Stereochemistry control of the alternating copolymerization of CO2 and propylene oxide catalyzed by SalenCrX complexes

Various poly(propylene carbonate)s (PPC)s with different head-to-tail linkages (from similar to 60% to 93%) could be synthesized by the copolymerization of racemic propylene oxide (rac-PO) and CO2 with chromium Salen complexes as catalyst. The axial X group and the Salen ligand framework of the chromium complexes and the properties of cocatalyst significantly affect stereochemistry of the resulting polymer. Continuous determination of a polymer chain end group (initiating and chain growth species) at various times was achieved by in situ electrospray ionization mass spectrometry (ESI-MS), which in combination with the terpolymerization of CO2 with R-PO and cyclohexene oxide (CHO) gave us an insight into the mechanistic understanding of CO2/PO copolymerization and polycarbonates stereochemistry control. Apart from the effect of the ligand set of the chromium Salen complexes on polymer stereochemistry, a "polymer chain-end control" mechanism also significantly affects the stereochemistry of the resulting polycarbonates.