Diethanol ammonium-borate based polybetaine with tunable UCST phase transition

A betaine-type styrene monomer with the quaternary ammonium-borate anion inner-salt pair was synthesized through the quaternization reaction and polymerized to afford the target polybetaine of poly(4-vinylbenzyl methyl-diethanol ammonium borate) (PVMAB). The chemical structures of the monomer and polymer were well demonstrated with 1 H-NMR and 11 B-NMR spectra analysis. The thermal-sensitive experiment showed that PVMAB in water afforded gradually welldefined sigmoidal transmittance-temperature ( T - t ) curves along with the increasing polymer concentration. However, the phase transition temperatures at the bottom of the S-shaped curves were always below 10 °C due to the very weak zwitterionic association of the ammonium-borate inner-salt pairs. The UCST phase transition could also be tuned by changing the ethanol content in the ethanol/water mixture. And the cytotoxicity experiment demonstrated the good biomimetic property of PVMAB. This study enriches the toolbox of polybetaines by introducing the quaternary ammonium-borate anion zwitterionic pair in the repeat units, therefore broadens the scope of synthetic polybetaines.