Regarding initial ring opening of propylene oxide in its copolymerization with CO2 catalyzed by a cobalt(III) porphyrin complex

Abstract

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 (1) H NMR 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 (1) H NMR spectra.

Keywords: NMR spectroscopy; carbon dioxide; onium salts; porphyrinoids; ring-opening polymerization.