Thermal stability of poly(trimethylene terephthalate)

The first-order thermal degradation rates of poly(trimethylene terephthalate) [PTT] at 240–280°C under non-oxidative conditions have been determined from the increase in allyl endgroups (1H NMR) which closely match the rates determined from the decrease in molecular weight (intrinsic viscosity). Consequently, the predominant thermal degradation mechanism of PTT is consistent with concerted, electrocyclic oxo retro-ene chain cleavage under conditions pertinent to viable polymerization processes and efficient downstream extrusion and spinning into fiber. Although catalysts, additives and other reaction variables can influence the thermo-oxidative stability of polyesters including PTT, these factors have been found to have little or no effect on PTT thermal degradation rates under non-oxidative environments. The thermal stability of poly(butylene terephthalate) [PBT] has also been determined from butenyl endgroups (NMR) and molecular weight (IV). The activation energies (Ea) for both PTT and PBT thermal chain cleavage are similar to the reported Eas for poly(ethylene terephthalate) [PET] degradation, which is further supported by semi-empirical molecular orbital calculations on model compounds. However, both PTT and PBT undergo molecular weight decrease faster than PET. The apparent slower chain cleavage of PET is attributed to the contribution of productive chain propagation reactions due to unstable vinyl endgroups which alters the equilibrium stoichiometry compared to the relatively stable endgroups of PTT and PBT.