If a noticeable imbalance in end groups exists in the reaction system, then the product may be a low-molecular-weight polymer with unwanted properties. Unfortunately, the loss of DPC, resulting from evaporation, through a distillation column has been reported to occur during the melt process,7,8 indicating that proper control of the molar ratio of reactive end groups in a reactor is not trivial. Therefore, the uncontrollable molar ratio strongly affects the characteristics of the resulting polymer. For practical purposes, a properly designed process and corresponding operating condition are required to yield PC with favorable properties. In this work, DMAP was used as a catalyst to elucidate the effects of operating conditions on polycondensation and the properties of the formed polymer. Experiments were conducted with various DPC/BPA ratios, catalyst concentrations, and operating conditions. The aim of this work was to yield useful information for producing high-quality PC. Reaction Kinetics The transesterification of DPC and BPA has been expressed as4

The melt transesterification of DPC and BPA involves com-

10.1021/ie050726+ CCC: $33.50 © 2006 American Chemical Society Published on Web 03/22/2006

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plicated reversible reactions. High-performance liquid chromatography analysis revealed that compounds of various molecular weights are present in the reacting mixture.6,7 Under ideal conditions, the system includes two kinds of terminal end groups. They are either phenolic (-AR-OH) or phenyl (-O-AR) end groups, indicating that polymer oligomers fall into three categories: An, Sn, and Bn. Apparently, eq 1 just represents a very special case of all reactions. Reactions among An, Sn, and Bn can be expressed as5

An + Sm h An+m + phenol

(2)

An + Bm h Sn+m+1 + phenol

(3)

Sn + Sm h Sn+m + phenol

(4)

Sn + Bm h Bn+m + phenol

(5)

When n and m equal zero, A0 is DPC and B0 is BPA. Equations 2-5 show all oligomer and polymer species involved in this complicated reaction system. Every one may be produced and may disappear during the reaction. It depends on the reaction conditions. If A0 (DPC) is in excess, the An- and Sn-type polymers would be dominant in the final product, and the Bntype polymer would be relatively low in concentration. Similarly, if BPA is in excess, the Bn- and Sn-type polymers would be dominant, and the An-type polymer would be relatively low in concentration. Thus, different monomer ratios lead to different polymer compositions. This also implies that the monomer ratio would affect polymer properties. It had been pointed out that branching, cross-linking, and other side reactions occurring temperatures greater than 150 °C can lead to discolored or degraded products.4,12 Thermal instability of BPA and a side reaction of the Kolbe-Schmitt type may take place in the presence of alkali at high temperature. Thus, BPA splits into isopropenyl phenol and phenol as

If these highly reactive unsaturated phenols are formed during transesterification, they will undergo polymerization and additional reactions before they can be removed from the reaction mixture by distillation. The polymerization and additional reactions of these phenols result in undesirable colored products. Experimental Section Materials. Diphenyl carbonate (DPC, >99.5%, mp ) 79.2 °C, INBO Chemical Engineering Co., China), 4,4′-dihydroxydiphenyl-2,2-propane (bisphenol A, >99.9%, Taiwan Prosperity Chemical Co., Taiwan), and 4-(dimethylamino)pyridine (DMAP, >99.9%, Aldrich) were used as received. Reactor system. With reference to Figure 1, melt polycondensation is carried out in a two-stage reaction system. The volume of the transesterification reactor (R-100) is around 50 L, and it is centrally mounted on a standard marine propeller with four symmetric baffles. The volume of the polymerization reactor (R-200) is around 50 L, and it is centrally mounted on a helical ribbon impeller. Both R-100 and R-200 are stainlesssteel-jacketed reactors heated by heating oil in the jacket. Condenser E-100 and receiver V-100 were cooled with roomtemperature cooling water. E-200 and E-300 were cooled with 5 °C chilling water. M-300 is a mechanical booster pump vacuum system. It is composed of a liquid ring vacuum pump and two mechanical boosters. At about 150 °C, the reactor pressure was gradually reduced. The liquid ring pump was first

