Novel Strategy for Polycondensation of Poly(ethylene terephthalate

Article pubs.acs.org/IECR

Novel Strategy for Polycondensation of Poly(ethylene terephthalate) Assisted by Supercritical Carbon Dioxide Tian Xia, Ye Feng, Yunlong Zhang, Zhenhao Xi, Tao Liu, and Ling Zhao* State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ABSTRACT: The polycondensation of poly(ethylene terephthalate) (PET) in both melt and solid states was conducted with high pressure CO2 and ambient pressure N2 sweeping to remove the volatile byproducts, revealing that only the solid state polycondensation (SSP) of PET could be facilitated by high pressure CO2 due to the substantial increase of free volume. With the comparison of SSP processes of PET in dynamic and static CO2 supplying modes, a periodical supercritical CO2 renewing strategy for SSP of PET was proposed. The SSP process was promoted significantly, and only 6 h were required from the degree of polymerization of 100 to 150, while it takes nearly 20 hours in the industrial SSP process of PET. The effects of CO2 renewing period, CO2 pressure, reaction temperature, and initial molecular weight on the newly proposed SSP strategy were investigated, and a semiempirical kinetic model was applied to fit the experimental data well. CO2 supplying mode.16 In a dynamic mode, scCO2 flows continuously over polymer matrix, while an amount of scCO2 is introduced at the beginning in a static one. The dynamic mode may be more effective than the static one since that fresh scCO2 is always in contact with the polymer. Depressurization is the final step, where pressure is reduced and scCO2 diffuses out of the polymer. Bruke et al. has reported the melt polycondensation of PET assisted by scCO2.15 However, the molecular weight of PET obtained with scCO2 sweeping was lower than that produced by the vacuum devolatilization. The solubility of EG (the main byproduct of PET polycondensation) in scCO2 was lower than that of phenol (the byproduct of PC polycondensation), which was considered to partially account for the lower effectiveness of scCO2 in PET polycondensation compared with PC polycondensation by Kendall et al.14 The reported process did not take full advantage of the promoting effect of scCO2 on PET polycondensation. In this work, the effects of scCO2 on the polycondensation of PET in both melt and solid states have been investigated, and then the SSP of PET carried out in different scCO2 supplying modes were compared, based on which a periodical gasrenewing strategy for SSP of PET was proposed and studied systematically.

1. INTRODUCTION Poly(ethylene terephthalate) (PET) is a kind of thermoplastic with high elastic moduli, high glass-transition temperature, and good solvent resistance, widely used in bottles, food-trays, fibers, and tire-cords.1 PET is synthesized from either direct esterification between terephthalate acid (TPA) and ethylene glycol (EG) or transesterification between dimethy terephthalate and EG. Conventionally, PET prepolymer with a degree of polymerization (DP) of 20−30 is further polymerized in melt to gain fiber grade products with a DP of approximately 100, and then solid-state polycondensation (SSP) is employed to produce molding grade PET having a DP about 150.2,3 The prepolymer with a DP of 20−30 can also be used as the starting material for SSP in the NG3 SSP technology developed by the Du Pont company.4−6 Since the polycondensation reactions of PET are reversible, the removal of volatile byproducts is essential to break the equilibrium and drive the chain growth reaction forward to obtain high molecular weight. Traditional devolatilization methods include vacuum and gas sweeping operations, but there always exist mass-transfer limitations of volatiles in the polymer matrix. Supercritical carbon dioxide (scCO2), a kind of environmentally friendly alternative to traditional solvents, has been successfully applied to facilitate the devolatilization of polymer7,8 and the SSP of polycarbonate (PC).9−12 The free volume of polymer swollen by scCO2 increases, which promotes the diffusion of volatiles in polymer matrix and the mobility of polymer chains as well as their end groups, resulting in higher polycondensation rate.13,14 Furthermore, many volatile byproducts of polycondensation display significant solubility in liquid or supercritical CO2,13,15 which may serve as more effective sweeping fluid to bring the volatiles away and drive the polycondensation forward. In a typical polycondensation process assisted by scCO2, the polymer is first exposed to high pressure CO2, followed by supercritical fluid extraction (SFE) coupling with CO 2 dissolving, polymer swelling, and polycondensation reaction.9−12 SFE can be implemented in either static or dynamic © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. The PET prepolymer (PET 1, DP = 23.3) used in the melt polycondensation was synthesized from TPA and EG with 450 ppm poly(antimony ethylene glycoxide) (Sb2(EG)3) as catalyst, which was 450 ppm based on TPA. And PET 1 was ground into powder. The PET prepolymers with DPs of 30.4 (PET 2) and 94.7 (PET 3) were kindly supplied by Received: August 24, 2014 Revised: October 27, 2014 Accepted: October 30, 2014

