Solid-State Polymerization of Poly(trimethylene terephthalate): Reaction Kinetics and Prepolymer Molecular Weight Effects

Abstract

The reaction kinetics and the effects of prepolymer molecular weight on the solid-state polymerization (SSP) of poly(trimethylene terephthalate) (PTT) were investigated using nitrogen as the sweep fluid. The synthetic conditions were carefully chosen to eliminate the influences of both internal and external diffusion of the reaction byproducts (1,3-propanediol and water), so that the reaction kinetics were controlled by the forward chain extension reaction. Higher forward reaction rate constants were consistently obtained for the SSP of the high-molecular-weight prepolymer, compared to that of the low-molecular-weight prepolymer. The activation energy of the polymerization reaction was determined to be 15–26 kcal/mol, depending on the prepolymer molecular weight. The slower reaction rate for polymerization of the low-molecular-weight prepolymer may be attributed to the inhibition of the chain-end mobility due to the higher crystallinity and larger lamellar thickness of the obtained polymers. In addition, the high concentration of carboxylic end groups in the low-molecular-weight prepolymer may also decrease the reaction rate by preferential transesterification between 3-hydroxyl propyl end groups over the esterification reaction between 3-hydroxyl propyl and carbonyl end groups. High-molecular-weight PTT with an intrinsic viscosity of 2.05 dL/g (that corresponds to a number average molecular weight of 57600 g/mol) can be obtained via the SSP of the high-molecular-weight prepolymer at a relatively low temperature of 190 °C under the conditions in which byproduct diffusion resistance is eliminated.