Figure 1. Experimental setup: DPC, diphenyl carbonate; BPA, bisphenol A; R-100 and R-200, 1st and 2nd stage reactors; M-100 and M-200, agitators of R-100 and R-200; V-100 and V-200, receivers of condensate; E-100, E-200, and E-300, condensers; M-300, vacuum pump; HOS, heating-oil supply; HOR, heating-oil return.

started to obtain a pressure range of 760-40 mmHg. A control valve was installed to regulate vapor flow. While pressure was below 40 mmHg, the first mechanical booster was started. At pressures below 3 mmHg, the full vacuum system was working. The reactor system was purged with high-purity nitrogen before the starting materials were charged. The starting materials, DPC and BPA, were weighed (about 17 kg ( 2 g) in a specific molar ratio. They were mixed with carefully weighed catalyst ((0.001 g) in a plastic drum. Then, all materials, in the powder form, were fed to R-100. But the agitator, M-100, cannot start immediately since the melting point of DPC is 79 °C. The reactor temperature gradually increased to around 120 °C, and the agitator was started when it could be smoothly rotated by hand. The rotation speed was kept at 150 rpm. Under this condition, it was assumed that all reactants were mixed uniformly. The reactor pressure was gradually reduced following a predetermined reducing rate. The phenol was liberated during transesterification under reduced pressure. The vapor was condensed by passed through the condenser (E-100), and the condensate was collected in a collection vessel (V-100) with a calibrated scale for reading the volume. The reacting mixture was transferred to R-200 by vapor pressure and gravity when the transesterification reached a certain temperature and pressure and the volume of the condensate no longer increased. A higher temperature and a stronger vacuum are required to promote polymerization. The current of the agitator was used to monitor the viscosity of the polymer melt. The agitation was stopped when the current reached the desired value. Finally, the molten polymer was ejected from the reactor by applying nitrogen under pressure. The polymer melt was cooled to form thin strings by passing it through a water bath; the polymer strings were pelletized into polymer chips. The molecular weight distributions of the PC samples were measured by gel permeation chromatography (GPC). The GPC system was a Waters LC system equipped with a Waters 610 fluid unit, a Waters 717 Plus automatic sampler, and a Waters 410 differential refractometer, using Waters Styragel HR1 and HR2 columns. The dry samples were dissolved in THF, which

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Figure 2. Difference between temperature of heating medium and that of reaction mixture: [ and O, difference between the temperature of the heating medium and that of the reaction mixture during reaction; solid curve, large temperature difference obtained by least-squares fitting using the equation large diff. ) -0.1665(temp - 374), 160 °C < temp < 200 °C; dashed curves, small temperature difference obtained by least-squares fitting using the equation small-diff. ) -0.1665(temp - 227), 160 °C < temp < 200 °C; [, large temperature difference; O, small temperature difference.

was also used as the eluent at a flow rate of 1 mL/min at 20 °C. The molecular weight was calibrated using narrow molecular weight (M) standard polystyrene and analyzed using Waters Millennium software. The molecular weights of the PC samples were also estimated by the viscometric method. Intrinsic viscosities (IV) were determined at 30 °C in a dilute methylene dichloride solution (0.5 g/dL) using an Ubbelohde Capillary viscometer. The Commission Internationale de l′Eclairage (CIE) organization determined standard values that are used worldwide to measure color. The values used by CIE are called L*, a*, and b*, and the color measurement method is called CIELAB. L* represents the difference between light (where L* ) 100) and dark (where L* ) 0); a* represents the difference between green (-a*) and red (+a*), and b* represents the difference between yellow (+b*) and blue (-b*). The CIELAB color values, L*, a*, and b*, were employed to represent the color index of PC. The color values of injection-molded PC bars were measured in the reflective mode using a spectrum colorimeter (Juki, model JP7000). A sample with a large L* and a small positive b* is a polymer with high transmittance and favorable color. Results and Discussion Energy is typically required to evaporate byproduct phenol in the melt process of PC. The heat-transfer rate depends on the temperature difference across the reactor wall. For a given reactor, a large temperature difference corresponds to a high heat-transfer rate. Figure 2 shows that the difference between the temperatures of the heating medium and the reaction mixture can be operated at high or low level. The high temperature difference generally exceeds 40-50 °C, and the low temperature difference is typically in the range of 5-15 °C. If a stainless steel reactor is heated by applying a high temperature difference, then the temperature of the reactor wall will greatly exceed the temperature of reaction mixture. As mentioned above, the thermal instability of BPA and a side reaction of the KolbeSchmitt type reaction may occur in the presence of the alkaline catalyst when the temperature exceeds 150 °C. Therefore, undesirable side reactions might occur at the reactor wall. A properly controlled temperature difference should be maintained to reduce these side reactions. A polymer produced with a large temperature difference is more likely to be dark brown, whereas one produced with a small temperature difference is more likely to be transparent and colorless. The results reveal that the reactor