A

dx.doi.org/10.1021/ie5033606 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Figure 1. Schematic of the PET polycondensation apparatus.

with 4 cm2 square bottom was used to load prepolymers. In melt polycondensation, 0.4 g PET 1 was loaded, and in SSP, 1.28 g PET 2 or PET 3 was loaded. 2.3. Characterization. Intrinsic viscosity (IV) of PET sample was measured with Ubbelohde viscometer using the mixture of phenol and tetrachloroethane (3/2, w/w) as solvent at 25 °C. Then the number-average molecular weight (Mn) and DP were calculated as follows:17

Sinopec Shanghai Petrochemical Co, Ltd., and used in the SSP, of which PET 3 was the starting material of industrial SSP process. The particle size of PET 2 and PET 3 was 40 mesh (0.45 mm). All prepolymers were dried at 110 °C in vacuum overnight to avoid the hydrolytic degradation. CO2 (purity: 99.9%, w/w) and N2 (purity: 99.99%, w/w) were commercially available from Shanghai Chenggong Gases Co, China. Phenol and tetrachloroethane were of analytical purity and purchased from Shanghai Lingfeng Chemical Reagent Co, Ltd. All gases and reagents were used without any further purification. 2.2. Polycondensation Process. A high-pressure autoclave with an internal volume of 115 mL was used in polycondensation process as demonstrated in Figure 1. The pressure of the autoclave was detected by a pressure transducer with an accuracy of ±0.1 MPa and maintained by the pressurization system. In a dynamic CO2 supplying process, the pressure was reduced to 0.35 MPa by a pressure reducing valve with a controllable flow rate, measured by the mass flowmeter. Two thermocouples were attached to the temperature control system, which could keep the temperature of autoclave with an accuracy of ±0.5 °C. Sweeping gas would be blown through a heating coil immersed in the oil bath before entering the autoclave to assist in keeping the system temperature at a stable level. In a PET polycondensation process, the polycondensation system loading with prepolymer was swept by low pressure sweeping gas several times. Subsequently, the autoclave was heated to the reaction temperature in about 5 min and then pressurized to the desired pressure. Simultaneously, the pressure-reducing valve was turned on and regulated to a certain flow rate in a dynamic gas supplying process. In a static process, the inlet and outlet valves of the autoclave were closed after pressurization. When the polycondensation process finished, the system was depressurized to ambient pressure, and immediately the autoclave was separated from the system and then cooled down quickly. Finally, sample was taken out for the subsequent analysis. A two-layer stainless sample cell

IV = 7.55 × 10−4Mn 0.685

(1)

DP = (M n−62.07)/192.17

(2)

The densities of the swollen PET melts under different gas atmospheres were determined by a high-temperature and -pressure view cell system, as described by Chen et al.18 For each measurement, after placing the cylindrical glass container filled with 1.00 g PET 1 in the view cell, the system was sealed and carefully swept by low pressure gas three times. Thereafter, the view cell was heated to 280 °C, charged to the desired pressure, and kept for 30 min. And then the image of PET sample was captured by the CCD camera and the volume of the swollen PET melt was determined by the software of ImagePro Plus (Media Cybernetics, Silver Spring, Maryland, U.S.A.), with which the density of the PET melt was calculated.

3. RESULTS AND DISCUSSION 3.1. PET Polycondensation in Both Melt and Solid States with Different Sweeping Gases. The polycondensation process of PET can be divided into four steps: the mobility of reactive end groups, the polycondensation reaction between end groups, the diffusion of volatile byproducts in polymer matrix (interior diffusion), and the diffusion of the byproducts from polymer surface to gas phase (external diffusion),19 of which the external diffusion is controlled by the flow rate of sweeping gas. As shown in Figure 2, DP of melt polycondensation samples showed an uptrend with the flow rate at first when both CO2 and N2 swept. Atmospheric N2 was B



