Transesterification and polymerization reactions of aliphatic polyesters in supercritical CO2 fluids without the presence of a catalyst

The reactions or stability of two crystalline polycaprolactones (PCL) and one amorphous poly(d,l-lactic acid) (dl-PLA) in supercritical CO2 fluid (scCO2) at 35 °C and 34.5 MPa without the presence of a catalyst were investigated using gel permeation chromatography (GPC), nuclear magnetic resonance spectroscopy (NMR), and differential scanning calorimetry (DSC). The two PCLs having 80,000 and 10,000 in number average molecular weights (M¯n), denoted by PCL8 and PCL1, respectively, and dl-PLA having 74,000 in M¯n were studied. Carbon 13 NMR data found these polyesters no formation of carbonate linkages after treatments in scCO2, indicating that the CO2 did not copolymerize with the PCLs and dl-PLA. GPC data found that the molecular weights of PCL8/PCL1 blends considerably varied with the treatment time while the components of the blends negligibly changed throughout 6 h of treatments in scCO2. These GPC data suggested that transesterification reactions occurred between PCL8 and PCL1. DSC data found that PCL8 co-crystallized with PCL1 and the treatments enhanced the melting temperatures and crystallinities of the blends. GPC data also found that the molecular weights of dl-PLA and PCL8/dl-PLA blends considerably varied with the treatment time in scCO2. From the variations of molecular weights, it could be suggested that transesterifications and polymerizations both occurred in the PCL8/dl-PLA blends, with the polymerizations predominating over the transesterifications. This predomination in polymerizations lowered with increasing PCL8 content in the blends. DSC data found that the amorphous dl-PLA could be induced to crystallize by scCO2, giving a heating endotherm at near 122 °C. Following scCO2 treatments, enhancements of the melting temperature and crystallinity of PCL8 in the PCL8/dl-PLA blends were found, although these enhancements fluctuated with the treatment time.