Figure 3. General operating conditions for transesterification stage: reactor temperature (°C), b; jacket temperature (°C), [; reaction pressure (mmHg), 4.

Figure 4. Dependence of removal rate of condensate on reactor pressure. Discrete symbols represent experimental data: dashed curve, removal rate obtained by least-squares fitting using the equation removal rate ) -8 × 10-6pressure + 0.0069, 0 mmHg < pressure < 760 mmHg.

should be designed with a large heat transfer area to supply sufficient energy to evaporate the phenol. The transesterification of DPC and BPA begins at around 150 °C, which is far below the normal boiling point of phenol. Therefore, reduced pressure is essential for the removal of phenol. The reactor temperature must be elevated to keep the reaction mixture in the molten state. This can be achieved by adopting, for example, the operation scheme illustrated in Figure 3, in which the pressure of the reactor was regulated by following a prespecified profile. When the pressure was reduced to around 150-200 mmHg, the reactor temperature was dropped. Clearly, the removal of heat by evaporation of phenol caused sudden cooling. Sudden cooling of the reactor should be prevented as much as possible because it may trigger local phase separation or heterogeneity in the reactor.9 Even though reactor pressure can be reduced as soon as the vacuum machine is applied, unexpected kinetic behavior occurs in the melt process. Also, the fact that the flash evaporation of phenol under reduced pressure causes the sudden cooling of the reactor is understandable. Although the heat-transfer rate can be improved by increasing the temperature gradient, doing so would aggravate the aforementioned thermal effect. Therefore, the reactor pressure should be adjusted to resist the sudden cooling of the reactor. A mild decline in pressure, as shown in Figure 3, is the preferred way to operate the reaction system smoothly. The transesterification of DPC and BPA is an equilibrium reaction. The removal of the byproduct is critical to increasing the molecular weight in this process. As previously mentioned, transesterification begins at a temperature that is below the normal boiling point of phenol. The pressure was reduced to remove phenol. A pressure gradient was reasonably imposed on the system. Figure 4 shows that reducing the reactor pressure, increases the pressure gradient, accelerating the removal of the condensate, which is defined as the volumetric flow rate per unit reaction volume (L/min/L). It reveals that the rate of removal of the condensate correlates closely with the operating

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Figure 5. Effect of initial monomer molar ratio on the IV of PC for polymer samples prepared at various DPC/BPA molar ratios. The DPC/BPA molar ratios are 1.02, [; 1.03, 9; 1.05, 2; and 1.10, b.

pressure. Although the condensate is a mixture of phenol and DPC, which could not be separated in this work, the overall reaction rate clearly depended upon the pressure. These observations indicate that mass transfer is important in the system. Molecular weight is one of the most important characteristics of a polymer. Theoretically, a (DPC/BPA) molar ratio closer to unity yields a polymer with a higher average molecular weight. The evaporation loss of DPC was such that the effect of the (DPC/BPA) molar ratio on the molecular weight was studied. Experiments were conducted with operating conditions similar to those shown in Figure 3. Figure 5 presents the effect of the initial (DPC/BPA) molar ratio on the IV of the PC sample. This figure indicates that, for (DPC/BPA) molar ratios of both 1.02 and 1.1, the IV of the product polymer is less than 0.3 cm3/g. If the (DPC/BPA) molar ratio is in the range of 1.031.05, PCs with higher IV can be obtained. When the ratio is 1.1, much of the DPC is present, even though its evaporation loss has been taken into account. However, if the ratio is 1.02, then the amount of DPC was not sufficient to compensate for its loss due to evaporation during the course of the reaction. Theoretically, the weight-fraction distribution of the PC prepared by the melt transesterification of DPC and BPA with stoichiometric equivalence can be expressed as10

