Controlling open-shell loading in norbornene-based radical polymers modulates the solid-state charge transport exponentially

Radical polymers are an emerging class of electronically active macromolecules; however, the fundamental mechanism by which charge is transferred in these polymers has yet to be established in full. To address this issue, well-defined norbornene-based nitroxide radical polymers were synthesized using the controlled ring-opening metathesis polymerization technique. These polymers were blended in solution with a quenched, electrically insulating hydroxylamine derivative to dilute the radical content of the system. Electron paramagnetic resonance spectroscopy data were used to characterize the radical content as well as to reveal that hydrogen atom transfer occurred between the open-shell and closed-shell polynorbornene derivatives when they were blended in solution. Using these platform macromolecules, we demonstrate that the systematic manipulation of the radical content in open-shell macromolecules leads to exponential changes in the macroscopic electrical conductivity. When coupled with the fact that these materials show a clear temperature-independent charge transport behavior, a picture emerges that charge transfer in radical polymers is dictated by a tunneling mechanism between localized donor and acceptor sites within the redox-active thin films. These results constitute the first experimental insight into the mechanism of solid-state electrical conduction in radical polymers, and this provides a design paradigm for open-shell macromolecular charge transport. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1516–1525