Novel biodegradable thermosensitive triblock copolymers of poly(D,L-3-methylglycolide)-block-poly(ethylene glycol)-block-poly(D,L-3-methylglycolide) (PMG-PEG-PMG) have been synthesized. Ring-opening polymerization of D,L-3-methyl-glycolide (MG) initiated with poly(ethylene glycol) (PEG) and Ca[N(SiMe3)(2)](2) (THF)(2) provided triblock copolymers with alternating lactyl/glycolyl sequences of controlled molecular weight, low polydispersity index, and uniform chain structure. At relatively low temperatures (similar to10degreesC) these copolymers formed clear solutions in water up to high concentrations (50 wt.-%). Depending on molecular mass ratios of PMG and PEG blocks, a sol-gel transition or an increase in viscosity without gel formation was observed upon increasing the temperature of the aqueous solutions. The temperature-induced gelation was ascertained by rheology and dynamic differential scanning calorimetry (DDSC). Phase diagram of PMG-PEG-PMG 1 400-1 450 1 400 in an aqueous solution.