Mechanism of nickel catalysed dehalogenative step polymerization of aryl dibromides

Dehalogenative polycondensation of 3,6-dibromo-N-alkylcarbazoles was catalysed by a Ni(O)-based complex generated in N,N-dimethylacetamide by electroreduction of NiBr 2 /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.