Thermo-Responsive Behaviors Of Water-Soluble Polymers Triggered By Photo-isomerization Of Their Oxime Groups

The light-triggered thermo-responsive behaviors of water-soluble polymers have been extensively studied because of their wide potential applications in biomedical and intelligent materials related fields. Inspired by the remarkable defference of the melting points between small molecular Z-type and E-type oxime compounds, this thesis explored a new approach for modulating the thermo-responsive behaviors simply via photo-isomerization of the polymeric oxime residues. To this end, a range of well-defined P(FPHPMA-ran-PEGMA) precursors, at similar degrees of polymerization but different compositions, were sythesized via visible light activating RAFT polymerization at2℃. P(HHMPPMA-ran-PEGMA) were achieved by further reacting of the aldelyde groups with hydroxyamine. The results demonstrated that attering the aldehyde groups to oxime groups decreased the cloud points and hydrodynamic diameters of aggregates. The thermo-responsive behaviors of P(HHMPPMA-ran-PEGMA) strongly depended on their unit ratios. Their E-type oxime groups could be isomerized to Z-type oxime groups upon irradiated with UV light. This light-triggered Z-isomerization slowed down on increasing the contents of PEGMA units. Z-isomerization of oxime groups didn’t change the cloud points of P(HHMPPMA-ran-PEGMA), but slightly changed the critical temperature of the compacting micelles formation. However, the sizes of the aggregates clearly increased on increasing the fraction of Z-type oxime groups. The copolymer chains pre-organized to micelles at2℃at a [HHMPPMA]:[PEGMA]-2:1, these micelles were stable on heating up to40℃. The light scattering studies and temperature-variable1H NMR analysis results demonstrated that the phase transition of P(HHMPPMA-ran-PEGMA) occured in the hydrated state, at either90%or24%E-type oxime groups. More importantly, further heating the solution up to20℃above the critical micellar temperature, the polymers at90%E-type oxime groups did not exhibit thermal hysteresis on cooling, while at24%E-type oxime groups showed a clear thermal hysteresis on cooling. Considering hydrogen bonding features of oxime compounds, i.e. in the confined mode at E-type or unlimited extension mode at Z-type, the thermo-responsive behaviors may be effectively regulated by changing their hydrogen bonding mode.This paper provides an approach toward novel water soluble polymers, whose thermo-responsive properties may be well tuned on irradiated with UV light.