f(x) )

wx ) xp(x-1)(1 - p)2 W

(7)

where x is the chain length of the polymer, p is a positive adjustable parameter characterizing the conversion of the functional group, wx is the weight of the polymer sample with chain length x, and W is the total weight of the polymer sample. A GPC curve is typically a plot of intensity against retention time. Such a curve can be transformed into a plot of intensity against molecular weight, using a calibration curve of polystyrene standards. The weight-fraction distributions of the PCs prepared with various DPC/BPA molar ratios can be expressed as10

f(x) )

wx (I/2.303x)(dt/d log M) ) ∞ W Idt

∫0

(8)

where I is the intensity of the GPC readout and is proportional to the mass concentration of the polymer sample, (dt/d log M) is the inverse slope of the calibration curve, and t is the retention time. Figure 6 plots the weight-fraction distribution curves calculated from eq 7 and those transformed from GPC data according to eq 8. The DPC/BPA molar ratios in Figure 6a-c are 1.05, 1.02, and 1.10 respectively, and the fitted values of p in eq 7 are 0.9775, 0.967, and 0.972, respectively. Parameter p

Figure 6. Weight-fraction distributions of PC samples. Discrete symbols represent the weight-fraction distribution of the PC sample transformed from the GPC readout according to eq 8. The solid curve shows the weightfraction distribution calculated from eq 7. The DPC/BPA molar ratios are (a) 1.05, 2; (b) 1.02, [; and (c) 1.10, 9. The values of p are 0.9775 in panel a, 0.967 in panel B, and 0.972 in panel C.

is a positive adjustable parameter characterizing the conversion of functional group. In this work, it represents the conversion of phenolic end groups (-ArOH). If DPC does not lost, the ultimate theoretical values of p are 0.95, 0.98, and 0.90. Because of dynamic loss in DPC, p does not reach these values. As can be seen at each molar ratio (DPC/BPA), the molecular weight distribution closely agrees with the most probable distribution. Notably, the GPC profile indicates a small shoulder peak at a chain length of around 70-110. It probably results from the branched polymer via a side reaction analogous to the KolbeSchmitt reaction that occurred at high temperature.4,11,12 Even though some catalyst systems have been discussed because of their catalytic activity in synthesizing PC by the melt transesterification of DPC and BPA,13 the effect of the catalyst on the hue of the PC resin seems to be of greater interest. The CIELAB tritimulus color value b* is used herein to represent the color index of PC. Figure 7 shows that the PC resin prepared by melt transesterification had a small b* when the catalyst concentration was low. The resin was almost colorless when b* was smaller than 2. However, b* increased with DMAP concentration. When the DMAP concentration was high, PC easily became colored. The results reveal that the DMAP concentration should not exceed 50 ppm to yield PC with a high transmittance and good color. A catalyst is used to accelerate the rate of reaction, but theoretically it does not affect the molecular weight of the resulting polymer. However, the catalytic concentration affected the IV of the PC samples, as presented in Figure 8, when the

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Acknowledgment The authors would like to thank the National Science Council of the Republic of China, Taiwan, for partially, financially supporting this work. Nomenclature An ) Figure 7. Effect of catalyst concentration on the b* value of PC samples. Discrete symbols show the b* at various catalyst concentrations when the DPC/BPA molar ratio is 1.05. The catalyst concentration is based on the amount of BPA. Dashed curve shows b* obtained by least-squares fitting with the equation b* ) 0.0229 (cat-conc + 35.5), 0 ppm < cat-conc < 200 ppm.

Bn )

Sn )

Figure 8. Effect of catalyst concentration on the IV of PC samples when the DPC/BPA molar ratio is 1.05. The catalyst concentration is based on the amount of BPA.