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plasticization effect of high-pressure CO2. According to Kulkarni,21 the fractional free volume vfree (T, P, ν)M of polymer/CO2 system depends on the temperature T, the pressure P applied to polymer/CO2, and the CO2 concentration ν as in the following equation: vfree(T , P , ν)M = vfree(Tg , Ps , ν = 0) + α(T − Tg) − β(P − Ps) + γν (3)

where vfree (Tg, Ps, ν = 0) is the fractional free volume of pure polymer at Tg and standard pressure Ps = 0.1 MPa, and the thermal expansion coefficient α, the compressibility β as well as the concentration coefficient γ are characteristic constants. And the fractional free volume vfree (T, P, ν)P of pure polymer (ν = 0) at temperature T and Ps = 0.1 MPa could be written as follows: vfree(T , P , ν)P = vfree(Tg , Ps , ν = 0) + α(T − Tg)

(4)

So the increasing ratio of free volume due to CO2 dissolution is as follows: vfree(T , P , ν)M − vfree(T , P , ν)p vfree(T , P , ν)p =

γν − β(P − PS) vfree(Tg , PS , ν = 0) + α(T − Tg)

(5)

With the decrease of T, the denominator of eq 5 decreases, which means that the free volume of pure polymer is low, while the numerator, representing the increase of free volume caused by CO2 dissolution, gets higher due to the elevating CO2 solubility.22 As a result, the increasing ratio of free volume becomes higher, which could facilitate the end groups mobility and interior diffusion. As presented in Figure 4, the SSP process of PET conducted at lower reaction temperature was promoted by scCO2, which implied that the facilitating effect of scCO2 on SSP of PET prevailed over the effect of the lower ability of scCO2 in removing byproducts. However, when the melt polycondensation of PET was carried out at higher reaction temperature, the increasing ratio of free volume would be slight. But the distance of the byproducts interior diffusion increased prominently since the density of the swollen PET melt decreased with CO2 pressure (shown in Table 1.), which did not change detectably in SSP process. In consequence, as illustrated in Figures 2 and 5, the DP of the melt polycondensation product obtained with atmospheric N2 as sweeping gas was higher than that gained with high pressure CO2 sweeping. But when the flow rates of sweeping gases were zero, the DP of melt polycondensation sample obtained under high pressure CO2 was higher than that obtained under atmospheric N2, as shown in Figure 2, which was ascribed to the relatively lower byproduct concentrations in CO2 due to the higher pressure. By the way, it was noteworthy that the effect of CO2 pressure on melt polycondensation was nonmonotonic, as shown in Figure 5. The density of the swollen PET melt reduced dramatically when CO2 pressure increased from 6 to 12 MPa, resulting in a lower polycondensation rate and DP. Then no obvious change of the density was observed as the CO2 pressure exceeded 12 MPa, but the free volume of PET still increased with pressure, leading to higher polycondensation rate and DP.

Figure 2. DP of PET obtained by melt polycondensation at 280 °C with different flow rates (a, reaction time = 40 min; b, reaction time = 20 min).

used as sweeping gas for comparison. It was known that the resistance of external diffusion was resulted from the concentration gradients of byproducts in the gas phase, which diminished gradually with the flow rate of sweeping gas.20 The concentration gradients vanished as the flow rate was high enough and the byproducts concentrations in sweeping gas maintained to be equilibrium values. As a result, DP reached a plateau with the continuing increase of flow rate. The flow rate, above which the promoting effect of increasing flow rate on DP tended to disappear, was defined as the turning point of sweeping gas. The turning point should shift to lower flow rate region when the surface concentration of the byproduct was lower and/or the diffusivity of the byproduct in sweeping gas was higher as indicated in Figure 3.20 More byproducts were generated when N2 swept, as inferred from the higher DP, and therefore, the surface concentrations were higher. And the turning point of N2 was lower than that of near- and supercritical CO2, as shown in Figure 2. Hence, the diffusivities of byproducts in ambient pressure N2 were higher than those in high pressure CO2, which indicated that compared with ambient pressure N2, high pressure CO2 presented lower ability in removing the volatile byproducts of PET polycondensation. In addition, the end group’s mobility and interior diffusion steps of PET polycondensation are influenced by the C



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Figure 3. Relationship between turning points, surface concentrations, and diffusivities of byproducts.

Figure 5. DP of melt polycondensation samples obtained with different sweeping gases (reaction temperature = 280 °C, flow rate = 6 standard liter per minute).

