Solid State Electrical Conductivity of Radical Polymers as a Function of Pendant Group Oxidation State

Abstract

We establish the relationship between pendant group chemical identity and the ability of a specific radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), to transport charge in the solid state. Radical polymers (i.e., macromolecules with aliphatic carbon backbones and pendant groups containing stable radical moieties) have attracted much attention in organic electronic applications due to straightforward synthetic methods, easily tunable electronic properties, and relatively high-performance with respect to charge transport. Because charge transport can occur only through the pendant group of these completely amorphous radical polymers, controlling the precise chemical nature of these functional groups is of key import. Specifically, we have determined that the deprotection step, which converts the pendant group functionality through a simple oxidation reaction, can lead to four distinct chemical functionalities along the radical polymer, as monitored by a range of complementary spectroscopic techniques. Of these four functionalities, only two (i.e., the stable free radical and the corresponding oxoammonium cation) are able to contribute positively to the charge transport ability of the macromolecule. As such, manipulating the reaction conditions for this deprotection step, and monitoring closely the resultant chemical functionalities, is critical in tuning the electrical properties of radical polymers. However, if these parameters are controlled well, we are able to generate transparent, conducting thin films of pristine (i.e., not doped) nonconjugated radical polymers with electrical conductivities as high as (1.5 ± 0.3) × 10–5 S cm–1.