Polymer bottlebrushes with a redox responsive backbone feel the heat: Synthesis and characterization of dual responsive poly(ferrocenylsilane)s with PNIPAM side chains

Molecular bottlebrushes, possessing a redox-responsive poly(ferrocenylsilane) (PFS) backbone and temperature-responsive poly(N-isopropylacrylamide) (PNIPAM) side chains, distributed homogeneously or as a gradient along the PFS main chain, were synthesized. Attachment of the PNIPAM chains via azide-alkyne click chemistry, or grafting from a PFS macroinitiator backbone by ATRP, resulted in cylindrical shaped molecular bottlebrushes. We found the bottlebrushes to be both redox and temperature responsive, with little influence of one responsiveness on the other. In an aqueous environment above 32°C the bottlebrushes collapsed to 70% of their original size due to the temperature sensitive side chains, and reversibly recovered their initial size upon cooling as revealed by Dynamic Light Scattering (DLS). Cyclic voltammograms showed electrochemical behavior typical of well solvated, single PFS chains. The backbone of the deposited molecules was in close proximity to the highly ordered pyrolytic graphite (HOPG) electrode surface and was accessible to the supporting electrolyte owing to the presence of the hydrophilic PNIPAM side chains. The bottlebrush molecules were deposited on HOPG surfaces for direct molecular visualization by atomic force microscopy (AFM). Molecular size data obtained by DLS and AFM showed good agreement. The bottlebrushes, reported here, are excellent candidates as addressable components for future devices e.g. to carry and deliver molecular payloads.