Figure 4. DP of SSP samples obtained with different sweeping gases (PET 2, reaction temperature = 220 °C, flow rate = 6 standard liter per minute).

Table 1. Density of Swollen PET Melt under Different Gas Atmospheres at 280 °C gas atmosphere

0.1 MPa N2

6 MPa CO2

12 MPa CO2

16 MPa CO2

ρ (g/cm3)

1.05

1.00

0.91

0.90

3.2. Solid State Polycondensation of PET with Different scCO2 Supplying Modes. When the flow rate of scCO2 was reduced to zero in the static SSP process of PET, the DP was lower than that obtained using the dynamic CO2 supplying mode, as demonstrated in Figure 6. Although the profiles of free volume were considered to be the same in static and dynamic processes, the concentrations of volatile byproducts in CO2 maintained to be lower equilibrium values in a dynamic process, while they increased continuously with reaction time in a static process as demonstrated in Figure 7, which weakened the mass-transfer impetus for the byproducts

Figure 6. DP of PET obtained with different SSP methods (PET 2, reaction temperature = 220 °C).

D



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Figure 7. Schematic of the changes in SSP with different CO2 supplying strategies (a, static CO2 supplying strategy; b, dynamic CO2 supplying strategy; and c, CO2 renewing strategy). E



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external diffusion, just as expected. But at the end of the first hour, the DP obtained using dynamic and static processes were almost the same as those shown in Figure 6, indicating that the byproduct concentrations (CS) of the static SSP process did not impose higher resistance to the external diffusion as compared with those of the dynamic SSP process. Therefore, the continuous CO2 sweeping would not be necessary if the byproducts concentrations in CO2 were kept below CS during the SSP process. As a result, a periodical scCO2 renewing strategy for SSP of PET was designed. In this strategy, as shown in Figure 7(c), after CO2 pressurization, the SSP process was operated in the static mode, followed by the depressurization at the end of the first gas-renewing period. And then the above process was repeated, including pressurization, SSP, and depressurization, until the desired DP was obtained. DP obtained using the gas-renewing strategy was much higher than that related to the scCO2 sweeping process (Figure 6). After depressurization, the relaxation of the polymer matrix to its unswollen state was very slow compared to the removal of CO2 from PET matrix.8 Hence, the free volume of the PET matrix was constant after it reached the maximum, as illustrated in Figure 7(c), which was the same as the free volume profile of the dynamic SSP process. Nevertheless, the byproduct concentrations in CO2 reduced to zero after every depressurization in the newly proposed SSP strategy, which facilitated the byproducts external diffusion significantly. As a consequence, the gas-renewing strategy was much more effective in promoting SSP process with lower energy and gas costs. It was noteworthy that the depressurization process was desired to be operated at a fast rate to avoid the relaxation of polymer to its unswollen state. The time of depressurization process was about 2 min in this work. And the PET particles would be blown off the sample cell and into the gas tubes, if the depressurization was faster. 3.3. Solid State Polycondensation of PET with the Periodical scCO2 Renewing Strategy. The conventional SSP process operated with gas sweeping was affected by many parameters, such as reaction temperature, gas flow rate, and initial molecular weight,19 all of which were still the operating variables of the newly proposed SSP strategy except the gas flow rate. In addition, CO2 pressure and gas renewing period were specific factors to the new SSP strategy. More times of depressurization were conducted and less byproducts were kept in CO2 as the gas-renewing period was shortened, both of which promoted the external diffusion of SSP. As presented in Figure 8, DP increased as the gasrenewing period was reduced. However, when the period decreased from 1 to 0.5 h, the decrease of byproduct concentrations did not facilitate the external diffusion any further and no obvious change of DP could be discovered. The DP of the new SSP strategy samples increased continuously with reaction time at different CO2 pressures. The promoting effect of higher CO2 pressure on SSP, resulting from the increasing free volume,11 was obvious only at lower CO2 pressure as shown in Figure 9, when the limitations of end group mobility and byproduct interior diffusion were very severe, caused by the lower free volume. As the CO2 pressure was higher than 8 MPa, the DP profiles obtained under different CO2 pressures almost overlapped, which was demonstrated in the form of end group concentration in Figure 12 of next section. Higher reaction temperature facilitated most steps of SSP, including the mobility of reactive end groups, the poly-

Figure 8. Effect of the gas-renewing period on DP of new strategy samples (PET 2, reaction temperature = 220 °C, CO2 pressure = 12 MPa).

