Regarding Initial Ring Opening of Propylene Oxide in its Copolymerization with CO₂ Catalyzed by a Cobalt(III) Porphyrin Complex

Cobalt(III) tetraphenylporphyrin chloride (TPPCoCl) was experimentally proved to be an active catalyst for poly(propylene carbonate) production. It was chosen as a model catalyst in the present work to investigate the initiation step of propylene oxide (PO)/CO₂ copolymerization, which is supposed to be the ring opening of the epoxide. Ring-opening intermediates (1-7) were detected by using ¹HNMR spectroscopy. A first-order reaction in TPPCoCl was determined. A combination of monometallic and bimetallic ring-opening pathways is proposed according to kinetics experiments. Addition of onium salts (e.g., bis(triphenylphosphine)iminium chloride, PPNCl) efficiently promoted the PO ring-opening rate. The existence of axial ligand exchange in the cobalt porphyrin complex in the presence of onium salts was suggested by analyzing collected ¹HNMR spectra. Monometallic or bimetallic? Ring-opening intermediates of propylene oxide (PO) catalyzed by a cobalt(III) porphyrin complex were identified by using ¹HNMR spectroscopy. The generated metal alkoxides underwent hydrolysis in the presence of trace amounts of water. A combined bimetallic and monometallic ring-opening pathway was suggested from the kinetics results. Addition of onium salts accelerated the PO ring opening. Ligand exchange on the cobalt center in the presence of onium salts was also detected.