Hydrolytic degradation of poly(ε-caprolactone) in the melt

Rubbery biodegradable polyester poly(ε-caprolactone) (PCL) having a relatively low melting temperature (ca. 60 °C) was hydrolysed in the melt in high-temperature and high-pressure water at a wide temperature range of 175-370 °C for periods up to 30 min and formation and decomposition of 6-hydroxycaproic acid (6-HCA) and molecular weight change of PCL were investigated. More than 90% of PCL was hydrolysed to water-soluble oligomers and monomer within 30 min when hydrolysed in the temperature range of 300-370 °C. Within 30 min, 6-HCA was obtained at the maximum yields exceeding 80% as far as hydrolysis was carried out in the temperature range of 300-370 °C, whereas the maximum yield decreased to 15% due to incomplete hydrolysis when hydrolysis temperature was lowered to 250 °C. The optimal temperatures for PCL (300-370 °C) were higher than those for poly(l-lactide) (PLLA) (≤270 °C) and poly[(R)-3-hydroxybutyrate] [R-P(3HB)] (≤200 °C), meaning the high stability of 6-HCA at high temperatures. Too long hydrolysis of PCL at 300-370 °C caused the decreased yield of 6-HCA due to its decomposition. The hydrolysis of PCL proceeds homogeneously and randomly via a bulk erosion mechanism. The molecular weight of PCL decreased exponentially without formation of specific low-molecular-weight chains originating from crystalline residues. The activation energy for the hydrolysis in the melt (ΔEh) obtained for PCL, 72.3 kJ mol−1 is higher than the 51.9 kJ mol−1 for PLLA, lower than 126 kJ mol−1 for R-P(3HB), and much lower than 233 kJ mol−1 for poly(ethylene terephthalate). This study revealed that the hydrolysis of PCL in the melt is an effective and simple method to obtain its monomer 6-HCA and to prepare PCL having different molecular weights without containing the specific low-molecular-weight chains, because of the removal of the effects caused by crystalline residues.