Effective, Selective Coupling of Propylene Oxide and Carbon Dioxide to Poly(Propylene Carbonate) Using (Salen)CrN₃ Catalysts

The copolymerization of propylene oxide and CO2 has been investigated employing Cr(salen)N3 complexes as catalysts. Unfortunately the reaction could not be studied in real time via in situ IR spectroscopy, thereby obtaining detailed kinetic data, because of the copolymer limited solubility in most solvents. Investigations employing batch reactor runs concentrating on varying the cocatalyst, the equivalents of cocatalyst, and the steric and electronic structure of the catalyst through modification of the salen ligand were undertaken. It was discovered that the optimal catalyst for copolymer selectivity vs the monomeric propylene carbonate was one that contained a salen ligand with an electron-withdrawing phenylene backbone and electron-donating tert-butyl groups in the phenolate rings. This catalyst was used to investigate the effect of altering the nature of the cocatalyst and its concentration, the three cocatalysts being tricyclohexylphosphine (PCy3), PPN+ N3(-), and PPN+ Cl-, where PPN+ is the large very weakly interacting bis(triphenylphosphoramylidene)ammonium cation. By utilization of more or less than 1 equiv of PCy3 as cocatalyst, the yield of polymer was reduced. On the other hand, the PPN+ salts showed the best activity when 0.5 equiv was employed, and produced only cyclic when using over 1 equiv.