Preparation and characterization of polymer micelles from poly(ethylene glycol)-poly(d,l-lactide) block copolymers as potential drug carrier

Poly(ethylene glycol)-poly(d,l-lactide) block copolymers (PEG-PLA) with varying composition were prepared through successive ring-opening polymerization of ethylene oxide and d,l-lactide using an anionic initiator, and their property of multimolecular micellization in aqueous milieu was examined in detail from the standpoint of designing carriers for hydrophobic drugs. The heterogeneity of PEG-PLA was found to crucially affect the size and distribution of micelles, and narrowly-distributed micelles with sizes of ∼30 nm in diameter were formed only from PEG-PLA with a substantially narrow molecular weight distribution and an appropriate balance in the length ratio of the PEG and PLA segments in PEG-PLA, indicating the importance of establishing a reliable synthetic route for the block copolymers. PEG-PLA micelles have a considerably low critical association concentration (∼1.0 mg/l) which is apparently an advantage in utilizing these micelles as drug carriers in an extremely diluted condition.