Star-shaped poly(l-lactide)s based on pentaerythritol: About crystallization and polycondensation plus disproportionation and ring formation in the solid state

Star-shaped poly(L-lactide)s (s-PLAs), were prepared by ring-opening polymerization of L-lactide (LA) initiated with pentaerythritol and catalyzed by tin(II) 2-ethylhyexanoate (SnOct 2 ). Two small series of ROPs were carried out in bulk at either 120 °C or 160 °C with variation of the LA/initiator ratio. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra revealed traces of cycles in 120 °C samples only at low initiator/catalyst ratios. In contrast, higher fractions of cycles were detected in samples prepared at 160 °C, suggesting a "back-biting" origin. The influence of arm length and thermal history on crystallinity was investigated by differential scanning calorimetry (DSC) and wide-angle x-ray scattering (WAXS). It was shown that annealing increased the melting temperature (T m ) and crystallinity. However, annealing at 140 °C for 14 d also caused significant solid-state transesterification. In addition, crystallites composed of s-PLAs with extended arms and, simultaneously, crystallites composed exclusively of extended rings were formed, identified by their typical "sawtooth" pattern in the MALDI mass spectra. Small angle x-ray scattering (SAXS) indicated that the crystal thickness was lower than that of linear PLAs annealed under identical conditions, but increased upon annealing at 140°C.