Poly(N-alkyl-3,6-carbazole)s via Suzuki–Miyaura Polymerization: From Macrocyclization toward End Functionalization

Abstract

The metal-catalyzed polymerization of 3,6-carbazoles is explored with emphasis of end-group fidelity of the telechelic polymers. The Suzuki–Miyaura coupling polymerization using asymmetric bifunctional (AB-type) monomers was selected to circumvent the inherent polycondensation character of symmetric (AA/BB-type) monomers. Common synthetic protocols were screened to facilitate the chain growth vs step growth character during the polymerization, e.g., several activation reagents for the boronic ester and various catalytic systems and their associated reaction conditions. The reactions were analyzed in detail by size exclusion chromatography (SEC), 1H NMR spectroscopy, and mass spectrometry including isotope analysis for end-group determination. It was found that the intermediate oligomers are susceptible for macrocycle formation, particularly at low concentration, while longer polymer chains continue to polymerize. The synthetic methodology was extended to utilize functionalized precatalysts and functional terminating agents in order to explore the scope of chain growth vs step growth. The concomitant formation of macrocycles was observed, which indicates the catalyst dissociation and, thus, also the polycondensation pathway. Importantly, the applied low concentration leads to trapping of the undesired nonfunctionalized chains as macrocycles, which can be readily removed from the telechelic linear polymer. In summary, the developed protocols demonstrate the versatility to prepare telechelic poly(3,6-carbazole)s via Suzuki–Miyaura coupling. The azide-decorated poly(carbazole) exemplifies the potential to serve as building blocks for further copper-mediated azide–alkyne cycloaddition reactions.