Cobalt(III)Salen-Acetate : Synthesis and Use as Organometallic-Mediated Radical Polymerization Initiator

Abstract

Metal-mediated polymerizations have largely contributed to the design of unprecedented (co)polymers with valuable properties for many applications in recent years. In particular, salen-type cobalt complexes have served as initiators and mediators for different types of polymerization methods including the organometallic-mediated radical polymerization (OMRP) of vinyl monomers and the ring-opening copolymerization (ROCoP) of epoxyde/anhydride leading to polyesters. The OMRP is based on the reversible cleavage of an alkyl-cobalt(III) species whereas ROCoP involves carboxy- and alkoxy-cobalt derivatives. In this work, we explore for the first time the potential of a carboxy-cobalt(III) complex, i.e. (R,R)-SalenCoOAc, as initiator and mediating species in the OMRP of vinyl monomers. In case of success, this should pave the way to the combination of OMRP and ROCoP and so to the synthesis of well-defined polyester-b-poly(vinyl monomer) block copolymers in minimal steps. In practice, (R,R)-SalenCoOAc was synthesized and tested as initiator of the methyl acrylate polymerization performed under thermal or photolytic treatments via different mechanistic regimes, namely reversible termination (RT) or degenerative chain transfer (DT). Reactions carried out under heating showed some characteristics of controlled radical polymerizations and poly(methyl acrylate)s with moderate dispersity were prepared accordingly. Overall, this study reports the first example of an OMRP process initiated from a carboxy-cobalt complex, which constitutes an important step to bridge the gap between OMRP and ROCoP.