Defining Role of a High-Molecular-Weight Population in Block Copolymers Based on Poly(α-benzyl carboxylate-ε-caprolactone) and Poly(ethylene glycol) on the Formation of Thermoreversible Hydrogels

Biodegradable thermoreversible hydrogels with sol-to-gel transition temperatures a few degrees below the physiological temperature have great potential for application in depot drug delivery and tissue engineering. This research aimed to investigate the formation of thermoreversible hydrogels from the triblock copolymers of poly(ethylene glycol) and poly(α-benzyl carboxylate-ε-caprolactone) (PBCL–PEG–PBCL) prepared by bulk and solution polymerization methods. For this purpose, PBCL–PEG—PBCL prepared at fixed PBCL-to-PEG ratios but with different polymerization times were characterized for their average molecular weights, molar mass dispersity, and intrinsic viscosity using 1H NMR and gel permeation chromatography (GPC). The inverse flow method was used to estimate the sol–gel transition temperature of aqueous polymer solutions. Rheological measurements and dynamic light scattering determined the viscoelastic behavior and aggregations of prepared structures in aqueous media as a function of temperature, respectively. The results indicated the production of a high-molecular-weight population with elevated intrinsic viscosity during the synthesis of block copolymers. The size and proportion of this population grew as a function of polymerization time in both bulk and solution polymerization methods. This increase was more gradual when solution polymerization was applied. Based on the indirect evidence provided by GPC analysis, the formation of this high-molecular-weight PBCL–PEG–PBCL population was attributed to the nonlinear architecture (partial cross-linking and/or branching) of the PBCL segment. Interestingly, around 40% mole concentration of this high-molecular-weight PBCL–PEG–PBCL population was required for thermoreversible micellar aggregation and gel formation in aqueous media.