Temperature induced self-assembly of amino acid-derived vinyl block copolymers via dual phase transitions.The unique thermoresponsive phase behaviors of diblock copolymers from amino acid-derived vinyl monomers have been demonstrated in view of variation in the aggregation state in water.  Amino acid-based block copolymers composed of N-acryloyl-Ala-methylester (NAAMe) and N-acryloyl-βAla-methylester (NAβAMe) are successfully synthesized by RAFT polymerization.  The resultant block copolymers poly(NAAMe48-b-NAβAMem) contain a constant degree of polymerization (DP=48) of the poly(NAAMe) block, but the DP of the poly(NAβAMe) block varies (m=80-122).  The turbidimetry subjected to these copolymer aqueous solutions exhibits two LCST transitions upon heating.  In the 1st LCST region, the block copolymer forms a relatively loose-molecular packing, while large aggregates due to partial dehydration of polymer molecules, which subsequently transform into a stable micelle structure in a region of 30-39°C.  Finally, a tight aggregate composed of the dehydrated micelles is formed.  Temperature-dependent 1H NMR spectroscopy of the diblock copolymers also supports such a postulation for the dual phase transitions and stable micelle structure formation.  In addition, a typical salting-out effect is observed in the thermal behavior of the polymer, but a serious cytotoxic effect is not observed in NIH/3T3 cells, suggesting that the novel diblock copolymers are relevant for biomedical applications.
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