Lower critical solution temperatures of thermo-responsive poly(N-isopropylacrylamide) copolymers with racemate or single enantiomer groups

BACKGROUND: Thermo-responsive copolymers with racemate or single enantiomer groups are attracting increasing attention due to their fascinating functional properties and potential applications. However, there is a lack of systematic information about the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide)-based thermo-responsive chiral recognition systems. In this study, a series of thermo-responsive chiral recognition copolymers, poly[(N-isopropylacrylamide)-co-(N-(S)-sec-butylacrylamide)](PN-S-8) and poly[(N-isopropylacrylamide)-co-(N-(R,S)-sec-butylacrylamide)] (PN-R,S-B), with different molar compositions, were prepared. The effects of heating and cooling processes, optical activity and amount of chiral recognition groups in the copolymers on the LCSTs of the prepared copolymers were systematically studied. RESULTS: LCST hysteresis phenomena are found in the phase transition processes of PN-S-B and PN-R,S-B copolymers in a heating and cooling cycle. The LCSTs of PN-S-B and PN-R,S-B during the heating process are higherthan those during the cooling process. With similar molar ratios of N-isopropylacrylamide groups in the copolymers, the LCST of the copolymer containing a single enantiomer (PN-S-B) is lower than that of the copolymer containing racemate (PN-R,S-B) due to the steric structural difference. The LCSTs of PN-R,S-B copolymers are in inverse proportion to the molar contents of the hydrophobic R,S-B moieties in these copolymers. CONCLUSION: The results provide valuable guidance for designing and fabricating thermo-responsive chiral recognition systems with desired LCSTs. (C) 2008 Society of Chemical Industry