Kinetic and mechanistic investigations of the copolymerization reaction of carbon dioxide and epoxides, using chromium chloride salen catalysts

A series of air-stable (salen)CrIIICl complexes, where H₂salen = N,N′-bis(3,5-di-t-butylsalicylidene)-1,2-di-(R)-1,2-ethylenediimine, have been shown to be effective catalysts for the coupling of cyclohexene oxide and propylene oxide with CO₂, to afford co-poly(cyclohexene carbonate) and co-poly(propylene carbonate), along with various quantities of the associated cyclic carbonate byproduct. The polycarbonate obtained for the reaction of cyclohexene oxide and CO₂, mediated by Jacobsen's catalyst (N,N′-bis(3,5-di-t-butylsalicylidene)-1,2-cyclohexanediimino Cr(III) chloride), exhibited >99% CO₂ incorporation and a Mn = 8900 g/mol with a polydispersity index (PDI) of 1.2 as determined by gel permeation chromatography. Further, Jacobsen's catalyst proved most active giving turnover number (TON) and turnover frequency (TOF) values of 250 moles of CHO consumed/mole of Cr and 10 mol CHO consumed/mol Cr/hour for reactions carried out at 80°C and 58.5 bar in CO₂ pressure. These values increased by threefold upon addition of 5 equivalents of N-methylimidazole relative to [Cr]. The polymer produced was completely atactic as evidenced by 13C NMR spectroscopy. The rate of carbonate chain growth and cyclic carbonate production are linearly dependent on [Cr]. This is demonstrated herein by way of in situ measurements. This was shown to hold for the use of CHO and PO substrates. Investigations of this (salen)CrIII-catalyzed system for the coupling of propylene oxide and CO₂ revealed that although cyclic carbonate was the major product at elevated temperatures, reactions carried out at ambient temperature produced predominantly co-poly(propylene carbonate). This characteristic difference between alicyclic and aliphatic substrates was investigated quantitatively by way of variable-temperature in situ measurements. Arrhenius analysis of the thus obtained rate data showed the activation energies for copolymer and cyclic carbonate formation in the PO/CO₂ coupling reaction to be 20 kJ/mol greater and 33 kJ/mol lower than that observed for the CHO/CO₂ coupling reaction, respectively.