Double Thermosensitive Diblock Copolymers of Vinyl Ethers with Pendant Oxyethylene Groups: Unique Physical Gelation
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Abstract
Diblock copolymers of vinyl ethers with two thermosensitive segments possessing different phase separation temperatures (TPS) have been synthesized by sequential living cationic copolymerization. Examples include 2-(2-ethoxy)ethoxyethyl vinyl ether (EOEOVE) and 2-methoxyethyl vinyl ether (MOVE), which were sequentially polymerized using a cationogen/Et1.5AlCl1.5 initiating system in the presence of tetrahydrofuran to give diblock copolymers with a very narrow molecular weight distribution. When an aqueous solution of a diblock copolymer (EOEOVE/MOVE = 200/400) was heated, four different viscoelastic stages were observed: clear liquid (sol, ≤40 °C), transparent gel (42−55 °C), hot clear liquid (sol, 57−63 °C), and opaque mixture by phase separation (>63 °C). Micelle formation during physical gelation was confirmed by the change in particle diameter. The temperatures of the first and third transitions corresponded to each TPS for two segments, whereas that of the second transition was dependent on factors such as structure (sequence, composition, and molecular weight distribution) and physical properties (concentration, additives). Freeze-fracture transmission electron microscopy revealed the physical gels to consist of a regular arrangement of spherical micelles with controlled size.
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