Figure 9. Effect of CO2 pressure on DP of new strategy samples (PET 2, reaction temperature = 230 °C, period = 1 h, reaction time = 10 h).

condensation reaction, and the byproduct interior diffusion.19 The inactive end groups, kept from polycondensation reaction by crystalline structure, reduced with reaction temperature.23 As demonstrated in Figure 10, DP increased greatly with the reaction temperature. The sticking problem occurred between particles when the reaction temperature exceeded 230 °C. PET 3, the starting material of industrial SSP processes, was also used as the raw material of the newly proposed SSP strategy, and only 6 h were required to reach the target DP

Figure 10. Effect of reaction temperature on DP of new strategy samples (PET 2, CO2 pressure = 12 MPa, period = 1 h). F



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(∼150), compared with about 20 hours needed in the industrial process with ambient pressure N2 as sweeping gas,24 which was a significant improvement. As shown in Figure 11, the DP

Figure 12. Curve fittings of C data (period = 1 h).

the temperature dependence of the forward reaction rate constant could be represented by the Arrhenius equation:

Figure 11. Effect of the initial molecular weight on DP of new strategy samples (reaction temperature = 230 °C, CO2 pressure = 8 MPa, period = 1 h).

⎛ −E ⎞ ka = A exp⎜ a ⎟ ⎝ RT ⎠

where A was the frequency factor, Ea was the apparent activation energy, and R was the universal gas constant. The pressure of CO2 was not treated as a parameter of the kinetic model as it exceeded 8 MPa. The effects of factors, such as CO2 pressure, gas-renewing period, and backward reactions, were lumped into three parameters, A, Ea, and Cai. As shown in Figure 12, Duh’s kinetic model fit all the experimental data of the newly proposed SSP strategy quite well, and the kinetic parameters are listed in Table 2. With the increasing reaction temperature, the mobility of the polymer chain and its end groups was enhanced, and some end groups trapped in the crystalline phase formerly were activated and able to take part in the polycondensation reaction. Therefore, the concentration of the inactive end group decreased with reaction temperature.23

profiles obtained with different initial molecular weights were nearly parallel, implying that the SSP processes using prepolymers with different initial molecular weights had the similar DP rising rate. 3.4. Semiempirical Kinetic Model for the Periodical Gas-Renewing SSP Strategy. Duh developed a semiempirical second-order kinetic model for fluid-bed SSP of finely divided PET prepolymers,23,25 which could fit the experimental data of SSP in most cases and was applied to fit the SSP data of the newly proposed strategy. The proposed rate equation was given as follows: −

dC = 2ka(C − Cai)2 dt

(6)

where t was the reaction time, ka was the forward reaction rate constant, Cai was the inactive end group concentration, and C was the total end group concentration and calculated by the relation: C=

2 × 106 Mn

4. CONCLUSIONS The polycondensation of PET in both melt and solid states have been performed using scCO2 as sweeping gas. Compared with ambient pressure N2, high pressure CO2 presented lower ability in removing the volatile byproducts of PET polycondensation. The increasing ratio of free volume casused by the dissolved CO2 reduced with the temperature, while the increase of the byproducts interior diffusion distance was prominent at higher temperature but undetectable at lower temperature. Therefore, only SSP of PET conducted at lower reaction temperature was facilitated with the high pressure CO2 as sweeping gas. The static CO2 supplying process was less effective in facilitating the SSP process of PET than the CO2 continuous sweeping process due to the higher concentrations of volatile byproducts in CO2, which weakened the byproducts external diffusion impetus. On the basis of the comparison between the static and dynamic CO2 supplying processes, a periodical CO2 renewing strategy was proposed, including a repeating cycle of pressurization, SSP operated in static CO2 supplying mode, and depressurization. The free volume profile of the newly proposed strategy was the same as that of the dynamic CO2 supplying process while the byproducts concentrations in CO 2 reduced to zeros after every depressurization, which promoted the SSP process significantly. Only 6 h were needed to reach a DP of 150 from 100, and

(7)

In this model, it was not necessary to distinguish two kinds of reactive end groups, hydroxyl and carboxyl end groups. The inactive end groups included chemical dead end groups and reactive end groups restricted by the crystalline structure, and the presence of Cai revealed that the SSP rate decreased with the reaction time, as shown in Figure 12. Intergrating eq 6 and using the initial condition C = C0 at t = 0, yielded: 1 1 − = 2kat C − Cai C0 − Cai (8) Co was 338.8 μmol/g for PET 2 and 109.6 μmol/g for PET 3. And rearranging the above equation to give the following: C0 − C = 2ka(C0 − Cai)C − 2ka(C0 − Cai)Cai t

