Effects of L-lactide and D,L-lactide in poly(lactide-co-glycolide)-poly(ethylene glycol)-poly(lactide-co-glycolide) on the bulk states of triblock copolymers, and their thermogellation and biodegradation in water

Two series of poly(L-lactide-co-glycolide)-poly(ethylene glycol)-poly(L-lactide-co-glycolide) (PLLGA-PEG-PLLGA) and poly(D, L-lactide-co-glycolide)-poly(ethylene glycol)-poly(D, L-lactide-co-glycolide) (PDLLGA-PEG- PDLLGA) triblock copolymers with similar molecular weights but different ratios of lactide (LA) and glycolide (GA) were synthesized. All of the PDLLGA-PEG-PDLLGA polymers were sticky pastes at dry state and their aqueous solutions underwent a sol-gel transition upon heating; while the PLLGA-PEG-PLLGA polymers presented various bulk states from sticky paste to powder and possessed different behaviors in water, which was dependent upon the L-LA/GA ratio. An appropriate L-LA/GA ratio not only led to a solid-like form in the bulk state, but also formed a stable sol in water prior to thermo-induced physical gelation for the obtained polymer. This feature was convenient for weighing, transferring and storing in the potential material applications. The effects of steric regularity in PLGA block on the thermogelling and degradation of triblock copolymers in water were further examined. At high LA/GA ratio, solid-like PLLGA-PEG-PLLGA showed significant difference in both thermogellation properties and degradation behaviors compared with sticky PDLLGA-PEG-PDLLGA. Consequently, the present study sheds light on the relationship between thermogelation and polymeric molecular structure and enriches the molecular design of the thermogelling systems as injectable biomaterials, based on commonly used monomers.