Synthesis and characterization of PCL/PEG/PCL triblock copolymers by using calcium catalyst

Triblock copolymer PCL-PEG-PCL was prepared by ring-opening polymerization of ε-caprolactone (CL) in the presence of poly(ethylene glycol) catalyzed by calcium ammoniate at 60 °C in xylene solution. The copolymer composition and triblock structure were confirmed by ¹H NMR and ¹³C NMR measurements. The differential scanning calorimetry and wide-angle X-ray diffraction analyses revealed the micro-domain structure in the copolymer. The melting temperature T_m and crystallization temperature T_c of the PEG domain were influenced by the relative length of the PCL blocks. This was caused by the strong covalent interconnection between the two domains. Aqueous micelles were prepared from the triblock copolymer. The critical micelle concentration was determined to be 0.4-1.2 mg/l by fluorescence technique using pyrene as probe, depending on the length of PCL blocks, and lower than that of corresponding PCL-PEG diblock copolymers. The ¹H NMR spectrum of the micelles in D₂O demonstrated only the -CH₂CH₂O- signal and thus confirmed the PCL-core/PEG-shell structure of the micelles.