Ring opening polymerization of ε-caprolactone in the presence of wet β-cyclodextrin: effect of the operative pressure and of water molecules in the β-cyclodextrin cavity

Abstract

The ring opening polymerization (ROP) of ε-caprolactone (CL) in the presence of β-cyclodextrin (β-CD) was performed in batch reactors both at room pressure and with the reaction system pressurized with CO₂, N₂ or Ar. Significant enhancements of the polymerization rate was observed when the ROP was carried out with wet β-CD under pressure. For example, after 24 hours at 120 °C with a β-CD/CL molar ratio of about 1/100, the monomer conversion increased from 4 to 98–99% when the pressure was changed from 0.1 to 12.5–13.0 MPa independent of the nature of the compressing gas. MALDI-TOF analyses indicated that a major fraction of polymer chains obtained in pressurized systems was initiated by water molecules. The collected results suggest that at 12–13 MPa wet β-CD can catalyse both the ring opening of ε-caprolactone and the polymerization of the obtained linear species and that high energy water molecules located inside the cavity of the cyclic oligosaccharide must play a role in initiating the polymerization. The trend of number average molecular weight and the results of MALDI-TOF analyses obtained in polymerizations performed for long reaction times and in a hydrolysis test of commercial poly(ε-caprolactone) indicate that wet β-CD can work as an artificial lipase enzyme under the adopted conditions.