Cobalt-Mediated Switchable Catalysis for One-Pot Synthesis of Cyclic Polymers.

Owing to the temporal/spatial control over reactions, switchable catalysis is emerging as a powerful tool to regulate polymer sequence and architecture directly from monomer mixtures. Herein, a cobalt salen pentenoate complex [salen = ( R,R )- N,N' -bis(3,5-di-tertbutylsalicylidene)-1,2-cyclohexanediamine ] is rationally designed as the catalyst for the ring-opening copolymerization (ROCOP) of epoxides/anhydrides/CO 2 . Via the migratory insertion of carbon monoxide (CO) into the Co-O bonds, the ROCOP active species α -alkene- ω -O-Co III (salen) can be rapidly and quantitatively transformed into α -alkene- ω -O 2 C-Co III (salen) telechelic linear precursors. Upon dilution of reaction mixtures, the homolytic cleavage of Co-C bonds induced by visible light generates α -alkene acyl radicals that spontaneously undergo intramolecular radical addition to afford organocobalt functionalized cyclic polyesters and CO 2 -based polycarbonates with excellent regioselectivity. The cyclic products can either react with radical scavengers to generate metal-free cyclic polymers or serve as the photo initiators for organometallic mediated radical polymerization (OMRP) to produce tadpole-shaped copolymers. This work opens a new avenue for the direct synthesis of cyclic polymers from monomer mixtures via switchable catalysis.