Control of association and gelation behavior of ABA triblock copolymers by means of conformational transition of B block

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Both-ends hydrophobically-modified water-soluble polypeptide, in which hydrophobic n-dodecyl (C12) groups are bonded at the both chain ends of poly[N^5-(2-hydroxyethyl) L-glutamine] (PHEG), has been prepared. Helix-coil transition and association behavior of this associative polypeptide in water/ethylene glycol (EG) mixed solvent, which is selective for PHEG block, have been investigated.1.In dilute aqueous solution, the middle PHEG chain is in random coil state, and isolated small-size micelles were formed by association of hydrophobic C12 groups. The micellar size increased drastically with the increase of EG content in the mixed solvent. These results suggest that the increase of the helix content of PHEG induces the increase of fraction of bridge conformation connecting different C12 associated cores, which results in the change of structure from an isolated associate to a large associate including multiple associated cores connected by PHEG chains.2.The above association scheme can explain process dependencies of sample preparation on the association in dilute solutions as well as gelation behavior of semi-concentrated solutions.3.Association behavior of one-end hydrophobically-modified PHEGs was also investigated. Association strength was increased with the increase of hydrophobicity of the terminal alkyl chain. Conformational transition of PHEG chain induced a change in micellar structure, i.e., spherical shape when PHEG is in random coil state and worm-like cylinder when PHEG in α-helix conformation.4.Block copolymer of PHEG and poly(ethylene glycol) was prepared, and its association in a selective solvent for poly(ethylene glycol) was investigated. When PHEG is in α-helical state, the associate is in hollow cylindrical shape because of a packing of rigid PHEG chain. When PHEG prefer to take random-coil conformation, formation of a normal core-corona type wormlike cylinder was suggested.