Facile Fabrication of Reversible Core Cross-Linked Micelles Possessing Thermosensitive Swellability

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Abstract

Poly(ethylene oxide)-b-poly(N-isopropylacrylamide-co-N-acryloxysuccinimide), PEO-b-P(NIPAM-co-NAS), was synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization in dioxane at 70 °C employing a poly(ethylene oxide) (PEO)-based macroRAFT agent. The obtained double hydrophilic block copolymer molecularly dissolves in aqueous solution at room temperature. Above the lower critical solution temperature (LCST) of P(NIPAM-co-NAS) block, it self-assembles into micelles consisting of thermoresponsive P(NIPAM-co-NAS) cores and well-solvated PEO coronas. Cross-linking of the P(NIPAM-co-NAS) cores was facilely achieved via the reaction of NAS residues with cystamine at elevated temperatures in aqueous media, forming structurally permanent core cross-linked micelles. Most importantly, the disulfide bonds within the cross-linker can be conveniently cleaved in the presence of dithiothreitol and re-formed again upon addition of cystamine as a thiol/disulfide exchange promoter, leading to the reversible core cross-linking of micelles. The P(NIPAM-co-NAS) cores of the obtained core cross-linked (CCL) micelles exhibit tunable swelling/deswelling behavior below and above the critical phase transition temperature. Dynamic laser light scattering, optical transmittance, 1H NMR, and transmission electron microscopy were used in combination to investigate the thermoresponsive micellization of PEO-b-P(NIPAM-co-NAS) and the subsequent reversible core cross-linking.

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