Synthesis of Double-responsive Core Cross-linked Star Polymers

A series of water-soluble double-responsive core cross-linked star polymers (PNSB@PAA-DMP), in which poly(3-acrylamidophenylboronic acid-co-acrylamide) (PAA-DMP) served as the pH-responsive arm and poly(N-isopropyl acrylamide-co-styrene-co-N,N'-methylenebisacrylamide) (PNSB) as the thermoresponsive core, have been successively prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization using the "arm-first" approach. Poly(AAPBA-co-AM) is first prepared via RAFT polymerization to give a well-defined linear hydrophilic macro-RAFT agent, which subsequently copolymerizes with thermosensitive monomer NIPAM (N-isopropyl acrylamide), crosslinker Bis (N,N'-methylenebisacrylamide) and St (styrene) to give the desired pH and thermoresponsive star polymers (PNSB@PAA-DMP). Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (H-1-NMR) are empolyed to characterize the structure and composition of the resultant polymers. The molecular weight of the polymers are obtained by gel permeation chromatography (GPC). The morphology and particle size of polymer under dry conditions are characterized by transmission electron microscopy (TEM). In addition, the phase transition behavior of the polymers in aqueous solutions at low concentration is investigate by dynamic light scattering (DLS) and ultraviolet-visible spectroscopy (UV-Vis). The results show that the obtained PNSB@PAA-DMP polymers exhibit reversible thermal-induced volume phase transition and pH responsibility. The lower critical solution temperature (LCST) of the core cross-linked star polymers in aqueous solutions can be tuned by changing the feeding ratio of the hydrophobic monomer St. In addition, the LCST of the polymer solution could also be affected by changing the pH of the polymer solution. What is most interesting is that sol-gel transition of the polymer solution can be achieved by controlling temperature and pH at a higher concentration. For example, a free flowing solution (20 mg/mL, pH = 9) is observed at 15 degrees C, but a white hydrogel is formed when this solution is heated to 30 degrees C. When pH = 12, the polymer solution (20 mg/mL), at any temperature, is always in the sol state, which quickly settles at the vial bottom when the sample vials is inverted.