ppm-Level Thermally Switchable Yttrium Phenoxide Catalysts for Moisture-Insensitive and Controllably Immortal Polymerization of rac-Lactide

Exploring the moisture-insensitive catalysis system for the ring-opening polymerization of cyclic esters is very valuable for the synthesis of polyesters in industry. To reduce metal residue in final polymers, using ppm-level catalysts for the moisture-insensitive and controllable ROP system is still a big challenge until now. In this work, as a general strategy, a series of D3 symmetrical binuclear yttrium phenoxides were applied in the moisture-insensitive ring-opening polymerization of rac-lactide. The structures of these yttrium complexes can be reversibly switched between highly active and less active species in the presence of water via changing temperature. These catalysts are so robust in air, the loading of catalyst can be as low as 15 ppm, and one molecular catalyst can mediate the synthesis of more than 1000 polymer chains. The low initiation efficiency of water was proved unambiguously; hence, the polymerizations are controllable with a highly efficient initiator of alcohol in moisture conditi...