Thermoplastic hydrogels of alternating multiblock copolymers, consisting of poly(ethylene glycol) (PEG) and poly(L-lactic acid) (PLLA), were synthesized. Dicarboxylated oligomeric PLLAs were synthesized by the condensation reaction of L-lactic acid in the presence of succinic acid. Changing the feed ratio of L-lactic acid to succinic acid controlled PLLA molecular weights. Alternating multiblock copolymers with different block lengths of PEG and PLLA were obtained from the polycondensation reaction between PEG and dicarboxylated-PLLA in the presence of dicyclohexyl carbodiimide and N-dimethyl aminopyridine as catalysts. The chemical compositions of dicarboxylated PLLAs and multiblock copolymers were verified by H-1-NMR and FT-IR, and the molecular weight and distribution were measured by gel permeation chromatography. DSC thermograms showed that there existed a high degree of phase mixing, as well as microphase separation in the multiblock copolymer, demonstrated by a large T-g peak from the amorphous phase-mixing domain and a small T-m peak from crystalline microdomains of the PLLA component. The block copolymers with low molecular weights were water-soluble and clouded at a temperature which is associated with a lower critical solution temperature (LCST) phenomenon. However, the high molecular weight polymers could swell in water and their optical transparency was influenced by temperature. This observation can be attributed to an enhanced hydrophobic interaction between hydrophobic moieties in the polymer chains, caused by an increase in temperature. The weight swelling ratio (absorbed water/polymer) of the polymers was controlled by polymer composition and molecular weight. These block copolymers may offer potential for applications in drug delivery and various other biomedical projects.