A series of biodegradable PCL-PEG-PCL block copolymers were successfully synthesized by ring-opening polymerization of epsilon-caprolactone initiated by poly(ethylene glycol) (PEG), which were characterized by (1)H NMR, (13)C NMR, and FTIR. Their aqueous solution displayed special gel-sol transition behavior with temperature increasing from 4 to 100 degrees C, when the polymer concentration was above corresponding critical gel concentration (CGC). The gel-sol phase diagram was recorded using test tube inverting method and DSC method, which depended not only on chemical composition of copolymers, but also on heating history of copolymer's aqueous solution. As a result, the gel-sol transition temperature could be adjusted, which might be very useful for its application in biomedical fields such as injectable drug delivery system. And the typical shell-core structure of PCL-PEG-PCL micelles was introduced. The micelle-packing and partial crystallization might be the key gelation machanism for this gel-sol transition behavior of PCL-PEG-PCL aqueous solution.

A series of low molecular weight poly(ethylene glycol)-polycaprolactone-poly(ethylene glycol) (PEG-PCL-PEG) biodegradable block copolymers were successfully synthesized using isophorone diisocyanate (IPDI) as the coupling agent, and were characterized using 1 H NMR and Fourier transform infrared spectroscopy. The aqueous solutions of the PEG-PCL-PEG copolymers displayed a special thermosensitive gel-sol transition when the concentration was above the corresponding critical gel concentration. Gel-sol phase diagrams were recorded using the test-tube-inversion method; they depended on the hydrophilic/hydrophobic balance in the macromolecular structure, as well as some other factors, including the heating history, volume, and the ageing time of the copolymer aqueous solutions and dissolution temperature of the copolymers. As a result, the gel-sol transition temperature range could be altered, which might be very useful for application in injectable drug delivery systems.

In this article, a new kind of biodegradable poly(e-caprolactone)-poly(ethylene glycol)-poly(e-caprolactone)-based polyurethane (PCEC-U) copolymers were successfully synthesized by melt-polycondensation method from e-caprolactone (e-CL), poly(ethylene glycol) (PEG), 1,4-butanediol (BD), and isophorone diisocyanate (IPDI). The obtained copolymers were characterized by 1 H-nuclear magnetic resonance ( 1 H-NMR), FTIR, and gel permeation chromatography (GPC). Thermal properties of PCEC-U copolymers were studied by DSC and TGA/DTG under nitrogen atmosphere. Water absorption and hydrolytic degradation behavior of these copolymers were also investigated. Hydrolytic degradation behavior was studied by weight loss method.1 H-NMR and GPC were also used to characterize the hydrolytic degradation behavior of PCEC-U copolymers. The molecular weight of PCL block and PEG block in soft segment and the content of hard segment strongly affected the water absorption and hydrolytic degradation behavior of PCEC-U copolymers.