Thermoreversible gelation of biodegradable poly(epsilon-caprolactone) and poly(ethylene glycol) multiblock copolymers in aqueous solutions.

The multiblock copolymers composed of poly(ethylene glycol)s (PEGs) and biodegradable poly(epsilon-caprolactone)s (PCLs) were synthesized through one-step condensation copolymerization with hexamethylene diisocyanate (HDI) as a coupling agent. The typical phase diagram of these multiblock copolymers in aqueous solution displayed a critical gel concentration (CGC) and an upper phase-transition temperature, which were mainly determined by the PEG/PCL block ratio, the PEG or PCL block lengths and the molecular weight. With decreasing PEG/PCL block ratio, the CGC decreased with an elevated sol-gel transition temperature on account of the enhanced hydrophobicity. The HDI/Diols ratio was used to control the molecular weight. At high molecular weights, the CGC decreased, related to the enhanced aggregation of PCL blocks and physical crosslinkage between PCL block domains due to the increased number of PCL blocks in each molecule. For the sample containing the long PCL(2000) block (M(n), 2000), the CGC dropped dramatically due to the high hydrophobicity and the poor compatibility between PCL and PEG. The dynamic phase transition process was observed by combining optical microscopy (OM) and differential scanning calorimetry (DSC) in a certain heating/cooling rate. Finally, a possible phase separation-induced gelation mechanism is suggested.