DPC/BPA molar ratio was 1.05. A high DMAP concentration seems to not favor the preparation of PC with a high molecular weight. Kinetic and transport interactions are inferred to have occurred in the reaction system. DPC was used in excess, and the monomer ratio was dynamically varied because of the evaporative loss of DPC, so when the DMAP concentration was high, DPC was rapidly consumed in the early stage of the reaction. The excess phenyl end groups remained in the resulting polymer. Additionally, the mild operation conditions applied in this work reduced the loss of DPC from the reactor. Not enough time sufficed to evaporate the excess DPC. Therefore, the increase in the molecular weight of PC was limited. Conclusion The effects of the operating conditions on polycondensation and on the properties of the resulting polymer were studied experimentally when DMAP was used as the catalyst. The thermal effect, which may cause side reactions, results from the large temperature difference in the heat-transfer operation. A proper temperature scheme can help to minimize the thermal effect. The reactor pressure is critical in the melt process. A reduced pressure must be applied to remove phenol. The overall reaction rate depends on the pressure. Logically, the masstransfer is important in melt transesterification. The DPC/BPA molar ratio profoundly affects the IV of PC. A DPC/BPA molar ratio between 1.03 and 1.05 can yield polymer with a high IV. The molecular weight distribution of PCs prepared by the melt process was the most probable distribution. However, a small shoulder peak probably corresponds to the branched polymer. Moreover, an almost colorless PC can be produced when the concentration of the catalyst is low, suggesting that the catalyst concentration should not exceed 50 ppm.

f(x) ) weight-fraction distribution of PC I ) intensity of GPC readout M ) molecular weight of standard polystyrene used for calibration p ) positive adjustable parameter characterizing the conversion of functional group t ) retention time of GPC (min) x ) chain length of polymer W ) weight of polymer sample wx ) weight of polymer sample with chain length x Literature Cited (1) Serini, V. Polycarbonates. In Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; Elvers, B., Hawkins, S., Schulz, G., Eds.; VCH: New York, 1992; A21, pp 207-215. (2) King, J. A., Jr. In Handbook of Polycarbonate Science and Technology; LeGrand, D. G., Bendler, J. T., Eds.; Marcel Dekker: New York, 2000; pp 7. (3) Komiya, K.; Fukuoka, S.; Aminaka, M.; Hasegawa, K.; Hachiya, H.; Okamato, H.; Watanabe, T.; Yoneda, H.; Fukawa, I.; Dozono, T. In Green Chemistry. Designing Chemistry for the EnVironment; Anastas, P. T., Williamson, T. C., Eds.; ACS Symposium Series 626; American Chemical Society: Washington, DC, 1996; pp 20-32. (4) Schnell, H. Chemistry and Physics of Polycarbonates; Polymer Reviews 9; Interscience Publishers: New York, 1964; Chapter III. (5) Hsu, J. P.; Wong, J. J. Polymer 2003, 44, 5851-5857. (6) Spivey, A. C.; Fekner, T.; Spey, S. E.; Adams, H. J. Org. Chem. 1999, 64, 9430-9443. (7) Kim, Y.; Choi, K. Y. J. Appl. Polym. Sci. 1993, 49, 747-764. (8) Woo, B. G.; Choi, K. Y.; Song, K. H.; Lee, S. H. J. Appl. Polym. Sci. 2001, 81, 1253-1266. (9) Turska, E.; Wro’bel, A. M. Polymer 1970, 11, 408-414. (10) Rosen, S. L. Fundamental Principles of Polymeric Materials, 2nd ed.; John Wiley & Sons: New York, 1993; Chapter VI. (11) Hagenarrs, A. C.; Pesce, J. J.; Bailly, Ch.; Wolf, B. A. Polymer 2001, 42, 7653-7661. (12) Akola, A.; Jones, R. O. Macromolecules 2003, 36, 1355-1360. (13) Ignatov, V. N.; Tartari, V.; Carraro, C.; Pippa, R.; Nadali, G.; Berti, C.; Fiorini, M. Macromol. Chem. Phys. 2001, 202, 9, 1941-1945.

ReceiVed for reView June 17, 2005 ReVised manuscript receiVed December 6, 2005 Accepted February 28, 2006 IE050726+