Diblock/Triblock Structural Transition and Sol-Gel Transition of Peptide/PEG Diblock Copolymer Having a Terminal Terpyridine Group Induced by Complexation with Metal Ion

Terpyridine-polyethyleneglycol-block-polyleucine block copolymer (tpy-PEG-PLeu) was synthesized by a ring-opening polymerization of L-leucine -carboxyanhydride. The copolymer complexed with ion and its aqueous solution showed a purple color as a result of the complexation. This complexation caused the diblock/triblock structural transition of the copolymer. The change of the aggregation behavior caused by the structural transition was observed by a dynamic light scattering apparatus. The diblock tpy-PEG-PLeu copolymer formed a micelle in the aqueous solution. On the other hand, the triblock-type copolymer, after the complexation, formed the micelle structures and huge aggregates, which is considered to be a network structure. The complexation of the diblock tpy-PEG-PLeu copolymer with Fe ion is consequently considered to be a trigger of the gelation. 1. Introduction The chloroform solution of the ABA-type triblock copolymer which consists of a polyleucine (PLeu) chain as the A segment and a polyethylene glycol (PEG) as the B segment has been reported to show a thermoresponsive sol-gel transition [1]. This sol-gel transition was not observed on the solution system of AB-type block copolymer having the same components. These results were attributed to the difference in the aggregation behavior between the AB-type and the ABA-type block copolymers. Since the two polyleucine segments of the triblock copolymer behave as the junction point due to the hydrophobic interaction to string the many copolymers together, the ABA-type triblock copolymer in the solution can form the network structure, resulting in the formation of the gel. On the other hand, the AB-type diblock copolymer could not form the gel like the ABA-type [2]. On the basis of the facts mentioned above, we would like to propose the novel sol-gel transition system made up of the AB-type diblock copolymer as a basic material. In this study, the terminal functionalized AB-type diblock copolymer consisting of PEG and polyleucine segments was synthesized. A terpyridine, which was known to make a complex with metal ion [3–6], was employed as a terminal functional group. The terminal functionalized AB-type diblock copolymer consisting of PEG and polyleucine segments and having the terpyridine group (tpy) at the end of PEG is, hereafter, referred to as tpy-PEG-PLeu. When the divalent metal ion is added to the solution of the tpy-PEG-PLeu, the metal ion should behave as the terminal connector of the diblock copolymer and make the tpy-PEG-PLeu into the ABA-type triblock copolymer. Therefore, the addition of