How to synthesize High-Tm Poly(butylene terephthalate)

Low-molecular mass poly(butylene terephthalate) - (PBT) was prepared in three ways, either by melt polycondensation of dimethyl terephthalate (DMT) with an excess of 1,4.butane diol or by degradation of high-molecular mass PBT prepared from DMT or from terephthalic acid (TA) with 1,4.butane diol. All three types of PBT were subjected to different annealing procedures (190 °C + 210 °C or 210 °C + 220 °C) with a variation of time and vacuum. The progress of the solid state polycondensation (SSP) was monitored by matrix-assisted laser desorption/ionization – time of flight (MALDI-TOF) mass spectrometry, differential scanning calorimetry (DSC) and size exclusion chromatography (SEC) measurements. Our results show that large fraction of CH 2 OH end groups, a pulverization of the PBT samples and the application of a vacuum accelerate SSP. Under optimum conditions, number-average molecular masses (M n ) increased by a factor of 7, and the melting temperature (T m ) increased to 239-240 °C. Furthermore, large fractions of even-numbered cycles were formed in the mass range below 4000 Da. Transformation of commercial PBTs from the normal low-T m morphology into the thermodynamically more stable high-T m morphology was not feasible. All results can be explained by efficient formation of small loops via transesterification between CH 2 OH end groups and ester groups across the surface of the crystallites. SAXS measurements further indicate that an increase in crystal thickness contributes to the development of the high-T m morphology.