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 1H-NMR
and 11B-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 ammoniumborate
anion zwitterionic pair in the repeat units, therefore broadens the scope of synthetic polybetaines.
