Synthesis of high-molecular-weight poly(ε-caprolactone) via heterogeneous zinc-cobalt(III) double metal cyanide complex

Abstract

The synthesis of high-molecular-weight poly(ε-caprolactone)s (PCLs) with enhanced mechanical properties remains challenging. Here we demonstrate the heterogeneous catalysis for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). Using a zinc-cobalt double-metal cyanide complex [Zn-Co(III) DMCC] in the presence of around 1% mol epoxides, PCLs are achieved with high number-average molecular weights (Mns) of up to 177.7 kDa determined by the gel permeation chromatography. The [Zn]-OH active site of the complex triggers the cleavage of the epoxide at first, which is considered as the rate-determining step of the polymerization. The formed zinc-alkoxide species then initiates the ROP of ε-CL, affording most of the PCLs possessing one epoxide molecule embedded in the polymer chain and dihydroxyl terminals as revealed by the 1H NMR and MALDI-TOF MS spectroscopy. The PCL specimen with a corrected Mn of 110.4 kDa exhibits the tensile strength and elongation at break of as high as 40.2 MPa and 1060%, respectively. This work provides a facile and practical method for the synthesis of high-molecular-weight PCLs.