Influence of amide versus ester linkages on the properties of eight-armed PEG-PLA star block copolymer hydrogels

Water-soluble eight-armed poly(ethylene glycol)-poly(l-lactide) star block copolymers linked by an amide or ester group between the PEG core and the PLA blocks (PEG-(NHCO)-(PLA)8 and PEG-(OCO)-(PLA)8) were synthesized by the stannous octoate catalyzed ring-opening polymerization of l-lactide using an amine- or hydroxyl-terminated eight-armed star PEG. At concentrations above the critical gel concentration, thermosensitive hydrogels were obtained, showing a reversible single gel-to-sol transition. At similar composition PEG-(NHCO)-(PLA)8 hydrogels were formed at significantly lower polymer concentrations and had higher storage moduli. Whereas the hydrolytic degradation/dissolution of the PEG-(OCO)-(PLA)8 takes place by preferential hydrolysis of the ester bond between the PEG and PLA block, the PEG-(NHCO)-(PLA)8 hydrogels degrade through hydrolysis of ester bonds in the PLA main chain. Because of their relatively good mechanical properties and slow degradation in vitro, PEG-(NHCO)-(PLA)8 hydrogels are interesting materials for biomedical applications such as controlled drug delivery systems and matrices for tissue engineering.