Investigation of Methyl Methacrylate and Vinyl Acetate Polymerization Promoted by Al(ⁱBu)₃/2,2‘-Bipyridine and Al(ⁱBu)₃/2,2‘-Bipyridine/TEMPO Complexes

Results of methyl methacrylate (MMA) and vinyl acetate (VOAc) polymerization using the Al(ⁱBu)₃/2,2‘-bipyridine/TEMPO initiating system at room temperature failed to confirm the “living” radical mechanism reported earlier. Several species apparently propagate without control in a normal free radical process and only above a critical initiator concentration. No evidence was found for the penta- and hexacoordinated aluminum species previously suggested, but high conversion to an alkoxyamine was observed. Additionally, mixing Al(ⁱBu)₃ and BIPY promotes a variety of reactions involving alkyl and hydride transfers to the aromatic rings forming reduced BIPY products, along with a persistent organoaluminum radical. All these results illustrate that the process is considerably more complicated than postulated before. Conversely, at −78 °C, a single active species excluding the participation of TEMPO has been put in evidence, displaying some “living” characteristics. Several experiments support its anionic nature, as previously suggested by Ikeda.