Investigation of the formation of poly(ethylene terephthalate) with model molecules. IV. Catalysis of the esterification of ethylene glycol with benzoic acid and of the condensation of ethylene glycol monobenzoate

Abstract

The esterfication of ethylene glycol (EG) with benzoic acid (BA) is an acid‐catalyzed reaction which leads, as a first intermediate, to ethylene glycol monobenzoate (EGMB). This compound has an OH group which is more active towards carboxylic groups than the first OH group of EG. Moreover, it is a key intermediate in the formation of sideproducts containing diethylene glycol (DEG) moieties. The condensation reaction of EGMB was particularly studied in the presence of carboxylic acids or metal derivatives as catalysts. Titanium is the most active. Sb, which is widely used in the industrial synthesis of PET, needs to be present at a high level of concentration to show an activity. The presence of carboxylic groups in the reaction medium does not affect its activity, but it is necessary to remove the ethylene glycol as soon as it is formed in order to keep it active during the condensation reaction.

Polyesters

“…The formation of strong complexes between EG and antimony has frequently been proposed to explain the low catalytic activity of this metal in alcohol-rich medium. 284,285 The presence of carboxyl groups has also been reported to inhibit the catalytic activity of various acetates and alkoxides. A reasonable mechanism should involve the coordination of the ester carbonyl to the metal atom, favoring the nucleophilic attack of alkoxy ligands or hydroxyl endgroups on the corresponding carbon.…”

Poly(Ethylene Terephthalate) Polymerization – Mechanism, Catalysis, Kinetics, Mass Transfer and Reactor Design

“…Calculations of the equilibrium constants for the system water, EG, DEG, and dioxane showed that dioxane is the most stable species in this system. More recent publications [24,38,57,59], however, favour the formation of DEG by direct etherification, which is condensation of two hydroxyl groups as 2R-OH → R-O-R + H 2 O. In the above temperature range, the equilibrium constant for the formation of DEG from EG is significantly lower, as shown in Figure 2.11.…”

“…It is generally accepted that compounds of antimony, tin and titanium are the most active catalysts for polycondensation. Otton and co-workers [18,[36][37][38] performed extensive studies on the catalyzed formation of PET with model systems. Hovenkamp [35] found that the latter catalysts are very active in media having both high and low hydroxyl group contents, but are easily poisoned by traces of carboxylic groups.…”

“…The equilibrium constant for the formation of dioxane from DEG is 72 500 at 310 • C and 143 100 at 180 • C. The reaction is slightly exothermic with a reaction enthalpy of approximately −7.1 kJ/mol. The work of Otton and Ratton [38] and Chen and Chen [59,[62][63][64][65] demonstrated that reactions involving different combinations of hydroxyl groups (free or terminal EG) have differing reaction rates and that the reaction between terminal EG and free EG is exceptionally fast. With values between 13 and 16, the equilibrium constant is still high enough to regard the formation of DEG from EG to be irreversible in an open industrial system.…”

Solubility and esterification kinetics of terephthalic acid in ethylene glycol III. The effects of functional groups

“…P E T prepolymer (DP of [1][2][3][4][5] is obtained from the esterification step and the prepolymer is processed for polycondensation at more severe conditions to obtain higher molecular weight, such as low pressure in the presence of a catalyst.…”