(10)

(9)

As the SSP data were fitted by the proposed rate equation, the (C0 − C)/t vs C plot was a straight line. And Cai and ka could be estimated from the slope and intercept. In addition, G



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Table 2. Estimated Kinetic Parameters ka( × 104(μmol/g)−1h−1)

Cai(μmol/g)

220 °C

225 °C

230 °C

A (μmol/g)−1h−1)

Ea(kJ/mol)

220 °C

225 °C

230 °C

6.48

7.13

7.82

8.25

38.75

58.12

46.09

26.11

(13) Jiang, C.-y.; Sun, Z.-j.; Pan, Q.-m.; Pi, J.-b. Solubility of Ethylene Glycol in Supercritical Carbon Dioxide at Pressures up to 19.0 MPa. J. Chem. Eng. Data 2012, 57, 1794−1802. (14) Kendall, J. L.; Canelas, D. A.; Young, J. L.; DeSimone, J. M. Polymerizations in Supercritical Carbon Dioxide. Chem. Rev. 1999, 99, 543−564. (15) C.Bruke, A. L.; Givens, R. D.; Jikei, M.; DeSimone, J. M. Use of CO2 in Step-Growth Polymerizations: From Plasticized Polymer Melts to Solid State Polymerizations. Polym. Prep. 1997, 38, 387−388. (16) Hedrick, J. L.; Mulcahey, L. J.; Taylor, L. T. Supercritical fluid extraction. Mikrochim. Acta 1992, 108, 115−132. (17) Xi, Z.; Zhao, L.; Liu, Z. New Falling Film Reactor for Melt Polycondensation Process. Macromol. Symp. 2007, 259, 10−16. (18) Chen, J.; Liu, T.; Zhao, L.; Yuan, W.-k. Determination of CO2 solubility in isotactic polypropylene melts with different polydispersities using magnetic suspension balance combined with swelling correction. Thermochim. Acta 2012, 530, 79−86. (19) Vouyiouka, S. N.; Karakatsani, E. K.; Papaspyrides, C. D. Solid state polymerization. Prog. Polym. Sci. 2005, 30, 10−37. (20) Huang, B.; Walsh, J. J. Solid-phase polymerization mechanism of poly(ethylene terephthalate) affected by gas flow velocity and particle size. Polymer 1998, 39, 6991−6999. (21) Kulkarni, S. S.; Stern, S. A. The diffusion of CO2, CH4, C2H4, and C3H8 in polyethylene at elevated pressures. J. Polym. Sci., Polym. Phys. 1983, 21, 441−465. (22) Zhong, H.; Sun, S.; Xi, Z.; Liu, T.; Zhao, L. Solubility of CO2 in molten poly(ethylene terephthalate). J. Chem. Ind. Eng. (in Chinese) 2013, 64, 1513−1519. (23) Duh, B. Semiempirical rate equation for solid state polymerization of poly(ethylene terephthalate). J. Appl. Polym. Sci. 2002, 84, 857−870. (24) Scheirs, J.; Long, T. E. Modern Polyesters: Chemistry and Technology of Polyesters and Copolyesters; John Wiley & Sons: London, U.K., 2005. (25) Duh, B. Reaction kinetics for solid-state polymerization of poly(ethylene terephthalate). J. Appl. Polym. Sci. 2001, 81, 1748−1761.

comparatively, it takes nearly 20 hours in the present industrial process using atmospheric N2 as sweeping gas. The rate of the newly proposed SSP increased with higher reaction temperature, higher CO2 pressure, and shorter gas-renewing period, but was not affected by the initial molecular weight. Duh’s semiempirical kinetic equation was adopted to model the data of the newly proposed SSP strategy well.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 21 64253175. Fax: +86 21 64253528. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (Grant No. 21176070), National Programs for High Technology Research and Development of China (863 Project, 2012AA040211), the joint research project for Yangtze River Delta (12195810900), Doctoral Program of Higher Education of China (20120074120019), Fundamental Research Funds for the Central Universities, and 111 Project (B08021).

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REFERENCES

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Novel Strategy for Polycondensation of Poly(ethylene terephthalate

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