Design of Hydroxy-Functionalized Thermoresponsive Copolymers: Improved Direct Radical Polymerization of Hydroxy-Functional Vinyl Ethers

The improved synthesis and aqueous solution properties of hydroxy-functional vinyl ether (OH-VE) copolymers with lower critical solution temperatures (LCST) in water are presented. A series of OH-VE copolymers were directly prepared by free radical polymerization. Effects of water and comonomers on the free radical polymerization using 2-hydroxyethyl vinyl ether (HEVE) were investigated in detail. The free radical polymerization of HEVE at 40–60 wt % in water proceeded with complete monomer conversion. The copolymerizations of HEVE with other vinyl ethers (VEs) or vinyl acetate (VAc) also proceeded smoothly. In each case, the primary key to success in the free radical copolymerization is proper hydrogen bonding between the VE oxygen and hydroxyl group in the pendant or nonacidic water as a solvent. In particular, the copolymerization of OH-VE and VE without a hydroxy group is enabled by a hydrogen-bonded monomer complex. Using the resulting copolymers, we demonstrated the existence of thermoresponsive behavior for the water-soluble copolymers. By choosing appropriate compositions for the copolymers, i.e., an appropriate hydrophilic/hydrophobic balance in the composition or in the side chain, LCST-type thermoresponsive behavior was observed in water. The product copolymers include poly(HEVE- co -VAc), the alkali-hydrolyzed poly(HEVE- co -vinyl alcohol), poly(diethylene glycol monovinyl ether- co -VAc), poly(HEVE- co -VE) [VE: 2-methoxyethyl vinyl ether, isobutyl vinyl ether, n -butyl vinyl ether, and 4-hydroxybutyl vinyl ether (HBVE)], and poly(HBVE- co -VAc). These direct radical copolymerizations allow the design of various thermoresponsive hydroxy-functional polymers with negligible cytotoxicity and tunable LCST.