Cyclic Polyester Formation with an [OSSO]‐Type Iron(III) Catalyst

The efficient formation of cyclic polyesters from the ring‐opening polymerization of lactide, ϵ‐caprolactone, and β‐butyrolactone catalyzed by a 1,4‐dithiabutanedyl‐2,2′‐bis(4,6‐dicumylphenol) [OSSO]‐FeCl complex activated with cyclohexene oxide was achieved. The catalyst was very active (initial turnover frequency up to 2718 h −1 ), robust, and worked with a monomer/Fe ratio up to 10 000. The formation of cyclic polymers was supported by using high‐resolution matrix‐assisted laser desorption ionization (MALDI) MS, and the average ring size (≈5 kDa for cyclic polylactide) independent of the reaction conditions. A monometallic ring‐opening polymerization/cyclization mechanism was proposed from the results of a kinetic investigation.