Effects of Molecular Weight Distribution of Amphiphilic Block Copolymers on Their Solubility, Micellization, and Temperature-Induced Sol–Gel Transition in Water

Poly(d,l-lactide-co-glycolide)-b-poly(ethylene glycol)-b-poly(d,l-lactide-co-glycolide) (PLGA–PEG–PLGA) was synthesized via ring-opening polymerization of d,l-lactide and glycolide in the presence of poly(ethylene glycol) (PEG), and then a series of triblock copolymers with different molar-mass dispersities (ĐMs) under similar weight-average molecular weight (Mw) or number-average molecular weight (Mn) were obtained via step precipitation and mixing of synthesized copolymers with a given PEG length and varied PLGA lengths. The effects of molecular weight distribution (MWD) on thermogelling and related behaviors of the copolymer aqueous solutions were investigated. The increase of ĐM, namely Mw/Mn, was found to enhance the solubility of the amphiphilic block copolymer in water and accelerate the solubilization process significantly. The critical micelle concentration (CMC) determined via a hydrophobic dye incorporation method also increased with ĐM, and the corresponding copolymer micelles became less hydrophobic as reflected by the fluorescence of 10-hydroxycamptothecin as a hydrophobic probe. Some of the copolymers with appropriate balance of hydrophilicity and hydrophobicity underwent sol–gel transition upon heating if the copolymer concentration was over its critical gel concentration (CGC). The CGC and sol–gel transition temperature Tgel increased with ĐM under both a given Mw and a given Mn. So, different components in polymers with broad MWD might cooperate with each other. The present study illustrates that ĐM serves as a regulator of the condensed state of amphiphilic block copolymers in a selective solvent.