Effects of the carboxyl concentration on the solid-state polymerization of poly(ethylene terephthalate)

There are two types of polycondensation reactions in the solid-state polymerization (SSP) of poly(ethylene terephthalate) (PET), namely, transesterification and esterification. Transesterification is the reaction between two hydroxyl ends with ethylene glycol as the byproduct, and esterification is the reaction between a carboxyl end and a hydroxyl end with water as the byproduct. The SSP of powdered PET in a fluid bed is practically a reaction-controlled process because of negligible or very small diffusion resistance. It can be proved mathematically that an optimal carboxyl concentration for reaction-controlled SSP exists only if k(2)/k(1) > 2, where k(2) and k(1) are the forward reaction rate constants of esterification and transesterification, respectively. Several interesting observations were made in fluid-bed SSP experiments of powdered PET: (1) the SSP rate increases monotonously with decreasing carboxyl concentration, (2) k(2)<k(1) in the presence of sufficient catalyst, (3) k(1) decreases with increasing carboxyl concentration if the catalyst concentration is insufficient, and (4) the minimum catalyst concentration required to achieve the highest SSP rate decreases with decreasing carboxyl concentration. In the SSP of pelletized PET, both reaction and diffusion are important, and there exists an optimal carboxyl concentration for the fastest SSP rate because esterification, which generates the faster diffusing byproduct, is retarded less than transesterification in the presence of substantial diffusion resistance. The optimal prepolymer carboxyl concentration, which ranges from 25 to 40% of the total end-group concentration in most commercial SSP processes, increases with increasing pellet size and product molecular weight. (C) 2002 John Wiley & Sons, Inc. J Appl Polym Sci 83: 1288-1304,2002.