Poly(N-alkyl-3,6-carbazole)s via Kumada Catalyst Transfer Polymerization: Impact of Metal–Halogen Exchange

Abstract

The preparation of poly(3,6-carbazole) via Kumada catalyst transfer polymerization (KCTP) is investigated and analyzed in detail by mass spectrometry to explore the scope of the applied protocols. Namely, common magnesium reagents were screened for the initial Grignard metathesis (GRIM) step and subsequently polymerized using [Ni(dppp)Cl2] (dppp is 1,3-bis(diphenylphosphanyl)propane). The metal–halogen exchange was monitored by GC, while the polymers were characterized by size exclusion chromatography (SEC) and 1H NMR spectroscopy. More importantly, the polymer end groups were investigated by means of mass spectrometry and isotope analysis, which revealed significant deviation from the chain-growth character of related poly(3-hexylthiophene). It was found that standard Grignard reagents led to incomplete GRIM even under extended reaction times and elevated temperatures, while the presence of LiCl greatly accelerated the metal–halogen exchange and also the KCTP. According to end-group analysis of the obtained polymers, side reactions occur that are attributed and explained in terms of catalyst dissociation and/or disproportionation. Namely, the replacement of the bromo-substituents by either alkyl groups via transmetalation or hydrogen via β-hydride elimination was observed. Finally, quantitative metal–halogen exchange was achieved using a magnesiate reagent, which also afforded the corresponding polymer in good isolated yield (66%). The end-group scrambling, dispersity, and observed molar masses support the proposed role of catalyst dissociation and/or disproportionation, even in the absence of excess of residual Grignard reagent.