Redox initiated free radical polymerization of 4-methylstyrene

A mononuclear cobalt(III) carboxylate complex with a fully O-based coordination sphere: CoIII-O bond homolysis and controlled radical polymerisation from [Co(acac)2(O2CPh)]

The addition of benzoyl peroxide to [Co^II(acac)₂] in a 1 : 2 ratio selectively produces [Co^III(acac)₂(O₂CPh)], a diamagnetic (NMR) mononuclear Co^III complex with an octahedral (X-ray diffraction) coordination geometry. It is the first reported mononuclear Co^III derivative with a chelated monocarboxylate ligand and an entirely O-based coordination sphere. The compound degrades in solution quite slowly by homolytic Co^III-O₂CPh bond cleavage upon warming above 40 °C to produce benzoate radicals and can serve as a unimolecular thermal initiator for the well-controlled radical polymerisation of vinyl acetate. Addition of ligands (L = py, NEt₃) induces benzoate chelate ring opening and formation of both cis and trans isomers of [Co^III(acac)₂(O₂CPh)(L)] for L = py under kinetic control, then converting quantitatively to the cis isomer, whereas the reaction is less selective and equilibrated for L = NEt₃. The py addition strengthens the Co^III-O₂CPh bond and lowers the initiator efficiency in radical polymerisation, whereas the NEt₃ addition results in benzoate radical quenching by a redox process. In addition to clarifying the mechanism of the radical polymerisation redox initiation by peroxides and rationalizing the quite low efficiency factor for the previously reported [Co^II(acac)₂]/peroxide-initiated organometallic-mediated radical polymerisation (OMRP) of vinyl acetate, this investigation provides relevant information on the Co^III-O homolytic bond cleavage process.