Intrinsic conductivity mechanisms of radical polymer films with conjugated and non-conjugated backbones

Radical polymers, which incorporate a stable radical pendent group such as 2,2,6,6-tetramethyl pipiridine-1-oxyl (TEMPO), form as class of non-toxic, environmentally friendly materials for battery electrodes. Their conductivity in the solid phase, while critical for battery electrode applications because of the need to collect current, have been controversial. Here we discuss our study of DC conductivity in several species of radical polymers. First, we prepared poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) using several synthetic methods to examine if the previously proposed redox hopping mechanism is sensitive to the preparation details. We found that PTMA is an insulator, implying that few radical sites participate in conductivity. In search of a radical polymer with higher conductivity, we also investigated the properties of a radical polymer with a conjugated backbone. Our results show that although introducing conjugation to the backbone is a route to introduce conductivity to a radical polymer, the steric hindrance from the TEMPO groups limit the size of the ordered conjugated crystal domains and thus severely reduce the conductivity as compared to the conjugated backbone without TEMPO.