Ultralow Self-Cross-Linked Poly(N-isopropylacrylamide) Microgels Prepared by Solvent Exchange

We found that the poly(N-isopropylacrylamide) (PNIPAm) synthesized by free-radical polymerization in organic phase could also form stable microgels in water through solvent exchange without chemical cross-linkers. Dynamic light scattering and transmission electron microscopy showed the larger swelling ratio and higher deformability of these microgels. Nuclear magnetic resonance and infrared spectroscopy indicated that the self-cross-linking structures in these microgels were attributed to the hydrogen atom abstraction both from the isopropyl tert-carbon atoms and the vinyl tert-carbon atoms in PNIPAm chains and the organic solvents were important assistants in the hydrogen abstraction behavior. Our discovery revealed that the self-cross-linking of PNIPAm chains is a common phenomenon within their free-radical polymerization process, whether in aqueous phase or in organic phase. Besides, the addition of second monomers will not affect the cross-linkage of the PNIPAm portion, which may be of great significance for the synthesis of various functional ultralow cross-linking PNIPAm microgels.