Anion-Radical Polymerization of Sulfur- and Selenium-Substituted N-Type Conjugated Polymers

Abstract

While controlled chain-growth polymerizations have been developed for a handful of p-type conjugated polymers, most n-type conjugated polymers are still synthesized through step-growth methods that offer little to no control over the polymerization. The anion-radical polymerization of thiophene-flanked naphthalene diimides has been shown to exhibit non-living chain-growth behavior; however, anion-radical polymerizations are limited to a few examples that exhibit varying degrees of control. Strategies to improve and expand the scope of this promising methodology have not been developed. In this study, we investigate the anion-radical polymerization of a series of oxygenated and thionated, thiophene- and selenophene-flanked naphthalene diimides. We show, through optical and spectroelectrochemical studies, that anion-radical monomers likely consist of a complex mixture of organic radicals with varying oxidation states. Surprisingly, subtle changes in monomer structure afforded by sulfur and selenium substitution result in significant changes in polymer synthesis. Notably, selenophene-flanked naphthalene diimides polymerize more rapidly to reach higher degrees of polymerization when compared to thiophene-flanked analogues. While thionated naphthalene diimides form anion-radical complexes and exhibit signs of catalyst insertion, they do not undergo polymerization. These results provide insights into the further development of anion-radical polymerization as a promising route to well-defined n-type conjugated polymers.