Poly(butylene terephthalate) – A deeper look into the solid-state polycondensation

Oligo(butylene terephthalate)s with degrees of polymerization (DP) ranging from 15 to 20 were synthesized via Ti(OEt) 4 -catalyzed transesterification in bulk, using either an excess of 1,4-butanediol or dimethyl terephthalate. The PBTs were annealed at 190 °C and 210 °C, either in the absence or presence of catalysts, such as tin(II) 2-ethyl hexanoate, Bu 2 SnO, and Zr(acac) 4 . The solid-state reactions were monitored by matrix-assisted laser desorption/ionization time-of-flight (MALDI TOF) mass spectrometry, which allowed differentiation between different end-group combinations. Furthermore, the reaction products were characterized using gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) measurements. Solid-state polycondensation (SSP) primarily proceeded by the reaction of chains with CH 2 OH end groups due to the formation of loops on the crystallite surface. In contrast, a predominance of methyl ester end groups proved unfavorable for the progress of the SSP. Under optimized conditions, SSP increased the molecular weight by a factor of eight and involved formation of cycles with a strong tendency toward formation of monodisperse extended-ring crystallites.