Hydrogel formation by the ‘topological conversion’ of cyclic PLA–PEO block copolymers

An amphiphilic cyclic block copolymer consisting of poly(L- or D-lactide) and poly(ethylene oxide), that is, PLLA–PEO or PDLA–PEO, was synthesized from its corresponding linear triblock precursor, PLLA–PEO–PLLA or PDLA–PEO–PDLA, respectively, with alkenyl end groups. A mixture of the micellar dispersions of linear PLLA–PEO–PLLA and linear PDLA–PEO–PDLA formed a gel upon heating, whereas a mixture of the cyclic counterparts did not undergo this phase transition. These results suggest that the gelation behavior is directed by the topology of the polymer components. Furthermore, cyclic PLLA–PEO and cyclic PDLA–PEO block copolymers incorporating photocleavable o-nitrobenzyl units were synthesized. A mixture of the micellar dispersions of these block copolymers formed a gel upon ultraviolet irradiation via the ‘topological conversion’.