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The chemical aspects of poly(ethylene terephthalate) synthesis via the antimony‐catalyzed polycondensation of hydroxy ethylene terephthalate end groups were studied to elucidate its mechanism. A polycondensation mechanism was proposed in which activation occurs by the formation of a chelate ligand on antimony composed of the hydroxyl end group and alcoholic oxygen of the ester of the same end group. The rate‐determining step of the polycondensation reaction was concluded to be the coordination of a second chain end to antimony. The low activity of antimony at a high concentration of hydroxyl end groups was proposed to result from the competition between hydroxyl end groups and the chelate structure leading to the transition state. The high selectivity of antimony is probably due to its relatively low Lewis acidity. Moreover, antimony was found to stabilize hydroxyl end groups against degradation by preventing their complexation to carbonyl functionalities. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1049‐1059, 2006

The esterification kinetics of terephthalic acid (TPA) and ethylene glycol (EG) in poly(ethylene terephthalate) (PET) synthesis were studied using a semibatch reactor. Rate constants were optimized by data fitting with the oligomeric chain length, the fraction of carboxyl groups in the terminal groups (α) and the water generation curve for different EG/TPA feed ratios. The influence of the TPA particle size distribution on the solid‐liquid mass transfer rate and on acid conversion (ε) was investigated. It was observed that conversion became more sensitive towards TPA particle size as the EG/TPA feed ratio was lowered. It is advantageous to use the model based on TPA particle size for mass transfer limited esterification reactors. The effect of the monomer feed ratio on conversion, chain length and system heterogeneity can be predicted with this model.magnified image

Summary: Hydrotalcite‐catalyzed polycondensation of BHET was studied by thermogravimetry to elucidate the kinetics. The reaction was found to follow second‐order kinetics with respect to hydroxyl end groups. The overall activation energy of the polycondensation was found to decrease with increasing catalyst concentration before it leveled out to the value of 93 kJ · mol−1 at high catalyst concentration. This is a result of the uncatalyzed reaction that takes place parallel to the catalyzed one. The activation energy of the uncatalyzed path was found to be 156 kJ · mol−1. IR spectroscopy and XRD showed that the monomer intercalates between the layers of hydrotalcite at the beginning of the reaction enabling the complexation of oxygen‐containing functional groups with the hydroxide groups of the catalyst. Based on these findings a polycondensation mechanism is proposed. One end group of the monomer is activated in the form of alkoxide that counterbalances the positive charge of the hydrotalcite layer. This alkoxide group attacks an ester carbonyl group fixed close to it, generating a new ester bond and a glycoxide species. The role of hydrotalcite is to activate the reactants, rendering the attacking hydroxyl group more nucleophilic and the ester carbonyl group more electrophilic, and at the same time fixing the reactant together in a favorable geometry.Intercalation of BHET in the gallery of HT layers in a twisted alkoxide form.magnified imageIntercalation of BHET in the gallery of HT layers in a twisted alkoxide form.

Summary: The hydrotalcite‐catalyzed synthesis of PET was studied to clarify the effect of hydrotalcite properties on its catalytic activity. Hydrotalcite was modified by various treatments to tune its activity as a polycondensation catalyst. Hydrotalcite activity was found to decrease upon calcination. However, rehydration of the calcinated hydrotalcite resumed the catalytic activity. The hydrotalcite activity depends on the ratio of magnesium to aluminum cations in its composition, and highest activity occurs at a molar ratio of 2:1. Replacement of the carbonate anions of hydrotalcite by more nucleophilic ones like hydroxide and alkoxide groups resulted in improved catalytic activity. Hydrotalcite has two assembly orders: primary lamination of sheets into plates and secondary agglomeration of plates into particles. Hydrotalcite with larger sheet size showed lower activity. On the other hand, milling of hydrotalcite particles did not affect its activity as it probably enters the reaction on a plate level, which is not affected by milling. Polycondensation resulted in expansion of the hydrotalcite sheets under the effect of formed polymer. Reuse of hydrotalcite after polycondensation followed by depolycondensation resulted in a large activity enhancement.High‐resolution SEM micrograph of calcinated HT after rehydration.magnified imageHigh‐resolution SEM micrograph of calcinated HT after rehydration.