Mechanism of nickel catalysed dehalogenative step polymerization of aryl dibromides

Abstract

AbstractDehalogenative polycondensation of 3,6‐dibromo‐N‐alkylcarbazoles was catalysed by a Ni(O)‐based complex generated in N,N‐dimethylacetamide by electroreduction of NiBr2/2,2′‐bipyridine. The process involves three different reaction steps, which are successively: electrolysis of Ni(II) to Ni(O); insertion of the latter into a CBr bond of monomer leading to CNiBr* active species; dehalogenative coupling between active species and monomer yielding oligomers. From the kinetics of electrogeneration of the catalytic system in absence of monomer, a first‐order reaction was determined. Insertion and coupling reactions were found to be second‐order and comparison of the values of the corresponding rate constants clearly showed that coupling is the rate‐determining step of the process. Simultaneous size exclusion chromatography and cyclic voltammetry measurements were carried out in the course of the electrosynthesis of poly(N‐alkylcarbazole)s. The change in the molar mass population distribution as a function of electrolysis time revealed a step polymerization process, while showing a termination of the chain growth due to the occurrence of monobrominated species. The study of the effect of the [monomer]/[catalyst] ratio on the molar mass population change indicated that the coupling of active species (Ni(O)‐activated monomer) is responsible for the chain growth.